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rfc:rfc3435

Network Working Group F. Andreasen Request for Comments: 3435 B. Foster Obsoletes: 2705 Cisco Systems Category: Informational January 2003

               Media Gateway Control Protocol (MGCP)
                            Version 1.0

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

IESG Note

 This document is being published for the information of the
 community.  It describes a protocol that is currently being deployed
 in a number of products.  Implementers should be aware of RFC 3015,
 which was developed in the IETF Megaco Working Group and the ITU-T
 SG16 and which is considered by the IETF and ITU-T to be the
 standards-based (including reviewed security considerations) way to
 meet the needs that MGCP was designed to address.

Abstract

 This document describes an application programming interface and a
 corresponding protocol (MGCP) which is used between elements of a
 decomposed multimedia gateway.  The decomposed multimedia gateway
 consists of a Call Agent, which contains the call control
 "intelligence", and a media gateway which contains the media
 functions, e.g., conversion from TDM voice to Voice over IP.
 Media gateways contain endpoints on which the Call Agent can create,
 modify and delete connections in order to establish and control media
 sessions with other multimedia endpoints.  Also, the Call Agent can
 instruct the endpoints to detect certain events and generate signals.
 The endpoints automatically communicate changes in service state to
 the Call Agent.  Furthermore, the Call Agent can audit endpoints as
 well as the connections on endpoints.

Andreasen & Foster Informational [Page 1] RFC 3435 MGCP 1.0 January 2003

 The basic and general MGCP protocol is defined in this document,
 however most media gateways will need to implement one or more MGCP
 packages, which define extensions to the protocol suitable for use
 with specific types of media gateways.  Such packages are defined in
 separate documents.

Table of Contents

 1.     Introduction.................................................5
 1.1    Relation with the H.323 Standards............................7
 1.2    Relation with the IETF Standards.............................8
 1.3    Definitions..................................................9
 1.4    Conventions used in this Document............................9
 2.     Media Gateway Control Interface.............................10
 2.1    Model and Naming Conventions................................10
 2.1.1  Types of Endpoints..........................................10
 2.1.2  Endpoint Identifiers........................................14
 2.1.3  Calls and Connections.......................................16
 2.1.4  Names of Call Agents and Other Entities.....................22
 2.1.5  Digit Maps..................................................23
 2.1.6  Packages....................................................26
 2.1.7  Events and Signals..........................................28
 2.2    Usage of SDP................................................33
 2.3    Gateway Control Commands....................................33
 2.3.1  Overview of Commands........................................33
 2.3.2  EndpointConfiguration.......................................36
 2.3.3  NotificationRequest.........................................37
 2.3.4  Notify......................................................44
 2.3.5  CreateConnection............................................46
 2.3.6  ModifyConnection............................................52
 2.3.7  DeleteConnection (from the Call Agent)......................54
 2.3.8  DeleteConnection (from the gateway).........................58
 2.3.9  DeleteConnection (multiple connections from the Call Agent) 59
 2.3.10 AuditEndpoint...............................................60
 2.3.11 AuditConnection.............................................65
 2.3.12 RestartInProgress...........................................66
 2.4    Return Codes and Error Codes................................69
 2.5    Reason Codes................................................74
 2.6    Use of Local Connection Options and Connection Descriptors..75
 2.7    Resource Reservations.......................................77
 3.     Media Gateway Control Protocol..............................77
 3.1    General Description.........................................78
 3.2    Command Header..............................................79
 3.2.1  Command Line................................................79
 3.2.2  Parameter Lines.............................................82
 3.3    Format of response headers.................................101
 3.3.1  CreateConnection Response..................................104
 3.3.2  ModifyConnection Response..................................105

Andreasen & Foster Informational [Page 2] RFC 3435 MGCP 1.0 January 2003

 3.3.3  DeleteConnection Response..................................106
 3.3.4  NotificationRequest Response...............................106
 3.3.5  Notify Response............................................106
 3.3.6  AuditEndpoint Response.....................................106
 3.3.7  AuditConnection Response...................................107
 3.3.8  RestartInProgress Response.................................108
 3.4    Encoding of the Session Description (SDP)..................108
 3.4.1  Usage of SDP for an Audio Service..........................110
 3.4.2  Usage of SDP for LOCAL Connections.........................110
 3.5    Transmission over UDP......................................111
 3.5.1  Providing the At-Most-Once Functionality...................112
 3.5.2  Transaction Identifiers and Three Ways Handshake...........113
 3.5.3  Computing Retransmission Timers............................114
 3.5.4  Maximum Datagram Size, Fragmentation and Reassembly........115
 3.5.5  Piggybacking...............................................116
 3.5.6  Provisional Responses......................................117
 4.     States, Failover and Race Conditions.......................119
 4.1    Failover Assumptions and Highlights........................119
 4.2    Communicating with Gateways................................121
 4.3    Retransmission, and Detection of Lost Associations:........122
 4.4    Race Conditions............................................126
 4.4.1  Quarantine List............................................127
 4.4.2  Explicit Detection.........................................133
 4.4.3  Transactional Semantics....................................134
 4.4.4  Ordering of Commands, and Treatment of Misorder............135
 4.4.5  Endpoint Service States....................................137
 4.4.6  Fighting the Restart Avalanche.............................140
 4.4.7  Disconnected Endpoints.....................................143
 4.4.8  Load Control in General....................................146
 5.     Security Requirements......................................147
 5.1    Protection of Media Connections............................148
 6.     Packages...................................................148
 6.1    Actions....................................................150
 6.2    BearerInformation..........................................150
 6.3    ConnectionModes............................................151
 6.4    ConnectionParameters.......................................151
 6.5    DigitMapLetters............................................151
 6.6    Events and Signals.........................................152
 6.6.1  Default and Reserved Events................................155
 6.7    ExtensionParameters........................................156
 6.8    LocalConnectionOptions.....................................157
 6.9    Reason Codes...............................................157
 6.10   RestartMethods.............................................158
 6.11   Return Codes...............................................158
 7.     Versions and Compatibility.................................158
 7.1    Changes from RFC 2705......................................158
 8.     Security Considerations....................................164
 9.     Acknowledgments............................................164

Andreasen & Foster Informational [Page 3] RFC 3435 MGCP 1.0 January 2003

 10.    References.................................................164
 Appendix A: Formal Syntax Description of the Protocol.............167
 Appendix B: Base Package..........................................175
 B.1    Events.....................................................175
 B.2    Extension Parameters.......................................176
 B.2.1  PersistentEvents...........................................176
 B.2.2  NotificationState..........................................177
 B.3    Verbs......................................................177
 Appendix C: IANA Considerations...................................179
 C.1    New MGCP Package Sub-Registry..............................179
 C.2    New MGCP Package...........................................179
 C.3    New MGCP LocalConnectionOptions Sub-Registry...............179
 Appendix D: Mode Interactions.....................................180
 Appendix E: Endpoint Naming Conventions...........................182
 E.1    Analog Access Line Endpoints...............................182
 E.2    Digital Trunks.............................................182
 E.3    Virtual Endpoints..........................................183
 E.4    Media Gateway..............................................184
 E.5    Range Wildcards............................................184
 Appendix F: Example Command Encodings.............................185
 F.1    NotificationRequest........................................185
 F.2    Notify.....................................................186
 F.3    CreateConnection...........................................186
 F.4    ModifyConnection...........................................189
 F.5    DeleteConnection (from the Call Agent).....................189
 F.6    DeleteConnection (from the gateway)........................190
 F.7    DeleteConnection (multiple connections
        from the Call Agent).......................................190
 F.8    AuditEndpoint..............................................191
 F.9    AuditConnection............................................192
 F.10   RestartInProgress..........................................193
 Appendix G: Example Call Flows....................................194
 G.1    Restart....................................................195
 G.1.1  Residential Gateway Restart................................195
 G.1.2  Call Agent Restart.........................................198
 G.2    Connection Creation........................................200
 G.2.1  Residential Gateway to Residential Gateway.................200
 G.3    Connection Deletion........................................206
 G.3.1  Residential Gateway to Residential Gateway.................206
 Authors' Addresses................................................209
 Full Copyright Statement..........................................210

Andreasen & Foster Informational [Page 4] RFC 3435 MGCP 1.0 January 2003

1. Introduction

 This document describes an abstract application programming interface
 (MGCI) and a corresponding protocol (MGCP) for controlling media
 gateways from external call control elements called media gateway
 controllers or Call Agents.  A media gateway is typically a network
 element that provides conversion between the audio signals carried on
 telephone circuits and data packets carried over the Internet or over
 other packet networks.  Examples of media gateways are:
  • Trunking gateways, that interface between the telephone network and

a Voice over IP network. Such gateways typically manage a large

   number of digital circuits.
  • Voice over ATM gateways, which operate much the same way as voice

over IP trunking gateways, except that they interface to an ATM

   network.
  • Residential gateways, that provide a traditional analog (RJ11)

interface to a Voice over IP network. Examples of residential

   gateways include cable modem/cable set-top boxes, xDSL devices, and
   broad-band wireless devices.
  • Access gateways, that provide a traditional analog (RJ11) or

digital PBX interface to a Voice over IP network. Examples of

   access gateways include small-scale voice over IP gateways.
  • Business gateways, that provide a traditional digital PBX interface

or an integrated "soft PBX" interface to a Voice over IP network.

  • Network Access Servers, that can attach a "modem" to a telephone

circuit and provide data access to the Internet. We expect that in

   the future, the same gateways will combine Voice over IP services
   and Network Access services.
  • Circuit switches, or packet switches, which can offer a control

interface to an external call control element.

 MGCP assumes a call control architecture where the call control
 "intelligence" is outside the gateways and handled by external call
 control elements known as Call Agents.  The MGCP assumes that these
 call control elements, or Call Agents, will synchronize with each
 other to send coherent commands and responses to the gateways under
 their control.  If this assumption is violated, inconsistent behavior
 should be expected.  MGCP does not define a mechanism for
 synchronizing Call Agents.  MGCP is, in essence, a master/slave
 protocol, where the gateways are expected to execute commands sent by
 the Call Agents.  In consequence, this document specifies in great

Andreasen & Foster Informational [Page 5] RFC 3435 MGCP 1.0 January 2003

 detail the expected behavior of the gateways, but only specifies
 those parts of a Call Agent implementation, such as timer management,
 that are mandated for proper operation of the protocol.
 MGCP assumes a connection model where the basic constructs are
 endpoints and connections.  Endpoints are sources and/or sinks of
 data and can be physical or virtual.  Examples of physical endpoints
 are:
  • An interface on a gateway that terminates a trunk connected to a

PSTN switch (e.g., Class 5, Class 4, etc.). A gateway that

   terminates trunks is called a trunking gateway.
  • An interface on a gateway that terminates an analog POTS connection

to a phone, key system, PBX, etc. A gateway that terminates

   residential POTS lines (to phones) is called a residential gateway.
 An example of a virtual endpoint is an audio source in an audio-
 content server.  Creation of physical endpoints requires hardware
 installation, while creation of virtual endpoints can be done by
 software.
 Connections may be either point to point or multipoint.  A point to
 point connection is an association between two endpoints with the
 purpose of transmitting data between these endpoints.  Once this
 association is established for both endpoints, data transfer between
 these endpoints can take place.  A multipoint connection is
 established by connecting the endpoint to a multipoint session.
 Connections can be established over several types of bearer networks,
 for example:
  • Transmission of audio packets using RTP and UDP over an IP network.
  • Transmission of audio packets using AAL2, or another adaptation

layer, over an ATM network.

  • Transmission of packets over an internal connection, for example

the TDM backplane or the interconnection bus of a gateway. This is

   used, in particular, for "hairpin" connections, connections that
   terminate in a gateway but are immediately rerouted over the
   telephone network.
 For point-to-point connections the endpoints of a connection could be
 in separate gateways or in the same gateway.

Andreasen & Foster Informational [Page 6] RFC 3435 MGCP 1.0 January 2003

1.1 Relation with the H.323 Standards

 MGCP is designed as an internal protocol within a distributed system
 that appears to the outside as a single VoIP gateway.  This system is
 composed of a Call Agent, that may or may not be distributed over
 several computer platforms, and of a set of gateways, including at
 least one "media gateway" that perform the conversion of media
 signals between circuits and packets, and at least one "signaling
 gateway" when connecting to an SS7 controlled network.  In a typical
 configuration, this distributed gateway system will interface on one
 side with one or more telephony (i.e., circuit) switches, and on the
 other side with H.323 conformant systems, as indicated in the
 following table:
  1. —————————————————————–

| Functional| Phone | Terminating | H.323 conformant |

 | Plane     |  switch    |  Entity         |  systems              |
 |-----------|------------|-----------------|-----------------------|
 | Signaling |  Signaling |  Call agent     |  Signaling exchanges  |
 | Plane     |  exchanges |                 |  with the Call Agent  |
 |           |  through   |                 |  through H.225/RAS and|
 |           |  SS7/ISUP  |                 |  H.225/Q.931.         |
 |-----------|------------|-----------------|-----------------------|
 |           |            |                 |  Possible negotiation |
 |           |            |                 |  of logical channels  |
 |           |            |                 |  and transmission     |
 |           |            |                 |  parameters through   |
 |           |            |                 |  H.245 with the call  |
 |           |            |                 |  agent.               |
 |-----------|------------|-----------------|-----------------------|
 |           |            |  Internal       |                       |
 |           |            |  synchronization|                       |
 |           |            |  through MGCP   |                       |
 |-----------|------------|-----------------|-----------------------|
 | Bearer    |  Connection|  Telephony      |  Transmission of VoIP |
 | Data      |  through   |  gateways       |  data using RTP       |
 | Transport |  high speed|                 |  directly between the |
 | Plane     |  trunk     |                 |  H.323 station and the|
 |           |  groups    |                 |  gateway.             |
  ------------------------------------------------------------------
 In the MGCP model, the gateways focus on the audio signal translation
 function, while the Call Agent handles the call signaling and call
 processing functions.  As a consequence, the Call Agent implements
 the "signaling" layers of the H.323 standard, and presents itself as
 an "H.323 Gatekeeper" or as one or more "H.323 Endpoints" to the
 H.323 systems.

Andreasen & Foster Informational [Page 7] RFC 3435 MGCP 1.0 January 2003

1.2 Relation with the IETF Standards

 While H.323 is the recognized standard for VoIP terminals, the IETF
 has also produced specifications for other types of multi-media
 applications.  These other specifications include:
  • the Session Description Protocol (SDP), RFC 2327
  • the Session Announcement Protocol (SAP), RFC 2974
  • the Session Initiation Protocol (SIP), RFC 3261
  • the Real Time Streaming Protocol (RTSP), RFC 2326.
 The latter three specifications are in fact alternative signaling
 standards that allow for the transmission of a session description to
 an interested party.  SAP is used by multicast session managers to
 distribute a multicast session description to a large group of
 recipients, SIP is used to invite an individual user to take part in
 a point-to-point or unicast session, RTSP is used to interface a
 server that provides real time data.  In all three cases, the session
 description is described according to SDP; when audio is transmitted,
 it is transmitted through the Real-time Transport Protocol, RTP.

Andreasen & Foster Informational [Page 8] RFC 3435 MGCP 1.0 January 2003

 The distributed gateway systems and MGCP will enable PSTN telephony
 users to access sessions set up using SAP, SIP or RTSP.  The Call
 Agent provides for signaling conversion, according to the following
 table:
  1. —————————————————————–

| Functional| Phone | Terminating | IETF conforming systems|

 | Plane     |  switch    |  Entity       |                         |
 |-----------|------------|---------------|-------------------------|
 | Signaling |  Signaling |  Call agent   |  Signaling exchanges    |
 | Plane     |  exchanges |               |  with the Call Agent    |
 |           |  through   |               |  through SAP, SIP or    |
 |           |  SS7/ISUP  |               |  RTSP.                  |
 |-----------|------------|---------------|-------------------------|
 |           |            |               |  Negotiation of session |
 |           |            |               |  description parameters |
 |           |            |               |  through SDP (telephony |
 |           |            |               |  gateway terminated but |
 |           |            |               |  passed via the call    |
 |           |            |               |  agent to and from the  |
 |           |            |               |  IETF conforming system)|
 |-----------|------------|---------------|-------------------------|
 |           |            | Internal syn- |                         |
 |           |            | chronization  |                         |
 |           |            | through MGCP  |                         |
 |-----------|------------|---------------|-------------------------|
 | Bearer    |  Connection|  Telephony    |  Transmission of VoIP   |
 | Data      |  through   |  gateways     |  data using RTP,        |
 | Transport |  high speed|               |  directly between the   |
 | Plane     |  trunk     |               |  remote IP end system   |
 |           |  groups    |               |  and the gateway.       |
  ------------------------------------------------------------------
 The SDP standard has a pivotal status in this architecture.  We will
 see in the following description that we also use it to carry session
 descriptions in MGCP.

1.3 Definitions

 Trunk:  A communication channel between two switching systems, e.g.,
 a DS0 on a T1 or E1 line.

1.4 Conventions used in this Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED, "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in BCP
 14, RFC 2119 [2].

Andreasen & Foster Informational [Page 9] RFC 3435 MGCP 1.0 January 2003

2. Media Gateway Control Interface

 The interface functions provide for connection control and endpoint
 control.  Both use the same system model and the same naming
 conventions.

2.1 Model and Naming Conventions

 The MGCP assumes a connection model where the basic constructs are
 endpoints and connections.  Connections are grouped in calls.  One or
 more connections can belong to one call.  Connections and calls are
 set up at the initiative of one or more Call Agents.

2.1.1 Types of Endpoints

 In the introduction, we presented several classes of gateways.  Such
 classifications, however, can be misleading.  Manufacturers can
 arbitrarily decide to provide several types of services in a single
 package.  A single product could well, for example, provide some
 trunk connections to telephony switches, some primary rate
 connections and some analog line interfaces, thus sharing the
 characteristics of what we described in the introduction as
 "trunking", "access" and "residential" gateways.  MGCP does not make
 assumptions about such groupings.  We simply assume that media
 gateways support collections of endpoints.  The type of the endpoint
 determines its functionality.  Our analysis, so far, has led us to
 isolate the following basic endpoint types:
  • Digital channel (DS0),
  • Analog line,
  • Announcement server access point,
  • Interactive Voice Response access point,
  • Conference bridge access point,
  • Packet relay,
  • ATM "trunk side" interface.
 In this section, we will describe the expected behavior of such
 endpoints.

Andreasen & Foster Informational [Page 10] RFC 3435 MGCP 1.0 January 2003

 This list is not final.  There may be other types of endpoints
 defined in the future, for example test endpoints that could be used
 to check network quality, or frame-relay endpoints that could be used
 to manage audio channels multiplexed over a frame-relay virtual
 circuit.

2.1.1.1 Digital Channel (DS0)

 Digital channels provide a 64 Kbps service.  Such channels are found
 in trunk and ISDN interfaces.  They are typically part of digital
 multiplexes, such as T1, E1, T3 or E3 interfaces.  Media gateways
 that support such channels are capable of translating the digital
 signals received on the channel, which may be encoded according to
 A-law or mu-law, using either the complete set of 8 bits per sample
 or only 7 of these bits, into audio packets.  When the media gateway
 also supports a Network Access Server (NAS) service, the gateway
 shall be capable of receiving either audio-encoded data (modem
 connection) or binary data (ISDN connection) and convert them into
 data packets.
                                       +-------
                         +------------+|
            (channel) ===|DS0 endpoint| -------- Connections
                         +------------+|
                                       +-------
 Media gateways should be able to establish several connections
 between the endpoint and the packet networks, or between the endpoint
 and other endpoints in the same gateway.  The signals originating
 from these connections shall be mixed according to the connection
 "mode", as specified later in this document.  The precise number of
 connections that an endpoint supports is a characteristic of the
 gateway, and may in fact vary according to the allocation of
 resources within the gateway.
 In some cases, digital channels are used to carry signaling.  This is
 the case for example for SS7 "F" links, or ISDN "D" channels.  Media
 gateways that support these signaling functions shall be able to send
 and receive the signaling packets to and from a Call Agent, using the
 "backhaul" procedures defined by the SIGTRAN working group of the
 IETF.  Digital channels are sometimes used in conjunction with
 channel associated signaling, such as "MF R2".  Media gateways that
 support these signaling functions shall be able to detect and produce
 the corresponding signals, such as for example "wink" or "A",
 according to the event signaling and reporting procedures defined in
 MGCP.

Andreasen & Foster Informational [Page 11] RFC 3435 MGCP 1.0 January 2003

2.1.1.2 Analog Line

 Analog lines can be used either as a "client" interface, providing
 service to a classic telephone unit, or as a "service" interface,
 allowing the gateway to send and receive analog calls.  When the
 media gateway also supports a NAS service, the gateway shall be
 capable of receiving audio-encoded data (modem connection) and
 convert them into data packets.
                                       +-------
                      +---------------+|
            (line) ===|analog endpoint| -------- Connections
                      +---------------+|
                                       +-------
 Media gateways should be able to establish several connections
 between the endpoint and the packet networks, or between the endpoint
 and other endpoints in the same gateway.  The audio signals
 originating from these connections shall be mixed according to the
 connection "mode", as specified later in this document.  The precise
 number of connections that an endpoint supports is a characteristic
 of the gateway, and may in fact vary according to the allocation of
 resources within the gateway.  A typical gateway should however be
 able to support two or three connections per endpoint, in order to
 support services such as "call waiting" or "three way calling".

2.1.1.3 Announcement Server Access Point

 An announcement server endpoint provides access to an announcement
 service.  Under requests from the Call Agent, the announcement server
 will "play" a specified announcement.  The requests from the Call
 Agent will follow the event signaling and reporting procedures
 defined in MGCP.
                +----------------------+
                | Announcement endpoint| -------- Connection
                +----------------------+
 A given announcement endpoint is not expected to support more than
 one connection at a time.  If several connections were established to
 the same endpoint, then the same announcements would be played
 simultaneously over all the connections.
 Connections to an announcement server are typically one way, or "half
 duplex" -- the announcement server is not expected to listen to the
 audio signals from the connection.

Andreasen & Foster Informational [Page 12] RFC 3435 MGCP 1.0 January 2003

2.1.1.4 Interactive Voice Response Access Point

 An Interactive Voice Response (IVR) endpoint provides access to an
 IVR service.  Under requests from the Call Agent, the IVR server will
 "play" announcements and tones, and will "listen" to responses, such
 as DTMF input or voice messages, from the user.  The requests from
 the Call Agent will follow the event signaling and reporting
 procedures defined in MGCP.
                    +-------------+
                    | IVR endpoint| -------- Connection
                    +-------------+
 A given IVR endpoint is not expected to support more than one
 connection at a time.  If several connections were established to the
 same endpoint, then the same tones and announcements would be played
 simultaneously over all the connections.

2.1.1.5 Conference Bridge Access Point

 A conference bridge endpoint is used to provide access to a specific
 conference.
                                         +-------
             +--------------------------+|
             |Conference bridge endpoint| -------- Connections
             +--------------------------+|
                                         +-------
 Media gateways should be able to establish several connections
 between the endpoint and the packet networks, or between the endpoint
 and other endpoints in the same gateway.  The signals originating
 from these connections shall be mixed according to the connection
 "mode", as specified later in this document.  The precise number of
 connections that an endpoint supports is a characteristic of the
 gateway, and may in fact vary according to the allocation of
 resources within the gateway.

2.1.1.6 Packet Relay

 A packet relay endpoint is a specific form of conference bridge, that
 typically only supports two connections.  Packets relays can be found
 in firewalls between a protected and an open network, or in
 transcoding servers used to provide interoperation between
 incompatible gateways, for example gateways that do not support
 compatible compression algorithms, or gateways that operate over
 different transmission networks such as IP and ATM.

Andreasen & Foster Informational [Page 13] RFC 3435 MGCP 1.0 January 2003

                                         +-------
                 +---------------------+ |
                 |Packet relay endpoint|  2 connections
                 +---------------------+ |
                                         +-------

2.1.1.7 ATM "trunk side" Interface

 ATM "trunk side" endpoints are typically found when one or several
 ATM permanent virtual circuits are used as a replacement for the
 classic "TDM" trunks linking switches.  When ATM/AAL2 is used,
 several trunks or channels are multiplexed on a single virtual
 circuit; each of these trunks correspond to a single endpoint.
                                        +-------
                    +------------------+|
        (channel) = |ATM trunk endpoint| -------- Connections
                    +------------------+|
                                        +-------
 Media gateways should be able to establish several connections
 between the endpoint and the packet networks, or between the endpoint
 and other endpoints in the same gateway.  The signals originating
 from these connections shall be mixed according to the connection
 "mode", as specified later in this document.  The precise number of
 connections that an endpoint supports is a characteristic of the
 gateway, and may in fact vary according to the allocation of
 resources within the gateway.

2.1.2 Endpoint Identifiers

 Endpoint identifiers have two components that both are case-
 insensitive:
  • the domain name of the gateway that is managing the endpoint
  • a local name within that gateway
 Endpoint names are of the form:
    local-endpoint-name@domain-name
 where domain-name is an absolute domain-name as defined in RFC 1034
 and includes a host portion, thus an example domain-name could be:
    mygateway.whatever.net

Andreasen & Foster Informational [Page 14] RFC 3435 MGCP 1.0 January 2003

 Also, domain-name may be an IP-address of the form defined for domain
 name in RFC 821, thus another example could be (see RFC 821 for
 details):
    [192.168.1.2]
 Both IPv4 and IPv6 addresses can be specified, however use of IP
 addresses as endpoint identifiers is generally discouraged.
 Note that since the domain name portion is part of the endpoint
 identifier, different forms or different values referring to the same
 entity are not freely interchangeable.  The most recently supplied
 form and value MUST always be used.
 The local endpoint name is case-insensitive.  The syntax of the local
 endpoint name is hierarchical, where the least specific component of
 the name is the leftmost term, and the most specific component is the
 rightmost term.  The precise syntax depends on the type of endpoint
 being named and MAY start with a term that identifies the endpoint
 type.  In any case, the local endpoint name MUST adhere to the
 following naming rules:
 1) The individual terms of the naming path MUST be separated by a
    single slash ("/", ASCII 2F hex).
 2) The individual terms are character strings composed of letters,
    digits or other printable characters, with the exception of
    characters used as delimiters ("/", "@"), characters used for
    wildcarding ("*", "$") and white spaces.
 3) Wild-carding is represented either by an asterisk ("*") or a
    dollar sign ("$") for the terms of the naming path which are to be
    wild-carded.  Thus, if the full local endpoint name is of the
    form:
        term1/term2/term3
    then the entity name field looks like this depending on which
    terms are wild-carded:
  • /term2/term3 if term1 is wild-carded

term1/*/term3 if term2 is wild-carded

        term1/term2/* if term3 is wild-carded
        term1/*/*     if term2 and term3 are wild-carded, etc.
    In each of these examples a dollar sign could have appeared
    instead of an asterisk.

Andreasen & Foster Informational [Page 15] RFC 3435 MGCP 1.0 January 2003

 4) A term represented by an asterisk ("*") is to be interpreted as:
    "use ALL values of this term known within the scope of the Media
    Gateway".  Unless specified otherwise, this refers to all
    endpoints configured for service, regardless of their actual
    service state, i.e., in-service or out-of-service.
 5) A term represented by a dollar sign ("$") is to be interpreted as:
    "use ANY ONE value of this term known within the scope of the
    Media Gateway".  Unless specified otherwise, this only refers to
    endpoints that are in-service.
 Furthermore, it is RECOMMENDED that Call Agents adhere to the
 following:
  • Wild-carding should only be done from the right, thus if a term is

wild-carded, then all terms to the right of that term should be

   wild-carded as well.
  • In cases where mixed dollar sign and asterisk wild-cards are used,

dollar-signs should only be used from the right, thus if a term had

   a dollar sign wild-card, all terms to the right of that term should
   also contain dollar sign wild-cards.
 The description of a specific command may add further criteria for
 selection within the general rules given above.
 Note, that wild-cards may be applied to more than one term in which
 case they shall be evaluated from left to right.  For example, if we
 have the endpoint names "a/1", "a/2", "b/1", and "b/2", then "$/*"
 (which is not recommended) will evaluate to either "a/1, a/2", or
 "b/1, b/2".  However, "*/$" may evaluate to "a/1, b/1", "a/1, b/2",
 "a/2, b/1", or "a/2, b/2".  The use of mixed wild-cards in a command
 is considered error prone and is consequently discouraged.
 A local name that is composed of only a wildcard character refers to
 either all (*) or any ($) endpoints within the media gateway.

2.1.3 Calls and Connections

 Connections are created on the Call Agent on each endpoint that will
 be involved in the "call".  In the classic example of a connection
 between two "DS0" endpoints (EP1 and EP2), the Call Agents
 controlling the endpoints will establish two connections (C1 and C2):
                +---+                            +---+
  (channel1) ===|EP1|--(C1)--...        ...(C2)--|EP2|===(channel2)
                +---+                            +---+

Andreasen & Foster Informational [Page 16] RFC 3435 MGCP 1.0 January 2003

 Each connection will be designated locally by an endpoint unique
 connection identifier, and will be characterized by connection
 attributes.
 When the two endpoints are located on gateways that are managed by
 the same Call Agent, the creation is done via the three following
 steps:
 1) The Call Agent asks the first gateway to "create a connection" on
    the first endpoint.  The gateway allocates resources to that
    connection, and responds to the command by providing a "session
    description".  The session description contains the information
    necessary for a third party to send packets towards the newly
    created connection, such as for example IP address, UDP port, and
    codec parameters.
 2) The Call Agent then asks the second gateway to "create a
    connection" on the second endpoint.  The command carries the
    "session description" provided by the first gateway.  The gateway
    allocates resources to that connection, and responds to the
    command by providing its own "session description".
 3) The Call Agent then uses a "modify connection" command to provide
    this second "session description" to the first endpoint.  Once
    this is done, communication can proceed in both directions.
 When the two endpoints are located on gateways that are managed by
 two different Call Agents, the Call Agents exchange information
 through a Call-Agent to Call-Agent signaling protocol, e.g., SIP [7],
 in order to synchronize the creation of the connection on the two
 endpoints.
 Once a connection has been established, the connection parameters can
 be modified at any time by a "modify connection" command.  The Call
 Agent may for example instruct the gateway to change the codec used
 on a connection, or to modify the IP address and UDP port to which
 data should be sent, if a connection is "redirected".
 The Call Agent removes a connection by sending a "delete connection"
 command to the gateway.  The gateway may also, under some
 circumstances, inform a gateway that a connection could not be
 sustained.
 The following diagram provides a view of the states of a connection,
 as seen from the gateway:

Andreasen & Foster Informational [Page 17] RFC 3435 MGCP 1.0 January 2003

         Create connection
            received
                |
                V
       +-------------------+
       |resource allocation|-(failed)-+
       +-------------------+          |
                |           (connection refused)
          (successful)
                |
                v
   +----------->+
   |            |
   |   +-------------------+
   |   |  remote session   |
   |   |   description     |----------(yes)--------+
   |   |    available ?    |                       |
   |   +-------------------+                       |
   |            |                                  |
   |          (no)                                 |
   |            |                                  |
   |      +-----------+                         +------+
   | +--->| half open |------> Delete   <-------| open |<----------+
   | |    |  (wait)   |      Connection         |(wait)|           |
   | |    +-----------+       received          +------+           |
   | |          |                 |                |               |
   | |   Modify Connection        |         Modify Connection      |
   | |      received              |            received            |
   | |          |                 |                |               |
   | | +--------------------+     |       +--------------------+   |
   | | |assess modification |     |       |assess modification |   |
   | | +--------------------+     |       +--------------------+   |
   | |    |             |         |          |             |       |
   | |(failed)     (successful)   |      (failed)     (successful) |
   | |    |             |         |          |             |       |
   | +<---+             |         |          +-------------+-------+
   |                    |         |
   +<-------------------+         |
                                  |
                         +-----------------+
                         | Free connection |
                         | resources.      |
                         | Report.         |
                         +-----------------+
                                  |
                                  V

Andreasen & Foster Informational [Page 18] RFC 3435 MGCP 1.0 January 2003

2.1.3.1 Names of Calls

 One of the attributes of each connection is the "call identifier",
 which as far as the MGCP protocol is concerned has little semantic
 meaning, and is mainly retained for backwards compatibility.
 Calls are identified by unique identifiers, independent of the
 underlying platforms or agents.  Call identifiers are hexadecimal
 strings, which are created by the Call Agent.  The maximum length of
 call identifiers is 32 characters.
 Call identifiers are expected to be unique within the system, or at a
 minimum, unique within the collection of Call Agents that control the
 same gateways.  From the gateway's perspective, the Call identifier
 is thus unique.  When a Call Agent builds several connections that
 pertain to the same call, either on the same gateway or in different
 gateways, these connections that belong to the same call should share
 the same call-id.  This identifier can then be used by accounting or
 management procedures, which are outside the scope of MGCP.

2.1.3.2 Names of Connections

 Connection identifiers are created by the gateway when it is
 requested to create a connection.  They identify the connection
 within the context of an endpoint.  Connection identifiers are
 treated in MGCP as hexadecimal strings.  The gateway MUST make sure
 that a proper waiting period, at least 3 minutes, elapses between the
 end of a connection that used this identifier and its use in a new
 connection for the same endpoint (gateways MAY decide to use
 identifiers that are unique within the context of the gateway).  The
 maximum length of a connection identifier is 32 characters.

2.1.3.3 Management of Resources, Attributes of Connections

 Many types of resources will be associated to a connection, such as
 specific signal processing functions or packetization functions.
 Generally, these resources fall in two categories:
 1) Externally visible resources, that affect the format of "the bits
    on the network" and must be communicated to the second endpoint
    involved in the connection.
 2) Internal resources, that determine which signal is being sent over
    the connection and how the received signals are processed by the
    endpoint.

Andreasen & Foster Informational [Page 19] RFC 3435 MGCP 1.0 January 2003

 The resources allocated to a connection, and more generally the
 handling of the connection, are chosen by the gateway under
 instructions from the Call Agent.  The Call Agent will provide these
 instructions by sending two sets of parameters to the gateway:
 1) The local directives instruct the gateway on the choice of
    resources that should be used for a connection,
 2) When available, the "session description" provided by the other
    end of the connection (referred to as the remote session
    description).
 The local directives specify such parameters as the mode of the
 connection (e.g., send-only, or send-receive), preferred coding or
 packetization methods, usage of echo cancellation or silence
 suppression.  (A detailed list can be found in the specification of
 the LocalConnectionOptions parameter of the CreateConnection
 command.)  Depending on the parameter, the Call Agent MAY either
 specify a value, a range of values, or no value at all.  This allows
 various implementations to implement various levels of control, from
 a very tight control where the Call Agent specifies minute details of
 the connection handling to a very loose control where the Call Agent
 only specifies broad guidelines, such as the maximum bandwidth, and
 lets the gateway choose the detailed values subject to the
 guidelines.
 Based on the value of the local directives, the gateway will
 determine the resources to allocate to the connection.  When this is
 possible, the gateway will choose values that are in line with the
 remote session description - but there is no absolute requirement
 that the parameters be exactly the same.
 Once the resources have been allocated, the gateway will compose a
 "session description" that describes the way it intends to send and
 receive packets.  Note that the session description may in some cases
 present a range of values.  For example, if the gateway is ready to
 accept one of several compression algorithms, it can provide a list
 of these accepted algorithms.

Andreasen & Foster Informational [Page 20] RFC 3435 MGCP 1.0 January 2003

               Local Directives
              (from Call Agent 1)
                      |
                      V
               +-------------+
               | resource    |
               | allocation  |
               | (gateway 1) |
               +-------------+
                 |         |
                 V         |
               Local       |
            Parameters     V
                 |      Session
                 |    Description               Local Directives
                 |         |                   (from Call Agent 2)
                 |         +---> Transmission----+      |
                 |                (CA to CA)     |      |
                 |                               V      V
                 |                           +-------------+
                 |                           | resource    |
                 |                           | allocation  |
                 |                           | (gateway 2) |
                 |                           +-------------+
                 |                               |      |
                 |                               |      V
                 |                               |    Local
                 |                               |  Parameters
                 |                            Session
                 |                          Description
                 |         +---- Transmission<---+
                 |         |      (CA to CA)
                 V         V
               +-------------+
               | modification|
               | (gateway 1) |
               +-------------+
                 |
                 V
               Local
            Parameters
  1. - Information flow: local directives & session descriptions –

Andreasen & Foster Informational [Page 21] RFC 3435 MGCP 1.0 January 2003

2.1.3.4 Special Case of Local Connections

 Large gateways include a large number of endpoints which are often of
 different types.  In some networks, we may often have to set-up
 connections between endpoints that are located within the same
 gateway.  Examples of such connections may be:
  • Connecting a call to an Interactive Voice-Response unit,
  • Connecting a call to a Conferencing unit,
  • Routing a call from one endpoint to another, something often

described as a "hairpin" connection.

 Local connections are much simpler to establish than network
 connections.  In most cases, the connection will be established
 through some local interconnecting device, such as for example a TDM
 bus.
 When two endpoints are managed by the same gateway, it is possible to
 specify the connection in a single command that conveys the names of
 the two endpoints that will be connected.  The command is essentially
 a "Create Connection" command which includes the name of the second
 endpoint in lieu of the "remote session description".

2.1.4 Names of Call Agents and Other Entities

 The media gateway control protocol has been designed to allow the
 implementation of redundant Call Agents, for enhanced network
 reliability.  This means that there is no fixed binding between
 entities and hardware platforms or network interfaces.
 Call Agent names consist of two parts, similar to endpoint names.
 Semantically, the local portion of the name does not exhibit any
 internal structure.  An example Call Agent name is:
    ca1@ca.whatever.net
 Note that both the local part and the domain name have to be
 supplied. Nevertheless, implementations are encouraged to accept call
 agent names consisting of only the domain name.
 Reliability can be improved by using the following procedures:
  • Entities such as endpoints or Call Agents are identified by their

domain name, not their network addresses. Several addresses can be

Andreasen & Foster Informational [Page 22] RFC 3435 MGCP 1.0 January 2003

   associated with a domain name.  If a command or a response cannot
   be forwarded to one of the network addresses, implementations MUST
   retry the transmission using another address.
  • Entities MAY move to another platform. The association between a

logical name (domain name) and the actual platform is kept in the

   domain name service.  Call Agents and Gateways MUST keep track of
   the time-to-live of the record they read from the DNS.  They MUST
   query the DNS to refresh the information if the time to live has
   expired.
 In addition to the indirection provided by the use of domain names
 and the DNS, the concept of "notified entity" is central to
 reliability and fail-over in MGCP.  The "notified entity" for an
 endpoint is the Call Agent currently controlling that endpoint.  At
 any point in time, an endpoint has one, and only one, "notified
 entity" associated with it.  The "notified entity" determines where
 the endpoint will send commands to; when the endpoint needs to send a
 command to the Call Agent, it MUST send the command to its current
 "notified entity".  The "notified entity" however does not determine
 where commands can be received from; any Call Agent can send commands
 to the endpoint.  Please refer to Section 5 for the relevant security
 considerations.
 Upon startup, the "notified entity" MUST be set to a provisioned
 value.  Most commands sent by the Call Agent include the ability to
 explicitly name the "notified entity" through the use of a
 "NotifiedEntity" parameter.  The "notified entity" will stay the same
 until either a new "NotifiedEntity" parameter is received or the
 endpoint does a warm or cold (power-cycle) restart.
 If a "NotifiedEntity" parameter is sent with an "empty" value, the
 "notified entity" for the endpoint will be set to empty.  If the
 "notified entity" for an endpoint is empty or has not been set
 explicitly (neither by a command nor by provisioning), the "notified
 entity" will then default to the source address (i.e., IP address and
 UDP port number) of the last successful non-audit command received
 for the endpoint.  Auditing will thus not change the "notified
 entity".  Use of an empty "NotifiedEntity" parameter value is
 strongly discouraged as it is error prone and eliminates the DNS-
 based fail-over and reliability mechanisms.

2.1.5 Digit Maps

 The Call Agent can ask the gateway to collect digits dialed by the
 user.  This facility is intended to be used with residential gateways
 to collect the numbers that a user dials; it can also be used with

Andreasen & Foster Informational [Page 23] RFC 3435 MGCP 1.0 January 2003

 trunking gateways and access gateways alike, to collect access codes,
 credit card numbers and other numbers requested by call control
 services.
 One procedure is for the gateway to notify the Call Agent of each
 individual dialed digit, as soon as they are dialed.  However, such a
 procedure generates a large number of interactions.  It is preferable
 to accumulate the dialed numbers in a buffer, and to transmit them in
 a single message.
 The problem with this accumulation approach, however, is that it is
 hard for the gateway to predict how many numbers it needs to
 accumulate before transmission.  For example, using the phone on our
 desk, we can dial the following numbers:
  1. —————————————————–

| 0 | Local operator |

     |  00                    |  Long distance operator     |
     |  xxxx                  |  Local extension number     |
     |  8xxxxxxx              |  Local number               |
     |  #xxxxxxx              |  Shortcut to local number at|
     |                        |  other corporate sites      |
     |  *xx                   |  Star services              |
     |  91xxxxxxxxxx          |  Long distance number       |
     |  9011 + up to 15 digits|  International number       |
      ------------------------------------------------------
 The solution to this problem is to have the Call Agent load the
 gateway with a digit map that may correspond to the dial plan.  This
 digit map is expressed using a syntax derived from the Unix system
 command, egrep.  For example, the dial plan described above results
 in the following digit map:
    (0T|00T|[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
 The formal syntax of the digit map is described by the DigitMap rule
 in the formal syntax description of the protocol (see Appendix A) -
 support for basic digit map letters is REQUIRED while support for
 extension digit map letters is OPTIONAL.  A gateway receiving a digit
 map with an extension digit map letter not supported SHOULD return
 error code 537 (unknown digit map extension).
 A digit map, according to this syntax, is defined either by a (case
 insensitive) "string" or by a list of strings.  Each string in the
 list is an alternative numbering scheme, specified either as a set of
 digits or timers, or as an expression over which the gateway will
 attempt to find a shortest possible match.  The following constructs
 can be used in each numbering scheme:

Andreasen & Foster Informational [Page 24] RFC 3435 MGCP 1.0 January 2003

  • Digit: A digit from "0" to "9".
  • Timer: The symbol "T" matching a timer expiry.
  • DTMF: A digit, a timer, or one of the symbols "A", "B", "C",

"D", "#", or "*". Extensions may be defined.

  • Wildcard: The symbol "x" which matches any digit ("0" to "9").
  • Range: One or more DTMF symbols enclosed between square brackets

("[" and "]").

  • Subrange: Two digits separated by hyphen ("-") which matches any

digit between and including the two. The subrange

             construct can only be used inside a range construct,
             i.e., between "[" and "]".
 * Position: A period (".") which matches an arbitrary number,
             including zero, of occurrences of the preceding
             construct.
 A gateway that detects events to be matched against a digit map MUST
 do the following:
 1) Add the event code as a token to the end of an internal state
    variable for the endpoint called the "current dial string".
 2) Apply the current dial string to the digit map table, attempting a
    match to each expression in the digit map.
 3) If the result is under-qualified (partially matches at least one
    entry in the digit map and doesn't completely match another
    entry), do nothing further.
 If the result matches an entry, or is over-qualified (i.e., no
 further digits could possibly produce a match), send the list of
 accumulated events to the Call Agent.  A match, in this
 specification, can be either a "perfect match," exactly matching one
 of the specified alternatives, or an impossible match, which occurs
 when the dial string does not match any of the alternatives.
 Unexpected timers, for example, can cause "impossible matches".  Both
 perfect matches and impossible matches trigger notification of the
 accumulated digits (which may include other events - see Section
 2.3.3).
 The following example illustrates the above.  Assume we have the
 digit map:
    (xxxxxxx|x11)
 and a current dial string of "41".  Given the input "1" the current
 dial string becomes "411".  We have a partial match with "xxxxxxx",
 but a complete match with "x11", and hence we send "411" to the Call
 Agent.

Andreasen & Foster Informational [Page 25] RFC 3435 MGCP 1.0 January 2003

 The following digit map example is more subtle:
   (0[12].|00|1[12].1|2x.#)
 Given the input "0", a match will occur immediately since position
 (".") allows for zero occurrences of the preceding construct.  The
 input "00" can thus never be produced in this digit map.
 Given the input "1", only a partial match exists.  The input "12" is
 also only a partial match, however both "11" and "121" are a match.
 Given the input "2", a partial match exists.  A partial match also
 exists for the input "23", "234", "2345", etc.  A full match does not
 occur here until a "#" is generated, e.g., "2345#".  The input "2#"
 would also have been a match.
 Note that digit maps simply define a way of matching sequences of
 event codes against a grammar.  Although digit maps as defined here
 are for DTMF input, extension packages can also be defined so that
 digit maps can be used for other types of input represented by event
 codes that adhere to the digit map syntax already defined for these
 event codes (e.g., "1" or "T").  Where such usage is envisioned, the
 definition of the particular event(s) SHOULD explicitly state that in
 the package definition.
 Since digit maps are not bounded in size, it is RECOMMENDED that
 gateways support digit maps up to at least 2048 bytes per endpoint.

2.1.6 Packages

 MGCP is a modular and extensible protocol, however with extensibility
 comes the need to manage, identify, and name the individual
 extensions.  This is achieved by the concept of packages, which are
 simply well-defined groupings of extensions.  For example, one
 package may support a certain group of events and signals, e.g.,
 off-hook and ringing, for analog access lines.  Another package may
 support another group of events and signals for analog access lines
 or for another type of endpoint such as video.  One or more packages
 may be supported by a given endpoint.
 MGCP allows the following types of extensions to be defined in a
 package:
  • BearerInformation
  • LocalConnectionOptions
  • ExtensionParameters

Andreasen & Foster Informational [Page 26] RFC 3435 MGCP 1.0 January 2003

  • ConnectionModes
  • Events
  • Signals
  • Actions
  • DigitMapLetters
  • ConnectionParameters
  • RestartMethods
  • ReasonCodes
  • Return codes
 each of which will be explained in more detail below.  The rules for
 defining each of these extensions in a package are described in
 Section 6, and the encoding and syntax are defined in Section 3 and
 Appendix A.
 With the exception of DigitMapLetters, a package defines a separate
 name space for each type of extension by adding the package name as a
 prefix to the extension, i.e.:
    package-name/extension
 Thus the package-name is followed by a slash ("/") and the name of
 the extension.
 An endpoint supporting one or more packages may define one of those
 packages as the default package for the endpoint.  Use of the package
 name for events and signals in the default package for an endpoint is
 OPTIONAL, however it is RECOMMENDED to always include the package
 name.  All other extensions, except DigitMapLetter, defined in the
 package MUST include the package-name when referring to the
 extension.
 Package names are case insensitive strings of letters, hyphens and
 digits, with the restriction that hyphens shall never be the first or
 last character in a name.  Examples of package names are "D", "T",
 and "XYZ".  Package names are not case sensitive - names such as
 "XYZ", "xyz", and "xYz" are equal.

Andreasen & Foster Informational [Page 27] RFC 3435 MGCP 1.0 January 2003

 Package definitions will be provided in other documents and with
 package names and extensions names registered with IANA.  For more
 details, refer to section 6.
 Implementers can gain experience by using experimental packages.  The
 name of an experimental package MUST start with the two characters
 "x-"; the IANA SHALL NOT register package names that start with these
 characters, or the characters "x+", which are reserved.  A gateway
 that receives a command referring to an unsupported package MUST
 return an error (error code 518 - unsupported package, is
 RECOMMENDED).

2.1.7 Events and Signals

 The concept of events and signals is central to MGCP.  A Call Agent
 may ask to be notified about certain events occurring in an endpoint
 (e.g., off-hook events) by including the name of the event in a
 RequestedEvents parameter (in a NotificationRequest command - see
 Section 2.3.3).
 A Call Agent may also request certain signals to be applied to an
 endpoint (e.g., dial-tone) by supplying the name of the event in a
 SignalRequests parameter.
 Events and signals are grouped in packages, within which they share
 the same name space which we will refer to as event names in the
 following.  Event names are case insensitive strings of letters,
 hyphens and digits, with the restriction that hyphens SHALL NOT be
 the first or last character in a name.  Some event codes may need to
 be parameterized with additional data, which is accomplished by
 adding the parameters between a set of parentheses.  Event names are
 not case sensitive - values such as "hu", "Hu", "HU" or "hU" are
 equal.
 Examples of event names can be "hu" (off hook or "hang-up"
 transition), "hf" (hook-flash) or "0" (the digit zero).
 The package name is OPTIONAL for events in the default package for an
 endpoint, however it is RECOMMENDED to always include the package
 name.  If the package name is excluded from the event name, the
 default package name for that endpoint MUST be assumed.  For example,
 for an analog access line which has the line package ("L") as a
 default with dial-tone ("dl") as one of the events in that package,
 the following two event names are equal:

Andreasen & Foster Informational [Page 28] RFC 3435 MGCP 1.0 January 2003

    L/dl
 and
    dl
 For any other non-default packages that are associated with that
 endpoint, (such as the generic package for an analog access
 endpoint-type for example), the package name MUST be included with
 the event name.  Again, unconditional inclusion of the package name
 is RECOMMENDED.
 Digits, or letters, are supported in some packages, notably "DTMF".
 Digits and letters are defined by the rules "Digit" and "Letter" in
 the definition of digit maps.  This definition refers to the digits
 (0 to 9), to the asterisk or star ("*") and orthotrope, number or
 pound sign ("#"), and to the letters "A", "B", "C" and "D", as well
 as the timer indication "T".  These letters can be combined in "digit
 string" that represents the keys that a user punched on a dial.  In
 addition, the letter "X" can be used to represent all digits (0 to
 9).  Also, extensions MAY define use of other letters.  The need to
 easily express the digit strings in earlier versions of the protocol
 has a consequence on the form of event names:
 An event name that does not denote a digit MUST always contain at
 least one character that is neither a digit, nor one of the letters
 A, B, C, D, T or X (such names also MUST NOT just contain the special
 signs "*", or "#").  Event names consisting of more than one
 character however may use any of the above.
 A Call Agent may often have to ask a gateway to detect a group of
 events.  Two conventions can be used to denote such groups:
  • The "*" and "all" wildcard conventions (see below) can be used to

detect any event belonging to a package, or a given event in many

   packages, or any event in any package supported by the gateway.
  • The regular expression Range notation can be used to detect a range

of digits.

 The star sign (*) can be used as a wildcard instead of a package
 name, and the keyword "all" can be used as a wildcard instead of an
 event name:
  • A name such as "foo/all" denotes all events in package "foo".
  • A name such as "*/bar" denotes the event "bar" in any package

supported by the gateway.

Andreasen & Foster Informational [Page 29] RFC 3435 MGCP 1.0 January 2003

  • The name "*/all" denotes all events supported by the endpoint.
 This specification purposely does not define any additional detail
 for the "all packages" and "all events" wildcards.  They provide
 limited benefits, but introduce significant complexity along with the
 potential for errors.  Their use is consequently strongly
 discouraged.
 The Call Agent can ask a gateway to detect a set of digits or letters
 either by individually describing those letters, or by using the
 "range" notation defined in the syntax of digit strings.  For
 example, the Call Agent can:
  • Use the letter "x" to denote" digits from 0 to 9.
  • Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound

sign.

 The individual event codes are still defined in a package though
 (e.g., the "DTMF" package).
 Events can by default only be generated and detected on endpoints,
 however events can be also be defined so they can be generated or
 detected on connections rather than on the endpoint itself (see
 Section 6.6).  For example, gateways may be asked to provide a
 ringback tone on a connection.  When an event is to be applied on a
 connection, the name of the connection MUST be added to the name of
 the event, using an "at" sign (@) as a delimiter, as in:
    G/rt@0A3F58
 where "G" is the name of the package and "rt" is the name of the
 event.  Should the connection be deleted while an event or signal is
 being detected or applied on it, that particular event detection or
 signal generation simply stops.  Depending on the signal, this may
 generate a failure (see below).
 The wildcard character "*" (star) can be used to denote "all
 connections".  When this convention is used, the gateway will
 generate or detect the event on all the connections that are
 connected to the endpoint.  This applies to existing as well as
 future connections created on the endpoint.  An example of this
 convention could be:
    R/qa@*
 where "R" is the name of the package and "qa" is the name of the
 event.

Andreasen & Foster Informational [Page 30] RFC 3435 MGCP 1.0 January 2003

 When processing a command using the "all connections" wildcard, the
 "*" wildcard character applies to all current and future connections
 on the endpoint, however it will not be expanded.  If a subsequent
 command either explicitly (e.g., by auditing) or implicitly (e.g., by
 persistence) refers to such an event, the "*" value will be used.
 However, when the event is actually observed, that particular
 occurrence of the event will include the name of the specific
 connection it occurred on.
 The wildcard character "$" can be used to denote "the current
 connection".  It can only be used by the Call Agent, when the event
 notification request is "encapsulated" within a connection creation
 or modification command.  When this convention is used, the gateway
 will generate or detect the event on the connection that is currently
 being created or modified.  An example of this convention is:
    G/rt@$
 When processing a command using the "current connection" wildcard,
 the "$" wildcard character will be expanded to the value of the
 current connection.  If a subsequent command either explicitly (e.g.,
 by auditing) or implicitly (e.g., by persistence) refers to such an
 event, the expanded value will be used.  In other words, the "current
 connection" wildcard is expanded once, which is at the initial
 processing of the command in which it was explicitly included.
 The connection id, or a wildcard replacement, can be used in
 conjunction with the "all packages" and "all events" conventions. For
 example, the notation:
  • /all@*
 can be used to designate all events on all current and future
 connections on the endpoint.  However, as mentioned before, the use
 of the "all packages" and "all events" wildcards are strongly
 discouraged.
 Signals are divided into different types depending on their behavior:
  • On/off (OO): Once applied, these signals last until they are

turned off. This can only happen as the result of a reboot/restart

   or a new SignalRequests where the signal is explicitly turned off
   (see later).  Signals of type OO are defined to be idempotent, thus
   multiple requests to turn a given OO signal on (or off) are

Andreasen & Foster Informational [Page 31] RFC 3435 MGCP 1.0 January 2003

   perfectly valid and MUST NOT result in any errors.  An On/Off
   signal could be a visual message-waiting indicator (VMWI).  Once
   turned on, it MUST NOT be turned off until explicitly instructed to
   by the Call Agent, or as a result of an endpoint restart, i.e.,
   these signals will not turn off as a result of the detection of a
   requested event.
  • Time-out (TO): Once applied, these signals last until they are

either cancelled (by the occurrence of an event or by not being

   included in a subsequent (possibly empty) list of signals), or a
   signal-specific period of time has elapsed.  A TO signal that times
   out will generate an "operation complete" event.  A TO signal could
   be "ringback" timing out after 180 seconds.  If an event occurs
   prior to the 180 seconds, the signal will, by default, be stopped
   (the "Keep signals active" action - see Section 2.3.3 - will
   override this behavior).  If the signal is not stopped, the signal
   will time out, stop and generate an "operation complete" event,
   about which the Call Agent may or may not have requested to be
   notified.  If the Call Agent has asked for the "operation complete"
   event to be notified, the "operation complete" event sent to the
   Call Agent SHALL include the name(s) of the signal(s) that timed
   out (note that if parameters were passed to the signal, the
   parameters will not be reported).  If the signal was generated on a
   connection, the name of the connection SHALL be included as
   described above.  Time-out signals have a default time-out value
   defined for them, which MAY be altered by the provisioning process.
   Also, the time-out period may be provided as a parameter to the
   signal (see Section 3.2.2.4).  A value of zero indicates that the
   time-out period is infinite.  A TO signal that fails after being
   started, but before having generated an "operation complete" event
   will generate an "operation failure" event which will include the
   name of the signal that failed.  Deletion of a connection with an
   active TO signal will result in such a failure.
  • Brief (BR): The duration of these signals is normally so short

that they stop on their own. If a signal stopping event occurs, or

   a new SignalRequests is applied, a currently active BR signal will
   not stop.  However, any pending BR signals not yet applied MUST be
   cancelled (a BR signal becomes pending if a NotificationRequest
   includes a BR signal, and there is already an active BR signal). As
   an example, a brief tone could be a DTMF digit. If the DTMF digit
   "1" is currently being played, and a signal stopping event occurs,
   the "1" would play to completion.  If a request to play DTMF digit
   "2" arrives before DTMF digit "1" finishes playing, DTMF digit "2"
   would become pending.
 Signal(s) generated on a connection MUST include the name of that
 connection.

Andreasen & Foster Informational [Page 32] RFC 3435 MGCP 1.0 January 2003

2.2 Usage of SDP

 The Call Agent uses the MGCP to provide the endpoint with the
 description of connection parameters such as IP addresses, UDP port
 and RTP profiles.  These descriptions will follow the conventions
 delineated in the Session Description Protocol which is now an IETF
 proposed standard, documented in RFC 2327.

2.3 Gateway Control Commands

2.3.1 Overview of Commands

 This section describes the commands of the MGCP.  The service
 consists of connection handling and endpoint handling commands.
 There are currently nine commands in the protocol:
  • The Call Agent can issue an EndpointConfiguration command to a

gateway, instructing the gateway about the coding characteristics

   expected by the "line-side" of the endpoint.
  • The Call Agent can issue a NotificationRequest command to a

gateway, instructing the gateway to watch for specific events such

   as hook actions or DTMF tones on a specified endpoint.
  • The gateway will then use the Notify command to inform the Call

Agent when the requested events occur.

  • The Call Agent can use the CreateConnection command to create a

connection that terminates in an "endpoint" inside the gateway.

  • The Call Agent can use the ModifyConnection command to change the

parameters associated with a previously established connection.

  • The Call Agent can use the DeleteConnection command to delete an

existing connection. The DeleteConnection command may also be used

   by a gateway to indicate that a connection can no longer be
   sustained.
  • The Call Agent can use the AuditEndpoint and AuditConnection

commands to audit the status of an "endpoint" and any connections

   associated with it.  Network management beyond the capabilities
   provided by these commands is generally desirable.  Such
   capabilities are expected to be supported by the use of the Simple
   Network Management Protocol (SNMP) and definition of a MIB which is
   outside the scope of this specification.

Andreasen & Foster Informational [Page 33] RFC 3435 MGCP 1.0 January 2003

  • The Gateway can use the RestartInProgress command to notify the

Call Agent that a group of endpoints managed by the gateway is

   being taken out-of-service or is being placed back in-service.
 These services allow a controller (normally, the Call Agent) to
 instruct a gateway on the creation of connections that terminate in
 an "endpoint" attached to the gateway, and to be informed about
 events occurring at the endpoint.  An endpoint may be for example:
  • A specific trunk circuit, within a trunk group terminating in a

gateway,

  • A specific announcement handled by an announcement server.
 Connections are logically grouped into "calls" (the concept of a
 "call" has however little semantic meaning in MGCP itself).  Several
 connections, that may or may not belong to the same call, can
 terminate in the same endpoint.  Each connection is qualified by a
 "mode" parameter, which can be set to "send only" (sendonly),
 "receive only" (recvonly), "send/receive" (sendrecv), "conference"
 (confrnce), "inactive" (inactive), "loopback", "continuity test"
 (conttest), "network loop back" (netwloop) or "network continuity
 test" (netwtest).
 Media generated by the endpoint is sent on connections whose mode is
 either "send only", "send/receive", or "conference", unless the
 endpoint has a connection in "loopback" or "continuity test" mode.
 However, media generated by applying a signal to a connection is
 always sent on the connection, regardless of the mode.
 The handling of the media streams received on connections is
 determined by the mode parameters:
  • Media streams received through connections in "receive",

"conference" or "send/receive" mode are mixed and sent to the

   endpoint, unless the endpoint has another connection in "loopback"
   or "continuity test" mode.
  • Media streams originating from the endpoint are transmitted over

all the connections whose mode is "send", "conference" or

   "send/receive", unless the endpoint has another connection in
   "loopback" or "continuity test" mode.
  • In addition to being sent to the endpoint, a media stream received

through a connection in "conference" mode is forwarded to all the

   other connections whose mode is "conference".  This also applies

Andreasen & Foster Informational [Page 34] RFC 3435 MGCP 1.0 January 2003

   when the endpoint has a connection in "loopback" or "continuity
   test" mode.  The details of this forwarding, e.g., RTP translator
   or mixer, is outside the scope of this document.
 Note that in order to detect events on a connection, the connection
 must by default be in one of the modes "receive", "conference",
 "send/receive", "network loopback" or "network continuity test".  The
 event detection only applies to the incoming media.  Connections in
 "sendonly", "inactive", "loopback", or "continuity test" mode will
 thus normally not detect any events, although requesting to do so is
 not considered an error.
 The "loopback" and "continuity test" modes are used during
 maintenance and continuity test operations.  An endpoint may have
 more than one connection in either "loopback" or "continuity test"
 mode.  As long as there is one connection in that particular mode,
 and no other connection on the endpoint is placed in a different
 maintenance or test mode, the maintenance or test operation shall
 continue undisturbed.  There are two flavors of continuity test, one
 specified by ITU and one used in the US.  In the first case, the test
 is a loopback test.  The originating switch will send a tone (the go
 tone) on the bearer circuit and expects the terminating switch to
 loopback the tone.  If the originating switch sees the same tone
 returned (the return tone), the COT has passed.  If not, the COT has
 failed.  In the second case, the go and return tones are different.
 The originating switch sends a certain go tone.  The terminating
 switch detects the go tone, it asserts a different return tone in the
 backwards direction.  When the originating switch detects the return
 tone, the COT is passed.  If the originating switch never detects the
 return tone, the COT has failed.
 If the mode is set to "loopback", the gateway is expected to return
 the incoming signal from the endpoint back into that same endpoint.
 This procedure will be used, typically, for testing the continuity of
 trunk circuits according to the ITU specifications.  If the mode is
 set to "continuity test", the gateway is informed that the other end
 of the circuit has initiated a continuity test procedure according to
 the GR specification (see [22]).  The gateway will place the circuit
 in the transponder mode required for dual-tone continuity tests.
 If the mode is set to "network loopback", the audio signals received
 from the connection will be echoed back on the same connection.  The
 media is not forwarded to the endpoint.
 If the mode is set to "network continuity test", the gateway will
 process the packets received from the connection according to the
 transponder mode required for dual-tone continuity test, and send the
 processed signal back on the connection.  The media is not forwarded

Andreasen & Foster Informational [Page 35] RFC 3435 MGCP 1.0 January 2003

 to the endpoint.  The "network continuity test" mode is included for
 backwards compatibility only and use of it is discouraged.

2.3.2 EndpointConfiguration

 The EndpointConfiguration command can be used to specify the encoding
 of the signals that will be received by the endpoint.  For example,
 in certain international telephony configurations, some calls will
 carry mu-law encoded audio signals, while others will use A-law.  The
 Call Agent can use the EndpointConfiguration command to pass this
 information to the gateway.  The configuration may vary on a call by
 call basis, but can also be used in the absence of any connection.
       ReturnCode,
       [PackageList]
       <-- EndpointConfiguration(EndpointId,
                                 [BearerInformation])
 EndpointId is the name of the endpoint(s) in the gateway where
 EndpointConfiguration executes.  The "any of" wildcard convention
 MUST NOT be used.  If the "all of" wildcard convention is used, the
 command applies to all the endpoints whose name matches the wildcard.
 BearerInformation is a parameter defining the coding of the data sent
 to and received from the line side.  The information is encoded as a
 list of sub-parameters.  The only sub-parameter defined in this
 version of the specification is the bearer encoding, whose value can
 be set to "A-law" or "mu-law".  The set of sub-parameters may be
 extended.
 In order to allow for extensibility, while remaining backwards
 compatible, the BearerInformation parameter is conditionally optional
 based on the following conditions:
  • if Extension Parameters (vendor, package or other) are not used,

the BearerInformation parameter is REQUIRED,

  • otherwise, the BearerInformation parameter is OPTIONAL.
 When omitted, BearerInformation MUST retain its current value.
 ReturnCode is a parameter returned by the gateway.  It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).

Andreasen & Foster Informational [Page 36] RFC 3435 MGCP 1.0 January 2003

2.3.3 NotificationRequest

 The NotificationRequest command is used to request the gateway to
 send notifications upon the occurrence of specified events in an
 endpoint.  For example, a notification may be requested for when a
 gateway detects that an endpoint is receiving tones associated with
 fax communication.  The entity receiving this notification may then
 decide to specify use of a different type of encoding method in the
 connections bound to this endpoint and instruct the gateway
 accordingly with a ModifyConnection Command.
       ReturnCode,
       [PackageList]
       <-- NotificationRequest(EndpointId,
                               [NotifiedEntity,]
                               [RequestedEvents,]
                               RequestIdentifier,
                               [DigitMap,]
                               [SignalRequests,]
                               [QuarantineHandling,]
                               [DetectEvents,]
                               [encapsulated EndpointConfiguration])
 EndpointId is the identifier for the endpoint(s) in the the gateway
 where the NotificationRequest executes.  The "any of" wildcard MUST
 NOT be used.
 NotifiedEntity is an optional parameter that specifies a new
 "notified entity" for the endpoint.
 RequestIdentifier is used to correlate this request with the
 notifications that it triggers.  It will be repeated in the
 corresponding Notify command.
 RequestedEvents is a list of events, possibly qualified by event
 parameters (see Section 3.2.2.4), that the gateway is requested to
 detect and report.  Such events may include, for example, fax tones,
 continuity tones, or on-hook transition.  Unless otherwise specified,
 events are detected on the endpoint, however some events can be
 detected on a connection.  A given event MUST NOT appear more than
 once in a RequestedEvents.  If the parameter is omitted, it defaults
 to empty.
 To each event is associated one or more actions, which can be:
  • Notify the event immediately, together with the accumulated list of

observed events,

Andreasen & Foster Informational [Page 37] RFC 3435 MGCP 1.0 January 2003

  • Swap audio,
  • Accumulate the event in an event buffer, but don't notify yet,
  • Accumulate according to Digit Map,
  • Keep Signal(s) active,
  • Process the Embedded Notification Request,
  • Ignore the event.
 Support for Notify, Accumulate, Keep Signal(s) Active, Embedded
 Notification Request, and Ignore is REQUIRED.  Support for Accumulate
 according to Digit Map is REQUIRED on any endpoint capable of
 detecting DTMF.  Support for any other action is OPTIONAL.  The set
 of actions can be extended.
 A given action can by default be specified for any event, although
 some actions will not make sense for all events.  For example, an
 off-hook event with the Accumulate according to Digit Map action is
 valid, but will of course immediately trigger a digit map mismatch
 when the off-hook event occurs.  Needless to say, such practice is
 discouraged.
 Some actions can be combined as shown in the table below, where "Y"
 means the two actions can be combined, and "N" means they cannot:
  1. ————————————————————-

| | Notif | Swap | Accum | AccDi | KeSiA | EmbNo | Ignor |

    |--------------------------------------------------------------|
    | Notif |   N   |   Y  |   N   |   N   |   Y   |   Y*  |   N   |
    | Swap  |   -   |   N  |   Y   |   N   |   N   |   N   |   Y   |
    | Accum |   -   |   -  |   N   |   N   |   Y   |   Y   |   N   |
    | AccDi |   -   |   -  |   -   |   N   |   Y   |   N   |   N   |
    | KeSiA |   -   |   -  |   -   |   -   |   N   |   Y   |   Y   |
    | EmbNo |   -   |   -  |   -   |   -   |   -   |   N   |   N   |
    | Ignor |   -   |   -  |   -   |   -   |   -   |   -   |   N   |
     --------------------------------------------------------------
    Note (*):  The "Embedded Notification Request" can only be
    combined with "Notify", if the gateway is allowed to issue more
    than one Notify command per Notification request (see below and
    Section 4.4.1).
 If no action is specified, the Notify action will be applied.  If one
 or more actions are specified, only those actions apply.  When two or
 more actions are specified, each action MUST be combinable with all

Andreasen & Foster Informational [Page 38] RFC 3435 MGCP 1.0 January 2003

 the other actions as defined by the table above - the individual
 actions are assumed to occur simultaneously.
 If a client receives a request with an invalid or unsupported action
 or an illegal combination of actions, it MUST return an error to the
 Call Agent (error code 523 - unknown or illegal combination of
 actions, is RECOMMENDED).
 In addition to the RequestedEvents parameter specified in the
 command, some MGCP packages may contain "persistent events" (this is
 generally discouraged though - see Appendix B for an alternative).
 Persistent events in a given package are always detected on an
 endpoint that implements that package.  If a persistent event is not
 included in the list of RequestedEvents, and the event occurs, the
 event will be detected anyway and processed like all other events, as
 if the persistent event had been requested with a Notify action.  A
 NotificationRequest MUST still be in place for a persistent event to
 trigger a Notify though. Thus, informally, persistent events can be
 viewed as always being implicitly included in the list of
 RequestedEvents with an action to Notify, although no glare
 detection, etc., will be performed.
 Non-persistent events are those events that need to be explicitly
 included in the RequestedEvents list. The (possibly empty) list of
 requested events completely replaces the previous list of requested
 events.  In addition to the persistent events, only the events
 specified in the requested events list will be detected by the
 endpoint.  If a persistent event is included in the RequestedEvents
 list, the action specified will replace the default action associated
 with the event for the life of the RequestedEvents list, after which
 the default action is restored.  For example, if "off-hook"was a
 persistent event, the "Ignore off-hook" action was specified, and a
 new request without any off-hook instructions were received, the
 default "Notify off-hook" operation would be restored.
 The gateway will detect the union of the persistent events and the
 requested events.  If an event is not included in either list, it
 will be ignored.
 The Call Agent can send a NotificationRequest with an empty (or
 omitted) RequestedEvents list to the gateway.  The Call Agent can do
 so, for example, to a gateway when it does not want to collect any
 more DTMF digits.  However, persistent events will still be detected
 and notified.
 The Swap Audio action can be used when a gateway handles more than
 one connection on an endpoint.  This will be the case for call
 waiting, and possibly other feature scenarios.  In order to avoid the

Andreasen & Foster Informational [Page 39] RFC 3435 MGCP 1.0 January 2003

 round-trip to the Call Agent when just changing which connection is
 attached to the audio functions of the endpoint, the
 NotificationRequest can map an event (usually hook flash, but could
 be some other event) to a local swap audio function, which selects
 the "next" connection in a round robin fashion.  If there is only one
 connection, this action is effectively a no-op.  If there are more
 than two connections, the order is undefined.  If the endpoint has
 exactly two connections, one of which is "inactive", the other of
 which is in "send/receive" mode, then swap audio will attempt to make
 the "send/receive" connection "inactive", and vice versa.  This
 specification intentionally does not provide any additional detail on
 the swap audio action.
 If signal(s) are desired to start when an event being looked for
 occurs, the "Embedded NotificationRequest" action can be used.  The
 embedded NotificationRequest may include a new list of
 RequestedEvents, SignalRequests and a new digit map as well.  The
 semantics of the embedded NotificationRequest is as if a new
 NotificationRequest was just received with the same NotifiedEntity,
 RequestIdentifier, QuarantineHandling and DetectEvents.  When the
 "Embedded NotificationRequest" is activated, the "current dial
 string" will be cleared; however the list of observed events and the
 quarantine buffer will be unaffected (if combined with a Notify, the
 Notify will clear the list of observed events though - see Section
 4.4.1).  Note, that the Embedded NotificationRequest action does not
 accumulate the triggering event, however it can be combined with the
 Accumulate action to achieve that.  If the Embedded
 NotificationRequest fails, an Embedded NotificationRequest failure
 event SHOULD be generated (see Appendix B).
 MGCP implementations SHALL be able to support at least one level of
 embedding.  An embedded NotificationRequest that respects this
 limitation MUST NOT contain another Embedded NotificationRequest.
 DigitMap is an optional parameter that allows the Call Agent to
 provision the endpoint with a digit map according to which digits
 will be accumulated.  If this optional parameter is absent, the
 previously defined value is retained.  This parameter MUST be
 defined, either explicitly or through a previous command, if the
 RequestedEvents parameter contains a request to "accumulate according
 to the digit map".  The collection of these digits will result in a
 digit string.  The digit string is initialized to a null string upon
 reception of the NotificationRequest, so that a subsequent
 notification only returns the digits that were collected after this
 request.  Digits that were accumulated according to the digit map are
 reported as any other accumulated event, in the order in which they
 occur.  It is therefore possible that other events accumulated are

Andreasen & Foster Informational [Page 40] RFC 3435 MGCP 1.0 January 2003

 found in between the list of digits.  If the gateway is requested to
 "accumulate according to digit map" and the gateway currently does
 not have a digit map for the endpoint in question, the gateway MUST
 return an error (error code 519 - endpoint does not have a digit map,
 is RECOMMENDED).
 SignalRequests is an optional parameter that contains the set of
 signals that the gateway is asked to apply.  When omitted, it
 defaults to empty.  When multiple signals are specified, the signals
 MUST be applied in parallel.  Unless otherwise specified, signals are
 applied to the endpoint.  However some signals can be applied to a
 connection.  Signals are identified by their name, which is an event
 name, and may be qualified by signal parameters (see Section
 3.2.2.4).  The following are examples of signals:
  • Ringing,
  • Busy tone,
  • Call waiting tone,
  • Off hook warning tone,
  • Ringback tones on a connection.
 Names and descriptions of signals are defined in the appropriate
 package.
 Signals are, by default, applied to endpoints.  If a signal applied
 to an endpoint results in the generation of a media stream (audio,
 video, etc.), then by default the media stream MUST NOT be forwarded
 on any connection associated with that endpoint, regardless of the
 mode of the connection.  For example, if a call-waiting tone is
 applied to an endpoint involved in an active call, only the party
 using the endpoint in question will hear the call-waiting tone.
 However, individual signals may define a different behavior.
 When a signal is applied to a connection that has received a
 RemoteConnectionDescriptor, the media stream generated by that signal
 will be forwarded on the connection regardless of the current mode of
 the connection (including loopback and continuity test).  If a
 RemoteConnectionDescriptor has not been received, the gateway MUST
 return an error (error code 527 - missing RemoteConnectionDescriptor,
 is RECOMMENDED).  Note that this restriction does not apply to
 detecting events on a connection.

Andreasen & Foster Informational [Page 41] RFC 3435 MGCP 1.0 January 2003

 When a (possibly empty) list of signal(s) is supplied, this list
 completely replaces the current list of active time-out signals.
 Currently active time-out signals that are not provided in the new
 list MUST be stopped and the new signal(s) provided will now become
 active.  Currently active time-out signals that are provided in the
 new list of signals MUST remain active without interruption, thus the
 timer for such time-out signals will not be affected.  Consequently,
 there is currently no way to restart the timer for a currently active
 time-out signal without turning the signal off first.  If the time-
 out signal is parameterized, the original set of parameters MUST
 remain in effect, regardless of what values are provided
 subsequently.  A given signal MUST NOT appear more than once in a
 SignalRequests.  Note that applying a signal S to an endpoint,
 connection C1 and connection C2, constitutes three different and
 independent signals.
 The action triggered by the SignalRequests is synchronized with the
 collection of events specified in the RequestedEvents parameter.  For
 example, if the NotificationRequest mandates "ringing" and the
 RequestedEvents asks to look for an "off-hook" event, the ringing
 SHALL stop as soon as the gateway detects an off-hook event.  The
 formal definition is that the generation of all "Time Out" signals
 SHALL stop as soon as one of the requested events is detected, unless
 the "Keep signals active" action is associated to the detected event.
 The RequestedEvents and SignalRequests may refer to the same event
 definitions.  In one case, the gateway is asked to detect the
 occurrence of the event, and in the other case it is asked to
 generate it.  The specific events and signals that a given endpoint
 can detect or perform are determined by the list of packages that are
 supported by that endpoint.  Each package specifies a list of events
 and signals that can be detected or performed.  A gateway that is
 requested to detect or perform an event belonging to a package that
 is not supported by the specified endpoint MUST return an error
 (error code 518 - unsupported or unknown package, is RECOMMENDED).
 When the event name is not qualified by a package name, the default
 package name for the endpoint is assumed.  If the event name is not
 registered in this default package, the gateway MUST return an error
 (error code 522 - no such event or signal, is RECOMMENDED).
 The Call Agent can send a NotificationRequest whose requested signal
 list is empty.  It will do so for example when a time-out signal(s)
 should stop.
 If signal(s) are desired to start as soon as a "looked-for" event
 occurs, the "Embedded NotificationRequest" action can be used.  The
 embedded NotificationRequest may include a new list of
 RequestedEvents, SignalRequests and a new Digit Map as well.  The
 embedded NotificationRequest action allows the Call Agent to set up a

Andreasen & Foster Informational [Page 42] RFC 3435 MGCP 1.0 January 2003

 "mini-script" to be processed by the gateway immediately following
 the detection of the associated event.  Any SignalRequests specified
 in the embedded NotificationRequest will start immediately.
 Considerable care must be taken to prevent discrepancies between the
 Call Agent and the gateway.  However, long-term discrepancies should
 not occur as a new SignalRequests completely replaces the old list of
 active time-out signals, and BR-type signals always stop on their
 own.  Limiting the number of On/Off-type signals is encouraged.  It
 is considered good practice for a Call Agent to occasionally turn on
 all On/Off signals that should be on, and turn off all On/Off signals
 that should be off.
 The Ignore action can be used to ignore an event, e.g., to prevent a
 persistent event from being notified.  However, the synchronization
 between the event and an active time-out signal will still occur by
 default (e.g., a time-out dial-tone signal will stop when an off-hook
 occurs even if off-hook was a requested event with action "Ignore").
 To prevent this synchronization from happening, the "Keep Signal(s)
 Active" action will have to be specified as well.
 The optional QuarantineHandling parameter specifies the handling of
 "quarantine" events, i.e., events that have been detected by the
 gateway before the arrival of this NotificationRequest command, but
 have not yet been notified to the Call Agent.  The parameter provides
 a set of handling options (see Section 4.4.1 for details):
  • whether the quarantined events should be processed or discarded

(the default is to process them).

  • whether the gateway is expected to generate at most one

notification (step by step), or multiple notifications (loop), in

   response to this request (the default is at most one).
 When the parameter is absent, the default value is assumed.
 We should note that the quarantine-handling parameter also governs
 the handling of events that were detected and processed but not yet
 notified when the command is received.
 DetectEvents is an optional parameter, possibly qualified by event
 parameters, that specifies a list of events that the gateway is
 requested to detect during the quarantine period.  When this
 parameter is absent, the events to be detected in the quarantine
 period are those listed in the last received DetectEvents list.  In
 addition, the gateway will also detect persistent events and the
 events specified in the RequestedEvents list, including those for
 which the "ignore" action is specified.

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 Some events and signals, such as the in-line ringback or the quality
 alert, are performed or detected on connections terminating in the
 endpoint rather than on the endpoint itself.  The structure of the
 event names (see Section 2.1.7) allows the Call Agent to specify the
 connection(s) on which the events should be performed or detected.
 The NotificationRequest command may carry an encapsulated
 EndpointConfiguration command, that will apply to the same
 endpoint(s).  When this command is present, the parameters of the
 EndpointConfiguration command are included with the normal parameters
 of the NotificationRequest, with the exception of the EndpointId,
 which is not replicated.
 The encapsulated EndpointConfiguration command shares the fate of the
 NotificationRequest command.  If the NotificationRequest is rejected,
 the EndpointConfiguration is not executed.
 ReturnCode is a parameter returned by the gateway.  It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).

2.3.4 Notify

 Notifications with the observed events are sent by the gateway via
 the Notify command when a triggering event occurs.
       ReturnCode,
       [PackageList]
       <-- Notify(EndpointId,
                  [NotifiedEntity,]
                  RequestIdentifier,
                  ObservedEvents)
 EndpointId is the name for the endpoint in the gateway which is
 issuing the Notify command.  The identifier MUST be a fully qualified
 endpoint identifier, including the domain name of the gateway.  The
 local part of the name MUST NOT use any of the wildcard conventions.
 NotifiedEntity is a parameter that identifies the entity which
 requested the notification.  This parameter is equal to the
 NotifiedEntity parameter of the NotificationRequest that triggered
 this notification.  The parameter is absent if there was no such
 parameter in the triggering request.  Regardless of the value of the
 NotifiedEntity parameter, the notification MUST be sent to the
 current "notified entity" for the endpoint.

Andreasen & Foster Informational [Page 44] RFC 3435 MGCP 1.0 January 2003

 RequestIdentifier is a parameter that repeats the RequestIdentifier
 parameter of the NotificationRequest that triggered this
 notification.  It is used to correlate this notification with the
 request that triggered it.  Persistent events will be viewed here as
 if they had been included in the last NotificationRequest.  An
 implicit NotificationRequest MAY be in place right after restart -
 the RequestIdentifier used for it will be zero ("0") - see Section
 4.4.1 for details.
 ObservedEvents is a list of events that the gateway detected and
 accumulated.  A single notification may report a list of events that
 will be reported in the order in which they were detected (FIFO).
 The list will only contain the identification of events that were
 requested in the RequestedEvents parameter of the triggering
 NotificationRequest.  It will contain the events that were either
 accumulated (but not notified) or treated according to digit map (but
 no match yet), and the final event that triggered the notification or
 provided a final match in the digit map.  It should be noted that
 digits MUST be added to the list of observed events as they are
 accumulated, irrespective of whether they are accumulated according
 to the digit map or not.  For example, if a user enters the digits
 "1234" and some event E is accumulated between the digits "3" and "4"
 being entered, the list of observed events would be "1, 2, 3, E, 4".
 Events that were detected on a connection SHALL include the name of
 that connection as in "R/qa@0A3F58" (see Section 2.1.7).
 If the list of ObservedEvents reaches the capacity of the endpoint,
 an ObservedEvents Full event (see Appendix B) SHOULD be generated
 (the endpoint shall ensure it has capacity to include this event in
 the list of ObservedEvents).  If the ObservedEvents Full event is not
 used to trigger a Notify, event processing continues as before
 (including digit map matching); however, the subsequent events will
 not be included in the list of ObservedEvents.
 ReturnCode is a parameter returned by the Call Agent.  It indicates
 the outcome of the command and consists of an integer number
 optionally followed by commentary.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).

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2.3.5 CreateConnection

 This command is used to create a connection between two endpoints.
       ReturnCode,
       [ConnectionId,]
       [SpecificEndPointId,]
       [LocalConnectionDescriptor,]
       [SecondEndPointId,]
       [SecondConnectionId,]
       [PackageList]
       <-- CreateConnection(CallId,
                            EndpointId,
                            [NotifiedEntity,]
                            [LocalConnectionOptions,]
                            Mode,
                            [{RemoteConnectionDescriptor |
                            SecondEndpointId}, ]
                            [Encapsulated NotificationRequest,]
                            [Encapsulated EndpointConfiguration])
 A connection is defined by its endpoints.  The input parameters in
 CreateConnection provide the data necessary to build a gateway's
 "view" of a connection.
 CallId is a parameter that identifies the call (or session) to which
 this connection belongs.  This parameter SHOULD, at a minimum, be
 unique within the collection of Call Agents that control the same
 gateways.  Connections that belong to the same call SHOULD share the
 same call-id.  The call-id has little semantic meaning in the
 protocol; however it can be used to identify calls for reporting and
 accounting purposes.  It does not affect the handling of connections
 by the gateway.
 EndpointId is the identifier for the connection endpoint in the
 gateway where CreateConnection executes.  The EndpointId can be
 fully-specified by assigning a value to the parameter EndpointId in
 the function call or it may be under-specified by using the "any of"
 wildcard convention.  If the endpoint is underspecified, the endpoint
 identifier SHALL be assigned by the gateway and its complete value
 returned in the SpecificEndPointId parameter of the response.  When
 the "any of" wildcard is used, the endpoint assigned MUST be in-
 service and MUST NOT already have any connections on it.  If no such
 endpoint is available, error code 410 (no endpoint available) SHOULD
 be returned.  The "all of" wildcard MUST NOT be used.
 The NotifiedEntity is an optional parameter that specifies a new
 "notified entity" for the endpoint.

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 LocalConnectionOptions is an optional structure used by the Call
 Agent to direct the handling of the connection by the gateway.  The
 fields contained in a LocalConnectionOptions structure may include
 one or more of the following (each field MUST NOT be supplied more
 than once):
  • Codec compression algorithm: One or more codecs, listed in order

of preference. For interoperability, it is RECOMMENDED to support

   G.711 mu-law encoding ("PCMU").  See Section 2.6 for details on the
   codec selection process.
  • Packetization period: A single millisecond value or a range may be

specified. The packetization period SHOULD NOT contradict the

   specification of the codec compression algorithm.  If a codec is
   specified that has a frame size which is inconsistent with the
   packetization period, and that codec is selected, the gateway is
   authorized to use a packetization period that is consistent with
   the frame size even if it is different from that specified.  In so
   doing, the gateway SHOULD choose a non-zero packetization period as
   close to that specified as possible.  If a packetization period is
   not specified, the endpoint SHOULD use the default packetization
   period(s) for the codec(s) selected.
  • Bandwidth: The allowable bandwidth, i.e., payload plus any header

overhead from the transport layer and up, e.g., IP, UDP, and RTP.

   The bandwidth specification SHOULD NOT contradict the specification
   of codec compression algorithm or packetization period.  If a codec
   is specified, then the gateway is authorized to use it, even if it
   results in the usage of a larger bandwidth than specified.  Any
   discrepancy between the bandwidth and codec specification will not
   be reported as an error.
  • Type of Service: This indicates the class of service to be used

for this connection. When the Type of Service is not specified,

   the gateway SHALL use a default value of zero unless provisioned
   otherwise.
  • Usage of echo cancellation: By default, the telephony gateways

always perform echo cancellation on the endpoint. However, it may

   be necessary, for some calls, to turn off these operations.  The
   echo cancellation parameter can have two values, "on" (when the
   echo cancellation is requested) and "off" (when it is turned off).
   The parameter is optional.  If the parameter is omitted when
   creating a connection and there are no other connections on the
   endpoint, the endpoint SHALL apply echo cancellation initially.  If
   the parameter is omitted when creating a connection and there are
   existing connections on the endpoint, echo cancellation is
   unchanged.  The endpoint SHOULD subsequently enable or disable echo

Andreasen & Foster Informational [Page 47] RFC 3435 MGCP 1.0 January 2003

   cancellation when voiceband data is detected - see e.g., ITU-T
   recommendation V.8, V.25, and G.168.  Following termination of
   voiceband data, the handling of echo cancellation SHALL then revert
   to the current value of the echo cancellation parameter.  It is
   RECOMMENDED that echo cancellation handling is left to the gateway
   rather than having this parameter specified by the Call Agent.
  • Silence Suppression: The telephony gateways may perform voice

activity detection, and avoid sending packets during periods of

   silence.  However, it is necessary, for example for modem calls, to
   turn off this detection.  The silence suppression parameter can
   have two values, "on" (when the detection is requested) and "off"
   (when it is not requested).  The default is "off" (unless
   provisioned otherwise).  Upon detecting voiceband data, the
   endpoint SHOULD disable silence suppression.  Following termination
   of voiceband data, the handling of silence suppression SHALL then
   revert to the current value of the silence suppression parameter.
  • Gain Control: The telephony gateways may perform gain control on

the endpoint, in order to adapt the level of the signal. However,

   it is necessary, for example for some modem calls, to turn off this
   function.  The gain control parameter may either be specified as
   "automatic", or as an explicit number of decibels of gain.  The
   gain specified will be added to media sent out over the endpoint
   (as opposed to the connection) and subtracted from media received
   on the endpoint.  The parameter is optional.  When there are no
   other connections on the endpoint, and the parameter is omitted,
   the default is to not perform gain control (unless provisioned
   otherwise), which is equivalent to specifying a gain of 0 decibels.
   If there are other connections on the endpoint, and the parameter
   is omitted, gain control is unchanged.  Upon detecting voiceband
   data, the endpoint SHOULD disable gain control if needed.
   Following termination of voiceband data, the handling of gain
   control SHALL then revert to the current value of the gain control
   parameter.  It should be noted, that handling of gain control is
   normally best left to the gateway and hence use of this parameter
   is NOT RECOMMENDED.
  • RTP security: The Call agent can request the gateway to enable

encryption of the audio Packets. It does so by providing a key

   specification, as specified in RFC 2327.  By default, encryption is
   not performed.
  • Network Type: The Call Agent may instruct the gateway to prepare

the connection on a specified type of network. If absent, the

   value is based on the network type of the gateway being used.

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  • Resource reservation: The Call Agent may instruct the gateway to

use network resource reservation for the connection. See Section

   2.7 for details.
 The Call Agent specifies the relevant fields it cares about in the
 command and leaves the rest to the discretion of the gateway.  For
 those of the above parameters that were not explicitly included, the
 gateway SHOULD use the default values if possible.  For a detailed
 list of local connection options included with this specification
 refer to section 3.2.2.10.  The set of local connection options can
 be extended.
 The Mode indicates the mode of operation for this side of the
 connection.  The basic modes are "send", "receive", "send/receive",
 "conference", "inactive", "loopback", "continuity test", "network
 loop back" and "network continuity test".  The expected handling of
 these modes is specified in the introduction of the "Gateway Control
 Commands", Section 2.3.  Note that signals applied to a connection do
 not follow the connection mode.  Some endpoints may not be capable of
 supporting all modes.  If the command specifies a mode that the
 endpoint does not support, an error SHALL be returned (error 517 -
 unsupported mode, is RECOMMENDED).  Also, if a connection has not yet
 received a RemoteConnectionDescriptor, an error MUST be returned if
 the connection is attempted to be placed in any of the modes "send
 only", "send/receive", "conference", "network loopback", "network
 continuity test", or if a signal (as opposed to detecting an event)
 is to be applied to the connection (error code 527 - missing
 RemoteConnectionDescriptor, is RECOMMENDED).  The set of modes can be
 extended.
 The gateway returns a ConnectionId, that uniquely identifies the
 connection within the endpoint, and a LocalConnectionDescriptor,
 which is a session description that contains information about the
 connection, e.g., IP address and port for the media, as defined in
 SDP.
 The SpecificEndPointId is an optional parameter that identifies the
 responding endpoint.  It is returned when the EndpointId argument
 referred to an "any of" wildcard name and the command succeeded.
 When a SpecificEndPointId is returned, the Call Agent SHALL use it as
 the EndpointId value in successive commands referring to this
 connection.
 The SecondEndpointId can be used instead of the
 RemoteConnectionDescriptor to establish a connection between two
 endpoints located on the same gateway.  The connection is by
 definition a local connection.  The SecondEndpointId can be fully-
 specified by assigning a value to the parameter SecondEndpointId in

Andreasen & Foster Informational [Page 49] RFC 3435 MGCP 1.0 January 2003

 the function call or it may be under-specified by using the "any of"
 wildcard convention.  If the SecondEndpointId is underspecified, the
 second endpoint identifier will be assigned by the gateway and its
 complete value returned in the SecondEndPointId parameter of the
 response.
 When a SecondEndpointId is specified, the command really creates two
 connections that can be manipulated separately through
 ModifyConnection and DeleteConnection commands.  In addition to the
 ConnectionId and LocalConnectionDescriptor for the first connection,
 the response to the creation provides a SecondConnectionId parameter
 that identifies the second connection.  The second connection is
 established in "send/receive" mode.
 After receiving a "CreateConnection" request that did not include a
 RemoteConnectionDescriptor parameter, a gateway is in an ambiguous
 situation.  Because it has exported a LocalConnectionDescriptor
 parameter, it can potentially receive packets.  Because it has not
 yet received the RemoteConnectionDescriptor parameter of the other
 gateway, it does not know whether the packets that it receives have
 been authorized by the Call Agent.  It must thus navigate between two
 risks, i.e., clipping some important announcements or listening to
 insane data.  The behavior of the gateway is determined by the value
 of the Mode parameter:
  • If the mode was set to ReceiveOnly, the gateway MUST accept the

media and transmit them through the endpoint.

  • If the mode was set to Inactive, Loopback, or Continuity Test, the

gateway MUST NOT transmit the media through to the endpoint.

 Note that the mode values SendReceive, Conference, SendOnly, Network
 Loopback and Network Continuity Test do not make sense in this
 situation.  They MUST be treated as errors, and the command MUST be
 rejected (error code 527 - missing RemoteConnectionDescriptor, is
 RECOMMENDED).
 The command may optionally contain an encapsulated Notification
 Request command, which applies to the EndpointId, in which case a
 RequestIdentifier parameter MUST be present, as well as, optionally,
 other parameters of the NotificationRequest with the exception of the
 EndpointId, which is not replicated.  The encapsulated
 NotificationRequest is executed simultaneously with the creation of
 the connection.  For example, when the Call Agent wants to initiate a
 call to a residential gateway, it could:

Andreasen & Foster Informational [Page 50] RFC 3435 MGCP 1.0 January 2003

  • ask the residential gateway to prepare a connection, in order to be

sure that the user can start speaking as soon as the phone goes off

   hook,
  • ask the residential gateway to start ringing,
  • ask the residential gateway to notify the Call Agent when the phone

goes off-hook.

 This can be accomplished in a single CreateConnection command, by
 also transmitting the RequestedEvents parameters for the off-hook
 event, and the SignalRequests parameter for the ringing signal.
 When these parameters are present, the creation and the
 NotificationRequest MUST be synchronized, which means that both MUST
 be accepted, or both MUST be refused.  In our example, the
 CreateConnection may be refused if the gateway does not have
 sufficient resources, or cannot get adequate resources from the local
 network access, and the off-hook NotificationRequest can be refused
 in the glare condition, if the user is already off-hook.  In this
 example, the phone must not ring if the connection cannot be
 established, and the connection must not be established if the user
 is already off-hook.
 The NotifiedEntity parameter, if present, defines the new "notified
 entity" for the endpoint.
 The command may carry an encapsulated EndpointConfiguration command,
 which applies to the EndpointId.  When this command is present, the
 parameters of the EndpointConfiguration command are included with the
 normal parameters of the CreateConnection with the exception of the
 EndpointId, which is not replicated.  The EndpointConfiguration
 command may be encapsulated together with an encapsulated
 NotificationRequest command.  Note that both of these apply to the
 EndpointId only.
 The encapsulated EndpointConfiguration command shares the fate of the
 CreateConnection command.  If the CreateConnection is rejected, the
 EndpointConfiguration is not executed.
 ReturnCode is a parameter returned by the gateway.  It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).

Andreasen & Foster Informational [Page 51] RFC 3435 MGCP 1.0 January 2003

2.3.6 ModifyConnection

 This command is used to modify the characteristics of a gateway's
 "view" of a connection.  This "view" of the call includes both the
 local connection descriptor as well as the remote connection
 descriptor.
       ReturnCode,
       [LocalConnectionDescriptor,]
       [PackageList]
       <-- ModifyConnection(CallId,
                            EndpointId,
                            ConnectionId,
                            [NotifiedEntity,]
                            [LocalConnectionOptions,]
                            [Mode,]
                            [RemoteConnectionDescriptor,]
                            [Encapsulated NotificationRequest,]
                            [Encapsulated EndpointConfiguration])
 The parameters used are the same as in the CreateConnection command,
 with the addition of a ConnectionId that identifies the connection
 within the endpoint.  This parameter was returned by the
 CreateConnection command, in addition to the local connection
 descriptor.  It uniquely identifies the connection within the context
 of the endpoint.  The CallId used when the connection was created
 MUST be included as well.
 The EndpointId MUST be a fully qualified endpoint identifier.  The
 local name MUST NOT use the wildcard conventions.
 The ModifyConnection command can be used to affect parameters of a
 connection in the following ways:
  • Provide information about the other end of the connection, through

the RemoteConnectionDescriptor. If the parameter is omitted, it

   retains its current value.
  • Activate or deactivate the connection, by changing the value of the

Mode parameter. This can occur at any time during the connection,

   with arbitrary parameter values.  If the parameter is omitted, it
   retains its current value.
  • Change the parameters of the connection through the

LocalConnectionOptions, for example by switching to a different

   coding scheme, changing the packetization period, or modifying the
   handling of echo cancellation.  If one or more
   LocalConnectionOptions parameters are omitted, then the gateway

Andreasen & Foster Informational [Page 52] RFC 3435 MGCP 1.0 January 2003

   SHOULD refrain from changing that parameter from its current value,
   unless another parameter necessitating such a change is explicitly
   provided.  For example, a codec change might require a change in
   silence suppression.  Note that if a RemoteConnectionDescriptor is
   supplied, then only the LocalConnectionOptions actually supplied
   with the ModifyConnection command will affect the codec negotiation
   (as described in Section 2.6).
 Connections can only be fully activated if the
 RemoteConnectionDescriptor has been provided to the gateway.  The
 receive-only mode, however, can be activated without the provision of
 this descriptor.
 The command will only return a LocalConnectionDescriptor if the local
 connection parameters, such as RTP ports, were modified.  Thus, if,
 for example, only the mode of the connection is changed, a
 LocalConnectionDescriptor will not be returned.  Note however, that
 inclusion of LocalConnectionOptions in the command is not a
 prerequisite for local connection parameter changes to occur.  If a
 connection parameter is omitted, e.g., silence suppression, the old
 value of that parameter will be retained if possible.  If a parameter
 change necessitates a change in one or more unspecified parameters,
 the gateway is free to choose suitable values for the unspecified
 parameters that must change.  This can for instance happen if the
 packetization period was not specified.  If the new codec supported
 the old packetization period, the value of this parameter would not
 change, as a change would not be necessary.  However, if it did not
 support the old packetization period, it would choose a suitable
 value.
 The command may optionally contain an encapsulated Notification
 Request command, in which case a RequestIdentifier parameter MUST be
 present, as well as, optionally, other parameters of the
 NotificationRequest with the exception of the EndpointId, which is
 not replicated.  The encapsulated NotificationRequest is executed
 simultaneously with the modification of the connection.  For example,
 when a connection is accepted, the calling gateway should be
 instructed to place the circuit in send-receive mode and to stop
 providing ringing tones.  This can be accomplished in a single
 ModifyConnection command, by also transmitting the RequestedEvents
 parameters, for the on-hook event, and an empty SignalRequests
 parameter, to stop the provision of ringing tones.
 When these parameters are present, the modification and the
 NotificationRequest MUST be synchronized, which means that both MUST
 be accepted, or both MUST be refused.

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 The NotifiedEntity parameter, if present, defines the new "notified
 entity" for the endpoint.
 The command may carry an encapsulated EndpointConfiguration command,
 that will apply to the same endpoint.  When this command is present,
 the parameters of the EndpointConfiguration command are included with
 the normal parameters of the ModifyConnection with the exception of
 the EndpointId, which is not replicated.  The EndpointConfiguration
 command may be encapsulated together with an encapsulated
 NotificationRequest command.
 The encapsulated EndpointConfiguration command shares the fate of the
 ModifyConnection command.  If the ModifyConnection is rejected, the
 EndpointConfiguration is not executed.
 ReturnCode is a parameter returned by the gateway.  It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).

2.3.7 DeleteConnection (from the Call Agent)

 This command is used to terminate a connection.  As a side effect, it
 collects statistics on the execution of the connection.
       ReturnCode,
       ConnectionParameters,
       [PackageList]
       <-- DeleteConnection(CallId,
                            EndpointId,
                            ConnectionId,
                            [NotifiedEntity,]
                            [Encapsulated NotificationRequest,]
                            [Encapsulated EndpointConfiguration])
 The endpoint identifier, in this form of the DeleteConnection
 command, SHALL be fully qualified.  Wildcard conventions SHALL NOT be
 used.
 The ConnectionId identifies the connection to be deleted.  The CallId
 used when the connection was created is included as well.
 The NotifiedEntity parameter, if present, defines the new "notified
 entity" for the endpoint.

Andreasen & Foster Informational [Page 54] RFC 3435 MGCP 1.0 January 2003

 In the case of IP multicast, connections can be deleted individually
 and independently.  However, in the unicast case where a connection
 has two ends, a DeleteConnection command has to be sent to both
 gateways involved in the connection.  After the connection has been
 deleted, media streams previously supported by the connection are no
 longer available.  Any media packets received for the old connection
 are simply discarded and no new media packets for the stream are
 sent.
 After the connection has been deleted, any loopback that has been
 requested for the connection must be cancelled (unless the endpoint
 has another connection requesting loopback).
 In response to the DeleteConnection command, the gateway returns a
 list of connection parameters that describe statistics for the
 connection.
 When the connection was for an Internet media stream, these
 parameters are:
 Number of packets sent:
    The total number of media packets transmitted by the sender since
    starting transmission on this connection.  In the case of RTP, the
    count is not reset if the sender changes its synchronization
    source identifier (SSRC, as defined in RTP), for example as a
    result of a ModifyConnection command.  The value is zero if the
    connection was always set in "receive only" mode and no signals
    were applied to the connection.
 Number of octets sent:
    The total number of payload octets (i.e., not including header or
    padding) transmitted in media packets by the sender since starting
    transmission on this connection.  In the case of RTP, the count is
    not reset if the sender changes its SSRC identifier, for example
    as a result of a ModifyConnection command.  The value is zero if
    the connection was always set in "receive only" mode and no
    signals were applied to the connection.
 Number of packets received:
    The total number of media packets received by the sender since
    starting reception on this connection.  In the case of RTP, the
    count includes packets received from different SSRC, if the sender
    used several values.  The value is zero if the connection was
    always set in "send only" mode.

Andreasen & Foster Informational [Page 55] RFC 3435 MGCP 1.0 January 2003

 Number of octets received:
    The total number of payload octets (i.e., not including header,
    e.g., RTP, or padding) transmitted in media packets by the sender
    since starting transmission on this connection.  In the case of
    RTP, the count includes packets received from different SSRC, if
    the sender used several values.  The value is zero if the
    connection was always set in "send only" mode.
 Number of packets lost:
    The total number of media packets that have been lost since the
    beginning of reception.  This number is defined to be the number
    of packets expected less the number of packets actually received,
    where the number of packets received includes any which are late
    or duplicates.  For RTP, the count includes packets received from
    different SSRC, if the sender used several values.  Thus packets
    that arrive late are not counted as lost, and the loss may be
    negative if there are duplicates.  The count includes packets
    received from different SSRC, if the sender used several values.
    The number of packets expected is defined to be the extended last
    sequence number received, as defined next, less the initial
    sequence number received.  The count includes packets received
    from different SSRC, if the sender used several values.  The value
    is zero if the connection was always set in "send only" mode.
 Interarrival jitter:
    An estimate of the statistical variance of the media packet
    interarrival time measured in milliseconds and expressed as an
    unsigned integer.  For RTP, the interarrival jitter J is defined
    to be the mean deviation (smoothed absolute value) of the
    difference D in packet spacing at the receiver compared to the
    sender for a pair of packets.  Detailed computation algorithms are
    found in RFC 1889.  The count includes packets received from
    different SSRC, if the sender used several values.  The value is
    zero if the connection was always set in "send only" mode.
 Average transmission delay:
    An estimate of the network latency, expressed in milliseconds. For
    RTP, this is the average value of the difference between the NTP
    timestamp indicated by the senders of the RTCP messages and the
    NTP timestamp of the receivers, measured when the messages are
    received.  The average is obtained by summing all the estimates,

Andreasen & Foster Informational [Page 56] RFC 3435 MGCP 1.0 January 2003

    then dividing by the number of RTCP messages that have been
    received.  When the gateway's clock is not synchronized by NTP,
    the latency value can be computed as one half of the round trip
    delay, as measured through RTCP.  When the gateway cannot compute
    the one way delay or the round trip delay, the parameter conveys a
    null value.
 For a detailed definition of these variables, refer to RFC 1889.
 When the connection was set up over a LOCAL interconnect, the meaning
 of these parameters is defined as follows:
 Number of packets sent:
    Not significant - MAY be omitted.
 Number of octets sent:
    The total number of payload octets transmitted over the local
    connection.
 Number of packets received:
    Not significant - MAY be omitted.
 Number of octets received:
    The total number of payload octets received over the connection.
 Number of packets lost:
    Not significant - MAY be omitted.  A value of zero is assumed.
 Interarrival jitter:
    Not significant - MAY be omitted.  A value of zero is assumed.
 Average transmission delay:
    Not significant - MAY be omitted.  A value of zero is assumed.
 The set of connection parameters can be extended.  Also, the meaning
 may be further defined by other types of networks which MAY
 furthermore elect to not return all, or even any, of the above
 specified parameters.
 The command may optionally contain an encapsulated Notification
 Request command, in which case a RequestIdentifier parameter MUST be
 present, as well as, optionally, other parameters of the
 NotificationRequest with the exception of the EndpointId, which is
 not replicated.  The encapsulated NotificationRequest is executed
 simultaneously with the deletion of the connection.  For example,
 when a user hang-up is notified, the gateway should be instructed to
 delete the connection and to start looking for an off-hook event.

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 This can be accomplished in a single DeleteConnection command, by
 also transmitting the RequestedEvents parameters, for the off-hook
 event, and an empty SignalRequests parameter.
 When these parameters are present, the DeleteConnection and the
 NotificationRequest must be synchronized, which means that both MUST
 be accepted, or both MUST be refused.
 The command may carry an encapsulated EndpointConfiguration command,
 that will apply to the same endpoint.  When this command is present,
 the parameters of the EndpointConfiguration command are included with
 the normal parameters of the DeleteConnection with the exception of
 the EndpointId, which is not replicated.  The EndpointConfiguration
 command may be encapsulated together with an encapsulated
 NotificationRequest command.
 The encapsulated EndpointConfiguration command shares the fate of the
 DeleteConnection command.  If the DeleteConnection is rejected, the
 EndpointConfiguration is not executed.
 ReturnCode is a parameter returned by the gateway.  It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).

2.3.8 DeleteConnection (from the gateway)

 In some rare circumstances, a gateway may have to clear a connection,
 for example because it has lost the resource associated with the
 connection, or because it has detected that the endpoint no longer is
 capable or willing to send or receive media.  The gateway may then
 terminate the connection by using a variant of the DeleteConnection
 command:
       ReturnCode,
       [PackageList]
       <-- DeleteConnection(CallId,
                            EndpointId,
                            ConnectionId,
                            ReasonCode,
                            Connection-parameters)
 The EndpointId, in this form of the DeleteConnection command, MUST be
 fully qualified.  Wildcard conventions MUST NOT be used.

Andreasen & Foster Informational [Page 58] RFC 3435 MGCP 1.0 January 2003

 The ReasonCode is a text string starting with a numeric reason code
 and optionally followed by a descriptive text string.  The reason
 code indicates the cause of the DeleteConnection.  A list of reason
 codes can be found in Section 2.5.
 In addition to the call, endpoint and connection identifiers, the
 gateway will also send the connection parameters that would have been
 returned to the Call Agent in response to a DeleteConnection command.
 ReturnCode is a parameter returned by the Call Agent.  It indicates
 the outcome of the command and consists of an integer number
 optionally followed by commentary.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).
 Note that use of this command is generally discouraged and should
 only be done as a last resort.  If a connection can be sustained,
 deletion of it should be left to the discretion of the Call Agent
 which is in a far better position to make intelligent decisions in
 this area.

2.3.9 DeleteConnection (multiple connections from the Call Agent)

 A variation of the DeleteConnection function can be used by the Call
 Agent to delete multiple connections at the same time.  Note that
 encapsulating other commands with this variation of the
 DeleteConnection command is not permitted.  The command can be used
 to delete all connections that relate to a Call for an endpoint:
       ReturnCode,
       [PackageList]
       <-- DeleteConnection(CallId,
                            EndpointId)
 The EndpointId, in this form of the DeleteConnection command, MUST
 NOT use the "any of" wildcard.  All connections for the endpoint(s)
 with the CallId specified will be deleted.  Note that the command
 will still succeed if there were no connections with the CallId
 specified, as long as the EndpointId was valid.  However, if the
 EndpointId is invalid, the command will fail.  The command does not
 return any individual statistics or call parameters.

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 It can also be used to delete all connections that terminate in a
 given endpoint:
       ReturnCode,
       [PackageList]
       <-- DeleteConnection(EndpointId)
 The EndpointId, in this form of the DeleteConnection command, MUST
 NOT use the "any of" wildcard.  Again, the command succeeds even if
 there were no connections on the endpoint(s).
 Finally, Call Agents can take advantage of the hierarchical structure
 of endpoint names to delete all the connections that belong to a
 group of endpoints.  In this case, the "local name" component of the
 EndpointId will be specified using the "all of" wildcarding
 convention.  The "any of" convention SHALL NOT be used.  For example,
 if endpoint names are structured as the combination of a physical
 interface name and a circuit number, as in "X35V3+A4/13", the Call
 Agent may replace the circuit number by the "all of" wild card
 character "*", as in "X35V3+A4/*".  This "wildcard" command instructs
 the gateway to delete all the connections that were attached to
 circuits connected to the physical interface "X35V3+A4".
 After all the connections have been deleted, any loopback that has
 been requested for the connections MUST be cancelled by the gateway.
 This command does not return any individual statistics or call
 parameters.
 ReturnCode is a parameter returned by the gateway.  It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).

2.3.10 AuditEndpoint

 The AuditEndPoint command can be used by the Call Agent to find out
 the status of a given endpoint.

Andreasen & Foster Informational [Page 60] RFC 3435 MGCP 1.0 January 2003

       ReturnCode,
       EndPointIdList,|{
       [RequestedEvents,]
       [QuarantineHandling,]
       [DigitMap,]
       [SignalRequests,]
       [RequestIdentifier,]
       [NotifiedEntity,]
       [ConnectionIdentifiers,]
       [DetectEvents,]
       [ObservedEvents,]
       [EventStates,]
       [BearerInformation,]
       [RestartMethod,]
       [RestartDelay,]
       [ReasonCode,]
       [MaxMGCPDatagram,]
       [Capabilities]}
       [PackageList]
       <-- AuditEndPoint(EndpointId,
                         [RequestedInfo])
 The EndpointId identifies the endpoint(s) being audited.  The "any
 of" wildcard convention MUST NOT be used.
 The EndpointId identifies the endpoint(s) being audited.  The "all
 of" wildcard convention can be used to start auditing of a group of
 endpoints (regardless of their service-state).  If this convention is
 used, the gateway SHALL return the list of endpoint identifiers that
 match the wildcard in the EndPointIdList parameter, which is simply
 one or more SpecificEndpointIds (each supplied separately).  In the
 case where the "all of" wildcard is used, RequestedInfo SHOULD NOT be
 included (if it is included, it MUST be ignored).  Note that the use
 of the "all of" wildcard can potentially generate a large
 EndPointIdList.  If the resulting EndPointIdList is considered too
 large, the gateway returns an error (error code 533 - response too
 large, is RECOMMENDED).
 When a non-wildcard EndpointId is specified, the (possibly empty)
 RequestedInfo parameter describes the information that is requested
 for the EndpointId specified.  The following endpoint info can be
 audited with this command:
    RequestedEvents, DigitMap, SignalRequests, RequestIdentifier,
    QuarantineHandling, NotifiedEntity, ConnectionIdentifiers,
    DetectEvents, ObservedEvents, EventStates, BearerInformation,
    RestartMethod, RestartDelay, ReasonCode, PackageList,
    MaxMGCPDatagram, and Capabilities.

Andreasen & Foster Informational [Page 61] RFC 3435 MGCP 1.0 January 2003

 The list may be extended by extension parameters.  The response will
 in turn include information about each of the items for which
 auditing info was requested.  Supported parameters with empty values
 MUST always be returned.  However, if an endpoint is queried about a
 parameter it does not understand, the endpoint MUST NOT generate an
 error; instead the parameter MUST be omitted from the response:
  • RequestedEvents: The current value of RequestedEvents the endpoint

is using including the action(s) and event parameters associated

   with each event - if no actions are included, the default action is
   assumed. Persistent events are included in the list. If an embedded
   NotificationRequest is active, the RequestedEvents will reflect the
   events requested in the embedded NotificationRequest, not any
   surrounding RequestedEvents (whether embedded or not).
  • DigitMap: The digit map the endpoint is currently using. The

parameter will be empty if the endpoint does not have a digit map.

  • SignalRequests: A list of the; Time-Out signals that are currently

active, On/Off signals that are currently "on" for the endpoint

   (with or without parameter), and any pending Brief signals.  Time-
   Out signals that have timed-out, and currently playing Brief
   signals are not included.  Any signal parameters included in the
   original SignalRequests will be included.
  • RequestIdentifier: The RequestIdentifier for the last

NotificationRequest received by this endpoint (includes

   NotificationRequests encapsulated in other commands).  If no
   NotificationRequest has been received since reboot/restart, the
   value zero will be returned.
  • QuarantineHandling: The QuarantineHandling for the last

NotificationRequest received by this endpoint. If

   QuarantineHandling was not included, or no notification request has
   been received, the default values will be returned.
  • DetectEvents: The value of the most recently received DetectEvents

parameter plus any persistent events implemented by the endpoint.

   If no DetectEvents parameter has been received, the (possibly
   empty) list only includes persistent events.
  • NotifiedEntity: The current "notified entity" for the endpoint.
  • ConnectionIdentifiers: The list of ConnectionIdentifiers for all

connections that currently exist for the specified endpoint.

  • ObservedEvents: The current list of observed events for the

endpoint.

Andreasen & Foster Informational [Page 62] RFC 3435 MGCP 1.0 January 2003

  • EventStates: For events that have auditable states associated with

them, the event corresponding to the state the endpoint is in,

   e.g., off-hook if the endpoint is off-hook.  Note that the
   definition of the individual events will state if the event in
   question has an auditable state associated with it.
  • BearerInformation: The value of the last received

BearerInformation parameter for this endpoint (this includes the

   case where BearerInformation was provisioned).  The parameter will
   be empty if the endpoint has not received a BearerInformation
   parameter and a value was also not provisioned.
  • RestartMethod: "restart" if the endpoint is in-service and

operation is normal, or if the endpoint is in the process of

   becoming in-service (a non-zero RestartDelay will indicate the
   latter).  Otherwise, the value of the restart method parameter in
   the last RestartInProgress command issued (or should have been
   issued) by the endpoint.  Note that a "disconnected" endpoint will
   thus only report "disconnected" as long as it actually is
   disconnected, and "restart" will be reported once it is no longer
   disconnected.  Similarly, "cancel-graceful" will not be reported,
   but "graceful" might (see Section 4.4.5 for further details).
  • RestartDelay: The value of the restart delay parameter if a

RestartInProgress command was to be issued by the endpoint at the

   time of this response, or zero if the command would not include
   this parameter.
  • ReasonCode: The value of the ReasonCode parameter in the last

RestartInProgress or DeleteConnection command issued by the gateway

   for the endpoint, or the special value 000 if the endpoint's state
   is normal.
  • PackageList: The packages supported by the endpoint including

package version numbers. For backwards compatibility, support for

   the parameter is OPTIONAL although implementations with package
   versions higher than zero SHOULD support it.
  • MaxMGCPDatagram: The maximum size of an MGCP datagram in bytes

that can be received by the endpoint (see Section 3.5.4). The

   value excludes any lower layer overhead.  For backwards
   compatibility, support for this parameter is OPTIONAL.  The default
   maximum MGCP datagram size SHOULD be assumed if a value is not
   returned.

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  • Capabilities: The capabilities for the endpoint similar to the

LocalConnectionOptions parameter and including packages and

   connection modes.  Extensions MAY be included as well.  If any
   unknown capabilities are reported, they MUST simply be ignored.  If
   there is a need to specify that some parameters, such as e.g.,
   silence suppression, are only compatible with some codecs, then the
   gateway MUST return several capability sets, each of which may
   include:
  1. Compression Algorithm: A list of supported codecs. The rest of

the parameters in the capability set will apply to all codecs

     specified in this list.
  1. Packetization Period: A single value or a range may be

specified.

  1. Bandwidth: A single value or a range corresponding to the range

for packetization periods may be specified (assuming no silence

     suppression).
  1. Echo Cancellation: Whether echo cancellation is supported or not

for the endpoint.

  1. Silence Suppression: Whether silence suppression is supported or

not.

  1. Gain Control: Whether gain control is supported or not.
  1. Type of Service: Whether type of service is supported or not.
  1. Resource Reservation: Whether resource reservation is supported

or not.

  1. Security: Whether media encryption is supported or not.
  1. Type of network: The type(s) of network supported.
  1. Packages: A list of packages supported. The first package in

the list will be the default package.

  1. Modes: A list of supported connection modes.
 The Call Agent may then decide to use the AuditConnection command to
 obtain further information about the connections.
 If no info was requested and the EndpointId refers to a valid
 endpoint (in-service or not), the gateway simply returns a positive
 acknowledgement.

Andreasen & Foster Informational [Page 64] RFC 3435 MGCP 1.0 January 2003

 ReturnCode is a parameter returned by the gateway.  It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.
 Note that PackageList MAY also be included with error code 518
 (unsupported package).

2.3.11 AuditConnection

 The AuditConnection command can be used by the Call Agent to retrieve
 the parameters attached to a connection.
       ReturnCode,
       [CallId,]
       [NotifiedEntity,]
       [LocalConnectionOptions,]
       [Mode,]
       [RemoteConnectionDescriptor,]
       [LocalConnectionDescriptor,]
       [ConnectionParameters,]
       [PackageList]
       <-- AuditConnection(EndpointId,
                           ConnectionId,
                           RequestedInfo)
 The EndpointId parameter specifies the endpoint that handles the
 connection.  The wildcard conventions SHALL NOT be used.
 The ConnectionId parameter is the identifier of the audited
 connection, within the context of the specified endpoint.
 The (possibly empty) RequestedInfo describes the information that is
 requested for the ConnectionId within the EndpointId specified.  The
 following connection info can be audited with this command:
    CallId, NotifiedEntity, LocalConnectionOptions, Mode,
    RemoteConnectionDescriptor, LocalConnectionDescriptor,
    ConnectionParameters
 The AuditConnection response will in turn include information about
 each of the items auditing info was requested for:
  • CallId, the CallId for the call the connection belongs to.
  • NotifiedEntity, the current "notified entity" for the Connection.

Note this is the same as the "notified entity" for the endpoint

   (included here for backwards compatibility).

Andreasen & Foster Informational [Page 65] RFC 3435 MGCP 1.0 January 2003

  • LocalConnectionOptions, the most recent LocalConnectionOptions

parameters that was actually supplied for the connection (omitting

   LocalConnectionOptions from a command thus does not change this
   value).  Note that default parameters omitted from the most recent
   LocalConnectionOptions will not be included.
   LocalConnectionOptions that retain their value across
   ModifyConnection commands and which have been included in a
   previous command for the connection are also included, regardless
   of whether they were supplied in the most recent
   LocalConnectionOptions or not.
  • Mode, the current mode of the connection.
  • RemoteConnectionDescriptor, the RemoteConnectionDescriptor that was

supplied to the gateway for the connection.

  • LocalConnectionDescriptor, the LocalConnectionDescriptor the

gateway supplied for the connection.

  • ConnectionParameters, the current values of the connection

parameters for the connection.

 If no info was requested and the EndpointId is valid, the gateway
 simply checks that the connection exists, and if so returns a
 positive acknowledgement.  Note, that by definition, the endpoint
 must be in-service for this to happen, as out-of-service endpoints do
 not have any connections.
 ReturnCode is a parameter returned by the gateway.  It indicates the
 outcome of the command and consists of an integer number optionally
 followed by commentary.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).

2.3.12 RestartInProgress

 The RestartInProgress command is used by the gateway to signal that
 an endpoint, or a group of endpoints, is put in-service or out-of-
 service.
       ReturnCode,
       [NotifiedEntity,]
       [PackageList]
       <-- RestartInProgress(EndPointId,
                             RestartMethod,
                             [RestartDelay,]
                             [ReasonCode])

Andreasen & Foster Informational [Page 66] RFC 3435 MGCP 1.0 January 2003

 The EndPointId identifies the endpoint(s) that are put in-service or
 out-of-service.  The "all of" wildcard convention may be used to
 apply the command to a group of endpoints managed by the same Call
 Agent, such as for example all endpoints that are attached to a
 specified interface, or even all endpoints that are attached to a
 given gateway.  The "any of" wildcard convention SHALL NOT be used.
 The RestartMethod parameter specifies the type of restart.  The
 following values have been defined:
  • A "graceful" restart method indicates that the specified endpoints

will be taken out-of-service after the specified delay. The

   established connections are not yet affected, but the Call Agent
   SHOULD refrain from establishing new connections, and SHOULD try to
   gracefully tear down the existing connections.
  • A "forced" restart method indicates that the specified endpoints

are taken abruptly out-of-service. The established connections, if

   any, are lost.
  • A "restart" method indicates that service will be restored on the

endpoints after the specified "restart delay", i.e., the endpoints

   will be in-service.  The endpoints are in their clean default state
   and there are no connections that are currently established on the
   endpoints.
  • A "disconnected" method indicates that the endpoint has become

disconnected and is now trying to establish connectivity (see

   Section 4.4.7).  The "restart delay" specifies the number of
   seconds the endpoint has been disconnected.  Established
   connections are not affected.
  • A "cancel-graceful" method indicates that a gateway is canceling a

previously issued "graceful" restart command. The endpoints are

   still in-service.
 The list of restart methods may be extended.
 The optional "restart delay" parameter is expressed as a number of
 seconds.  If the number is absent, the delay value MUST be considered
 null (i.e., zero).  In the case of the "graceful" method, a null
 delay indicates that the Call Agent SHOULD simply wait for the
 natural termination of the existing connections, without establishing
 new connections.  The restart delay is always considered null in the
 case of the "forced" and "cancel-graceful" methods, and hence the
 "restart delay" parameter MUST NOT be used with these restart
 methods.  When the gateway sends a "restart" or "graceful"

Andreasen & Foster Informational [Page 67] RFC 3435 MGCP 1.0 January 2003

 RestartInProgress message with a non-zero restart delay, the gateway
 SHOULD send an updated RestartInProgress message after the "restart
 delay" has passed.
 A restart delay of null for the "restart" method indicates that
 service has already been restored.  This typically will occur after
 gateway startup/reboot.  To mitigate the effects of a gateway IP
 address change as a result of a re-boot, the Call Agent MAY wish to
 either flush its DNS cache for the gateway's domain name or resolve
 the gateway's domain name by querying the DNS regardless of the TTL
 of a current DNS resource record for the restarted gateway.
 The optional reason code parameter indicates the cause of the
 restart.
 Gateways SHOULD send a "graceful" or "forced" RestartInProgress
 message (for the relevant endpoints) as a courtesy to the Call Agent
 when they are taken out-of-service, e.g., by being shutdown, or taken
 out-of-service by a network management system, however the Call Agent
 cannot rely on always receiving such a message.  Gateways MUST send a
 "restart" RestartInProgress message (for the relevant endpoints) with
 a null delay to their Call Agent when they are back in-service
 according to the restart procedure specified in Section 4.4.6 - Call
 Agents can rely on receiving this message.  Also, gateways MUST send
 a "disconnected" RestartInProgress message (for the relevant
 endpoints) to their current "notified entity" according to the
 "disconnected" procedure specified in Section 4.4.7.
 The RestartInProgress message will be sent to the current "notified
 entity" for the EndpointId in question.  It is expected that a
 default Call Agent, i.e., "notified entity", has been provisioned so
 that after a reboot/restart, the default Call Agent will always be
 the "notified entity" for the endpoint.  Gateways SHOULD take full
 advantage of wild-carding to minimize the number of RestartInProgress
 messages generated when multiple endpoints in a gateway restart and
 the endpoints are managed by the same Call Agent.
 ReturnCode is a parameter returned by the Call Agent.  It indicates
 the outcome of the command and consists of an integer number
 optionally followed by commentary.
 A NotifiedEntity may additionally be returned with the response to
 the RestartInProgress from the Call Agent - this SHOULD normally only
 be done in response to "restart" or "disconnected" (see also Section
 4.4.6 and 4.4.7):

Andreasen & Foster Informational [Page 68] RFC 3435 MGCP 1.0 January 2003

  • If the response indicated success (return code 200 - transaction

executed), the restart in question completed successfully, and the

   NotifiedEntity returned is the new "notified entity" for the
   endpoint(s).
  • If the response from the Call Agent indicated an error, the restart

in question did not complete successfully. If a NotifiedEntity

   parameter was included in the response returned, it specifies a new
   "notified entity" for the endpoint(s), which MUST be used when
   retrying the restart in question (as a new transaction).  This
   SHOULD only be done with error code 521 (endpoint redirected).
 Note that the above behavior for returning a NotifiedEntity in the
 response is only defined for RestartInProgress responses and SHOULD
 NOT be done for responses to other commands.  Any other behavior is
 undefined.
 PackageList is a list of supported packages that MAY be included with
 error code 518 (unsupported package).

2.4 Return Codes and Error Codes

 All MGCP commands are acknowledged.  The acknowledgment carries a
 return code, which indicates the status of the command.  The return
 code is an integer number, for which the following ranges of values
 have been defined:
  • values between 000 and 099 indicate a response acknowledgement
  • values between 100 and 199 indicate a provisional response
  • values between 200 and 299 indicate a successful completion
  • values between 400 and 499 indicate a transient error
  • values between 500 and 599 indicate a permanent error
  • values between 800 and 899 are package specific response codes.
 A broad description of transient errors (4XX error codes) versus
 permanent errors (5XX error codes) is as follows:

Andreasen & Foster Informational [Page 69] RFC 3435 MGCP 1.0 January 2003

  • If a Call Agent receives a transient error, there is the

expectation of the possibility that a future similar request will

   be honored by the endpoint.  In some cases, this may require some
   state change in the environment of the endpoint (e.g., hook state
   as in the case of error codes 401 or 402; resource availability as
   in the case of error code 403, or bandwidth availability as in the
   case of error code 404).
  • Permanent errors (error codes 500 to 599) indicate one or more

permanent conditions either due to protocol error or

   incompatibility between the endpoint and the Call Agent, or because
   of some error condition over which the Call Agent has no control.
   Examples are protocol errors, requests for endpoint capabilities
   that do not exist, errors on interfaces associated with the
   endpoint, missing or incorrect information in the request or any
   number of other conditions which will simply not disappear with
   time.
 The values that have been already defined are the following:
 000 Response Acknowledgement.
 100 The transaction is currently being executed.  An actual
     completion message will follow later.
 101 The transaction has been queued for execution.  An actual
     completion message will follow later.
 200 The requested transaction was executed normally.  This return
     code can be used for a successful response to any command.
 250 The connection was deleted.  This return code can only be used
     for a successful response to a DeleteConnection command.
 400 The transaction could not be executed, due to some unspecified
     transient error.
 401 The phone is already off hook.
 402 The phone is already on hook.
 403 The transaction could not be executed, because the endpoint does
     not have sufficient resources at this time.
 404 Insufficient bandwidth at this time.
 405 The transaction could not be executed, because the endpoint is
     "restarting".

Andreasen & Foster Informational [Page 70] RFC 3435 MGCP 1.0 January 2003

 406 Transaction time-out.  The transaction did not complete in a
     reasonable period of time and has been aborted.
 407 Transaction aborted.  The transaction was aborted by some
     external action, e.g., a ModifyConnection command aborted by a
     DeleteConnection command.
 409 The transaction could not be executed because of internal
     overload.
 410 No endpoint available.  A valid "any of" wildcard was used,
     however there was no endpoint available to satisfy the request.
 500 The transaction could not be executed, because the endpoint is
     unknown.
 501 The transaction could not be executed, because the endpoint is
     not ready.  This includes the case where the endpoint is out-of-
     service.
 502 The transaction could not be executed, because the endpoint does
     not have sufficient resources (permanent condition).
 503 "All of" wildcard too complicated.
 504 Unknown or unsupported command.
 505 Unsupported RemoteConnectionDescriptor.  This SHOULD be used when
     one or more mandatory parameters or values in the
     RemoteConnectionDescriptor is not supported.
 506 Unable to satisfy both LocalConnectionOptions and
     RemoteConnectionDescriptor.  This SHOULD be used when the
     LocalConnectionOptions and RemoteConnectionDescriptor contain one
     or more mandatory parameters or values that conflict with each
     other and/or cannot be supported at the same time (except for
     codec negotiation failure - see error code 534).
 507 Unsupported functionality. Some unspecified functionality
     required to carry out the command is not supported. Note that
     several other error codes have been defined for specific areas of
     unsupported functionality (e.g. 508, 511, etc.), and this error
     code SHOULD only be used if there is no other more specific error
     code for the unsupported functionality.
 508 Unknown or unsupported quarantine handling.

Andreasen & Foster Informational [Page 71] RFC 3435 MGCP 1.0 January 2003

 509 Error in RemoteConnectionDescriptor.  This SHOULD be used when
     there is a syntax or semantic error in the
     RemoteConnectionDescriptor.
 510 The transaction could not be executed, because some unspecified
     protocol error was detected.  Automatic recovery from such an
     error will be very difficult, and hence this code SHOULD only be
     used as a last resort.
 511 The transaction could not be executed, because the command
     contained an unrecognized extension.  This code SHOULD be used
     for unsupported critical parameter extensions ("X+").
 512 The transaction could not be executed, because the gateway is not
     equipped to detect one of the requested events.
 513 The transaction could not be executed, because the gateway is not
     equipped to generate one of the requested signals.
 514 The transaction could not be executed, because the gateway cannot
     send the specified announcement.
 515 The transaction refers to an incorrect connection-id (may have
     been already deleted).
 516 The transaction refers to an unknown call-id, or the call-id
     supplied is incorrect (e.g., connection-id not associated with
     this call-id).
 517 Unsupported or invalid mode.
 518 Unsupported or unknown package.  It is RECOMMENDED to include a
     PackageList parameter with the list of supported packages in the
     response, especially if the response is generated by the Call
     Agent.
 519 Endpoint does not have a digit map.
 520 The transaction could not be executed, because the endpoint is
     "restarting".  In most cases this would be a transient error, in
     which case, error code 405 SHOULD be used instead.  The error
     code is only included here for backwards compatibility.
 521 Endpoint redirected to another Call Agent.  The associated
     redirection behavior is only well-defined when this response is
     issued for a RestartInProgress command.

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 522 No such event or signal.  The request referred to an event or
     signal that is not defined in the relevant package (which could
     be the default package).
 523 Unknown action or illegal combination of actions.
 524 Internal inconsistency in LocalConnectionOptions.
 525 Unknown extension in LocalConnectionOptions.  This code SHOULD be
     used for unsupported mandatory vendor extensions ("x+").
 526 Insufficient bandwidth.  In cases where this is a transient
     error, error code 404 SHOULD be used instead.
 527 Missing RemoteConnectionDescriptor.
 528 Incompatible protocol version.
 529 Internal hardware failure.
 530 CAS signaling protocol error.
 531 Failure of a grouping of trunks (e.g., facility failure).
 532 Unsupported value(s) in LocalConnectionOptions.
 533 Response too large.
 534 Codec negotiation failure.
 535 Packetization period not supported.
 536 Unknown or unsupported RestartMethod.
 537 Unknown or unsupported digit map extension.
 538 Event/signal parameter error (e.g., missing, erroneous,
     unsupported, unknown, etc.).
 539 Invalid or unsupported command parameter. This code SHOULD only
     be used when the parameter is neither a package or vendor
     extension parameter.
 540 Per endpoint connection limit exceeded.
 541 Invalid or unsupported LocalConnectionOptions. This code SHOULD
     only be used when the LocalConnectionOptions is neither a package
     nor a vendor extension LocalConnectionOptions.

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 The set of return codes may be extended in a future version of the
 protocol.  Implementations that receive an unknown or unsupported
 return code SHOULD treat the return code as follows:
  • Unknown 0xx code treated as 000.
  • Unknown 1xx code treated as 100.
  • Unknown 2xx code treated as 200.
  • Unknown 3xx code treated as 521.
  • Unknown 4xx code treated as 400.
  • Unknown 5xx-9xx code treated as 510.

2.5 Reason Codes

 Reason codes are used by the gateway when deleting a connection to
 inform the Call Agent about the reason for deleting the connection.
 They may also be used in a RestartInProgress command to inform the
 Call Agent of the reason for the RestartInProgress.
 The reason code is an integer number, and the following values have
 been defined:
 000 Endpoint state is normal (this code is only used in response to
     audit requests).
 900 Endpoint malfunctioning.
 901 Endpoint taken out-of-service.
 902 Loss of lower layer connectivity (e.g., downstream sync).
 903 QoS resource reservation was lost.
 904 Manual intervention.
 905 Facility failure (e.g., DS-0 failure).
 The set of reason codes can be extended.

Andreasen & Foster Informational [Page 74] RFC 3435 MGCP 1.0 January 2003

2.6 Use of Local Connection Options and Connection Descriptors

 As indicated previously, the normal sequence in setting up a bi-
 directional connection involves at least 3 steps:
 1) The Call Agent asks the first gateway to "create a connection" on
    an endpoint.  The gateway allocates resources to that connection,
    and responds to the command by providing a "session description"
    (referred to as its LocalConnectionDescriptor).  The session
    description contains the information necessary for another party
    to send packets towards the newly created connection.
 2) The Call Agent then asks the second gateway to "create a
    connection" on an endpoint.  The command carries the "session
    description" provided by the first gateway (now referred to as the
    RemoteConnectionDescriptor).  The gateway allocates resources to
    that connection, and responds to the command by providing its own
    "session description" (LocalConnectionDescriptor).
 3) The Call Agent uses a "modify connection" command to provide this
    second "session description" (now referred to as the
    RemoteConnectionDescriptor ) to the first endpoint.  Once this is
    done, communication can proceed in both directions.
 When the Call Agent issues a Create or Modify Connection command,
 there are thus three parameters that determine the media supported by
 that connection:
  • LocalConnectionOptions: Supplied by the Call Agent to control the

media parameters used by the gateway for the connection. When

   supplied, the gateway MUST conform to these media parameters until
   either the connection is deleted, or a ModifyConnection command
   with new media parameters (LocalConnectionOptions or
   RemoteConnectionDescriptor) is received.
  • RemoteConnectionDescriptor: Supplied by the Call Agent to convey

the media parameters supported by the other side of the connection.

   When supplied, the gateway MUST conform to these media parameters
   until either the connection is deleted, or a ModifyConnection
   command with new media parameters (LocalConnectionOptions or
   RemoteConnectionDescriptor) is received.
  • LocalConnectionDescriptor: Supplied by the gateway to the Call

Agent to convey the media parameters it supports for the

   connection. When supplied, the gateway MUST honor the media
   parameters until either the connection is deleted, or the gateway
   issues a new LocalConnectionDescriptor for that connection.

Andreasen & Foster Informational [Page 75] RFC 3435 MGCP 1.0 January 2003

 In determining which codec(s) to provide in the
 LocalConnectionDescriptor, there are three lists of codecs that a
 gateway needs to consider:
  • A list of codecs allowed by the LocalConnectionOptions in the

current command (either explicitly by encoding method or implicitly

   by bandwidth and/or packetization period).
  • A list of codecs in the RemoteConnectionDescriptor in the current

command.

  • An internal list of codecs that the gateway can support for the

connection. A gateway MAY support one or more codecs for a given

   connection.
 Codec selection (including all relevant media parameters) can then be
 described by the following steps:
 1. An approved list of codecs is formed by taking the intersection of
    the internal list of codecs and codecs allowed by the
    LocalConnectionOptions. If LocalConnectionOptions were not
    provided in the current command, the approved list of codecs thus
    contains the internal list of codecs.
 2. If the approved list of codecs is empty, a codec negotiation
    failure has occurred and an error response is generated (error
    code 534 - codec negotiation failure, is RECOMMENDED).
 3. Otherwise, a negotiated list of codecs is formed by taking the
    intersection of the approved list of codecs and codecs allowed by
    the RemoteConnectionDescriptor. If a RemoteConnectionDescriptor
    was not provided in the current command, the negotiated list of
    codecs thus contains the approved list of codecs.
 4. If the negotiated list of codecs is empty, a codec negotiation
    failure has occurred and an error response is generated (error
    code 534 - codec negotiation failure, is RECOMMENDED).
 5. Otherwise, codec negotiation has succeeded, and the negotiated
    list of codecs is returned in the LocalConnectionDescriptor.
 Note that both LocalConnectionOptions and the
 RemoteConnectionDescriptor can contain a list of codecs ordered by
 preference. When both are supplied in the current command, the
 gateway MUST adhere to the preferences provided in the
 LocalConnectionOptions.

Andreasen & Foster Informational [Page 76] RFC 3435 MGCP 1.0 January 2003

2.7 Resource Reservations

 The gateways can be instructed to perform a reservation, for example
 using RSVP, on a given connection. When a reservation is needed, the
 call agent will specify the reservation profile to be used, which is
 either "controlled load" or "guaranteed service". The absence of
 reservation can be indicated by asking for the "best effort" service,
 which is the default value of this parameter in a CreateConnection
 command. For a ModifyConnection command, the default is simply to
 retain the current value. When reservation has been asked on a
 connection, the gateway will:
  • start emitting RSVP "PATH" messages if the connection is in "send-

only", "send-receive", "conference", "network loop back" or

   "network continuity test" mode (if a suitable remote connection
   descriptor has been received,).
  • start emitting RSVP "RESV" messages as soon as it receives "PATH"

messages if the connection is in "receive-only", "send-receive",

   "conference", "network loop back" or "network continuity test"
   mode.
 The RSVP filters will be deduced from the characteristics of the
 connection. The RSVP resource profiles will be deduced from the
 connection's codecs, bandwidth and packetization period.

3. Media Gateway Control Protocol

 The Media Gateway Control Protocol (MGCP) implements the media
 gateway control interface as a set of transactions. The transactions
 are composed of a command and a mandatory response. There are nine
 commands:
  • EndpointConfiguration
  • CreateConnection
  • ModifyConnection
  • DeleteConnection
  • NotificationRequest
  • Notify
  • AuditEndpoint
  • AuditConnection

Andreasen & Foster Informational [Page 77] RFC 3435 MGCP 1.0 January 2003

  • RestartInProgress
 The first five commands are sent by the Call Agent to a gateway. The
 Notify command is sent by the gateway to the Call Agent. The gateway
 may also send a DeleteConnection as defined in Section 2.3.8.  The
 Call Agent may send either of the Audit commands to the gateway, and
 the gateway may send a RestartInProgress command to the Call Agent.

3.1 General Description

 All commands are composed of a Command header, optionally followed by
 a session description.
 All responses are composed of a Response header, optionally followed
 by session description information.
 Headers and session descriptions are encoded as a set of text lines,
 separated by a carriage return and line feed character (or,
 optionally, a single line-feed character). The session descriptions
 are preceded by an empty line.
 MGCP uses a transaction identifier to correlate commands and
 responses. The transaction identifier is encoded as a component of
 the command header and repeated as a component of the response header
 (see sections 3.2.1.2 and 3.3).
 Note that an ABNF grammar for MGCP is provided in Appendix A.
 Commands and responses SHALL be encoded in accordance with the
 grammar, which, per RFC 2234, is case-insensitive except for the SDP
 part.  Similarly, implementations SHALL be capable of decoding
 commands and responses that follow the grammar.  Additionally, it is
 RECOMMENDED that implementations tolerate additional linear white
 space.
 Some productions allow for use of quoted strings, which can be
 necessary to avoid syntax problems.  Where the quoted string form is
 used, the contents will be UTF-8 encoded [20], and the actual value
 provided is the unquoted string (UTF-8 encoded).  Where both a quoted
 and unquoted string form is allowed, either form can be used provided
 it does not otherwise violate the grammar.
 In the following, we provide additional detail on the format of MGCP
 commands and responses.

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3.2 Command Header

 The command header is composed of:
  • A command line, identifying the requested action or verb, the

transaction identifier, the endpoint towards which the action is

    requested, and the MGCP protocol version,
  • A set of zero or more parameter lines, composed of a parameter

name followed by a parameter value.

 Unless otherwise noted or dictated by other referenced standards
 (e.g., SDP), each component in the command header is case
 insensitive.  This goes for verbs as well as parameters and values,
 and hence all comparisons MUST treat upper and lower case as well as
 combinations of these as being equal.

3.2.1 Command Line

 The command line is composed of:
  • The name of the requested verb,
  • The identification of the transaction,
  • The name of the endpoint(s) that are to execute the command (in

notifications or restarts, the name of the endpoint(s) that is

   issuing the command),
  • The protocol version.
   These four items are encoded as strings of printable ASCII
   characters, separated by white spaces, i.e., the ASCII space (0x20)
   or tabulation (0x09) characters.  It is RECOMMENDED to use exactly
   one ASCII space separator.  However, MGCP entities MUST be able to
   parse messages with additional white space characters.

Andreasen & Foster Informational [Page 79] RFC 3435 MGCP 1.0 January 2003

3.2.1.1 Coding of the Requested Verb

 The verbs that can be requested are encoded as four letter upper or
 lower case ASCII codes (comparisons SHALL be case insensitive) as
 defined in the following table:
  1. —————————-

| Verb | Code |

               |----------------------|------|
               | EndpointConfiguration| EPCF |
               | CreateConnection     | CRCX |
               | ModifyConnection     | MDCX |
               | DeleteConnection     | DLCX |
               | NotificationRequest  | RQNT |
               | Notify               | NTFY |
               | AuditEndpoint        | AUEP |
               | AuditConnection      | AUCX |
               | RestartInProgress    | RSIP |
                -----------------------------
 The transaction identifier is encoded as a string of up to 9 decimal
 digits.  In the command line, it immediately follows the coding of
 the verb.
 New verbs may be defined in further versions of the protocol.  It may
 be necessary, for experimentation purposes, to use new verbs before
 they are sanctioned in a published version of this protocol.
 Experimental verbs MUST be identified by a four letter code starting
 with the letter X, such as for example XPER.

3.2.1.2 Transaction Identifiers

 MGCP uses a transaction identifier to correlate commands and
 responses.  A gateway supports two separate transaction identifier
 name spaces:
  • a transaction identifier name space for sending transactions, and
  • a transaction identifier name space for receiving transactions.
 At a minimum, transaction identifiers for commands sent to a given
 gateway MUST be unique for the maximum lifetime of the transactions
 within the collection of Call Agents that control that gateway.
 Thus, regardless of the sending Call Agent, gateways can always
 detect duplicate transactions by simply examining the transaction
 identifier.  The coordination of these transaction identifiers
 between Call Agents is outside the scope of this specification
 though.

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 Transaction identifiers for all commands sent from a given gateway
 MUST be unique for the maximum lifetime of the transactions
 regardless of which Call Agent the command is sent to.  Thus, a Call
 Agent can always detect a duplicate transaction from a gateway by the
 combination of the domain-name of the endpoint and the transaction
 identifier.
 The transaction identifier is encoded as a string of up to nine
 decimal digits.  In the command lines, it immediately follows the
 coding of the verb.
 Transaction identifiers have values between 1 and 999,999,999 (both
 included).  Transaction identifiers SHOULD NOT use any leading
 zeroes, although equality is based on numerical value, i.e., leading
 zeroes are ignored.  An MGCP entity MUST NOT reuse a transaction
 identifier more quickly than three minutes after completion of the
 previous command in which the identifier was used.

3.2.1.3 Coding of the Endpoint Identifiers and Entity Names

 The endpoint identifiers and entity names are encoded as case
 insensitive e-mail addresses, as defined in RFC 821, although with
 some syntactic restrictions on the local part of the name.
 Furthermore, both the local endpoint name part and the domain name
 part can each be up to 255 characters.  In these addresses, the
 domain name identifies the system where the endpoint is attached,
 while the left side identifies a specific endpoint or entity on that
 system.
 Examples of such addresses are:
  1. —————————————————————–

| hrd4/56@gw23.example.net | Circuit number 56 in |

 |                              |  interface "hrd4" of the Gateway  |
 |                              |  23 of the "Example" network      |
 | Call-agent@ca.example.net    |  Call Agent for the               |
 |                              |  "example" network                |
 | Busy-signal@ann12.example.net|  The "busy signal" virtual        |
 |                              |  endpoint in the announcement     |
 |                              |  server number 12.                |
  ------------------------------------------------------------------
 The name of a notified entity is expressed with the same syntax, with
 the possible addition of a port number as in:
    Call-agent@ca.example.net:5234

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 In case the port number is omitted from the notified entity, the
 default MGCP Call Agent port (2727) MUST be used.

3.2.1.4 Coding of the Protocol Version

 The protocol version is coded as the keyword MGCP followed by a white
 space and the version number, and optionally followed by a profile
 name.  The version number is composed of a major version, coded by a
 decimal number, a dot, and a minor version number, coded as a decimal
 number.  The version described in this document is version 1.0.
 The profile name, if present, is represented by white-space separated
 strings of visible (printable) characters extending to the end of the
 line.  Profile names may be defined for user communities who want to
 apply restrictions or other profiling to MGCP.
 In the initial messages, the version will be coded as:
    MGCP 1.0
 An entity that receives a command with a protocol version it does not
 support, MUST respond with an error (error code 528 - incompatible
 protocol version, is RECOMMENDED).  Note that this applies to
 unsupported profiles as well.

3.2.2 Parameter Lines

 Parameter lines are composed of a parameter name, which in most cases
 is composed of one or two characters, followed by a colon, optional
 white space(s) and the parameter value.  The parameters that can be
 present in commands are defined in the following table:

Andreasen & Foster Informational [Page 82] RFC 3435 MGCP 1.0 January 2003

  1. —————————————————————–

|Parameter name | Code | Parameter value |

 |----------------------|------|------------------------------------|
 |BearerInformation     |   B  |  See description (3.2.2.1).        |
 |CallId                |   C  |  See description (3.2.2.2).        |
 |Capabilities          |   A  |  See description (3.2.2.3).        |
 |ConnectionId          |   I  |  See description (3.2.2.5).        |
 |ConnectionMode        |   M  |  See description (3.2.2.6).        |
 |ConnectionParameters  |   P  |  See description (3.2.2.7).        |
 |DetectEvents          |   T  |  See description (3.2.2.8).        |
 |DigitMap              |   D  |  A text encoding of a digit map.   |
 |EventStates           |   ES |  See description (3.2.2.9).        |
 |LocalConnectionOptions|   L  |  See description (3.2.2.10).       |
 |MaxMGCPDatagram       |   MD |  See description (3.2.2.11).       |
 |NotifiedEntity        |   N  |  An identifier, in RFC 821 format, |
 |                      |      |  composed of an arbitrary string   |
 |                      |      |  and of the domain name of the     |
 |                      |      |  requesting entity, possibly com-  |
 |                      |      |  pleted by a port number, as in:   |
 |                      |      |    Call-agent@ca.example.net:5234  |
 |                      |      |  See also Section 3.2.1.3.         |
 |ObservedEvents        |   O  |  See description (3.2.2.12).       |
 |PackageList           |   PL |  See description (3.2.2.13).       |
 |QuarantineHandling    |   Q  |  See description (3.2.2.14).       |
 |ReasonCode            |   E  |  A string with a 3 digit integer   |
 |                      |      |  optionally followed by a set of   |
 |                      |      |  arbitrary characters (3.2.2.15).  |
 |RequestedEvents       |   R  |  See description (3.2.2.16).       |
 |RequestedInfo         |   F  |  See description (3.2.2.17).       |
 |RequestIdentifier     |   X  |  See description (3.2.2.18).       |
 |ResponseAck           |   K  |  See description (3.2.2.19).       |
 |RestartDelay          |   RD |  A number of seconds, encoded as   |
 |                      |      |  a decimal number.                 |
 |RestartMethod         |   RM |  See description (3.2.2.20).       |
 |SecondConnectionId    |   I2 |  Connection Id.                    |
 |SecondEndpointId      |   Z2 |  Endpoint Id.                      |
 |SignalRequests        |   S  |  See description (3.2.2.21).       |
 |SpecificEndPointId    |   Z  |  An identifier, in RFC 821 format, |
 |                      |      |  composed of an arbitrary string,  |
 |                      |      |  followed by an "@" followed by    |
 |                      |      |  the domain name of the gateway to |
 |                      |      |  which this endpoint is attached.  |
 |                      |      |  See also Section 3.2.1.3.         |
 |----------------------|------|------------------------------------|

Andreasen & Foster Informational [Page 83] RFC 3435 MGCP 1.0 January 2003

 |RemoteConnection-     |   RC |  Session Description.              |
 |         Descriptor   |      |                                    |
 |LocalConnection-      |   LC |  Session Description.              |
 |         Descriptor   |      |                                    |
  ------------------------------------------------------------------
 The parameters are not necessarily present in all commands.  The
 following table provides the association between parameters and
 commands.  The letter M stands for mandatory, O for optional and F
 for forbidden.  Unless otherwise specified, a parameter MUST NOT be
 present more than once.

Andreasen & Foster Informational [Page 84] RFC 3435 MGCP 1.0 January 2003

  1. —————————————————————–

| Parameter name | EP | CR | MD | DL | RQ | NT | AU | AU | RS |

 |                     | CF | CX | CX | CX | NT | FY | EP | CX | IP |
 |---------------------|----|----|----|----|----|----|----|----|----|
 | BearerInformation   |  O*|  O |  O |  O |  O |  F |  F |  F |  F |
 | CallId              |  F |  M |  M |  O |  F |  F |  F |  F |  F |
 | Capabilities        |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | ConnectionId        |  F |  F |  M |  O |  F |  F |  F |  M |  F |
 | ConnectionMode      |  F |  M |  O |  F |  F |  F |  F |  F |  F |
 | Connection-         |  F |  F |  F |  O*|  F |  F |  F |  F |  F |
 |   Parameters        |    |    |    |    |    |    |    |    |    |
 | DetectEvents        |  F |  O |  O |  O |  O |  F |  F |  F |  F |
 | DigitMap            |  F |  O |  O |  O |  O |  F |  F |  F |  F |
 | EventStates         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | LocalConnection-    |  F |  O |  O |  F |  F |  F |  F |  F |  F |
 |            Options  |    |    |    |    |    |    |    |    |    |
 | MaxMGCPDatagram     |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | NotifiedEntity      |  F |  O |  O |  O |  O |  O |  F |  F |  F |
 | ObservedEvents      |  F |  F |  F |  F |  F |  M |  F |  F |  F |
 | PackageList         |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | QuarantineHandling  |  F |  O |  O |  O |  O |  F |  F |  F |  F |
 | ReasonCode          |  F |  F |  F |  O |  F |  F |  F |  F |  O |
 | RequestedEvents     |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
 | RequestIdentifier   |  F |  O*|  O*|  O*|  M |  M |  F |  F |  F |
 | RequestedInfo       |  F |  F |  F |  F |  F |  F |  O |  M |  F |
 | ResponseAck         |  O |  O |  O |  O |  O |  O |  O |  O |  O |
 | RestartDelay        |  F |  F |  F |  F |  F |  F |  F |  F |  O |
 | RestartMethod       |  F |  F |  F |  F |  F |  F |  F |  F |  M |
 | SecondConnectionId  |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | SecondEndpointId    |  F |  O |  F |  F |  F |  F |  F |  F |  F |
 | SignalRequests      |  F |  O |  O |  O |  O*|  F |  F |  F |  F |
 | SpecificEndpointId  |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 |---------------------|----|----|----|----|----|----|----|----|----|
 | RemoteConnection-   |  F |  O |  O |  F |  F |  F |  F |  F |  F |
 |          Descriptor |    |    |    |    |    |    |    |    |    |
 | LocalConnection-    |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 |          Descriptor |    |    |    |    |    |    |    |    |    |
  ------------------------------------------------------------------
 Notes (*):
  • The BearerInformation parameter is only conditionally optional as

explained in Section 2.3.2.

  • The RequestIdentifier parameter is optional in connection creation,

modification and deletion commands, however it becomes REQUIRED if

   the command contains an encapsulated notification request.

Andreasen & Foster Informational [Page 85] RFC 3435 MGCP 1.0 January 2003

  • The RequestedEvents and SignalRequests parameters are optional in

the NotificationRequest. If these parameters are omitted the

   corresponding lists will be considered empty.
  • The ConnectionParameters parameter is only valid in a

DeleteConnection request sent by the gateway.

 The set of parameters can be extended in two different ways:
  • Package Extension Parameters (preferred)
  • Vendor Extension Parameters
 Package Extension Parameters are defined in packages which provides
 the following benefits:
  • a registration mechanism (IANA) for the package name.
  • a separate name space for the parameters.
  • a convenient grouping of the extensions.
  • a simple way to determine support for them through auditing.
 The package extension mechanism is the preferred extension method.
 Vendor extension parameters can be used if implementers need to
 experiment with new parameters, for example when developing a new
 application of MGCP.  Vendor extension parameters MUST be identified
 by names that start with the string "X-" or "X+", such as for
 example:
    X-Flower: Daisy
 Parameter names that start with "X+" are critical parameter
 extensions.  An MGCP entity that receives a critical parameter
 extension that it cannot understand MUST refuse to execute the
 command.  It SHOULD respond with error code 511 (unrecognized
 extension).
 Parameter names that start with "X-" are non-critical parameter
 extensions.  An MGCP entity that receives a non-critical parameter
 extension that it cannot understand MUST simply ignore that
 parameter.

Andreasen & Foster Informational [Page 86] RFC 3435 MGCP 1.0 January 2003

 Note that vendor extension parameters use an unmanaged name space,
 which implies a potential for name clashing.  Vendors are
 consequently encouraged to include some vendor specific string, e.g.,
 vendor name, in their vendor extensions.

3.2.2.1 BearerInformation

 The values of the bearer information are encoded as a comma separated
 list of attributes, which are represented by an attribute name, and
 possibly followed by a colon and an attribute value.
 The only attribute that is defined is the "encoding" (code "e")
 attribute, which MUST have one of the values "A" (A-law) or "mu"
 (mu-law).
 An example of bearer information encoding is:
    B: e:mu
 The set of bearer information attributes may be extended through
 packages.

3.2.2.2 CallId

 The Call Identifier is encoded as a hexadecimal string, at most 32
 characters in length.  Call Identifiers are compared as strings
 rather than numerical values.

3.2.2.3 Capabilities

 Capabilities inform the Call Agent about endpoints' capabilities when
 audited.  The encoding of capabilities is based on the Local
 Connection Options encoding for the parameters that are common to
 both, although a different parameter line code is used ("A").  In
 addition, capabilities can also contain a list of supported packages,
 and a list of supported modes.
 The parameters used are:
 A list of supported codecs.
    The following parameters will apply to all codecs specified in
    this list.  If there is a need to specify that some parameters,
    such as e.g., silence suppression, are only compatible with some
    codecs, then the gateway will return several Capability
    parameters; one for each set of codecs.
 Packetization Period:
    A range may be specified.

Andreasen & Foster Informational [Page 87] RFC 3435 MGCP 1.0 January 2003

 Bandwidth:
    A range corresponding to the range for packetization periods may
    be specified (assuming no silence suppression).  If absent, the
    values will be deduced from the codec type.
 Echo Cancellation:
    "on" if echo cancellation is supported, "off" otherwise.  The
    default is support.
 Silence Suppression:
    "on" if silence suppression is supported for this codec, "off"
    otherwise.  The default is support.
 Gain Control:
    "0" if gain control is not supported, all other values indicate
    support for gain control.  The default is support.
 Type of Service:
    The value "0" indicates no support for type of service, all other
    values indicate support for type of service.  The default is
    support.
 Resource Reservation Service:
    The parameter indicates the reservation services that are
    supported, in addition to best effort.  The value "g" is encoded
    when the gateway supports both the guaranteed and the controlled
    load service, "cl" when only the controlled load service is
    supported.  The default is "best effort".
 Encryption Key:
    Encoding any value indicates support for encryption.  Default is
    no support which is implied by omitting the parameter.
 Type of network:
    The keyword "nt", followed by a colon and a semicolon separated
    list of supported network types.  This parameter is optional.
 Packages:
    The packages supported by the endpoint encoded as the keyword "v",
    followed by a colon and a character string.  If a list of values
    is specified, these values will be separated by a semicolon.  The
    first value specified will be the default package for the
    endpoint.
 Modes:
    The modes supported by this endpoint encoded as the keyword "m",
    followed by a colon and a semicolon-separated list of supported
    connection modes for this endpoint.

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 Lack of support for a capability can also be indicated by excluding
 the parameter from the capability set.
 An example capability is:
   A: a:PCMU;G728, p:10-100, e:on, s:off, t:1, v:L,
                            m:sendonly;recvonly;sendrecv;inactive
 The carriage return above is included for formatting reasons only and
 is not permissible in a real implementation.
 If multiple capabilities are to be returned, each will be returned as
 a separate capability line.
 Since Local Connection Options can be extended, the list of
 capability parameters can also be extended.  Individual extensions
 may define how they are reported as capabilities.  If no such
 definition is provided, the following defaults apply:
  • Package Extension attributes: The individual attributes are not

reported. Instead, the name of the package is simply reported in

   the list of supported packages.
  • Vendor Extension attributes: The name of the attribute is reported

without any value.

  • Other Extension attributes: The name of the attribute is reported

without any value.

3.2.2.4 Coding of Event Names

 Event names are composed of an optional package name, separated by a
 slash (/) from the name of the actual event (see Section 2.1.7).  The
 wildcard character star ("*") can be use to refer to all packages.
 The event name can optionally be followed by an at sign (@) and the
 identifier of a connection (possibly using a wildcard) on which the
 event should be observed.  Event names are used in the
 RequestedEvents, SignalRequests, ObservedEvents, DetectEvents, and
 EventStates parameters.
 Events and signals may be qualified by parameters defined for the
 event/signal.  Such parameters may be enclosed in double-quotes (in
 fact, some parameters MUST be enclosed in double-quotes due to
 syntactic restrictions) in which case they are UTF-8 encoded [20].
 The parameter name "!" (exclamation point) is reserved for future use
 for both events and signals.

Andreasen & Foster Informational [Page 89] RFC 3435 MGCP 1.0 January 2003

 Each signal has one of the following signal-types associated with it:
 On/Off (OO), Time-out (TO), or Brief (BR).  (These signal types are
 specified in the package definitions, and are not present in the
 messages.) On/Off signals can be parameterized with a "+" to turn the
 signal on, or a "-" to turn the signal off.  If an on/off signal is
 not parameterized, the signal is turned on.  Both of the following
 will turn the vmwi signal (from the line package "L") on:
    L/vmwi(+)
    L/vmwi
 In addition to "!", "+" and "-", the signal parameter "to" is
 reserved as well.  It can be used with Time-Out signals to override
 the default time-out value for the current request.  A decimal value
 in milliseconds will be supplied.  The individual signal and/or
 package definition SHOULD indicate if this parameter is supported for
 one or more TO signals in the package.  If not indicated, TO signals
 in package version zero are assumed to not support it, whereas TO
 signals in package versions one or higher are assumed to support it.
 By default, a supplied time-out value MAY be rounded to the nearest
 non-zero value divisible by 1000, i.e., whole second.  The individual
 signal and/or package definition may define other rounding rules. All
 new package and TO signal definitions are strongly encouraged to
 support the "to" signal parameter.
 The following example illustrates how the "to" parameter can be used
 to apply a signal for 6 seconds:
    L/rg(to=6000)
    L/rg(to(6000))
 The following are examples of event names:
  1. ———————————————————-

| L/hu | on-hook transition, in the line package |

   | F/0         |   digit 0 in the MF package                 |
   | hf          |   Hook-flash, assuming that the line package|
   |             |   is the default package for the endpoint.  |
   | G/rt@0A3F58 |   Ring back signal on connection "0A3F58"   |
    -----------------------------------------------------------
 In addition, the range and wildcard notation of events can be used,
 instead of individual names, in the RequestedEvents and DetectEvents
 parameters.  The event code "all" is reserved and refers to all
 events or signals in a package.  The star sign ("*") can be used to
 denote "all connections", and the dollar sign ("$") can be used to
 denote the "current" connection (see Section 2.1.7 for details).

Andreasen & Foster Informational [Page 90] RFC 3435 MGCP 1.0 January 2003

 The following are examples of such notations:
  1. ——————————————————–

| M/[0-9] | Digits 0 to 9 in the MF package. |

   | hf        |   Hook-flash, assuming that the line package|
   |           |   is a default package for the endpoint.    |
   | [0-9*#A-D]|   All digits and letters in the DTMF        |
   |           |   packages (default for endpoint).          |
   | T/all     |   All events in the trunk package.          |
   | R/qa@*    |   The quality alert event on all            |
   |           |   connections.                              |
   | G/rt@$    |   Ringback on current connection.           |
    ---------------------------------------------------------

3.2.2.5 ConnectionId

 The Connection Identifier is encoded as a hexadecimal string, at most
 32 characters in length.  Connection Identifiers are compared as
 strings rather than numerical values.

3.2.2.6 ConnectionMode

 The connection mode describes the mode of operation of the
 connection.  The possible values are:
  1. ——————————————————-

| Mode | Meaning |

   |-------------|------------------------------------------|
   | M: sendonly |  The gateway should only send packets    |
   | M: recvonly |  The gateway should only receive packets |
   | M: sendrecv |  The gateway should send                 |
   |             |  and receive packets                     |
   | M: confrnce |  The gateway should place                |
   |             |  the connection in conference mode       |
   | M: inactive |  The gateway should neither              |
   |             |  send nor receive packets                |
   | M: loopback |  The gateway should place                |
   |             |  the circuit in loopback mode.           |
   | M: conttest |  The gateway should place                |
   |             |  the circuit in test mode.               |
   | M: netwloop |  The gateway should place                |
   |             |  the connection in network loopback mode.|
   | M: netwtest |  The gateway should place the connection |
   |             |  in network continuity test mode.        |
    --------------------------------------------------------

Andreasen & Foster Informational [Page 91] RFC 3435 MGCP 1.0 January 2003

 Note that irrespective of the connection mode, signals applied to the
 connection will still result in packets being sent (see Section
 2.3.1).
 The set of connection modes can be extended through packages.

3.2.2.7 ConnectionParameters

 Connection parameters are encoded as a string of type and value
 pairs, where the type is either a two-letter identifier of the
 parameter or an extension type, and the value a decimal integer.
 Types are separated from value by an '=' sign.  Parameters are
 separated from each other by a comma.  Connection parameter values
 can contain up to nine digits.  If the maximum value is reached, the
 counter is no longer updated, i.e., it doesn't wrap or overflow.
 The connection parameter types are specified in the following table:
  1. —————————————————————-

| Connection parameter| Code | Connection parameter |

 | name                |      |  value                             |
 |---------------------|------|------------------------------------|
 | Packets sent        |  PS  |  The number of packets that        |
 |                     |      |  were sent on the connection.      |
 | Octets sent         |  OS  |  The number of octets that         |
 |                     |      |  were sent on the connection.      |
 | Packets received    |  PR  |  The number of packets that        |
 |                     |      |  were received on the connection.  |
 | Octets received     |  OR  |  The number of octets that         |
 |                     |      |  were received on the connection.  |
 | Packets lost        |  PL  |  The number of packets that        |
 |                     |      |  were lost on the connection       |
 |                     |      |  as deduced from gaps in the       |
 |                     |      |  RTP sequence number.              |
 | Jitter              |  JI  |  The average inter-packet arrival  |
 |                     |      |  jitter, in milliseconds,          |
 |                     |      |  expressed as an integer number.   |
 | Latency             |  LA  |  Average latency, in milliseconds, |
 |                     |      |  expressed as an integer number.   |
  -----------------------------------------------------------------
 The set of connection parameters can be extended in two different
 ways:
  • Package Extension Parameters (preferred)
  • Vendor Extension Parameters

Andreasen & Foster Informational [Page 92] RFC 3435 MGCP 1.0 January 2003

 Package Extension Connection Parameters are defined in packages which
 provides the following benefits:
  • A registration mechanism (IANA) for the package name.
  • A separate name space for the parameters.
  • A convenient grouping of the extensions.
  • A simple way to determine support for them through auditing.
 The package extension mechanism is the preferred extension method.
 Vendor extension parameters names are composed of the string "X-"
 followed by a two or more letters extension parameter name.
 Call agents that receive unrecognized package or vendor connection
 parameter extensions SHALL silently ignore these parameters.
 An example of connection parameter encoding is:
    P: PS=1245, OS=62345, PR=0, OR=0, PL=0, JI=0, LA=48

3.2.2.8 DetectEvents

 The DetectEvents parameter is encoded as a comma separated list of
 events (see Section 3.2.2.4), such as for example:
    T: L/hu,L/hd,L/hf,D/[0-9#*]
 It should be noted, that no actions can be associated with the
 events, however event parameters may be provided.

3.2.2.9 EventStates

 The EventStates parameter is encoded as a comma separated list of
 events (see Section 3.2.2.4), such as for example:
    ES: L/hu
 It should be noted, that no actions can be associated with the
 events, however event parameters may be provided.

3.2.2.10 LocalConnectionOptions

 The local connection options describe the operational parameters that
 the Call Agent provides to the gateway in connection handling
 commands.  These include:

Andreasen & Foster Informational [Page 93] RFC 3435 MGCP 1.0 January 2003

  • The allowed codec(s), encoded as the keyword "a", followed by a

colon and a character string. If the Call Agent specifies a list

   of values, these values will be separated by a semicolon.  For RTP,
   audio codecs SHALL be specified by using encoding names defined in
   the RTP AV Profile [4] or its replacement, or by encoding names
   registered with the IANA.  Non-audio media registered as a MIME
   type MUST use the "<MIME type>/<MIME subtype>" form, as in
   "image/t38".
  • The packetization period in milliseconds, encoded as the keyword

"p", followed by a colon and a decimal number. If the Call Agent

   specifies a range of values, the range will be specified as two
   decimal numbers separated by a hyphen (as specified for the "ptime"
   parameter for SDP).
  • The bandwidth in kilobits per second (1000 bits per second),

encoded as the keyword "b", followed by a colon and a decimal

   number.  If the Call Agent specifies a range of values, the range
   will be specified as two decimal numbers separated by a hyphen.
  • The type of service parameter, encoded as the keyword "t", followed

by a colon and the value encoded as two hexadecimal digits. When

   the connection is transmitted over an IP network, the parameters
   encode the 8-bit type of service value parameter of the IP header
   (a.k.a. DiffServ field).  The left-most "bit" in the parameter
   corresponds to the least significant bit in the IP header.
  • The echo cancellation parameter, encoded as the keyword "e",

followed by a colon and the value "on" or "off".

  • The gain control parameter, encoded as the keyword "gc", followed

by a colon and a value which can be either the keyword "auto" or a

   decimal number (positive or negative) representing the number of
   decibels of gain.
  • The silence suppression parameter, encoded as the keyword "s",

followed by a colon and the value "on" or "off".

  • The resource reservation parameter, encoded as the keyword "r",

followed by a colon and the value "g" (guaranteed service), "cl"

   (controlled load) or "be" (best effort).
  • The encryption key, encoded as the keyword "k" followed by a colon

and a key specification, as defined for the parameter "K" in SDP

   (RFC 2327).

Andreasen & Foster Informational [Page 94] RFC 3435 MGCP 1.0 January 2003

  • The type of network, encoded as the keyword "nt" followed by a

colon and the type of network encoded as the keyword "IN"

   (internet), "ATM", "LOCAL" (for a local connection), or possibly
   another type of network registered with the IANA as per SDP (RFC
   2327).
  • The resource reservation parameter, encoded as the keyword "r",

followed by a colon and the value "g" (guaranteed service), "cl"

   (controlled load) or "be" (best effort).
 The encoding of the first three attributes, when they are present,
 will be compatible with the SDP and RTP profiles.  Note that each of
 the attributes is optional.  When several attributes are present,
 they are separated by a comma.
 Examples of local connection options are:
    L: p:10, a:PCMU
    L: p:10, a:G726-32
    L: p:10-20, b:64
    L: b:32-64, e:off
 The set of Local Connection Options attributes can be extended in
 three different ways:
  • Package Extension attributes (preferred)
  • Vendor Extension attributes
  • Other Extension attributes
 Package Extension Local Connection Options attributes are defined in
 packages which provides the following benefits:
  • A registration mechanism (IANA) for the package name.
  • A separate name space for the attributes.
  • A convenient grouping of the extensions.
  • A simple way to determine support for them through auditing.
 The package extension mechanism is the preferred extension method.

Andreasen & Foster Informational [Page 95] RFC 3435 MGCP 1.0 January 2003

 Vendor extension attributes are composed of an attribute name, and
 possibly followed by a colon and an attribute value.  The attribute
 name MUST start with the two characters "x+", for a mandatory
 extension, or "x-", for a non-mandatory extension.  If a gateway
 receives a mandatory extension attribute that it does not recognize,
 it MUST reject the command (error code 525 - unknown extension in
 LocalConnectionOptions, is RECOMMENDED).
 Note that vendor extension attributes use an unmanaged name space,
 which implies a potential for name clashing.  Vendors are
 consequently encouraged to include some vendor specific string, e.g.,
 vendor name, in their vendor extensions.
 Finally, for backwards compatibility with some existing
 implementations, MGCP allows for other extension attributes as well
 (see grammar in Appendix A).  Note however, that these attribute
 extensions do not provide the package extension attribute benefits.
 Use of this mechanism for new extensions is discouraged.

3.2.2.11 MaxMGCPDatagram

 The MaxMGCPDatagram can only be used for auditing, i.e., it is a
 valid RequestedInfo code and can be provided as a response parameter.
 In responses, the MaxMGCPDatagram value is encoded as a string of up
 to nine decimal digits -- leading zeroes are not permitted.  The
 following example illustrates the use of this parameter:
    MD: 8100

3.2.2.12 ObservedEvents

 The observed events parameter provides the list of events that have
 been observed.  The event codes are the same as those used in the
 NotificationRequest.  Events that have been accumulated according to
 the digit map may be grouped in a single string, however such
 practice is discouraged; they SHOULD be reported as lists of isolated
 events if other events were detected during the digit accumulation.
 Examples of observed events are:
    O: L/hu
    O: D/8295555T
    O: D/8,D/2,D/9,D/5,D/5,L/hf,D/5,D/5,D/T
    O: L/hf, L/hf, L/hu

Andreasen & Foster Informational [Page 96] RFC 3435 MGCP 1.0 January 2003

3.2.2.13 PackageList

 The Package List can only be used for auditing, i.e., it is a valid
 RequestedInfo code and can be provided as a response parameter.
 The response parameter will consist of a comma separated list of
 packages supported.  The first package returned in the list is the
 default package.  Each package in the list consists of the package
 name followed by a colon, and the highest version number of the
 package supported.
 An example of a package list is:
   PL: L:1,G:1,D:0,FOO:2,T:1
 Note that for backwards compatibility, support for this parameter is
 OPTIONAL.

3.2.2.14 QuarantineHandling

 The quarantine handling parameter contains a list of comma separated
 keywords:
  • The keyword "process" or "discard" to indicate the treatment of

quarantined and observed events. If neither "process" or "discard"

   is present, "process" is assumed.
  • The keyword "step" or "loop" to indicate whether at most one

notification per NotificationRequest is allowed, or whether

   multiple notifications per NotificationRequest are allowed.  If
   neither "step" nor "loop" is present, "step" is assumed.
 The following values are valid examples:
    Q: loop
    Q: process
    Q: loop,discard

3.2.2.15 ReasonCode

 Reason codes are three-digit numeric values.  The reason code is
 optionally followed by a white space and commentary, e.g.:
    E: 900 Endpoint malfunctioning
 A list of reason codes can be found in Section 2.5.
 The set of reason codes can be extended through packages.

Andreasen & Foster Informational [Page 97] RFC 3435 MGCP 1.0 January 2003

3.2.2.16 RequestedEvents

 The RequestedEvents parameter provides the list of events that are
 requested.  The event codes are described in Section 3.2.2.4.
 Each event can be qualified by a requested action, or by a list of
 actions.  The actions, when specified, are encoded as a list of
 keywords, enclosed in parenthesis and separated by commas.  The codes
 for the various actions are:
  1. ————————————

| Action | Code |

             |------------------------------|------|
             | Notify immediately           |  N   |
             | Accumulate                   |  A   |
             | Treat according to digit map |  D   |
             | Swap                         |  S   |
             | Ignore                       |  I   |
             | Keep Signal(s) active        |  K   |
             | Embedded Notification Request|  E   |
              -------------------------------------
 When no action is specified, the default action is to notify the
 event.  This means that, for example, ft and ft(N) are equivalent.
 Events that are not listed are ignored (unless they are persistent).
 The digit-map action SHOULD only be specified for the digits, letters
 and interdigit timers in packages that define the encoding of digits,
 letters, and timers (including extension digit map letters).
 The requested events list is encoded on a single line, with
 event/action groups separated by commas.  Examples of RequestedEvents
 encodings are:
    R: L/hu(N), L/hf(S,N)
    R: L/hu(N), D/[0-9#T](D)
 In the case of the "Embedded Notification Request" action, the
 embedded notification request parameters are encoded as a list of up
 to three parameter groups separated by commas.  Each group starts by
 a one letter identifier, followed by a list of parameters enclosed
 between parentheses. The first optional parameter group, identified
 by the letter "R", is the value of the embedded RequestedEvents
 parameter.  The second optional group, identified by the letter "S",
 is the embedded value of the SignalRequests parameter.  The third

Andreasen & Foster Informational [Page 98] RFC 3435 MGCP 1.0 January 2003

 optional group, identified by the letter "D", is the embedded value
 of the DigitMap.  (Note that some existing implementations and
 profiles may encode these three components in a different order.
 Implementers are encouraged to accept such encodings, but they SHOULD
 NOT generate them.)
 If the RequestedEvents parameter is not present, the parameter will
 be set to a null value.  If the SignalRequests parameter is not
 present, the parameter will be set to a null value.  If the DigitMap
 is absent, the current value MUST be used.  The following are valid
 examples of embedded requests:
    R: L/hd(E(R(D/[0-9#T](D),L/hu(N)),S(L/dl),D([0-9].[#T])))
    R: L/hd(E(R(D/[0-9#T](D),L/hu(N)),S(L/dl)))
 Some events can be qualified by additional event parameters.  Such
 event parameters will be separated by commas and enclosed within
 parentheses.  Event parameters may be enclosed in double-quotes (in
 fact, some event parameters MUST be enclosed in double-quotes due to
 syntactic restrictions), in which case the quoted string itself is
 UTF-8 encoded.  Please refer to Section 3.2.2.4 for additional detail
 on event parameters.
 The following example shows the foobar event with an event parameter
 "epar":
    R: X/foobar(N)(epar=2)
 Notice that the Action was included even though it is the default
 Notify action - this is required by the grammar.

3.2.2.17 RequestedInfo

 The RequestedInfo parameter contains a comma separated list of
 parameter codes, as defined in Section 3.2.2.  For example, if one
 wants to audit the value of the NotifiedEntity, RequestIdentifier,
 RequestedEvents, SignalRequests, DigitMap, QuarantineHandling and
 DetectEvents parameters, the value of the RequestedInfo parameter
 will be:
    F: N,X,R,S,D,Q,T
 Note that extension parameters in general can be audited as well.
 The individual extension will define the auditing operation.

Andreasen & Foster Informational [Page 99] RFC 3435 MGCP 1.0 January 2003

 The capabilities request, in the AuditEndPoint command, is encoded by
 the parameter code "A", as in:
    F: A

3.2.2.18 RequestIdentifier

 The request identifier correlates a Notify command with the
 NotificationRequest that triggered it.  A RequestIdentifier is a
 hexadecimal string, at most 32 characters in length.
 RequestIdentifiers are compared as strings rather than numerical
 value.  The string "0" is reserved for reporting of persistent events
 in the case where a NotificationRequest has not yet been received
 after restart.

3.2.2.19 ResponseAck

 The response acknowledgement parameter is used to manage the "at-
 most-once" facility described in Section 3.5.  It contains a comma
 separated list of "confirmed transaction-id ranges".
 Each "confirmed transaction-id range" is composed of either one
 decimal number, when the range includes exactly one transaction, or
 two decimal numbers separated by a single hyphen, describing the
 lower and higher transaction identifiers included in the range.
 An example of a response acknowledgement is:
    K: 6234-6255, 6257, 19030-19044

3.2.2.20 RestartMethod

 The RestartMethod parameter is encoded as one of the keywords
 "graceful", "forced", "restart", "disconnected" or "cancel-graceful"
 as for example:
    RM: restart
 The set of restart methods can be extended through packages.

3.2.2.21 SignalRequests

 The SignalRequests parameter provides the name of the signal(s) that
 have been requested.  Each signal is identified by a name, as
 described in Section 3.2.2.4.
 Some signals, such as for example announcement or ADSI display, can
 be qualified by additional parameters, e.g.:

Andreasen & Foster Informational [Page 100] RFC 3435 MGCP 1.0 January 2003

  • the name and parameters of the announcement,
  • the string that should be displayed.
 Such parameters will be separated by commas and enclosed within
 parenthesis, as in:
    S: L/adsi("123456 Francois Gerard")
    S: A/ann(http://ann.example.net/no-such-number.au, 1234567)
 When a quoted-string is provided, the string itself is UTF-8 encoded
 [20].
 When several signals are requested, their codes are separated by a
 comma, as in:
    S: L/adsi("123456 Your friend"), L/rg
 Please refer to Section 3.2.2.4 for additional detail on signal
 parameters.

3.3 Format of response headers

 The response header is composed of a response line, optionally
 followed by headers that encode the response parameters.
 An example of a response header could be:
    200 1203 OK
 The response line starts with the response code, which is a three
 digit numeric value.  The code is followed by a white space, and the
 transaction identifier.  Response codes defined in packages (8xx) are
 followed by white space, a slash ("/") and the package name.  All
 response codes may furthermore be followed by optional commentary
 preceded by a white space.
 The following table describes the parameters whose presence is
 mandatory or optional in a response header, as a function of the
 command that triggered the response.  The letter M stands for
 mandatory, O for optional and F for forbidden.  Unless otherwise
 specified, a parameter MUST NOT be present more than once.  Note that
 the table only reflects the default for responses that have not
 defined any other behavior.  If a response is received with a
 parameter that is either not understood or marked as forbidden, the
 offending parameter(s) MUST simply be ignored.

Andreasen & Foster Informational [Page 101] RFC 3435 MGCP 1.0 January 2003

  1. —————————————————————–

| Parameter name | EP | CR | MD | DL | RQ | NT | AU | AU | RS |

 |                     | CF | CX | CX | CX | NT | FY | EP | CX | IP |
 |---------------------|----|----|----|----|----|----|----|----|----|
 | BearerInformation   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | CallId              |  F |  F |  F |  F |  F |  F |  F |  O |  F |
 | Capabilities        |  F |  F |  F |  F |  F |  F |  O*|  F |  F |
 | ConnectionId        |  F |  O*|  F |  F |  F |  F |  O*|  F |  F |
 | ConnectionMode      |  F |  F |  F |  F |  F |  F |  F |  O |  F |
 | Connection-         |  F |  F |  F |  O*|  F |  F |  F |  O |  F |
 |   Parameters        |    |    |    |    |    |    |    |    |    |
 | DetectEvents        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | DigitMap            |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | EventStates         |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | LocalConnection-    |  F |  F |  F |  F |  F |  F |  F |  O |  F |
 |            Options  |    |    |    |    |    |    |    |    |    |
 | MaxMGCPDatagram     |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | NotifiedEntity      |  F |  F |  F |  F |  F |  F |  O |  O |  O |
 | ObservedEvents      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | QuarantineHandling  |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | PackageList         |  O*|  O*|  O*|  O*|  O*|  O*|  O |  O*|  O*|
 | ReasonCode          |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | RequestIdentifier   |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | ResponseAck         |  O*|  O*|  O*|  O*|  O*|  O*|  O*|  O*|  O*|
 | RestartDelay        |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | RestartMethod       |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | RequestedEvents     |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | RequestedInfo       |  F |  F |  F |  F |  F |  F |  F |  F |  F |
 | SecondConnectionId  |  F |  O |  F |  F |  F |  F |  F |  F |  F |
 | SecondEndpointId    |  F |  O |  F |  F |  F |  F |  F |  F |  F |
 | SignalRequests      |  F |  F |  F |  F |  F |  F |  O |  F |  F |
 | SpecificEndpointId  |  F |  O |  F |  F |  F |  F |  O*|  F |  F |
 |---------------------|----|----|----|----|----|----|----|----|----|
 | LocalConnection-    |  F |  O*|  O |  F |  F |  F |  F |  O*|  F |
 |         Descriptor  |    |    |    |    |    |    |    |    |    |
 | RemoteConnection-   |  F |  F |  F |  F |  F |  F |  F |  O*|  F |
 |         Descriptor  |    |    |    |    |    |    |    |    |    |
  ------------------------------------------------------------------
 Notes (*):
  • The PackageList parameter is only allowed with return code 518

(unsupported package), except for AuditEndpoint, where it may also

   be returned if audited.

Andreasen & Foster Informational [Page 102] RFC 3435 MGCP 1.0 January 2003

  • The ResponseAck parameter MUST NOT be used with any other responses

than a final response issued after a provisional response for the

   transaction in question.  In that case, the presence of the
   ResponseAck parameter SHOULD trigger a Response Acknowledgement -
   any ResponseAck values provided will be ignored.
  • In the case of a CreateConnection message, the response line is

followed by a Connection-Id parameter and a

   LocalConnectionDescriptor.  It may also be followed a Specific-
   Endpoint-Id parameter, if the creation request was sent to a
   wildcarded Endpoint-Id.  The connection-Id and
   LocalConnectionDescriptor parameter are marked as optional in the
   Table.  In fact, they are mandatory with all positive responses,
   when a connection was created, and forbidden when the response is
   negative, and no connection was created.
  • A LocalConnectionDescriptor MUST be transmitted with a positive

response (code 200) to a CreateConnection. It MUST also be

   transmitted in response to a ModifyConnection command, if the
   modification resulted in a modification of the session parameters.
   The LocalConnectionDescriptor is encoded as a "session
   description", as defined in section 3.4.  It is separated from the
   response header by an empty line.
  • Connection-Parameters are only valid in a response to a non-

wildcarded DeleteConnection command sent by the Call Agent.

  • Multiple ConnectionId, SpecificEndpointId, and Capabilities

parameters may be present in the response to an AuditEndpoint

   command.
  • When several session descriptors are encoded in the same response,

they are encoded one after each other, separated by an empty line.

   This is the case for example when the response to an audit
   connection request carries both a local session description and a
   remote session description, as in:

Andreasen & Foster Informational [Page 103] RFC 3435 MGCP 1.0 January 2003

        200 1203 OK
        C: A3C47F21456789F0
        N: [128.96.41.12]
        L: p:10, a:PCMU;G726-32
        M: sendrecv
        P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,LA=48
        v=0
        o=- 25678 753849 IN IP4 128.96.41.1
        s=-
        c=IN IP4 128.96.41.1
        t=0 0
        m=audio 1296 RTP/AVP 0
        v=0
        o=- 33343 346463 IN IP4 128.96.63.25
        s=-
        c=IN IP4 128.96.63.25
        t=0 0
        m=audio 1296 RTP/AVP 0 96
        a=rtpmap:96 G726-32/8000
   In this example, according to the SDP syntax, each description
   starts with a "version" line, (v=...).  The local description is
   always transmitted before the remote description.  If a connection
   descriptor is requested, but it does not exist for the connection
   audited, that connection descriptor will appear with the SDP
   protocol version field only.
 The response parameters are described for each of the commands in the
 following.

3.3.1 CreateConnection Response

 In the case of a CreateConnection message, the response line is
 followed by a Connection-Id parameter with a successful response
 (code 200).  A LocalConnectionDescriptor is furthermore transmitted
 with a positive response.  The LocalConnectionDescriptor is encoded
 as a "session description", as defined by SDP (RFC 2327).  It is
 separated from the response header by an empty line, e.g.:

Andreasen & Foster Informational [Page 104] RFC 3435 MGCP 1.0 January 2003

    200 1204 OK
    I: FDE234C8
    v=0
    o=- 25678 753849 IN IP4 128.96.41.1
    s=-
    c=IN IP4 128.96.41.1
    t=0 0
    m=audio 3456 RTP/AVP 96
    a=rtpmap:96 G726-32/8000
 When a provisional response has been issued previously, the final
 response SHOULD furthermore contain the Response Acknowledgement
 parameter (final responses issued by entities adhering to this
 specification will include the parameter, but older RFC 2705
 implementations MAY not):
    200 1204 OK
    K:
    I: FDE234C8
    v=0
    o=- 25678 753849 IN IP4 128.96.41.1
    s=-
    c=IN IP4 128.96.41.1
    t=0 0
    m=audio 3456 RTP/AVP 96
    a=rtpmap:96 G726-32/8000
 The final response SHOULD then be acknowledged by a Response
 Acknowledgement:
    000 1204

3.3.2 ModifyConnection Response

 In the case of a successful ModifyConnection message, the response
 line is followed by a LocalConnectionDescriptor, if the modification
 resulted in a modification of the session parameters (e.g., changing
 only the mode of a connection does not alter the session parameters).
 The LocalConnectionDescriptor is encoded as a "session description",
 as defined by SDP.  It is separated from the response header by an
 empty line.

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    200 1207 OK
    v=0
    o=- 25678 753849 IN IP4 128.96.41.1
    s=-
    c=IN IP4 128.96.41.1
    t=0 0
    m=audio 3456 RTP/AVP 0
 When a provisional response has been issued previously, the final
 response SHOULD furthermore contain the Response Acknowledgement
 parameter as in:
    200 1207 OK
    K:
 The final response SHOULD then be acknowledged by a Response
 Acknowledgement:
    000 1207 OK

3.3.3 DeleteConnection Response

 Depending on the variant of the DeleteConnection message, the
 response line may be followed by a Connection Parameters parameter
 line, as defined in Section 3.2.2.7.
    250 1210 OK
    P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48

3.3.4 NotificationRequest Response

 A successful NotificationRequest response does not include any
 additional response parameters.

3.3.5 Notify Response

 A successful Notify response does not include any additional response
 parameters.

3.3.6 AuditEndpoint Response

 In the case of a successful AuditEndPoint the response line may be
 followed by information for each of the parameters requested - each
 parameter will appear on a separate line.  Parameters for which no

Andreasen & Foster Informational [Page 106] RFC 3435 MGCP 1.0 January 2003

 value currently exists, e.g., digit map, will still be provided but
 with an empty value.  Each local endpoint name "expanded" by a
 wildcard character will appear on a separate line using the
 "SpecificEndPointId" parameter code, e.g.:
    200 1200 OK
    Z: aaln/1@rgw.whatever.net
    Z: aaln/2@rgw.whatever.net
 When connection identifiers are audited and multiple connections
 exist on the endpoint, a comma-separated list of connection
 identifiers SHOULD be returned as in:
    200 1200 OK
    I: FDE234C8, DFE233D1
 Alternatively, multiple connection id parameter lines may be returned
 - the two forms should not be mixed although doing so does not
 constitute an error.
 When capabilities are audited, the response may include multiple
 capabilities parameter lines as in:
    200 1200 OK
    A: a:PCMU;G728, p:10-100, e:on, s:off, t:1, v:L,
        m:sendonly;recvonly;sendrecv;inactive
    A: a:G729, p:30-90, e:on, s:on, t:1, v:L,
        m:sendonly;recvonly;sendrecv;inactive;confrnce
 Note:  The carriage return for Capabilities shown above is present
 for formatting reasons only.  It is not permissible in a real command
 encoding.

3.3.7 AuditConnection Response

 In the case of a successful AuditConnection, the response may be
 followed by information for each of the parameters requested.
 Parameters for which no value currently exists will still be
 provided.  Connection descriptors will always appear last and each
 will be preceded by an empty line, as for example:

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    200 1203 OK
    C: A3C47F21456789F0
    N: [128.96.41.12]
    L: p:10, a:PCMU;G728
    M: sendrecv
    P: PS=622, OS=31172, PR=390, OR=22561, PL=5, JI=29, LA=50
    v=0
    o=- 4723891 7428910 IN IP4 128.96.63.25
    s=-
    c=IN IP4 128.96.63.25
    t=0 0
    m=audio 1296 RTP/AVP 96
    a=rtpmap:96 G726-32/8000
 If both a local and a remote connection descriptor are provided, the
 local connection descriptor will be the first of the two.  If a
 connection descriptor is requested, but it does not exist for the
 connection audited, that connection descriptor will appear with the
 SDP protocol version field only ("v=0"), as for example:
    200 1203 OK
    v=0

3.3.8 RestartInProgress Response

 A successful RestartInProgress response may include a NotifiedEntity
 parameter, but otherwise does not include any additional response
 parameters.
 Also, a 521 response to a RestartInProgress MUST include a
 NotifiedEntity parameter with the name of another Call Agent to
 contact when the first Call Agent redirects the endpoint to another
 Call Agent as in:
    521 1204 Redirect
    N: CA-1@whatever.net

3.4 Encoding of the Session Description (SDP)

 The session description (SDP) is encoded in conformance with the
 session description protocol, SDP.  MGCP implementations are REQUIRED
 to be fully capable of parsing any conformant SDP message, and MUST
 send session descriptions that strictly conform to the SDP standard.

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 The general description and explanation of SDP parameters can be
 found in RFC 2327 (or its successor).  In particular, it should be
 noted that the
  • Origin ("o="),
  • Session Name ("s="), and
  • Time active ("t=")
 are all mandatory in RFC 2327.  While they are of little use to MGCP,
 they MUST be provided in conformance with RFC 2327 nevertheless.  The
 following suggests values to be used for each of the fields, however
 the reader is encouraged to consult RFC 2327 (or its successor) for
 details:
 Origin
 o = <username> <session id> <version> <network type> <address type>
     <address>
  • The username SHOULD be set to hyphen ("-").
  • The session id is RECOMMENDED to be an NTP timestamp as suggested

in RFC 2327.

  • The version is a version number that MUST increment with each

change to the SDP. A counter initialized to zero or an NTP

   timestamp as suggested in RFC 2327 is RECOMMENDED.
  • The network type defines the type of network. For RTP sessions the

network type SHOULD be "IN".

  • The address type defines the type of address. For RTP sessions the

address type SHOULD be "IP4" (or "IP6").

  • The address SHOULD be the same address as provided in the

connection information ("c=") field.

 Session Name
 s = <session name>
 The session name should be hyphen ("-").
 Time active
 t = <start time> <stop time>

Andreasen & Foster Informational [Page 109] RFC 3435 MGCP 1.0 January 2003

  • The start time may be set to zero.
  • The stop time should be set to zero.
 Each of the three fields can be ignored upon reception.
 To further accommodate the extensibility principles of MGCP,
 implementations are ENCOURAGED to support the PINT "a=require"
 attribute - please refer to RFC 2848 for further details.
 The usage of SDP actually depends on the type of session that is
 being established.  Below we describe usage of SDP for an audio
 service using the RTP/AVP profile [4], or the LOCAL interconnect
 defined in this document.  In case of any conflicts between what is
 described below and SDP (RFC 2327 or its successor), the SDP
 specification takes precedence.

3.4.1 Usage of SDP for an Audio Service

 In a telephony gateway, we only have to describe sessions that use
 exactly one media, audio.  The usage of SDP for this is
 straightforward and described in detail in RFC 2327.
 The following is an example of an RFC 2327 conformant session
 description for an audio connection:
    v=0
    o=- A7453949499 0 IN IP4 128.96.41.1
    s=-
    c=IN IP4 128.96.41.1
    t=0 0
    m=audio 3456 RTP/AVP 0 96
    a=rtpmap:96 G726-32/8000

3.4.2 Usage of SDP for LOCAL Connections

 When MGCP is used to set up internal connections within a single
 gateway, the SDP format is used to encode the parameters of that
 connection.  The connection and media parameters will be used as
 follows:
  • The connection parameter (c=) will specify that the connection is

local, using the keyword "LOCAL" as network type, the keyword "EPN"

   (endpoint name) as address type, and the local name of the endpoint
   as the connection-address.

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  • The "m=audio" parameter will specify a port number, which will

always be set to 0, the type of protocol, always set to the keyword

   LOCAL, and the type of encoding, using the same conventions used
   for the RTP AVP profile (RTP payload numbers).  The type of
   encoding should normally be set to 0 (PCMU).
 A session-level attribute identifying the connection MAY furthermore
 be present.  This enables endpoints to support multiple LOCAL
 connections.  Use of this attribute is OPTIONAL and indeed
 unnecessary for endpoints that only support a single LOCAL
 connection.  The attribute is defined as follows:
 a=MGCPlocalcx:<ConnectionID>
    The MGCP Local Connection attribute is a session level only case-
    insensitive attribute that identifies the MGCP LOCAL connection,
    on the endpoint identified in the connection information, to which
    the SDP applies.  The ConnectionId is a hexadecimal string
    containing at most 32 characters.  The ConnectionId itself is
    case-insensitive.  The MGCP Local Connection attribute is not
    subject to the charset attribute.
 An example of a LOCAL session description could be:
    v=0
    o=- A7453949499 0 LOCAL EPN X35V3+A4/13
    s=-
    c=LOCAL EPN X35V3+A4/13
    t=0 0
    a=MGCPlocalcx:FDE234C8
    m=audio 0 LOCAL 0
 Note that the MGCP Local Connection attribute is specified at the
 session level and that it could have been omitted in case only a
 single LOCAL connection per endpoint is supported.

3.5 Transmission over UDP

 MGCP messages are transmitted over UDP.  Commands are sent to one of
 the IP addresses defined in the DNS for the specified endpoint.  The
 responses are sent back to the source address (i.e., IP address and
 UDP port number) of the commands - the response may or may not arrive
 from the same address as the command was sent to.

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 When no port is specified for the endpoint, the commands MUST by
 default be sent:
  • by the Call Agents, to the default MGCP port for gateways, 2427.
  • by the Gateways, to the default MGCP port for Call Agents, 2727.

3.5.1 Providing the At-Most-Once Functionality

 MGCP messages, being carried over UDP, may be subject to losses.  In
 the absence of a timely response, commands are retransmitted.  Most
 MGCP commands are not idempotent.  The state of the gateway would
 become unpredictable if, for example, CreateConnection commands were
 executed several times.  The transmission procedures MUST thus
 provide an "at-most-once" functionality.
 MGCP entities are expected to keep in memory a list of the responses
 that they sent to recent transactions, and a list of the transactions
 that are currently being executed.  The numerical value of
 transaction identifiers of incoming commands are compared to the
 transaction identifiers of the recent responses.  If a match is
 found, the MGCP entity does not execute the transaction again, but
 simply resends the response.  The remaining commands will be compared
 to the list of current transactions, i.e., transactions received
 previously which have not yet finished executing.  If a match is
 found, the MGCP entity does not execute the transaction again, but a
 provisional response (Section 3.5.5) SHOULD be issued to acknowledge
 receipt of the command.
 The procedure uses a long timer value, noted T-HIST in the following.
 The timer MUST be set larger than the maximum duration of a
 transaction, which MUST take into account the maximum number of
 repetitions, the maximum value of the repetition timer and the
 maximum propagation delay of a packet in the network.  A suggested
 value is 30 seconds.
 The copy of the responses MAY be destroyed either T-HIST seconds
 after the response is issued, or when the gateway (or the Call Agent)
 receives a confirmation that the response has been received, through
 the "Response Acknowledgement".  For transactions that are
 acknowledged through this attribute, the gateway SHALL keep a copy of
 the transaction-id (as opposed to the entire transaction response)
 for T-HIST seconds after the response is issued, in order to detect
 and ignore duplicate copies of the transaction request that could be
 produced by the network.

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3.5.2 Transaction Identifiers and Three Ways Handshake

 Transaction identifiers are integer numbers in the range from 1 to
 999,999,999 (both included).  Call-agents may decide to use a
 specific number space for each of the gateways that they manage, or
 to use the same number space for all gateways that belong to some
 arbitrary group.  Call agents may decide to share the load of
 managing a large gateway between several independent processes.
 These processes MUST then share the transaction number space.  There
 are multiple possible implementations of this sharing, such as having
 a centralized allocation of transaction identifiers, or pre-
 allocating non-overlapping ranges of identifiers to different
 processes.  The  implementations MUST guarantee that unique
 transaction identifiers are allocated to all transactions that
 originate from a logical call agent, as defined in Section 4.
 Gateways can simply detect duplicate transactions by looking at the
 transaction identifier only.
 The Response Acknowledgement Attribute can be found in any command.
 It carries a set of "confirmed transaction-id ranges" for final
 responses received - provisional responses MUST NOT be confirmed.  A
 given response SHOULD NOT be confirmed in two separate messages.
 MGCP entities MAY choose to delete the copies of the responses (but
 not the transaction-id) to transactions whose id is included in
 "confirmed transaction-id ranges" received in the Response
 Confirmation messages (command or response).  They SHOULD then
 silently discard further commands from that entity when the
 transaction-id falls within these ranges, and the response was issued
 less than T-HIST seconds ago.
 Entities MUST exercise due caution when acknowledging responses.  In
 particular, a response SHOULD only be acknowledged if the response
 acknowledgement is sent to the same entity as the corresponding
 command (i.e., the command whose response is being acknowledged) was
 sent to.
 Likewise, entities SHOULD NOT blindly accept a response
 acknowledgement for a given response.  However it is considered safe
 to accept a response acknowledgement for a given response, when that
 response acknowledgement is sent by the same entity as the command
 that generated that response.
 It should be noted, that use of response acknowledgments in commands
 (as opposed to the Response Acknowledgement response following a
 provisional response) is OPTIONAL.  The benefit of using it is that
 it reduces overall memory consumption.  However, in order to avoid
 large messages, implementations SHOULD NOT generate large response

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 acknowledgement lists.  One strategy is to manage responses to
 commands on a per endpoint basis.  A command for an endpoint can
 confirm a response to an older command for that same endpoint.
 Responses to commands with wildcarded endpoint names can be confirmed
 selectively with due consideration to message sizes, or alternatively
 simply not be acknowledged (unless the response explicitly required a
 Response Acknowledgement).  Care must be taken to not confirm the
 same response twice or a response that is more than T-HIST seconds
 old.
 The "confirmed transaction-id ranges" values SHALL NOT be used if
 more than T-HIST seconds have elapsed since the entity issued its
 last response to the other entity, or when an entity resumes
 operation.  In this situation, commands MUST be accepted and
 processed, without any test on the transaction-id.
 Commands that carry the "Response Acknowledgement attribute" may be
 transmitted in disorder.  The union of the "confirmed transaction-id
 ranges" received in recent messages SHALL be retained.

3.5.3 Computing Retransmission Timers

 It is the responsibility of the requesting entity to provide suitable
 time outs for all outstanding commands, and to retry commands when
 time outs have been exceeded.  Furthermore, when repeated commands
 fail to be acknowledged, it is the responsibility of the requesting
 entity to seek redundant services and/or clear existing or pending
 associations.
 The specification purposely avoids specifying any value for the
 retransmission timers.  These values are typically network dependent.
 The retransmission timers SHOULD normally estimate the timer by
 measuring the time spent between the sending of a command and the
 return of the first response to the command.  At a minimum, a
 retransmission strategy involving exponential backoff MUST be
 implemented.  One possibility is to use the algorithm implemented in
 TCP/IP, which uses two variables:
  • the average acknowledgement delay, AAD, estimated through an

exponentially smoothed average of the observed delays,

  • the average deviation, ADEV, estimated through an exponentially

smoothed average of the absolute value of the difference between

   the observed delay and the current average.

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 The retransmission timer, RTO, in TCP, is set to the sum of the
 average delay plus N times the average deviation, where N is a
 constant.  In MGCP, the maximum value of the timer SHOULD however be
 bounded, in order to guarantee that no repeated packet will be
 received by the gateways after T-HIST seconds.  A suggested maximum
 value for RTO (RTO-MAX) is 4 seconds.  Implementers SHOULD consider
 bounding the minimum value of this timer as well [19].
 After any retransmission, the MGCP entity SHOULD do the following:
  • It should double the estimated value of the acknowledgement delay

for this transaction, T-DELAY.

  • It should compute a random value, uniformly distributed between 0.5

T-DELAY and T-DELAY.

  • It should set the retransmission timer (RTO) to the minimum of:
    1. the sum of that random value and N times the average deviation,
    2. RTO-MAX.
 This procedure has two effects.  Because it includes an exponentially
 increasing component, it will automatically slow down the stream of
 messages in case of congestion.  Because it includes a random
 component, it will break the potential synchronization between
 notifications triggered by the same external event.
 Note that the estimators AAD and ADEV SHOULD NOT be updated for
 transactions that involve retransmissions.  Also, the first new
 transmission following a successful retransmission SHOULD use the RTO
 for that last retransmission.  If this transmission succeeds without
 any retransmissions, the AAD and ADEV estimators are updated and RTO
 is determined as usual again.  See, e.g., [18] for further details.

3.5.4 Maximum Datagram Size, Fragmentation and Reassembly

 MGCP messages being transmitted over UDP rely on IP for fragmentation
 and reassembly of large datagrams.  The maximum theoretical size of
 an IP datagram is 65535 bytes.  With a 20-byte IP header and an 8-
 byte UDP header, this leaves us with a maximum theoretical MGCP
 message size of 65507 bytes when using UDP.
 However, IP does not require a host to receive IP datagrams larger
 than 576 bytes [21], which would provide an unacceptably small MGCP
 message size.  Consequently, MGCP mandates that implementations MUST
 support MGCP datagrams up to at least 4000 bytes, which requires the

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 corresponding IP fragmentation and reassembly to be supported.  Note,
 that the 4000 byte limit applies to the MGCP level.  Lower layer
 overhead will require support for IP datagrams that are larger than
 this:  UDP and IP overhead will be at least 28 bytes, and, e.g., use
 of IPSec will add additional overhead.
 It should be noted, that the above applies to both Call Agents and
 endpoints.  Call Agents can audit endpoints to determine if they
 support larger MGCP datagrams than specified above.  Endpoints do
 currently not have a similar capability to determine if a Call Agent
 supports larger MGCP datagram sizes.

3.5.5 Piggybacking

 There are cases when a Call Agent will want to send several messages
 at the same time to the same gateways, and vice versa.  When several
 MGCP messages have to be sent in the same datagram, they MUST be
 separated by a line of text that contains a single dot, as in for
 example:
    200 2005 OK
    .
    DLCX 1244 card23/21@tgw-7.example.net MGCP 1.0
    C: A3C47F21456789F0
    I: FDE234C8
 The piggybacked messages MUST be processed exactly as if they had
 been received one at a time in several separate datagrams.  Each
 message in the datagram MUST be processed to completion and in order
 starting with the first message, and each command MUST be responded
 to.  Errors encountered in a message that was piggybacked MUST NOT
 affect any of the other messages received in that datagram - each
 message is processed on its own.
 Piggybacking can be used to achieve two things:
  • Guaranteed in-order delivery and processing of messages.
  • Fate sharing of message delivery.
 When piggybacking is used to guarantee in-order delivery of messages,
 entities MUST ensure that this in-order delivery property is retained
 on retransmissions of the individual messages.  An example of this is
 when multiple Notify's are sent using piggybacking (as described in
 Section 4.4.1).

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 Fate sharing of message delivery ensures that either all the messages
 are delivered, or none of them are delivered.  When piggybacking is
 used to guarantee this fate-sharing, entities MUST also ensure that
 this property is retained upon retransmission.  For example, upon
 receiving a Notify from an endpoint operating in lockstep mode, the
 Call Agent may wish to send the response and a new
 NotificationRequest command in a single datagram to ensure message
 delivery fate-sharing of the two.

3.5.6 Provisional Responses

 Executing some transactions may require a long time.  Long execution
 times may interact with the timer based retransmission procedure.
 This may result either in an inordinate number of retransmissions, or
 in timer values that become too long to be efficient.
 Gateways (and Call Agents) that can predict that a transaction will
 require a long execution time SHOULD send a provisional response with
 response code 100.  As a guideline, a transaction that requires
 external communication to complete, e.g., network resource
 reservation, SHOULD issue a provisional response.  Furthermore
 entities SHOULD send a provisional response if they receive a
 repetition of a transaction that has not yet finished executing.
 Gateways (or Call Agents) that start building up queues of
 transactions to be executed may send a provisional response with
 response code 101 to indicate this (see Section 4.4.8 for further
 details).
 Pure transactional semantics would imply, that provisional responses
 SHOULD NOT return any other information than the fact that the
 transaction is currently executing, however an optimistic approach
 allowing some information to be returned enables a reduction in the
 delay that would otherwise be incurred in the system.
 In order to reduce the delay in the system, it is RECOMMENDED to
 include a connection identifier and session description in a 100
 provisional response to the CreateConnection command.  If a session
 description would be returned by the ModifyConnection command, the
 session description SHOULD be included in the provisional response
 here as well.  If the transaction completes successfully, the
 information returned in the provisional response MUST be repeated in
 the final response.  It is considered a protocol error not to repeat
 this information or to change any of the previously supplied
 information in a successful response.  If the transaction fails, an
 error code is returned - the information returned previously is no
 longer valid.

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 A currently executing CreateConnection or ModifyConnection
 transaction MUST be cancelled if a DeleteConnection command for the
 endpoint is received.  In that case, a final response for the
 cancelled transaction SHOULD still be returned automatically (error
 code 407 - transaction aborted, is RECOMMENDED), and a final response
 for the cancelled transaction MUST be returned if a retransmission of
 the cancelled transaction is detected (see also Section 4.4.4).
 MGCP entities that receive a provisional response SHALL switch to a
 longer repetition timer (LONGTRAN-TIMER) for that transaction.  The
 purpose of this timer is primarily to detect processing failures.
 The default value of LONGTRAN-TIMER is 5 seconds, however the
 provisioning process may alter this.  Note, that retransmissions MUST
 still satisfy the timing requirements specified in Section 3.5.1 and
 3.5.3.  Consequently LONGTRAN-TIMER MUST be smaller than T-HIST (it
 should in fact be considerably smaller).  Also, entities MUST NOT let
 a transaction run forever.  A transaction that is timed out by the
 entity SHOULD return error code 406 (transaction time-out).  Per the
 definition of T-HIST (Section 3.5.1), the maximum transaction
 execution time is smaller than T-HIST (in a network with low delay,
 it can reasonably safely be approximated as T-HIST minus T-MAX), and
 a final response should be received no more than T-HIST seconds after
 the command was sent initially.  Nevertheless, entities SHOULD wait
 for 2*T-HIST seconds before giving up on receiving a final response.
 Retransmission of the command MUST still cease after T-MAX seconds
 though.  If a response is not received, the outcome of the
 transaction is not known.  If the entity sending the command was a
 gateway, it now becomes "disconnected" and SHALL initiate the
 "disconnected" procedure (see Section 4.4.7).
 When the transaction finishes execution, the final response is sent
 and the by now obsolete provisional response is deleted.  In order to
 ensure rapid detection of a lost final response, final responses
 issued after provisional responses for a transaction SHOULD be
 acknowledged (unfortunately older RFC 2705 implementations may not do
 this, which is the only reason it is not an absolute requirement).
 The endpoint SHOULD therefore include an empty "ResponseAck"
 parameter in those, and only those, final responses.  The presence of
 the "ResponseAck" parameter in the final response SHOULD trigger a
 "Response Acknowledgement" response to be sent back to the endpoint.
 The Response Acknowledgement" response will then include the
 transaction-id of the response it acknowledges in the response
 header.  Note that, for backwards compatibility, entities cannot
 depend on receiving such a "response acknowledgement", however it is
 strongly RECOMMENDED to support this behavior, as excessive delays in
 case of packet loss as well as excessive retransmissions may occur
 otherwise.

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 Receipt of a "Response Acknowledgement" response is subject to the
 same time-out and retransmission strategies and procedures as
 responses to commands, i.e., the sender of the final response will
 retransmit it if a "Response Acknowledgement" is not received in
 time.  For backwards compatibility, failure to receive a "response
 acknowledgement" SHOULD NOT affect the roundtrip time estimates for
 subsequent commands, and furthermore MUST NOT lead to the endpoint
 becoming "disconnected".  The "Response Acknowledgment" response is
 never acknowledged.

4. States, Failover and Race Conditions

 In order to implement proper call signaling, the Call Agent must keep
 track of the state of the endpoint, and the gateway must make sure
 that events are properly notified to the Call Agent.  Special
 conditions exist when the gateway or the Call Agent are restarted:
 the gateway must be redirected to a new Call Agent during "failover"
 procedures, the Call Agent must take special action when the gateway
 is taken offline, or restarted.

4.1 Failover Assumptions and Highlights

 The following protocol highlights are important to understanding Call
 Agent fail-over mechanisms:
  • Call Agents are identified by their domain name (and optional

port), not their network addresses, and several addresses can be

   associated with a domain name.
  • An endpoint has one and only one Call Agent associated with it at

any given point in time. The Call Agent associated with an

   endpoint is the current value of the "notified entity".  The
   "notified entity" determines where the gateway will send it's
   commands.  If the "notified entity" does not include a port number,
   the default Call Agent port number (2727) is assumed.
  • NotifiedEntity is a parameter sent by the Call Agent to the gateway

to set the "notified entity" for the endpoint.

  • The "notified entity" for an endpoint is the last value of the

NotifiedEntity parameter received for this endpoint. If no

   explicit NotifiedEntity parameter has ever been received, the
   "notified entity" defaults to a provisioned value.  If no value was
   provisioned or an empty NotifiedEntity parameter was provided (both
   strongly discouraged) thereby making the "notified entity" empty,
   the "notified entity" is set to the source address of the last
   non-audit command for the endpoint.  Thus auditing will not change
   the "notified entity".

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  • Responses to commands are sent to the source address of the

command, regardless of the current "notified entity". When a

   Notify message needs to be piggybacked with the response, the
   datagram is still sent to the source address of the new command
   received, regardless of the current "notified entity".
 The ability for the "notified entity" to resolve to multiple network
 addresses, allows a "notified entity" to represent a Call Agent with
 multiple physical interfaces on it and/or a logical Call Agent made
 up of multiple physical systems.  The order of network addresses when
 a DNS name resolves to multiple addresses is non-deterministic so
 Call Agent fail-over schemes MUST NOT depend on any order (e.g., a
 gateway MUST be able to send a "Notify" to any of the resolved
 network addresses).  On the other hand, the system is likely to be
 most efficient if the gateway sends commands to the interface with
 which it already has a current association.  It is RECOMMENDED that
 gateways use the following algorithm to achieve that goal:
  • If the "notified entity" resolves to multiple network addresses,

and the source address of the request is one of those addresses,

   that network address is the preferred destination address for
   commands.
  • If on the other hand, the source address of the request is not one

of the resolved addresses, the gateway must choose one of the

   resolved addresses for commands.
  • If the gateway fails to contact the network address chosen, it MUST

try the alternatives in the resolved list as described in Section

   4.3.
 If an entire Call Agent becomes unavailable, the endpoints managed by
 that Call Agent will eventually become "disconnected".  The only way
 for these endpoints to become connected again is either for the
 failed Call Agent to become available, or for a backup call agent to
 contact the affected endpoints with a new "notified entity".
 When a backup Call Agent has taken over control of a group of
 endpoints, it is assumed that the failed Call Agent will communicate
 and synchronize with the backup Call Agent in order to transfer
 control of the affected endpoints back to the original Call Agent.
 Alternatively, the failed Call Agent could simply become the backup
 Call Agent.

Andreasen & Foster Informational [Page 120] RFC 3435 MGCP 1.0 January 2003

 We should note that handover conflict resolution between separate
 CA's is not in place - we are relying strictly on the CA's knowing
 what they are doing and communicating with each other (although
 AuditEndpoint can be used to learn about the current "notified
 entity").  If this is not the case, unexpected behavior may occur.
 Note that as mentioned earlier, the default "notified entity" is
 provisioned and may include both domain name and port.  For small
 gateways, provisioning may be done on a per endpoint basis.  For much
 larger gateways, a single provisioning element may be provided for
 multiple endpoints or even for the entire gateway itself.  In either
 case, once the gateway powers up, each endpoint MUST have its own
 "notified entity", so provisioned values for an aggregation of
 endpoints MUST be copied to the "notified entity" for each endpoint
 in the aggregation before operation proceeds.  Where possible, the
 RestartInProgress command on restart SHOULD be sent to the
 provisioned "notified entity" based on an aggregation that allows the
 "all of" wild-card to be used.  This will reduce the number of
 RestartInProgress messages.
 Another way of viewing the use of "notified entity" is in terms of
 associations between gateways and Call Agents.  The "notified entity"
 is a means to set up that association, and governs where the gateway
 will send commands to.  Commands received by the gateway however may
 come from any source.  The association is initially provisioned with
 a provisioned "notified entity", so that on power up
 RestartInProgress and persistent events that occur prior to the first
 NotificationRequest from Call Agents will be sent to the provisioned
 Call Agent.  Once a Call Agent makes a request, however it may
 include the NotifiedEntity parameter and set up a new association.
 Since the "notified entity" persists across calls, the association
 remains intact until a new "notified entity" is provided.

4.2 Communicating with Gateways

 Endpoint names in gateways include a local name indicating the
 specific endpoint and a domain name indicating the host/gateway where
 the endpoint resides.  Gateways may have several interfaces for
 redundancy.
 In gateways that have routing capability, the domain name may resolve
 to a single network address with internal routing to that address
 from any of the gateway's interfaces.  In others, the domain name may
 resolve to multiple network addresses, one for each interface.  In
 the latter case, if a Call Agent fails to contact the gateway on one
 of the addresses, it MUST try the alternates.

Andreasen & Foster Informational [Page 121] RFC 3435 MGCP 1.0 January 2003

4.3 Retransmission, and Detection of Lost Associations:

 The media gateway control protocol is organized as a set of
 transactions, each of which is composed of a command and a response,
 commonly referred to as an acknowledgement.  The MGCP messages, being
 carried over UDP, may be subject to losses.  In the absence of a
 timely response, commands are retransmitted.  MGCP entities MUST keep
 in memory a list of the responses that they sent to recent
 transactions, i.e., a list of all the responses they sent over the
 last T-HIST seconds, and a list of the transactions that have not yet
 finished executing.
 The transaction identifiers of incoming commands are compared to the
 transaction identifiers of the recent responses.  If a match is
 found, the MGCP entity does not execute the transaction, but simply
 repeats the response.  If a match to a previously responded to
 transaction is not found, the transaction identifier of the incoming
 command is compared to the list of transactions that have not yet
 finished executing.  If a match is found, the MGCP entity does not
 execute the transaction again, but SHOULD simply send a provisional
 response - a final response will be provided when the execution of
 the command is complete (see Section 3.5.6 for further detail).
 The repetition mechanism is used to guard against four types of
 possible errors:
  • transmission errors, when for example a packet is lost due to noise

on a line or congestion in a queue,

  • component failure, when for example an interface to a Call Agent

becomes unavailable,

  • Call Agent failure, when for example an entire Call Agent becomes

unavailable,

  • failover, when a new Call Agent is "taking over" transparently.
 The elements should be able to derive from the past history an
 estimate of the packet loss rate due to transmission errors.  In a
 properly configured system, this loss rate should be very low,
 typically less than 1%.  If a Call Agent or a gateway has to repeat a
 message more than a few times, it is very legitimate to assume that
 something other than a transmission error is occurring.  For example,
 given a loss rate of 1%, the probability that 5 consecutive
 transmission attempts fail is 1 in 100 billion, an event that should
 occur less than once every 10 days for a Call Agent that processes
 1,000 transactions per second.  (Indeed, the number of
 retransmissions that is considered excessive should be a function of

Andreasen & Foster Informational [Page 122] RFC 3435 MGCP 1.0 January 2003

 the prevailing packet loss rate.)  We should note that the "suspicion
 threshold", which we will call "Max1", is normally lower than the
 "disconnection threshold", which we will call "Max2".  Max2 MUST be
 set to a larger value than Max1.
 The MGCP retransmission algorithm is illustrated in the Figure below
 and explained further in the following:

Andreasen & Foster Informational [Page 123] RFC 3435 MGCP 1.0 January 2003

    Command issued: N=0, T=0
            |
            |  +------------ retransmission: N++ <--------------+
            |  |                                                |
            |  |     if T <= T-Max then                         |
            |  |      transmission                              |
            |  |  +-- to new address, <-+<----------------------|--+
            |  |  |       N=0           |                       |  |
            V  V  V                     |                       |  |
        +-----------+                   |                       |  |
    +-->| awaiting  |- new Call Agent ->+   +------------+      |  |
    |   |  response |--- timer elapsed  --->| T > T-Max ?|      |  |
    |   +-----------+                       +------------+      ^  ^
    |          |                             |    |             |  |
    |          v             +-----(yes)-----+   (no)           |  |
    |      (response         |                    |             |  |
    |       received)        |              +------------+      |  |
    |          |             |              | N >= Max1 ?|-(no)>+  |
    |          v             |              +------------+      ^  ^
    |      +--------+        |                    |             |  |
    +<(no)-| final ?|        |                  (yes)           |  |
    ^      +--------+        |                    |             |  |
    |          |             |     (if first address & N=Max1,  |  |
    |          v             |      or last address & N=Max2    |  |
    |        (yes)           |               check DNS)         |  |
    |          |             |                    |             |  |
    |          v             V           +---------------+      |  |
    |        (end)           |           |more addresses?|(yes)-|->+
    |                        |           +---------------+      |
    |                        |                    |             ^
    |                        |                  (no)            |
    |                        |                    |             |
    |                        |              +------------+      |
    |                        |              | N >= Max2 ?|(no)--+
    |                        |              +------------+
    |                        |                    |
    |                        |                  (yes)
    |                        |                    |
    |                        |            +----------------+
    |                        +----------->| T >= 2*T-HIST ?|
    |                                     +----------------+
    |                                       |       |
    |                                     (no)    (yes)
    +---------------<-----------------------+       |
                                                    v
                                              (disconnected)

Andreasen & Foster Informational [Page 124] RFC 3435 MGCP 1.0 January 2003

 A classic retransmission algorithm would simply count the number of
 successive repetitions, and conclude that the association is broken
 after re-transmitting the packet an excessive number of times
 (typically between 7 and 11 times). In order to account for the
 possibility of an undetected or in-progress "failover", we modify the
 classic algorithm as follows:
  • We require that the gateway always checks for the presence of a new

Call Agent. It can be noticed either by:

  1. receiving a command where the NotifiedEntity points to the new

Call Agent, or

  1. receiving a redirection response pointing to a new Call Agent.
   If a new Call Agent is detected, the gateway MUST start
   retransmitting outstanding commands for the endpoint(s) redirected
   to that new Call Agent.  Responses to new or old commands are still
   transmitted to the source address of the command.
  • Prior to any retransmission, it is checked that the time elapsed

since the sending of the initial datagram is no greater than T-MAX.

   If more than T-MAX time has elapsed, then retransmissions MUST
   cease.  If more than 2*T-HIST has elapsed, then the endpoint
   becomes disconnected.
  • If the number of repetitions for this Call Agent is equal to

"Max1", and its domain name was not resolved recently (e.g., within

   the last 5 seconds or otherwise provisioned), and it is not in the
   process of being resolved, then the gateway MAY actively query the
   domain name server in order to detect the possible change of the
   Call Agent interfaces.  Note that the first repetition is the
   second transmission.
  • The gateway may have learned several IP addresses for the call

agent. If the number of repetitions for this IP address is greater

   than or equal to "Max1" and lower than "Max2", and there are more
   addresses that have not been tried, then the gateway MUST direct
   the retransmissions to alternate addresses.  Also, receipt of
   explicit network notifications such as, e.g., ICMP network, host,
   protocol, or port unreachable SHOULD lead the gateway to try
   alternate addresses (with due consideration to possible security
   issues).

Andreasen & Foster Informational [Page 125] RFC 3435 MGCP 1.0 January 2003

  • If there are no more interfaces to try, and the number of

repetitions for this address is Max2, then the gateway SHOULD

   contact the DNS one more time to see if any other interfaces have
   become available, unless the domain name was resolved recently
   (e.g., within the last 5 seconds or otherwise provisioned), or it
   is already in the process of being resolved.  If there still are no
   more interfaces to try, the gateway is then disconnected and MUST
   initiate the "disconnected" procedure (see Section 4.4.7).
 In order to automatically adapt to network load, MGCP specifies
 exponentially increasing timers.  If the initial timer is set to 200
 milliseconds, the loss of a fifth retransmission will be detected
 after about 6 seconds.  This is probably an acceptable waiting delay
 to detect a failover.  The repetitions should continue after that
 delay not only in order to perhaps overcome a transient connectivity
 problem, but also in order to allow some more time for the execution
 of a failover - waiting a total delay of 30 seconds is probably
 acceptable.
 It is however important that the maximum delay of retransmissions be
 bounded.  Prior to any retransmission, it is checked that the time
 (T) elapsed since the sending of the initial datagram is no greater
 than T-MAX.  If more than T-MAX time has elapsed, retransmissions
 MUST cease.  If more than 2*T-HIST time has elapsed, the endpoint
 becomes disconnected.  The value T-MAX is related to the T-HIST
 value:  the T-HIST value MUST be greater than or equal to T-MAX plus
 the maximum propagation delay in the network.
 The default value for T-MAX is 20 seconds.  Thus, if the assumed
 maximum propagation delay is 10 seconds, then responses to old
 transactions would have to be kept for a period of at least 30
 seconds.  The importance of having the sender and receiver agree on
 these values cannot be overstated.
 The default value for Max1 is 5 retransmissions and the default value
 for Max2 is 7 retransmissions.  Both of these values may be altered
 by the provisioning process.
 The provisioning process MUST be able to disable one or both of the
 Max1 and Max2 DNS queries.

4.4 Race Conditions

 MGCP deals with race conditions through the notion of a "quarantine
 list" and through explicit detection of desynchronization, e.g., for
 mismatched hook state due to glare for an endpoint.

Andreasen & Foster Informational [Page 126] RFC 3435 MGCP 1.0 January 2003

 MGCP does not assume that the transport mechanism will maintain the
 order of commands and responses.  This may cause race conditions,
 that may be obviated through a proper behavior of the Call Agent.
 (Note that some race conditions are inherent to distributed systems;
 they would still occur, even if the commands were transmitted in
 strict order.)
 In some cases, many gateways may decide to restart operation at the
 same time.  This may occur, for example, if an area loses power or
 transmission capability during an earthquake or an ice storm.  When
 power and transmission are reestablished, many gateways may decide to
 send "RestartInProgress" commands simultaneously, leading to very
 unstable operation.

4.4.1 Quarantine List

 MGCP controlled gateways will receive "notification requests" that
 ask them to watch for a list of "events".  The protocol elements that
 determine the handling of these events are the "Requested Events"
 list, the "Digit Map", the "Quarantine Handling", and the "Detect
 Events" list.
 When the endpoint is initialized, the requested events list only
 consists of persistent events for the endpoint, and the digit map is
 assumed empty.  At this point, the endpoint MAY use an implicit
 NotificationRequest with the reserved RequestIdentifier zero ("0") to
 detect and report a persistent event, e.g., off-hook.  A pre-existing
 off-hook condition MUST here result in the off-hook event being
 generated as well.
 The endpoint awaits the reception of a NotificationRequest command,
 after which the gateway starts observing the endpoint for occurrences
 of the events mentioned in the list, including persistent events.
 The events are examined as they occur.  The action that follows is
 determined by the "action" parameter associated with the event in the
 list of requested events, and also by the digit map.  The events that
 are defined as "accumulate" or "accumulate according to digit map"
 are accumulated in a list of events, the events that are marked as
 "accumulate according to the digit map" will additionally be
 accumulated in the "current dial string".  This will go on until one
 event is encountered that triggers a notification which will be sent
 to the current "notified entity".
 The gateway, at this point, will transmit the Notify command and will
 place the endpoint in a "notification" state.  As long as the
 endpoint is in this notification state, the events that are to be
 detected on the endpoint are stored in a "quarantine" buffer (FIFO)

Andreasen & Foster Informational [Page 127] RFC 3435 MGCP 1.0 January 2003

 for later processing.  The events are, in a sense, "quarantined".
 All events that are specified by the union of the RequestedEvents
 parameter and the most recently received DetectEvents parameter or,
 in the absence of the latter, all events that are referred to in the
 RequestedEvents, SHALL be detected and quarantined, regardless of the
 action associated with the event.  Persistent events are here viewed
 as implicitly included in RequestedEvents.  If the quarantine buffer
 reaches the capacity of the endpoint, a Quarantine Buffer Overflow
 event (see Appendix B) SHOULD be generated (when this event is
 supported, the endpoint MUST ensure it has capacity to include the
 event in the quarantine buffer).  Excess events will now be
 discarded.
 The endpoint exits the "notification state" when the response
 (whether success or failure) to the Notify command is received.  The
 Notify command may be retransmitted in the "notification state", as
 specified in Section 3.5 and 4.  If the endpoint is or becomes
 disconnected (see Section 4.3) during this, a response to the Notify
 command will never be received.  The Notify command is then lost and
 hence no longer considered pending, yet the endpoint is still in the
 "notification state".  Should that occur, completion of the
 disconnected procedure specified in Section 4.4.7 SHALL then lead the
 endpoint to exit the "notification state".
 When the endpoint exits the "notification state" it resets the list
 of observed events and the "current dial string" of the endpoint to a
 null value.
 Following that point, the behavior of the gateway depends on the
 value of the QuarantineHandling parameter in the triggering
 NotificationRequest command:
 If the Call Agent had specified, that it expected at most one
 notification in response to the notification request command, then
 the gateway SHALL simply keep on accumulating events in the
 quarantine buffer until it receives the next notification request
 command.
 If, however, the gateway is authorized to send multiple successive
 Notify commands, it will proceed as follows.  When the gateway exits
 the "notification state", it resets the list of observed events and
 the "current dial string" of the endpoint to a null value and starts
 processing the list of quarantined events, using the already received
 list of requested events and digit map.  When processing these
 events, the gateway may encounter an event which triggers a Notify
 command to be sent.  If that is the case, the gateway can adopt one
 of the two following behaviors:

Andreasen & Foster Informational [Page 128] RFC 3435 MGCP 1.0 January 2003

  • it can immediately transmit a Notify command that will report all

events that were accumulated in the list of observed events until

   the triggering event, included, leaving the unprocessed events in
   the quarantine buffer,
  • or it can attempt to empty the quarantine buffer and transmit a

single Notify command reporting several sets of events (in a single

   list of observed events) and possibly several dial strings.  The
   "current dial string" is reset to a null value after each
   triggering event.  The events that follow the last triggering event
   are left in the quarantine buffer.
 If the gateway transmits a Notify command, the endpoint will reenter
 and remain in the "notification state" until the acknowledgement is
 received (as described above).  If the gateway does not find a
 quarantined event that triggers a Notify command, it places the
 endpoint in a normal state.  Events are then processed as they come,
 in exactly the same way as if a Notification Request command had just
 been received.
 A gateway may receive at any time a new Notification Request command
 for the endpoint, including the case where the endpoint is
 disconnected.  Activating an embedded Notification Request is here
 viewed as receiving a new Notification Request as well, except that
 the current list of ObservedEvents remains unmodified rather than
 being processed again.  When a new notification request is received
 in the notification state, the gateway SHALL ensure that the pending
 Notify is received by the Call Agent prior to a new Notify (note that
 a Notify that was lost due to being disconnected, is no longer
 considered pending).  It does so by using the "piggybacking"
 functionality of the protocol.  The messages will then be sent in a
 single packet to the current "notified entity".  The steps involved
 are the following:
 a) the gateway sends a response to the new notification request.
 b) the endpoint is then taken out of the "notification state" without
    waiting for the acknowledgement of the pending Notify command.
 c) a copy of the unacknowledged Notify command is kept until an
    acknowledgement is received.  If a timer elapses, the Notify will
    be retransmitted.
 d) If the gateway has to transmit a new Notify before the previous
    Notify(s) is acknowledged, it constructs a packet that piggybacks
    a repetition of the old Notify(s) and the new Notify (ordered by
    age with the oldest first).  This datagram will be sent to the
    current "notified entity".

Andreasen & Foster Informational [Page 129] RFC 3435 MGCP 1.0 January 2003

 f) Gateways that cannot piggyback several messages in the same
    datagram and hence guarantee in-order delivery of two (or more)
    Notify's SHALL leave the endpoint in the "notification" state as
    long as the last Notify is not acknowledged.

Andreasen & Foster Informational [Page 130] RFC 3435 MGCP 1.0 January 2003

 The procedure is illustrated by the following diagram:
  +-------------------+
  | Processing Events |<--------------------------------------+
  +-------------------+                                       |
           |                                                  |
   Need to send NTFY                                          |
           |                                                  |
           v                                                  |
  +-------------------+                                       |
  | Outstanding NTFY  |---- No -------+                       |
  |                   |               |                       |
  +-------------------+               v                       |
           |                    +-----------+                 |
          Yes                   | Send NTFY |                 |
           |                    +-----------+                 |
           v                          |                       |
  +--------------------+              v                       |
  | Piggyback new NTFY |     +--------------------+           |
  | w. old outstanding |---->| Notification State |           |
  | NTFY(s)            |     +--------------------+           |
  +--------------------+       |               |              |
                           new RQNT        NTFY response      |
                           received        received           |
                               |               |              |
                               |               v              |
                               |        +-------------+       |
                               |        | Step mode ? |- No ->+
                               |        +-------------+       ^
                               |               |              |
                               |              Yes             |
                               |               |              |
                               |               v              |
                               |       +---------------+      |
                               |       | Wait for RQNT |      |
                               |       +---------------+      |
                               |               |              |
                               |         RQNT received        |
                               |               |              |
                               |               v              |
                               |       +---------------+      |
                               +------>| Apply RQNT and|----->+
                                       | send response |
                                       +---------------+

Andreasen & Foster Informational [Page 131] RFC 3435 MGCP 1.0 January 2003

 Gateways may also attempt to deliver the pending Notify prior to a
 successful response to the new NotificationRequest by using the
 "piggybacking" functionality of the protocol.  This was in fact
 required behavior in RFC 2705, however there are several
 complications in doing this, and the benefits are questionable.  In
 particular, the RFC 2705 mechanism did not guarantee in-order
 delivery of Notify's and responses to NotificationRequests in
 general, and hence Call Agents had to handle out-of-order delivery of
 these messages anyway.  The change to optional status is thus
 backwards compatible while greatly reducing complexity.
 After receiving the Notification Request command, the requested
 events list and digit map (if a new one was provided) are replaced by
 the newly received parameters, and the current dial string is reset
 to a null value.  Furthermore, when the Notification Request was
 received in the "notification state", the list of observed events is
 reset to a null value.  The subsequent behavior is conditioned by the
 value of the QuarantineHandling parameter.  The parameter may specify
 that quarantined events (and observed events which in this case is
 now an empty list), should be discarded, in which case they will be.
 If the parameter specifies that the quarantined (and observed) events
 are to be processed, the gateway will start processing the list of
 quarantined (and observed) events, using the newly received list of
 requested events and digit map (if provided).  When processing these
 events, the gateway may encounter an event which requires a Notify
 command to be sent.  If that is the case, the gateway will
 immediately transmit a Notify command that will report all events
 that were accumulated in the list of observed events until the
 triggering event, included leaving the unprocessed events in the
 quarantine buffer, and will enter the "notification state".
 A new notification request may be received while the gateway has
 accumulated events according to the previous notification request,
 but has not yet detected a notification-triggering events, i.e., the
 endpoint is not in the "notification state".  The handling of not-
 yet-notified events is determined, as with the quarantined events, by
 the quarantine handling parameter:
  • If the quarantine-handling parameter specifies that quarantined

events shall be ignored, the observed events list is simply reset.

  • If the quarantine-handling parameter specifies that quarantined

events shall be processed, the observed event list is transferred

   to the quarantined event list.  The observed event list is then
   reset, and the quarantined event list is processed.

Andreasen & Foster Informational [Page 132] RFC 3435 MGCP 1.0 January 2003

 Call Agents controlling endpoints in lockstep mode SHOULD provide the
 response to a successful Notify message and the new
 NotificationRequest in the same datagram using the piggybacking
 mechanism.

4.4.2 Explicit Detection

 A key element of the state of several endpoints is the position of
 the hook.  A race condition may occur when the user decides to go
 off-hook before the Call Agent has the time to ask the gateway to
 notify an off-hook event (the "glare" condition well known in
 telephony), or if the user goes on-hook before the Call Agent has the
 time to request the event's notification.
 To avoid this race condition, the gateway MUST check the condition of
 the endpoint before acknowledging a NotificationRequest.  It MUST
 return an error:
 1. If the gateway is requested to notify an "off-hook" transition
    while the phone is already off-hook, (error code 401 - phone off
    hook)
 2. If the gateway is requested to notify an "on-hook" or "flash hook"
    condition while the phone is already on-hook (error code 402 -
    phone on hook).
 Additionally, individual signal definitions can specify that a signal
 will only operate under certain conditions, e.g., ringing may only be
 possible if the phone is already off-hook.  If such prerequisites
 exist for a given signal, the gateway MUST return the error specified
 in the signal definition if the prerequisite is not met.
 It should be noted, that the condition check is performed at the time
 the notification request is received, whereas the actual event that
 caused the current condition may have either been reported, or
 ignored earlier, or it may currently be quarantined.
 The other state variables of the gateway, such as the list of
 RequestedEvents or list of requested signals, are entirely replaced
 after each successful NotificationRequest, which prevents any long
 term discrepancy between the Call Agent and the gateway.
 When a NotificationRequest is unsuccessful, whether it is included in
 a connection-handling command or not, the gateway MUST simply
 continue as if the command had never been received.  As all other
 transactions, the NotificationRequest MUST operate as an atomic
 transaction, thus any changes initiated as a result of the command
 MUST be reverted.

Andreasen & Foster Informational [Page 133] RFC 3435 MGCP 1.0 January 2003

 Another race condition may occur when a Notify is issued shortly
 before the reception by the gateway of a NotificationRequest.  The
 RequestIdentifier is used to correlate Notify commands with
 NotificationRequest commands thereby enabling the Call Agent to
 determine if the Notify command was generated before or after the
 gateway received the new NotificationRequest.  This is especially
 important to avoid deadlocks in "step" mode.

4.4.3 Transactional Semantics

 As the potential transaction completion times increase, e.g., due to
 external resource reservations, a careful definition of the
 transactional semantics becomes increasingly important.  In
 particular the issue of race conditions, e.g., as it relates to
 hook-state, must be defined carefully.
 An important point to consider is, that the status of a pre-condition
 (e.g., hook-state) may in fact change between the time a transaction
 starts and the time it either completes successfully (transaction
 commit) or fails.  In general, we can say that the successful
 execution of a transaction depends on one or more pre-conditions
 where the status of one or more of the pre-conditions may change
 dynamically between the transaction start and transaction commit.
 The simplest semantics for this is simply to require that all pre-
 conditions be met from the time the transaction is initiated until
 the transaction commits.  If any pre-condition is not met before the
 completion of the transaction, the transaction will also fail.
 As an example, consider a transaction that includes a request for the
 "off-hook" event.  When the transaction is initiated the phone is
 "on-hook" and this pre-condition is therefore met.  If the hook-state
 changes to "off-hook" before the transaction completes, the pre-
 condition is no longer met, and the transaction therefore immediately
 fails.
 Finally, we need to consider the point in time when a new transaction
 takes effect and endpoint processing according to an old transaction
 stops.  For example, assume that transaction T1 has been executed
 successfully and event processing is currently being done according
 to transaction T1.  Now we receive a new transaction T2 specifying
 new event processing (for example a CreateConnection with an
 encapsulated NotificationRequest).  Since we don't know whether T2
 will complete successfully or not, we cannot start processing events
 according to T2 until the outcome of T2 is known.  While we could
 suspend all event processing until the outcome of T2 is known, this
 would make for a less responsive system and hence SHOULD NOT be done.
 Instead, when a new transaction Ty is received and Ty modifies

Andreasen & Foster Informational [Page 134] RFC 3435 MGCP 1.0 January 2003

 processing according to an old transaction Tx, processing according
 to Tx SHOULD remain active for as long as possible, until a
 successful outcome of Ty is known to occur.  If Ty fails, then
 processing according to Tx will of course continue as usual.  Any
 changes incurred by Ty logically takes effect when Ty commits.  Thus,
 if the endpoint was in the notification state when Ty commits, and Ty
 contained a NotificationRequest, the endpoint will be taken out of
 the notification state when Ty commits.  Note that this is
 independent of whether the endpoint was in the notification state
 when Ty was initiated.  For example, a Notify could be generated due
 to processing according to Tx between the start and commit of Ty.  If
 the commit of Ty leads to the endpoint entering the notification
 state, a new NotificationRequest (Tz) is needed to exit the
 notification state.  This follows from the fact that transaction
 execution respects causal order.
 Another related issue is the use of wildcards, especially the "all
 of" wildcard, which may match more than one endpoint.  When a command
 is requested, and the endpoint identifier matches more than one
 endpoint, transactional semantics still apply.  Thus, the command
 MUST either succeed for all the endpoints, or it MUST fail for all of
 them.  A single response is consequently always issued.

4.4.4 Ordering of Commands, and Treatment of Misorder

 MGCP does not mandate that the underlying transport protocol
 guarantees in-order delivery of commands to a gateway or an endpoint.
 This property tends to maximize the timeliness of actions, but it has
 a few drawbacks.  For example:
  • Notify commands may be delayed and arrive at the Call Agent after

the transmission of a new Notification Request command,

  • If a new NotificationRequest is transmitted before a previous one

is acknowledged, there is no guarantee that the previous one will

   not be received and executed after the new one.
 Call Agents that want to guarantee consistent operation of the
 endpoints can use the following rules:
 1) When a gateway handles several endpoints, commands pertaining to
    the different endpoints can be sent in parallel, for example
    following a model where each endpoint is controlled by its own
    process or its own thread.
 2) When several connections are created on the same endpoint,
    commands pertaining to different connections can be sent in
    parallel.

Andreasen & Foster Informational [Page 135] RFC 3435 MGCP 1.0 January 2003

 3) On a given connection, there should normally be only one
    outstanding command (create or modify).  However, a
    DeleteConnection command can be issued at any time.  In
    consequence, a gateway may sometimes receive a ModifyConnection
    command that applies to a previously deleted connection.  Such
    commands will fail, and an error code MUST be returned (error code
    515 - incorrect connection-id, is RECOMMENDED).
 4) On a given endpoint, there should normally be only one outstanding
    NotificationRequest command at any time.  The RequestId parameter
    MUST be used to correlate Notify commands with the triggering
    notification request.
 5) In some cases, an implicitly or explicitly wildcarded
    DeleteConnection command that applies to a group of endpoints can
    step in front of a pending CreateConnection command.  The Call
    Agent should individually delete all connections whose completion
    was pending at the time of the global DeleteConnection command.
    Also, new CreateConnection commands for endpoints named by the
    wild-carding SHOULD NOT be sent until the wild-carded
    DeleteConnection command is acknowledged.
 6) When commands are embedded within each other, sequencing
    requirements for all commands must be adhered to.  For example a
    Create Connection command with a Notification Request in it must
    adhere to the sequencing requirements associated with both
    CreateConnection and NotificationRequest at the same time.
 7) AuditEndpoint and AuditConnection are not subject to any
    sequencing requirements.
 8) RestartInProgress MUST always be the first command sent by an
    endpoint as defined by the restart procedure.  Any other command
    or non-restart response (see Section 4.4.6), except for responses
    to auditing, MUST be delivered after this RestartInProgress
    command (piggybacking allowed).
 9) When multiple messages are piggybacked in a single packet, the
    messages are always processed in order.
 10) On a given endpoint, there should normally be only one
    outstanding EndpointConfiguration command at any time.
 Gateways MUST NOT make any assumptions as to whether Call Agents
 follow these rules or not.  Consequently gateways MUST always respond
 to commands, regardless of whether they adhere to the above rules or
 not.  To ensure consistent operation, gateways SHOULD behave as
 specified below when one or more of the above rules are not followed:

Andreasen & Foster Informational [Page 136] RFC 3435 MGCP 1.0 January 2003

  • Where a single outstanding command is expected (ModifyConnection,

NotificationRequest, and EndpointConfiguration), but the same

   command is received in a new transaction before the old finishes
   executing, the gateway SHOULD fail the previous command.  This
   includes the case where one or more of the commands were
   encapsulated.  The use of error code 407 (transaction aborted) is
   RECOMMENDED.
  • If a ModifyConnection command is received for a pending

CreateConnection command, the ModifyConnection command SHOULD

   simply be rejected.  The use of error code 400 (transient error) is
   RECOMMENDED.  Note that this situation constitutes a Call Agent
   programming error.
  • If a DeleteConnection command is received for a pending

CreateConnection or ModifyConnection command, the pending command

   MUST be aborted.  The use of error code 407 (transaction aborted)
   is RECOMMENDED.
 Note, that where reception of a new command leads to aborting an old
 command, the old command SHOULD be aborted regardless of whether the
 new command succeeds or not.  For example, if a ModifyConnection
 command is aborted by a DeleteConnection command which itself fails
 due to an encapsulated NotificationRequest, the ModifyConnection
 command is still aborted.

4.4.5 Endpoint Service States

 As described earlier, endpoints configured for operation may be
 either in-service or out-of-service.  The actual service-state of the
 endpoint is reflected by the combination of the RestartMethod and
 RestartDelay parameters, which are sent with RestartInProgress
 commands (Section 2.3.12) and furthermore may be audited in
 AuditEndpoint commands (Section 2.3.10).
 The service-state of an endpoint affects how it processes a command.
 An endpoint in-service MUST process any command received, whereas an
 endpoint that is out-of-service MUST reject non-auditing commands,
 but SHOULD process auditing commands if possible.  For backwards
 compatibility, auditing commands for an out-of-service endpoint may
 alternatively be rejected as well.  Any command rejected due to an
 endpoint being out-of-service SHOULD generate error code 501
 (endpoint not ready/out-of-service).
 Note that (per Section 2.1.2), unless otherwise specified for a
 command, endpoint names containing the "any of" wildcard only refer
 to endpoints in-service, whereas endpoint names containing the "all
 of" wildcard refer to all endpoints, regardless of service state.

Andreasen & Foster Informational [Page 137] RFC 3435 MGCP 1.0 January 2003

 The above relationships are illustrated in the table below which
 shows the current service-states and gateway processing of commands
 as a function of the RestartInProgress command sent and the response
 (if any) received to it.  The last column also lists (in parentheses)
 the RestartMethod to be returned if audited:

Andreasen & Foster Informational [Page 138] RFC 3435 MGCP 1.0 January 2003

  1. —————————————————————–

| Restart- | Restart- | 2xx | Service- | Response to |

 |    Method |    Delay | received ?|    State |   new command      |
 |------------------------------------------------------------------|
 | graceful  |   zero   |   Yes/No  |   In     | non-audit: 2xx     |
 |           |          |           |          | audit:     2xx     |
 |           |          |           |          |        (graceful)  |
 |-----------+----------+-----------+----------+--------------------|
 | graceful  | non-zero |   Yes/No  |   In*    | non-audit: 2xx     |
 |           |          |           |          | audit:     2xx     |
 |           |          |           |          |        (graceful)  |
 |-----------+----------+-----------+----------+--------------------|
 | forced    |   N/A    |   Yes/No  |   Out    | non-audit: 501     |
 |           |          |           |          | audit:     2xx     |
 |           |          |           |          |         (forced)   |
 |-----------+----------+-----------+----------+--------------------|
 | restart   |   zero   |    No     |   In     | non-audit: 2xx,405*|
 |           |          |           |          | audit:     2xx     |
 |           |          |           |          |         (restart)  |
 |-----------+----------+-----------+----------+--------------------|
 | restart   |   zero   |    Yes    |   In     | non-audit: 2xx     |
 |           |          |           |          | audit:     2xx     |
 |           |          |           |          |         (restart)  |
 |-----------+----------+-----------+----------+--------------------|
 | restart   | non-zero |    No     |   Out*   | non-audit: 501*    |
 |           |          |           |          | audit:     2xx     |
 |           |          |           |          |         (restart)  |
 |-----------+----------+-----------+----------+--------------------|
 | restart   | non-zero |    Yes    |   Out*   | non-audit: 501*    |
 |           |          |           |          | audit:     2xx     |
 |           |          |           |          |         (restart)  |
 |-----------+----------+-----------+----------+--------------------|
 | discon-   |   zero/  |    No     |   In     | non-audit: 2xx,    |
 |    nected | non-zero |           |          | audit:     2xx     |
 |           |          |           |          |      (disconnected)|
 |-----------+----------+-----------+----------+--------------------|
 | discon-   |   zero/  |    Yes    |   In     | non-audit: 2xx     |
 |    nected | non-zero |           |          | audit:     2xx     |
 |           |          |           |          |         (restart)  |
 |-----------+----------+-----------+----------+--------------------|
 | cancel-   |   N/A    |   Yes/No  |   In     | non-audit: 2xx     |
 |  graceful |          |           |          | audit:     2xx     |
 |           |          |           |          |         (restart)  |
  ------------------------------------------------------------------

Andreasen & Foster Informational [Page 139] RFC 3435 MGCP 1.0 January 2003

 Notes (*):
  • The three service-states marked with "*" will change after the

expiration of the RestartDelay at which time an updated

   RestartInProgress command SHOULD be sent.
  • If the endpoint returns 2xx when the restart procedure has not yet

completed, then in-order delivery MUST still be satisfied, i.e.,

   piggy-backing is to be used.  If instead, the command is not
   processed, 405 SHOULD be returned.
  • Following a "restart" RestartInProgress with a non-zero

RestartDelay, error code 501 is only returned until the endpoint

   goes in-service, i.e., until the expiration of the RestartDelay.

4.4.6 Fighting the Restart Avalanche

 Let's suppose that a large number of gateways are powered on
 simultaneously.  If they were to all initiate a RestartInProgress
 transaction, the Call Agent would very likely be swamped, leading to
 message losses and network congestion during the critical period of
 service restoration.  In order to prevent such avalanches, the
 following behavior is REQUIRED:
 1) When a gateway is powered on, it MUST initiate a restart timer to
    a random value, uniformly distributed between 0 and a maximum
    waiting delay (MWD).  Care should be taken to avoid synchronicity
    of the random number generation between multiple gateways that
    would use the same algorithm.
 2) The gateway MUST then wait for either the end of this timer, the
    reception of a command from the Call Agent, or the detection of a
    local user activity, such as for example an off-hook transition on
    a residential gateway.
 3) When the timer elapses, when a command is received, or when an
    activity is detected, the gateway MUST initiate the restart
    procedure.
 The restart procedure simply requires the endpoint to guarantee that
 the first
  • non-audit command, or
  • non-restart response (i.e., error codes other than 405, 501, and

520) to a non-audit command

Andreasen & Foster Informational [Page 140] RFC 3435 MGCP 1.0 January 2003

 that the Call Agent sees from this endpoint is a "restart"
 RestartInProgress command.  The endpoint is free to take full
 advantage of piggybacking to achieve this.  Endpoints that are
 considered in-service will have a RestartMethod of "restart", whereas
 endpoints considered out-of-service will have a RestartMethod of
 "forced" (also see Section 4.4.5).  Commands rejected due to an
 endpoint not yet having completed the restart procedure SHOULD use
 error code 405 (endpoint "restarting").
 The restart procedure is complete once a success response has been
 received.  If an error response is received, the subsequent behavior
 depends on the error code in question:
  • If the error code indicates a transient error (4xx), then the

restart procedure MUST be initiated again (as a new transaction).

  • If the error code is 521, then the endpoint is redirected, and the

restart procedure MUST be initiated again (as a new transaction).

   The 521 response MUST have included a NotifiedEntity which then is
   the "notified entity" towards which the restart is initiated.  If
   it did not include a NotifiedEntity, the response is treated as any
   other permanent error (see below).
  • If the error is any other permanent error (5xx), and the endpoint

is not able to rectify the error, then the endpoint no longer

   initiates the restart procedure on its own (until
   rebooted/restarted) unless otherwise specified.  If a command is
   received for the endpoint, the endpoint MUST initiate the restart
   procedure again.
 Note that if the RestartInProgress is piggybacked with the response
 (R) to a command received while restarting, then retransmission of
 the RestartInProgress does not require piggybacking of the response
 R.  However, while the endpoint is restarting, a resend of the
 response R does require the RestartInProgress to be piggybacked to
 ensure in-order delivery of the two.
 Should the gateway enter the "disconnected" state while carrying out
 the restart procedure, the disconnected procedure specified in
 Section 4.4.7 MUST be carried out, except that a "restart" rather
 than "disconnected" message is sent during the procedure.
 Each endpoint in a gateway will have a provisionable Call Agent,
 i.e., "notified entity", to direct the initial restart message
 towards.  When the collection of endpoints in a gateway is managed by
 more than one Call Agent, the above procedure MUST be performed for
 each collection of endpoints managed by a given Call Agent.  The
 gateway MUST take full advantage of wild-carding to minimize the

Andreasen & Foster Informational [Page 141] RFC 3435 MGCP 1.0 January 2003

 number of RestartInProgress messages generated when multiple
 endpoints in a gateway restart and the endpoints are managed by the
 same Call Agent.  Note that during startup, it is possible for
 endpoints to start out as being out-of-service, and then become in-
 service as part of the gateway initialization procedure.  A gateway
 may thus choose to send first a "forced" RestartInProgress for all
 its endpoints, and subsequently a "restart" RestartInProgress for the
 endpoints that come in-service.  Alternatively, the gateway may
 simply send "restart" RestartInProgress for only those endpoints that
 are in-service, and "forced" RestartInProgress for the specific
 endpoints that are out-of-service.  Wild-carding MUST still be used
 to minimize the number of messages sent though.
 The value of MWD is a configuration parameter that depends on the
 type of the gateway.  The following reasoning can be used to
 determine the value of this delay on residential gateways.
 Call agents are typically dimensioned to handle the peak hour traffic
 load, during which, in average, 10% of the lines will be busy,
 placing calls whose average duration is typically 3 minutes.  The
 processing of a call typically involves 5 to 6 MGCP transactions
 between each endpoint and the Call Agent.  This simple calculation
 shows that the Call Agent is expected to handle 5 to 6 transactions
 for each endpoint, every 30 minutes on average, or, to put it
 otherwise, about one transaction per endpoint every 5 to 6 minutes on
 average.  This suggest that a reasonable value of MWD for a
 residential gateway would be 10 to 12 minutes.  In the absence of
 explicit configuration, residential gateways should adopt a value of
 600 seconds for MWD.
 The same reasoning suggests that the value of MWD should be much
 shorter for trunking gateways or for business gateways, because they
 handle a large number of endpoints, and also because the usage rate
 of these endpoints is much higher than 10% during the peak busy hour,
 a typical value being 60%.  These endpoints, during the peak hour,
 are thus expected to contribute about one transaction per minute to
 the Call Agent load.  A reasonable algorithm is to make the value of
 MWD per "trunk" endpoint six times shorter than the MWD per
 residential gateway, and also inversely proportional to the number of
 endpoints that are being restarted.  For example MWD should be set to
 2.5 seconds for a gateway that handles a T1 line, or to 60
 milliseconds for a gateway that handles a T3 line.

Andreasen & Foster Informational [Page 142] RFC 3435 MGCP 1.0 January 2003

4.4.7 Disconnected Endpoints

 In addition to the restart procedure, gateways also have a
 "disconnected" procedure, which MUST be initiated when an endpoint
 becomes "disconnected" as described in Section 4.3.  It should here
 be noted, that endpoints can only become disconnected when they
 attempt to communicate with the Call Agent.  The following steps MUST
 be followed by an endpoint that becomes "disconnected":
 1. A "disconnected" timer is initialized to a random value, uniformly
    distributed between 1 and a provisionable "disconnected" initial
    waiting delay (Tdinit), e.g., 15 seconds.  Care MUST be taken to
    avoid synchronicity of the random number generation between
    multiple gateways and endpoints that would use the same algorithm.
 2. The gateway then waits for either the end of this timer, the
    reception of a command for the endpoint from the Call Agent, or
    the detection of a local user activity for the endpoint, such as
    for example an off-hook transition.
 3. When the "disconnected" timer elapses for the endpoint, when a
    command is received for the endpoint, or when local user activity
    is detected for the endpoint, the gateway initiates the
    "disconnected" procedure for the endpoint - if a disconnected
    procedure was already in progress for the endpoint, it is simply
    replaced by the new one.  Furthermore, in the case of local user
    activity, a provisionable "disconnected" minimum waiting delay
    (Tdmin) MUST have elapsed since the endpoint became disconnected
    or the last time it ended the "disconnected" procedure in order to
    limit the rate at which the procedure is performed.  If Tdmin has
    not passed, the endpoint simply proceeds to step 2 again, without
    affecting any disconnected procedure already in progress.
 4. If the "disconnected" procedure still left the endpoint
    disconnected, the "disconnected" timer is then doubled, subject to
    a provisionable "disconnected" maximum waiting delay (Tdmax),
    e.g., 600 seconds, and the gateway proceeds with step 2 again
    (using a new transaction-id).
 The "disconnected" procedure is similar to the restart procedure in
 that it simply states that the endpoint MUST send a RestartInProgress
 command to the Call Agent informing it that the endpoint was
 disconnected.  Furthermore, the endpoint MUST guarantee that the
 first non-audit message (non-audit command or response to non-audit
 command) that the Call Agent sees from this endpoint MUST inform the
 Call Agent that the endpoint is disconnected (unless the endpoint
 goes out-of-service).  When a command (C) is received, this is
 achieved by sending a piggy-backed datagram with a "disconnected"

Andreasen & Foster Informational [Page 143] RFC 3435 MGCP 1.0 January 2003

 RestartInProgress command and the response to command C to the source
 address of command C as opposed to the current "notified entity".
 This piggy-backed RestartInProgress is not automatically
 retransmitted by the endpoint but simply relies on fate-sharing with
 the piggy-backed response to guarantee the in-order delivery
 requirement.  The Call Agent still sends a response to the piggy-
 backed RestartInProgress, however, as usual, the response may be
 lost.  In addition to the piggy-backed RestartInProgress command, a
 new "disconnected" procedure is triggered by the command received.
 This will lead to a non piggy-backed copy (i.e., same transaction) of
 the "disconnected" RestartInProgress command being sent reliably to
 the current "notified entity".
 When the Call Agent learns that the endpoint is disconnected, the
 Call Agent may then for instance decide to audit the endpoint, or
 simply clear all connections for the endpoint.  Note that each such
 "disconnected" procedure will result in a new RestartInProgress
 command, which will be subject to the normal retransmission
 procedures specified in Section 4.3.  At the end of the procedure,
 the endpoint may thus still be "disconnected".  Should the endpoint
 go out-of-service while being disconnected, it SHOULD send a "forced"
 RestartInProgress message as described in Section 2.3.12.
 The disconnected procedure is complete once a success response has
 been received.  Error responses are handled similarly to the restart
 procedure (Section 4.4.6).  If the "disconnected" procedure is to be
 initiated again following an error response, the rate-limiting timer
 considerations specified above still apply.
 Note, that if the RestartInProgress is piggybacked with the response
 (R) to a command received while being disconnected, then
 retransmission of this particular RestartInProgress does not require
 piggybacking of the response R.  However, while the endpoint is
 disconnected, resending the response R does require the
 RestartInProgress to be piggybacked with the response to ensure the
 in-order delivery of the two.
 If a set of disconnected endpoints have the same "notified entity",
 and the set of endpoints can be named with a wildcard, the gateway
 MAY replace the individual disconnected procedures with a suitably
 wildcarded disconnected procedure instead.  In that case, the Restart
 Delay for the wildcarded "disconnected" RestartInProgress command
 SHALL be the Restart Delay corresponding to the oldest disconnected
 procedure replaced.  Note that if only a subset of these endpoints
 subsequently have their "notified entity" changed and/or are no
 longer disconnected, then that wildcarded disconnected procedure can
 no longer be used.  The remaining individual disconnected procedures
 MUST then be resumed again.

Andreasen & Foster Informational [Page 144] RFC 3435 MGCP 1.0 January 2003

 A disconnected endpoint may wish to send a command (besides
 RestartInProgress) while it is disconnected.  Doing so will only
 succeed once the Call Agent is reachable again, which raises the
 question of what to do with such a command meanwhile.  At one
 extreme, the endpoint could drop the command right away, however that
 would not work very well when the Call Agent was in fact available,
 but the endpoint had not yet completed the "disconnected" procedure
 (consider for example the case where a NotificationRequest was just
 received which immediately resulted in a Notify being generated).  To
 prevent such scenarios, disconnected endpoints SHALL NOT blindly drop
 new commands to be sent for a period of T-MAX seconds after they
 receive a non-audit command.
 One way of satisfying this requirement is to employ a temporary
 buffering of commands to be sent, however in doing so, the endpoint
 MUST ensure, that it:
  • does not build up a long queue of commands to be sent,
  • does not swamp the Call Agent by rapidly sending too many commands

once it is connected again.

 Buffering commands for T-MAX seconds and, once the endpoint is
 connected again, limiting the rate at which buffered commands are
 sent to one outstanding command per endpoint is considered acceptable
 (see also Section 4.4.8, especially if using wildcards).  If the
 endpoint is not connected within T-MAX seconds, but a "disconnected"
 procedure is initiated within T-MAX seconds, the endpoint MAY
 piggyback the buffered command(s) with that RestartInProgress.  Note,
 that once a command has been sent, regardless of whether it was
 buffered initially, or piggybacked earlier, retransmission of that
 command MUST cease T-MAX seconds after the initial send as described
 in Section 4.3.
 This specification purposely does not specify any additional behavior
 for a disconnected endpoint.  Vendors MAY for instance choose to
 provide silence, play reorder tone, or even enable a downloaded wav
 file to be played.
 The default value for Tdinit is 15 seconds, the default value for
 Tdmin, is 15 seconds, and the default value for Tdmax is 600 seconds.

Andreasen & Foster Informational [Page 145] RFC 3435 MGCP 1.0 January 2003

4.4.8 Load Control in General

 The previous sections have described several MGCP mechanisms to deal
 with congestion and overload, namely:
  • the UDP retransmission strategy which adapts to network and call

agent congestion on a per endpoint basis,

  • the guidelines on the ordering of commands which limit the number

of commands issued in parallel,

  • the restart procedure which prevents flooding in case of a restart

avalanche, and

  • the disconnected procedure which prevents flooding in case of a

large number of disconnected endpoints.

 It is however still possible for a given set of endpoints, either on
 the same or different gateways, to issue one or more commands at a
 given point in time.  Although it can be argued, that Call Agents
 should be sized to handle one message per served endpoint at any
 given point in time, this may not always be the case in practice.
 Similarly, gateways may not be able to handle a message for all of
 its endpoints at any given point in time.  In general, such issues
 can be dealt with through the use of a credit-based mechanism, or by
 monitoring and automatically adapting to the observed behavior.  We
 opt for the latter approach as follows.
 Conceptually, we assume that Call Agents and gateways maintain a
 queue of incoming transactions to be executed.  Associated with this
 transaction queue is a high-water and a low-water mark.  Once the
 queue length reaches the high-water mark, the entity SHOULD start
 issuing 101 provisional responses (transaction queued) until the
 queue length drops to the low-water mark.  This applies to new
 transactions as well as to retransmissions.  If the entity is unable
 to process any new transactions at this time, it SHOULD return error
 code 409 (processing overload).
 Furthermore, gateways SHOULD adjust the sending rate of new commands
 to a given Call Agent by monitoring the observed response times from
 that Call Agent to a *set* of endpoints.  If the observed smoothed
 average response time suddenly rises significantly over some
 threshold, or the gateway receives a 101 (transaction queued) or 409
 (overload) response, the gateway SHOULD adjust the sending rate of
 new commands to that Call Agent accordingly.  The details of the
 smoothing average algorithm, the rate adjustments, and the thresholds
 involved are for further study, however they MUST be configurable.

Andreasen & Foster Informational [Page 146] RFC 3435 MGCP 1.0 January 2003

 Similarly, Call Agents SHOULD adjust the sending rate of new
 transactions to a given gateway by monitoring the observed response
 times from that gateway for a *set* of endpoints.  If the observed
 smoothed average response time suddenly rises significantly over some
 threshold, or the Call Agent receives a 101 (transaction queued) or
 409 (overloaded), the Call Agent SHOULD adjust the sending rate of
 new commands to that gateway accordingly.  The details of the
 smoothing average algorithm, the rate adjustments, and the thresholds
 involved are for further study, however they MUST be configurable.

5. Security Requirements

 Any entity can send a command to an MGCP endpoint.  If unauthorized
 entities could use the MGCP, they would be able to set-up
 unauthorized calls, or to interfere with authorized calls.  We expect
 that MGCP messages will always be carried over secure Internet
 connections, as defined in the IP security architecture as defined in
 RFC 2401, using either the IP Authentication Header, defined in RFC
 2402, or the IP Encapsulating Security Payload, defined in RFC 2406.
 The complete MGCP protocol stack would thus include the following
 layers:
  1. ——————————

| MGCP |

             |-------------------------------|
             |              UDP              |
             |-------------------------------|
             |          IP security          |
             | (authentication or encryption)|
             |-------------------------------|
             |              IP               |
             |-------------------------------|
             |       transmission media      |
              -------------------------------
 Adequate protection of the connections will be achieved if the
 gateways and the Call Agents only accept messages for which IP
 security provided an authentication service.  An encryption service
 will provide additional protection against eavesdropping, thus
 preventing third parties from monitoring the connections set up by a
 given endpoint.
 The encryption service will also be requested if the session
 descriptions are used to carry session keys, as defined in SDP.

Andreasen & Foster Informational [Page 147] RFC 3435 MGCP 1.0 January 2003

 These procedures do not necessarily protect against denial of service
 attacks by misbehaving gateways or misbehaving Call Agents.  However,
 they will provide an identification of these misbehaving entities,
 which should then be deprived of their authorization through
 maintenance procedures.

5.1 Protection of Media Connections

 MGCP allows Call Agent to provide gateways with "session keys" that
 can be used to encrypt the audio messages, protecting against
 eavesdropping.
 A specific problem of packet networks is "uncontrolled barge-in".
 This attack can be performed by directing media packets to the IP
 address and UDP port used by a connection.  If no protection is
 implemented, the packets will be decoded and the signals will be
 played on the "line side".
 A basic protection against this attack is to only accept packets from
 known sources, however this tends to conflict with RTP principles.
 This also has two inconveniences:  it slows down connection
 establishment and it can be fooled by source spoofing:
  • To enable the address-based protection, the Call Agent must obtain

the source address of the egress gateway and pass it to the ingress

   gateway.  This requires at least one network round trip, and leaves
   us with a dilemma:  either allow the call to proceed without
   waiting for the round trip to complete, and risk for example
   "clipping" a remote announcement, or wait for the full round trip
   and settle for slower call-set-up procedures.
  • Source spoofing is only effective if the attacker can obtain valid

pairs of source and destination addresses and ports, for example by

   listening to a fraction of the traffic.  To fight source spoofing,
   one could try to control all access points to the network.  But
   this is in practice very hard to achieve.
 An alternative to checking the source address is to encrypt and
 authenticate the packets, using a secret key that is conveyed during
 the call set-up procedure.  This will not slow down the call set-up,
 and provides strong protection against address spoofing.

6. Packages

 As described in Section 2.1.6, packages are the preferred way of
 extending MGCP.  In this section we describe the requirements
 associated with defining a package.

Andreasen & Foster Informational [Page 148] RFC 3435 MGCP 1.0 January 2003

 A package MUST have a unique package name defined.  The package name
 MUST be registered with the IANA, unless it starts with the
 characters "x-" or "x+" which are reserved for experimental packages.
 Please refer to Appendix C for IANA considerations.
 A package MUST also have a version defined which is simply a non-
 negative integer.  The default and initial version of a package is
 zero, the next version is one, etc.  New package versions MUST be
 completely backwards compatible, i.e., a new version of a package
 MUST NOT redefine or remove any of the extensions provided in an
 earlier version of the package.  If such a need arises, a new package
 name MUST be used instead.
 Packages containing signals of type time-out MAY indicate if the "to"
 parameter is supported for all the time-out signals in the package as
 well as the default rounding rules associated with these (see Section
 3.2.2.4).  If no such definition is provided, each time-out signal
 SHOULD provide these definitions.
 A package defines one or more of the following extensions:
  • Actions
  • BearerInformation
  • ConnectionModes
  • ConnectionParameters
  • DigitMapLetters
  • Events and Signals
  • ExtensionParameters
  • LocalConnectionOptions
  • ReasonCodes
  • RestartMethods
  • Return codes
 For each of the above types of extensions supported by the package,
 the package definition MUST contain a description of the extension as
 defined in the following sections.  Please note, that package
 extensions, just like any other extension, MUST adhere to the MGCP
 grammar.

Andreasen & Foster Informational [Page 149] RFC 3435 MGCP 1.0 January 2003

6.1 Actions

 Extension Actions SHALL include:
  • The name and encoding of the extension action.
  • If the extension action takes any action parameters, then the name,

encoding, and possible values of those parameters.

  • A description of the operation of the extension action.
  • A listing of the actions in this specification the extension can be

combined with. If such a listing is not provided, it is assumed

   that the extension action cannot be combined with any other action
   in this specification.
  • If more than one extension action is defined in the package, then a

listing of the actions in the package the extension can be combined

   with.  If such a listing is not provided, it is assumed that the
   extension action cannot be combined with any other action in the
   package.
 Extension actions defined in two or more different packages SHOULD
 NOT be used simultaneously, unless very careful consideration to
 their potential interaction and side-effects has been given.

6.2 BearerInformation

 BearerInformation extensions SHALL include:
  • The name and encoding of the BearerInformation extension.
  • The possible values and encoding of those values that can be

assigned to the BearerInformation extension.

  • A description of the operation of the BearerInformation extension.

As part of this description the default value (if any) if the

   extension is omitted in an EndpointConfiguration command MUST be
   defined.  It may be necessary to make a distinction between the
   default value before and after the initial application of the
   parameter, for example if the parameter retains its previous value
   once specified, until explicitly altered.  If default values are
   not described, then the extension parameter simply defaults to
   empty in all EndpointConfiguration commands.
 Note that the extension SHALL be included in the result for an
 AuditEndpoint command auditing the BearerInformation.

Andreasen & Foster Informational [Page 150] RFC 3435 MGCP 1.0 January 2003

6.3 ConnectionModes

 Extension Connection Modes SHALL include:
  • The name and encoding of the extension connection mode.
  • A description of the operation of the extension connection mode.
  • A description of the interaction a connection in the extension

connection mode will have with other connections in each of the

   modes defined in this specification.  If such a description is not
   provided, the extension connection mode MUST NOT have any
   interaction with other connections on the endpoint.
 Extension connection modes SHALL NOT be included in the list of modes
 in a response to an AuditEndpoint for Capabilities, since the package
 will be reported in the list of packages.

6.4 ConnectionParameters

 Extension Connection Parameters SHALL include:
  • The name and encoding of the connection parameter extension.
  • The possible values and encoding of those values that can be

assigned to the connection parameter extension.

  • A description of how those values are derived.
 Note that the extension connection parameter MUST be included in the
 result for an AuditConnection command auditing the connection
 parameters.

6.5 DigitMapLetters

 Extension Digit Map Letters SHALL include:
  • The name and encoding of the extension digit map letter(s).
  • A description of the meaning of the extension digit map letter(s).
 Note that extension DigitMapLetters in a digit map do not follow the
 normal naming conventions for extensions defined in packages.  More
 specifically the package name and slash ("/") will not be part of the
 extension name, thereby forming a flat and limited name space with
 potential name clashing.

Andreasen & Foster Informational [Page 151] RFC 3435 MGCP 1.0 January 2003

 Therefore, a package SHALL NOT define a digit map letter extension
 whose encoding has already been used in another package.  If two
 packages have used the same encoding for a digit map letter
 extension, and those two packages are supported by the same endpoint,
 the result of using that digit map letter extension is undefined.
 Note that although an extension DigitMapLetter does not include the
 package name prefix and slash ("/") as part of the extension name
 within a digit map, the package name prefix and slash are included
 when the event code for the event that matched the DigitMapLetter is
 reported as an observed event.  In other words, the digit map just
 define the matching rule(s), but the event is still reported like any
 other event.

6.6 Events and Signals

 The event/signal definition SHALL include the precise name of the
 event/signal (i.e., the code used in MGCP), a plain text definition
 of the event/signal, and, when appropriate, the precise definition of
 the corresponding events/signals, for example the exact frequencies
 of audio signals such as dial tones or DTMF tones.
 The package description MUST provide, for each event/signal, the
 following information:
  • The description of the event/signal and its purpose, which SHOULD

include the actual signal that is generated by the client (e.g., xx

   ms FSK tone) as well as the resulting user observed result (e.g.,
   Message Waiting light on/off).
 The event code used for the event/signal.
  • The detailed characteristics of the event/signal, such as for

example frequencies and amplitude of audio signals, modulations and

   repetitions.  Such details may be country specific.
  • The typical and maximum duration of the event/signal if applicable.
  • If the signal or event can be applied to a connection (across a

media stream), it MUST be indicated explicitly. If no such

   indication is provided, it is assumed that the signal or event
   cannot be applied to a connection.
 For events, the following MUST be provided as well:
  • An indication if the event is persistent. By default, events are

not persistent - defining events as being persistent is discouraged

   (see Appendix B for a preferred alternative).  Note that persistent

Andreasen & Foster Informational [Page 152] RFC 3435 MGCP 1.0 January 2003

   events will automatically trigger a Notify when they occur, unless
   the Call Agent explicitly instructed the endpoint otherwise.  This
   not only violates the normal MGCP model, but also assumes the Call
   Agent supports the package in question.  Such an assumption is
   unlikely to hold in general.
  • An indication if there is an auditable event-state associated with

the event. By default, events do not have auditable event-states.

  • If event parameters are supported, it MUST be stated explicitly.

The precise syntax and semantics of these MUST then be provided

   (subject to the grammar provided in Appendix A).  It SHOULD also be
   specified whether these parameters apply to RequestedEvents,
   ObservedEvents, DetectEvents and EventStates.  If not specified
   otherwise, it is assumed that:
  • they do not apply to RequestedEvents,
  • they do apply to ObservedEvents,
  • they apply in the same way to DetectEvents as they do to

RequestedEvents for a given event parameter,

  • they apply in the same way to EventStates as they do to

ObservedEvents for a given event parameter.

  • If the event is expected to be used in digit map matching, it

SHOULD explicitly state so. Note that only events with single

   letter or digit parameter codes can do this.  See Section 2.1.5 for
   further details.
 For signals, the following MUST be provided as well:
  • The type of signal (OO, TO, BR).
  • Time-Out signals SHOULD have an indication of the default time-out

value. In some cases, time-out values may be variable (if

   dependent on some action to complete such as out-pulsing digits).
  • If signal parameters are supported, it MUST be stated explicitly.

The precise syntax and semantics of these MUST then be provided

   (subject to the grammar provided in Appendix A).
  • Time-Out signals may also indicate whether the "to" parameter is

supported or not as well as what the rounding rules associated with

   them are.  If omitted from the signal definition, the package-wide
   definition is assumed (see Section 6).  If the package definition
   did not specify this, rounding rules default to the nearest non-

Andreasen & Foster Informational [Page 153] RFC 3435 MGCP 1.0 January 2003

   zero second, whereas support for the "to" parameter defaults to
   "no" for package version zero, and "yes" for package versions one
   and higher.
 The following format is RECOMMENDED for defining events and signals
 in conformance with the above:
  1. —————————————————————–

| Symbol | Definition | R | S Duration |

 |---------|----------------------------|-----|---------------------|
 |         |                            |     |                     |
 |         |                            |     |                     |
  ------------------------------------------------------------------
 where:
  • Symbol indicates the event code used for the event/signal, e.g.,

"hd".

  • Definition gives a brief definition of the event/signal
  • R contains an "x" if the event can be detected or one or more of

the following symbols:

  1. "P" if the event is persistent.
  1. "S" if the events is an event-state that may be audited.
  1. "C" if the event can be detected on a connection.
  • S contains one of the following if it is a signal:
  1. "OO" if the signal is On/Off signal.
  1. "TO" if the signal is a Time-Out signal.
  1. "BR" if the signal is a Brief signal.
  • S also contains:
  1. "C" if the signal can be applied on a connection.
 The table SHOULD then be followed by a more comprehensive description
 of each event/signal defined.

Andreasen & Foster Informational [Page 154] RFC 3435 MGCP 1.0 January 2003

6.6.1 Default and Reserved Events

 All packages that contain Time-Out type signals contain the operation
 failure ("of") and operation complete ("oc") events, irrespective of
 whether they are provided as part of the package description or not.
 These events are needed to support Time-Out signals and cannot be
 overridden in packages with Time-Out signals.  They MAY be extended
 if necessary, however such practice is discouraged.
 If a package without Time-Out signals does contain definitions for
 the "oc" and "of" events, the event definitions provided in the
 package MAY over-ride those indicated here.  Such practice is however
 discouraged and is purely allowed to avoid potential backwards
 compatibility problems.
 It is considered good practice to explicitly mention that the two
 events are supported in accordance with their default definitions,
 which are as follows:
  1. —————————————————————–

| Symbol | Definition | R | S Duration |

 |---------|----------------------------|-----|---------------------|
 | oc      | Operation Complete         |  x  |                     |
 | of      | Operation Failure          |  x  |                     |
  ------------------------------------------------------------------
 Operation complete (oc):  The operation complete event is generated
 when the gateway was asked to apply one or several signals of type TO
 on the endpoint or connection, and one or more of those signals
 completed without being stopped by the detection of a requested event
 such as off-hook transition or dialed digit.  The completion report
 should carry as a parameter the name of the signal that came to the
 end of its live time, as in:
    O: G/oc(G/rt)
 In this case, the observed event occurred because the "rt" signal in
 the "G" package timed out.
 If the reported signal was applied on a connection, the parameter
 supplied will include the name of the connection as well, as in:
    O: G/oc(G/rt@0A3F58)
 When the operation complete event is requested, it cannot be
 parameterized with any event parameters.  When the package name is
 omitted (which is discouraged) as part of the signal name, the
 default package is assumed.

Andreasen & Foster Informational [Page 155] RFC 3435 MGCP 1.0 January 2003

 Operation failure (of):  The operation failure event is generated
 when the endpoint was asked to apply one or several signals of type
 TO on the endpoint or connection, and one or more of those signals
 failed prior to timing out.  The completion report should carry as a
 parameter the name of the signal that failed, as in:
    O: G/of(G/rt)
 In this case a failure occurred in producing the "rt" signal in the
 "G" package.
 When the reported signal was applied on a connection, the parameter
 supplied will include the name of the connection as well, as in:
    O: G/of(G/rt@0A3F58)
 When the operation failure event is requested, event parameters can
 not be specified.  When the package name is omitted (which is
 discouraged), the default package name is assumed.

6.7 ExtensionParameters

 Extension parameter extensions SHALL include:
  • The name and encoding of the extension parameter.
  • The possible values and encoding of those values that can be

assigned to the extension parameter.

  • For each of the commands defined in this specification, whether the

extension parameter is Mandatory, Optional, or Forbidden in

   requests as well as responses.  Note that extension parameters
   SHOULD NOT normally be mandatory.
  • A description of the operation of the extension parameter. As part

of this description the default value (if any) if the extension is

   omitted in a command MUST be defined.  It may be necessary to make
   a distinction between the default value before and after the
   initial application of the parameter, for example if the parameter
   retains its previous value once specified, until explicitly
   altered.  If default values are not described, then the extension
   parameter simply defaults to empty in all commands.
  • Whether the extension can be audited in AuditEndpoint and/or

AuditConnection as well as the values returned. If nothing is

   specified, then auditing of the extension parameter can only be
   done for AuditEndpoint, and the value returned SHALL be the current
   value for the extension.  Note that this may be empty.

Andreasen & Foster Informational [Page 156] RFC 3435 MGCP 1.0 January 2003

6.8 LocalConnectionOptions

 LocalConnectionOptions extensions SHALL include:
  • The name and encoding of the LocalConnectionOptions extension.
  • The possible values and encoding of those values that can be

assigned to the LocalConnectionOptions extension.

  • A description of the operation of the LocalConnectionOptions

extension. As part of this description the following MUST be

   specified:
  1. The default value (if any) if the extension is omitted in a

CreateConnection command.

  1. The default value if omitted in a ModifyConnection command. This

may be to simply retain the previous value (if any) or to apply

     the default value.  If nothing is specified, the current value is
     retained if possible.
  1. If Auditing of capabilities will result in the extension being

returned, then a description to that effect as well as with what

     possible values and their encoding (note that the package itself
     will always be returned).  If nothing is specified, the extension
     SHALL NOT be returned when auditing capabilities.
 Also note, that the extension MUST be included in the result for an
 AuditConnection command auditing the LocalConnectionOptions.

6.9 Reason Codes

 Extension reason codes SHALL include:
  • The number for the reason code. The number MUST be in the range

800 to 899.

  • A description of the extension reason code including the

circumstances that leads to the generation of the reason code.

   Those circumstances SHOULD be limited to events caused by another
   extension defined in the package to ensure the recipient will be
   able to interpret the extension reason code correctly.
 Note that the extension reason code may have to be provided in the
 result for an AuditEndpoint command auditing the reason code.

Andreasen & Foster Informational [Page 157] RFC 3435 MGCP 1.0 January 2003

6.10 RestartMethods

 Extension Restart Methods SHALL include:
  • The name and encoding for the restart method.
  • A description of the restart method including the circumstances

that leads to the generation of the restart method. Those

   circumstances SHOULD be limited to events caused by another
   extension defined in the package to ensure the recipient will be
   able to interpret the extension restart method correctly.
  • An indication of whether the RestartDelay parameter is to be used

with the extension. If nothing is specified, it is assumed that it

   is not to be used.  In that case, RestartDelay MUST be ignored if
   present.
  • If the restart method defines a service state, the description MUST

explicitly state and describe this. In that case, the extension

   restart method can then be provided in the result for an
   AuditEndpoint command auditing the restart method.

6.11 Return Codes

 Extension Return Codes SHALL include:
  • The number for the extension return code. The number MUST be in

the range 800 to 899.

  • A description of the extension return code including the

circumstances that leads to the generation of the extension return

   code.  Those circumstances SHOULD be limited to events caused by
   another extension defined in the package to ensure the recipient
   will be able to interpret the extension return code correctly.

7. Versions and Compatibility

7.1 Changes from RFC 2705

 RFC 2705 was issued in October 1999, as the last update of draft
 version 0.5.  This updated document benefits from further
 implementation experience.  The main changes from RFC 2705 are:
  • Contains several clarifications, editorial changes and resolution

of known inconsistencies.

  • Firmed up specification language in accordance with RFC 2119 and

added RFC 2119 conventions section.

Andreasen & Foster Informational [Page 158] RFC 3435 MGCP 1.0 January 2003

  • Clarified behavior of mixed wild-carding in endpoint names.
  • Deleted naming requirement about having first term identify the

physical gateway when the gateway consists of multiple physical

   gateways.  Also added recommendations on wild-carding naming usage
   from the right only, as well as mixed wildcard usage.
  • Clarified that synonymous forms and values for endpoint names are

not freely interchangeable.

  • Allowed IPv6 addresses in endpoint names.
  • Clarified Digit Map matching rules.
  • Added missing semantics for symbols used in digit maps.
  • Added Timer T description in Digit Maps.
  • Added recommendation to support digit map sizes of at least 2048

bytes per endpoint.

  • Clarified use of wildcards in several commands.
  • Event and Signal Parameters formally defined for events and

signals.

  • Persistent events now allowed in base MGCP protocol.
  • Added additional detail on connection wildcards.
  • Clarified behavior of loopback, and continuity test connection

modes for mixing and multiple connections in those modes.

  • Modified BearerInformation to be conditional optional in the

EndpointConfiguration command.

  • Clarified "swap audio" action operation for one specific scenario

and noted that operation for other scenarios is undefined.

  • Added recommendation that all implementations support PCMU encoding

for interoperability.

  • Changed Bandwidth LocalConnectionOptions value from excluding to

including overhead from the IP layer and up for consistency with

   SDP.
  • Clarified that mode of second connection in a CreateConnection

command will be set to "send/receive".

Andreasen & Foster Informational [Page 159] RFC 3435 MGCP 1.0 January 2003

  • Type of service default changed to zero.
  • Additional detail on echo cancellation, silence suppression, and

gain control. Also added recommendation for Call Agents not to

   specify handling of echo cancellation and gain control.
  • Added requirement for a connection to have a

RemoteConnectionDescriptor in order to use the "network loopback"

   and "network continuity test" modes.
  • Removed procedures and specification for NAS's (will be provided as

package instead).

  • Removed procedures and specification for ATM (will be provided as

package instead).

  • Added missing optional NotifiedEntity parameter to the

DeleteConnection (from the Call Agent) MGCI command.

  • Added optional new MaxMGCPDatagram RequestedInfo code for

AuditEndpoint to enable auditing of maximum size of MGCP datagrams

   supported.
  • Added optional new PackageList RequestedInfo code for AuditEndpoint

to enable auditing of packages with a package version number.

   PackageList parameter also allowed with return code 518
   (unsupported package).
  • Added missing attributes in Capabilities.
  • Clarified that at the expiration of a non-zero restart delay, an

updated RestartInProgress should be sent. Also clarified that a

   new NotifiedEntity can only be returned in response to a
   RestartInProgress command.
  • Added Response Acknowledgement response (return code 000) and

included in three-way handshake.

  • ResponseAck parameter changed to be allowed in all commands.
  • Added return codes 101, 405, 406, 407, 409, 410, 503, 504, 505,

506, 507, 508, 509, 533, 534, 535, 536, 537, 538, 539, 540, 541,

   and defined return codes in range 800-899 to be package specific
   return codes.  Additional text provided for some return codes and
   additional detail on how to handle unknown return codes added.
  • Added reason code 903, 904, 905 and defined reason codes 800-899 to

be package specific reason codes.

Andreasen & Foster Informational [Page 160] RFC 3435 MGCP 1.0 January 2003

  • Added section clarifying codec negotiation procedure.
  • Clarified that resource reservation parameters in a

ModifyConnection command defaults to the current value used.

  • Clarified that connection mode is optional in ModifyConnection

commands.

  • Corrected LocalConnectionDescriptor to be optional in response to

CreateConnection commands (in case of failure).

  • Clarified that quoted-strings are UTF-8 encoded and

interchangeability of quoted strings and unquoted strings.

  • Clarified that Transaction Identifiers are compared as numerical

values.

  • Clarified bit-ordering for Type Of Service LocalConnectionOptions.
  • Clarified the use of RequestIdentifier zero.
  • Added example sections for commands, responses, and some call

flows.

  • Corrected usage of and requirements for SDP to be strictly RFC 2327

compliant.

  • Added requirement that all MGCP implementations must support MGCP

datagrams up to at least 4000 bytes. Also added new section on

   Maximum Datagram Size, Fragmentation and reassembly.
  • Generalized piggybacking retransmission scheme to only state

underlying requirements to be satisfied.

  • Clarified the section on computing retransmission timers.
  • Clarified operation of long-running transactions, including

provisional responses, retransmissions and failures.

  • Enhanced description of provisional responses and interaction with

three-way handshake.

  • Enhanced description of fail-over and the role of "notified

entity". An empty "notified entity" has been allowed, although

   strongly discouraged.

Andreasen & Foster Informational [Page 161] RFC 3435 MGCP 1.0 January 2003

  • Clarified retransmission procedure and removed "wrong key"

considerations from it. Also fixed inconsistencies between Max1

   and Max2 retransmission boundaries and the associated flow diagram.
  • Updated domain name resolution for retransmission procedure to

incur less overhead when multiple endpoints are retransmitting.

  • Removed requirement for in-order delivery of NotificationRequests

response and Notify commands. Notify commands are still delivered

   in-order.
  • Clarified that activating an embedded Notification Request does not

clear the list of ObservedEvents.

  • Defined interactions between disconnected state and notification

state.

  • Added section on transactional semantics.
  • Defined gateway behavior when multiple interacting transactions are

received.

  • Additional details provided on service states. Clarified

relationship between endpoint service states, restart methods, and

   associated processing of commands.
  • Clarified operation for transitioning from "restart procedure" to

"disconnected state".

  • Allowed auditing commands and responses to bypass the "restart" and

"disconnected" procedures.

  • Clarified operation of "disconnected procedure" and in particular

the operation of piggy-backed "disconnected" RestartInProgress

   messages.
  • Added option to aggregate "disconnected" RestartInProgress messages

under certain conditions to reduce message volume.

  • Defined additional behavior for endpoints wishing to send commands

while in the "disconnected" state.

  • Added new section on Load Control in General which includes two new

error codes (101 and 409) to handle overload.

  • Deleted the "Proposed MoveConnection command".

Andreasen & Foster Informational [Page 162] RFC 3435 MGCP 1.0 January 2003

  • Removed packages from protocol specification (will be provided in

separate documents instead).

  • Package concept formally extended to be primary extension mechanism

now allowing extensions for the following to be defined in packages

   as well:
  1. BearerInformation
  1. LocalConnectionOptions
  1. ExtensionParameters
  1. Connection Modes
  1. Actions
  1. Digit Map Letters
  1. Connection Parameters
  1. Restart Methods
  1. Reason Codes
  1. Return Codes
  • Requirements and suggested format for package definitions added.
  • Defined "operation complete" and "operation failure" events to be

automatically present in packages with Time-Out signals.

  • Deleted list of differences that were prior to RFC 2705.
  • Added Base Package to deal with quarantine buffer overflow,

ObservedEvents overflow, embedded NotificationRequest failure, and

   to enable events to be requested persistently.  A new "Message"
   command is included as well.
  • IANA registration procedures for packages and other extensions

added.

  • Updated grammar to fix known errors and support new extensions in a

backwards compatible manner. Added new (optional) PackageList and

   MaxMGCPDatagram for auditing.  Changed explicit white space rules
   in some productions to make grammar more consistent.
  • Connection Mode interaction table added.

Andreasen & Foster Informational [Page 163] RFC 3435 MGCP 1.0 January 2003

  • Added additional detail on virtual endpoint naming conventions.

Also added suggested gateway endpoint convention and a "Range

   Wildcard" option to the Endpoint Naming Conventions.

8. Security Considerations

 Security issues are discussed in section 5.

9. Acknowledgements

 Special thanks are due to the authors of the original MGCP 1.0
 specification:  Mauricio Arango, Andrew Dugan, Isaac Elliott,
 Christian Huitema, and Scott Picket.
 We also want to thank the many reviewers who provided advice on the
 design of SGCP and then MGCP, notably Sankar Ardhanari, Francois
 Berard, David Auerbach, Bob Biskner, David Bukovinsky, Charles Eckel,
 Mario Edini, Ed Guy, Barry Hoffner, Jerry Kamitses, Oren Kudevitzki,
 Rajesh Kumar, Troy Morley, Dave Oran, Jeff Orwick, John Pickens, Lou
 Rubin, Chip Sharp, Paul Sijben, Kurt Steinbrenner, Joe Stone, and
 Stuart Wray.
 The version 0.1 of MGCP was heavily inspired by the "Internet
 Protocol Device Control" (IPDC) designed by the Technical Advisory
 Committee set up by Level 3 Communications.  Whole sets of text were
 retrieved from the IP Connection Control protocol, IP Media Control
 protocol, and IP Device Management.  The authors wish to acknowledge
 the contribution to these protocols made by Ilya Akramovich, Bob
 Bell, Dan Brendes, Peter Chung, John Clark, Russ Dehlinger, Andrew
 Dugan, Isaac Elliott, Cary FitzGerald, Jan Gronski, Tom Hess, Geoff
 Jordan, Tony Lam, Shawn Lewis, Dave Mazik, Alan Mikhak, Pete
 O'Connell, Scott Pickett, Shyamal Prasad, Eric Presworsky, Paul
 Richards, Dale Skran, Louise Spergel, David Sprague, Raj Srinivasan,
 Tom Taylor and Michael Thomas.

10. References

 [1]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.
 [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [3]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
      "RTP:  A Transport Protocol for Real-Time Applications", RFC
      1889, January 1996.

Andreasen & Foster Informational [Page 164] RFC 3435 MGCP 1.0 January 2003

 [4]  Schulzrinne, H., "RTP Profile for Audio and Video Conferences
      with Minimal Control", RFC 1890, January 1996.
 [5]  Handley, M. and V. Jacobson, "SDP: Session Description
      Protocol", RFC 2327, April 1998.
 [6]  Handley, M., Perkins, C. and E. Whelan, "Session Announcement
      Protocol", RFC 2974, October 2000.
 [7]  Rosenberg, J., Camarillo, G., Johnston, A., Peterson, J.,
      Sparks, R., Handley, M., Schulzrinne, H. and E. Schooler,
      "Session Initiation Protocol (SIP)", RFC 3261, June 2002.
 [8]  Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
      Protocol (RTSP)", RFC 2326, April 1998.
 [9]  ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE ISDN
      USER PART OF SIGNALING SYSTEM No. 7", (Malaga-Torremolinos,
      1984; modified at Helsinki, 1993).
 [10] ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND
      SIGNALS OF THE ISDN USER PART OF SIGNALING SYSTEM No. 7",
      (MalagaTorremolinos, 1984; modified at Helsinki, 1993).
 [11] ITU-T, Recommendation H.323 (02/98), "PACKET-BASED MULTIMEDIA
      COMMUNICATIONS SYSTEMS".
 [12] ITU-T, Recommendation H.225, "Call Signaling Protocols and Media
      Stream Packetization for Packet Based Multimedia Communications
      Systems".
 [13] ITU-T, Recommendation H.245 (02/98), "CONTROL PROTOCOL FOR
      MULTIMEDIA COMMUNICATION".
 [14] Kent, S. and R. Atkinson, "Security Architecture for the
      Internet Protocol", RFC 2401, November 1998.
 [15] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
      November 1998.
 [16] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
      (ESP)", RFC 2406, November 1998.
 [17] Crocker, D. and P. Overell, "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, November 1997.
 [18] Stevens, W. Richard, "TCP/IP Illustrated, Volume 1, The
      Protocols", Addison-Wesley, 1994.

Andreasen & Foster Informational [Page 165] RFC 3435 MGCP 1.0 January 2003

 [19] Allman, M., Paxson, V. "On Estimating End-to-End Network Path
      Properties", Proc. SIGCOMM'99, 1999.
 [20] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
      2279, January 1998.
 [21] Braden, R., "Requirements for Internet Hosts -- Communication
      Layers", STD 3, RFC 1122, October 1989.
 [22] Bellcore, "LSSGR: Switching System Generic Requirements for Call
      Control Using the Integrated Services Digital Network User Part
      (ISDNUP)", GR-317-CORE, Issue 2, December 1997.
 [23] Narten, T., and Alvestrand H., "Guidelines for Writing an IANA
      Considerations Section in RFCs", RFC 2434, October 1998.

Andreasen & Foster Informational [Page 166] RFC 3435 MGCP 1.0 January 2003

Appendix A: Formal Syntax Description of the Protocol

 In this section, we provide a formal description of the protocol
 syntax, following the "Augmented BNF for Syntax Specifications"
 defined in RFC 2234.  The syntax makes use of the core rules defined
 in RFC 2234, Section 6.1, which are not included here.  Furthermore,
 the syntax follows the case-sensitivity rules of RFC 2234, i.e., MGCP
 is case-insensitive (but SDP is not).  It should be noted, that ABNF
 does not provide for implicit specification of linear white space and
 MGCP messages MUST thus follow the explicit linear white space rules
 provided in the grammar below.  However, in line with general
 robustness principles, implementers are strongly encouraged to
 tolerate additional linear white space in messages received.

MGCPMessage = MGCPCommand / MGCPResponse

MGCPCommand = MGCPCommandLine 0*(MGCPParameter) [EOL *SDPinformation]

MGCPCommandLine = MGCPVerb 1*(WSP) transaction-id 1*(WSP)

                      endpointName 1*(WSP) MGCPversion EOL

MGCPVerb = "EPCF" / "CRCX" / "MDCX" / "DLCX" / "RQNT"

       / "NTFY" / "AUEP" / "AUCX" / "RSIP" / extensionVerb

extensionVerb = ALPHA 3(ALPHA / DIGIT) ; experimental starts with X

transaction-id = 1*9(DIGIT)

endpointName = LocalEndpointName "@" DomainName LocalEndpointName = LocalNamePart 0*("/" LocalNamePart) LocalNamePart = AnyName / AllName / NameString AnyName = "$" AllName = "*" NameString = 1*(range-of-allowed-characters) ; VCHAR except "$", "*", "/", "@" range-of-allowed-characters = %x21-23 / %x25-29 / %x2B-2E

                           / %x30-3F / %x41-7E

DomainName = 1*255(ALPHA / DIGIT / "." / "-") ; as defined

         / "#" number                          ; in RFC 821
         / "[" IPv4address / IPv6address "]"   ; see RFC 2373

; Rewritten to ABNF from RFC 821 number = 1*DIGIT

;From RFC 2373 IPv6address = hexpart [ ":" IPv4address ] IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT

Andreasen & Foster Informational [Page 167] RFC 3435 MGCP 1.0 January 2003

; this production, while occurring in RFC2373, is not referenced ; IPv6prefix = hexpart "/" 1*2DIGIT hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ] hexseq = hex4 *( ":" hex4) hex4 = 1*4HEXDIG

MGCPversion = "MGCP" 1*(WSP) 1*(DIGIT) "." 1*(DIGIT)

                          [1*(WSP) ProfileName]

ProfileName = VCHAR *( WSP / VCHAR)

MGCPParameter = ParameterValue EOL

; Check infoCode if more parameter values defined ; Most optional values can only be omitted when auditing ParameterValue = ("K" ":" 0*(WSP) [ResponseAck])

             / ("B"  ":" 0*(WSP)  [BearerInformation])
             / ("C"  ":" 0*(WSP)  CallId)
             / ("I"  ":" 0*(WSP)  [ConnectionId])
             / ("N"  ":" 0*(WSP)  [NotifiedEntity])
             / ("X"  ":" 0*(WSP)  [RequestIdentifier])
             / ("L"  ":" 0*(WSP)  [LocalConnectionOptions])
             / ("M"  ":" 0*(WSP)  ConnectionMode)
             / ("R"  ":" 0*(WSP)  [RequestedEvents])
             / ("S"  ":" 0*(WSP)  [SignalRequests])
             / ("D"  ":" 0*(WSP)  [DigitMap])
             / ("O"  ":" 0*(WSP)  [ObservedEvents])
             / ("P"  ":" 0*(WSP)  [ConnectionParameters])
             / ("E"  ":" 0*(WSP)  ReasonCode)
             / ("Z"  ":" 0*(WSP)  [SpecificEndpointID])
             / ("Z2" ":" 0*(WSP)  SecondEndpointID)
             / ("I2" ":" 0*(WSP)  SecondConnectionID)
             / ("F"  ":" 0*(WSP)  [RequestedInfo])
             / ("Q"  ":" 0*(WSP)  QuarantineHandling)
             / ("T"  ":" 0*(WSP)  [DetectEvents])
             / ("RM" ":" 0*(WSP)  RestartMethod)
             / ("RD" ":" 0*(WSP)  RestartDelay)
             / ("A"  ":" 0*(WSP)  [Capabilities])
             / ("ES" ":" 0*(WSP)  [EventStates])
             / ("PL" ":" 0*(WSP)  [PackageList])    ; Auditing only
             / ("MD" ":" 0*(WSP)  MaxMGCPDatagram)  ; Auditing only
             / (extensionParameter ":" 0*(WSP) [parameterString])

; A final response may include an empty ResponseAck ResponseAck = confirmedTransactionIdRange

  • ( "," 0*(WSP) confirmedTransactionIdRange )

confirmedTransactionIdRange = transaction-id ["-" transaction-id]

Andreasen & Foster Informational [Page 168] RFC 3435 MGCP 1.0 January 2003

BearerInformation = BearerAttribute 0*("," 0*(WSP) BearerAttribute) BearerAttribute = ("e" ":" BearerEncoding)

                / (BearerExtensionName [":" BearerExtensionValue])

BearerExtensionName = PackageLCOExtensionName BearerExtensionValue = LocalOptionExtensionValue BearerEncoding = "A" / "mu"

CallId = 1*32(HEXDIG)

; The audit request response may include a list of identifiers ConnectionId = 1*32(HEXDIG) 0*("," 0*(WSP) 1*32(HEXDIG)) SecondConnectionID = ConnectionId

NotifiedEntity = [LocalName "@"] DomainName [":" portNumber] LocalName = LocalEndpointName ; No internal structure

portNumber = 1*5(DIGIT)

RequestIdentifier = 1*32(HEXDIG)

LocalConnectionOptions = LocalOptionValue 0*(WSP)

                         0*("," 0*(WSP) LocalOptionValue 0*(WSP))

LocalOptionValue = ("p" ":" packetizationPeriod)

               / ("a"  ":" compressionAlgorithm)
               / ("b"  ":" bandwidth)
               / ("e"  ":" echoCancellation)
               / ("gc" ":" gainControl)
               / ("s"  ":" silenceSuppression)
               / ("t"  ":" typeOfService)
               / ("r"  ":" resourceReservation)
               / ("k"  ":" encryptiondata)
               / ("nt" ":" ( typeOfNetwork /
                                  supportedTypeOfNetwork))
               / (LocalOptionExtensionName
                       [":" LocalOptionExtensionValue])

Capabilities = CapabilityValue 0*(WSP)

                   0*("," 0*(WSP) CapabilityValue 0*(WSP))

CapabilityValue = LocalOptionValue

              / ("v" ":" supportedPackages)
              / ("m" ":" supportedModes)

PackageList = pkgNameAndVers 0*("," pkgNameAndVers) pkgNameAndVers = packageName ":" packageVersion packageVersion = 1*(DIGIT)

packetizationPeriod = 1*4(DIGIT) ["-" 1*4(DIGIT)] compressionAlgorithm = algorithmName 0*(";" algorithmName)

Andreasen & Foster Informational [Page 169] RFC 3435 MGCP 1.0 January 2003

algorithmName = 1*(SuitableLCOCharacter) bandwidth = 1*4(DIGIT) ["-" 1*4(DIGIT)] echoCancellation = "on" / "off" gainControl = "auto" / ["-"] 1*4(DIGIT) silenceSuppression = "on" / "off" typeOfService = 1*2(HEXDIG) ; 1 hex only for capabilities resourceReservation = "g" / "cl" / "be"

;encryption parameters are coded as in SDP (RFC 2327) ;NOTE: encryption key may contain an algorithm as specified in RFC 1890 encryptiondata = ( "clear" ":" encryptionKey )

             / ( "base64" ":" encodedEncryptionKey )
             / ( "uri" ":" URItoObtainKey )
             / ( "prompt" ) ; defined in SDP, not usable in MGCP!

encryptionKey = 1*(SuitableLCOCharacter) / quotedString ; See RFC 2045 encodedEncryptionKey = 1*(ALPHA / DIGIT / "+" / "/" / "=") URItoObtainKey = 1*(SuitableLCOCharacter) / quotedString

typeOfNetwork = "IN" / "ATM" / "LOCAL" / OtherTypeOfNetwork ; Registered with IANA - see RFC 2327 OtherTypeOfNetwork = 1*(SuitableLCOCharacter) supportedTypeOfNetwork = typeOfNetwork *(";" typeOfNetwork)

supportedModes = ConnectionMode 0*(";" ConnectionMode)

supportedPackages = packageName 0*(";" packageName)

packageName = 1*(ALPHA / DIGIT / HYPHEN) ; Hyphen neither first or last

LocalOptionExtensionName = VendorLCOExtensionName

                       / PackageLCOExtensionName
                       / OtherLCOExtensionName

VendorLCOExtensionName = "x" ("+"/"-") 1*32(SuitableExtLCOCharacter) PackageLCOExtensionName = packageName "/"

                          1*32(SuitablePkgExtLCOCharacter)

; must not start with "x-" or "x+" OtherLCOExtensionName = 1*32(SuitableExtLCOCharacter)

LocalOptionExtensionValue = (1*(SuitableExtLCOValChar)

                                                  / quotedString)
                            *(";" (1*(SuitableExtLCOValChar)
                                                    / quotedString))

;Note: No "data" mode. ConnectionMode = "sendonly" / "recvonly" / "sendrecv"

             / "confrnce" / "inactive" / "loopback"

Andreasen & Foster Informational [Page 170] RFC 3435 MGCP 1.0 January 2003

             / "conttest" / "netwloop" / "netwtest"
             / ExtensionConnectionMode

ExtensionConnectionMode = PkgExtConnectionMode PkgExtConnectionMode = packageName "/" 1*(ALPHA / DIGIT)

RequestedEvents = requestedEvent 0*("," 0*(WSP) requestedEvent) requestedEvent = (eventName ["(" requestedActions ")"])

              / (eventName "(" requestedActions ")"
                                     "(" eventParameters ")" )

eventName = [(packageName / "*") "/"]

              (eventId / "all" / eventRange
                                      / "*" / "#") ; for DTMF
                            ["@" (ConnectionId / "$" / "*")]

eventId = 1*(ALPHA / DIGIT / HYPHEN) ; Hyphen neither first nor last eventRange = "[" 1*(DigitMapLetter / (DIGIT "-" DIGIT) /

                      (DTMFLetter "-" DTMFLetter)) "]"

DTMFLetter = "A" / "B" / "C" / "D"

requestedActions = requestedAction 0*("," 0*(WSP) requestedAction) requestedAction = "N" / "A" / "D" / "S" / "I" / "K"

               / "E" "(" EmbeddedRequest ")"
               / ExtensionAction

ExtensionAction = PackageExtAction PackageExtAction = packageName "/" Action ["(" ActionParameters ")"] Action = 1*ALPHA ActionParameters = eventParameters ; May contain actions

;NOTE: Should tolerate different order when receiving, e.g., for NCS. EmbeddedRequest = ( "R" "(" EmbeddedRequestList ")"

                  ["," 0*(WSP) "S" "(" EmbeddedSignalRequest ")"]
                  ["," 0*(WSP) "D" "(" EmbeddedDigitMap ")"]      )
              / (      "S" "(" EmbeddedSignalRequest ")"
                  ["," 0*(WSP) "D" "(" EmbeddedDigitMap ")"] )
              / (      "D" "(" EmbeddedDigitMap ")" )

EmbeddedRequestList = RequestedEvents EmbeddedSignalRequest = SignalRequests EmbeddedDigitMap = DigitMap

SignalRequests = SignalRequest 0*("," 0*(WSP) SignalRequest ) SignalRequest = eventName [ "(" eventParameters ")" ]

eventParameters = eventParameter 0*("," 0*(WSP) eventParameter) eventParameter = eventParameterValue

               / eventParameterName "=" eventParameter
               / eventParameterName "(" eventParameters ")"

eventParameterString = 1*(SuitableEventParamCharacter) eventParameterName = eventParameterString

Andreasen & Foster Informational [Page 171] RFC 3435 MGCP 1.0 January 2003

eventParameterValue = eventParameterString / quotedString

DigitMap = DigitString / "(" DigitStringList ")" DigitStringList = DigitString 0*( "|" DigitString ) DigitString = 1*(DigitStringElement) DigitStringElement = DigitPosition ["."] DigitPosition = DigitMapLetter / DigitMapRange ; NOTE "X" is now included DigitMapLetter = DIGIT / "#" / "*" / "A" / "B" / "C" / "D" / "T"

                 / "X" / ExtensionDigitMapLetter

ExtensionDigitMapLetter = "E" / "F" / "G" / "H" / "I" / "J" / "K"

                      / "L" / "M" / "N" / "O" / "P" / "Q" / "R"
                      / "S" / "U" / "V" / "W" / "Y" / "Z"

; NOTE "[x]" is now allowed DigitMapRange = "[" 1*DigitLetter "]" DigitLetter = *1)

1)
DIGIT "-" DIGIT) / DigitMapLetter) ObservedEvents = SignalRequests EventStates = SignalRequests ConnectionParameters = ConnectionParameter
                      0*( "," 0*(WSP) ConnectionParameter )
ConnectionParameter = ( "PS" "=" packetsSent )
                   / ( "OS" "=" octetsSent )
                   / ( "PR" "=" packetsReceived )
                   / ( "OR" "=" octetsReceived )
                   / ( "PL" "=" packetsLost )
                   / ( "JI" "=" jitter )
                   / ( "LA" "=" averageLatency )
                   / ( ConnectionParameterExtensionName
                            "=" ConnectionParameterExtensionValue )
packetsSent = 1*9(DIGIT) octetsSent = 1*9(DIGIT) packetsReceived = 1*9(DIGIT) octetsReceived = 1*9(DIGIT) packetsLost = 1*9(DIGIT) jitter = 1*9(DIGIT) averageLatency = 1*9(DIGIT) ConnectionParameterExtensionName = VendorCPExtensionName
                               /    PackageCPExtensionName
VendorCPExtensionName = "X" "-" 2*ALPHA PackageCPExtensionName = packageName "/" CPName CPName = 1*(ALPHA / DIGIT / HYPHEN) ConnectionParameterExtensionValue = 1*9(DIGIT) Andreasen & Foster Informational [Page 172] RFC 3435 MGCP 1.0 January 2003 MaxMGCPDatagram = 1*9(DIGIT) ReasonCode = 3DIGIT
           [1*(WSP) "/" packageName]     ; Only for 8xx
           [WSP 1*(%x20-7E)]
SpecificEndpointID = endpointName SecondEndpointID = endpointName RequestedInfo = infoCode 0*("," 0*(WSP) infoCode) infoCode = "B" / "C" / "I" / "N" / "X" / "L" / "M" / "R" / "S"
       / "D" / "O" / "P" / "E" / "Z" / "Q" / "T" / "RC" / "LC"
       / "A" / "ES" / "RM" / "RD" / "PL" / "MD" / extensionParameter
QuarantineHandling = loopControl / processControl
                 / (loopControl "," 0*(WSP) processControl )
loopControl = "step" / "loop" processControl = "process" / "discard" DetectEvents = SignalRequests RestartMethod = "graceful" / "forced" / "restart" / "disconnected"
            / "cancel-graceful" / extensionRestartMethod
extensionRestartMethod = PackageExtensionRM PackageExtensionRM = packageName "/" 1*32(ALPHA / DIGIT / HYPHEN) RestartDelay = 1*6(DIGIT) extensionParameter = VendorExtensionParameter
                 / PackageExtensionParameter
                 / OtherExtensionParameter
VendorExtensionParameter = "X" ("-"/"+") 1*6(ALPHA / DIGIT) PackageExtensionParameter = packageName "/"
                          1*32(ALPHA / DIGIT / HYPHEN)
; must not start with "x-" or x+" OtherExtensionParameter = 1*32(ALPHA / DIGIT / HYPHEN) ;If first character is a double-quote, then it is a quoted-string parameterString = (%x21 / %x23-7F) *(%x20-7F) ; first and last must not
                                            ; be white space
                  / quotedString
MGCPResponse = MGCPResponseLine 0*(MGCPParameter)
  • 2(EOL *SDPinformation)
MGCPResponseLine = responseCode 1*(WSP) transaction-id
                      [1*(WSP) "/" packageName]    ; Only for 8xx
                           [WSP responseString] EOL
Andreasen & Foster Informational [Page 173] RFC 3435 MGCP 1.0 January 2003 responseCode = 3DIGIT responseString = *(%x20-7E) SuitablePkgExtLCOCharacter = SuitableLCOCharacter SuitableExtLCOCharacter = DIGIT / ALPHA / "+" / "-" / "_" / "&"
            / "!" / "'" / "|" / "=" / "#" / "?"
            / "." / "$" / "*" /       "@" / "[" / "]"
            / "^" / "`" / "{" / "}" / "~"
SuitableLCOCharacter = SuitableExtLCOCharacter / "/" SuitableExtLCOValChar = SuitableLCOCharacter / ":" ; VCHAR except """, "(", ")", ",", and "=" SuitableEventParamCharacter = %x21 / %x23-27 / %x2A-2B
                          / %x2D-3C / %x3E-7E
; NOTE: UTF8 encoded quotedString = DQUOTE 0*(quoteEscape / quoteChar) DQUOTE quoteEscape = DQUOTE DQUOTE quoteChar = (%x00-21 / %x23-FF) EOL = CRLF / LF HYPHEN = "-" ; See RFC 2327 for proper SDP grammar instead. SDPinformation = SDPLine CRLF *(SDPLine CRLF) ; see RFC 2327 SDPLine = 1*(%x01-09 / %x0B / %x0C / %x0E-FF) ; for proper def. Andreasen & Foster Informational [Page 174] RFC 3435 MGCP 1.0 January 2003 Appendix B: Base Package
 Package name: B
 Version: 0
 The MGCP specification defines a base package which contains a set of
 events and extension parameters that are of general use to the
 protocol.  Although not required, it is highly RECOMMENDED to support
 this package as it provides important functionality for the base
 protocol.
B.1 Events
 The table below lists the events:
  1. —————————————————————–
| Symbol | Definition | R | S Duration |
 |---------|----------------------------|-----|---------------------|
 | enf(##) | embedded RQNT failure      |  x  |                     |
 | oef     | observed events full       |  x  |                     |
 | qbo     | quarantine buffer overflow |  x  |                     |
  ------------------------------------------------------------------
 The events are defined as follows:
 Embedded NotificationRequest failure (enf):
   The Embedded NotificationRequest Failure (enf) event is generated
   when an embedded Notification Request failure occurs.  When the
   event is requested, it should be as part of the Embedded
   NotificationRequest itself.  When the event is reported, it may be
   parameterized with an error code (see Section 2.4) detailing the
   error that occurred.  When requested, it cannot be parameterized.
 Observed events full (oef):
   The event is generated when the endpoint is unable to accumulate
   any more events in the list of ObservedEvents.  If this event
   occurs, and it is not used to trigger a Notify, subsequent events
   that should have been added to the list will be lost.
 Quarantine buffer overflow (qbo):
   The event is generated when the quarantine buffer overflows and one
   or more events have been lost.
Andreasen & Foster Informational [Page 175] RFC 3435 MGCP 1.0 January 2003 B.2 Extension Parameters B.2.1 PersistentEvents
 PersistentEvents:  A list of events that the gateway is requested to
 detect and report persistently.  The parameter is optional but can be
 provided in any command where the DetectEvents parameter can be
 provided.  The initial default value of the parameter is empty.  When
 the parameter is omitted from a command, it retains its current
 value.  When the parameter is provided, it completely replaces the
 current value.  Providing an event in this list, is similar (but
 preferable) to defining that particular event as being persistent.
 The current list of PersistentEvents will implicitly apply to the
 current as well as subsequent NotificationRequests, however no glare
 detection etc. will be performed (similarly to DetectEvents).  If an
 event provided in this list is included in a RequestedEvents list,
 the action and event parameters used in the RequestedEvents will
 replace the action and event parameters associated with the event in
 the PersistentEvents list for the life of the RequestedEvents list,
 after which the PersistentEvents action and event parameters are
 restored.  Events with event states requested through this parameter
 will be included in the list of EventStates if audited.
 PersistentEvents can also be used to detect events on connections.
 Use of the "all connections" wildcard is straightforward, whereas
 using PersistentEvents with one or more specific connections must be
 considered carefully.  Once the connection in question is deleted, a
 subsequent NotificationRequest without a new PersistentEvents value
 will fail (error code 515 - incorrect connection-id, is RECOMMENDED),
 as it implicitly refers to the deleted connection.
 The parameter generates the relevant error codes from the base
 protocol, e.g., error code 512 if an unknown event is specified.
 The PersistentEvents parameter can be audited, in which case it will
 return its current value.  Auditing of RequestedEvents is not
 affected by this extension, i.e., events specified in this list are
 not automatically reported when auditing RequestedEvents.
 The parameter name for PersistentEvents is "PR" and it is defined by
 the production:
   PersistentEvents = "PR" ":" 0*WSP  [RequestedEvents]
Andreasen & Foster Informational [Page 176] RFC 3435 MGCP 1.0 January 2003
 The following example illustrates the use of the parameter:
   B/PR: L/hd(N), L/hf(N), L/hu(N), B/enf, B/oef, B/qbo
 which instructs the endpoint to persistently detect and report off-
 hook, hook-flash, and on-hook.  It also instructs the endpoint to
 persistently detect and report Embedded Notification Request failure,
 Observed events full, and Quarantine buffer overflow.
B.2.2 NotificationState
 NotificationState is a RequestedInfo parameter that can be audited
 with the AuditEndpoint command.  It can be used to determine if the
 endpoint is in the notification state or not.
 The parameter is forbidden in any command.  In responses, it is a
 valid response parameter for AuditEndpoint only.
 It is defined by the following grammar:
   NotificationState        = "NS" ":" 0*WSP NotificationStateValue
   NotificationStateValue   = "ns" / "ls" / "o"
 It is requested as part of auditing by including the parameter code
 in RequestedInfo, as in:
   F: B/NS
 The response parameter will contain the value "ns" if the endpoint is
 in the "notification state", the value "ls" if the endpoint is in the
 "lockstep state" (i.e., waiting for an RQNT after a response to a
 NTFY has been received when operating in "step" mode), or the value
 "o" otherwise, as for example:
   B/NS: ns
B.3 Verbs
 MGCP packages are not intended to define new commands, however an
 exception is made in this case in order to add an important general
 capability currently missing, namely the ability for the gateway to
 send a generic message to the Call Agent.
 The definition of the new command is:
        ReturnCode
        <-- Message(EndpointId
                       [, ...])
Andreasen & Foster Informational [Page 177] RFC 3435 MGCP 1.0 January 2003
 EndpointId is the name for the endpoint(s) in the gateway which is
 issuing the Message command.  The identifier MUST be a fully
 qualified endpoint identifier, including the domain name of the
 gateway.  The local part of the endpoint name MUST NOT use the "any
 of" wildcard.
 The only parameter specified in the definition of the Message command
 is the EndpointId, however, it is envisioned that extensions will
 define additional parameters to be used with the Message command.
 Such extensions MUST NOT alter or otherwise interfere with the normal
 operation of the basic MGCP protocol.  They may however define
 additional capabilities above and beyond that provided by the basic
 MGCP protocol.  For example, an extension to enable the gateway to
 audit the packages supported by the Call Agent could be defined,
 whereas using the Message command as an alternative way of reporting
 observed events would be illegal, as that would alter the normal MGCP
 protocol behavior.
 In order to not interfere with normal MGCP operation, lack of a
 response to the Message command MUST NOT lead the endpoint to become
 disconnected.  The endpoint(s) MUST be prepared to handle this
 transparently and continue normal processing unaffected.
 If the endpoint(s) receive a response indicating that the Call Agent
 does not support the Message command, the endpoint(s) MUST NOT send a
 Message command again until the current "notified entity" has
 changed.  Similarly, if the endpoint(s) receive a response indicating
 that the Call Agent does not support one or more parameters in the
 Message command, the endpoint(s) MUST NOT send a Message command with
 those parameters again until the current "notified entity" has
 changed.
 The Message command is encoded as MESG, as shown in the following
 example:
    MESG 1200 aaln/1@rgw.whatever.net MGCP 1.0
Andreasen & Foster Informational [Page 178] RFC 3435 MGCP 1.0 January 2003 Appendix C: IANA Considerations C.1 New MGCP Package Sub-Registry
 The IANA has established a new sub-registry for MGCP packages under
 http://www.iana.org/assignments/mgcp-packages.
 Packages can be registered with the IANA according to the following
 procedure:
 The package MUST have a unique string name which MUST NOT start with
 the two characters "x-" or "x+".
 The package title, name, and version (zero assumed by default) MUST
 be registered with IANA as well as a reference to the document that
 describes the package.  The document MUST have a stable URL and MUST
 be contained on a public web server.
 Packages may define one or more Extension Digit Map Letters, however
 these are taken from a limited and flat name space.  To prevent name
 clashing, IANA SHALL NOT register a package that defines an Extension
 Digit Map Letter already defined in another package registered by
 IANA.  To ease this task, such packages SHALL contain the line
 "Extension Digit Map Letters:  " followed by a list of the Extension
 Digit Map Letters defined in the package at the beginning of the
 package definition.
 A contact name, e-mail and postal address for the package MUST be
 provided.  The contact information SHALL be updated by the defining
 organization as necessary.
 Finally, prior to registering a package, the IANA MUST have a
 designated expert [23] review the package. The expert reviewer will
 send e-mail to the IANA on the overall review determination.
C.2 New MGCP Package
 This document defines a new MGCP Base Package in Appendix B, which
 has been registered by IANA.
C.3 New MGCP LocalConnectionOptions Sub-Registry
 The IANA has established a new sub-registry for MGCP
 LocalConnectionOptions under http://www.iana.org/assignments/mgcp-
 localconnectionoptions.
Andreasen & Foster Informational [Page 179] RFC 3435 MGCP 1.0 January 2003
 Packages are the preferred extension mechanism, however for backwards
 compatibility, local connection options beyond those provided in this
 specification can be registered with IANA.  Each such local
 connection option MUST have a unique string name which MUST NOT start
 with "x-" or "x+".  The local connection option field name and
 encoding name MUST be registered with IANA as well as a reference to
 the document that describes the local connection option.  The
 document MUST have a stable URL and MUST be contained on a public web
 server.
 A contact name, e-mail and postal address for the local connection
 option MUST be provided.  The contact information SHALL be updated by
 the defining organization as necessary.
 Finally, prior to registering a LocalConnectionOption, the IANA MUST
 have a designated expert [23] review the LocalConnectionOption. The
 expert reviewer will send e-mail to the IANA on the overall review
 determination.
Appendix D: Mode Interactions
 An MGCP endpoint can establish one or more media streams.  These
 streams are either incoming (from a remote endpoint) or outgoing
 (generated at the handset microphone).  The "connection mode"
 parameter establishes the direction and generation of these streams.
 When there is only one connection to an endpoint, the mapping of
 these streams is straightforward; the handset plays the incoming
 stream over the handset speaker and generates the outgoing stream
 from the handset microphone signal, depending on the mode parameter.
 However, when several connections are established to an endpoint,
 there can be many incoming and outgoing streams.  Depending on the
 connection mode used, these streams may interact differently with
 each other and the streams going to/from the handset.
 The table below describes how different connections SHALL be mixed
 when one or more connections are concurrently "active".  An active
 connection is here defined as a connection that is in one of the
 following modes:
  • "send/receive"
  • "send only"
  • "receive only"
  • "conference"
 Connections in "network loopback", "network continuity test", or
 "inactive" modes are not affected by connections in the "active"
 modes.  The Table uses the following conventions:
Andreasen & Foster Informational [Page 180] RFC 3435 MGCP 1.0 January 2003
  • Ai is the incoming media stream from Connection A
  • Bi is the incoming media stream from Connection B
  • Hi is the incoming media stream from the Handset Microphone
  • Ao is the outgoing media stream to Connection A
  • Bo is the outgoing media stream to Connection B
  • Ho is the outgoing media stream to the Handset earpiece
  • NA indicates no stream whatsoever (assuming there are no signals
applied on the connection)
 "netw" in the following table indicates either "netwloop" or
 "netwtest" mode.
  1. ————————————————————
| | Connection A Mode |
  |       |-----------------------------------------------------
  |       |sendonly|recvonly|sendrecv|confrnce|inactive|  netw  |
  |-------|-----------------------------------------------------|
  | |Send | Ao=Hi  | Ao=NA  | Ao=Hi  | Ao=Hi  | Ao=NA  | Ao=Ai  |
  |C|only | Bo=Hi  | Bo=Hi  | Bo=Hi  | Bo=Hi  | Bo=Hi  | Bo=Hi  |
  |o|     | Ho=NA  | Ho=Ai  | Ho=Ai  | Ho=Ai  | Ho=NA  | Ho=NA  |
  |n|-----------------------------------------------------------
  |n|recv |        |Ao=NA   |Ao=Hi   |Ao=Hi   | Ao=NA  | Ao=Ai  |
  |e|only |        |Bo=NA   |Bo=NA   |Bo=NA   | Bo=NA  | Bo=NA  |
  |c|     |        |Ho=Ai+Bi|Ho=Ai+Bi|Ho=Ai+Bi| Ho=Bi  | Ho=Bi  |
  |t|-----------------------------------------------------------|
  |i|send |        |        |Ao=Hi   |Ao=Hi   | Ao=NA  | Ao=Ai  |
  |o|recv |        |        |Bo=Hi   |Bo=Hi   | Bo=Hi  | Bo=Hi  |
  |n|     |        |        |Ho=Ai+Bi|Ho=Ai+Bi| Ho=Bi  | Ho=Bi  |
  | |-----------------------------------------------------------|
  |B|conf |        |        |        |Ao=Hi+Bi| Ao=NA  | Ao=Ai  |
  | |rnce |        |        |        |Bo=Hi+Ai| Bo=Hi  | Bo=Hi  |
  |M|     |        |        |        |Ho=Ai+Bi| Ho=Bi  | Ho=Bi  |
  |o|-----------------------------------------------------------|
  |d|Inac |        |        |        |        | Ao=NA  | Ao=Ai  |
  |e|tive |        |        |        |        | Bo=NA  | Bo=NA  |
  | |     |        |        |        |        | Ho=NA  | Ho=NA  |
  | |-----------------------------------------------------------|
  | |netw |        |        |        |        |        | Ao=Ai  |
  | |     |        |        |        |        |        | Bo=Bi  |
  | |     |        |        |        |        |        | Ho=NA  |
   -------------------------------------------------------------
 If there are three or more "active" connections they will still
 interact as defined in the table above with the outgoing media
 streams mixed for each interaction (union of all streams).  If
 internal resources are used up and the streams cannot be mixed, the
 gateway MUST return an error (error code 403 or 502, not enough
 resources, are RECOMMENDED).
Andreasen & Foster Informational [Page 181] RFC 3435 MGCP 1.0 January 2003 Appendix E: Endpoint Naming Conventions
 The following sections provide some RECOMMENDED endpoint naming
 conventions.
E.1 Analog Access Line Endpoints
 The string "aaln", should be used as the first term in a local
 endpoint name for analog access line endpoints.  Terms following
 "aaln" should follow the physical hierarchy of the gateway so that if
 the gateway has a number of RJ11 ports, the local endpoint name could
 look like the following:
    aaln/#
 where "#" is the number of the analog line (RJ11 port) on the
 gateway.
 On the other hand, the gateway may have a number of physical plug-in
 units, each of which contain some number of RJ11 ports, in which
 case, the local endpoint name might look like the following:
    aaln/<unit #>/#
 where <unit #> is the number of the plug in unit in the gateway and
 "#" is the number of the analog line (RJ11 port) on that unit.
 Leading zeroes MUST NOT be used in any of the numbers ("#") above.
E.2 Digital Trunks
 The string "ds" should be used for the first term of digital
 endpoints with a naming convention that follows the physical and
 digital hierarchy such as:
    ds/<unit-type1>-<unit #>/<unit-type2>-<unit #>/.../<channel #>
 where:  <unit-type> identifies the particular hierarchy level.  Some
 example values of <unit-type> are:  "s", "su", "oc3", "ds3", "e3",
 "ds2", "e2", "ds1", "e1" where "s" indicates a slot number and "su"
 indicates a sub-unit within a slot.  Leading zeroes MUST NOT be used
 in any of the numbers ("#") above.
 The <unit #> is a decimal number which is used to reference a
 particular instance of a <unit-type> at that level of the hierarchy.
 The number of levels and naming of those levels is based on the
 physical hierarchy within the media gateway.
Andreasen & Foster Informational [Page 182] RFC 3435 MGCP 1.0 January 2003 E.3 Virtual Endpoints
 Another type of endpoint is one that is not associated with a
 physical interface (such as an analog or digital endpoint).  This
 type of endpoint is called a virtual endpoint and is often used to
 represent some DSP resources that gives the endpoint some capability.
 Examples are announcement, IVR or conference bridge devices.  These
 devices may have multiple instances of DSP functions so that a
 possible naming convention is:
    <virtual-endpoint-type>/<endpoint-#>
 where <virtual-endpoint-type> may be some string representing the
 type of endpoint (such as "ann" for announcement server or "cnf" for
 conference server) and <endpoint-#> would identify a particular
 virtual endpoint within the device.  Leading zeroes MUST NOT be used
 in the number ("#") above.  If the physical hierarchy of the server
 includes plug-in DSP cards, another level of hierarchy in the local
 endpoint name may be used to describe the plug in unit.
 A virtual endpoint may be created as the result of using the "any of"
 wildcard.  Similarly, a virtual endpoint may cease to exist once the
 last connection on the virtual endpoint is deleted.  The definition
 of the virtual endpoint MUST detail both of these aspects.
 When a <virtual-endpoint-type> creates and deletes virtual endpoints
 automatically, there will be cases where no virtual endpoints exist
 at the time a RestartInProgress command is to be issued.  In such
 cases, the gateway SHOULD simply use the "all of" wildcard in lieu of
 any specific <endpoint-#> as in, e.g.:
   ann/*@mygateway.whatever.net
 If the RestartInProgress command refers to all endpoints in the
 gateway (virtual or not), the <virtual-endpoint-id> can be omitted as
 in, e.g.:
  • @mygateway.whatever.net
 Commands received by the gateway will still have to refer to an
 actual endpoint (possibly created by that command by use of the "any
 of" wildcard) in order for the command to be processed though.
Andreasen & Foster Informational [Page 183] RFC 3435 MGCP 1.0 January 2003 E.4 Media Gateway
 MGCP only defines operation on endpoints in a media gateway.  It may
 be beneficial to define an endpoint that represents the gateway
 itself as opposed to the endpoints managed by the gateway.
 Implementations that wish to do so should use the local endpoint name
 "mg" (for media gateway) as in:
   mg@mygateway.whatever.net
 Note that defining such an endpoint does not change any of the
 protocol semantics, i.e., the "mg" endpoint and other endpoints
 (e.g., digital trunks) in the gateway are still independent endpoints
 and MUST be treated as such.  For example, RestartInProgress commands
 MUST still be issued for all endpoints in the gateway as usual.
E.5 Range Wildcards
 As described in Section 2.1.2, the MGCP endpoint naming scheme
 defines the "all of" and "any of" wildcards for the individual terms
 in a local endpoint name.  While the "all of" wildcard is very useful
 for reducing the number of messages, it can by definition only be
 used when we wish to refer to all instances of a given term in the
 local endpoint name.  Furthermore, in the case where a command is to
 be sent by the gateway to the Call Agent, the "all of" wildcard can
 only be used if all of the endpoints named by it have the same
 "notified entity".  Implementations that prefer a finer-grained
 wildcarding scheme can use the range wildcarding scheme described
 here.
 A range wildcard is defined as follows:
 RangeWildcard    = "[" NumericalRange *( "," NumericalRange ) "]"
 NumericalRange   = 1*(DIGIT) [ "-" 1*(DIGIT) ]
 Note that white space is not permitted.  Also, since range wildcards
 use the character "[" to indicate the start of a range, the "["
 character MUST NOT be used in endpoint names that use range
 wildcards.  The length of a range wildcard SHOULD be bounded to a
 reasonably small value, e.g., 128 characters.
 Range wildcards can be used anywhere an "all of" wildcard can be
 used.  The semantics are identical for the endpoints named.  However,
 it MUST be noted, that use of the range wildcarding scheme requires
 support on both the gateway and the Call Agent.  Therefore, a gateway
 MUST NOT assume that it's Call Agent supports range wildcarding and
 vice versa.  In practice, this typically means that both the gateway
 and Call Agent will need to be provisioned consistently in order to
Andreasen & Foster Informational [Page 184] RFC 3435 MGCP 1.0 January 2003
 use range wildcards.  Also, if a gateway or Call Agent using range
 wildcards receives an error response that could indicate a possible
 endpoint naming problem, they MUST be able to automatically revert to
 not using range wildcards.
 The following examples illustrates the use of range wildcards:
    ds/ds1-1/[1-12]
    ds/ds1-1/[1,3,20-24]
    ds/ds1-[1-2]/*
    ds/ds3-1/[1-96]
 The following example illustrates how to use it in a command:
    RSIP 1204 ds/ds3-1/[1-96]@tgw-18.whatever.net MGCP 1.0
    RM: restart
    RD: 0
Appendix F: Example Command Encodings
 This appendix provides examples of commands and responses shown with
 the actual encoding used.  Examples are provided for each command.
 All commentary shown in the commands and responses is optional.
F.1 NotificationRequest
 The first example illustrates a NotificationRequest that will ring a
 phone and look for an off-hook event:
    RQNT 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0
    N: ca@ca1.whatever.net:5678
    X: 0123456789AC
    R: l/hd(N)
    S: l/rg
 The response indicates that the transaction was successful:
    200 1201 OK
 The second example illustrates a NotificationRequest that will look
 for and accumulate an off-hook event, and then provide dial-tone and
 accumulate digits according to the digit map provided.  The "notified
 entity" is set to "ca@ca1.whatever.net:5678", and since the
 SignalRequests parameter is empty (it could have been omitted as
 well), all currently active TO signals will be stopped.  All events
 in the quarantine buffer will be processed, and the list of events to
 detect in the "notification" state will include fax tones in addition
 to the "requested events" and persistent events:
Andreasen & Foster Informational [Page 185] RFC 3435 MGCP 1.0 January 2003
    RQNT 1202 aaln/1@rgw-2567.whatever.net MGCP 1.0
    N: ca@ca1.whatever.net:5678
    X: 0123456789AC
    R: L/hd(A, E(S(L/dl),R(L/oc, L/hu, D/[0-9#*T](D))))
    D: (0T|00T|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
    S:
    Q: process
    T: G/ft
 The response indicates that the transaction was successful:
    200 1202 OK
F.2 Notify
 The example below illustrates a Notify message that notifies an off-
 hook event followed by a 12-digit number beginning with "91".  A
 transaction identifier correlating the Notify with the
 NotificationRequest it results from is included.  The command is sent
 to the current "notified entity", which typically will be the actual
 value supplied in the NotifiedEntity parameter, i.e.,
 "ca@ca1.whatever.net:5678" - a failover situation could have changed
 this:
    NTFY 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0
    N: ca@ca1.whatever.net:5678
    X: 0123456789AC
    O: L/hd,D/9,D/1,D/2,D/0,D/1,D/8,D/2,D/9,D/4,D/2,D/6,D/6
 The Notify response indicates that the transaction was successful:
    200 2002 OK
F.3 CreateConnection
 The first example illustrates a CreateConnection command to create a
 connection on the endpoint specified.  The connection will be part of
 the specified CallId.  The LocalConnectionOptions specify that G.711
 mu-law will be the codec used and the packetization period will be 10
 ms.  The connection mode will be "receive only":
    CRCX 1204 aaln/1@rgw-2567.whatever.net MGCP 1.0
    C: A3C47F21456789F0
    L: p:10, a:PCMU
    M: recvonly
Andreasen & Foster Informational [Page 186] RFC 3435 MGCP 1.0 January 2003
 The response indicates that the transaction was successful, and a
 connection identifier for the newly created connection is therefore
 included.  A session description for the new connection is included
 as well - note that it is preceded by an empty line.
    200 1204 OK
    I: FDE234C8
    v=0
    o=- 25678 753849 IN IP4 128.96.41.1
    s=-
    c=IN IP4 128.96.41.1
    t=0 0
    m=audio 3456 RTP/AVP 0
 The second example illustrates a CreateConnection command containing
 a notification request and a RemoteConnectionDescriptor:
    CRCX 1205 aaln/1@rgw-2569.whatever.net MGCP 1.0
    C: A3C47F21456789F0
    L: p:10, a:PCMU
    M: sendrecv
    X: 0123456789AD
    R: L/hd
    S: L/rg
    v=0
    o=- 25678 753849 IN IP4 128.96.41.1
    s=-
    c=IN IP4 128.96.41.1
    t=0 0
    m=audio 3456 RTP/AVP 0
 The response indicates that the transaction failed, because the phone
 was already off-hook.  Consequently, neither a connection-id nor a
 session description is returned:
    401 1205 Phone off-hook
 Our third example illustrates the use of the provisional response and
 the three-way handshake.  We create another connection and
 acknowledge the previous response received by using the response
 acknowledgement parameter:
Andreasen & Foster Informational [Page 187] RFC 3435 MGCP 1.0 January 2003
    CRCX 1206 aaln/1@rgw-2569.whatever.net MGCP 1.0
    K: 1205
    C: A3C47F21456789F0
    L: p:10, a:PCMU
    M: inactive
    v=0
    o=- 25678 753849 IN IP4 128.96.41.1
    s=-
    c=IN IP4 128.96.41.1
    t=0 0
    m=audio 3456 RTP/AVP 0
 A provisional response is returned initially:
    100 1206 Pending
    I: DFE233D1
    v=0
    o=- 4723891 7428910 IN IP4 128.96.63.25
    s=-
    c=IN IP4 128.96.63.25
    t=0 0
    m=audio 3456 RTP/AVP 0
 A little later, the final response is received:
    200 1206 OK
    K:
    I: DFE233D1
    v=0
    o=- 4723891 7428910 IN IP4 128.96.63.25
    s=-
    c=IN IP4 128.96.63.25
    t=0 0
    m=audio 3456 RTP/AVP 0
 The Call Agent acknowledges the final response as requested:
    000 1206
 and the transaction is complete.
Andreasen & Foster Informational [Page 188] RFC 3435 MGCP 1.0 January 2003 F.4 ModifyConnection
 The first example shows a ModifyConnection command that simply sets
 the connection mode of a connection to "send/receive" - the "notified
 entity" is set as well:
    MDCX 1209 aaln/1@rgw-2567.whatever.net MGCP 1.0
    C: A3C47F21456789F0
    I: FDE234C8
    N: ca@ca1.whatever.net
    M: sendrecv
 The response indicates that the transaction was successful:
    200 1209 OK
 In the second example, we pass a session description and include a
 notification request with the ModifyConnection command.  The endpoint
 will start playing ring-back tones to the user:
    MDCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
    C: A3C47F21456789F0
    I: FDE234C8
    M: recvonly
    X: 0123456789AE
    R: L/hu
    S: G/rt
    v=0
    o=- 4723891 7428910 IN IP4 128.96.63.25
    s=-
    c=IN IP4 128.96.63.25
    t=0 0
    m=audio 3456 RTP/AVP 0
 The response indicates that the transaction was successful:
    200 1206 OK
F.5 DeleteConnection (from the Call Agent)
 In this example, the Call Agent simply instructs the gateway to
 delete the connection "FDE234C8" on the endpoint specified:
    DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
    C: A3C47F21456789F0
    I: FDE234C8
Andreasen & Foster Informational [Page 189] RFC 3435 MGCP 1.0 January 2003
 The response indicates success, and that the connection was deleted.
 Connection parameters for the connection are therefore included as
 well:
    250 1210 OK
    P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48
F.6 DeleteConnection (from the gateway)
 In this example, the gateway sends a DeleteConnection command to the
 Call Agent to instruct it that a connection on the specified endpoint
 has been deleted.  The ReasonCode specifies the reason for the
 deletion, and Connection Parameters for the connection are provided
 as well:
    DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
    C: A3C47F21456789F0
    I: FDE234C8
    E: 900 - Hardware error
    P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48
 The Call Agent sends a success response to the gateway:
    200 1210 OK
F.7 DeleteConnection (multiple connections from the Call Agent)
 In the first example, the Call Agent instructs the gateway to delete
 all connections related to call "A3C47F21456789F0" on the specified
 endpoint:
    DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
    C: A3C47F21456789F0
 The response indicates success and that the connection(s) were
 deleted:
    250 1210 OK
 In the second example, the Call Agent instructs the gateway to delete
 all connections related to all of the endpoints specified:
    DLCX 1210 aaln/*@rgw-2567.whatever.net MGCP 1.0
 The response indicates success:
    250 1210 OK
Andreasen & Foster Informational [Page 190] RFC 3435 MGCP 1.0 January 2003 F.8 AuditEndpoint
 In the first example, the Call Agent wants to learn what endpoints
 are present on the gateway specified, hence the use of the "all of"
 wild-card for the local portion of the endpoint-name:
    AUEP 1200 *@rgw-2567.whatever.net MGCP 1.0
 The gateway indicates success and includes a list of endpoint names:
    200 1200 OK
    Z: aaln/1@rgw-2567.whatever.net
    Z: aaln/2@rgw-2567.whatever.net
 In the second example, the capabilities of one of the endpoints is
 requested:
    AUEP 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0
    F: A
 The response indicates success and the capabilities as well.  Two
 codecs are supported, however with different capabilities.
 Consequently two separate capability sets are returned:
    200 1201 OK
    A: a:PCMU, p:10-100, e:on, s:off, v:L;S, m:sendonly;
             recvonly;sendrecv;inactive;netwloop;netwtest
    A: a:G729, p:30-90, e:on, s:on, v:L;S, m:sendonly;
             recvonly;sendrecv;inactive;confrnce;netwloop
 Note that the carriage return in the Capabilities lines are shown for
 formatting reasons only - they are not permissible in a real
 implementation.
 In the third example, the Call Agent audits several types of
 information for the endpoint:
    AUEP 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0
    F: R,D,S,X,N,I,T,O,ES
Andreasen & Foster Informational [Page 191] RFC 3435 MGCP 1.0 January 2003
 The response indicates success:
    200 2002 OK
    R: L/hu,L/oc(N),D/[0-9](N)
    D:
    S: L/vmwi(+)
    X: 0123456789B1
    N: [128.96.41.12]
    I: 32F345E2
    T: G/ft
    O: L/hd,D/9,D/1,D/2
    ES: L/hd
 The list of requested events contains three events.  Where no package
 name is specified, the default package is assumed.  The same goes for
 actions, so the default action - Notify - must therefore be assumed
 for the "L/hu" event.  The omission of a value for the "digit map"
 means the endpoint currently does not have a digit map.  There are
 currently no active time-out signals, however the OO signal "vmwi" is
 currently on and is consequently included - in this case it was
 parameterized, however the parameter could have been excluded.  The
 current "notified entity" refers to an IP-address and only a single
 connection exists for the endpoint.  The current value of
 DetectEvents is "G/ft", and the list of ObservedEvents contains the
 four events specified.  Finally, the event-states audited reveals
 that the phone was off-hook at the time the transaction was
 processed.
F.9 AuditConnection
 The first example shows an AuditConnection command where we audit the
 CallId, NotifiedEntity, LocalConnectionOptions, Connection Mode,
 LocalConnectionDescriptor, and the Connection Parameters:
    AUCX 2003 aaln/1@rgw-2567.whatever.net MGCP 1.0
    I: 32F345E2
    F: C,N,L,M,LC,P
Andreasen & Foster Informational [Page 192] RFC 3435 MGCP 1.0 January 2003
 The response indicates success and includes information for the
 RequestedInfo:
    200 2003 OK
    C: A3C47F21456789F0
    N: ca@ca1.whatever.net
    L: p:10, a:PCMU
    M: sendrecv
    P: PS=395, OS=22850, PR=615, OR=30937, PL=7, JI=26, LA=47
    v=0
    o=- 4723891 7428910 IN IP4 128.96.63.25
    s=-
    c=IN IP4 128.96.63.25
    t=0 0
    m=audio 1296 RTP/AVP 0
 In the second example, we request to audit RemoteConnectionDescriptor
 and LocalConnectionDescriptor:
    AUCX 1203 aaln/2@rgw-2567.whatever.net MGCP 1.0
    I: FDE234C8
    F: RC,LC
 The response indicates success, and includes information for the
 RequestedInfo.  In this case, no RemoteConnectionDescriptor exists,
 hence only the protocol version field is included for the
 RemoteConnectionDescriptor:
    200 1203 OK
    v=0
    o=- 4723891 7428910 IN IP4 128.96.63.25
    s=-
    c=IN IP4 128.96.63.25
    t=0 0
    m=audio 1296 RTP/AVP 0
    v=0
F.10 RestartInProgress
 The first example illustrates a RestartInProgress message sent by an
 gateway to inform the Call Agent that the specified endpoint will be
 taken out-of-service in 300 seconds:
Andreasen & Foster Informational [Page 193] RFC 3435 MGCP 1.0 January 2003
    RSIP 1200 aaln/1@rgw-2567.whatever.net MGCP 1.0
    RM: graceful
    RD: 300
 The Call Agent's response indicates that the transaction was
 successful:
    200 1200 OK
 In the second example, the RestartInProgress message sent by the
 gateway informs the Call Agent, that all of the gateway's endpoints
 are being placed in-service in 0 seconds, i.e., they are currently in
 service.  The restart delay could have been omitted as well:
    RSIP 1204 *@rgw-2567.whatever.net MGCP 1.0
    RM: restart
    RD: 0
 The Call Agent's response indicates success, and furthermore provides
 the endpoints in question with a new "notified entity":
    200 1204 OK
    N: CA-1@whatever.net
 Alternatively, the command could have failed with a new "notified
 entity" as in:
    521 1204 OK
    N: CA-1@whatever.net
 In that case, the command would then have to be retried in order to
 satisfy the "restart procedure", this time going to Call Agent "CA-
 1@whatever.net".
Appendix G: Example Call Flows
 The message flow tables in this section use the following
 abbreviations:
  • rgw = Residential Gateway
  • ca = Call Agent
  • n+ = step 'n' is repeated one or more times
Andreasen & Foster Informational [Page 194] RFC 3435 MGCP 1.0 January 2003
 Note that any use of upper and lower case within the text of the
 messages is to aid readability and is not in any way a requirement.
 The only requirement involving case is to be case insensitive at all
 times.
G.1 Restart G.1.1 Residential Gateway Restart
 The following table shows a message sequence that might occur when a
 call agent (ca) is contacted by two independent residential gateways
 (rgw1 and rgw2) which have restarted.
                Table F.1: Residential Gateway Restart

step# usr1 rgw1 ca rgw2 usr2
================================================================
1 rsip →
← ack
—–———————————————————–
2 ← auep
ack →
—–———————————————————–
3+ ← rqnt
ack →
—–———————————————————–
4 ← rsip
ack →
—–———————————————————–
5 auep →
← ack
—–———————————————————–
6+ rqnt →
← ack
———————————————————————
 Step 1 - RestartInProgress (rsip) from rgw1 to ca
 rgw1 uses DNS to determine the domain name of ca and send to the
 default port of 2727.  The command consists of the following:
    rsip 1 *@rgw1.whatever.net mgcp 1.0
    rm: restart
Andreasen & Foster Informational [Page 195] RFC 3435 MGCP 1.0 January 2003
 The "*" is used to inform ca that all endpoints of rgw1 are being
 restarted, and "restart" is specified as the restart method.  The
 Call Agent "ca" acknowledges the command with an acknowledgement
 message containing the transaction-id (in this case 1) for the
 command.  It sends the acknowledgement to rgw1 using the same port
 specified as the source port for the rsip.  If none was indicated, it
 uses the default port of 2727.
    200 1 ok
 A response code is mandatory.  In this case, "200", indicates "the
 requested transaction was executed normally".  The response string is
 optional.  In this case, "ok" is included as an additional
 description.
 Step 2 - AuditEndpoint (auep) from ca to rgw1
 The command consists of the following:
    auep 153 *@rgw1.whatever.net mgcp 1.0
 The "*" is used to request audit information from rgw1 of all its
 endpoints.  rgw1 acknowledges the command with an acknowledgement
 message containing the transaction-id (in this case 153) of the
 command, and it includes a list of its endpoints.  In this example,
 rgw1 has two endpoints, aaln/1 and aaln/2.
    200 153 ok
    Z: aaln/1@rgw1.whatever.net
    Z: aaln/2@rgw1.whatever.net
 Once it has the list of endpoint ids, ca may send individual
 AuditEndpoint commands in which the "*" is replaced by the id of the
 given endpoint.  As its response, rgw1 would replace the endpoint id
 list returned in the example with the info requested for the
 endpoint.  This optional message exchange is not shown in this
 example.
 Step 3 - NotificationRequest (rqnt) from ca to each endpoint of rgw1
 In this case, ca sends two rqnts, one for aaln/1:
    rqnt 154 aaln/1@rgw1.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 3456789a0
Andreasen & Foster Informational [Page 196] RFC 3435 MGCP 1.0 January 2003
 and a second for aaln/2:
    rqnt 155 aaln/2@rgw1.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 3456789a1
 Note that in the requested events parameter line, the event is fully
 specified as "l/hd", i.e., with the package name, in order to avoid
 any potential ambiguity.  This is the recommended behavior.  For the
 sake of clarity, the action, which in this case is to Notify, is
 explicitly specified by including the "(n)".  If no action is
 specified, Notify is assumed as the default regardless of the event.
 If any other action is desired, it must be stated explicitly.
 The expected response from rgw1 to these requests is an
 acknowledgement from aaln/1 as follows:
    200 154 ok
 and from aaln/2:
    200 155 ok
 Step 4 RestartInProgress (rsip) from rgw2 to ca
    rsip 0 *@rgw2.whatever.net mgcp 1.0
    rm: restart
 followed by the acknowledgement from ca:
    200 0 ok
 Step 5 - AuditEndpoint (auep) from ca to rgw2
    auep 156 *@rgw2.whatever.net mgcp 1.0
 followed by an acknowledgement from rgw2:
    200 156 ok
    z: aaln/1@rgw2.whatever.net
    z: aaln/2@rgw2.whatever.net
 Step 6 - NotificationRequest (rqnt) from ca to each endpoint of rgw2
    rqnt 157 aaln/1@rgw2.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 3456789a2
Andreasen & Foster Informational [Page 197] RFC 3435 MGCP 1.0 January 2003
 followed by:
    rqnt 158 aaln/2@rgw2.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 3456789a3
 with rgw2 acknowledging for aaln/1:
    200 157 ok
 and for aaln/2:
    200 158 ok
G.1.2 Call Agent Restart
 The following table shows the message sequence which occurs when a
 call agent (ca) restarts.  How it determines the address information
 of the gateways, in this case rgw1 and rgw2, is not covered in this
 document.  For interoperability, it is RECOMMENDED to provide the
 ability to configure the call agent to send AUEP (*) to specific
 addresses and ports.
                Table F.2: Residential Gateway Restart

# usr1 rgw1 ca rgw2 usr2
================================================================
1 ← auep
ack →
————-————————————————
2+ ← rqnt
ack →
————-————————————————
3 auep →
← ack
————-————————————————
4+ rqnt →
← ack
———————————————————————
 Step 1 - AuditEndpoint (auep) from ca to rgw1
 The command consists of the following:
    auep 0 *@rgw1.whatever.net mgcp 1.0
Andreasen & Foster Informational [Page 198] RFC 3435 MGCP 1.0 January 2003
 The "*" is used to request audit information from rgw1 of all its
 endpoints.  rgw1 acknowledges the command with an acknowledgement
 message containing the transaction id (in this case 0) of the
 command, and it includes a list of its endpoints.  In this example,
 rgw1 has two endpoints, aaln/1 and aaln/2.
    200 0 ok
    z: aaln/1@rgw1.whatever.net
    z: aaln/2@rgw1.whatever.net
 Once it has the list of endpoint ids, ca may send individual
 AuditEndpoint commands in which the "*" is replaced by the id of the
 given endpoint.  As its response, rgw1 would replace the endpoint id
 list returned in the example with the info requested for the
 endpoint.  This optional message exchange is not shown in this
 example.
 Step 2 - NotificationRequest (rqnt) off-hook from ca to rgw1
 In this case, ca sends two rqnts, one for aaln/1:
    rqnt 1 aaln/1@rgw1.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 234567890
 and a second for aaln/2:
    rqnt 2 aaln/2@rgw1.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 234567891
 The expected response from rgw1 to these requests is an
 acknowledgement from aaln/1 as follows:
    200 1 ok
 and from aaln/2:
    200 2 ok
 Step 3 - AuditEndpoint (auep) from ca to rgw2
    auep 3 *@rgw2.whatever.net mgcp 1.0
Andreasen & Foster Informational [Page 199] RFC 3435 MGCP 1.0 January 2003
 followed by an acknowledgement from rgw2:
    200 3 ok
    z: aaln/1@rgw2.whatever.net
    z: aaln/2@rgw2.whatever.net
 Step 4 - NotificationRequest (rqnt) from ca to each endpoint of rgw2
    rqnt 4 aaln/1@rgw2.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 234567892
 followed by:
    rqnt 5 aaln/2@rgw2.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 234567893
 with rgw2 acknowledging for aaln/1:
    200 4 ok
 and for aaln/2:
    200 5 ok
G.2 Connection Creation G.2.1 Residential Gateway to Residential Gateway
 The following table shows the message sequence which occurs when a
 user (usr1) makes a call through a residential gateway (rgw1) to a
 user served by another residential gateway (rgw2).  This example
 illustrates the communication between the residential gateways and
 the call agent (ca) only.  The local name of the endpoints in this
 example is aaln/1 for both gateways, and references within the
 description of the steps to rgw1 and rgw2 can be assumed to refer to
 aaln/1 of rgw1 and aaln/1 of rgw2.  Note that this is only an example
 and is not the only legal call scenario.
Andreasen & Foster Informational [Page 200] RFC 3435 MGCP 1.0 January 2003
          Table F.3: Residential Gateway Connection Creation

# usr1 rgw1 ca rgw2 usr2
================================================================
1 offhook → ntfy →
← ack
————-————————————————
2 ← dialtone ← rqnt
ack →
————-————————————————
3 digits → ntfy →
← ack
————-————————————————
4 ← rqnt
ack →
————-————————————————
5 ← recvonly ← crcx
ack →
————-————————————————
6 crcx → sendrcv →
← ack
————-————————————————
7 ← recvonly ← mdcx
ack →
————-————————————————
8 ← ringback ← rqnt
ack →
————-————————————————
9 rqnt → ringing →
← ack
————-————————————————
10 ← ntfy ← offhook
ack →
————-————————————————
11 rqnt →
← ack
————-————————————————
12 ← rqnt
ack →
————-————————————————
13 ← sendrcv ← mdcx
ack →
———————————————————————
 Step 1 - Notify (ntfy) offhook from rgw1 to ca
Andreasen & Foster Informational [Page 201] RFC 3435 MGCP 1.0 January 2003
 This ntfy is the result of usr1 going offhook and assumes ca had
 previously sent an rqnt with RequestId "445678944" to rgw1 requesting
 notification in the event of an offhook:
    ntfy 12 aaln/1@rgw1.whatever.net mgcp 1.0
    o: l/hd
    x: 445678944
 Acknowledgement from ca:
    200 12 ok
 Step 2 - Request Notification (rqnt) for digits from ca to rgw1
 Request rgw1 to notify if on-hook and collect digits according to the
 digit map, and to provide dialtone:
    rqnt 1057 aaln/1@rgw1.whatever.net mgcp 1.0
    r: l/hu(n), d/[0-9#*T](d)
    s: l/dl
    x: 445678945
    d: 5xxx
 Acknowledgement from rgw1:
    200 1057 ok
 Step 3 - Notify (ntfy) digits from rgw1 to ca
    ntfy 13 aaln/1@rgw1.whatever.net mgcp 1.0
    o: d/5, d/0, d/0, d/1
    x: 445678945
 Acknowledgement from ca:
    200 13 ok
 Step 4 - Request Notification (rqnt) from ca to rgw1
 Request rgw1 to notify in the event of an on-hook transition:
    rqnt 1058 aaln/1@rgw1.whatever.net mgcp 1.0
    r: l/hu(n)
    x: 445678946
Andreasen & Foster Informational [Page 202] RFC 3435 MGCP 1.0 January 2003
 Acknowledgement from rgw1:
    200 1058 ok
 Step 5 - Create Connection (crcx) from ca to rgw1
 Request a new connection on rgw1 with the specified local connection
 options, including 20 msec as the packetization period, G.711 mu-law
 as the codec, and receive only as the mode:
    crcx 1059 aaln/1@rgw1.whatever.net mgcp 1.0
    c: 9876543210abcdef
    l: p:20, a:PCMU
    m: recvonly
 Acknowledgement from rgw1 that a new connection, "456789fedcba5", has
 been created, followed by a blank line and then the SDP parameters:
    200 1059 ok
    i: 456789fedcba5
    v=0
    o=- 23456789 98765432 IN IP4 192.168.5.7
    s=-
    c=IN IP4 192.168.5.7
    t=0 0
    m=audio 6058 RTP/AVP 0
 Step 6 - Create Connection (crcx) from ca to rgw2
 Request a new connection on rgw2.  The request includes the session
 description returned by rgw1 such that a two way connection can be
 initiated:
    crcx 2052 aaln/1@rgw2.whatever.net mgcp 1.0
    c: 9876543210abcdef
    l: p:20, a:PCMU
    m: sendrecv
    v=0
    o=- 23456789 98765432 IN IP4 192.168.5.7
    s=-
    c=IN IP4 192.168.5.7
    t=0 0
    m=audio 6058 RTP/AVP 0
Andreasen & Foster Informational [Page 203] RFC 3435 MGCP 1.0 January 2003
 Acknowledgement from rgw2 that a new connection, "67890af54c9", has
 been created; followed by a blank line and then the SDP parameters:
    200 2052 ok
    i: 67890af54c9
    v=0
    o=- 23456889 98865432 IN IP4 192.168.5.8
    s=-
    c=IN IP4 192.168.5.8
    t=0 0
    m=audio 6166 RTP/AVP 0
 Step 7 - Modify Connection (mdcx) from ca to rgw1
 Request rgw1 to modify the existing connection, "456789fedcba5", to
 use the session description returned by rgw2 establishing a half
 duplex connection which, though not used in this example, could be
 used to provide usr1 with in band ringback tone, announcements, etc:
    mdcx 1060 aaln/1@rgw1.whatever.net mgcp 1.0
    c: 9876543210abcdef
    i: 456789fedcba5
    l: p:20, a:PCMU
    M: recvonly
    v=0
    o=- 23456889 98865432 IN IP4 192.168.5.8
    s=-
    c=IN IP4 192.168.5.8
    t=0 0
    m=audio 6166 RTP/AVP 0
 Acknowledgement from rgw1:
    200 1060 ok
 Step 8 - Request Notification (rqnt) from ca for rgw1 to provide
 ringback
 Request rgw1 to notify in the event of an on-hook transition, and
 also to provide ringback tone:
    rqnt 1061 aaln/1@rgw1.whatever.net mgcp 1.0
    r: l/hu(n)
    s: g/rt
    x: 445678947
Andreasen & Foster Informational [Page 204] RFC 3435 MGCP 1.0 January 2003
 Acknowledgement from rgw1:
    200 1061 ok
 Step 9 - Request Notification (rqnt) from ca to rgw2 to provide
 ringing
 Request rgw2 to continue to look for offhook and provide ringing:
    rqnt 2053 aaln/1@rgw2.whatever.net mgcp 1.0
    r: l/hd(n)
    s: l/rg
    x: 445678948
 Acknowledgement from rgw2:
    200 2053 ok
 Step 10 - Notify (ntfy) offhook from rgw2 to ca
    ntfy 27 aaln/1@rgw2.whatever.net mgcp 1.0
    o: l/hd
    x: 445678948
 Acknowledgement from ca:
    200 27 ok
 Step 11 - Request Notification (rqnt) of on-hook from ca to rgw2
    rqnt 2054 aaln/1@rgw2.whatever.net mgcp 1.0
    r: l/hu(n)
    x: 445678949
 Acknowledgement from rgw2:
    200 2054 ok
 Step 12 - Request Notification (rqnt) of on-hook from ca to rgw1
    rqnt 1062 aaln/1@rgw1.whatever.net mgcp 1.0
    r: l/hu(n)
    x: 445678950
 Acknowledgement from rgw1:
    200 1062 ok
Andreasen & Foster Informational [Page 205] RFC 3435 MGCP 1.0 January 2003
 Step 13 - Modify Connection (mdcx) from ca to rgw1
 Request rgw1 to modify the existing connection, "456789fedcba5", to
 sendrecv such that a full duplex connection is initiated:
    mdcx 1063 aaln/1@rgw1.whatever.net mgcp 1.0
    c: 9876543210abcdef
    i: 456789fedcba5
    m: sendrecv
 Acknowledgement from rgw1:
    200 1063 ok
G.3 Connection Deletion G.3.1 Residential Gateway to Residential Gateway
 The following table shows the message sequence which occurs when a
 user (usr2) initiates the deletion of an existing connection on a
 residential gateway (rgw2) with a user served by another residential
 gateway (rgw1).  This example illustrates the communication between
 the residential gateways and the call agent (ca) only.  The local
 name of the endpoints in this example is aaln/1 for both gateways,
 and references within the description of the steps to rgw1 and rgw2
 can be assumed to refer to aaln/1 of rgw1 and aaln/1 of rgw2.
Andreasen & Foster Informational [Page 206] RFC 3435 MGCP 1.0 January 2003
          Table F.4: Residential Gateway Connection Deletion

# usr1 rgw1 ca rgw2 usr2
================================================================
1 ← ntfy ← on-hook
ack →
————-————————————————
2 dlcx →
← ack
————-————————————————
3 ← dlcx
ack →
————-————————————————
4 rqnt →
← ack
————-————————————————
5 on-hook → ntfy →
← ack
————-————————————————
6 ← rqnt
ack →
———————————————————————
 Step 1 - Notify (ntfy) offhook from rgw1 to ca
 This ntfy is the result of usr2 going on-hook and assumes that ca had
 previously sent an rqnt to rgw2 requesting notification in the event
 of an on-hook (see end of Connection Creation sequence):
    ntfy 28 aaln/1@rgw2.whatever.net mgcp 1.0
    o: l/hu
    x: 445678949
 Acknowledgement from ca:
    200 28 ok
 Step 2 - Delete Connection (dlcx) from ca to rgw2
 Requests rgw2 to delete the connection "67890af54c9":
    dlcx 2055 aaln/1@rgw1.whatever.net mgcp 1.0
    c: 9876543210abcdef
    i: 67890af54c9
Andreasen & Foster Informational [Page 207] RFC 3435 MGCP 1.0 January 2003
 Acknowledgement from rgw2.  Note the response code of "250" meaning
 "the connection was deleted":
    250 2055 ok
 Step 3 - Delete Connection (dlcx) from ca to rgw1
 Requests rgw1 to delete the connection "456789fedcba5":
    dlcx 1064 aaln/1@rgw1.whatever.net mgcp 1.0
    c: 9876543210abcdef
    i: 456789fedcba5
 Acknowledgement from rgw1:
    250 1064 ok
 Step 4 - NotificationRequest (rqnt) from ca to rgw2
 Requests rgw2 to notify ca in the event of an offhook transition:
    rqnt 2056 aaln/1@rgw2.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 445678951
 Acknowledgement from rgw2:
    200 2056 ok
 Step 5 - Notify (ntfy) on-hook from rgw1 to ca
 Notify ca that usr1 at rgw1 went back on-hook:
    ntfy 15 aaln/1@rgw1.whatever.net mgcp 1.0
    o: l/hu
    x: 445678950
 Acknowledgement from ca:
    200 15 ok
 Step 6 - NotificationRequest (rqnt) offhook from ca to rgw1
 Requests rgw1 to notify ca in the event of an offhook transition:
    rqnt 1065 aaln/1@rgw1.whatever.net mgcp 1.0
    r: l/hd(n)
    x: 445678952
Andreasen & Foster Informational [Page 208] RFC 3435 MGCP 1.0 January 2003
 Acknowledgement from rgw1:
    200 1065 ok
Authors' Addresses
 Flemming Andreasen
 Cisco Systems
 499 Thornall Street, 8th Floor
 Edison, NJ 08837
 EMail: fandreas@cisco.com
 Bill Foster
 Cisco Systems
 771 Alder Drive
 Milpitas, CA 95035
 EMail: bfoster@cisco.com
Andreasen & Foster Informational [Page 209] RFC 3435 MGCP 1.0 January 2003 Full Copyright Statement
 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
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 or assist in its implementation may be prepared, copied, published
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 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
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 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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Acknowledgement
 Funding for the RFC Editor function is currently provided by the
 Internet Society.
Andreasen & Foster Informational [Page 210]
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