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rfc:rfc4233

Network Working Group K. Morneault Request for Comments: 4233 Cisco Systems Obsoletes: 3057 S. Rengasami Category: Standards Track Tridea Works

                                                              M. Kalla
                                                Telcordia Technologies
                                                         G. Sidebottom
                                                 Signatus Technologies
                                                          January 2006
             Integrated Services Digital Network (ISDN)
                    Q.921-User Adaptation Layer

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 This document defines a protocol for backhauling of Integrated
 Services Digital Network (ISDN) Q.921 User messages over IP using the
 Stream Control Transmission Protocol (SCTP).  This protocol would be
 used between a Signaling Gateway (SG) and Media Gateway Controller
 (MGC).  It is assumed that the SG receives ISDN signaling over a
 standard ISDN interface.
 This document obsoletes RFC 3057.

Morneault, et al. Standards Track [Page 1] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

Table of Contents

 1. Introduction ....................................................3
    1.1. Scope ......................................................3
    1.2. Terminology ................................................3
    1.3. IUA Overview ...............................................5
    1.4. Services Provided by the IUA Layer .........................7
    1.5. Functions Implemented by the IUA Layer ....................10
    1.6. Definition of IUA Boundaries ..............................12
 2. Conventions ....................................................15
 3. Protocol Elements ..............................................15
    3.1. Common Message Header .....................................15
    3.2. IUA Message Header ........................................19
    3.3. IUA Messages ..............................................21
 4. Procedures .....................................................46
    4.1. Procedures to Support Service in Section 1.4.1 ............46
    4.2. Procedures to Support Service in Section 1.4.2 ............46
    4.3. Procedures to Support Service in Section 1.4.3 ............48
 5. Examples .......................................................58
    5.1. Establishment of Association and Traffic between
         SGs and ASPs ..............................................58
    5.2. ASP Traffic Fail-over Examples ............................62
    5.3. Q.921/Q.931 Primitives Backhaul Examples ..................63
    5.4. Layer Management Communication Examples ...................64
 6. Security .......................................................65
 7. IANA Considerations ............................................65
    7.1. SCTP Payload Protocol Identifier ..........................65
    7.2. IUA Protocol Extensions ...................................65
 8. Timer Values ...................................................67
 9. Acknowledgements ...............................................67
 10. References ....................................................67
    10.1. Normative References .....................................67
    10.2. Informative References ...................................67
 11. Change Log ....................................................68
 Appendix A ........................................................69
    A.1. Signaling Network Architecture ............................69
    A.2. Application Server Process Redundancy .....................70

Morneault, et al. Standards Track [Page 2] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

1. Introduction

 In this document, the term Q.921-User refers to an upper layer that
 uses the services of Q.921, not the user side of ISDN interface [1].
 Examples of the upper layer would be Q.931 and QSIG.
 This section describes the need for ISDN Q.921-User Adaptation (IUA)
 layer protocol as well as how this protocol shall be implemented.

1.1. Scope

 There is a need for Switched Circuit Network (SCN) signaling protocol
 delivery from an ISDN Signaling Gateway (SG) to a Media Gateway
 Controller (MGC) as described in the Framework Architecture for
 Signaling Transport [5].  The delivery mechanism SHOULD meet the
 following criteria:
  • Support for transport of the Q.921/Q.931 boundary primitives
  • Support for communication between Layer Management modules on SG

and MGC

  • Support for management of SCTP active associations between SG

and MGC

 This document supports both ISDN Primary Rate Access (PRA) as well as
 Basic Rate Access (BRA) including the support for both point-to-point
 and point-to-multipoint modes of communication.  This support
 includes Facility Associated Signaling (FAS), Non-Facility Associated
 Signaling (NFAS), and NFAS with backup D channel.  QSIG adaptation
 layer requirements do not differ from Q.931 adaptation layer; hence,
 the procedures described in this document are also applicable for a
 QSIG adaptation layer.  For simplicity, only Q.931 will be mentioned
 in the rest of this document.

1.2. Terminology

 Application Server (AS) - A logical entity serving a specific
 application instance.  An example of an Application Server is a MGC
 handling the Q.931 and call processing for D channels terminated by
 the Signaling Gateways.  Practically speaking, an AS is modeled at
 the SG as an ordered list of one or more related Application Server
 Processes (e.g., primary, secondary, tertiary).
 Application Server Process (ASP) - A process instance of an
 Application Server.  Examples of Application Server Processes are
 primary or backup MGC instances.

Morneault, et al. Standards Track [Page 3] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 Association - An association refers to an SCTP association.  The
 association will provide the transport for the delivery of Q.921-User
 protocol data units and IUA adaptation layer peer messages.
 Backhaul - A SG terminates the lower layers of an SCN protocol and
 backhauls the upper layer(s) to MGC for call processing.  For the
 purposes of this document, the SG terminates Q.921 and backhauls
 Q.931 to MGC.
 Fail-over - The capability to re-route signaling traffic as required
 between related ASPs in the event of failure or unavailability of the
 currently used ASP (e.g., from primary MGC to backup MGC).  Fail-over
 also applies upon the return to service of a previously unavailable
 process.
 Host - The computing platform that the ASP process is running on.
 Interface - For the purposes of this document, an interface supports
 the relevant ISDN signaling channel.  This signaling channel MAY be a
 16-kbps D channel for an ISDN BRA as well as 64-kbps primary or
 backup D channel for an ISDN PRA.  For QSIG, the signaling channel is
 a Qc channel.
 Interface Identifier - The Interface Identifier identifies the
 physical interface at the SG for which the signaling messages are
 sent/received.  The format of the Interface Identifier parameter can
 be text or integer, the values of which are assigned according to
 network operator policy.  The values used are of local significance
 only, coordinated between the SG and ASP.  Significance is not
 implied across SGs served by an AS.
 Layer Management - Layer Management is a nodal function that handles
 the inputs and outputs between the IUA layer and a local management
 entity.
 Network Byte Order - Most significant byte first, a.k.a big endian.
 Stream - A stream refers to an SCTP stream: a uni-directional logical
 channel established from one SCTP endpoint to another associated SCTP
 endpoint, within which all user messages are delivered in sequence
 except for those submitted to the un-ordered delivery service.
 Q.921-User - Any protocol normally using the services of the ISDN
 Q.921 (e.g., Q.931, QSIG, etc.).

Morneault, et al. Standards Track [Page 4] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

1.3. IUA Overview

 The architecture that has been defined [5] for SCN signaling
 transport over IP uses multiple components, including an IP transport
 protocol, a signaling common transport protocol, and an adaptation
 module to support the services expected by a particular SCN signaling
 protocol from its underlying protocol layer.
 This document defines an adaptation module that is suitable for the
 transport of ISDN Q.921-User (e.g., Q.931) messages.

1.3.1. Example: SG to MGC

 In a Signaling Gateway (SG), it is expected that the ISDN signaling
 is received over a standard ISDN network termination.  The SG then
 provides interworking of transport functions with IP Signaling
 Transport, in order to transport the Q.931 signaling messages to the
 MGC where the peer Q.931 protocol layer exists, as shown below:
  • * ISDN IP *
  • EP *—————* SG *————–* MGC *
  • * *
          +-----+                                  +-----+
          |Q.931|              (NIF)               |Q.931|
          +-----+           +----------+           +-----+
          |     |           |     | IUA|           | IUA |
          |     |           |     +----+           +-----+
          |Q.921|           |Q.921|SCTP|           |SCTP |
          |     |           |     +----+           +-----+
          |     |           |     | IP |           | IP  |
          +-----+           +-----+----+           +-----+
          NIF  - Nodal Interworking Function
          EP   - ISDN End Point
          SCTP - Stream Control Transmission Protocol (Refer to [4,8])
          IUA  - ISDN User Adaptation Layer Protocol
         Figure 1.  IUA in the SG to MGC Application
 It is recommended that the IUA use the services of the Stream Control
 Transmission Protocol (SCTP) as the underlying reliable common
 signaling transport protocol.  The use of SCTP provides the following
 features:
  1. explicit packet-oriented delivery (not stream-oriented)

Morneault, et al. Standards Track [Page 5] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

  1. sequenced delivery of user messages within multiple streams,

with an option for order-of-arrival delivery of individual user

       messages,
    -  optional multiplexing of user messages into SCTP datagrams,
    -  network-level fault tolerance through support of multi-homing
       at either or both ends of an association,
    -  resistance to flooding and masquerade attacks, and
    -  data segmentation to conform to discovered path MTU size.
 There are scenarios without redundancy requirements and scenarios in
 which redundancy is supported below the transport layer.  In these
 cases, the SCTP functions above MAY be determined to not be required
 and TCP MAY be used as the underlying common transport protocol.

1.3.2. Support for the Management of SCTP Associations between the SG

      and ASPs
 The IUA layer at the SG maintains the availability state of all
 dynamically registered remote ASPs, in order to manage the SCTP
 associations and the traffic between the SG and ASPs.  As well, the
 active/inactive states of remote ASP(s) are maintained.  Active ASPs
 are those currently receiving traffic from the SG.
 The IUA layer MAY be instructed by local management to establish an
 SCTP association to a peer IUA node.  This can be achieved using the
 M-SCTP ESTABLISH primitive to request, indicate, and confirm the
 establishment of an SCTP association with a peer IUA node.
 The IUA layer MAY also need to inform local management of the status
 of the underlying SCTP associations using the M-SCTP STATUS request
 and indication primitive.  For example, the IUA MAY inform local
 management of the reason for the release of an SCTP association,
 determined either locally within the IUA layer or by a primitive from
 the SCTP.

1.3.3. ASP Fail-over Model and Terminology

 The IUA layer supports ASP fail-over functions in order to support a
 high availability of call processing capability.  All Q.921-User
 messages incoming to an SG are assigned to a unique Application
 Server, based on the Interface Identifier of the message.
 The Application Server is, in practical terms, a list of all ASPs
 configured to process Q.921-User messages from certain Interface
 Identifiers.  One or more ASPs in the list are normally active (i.e.,
 handling traffic) while any others MAY be unavailable or inactive, to
 be possibly used in the event of failure or unavailability of the
 active ASP(s).

Morneault, et al. Standards Track [Page 6] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The IUA layer supports an n+k redundancy model (active-standby, load
 sharing, broadcast) where n is the minimum number of redundant ASPs
 required to handle traffic and k ASPs are available to take over for
 a failed or unavailable ASP.  Note that 1+1 active/standby redundancy
 is a subset of this model.  A simplex 1+0 model is also supported as
 a subset, with no ASP redundancy.

1.3.4. Client/Server Model

 It is recommended that the SG and ASP be able to support both client
 and server operation.  The peer endpoints using IUA SHOULD be
 configured so that one always takes on the role of client and the
 other the role of server for initiating SCTP associations.  The
 default orientation would be for the SG to take on the role of server
 while the ASP is the client.  In this case, ASPs SHOULD initiate the
 SCTP association to the SG.
 The SCTP and TCP Registered User Port Number Assignment for IUA is
 9900.

1.4. Services Provided by the IUA Layer

1.4.1. Support for Transport of Q.921/Q.931 Boundary Primitives

 In the backhaul scenario, the Q.921/Q.931 boundary primitives are
 exposed.  IUA layer needs to support all of the primitives of this
 boundary to successfully backhaul Q.931.
 This includes the following primitives [1]:
 DL-ESTABLISH
 The DL-ESTABLISH primitives are used to request, indicate, and
 confirm the outcome of the procedures for establishing multiple frame
 operation.
 DL-RELEASE
 DL-RELEASE primitives are used to request, indicate, and confirm the
 outcome of the procedures for terminating a previously established
 multiple frame operation, or for reporting an unsuccessful
 establishment attempt.

Morneault, et al. Standards Track [Page 7] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 DL-DATA
 The DL-DATA primitives are used to request and indicate layer 3
 (Q.931) messages that are to be transmitted, or have been received,
 by the Q.921 layer using the acknowledged information transfer
 service.
 DL-UNIT DATA
 The DL-UNIT DATA primitives are used to request and indicate layer 3
 (Q.931) messages that are to be transmitted, by the Q.921 layer using
 the unacknowledged information transfer service.

1.4.2. Support for Communication between Layer Management Modules on SG

      and MGC
 It is envisioned that the IUA layer needs to provide some services
 that will facilitate communication between Layer Management modules
 on the SG and MGC.  These primitives are shown below:
 M-TEI STATUS
 The M-TEI STATUS primitives are used to request, confirm, and
 indicate the status (assigned/unassigned) of an ISDN Terminal
 Endpoint Identifier (TEI).
 M-ERROR
 The M-ERROR primitive is used to indicate an error with a received
 IUA message (e.g., interface identifier value is not known to the
 SG).

1.4.3. Support for Management of Active Associations between SG and MGC

 A set of primitives between the IUA layer and the Layer Management is
 defined below to help the Layer Management manage the SCTP
 association(s) between the SG and MGC.  The IUA layer can be
 instructed by the Layer Management to establish an SCTP association
 to a peer IUA node.  This procedure can be achieved using the M-SCTP
 ESTABLISH primitive.
 M-SCTP ESTABLISH
 The M-SCTP ESTABLISH primitives are used to request, indicate, and
 confirm the establishment of an SCTP association to a peer IUA node.

Morneault, et al. Standards Track [Page 8] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 M-SCTP RELEASE
 The M-SCTP RELEASE primitives are used to request, indicate, and
 confirm the release of an SCTP association to a peer IUA node.
 The IUA layer MAY also need to inform the status of the SCTP
 associations to the Layer Management.  This can be achieved using the
 M-SCTP STATUS primitive.
 M-SCTP STATUS
 The M-SCTP STATUS primitives are used to request and indicate the
 status of the underlying SCTP association(s).
 The Layer Management MAY need to inform the IUA layer of an AS/ASP
 status (i.e., failure, active, etc.), so that messages can be
 exchanged between IUA layer peers to stop traffic to the local IUA
 user.  This can be achieved using the M-ASP STATUS primitive.
 M-ASP STATUS
 The ASP status is stored inside IUA layer on both the SG and MGC
 sides.  The M-ASP STATUS primitive can be used by Layer Management to
 request the status of the Application Server Process from the IUA
 layer.  This primitive can also be used to indicate the status of the
 Application Server Process.
 M-ASP-UP
 The M-ASP-UP primitive can be used by Layer Management to send a ASP
 Up message for the Application Server Process.  It can also be used
 to generate an ASP Up Acknowledgement.
 M-ASP-DOWN
 The M-ASP-DOWN primitive can be used by Layer Management to send a
 ASP Down message for the Application Server Process.  It can also be
 used to generate an ASP Down Acknowledgement.
 M-ASP-ACTIVE
 The M-ASP-UP primitive can be used by Layer Management to send a ASP
 Active message for the Application Server Process.  It can also be
 used to generate an ASP Active Acknowledgement.

Morneault, et al. Standards Track [Page 9] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 M-ASP-INACTIVE
 The M-ASP-UP primitive can be used by Layer Management to send a ASP
 Inactive message for the Application Server Process.  It can also be
 used to generate an ASP Inactive Acknowledgement.
 M-AS STATUS
 The M-AS STATUS primitive can be used by Layer Management to request
 the status of the Application Server.  This primitive can also be
 used to indicate the status of the Application Server.

1.5. Functions Implemented by the IUA Layer

1.5.1. Mapping

 The IUA layer MUST maintain a map of the Interface Identifier to a
 physical interface on the Signaling Gateway.  A physical interface
 would be a T1 line, E1 line, etc., and could include the Time-
 Division Multiplexing (TDM) timeslot.  In addition, for a given
 interface the SG MUST be able to identify the associated signaling
 channel.  IUA layers on both SG and MGC MAY maintain the status of
 ISDN Terminal Endpoint Identifiers (TEIs) and Service Access Point
 Identifiers (SAPIs).
 The SG maps an Interface Identifier to an SCTP association/stream
 only when an ASP sends an ASP Active message for a particular
 Interface Identifier.  It MUST be noted, however, that this mapping
 is dynamic and could change at any time due to a change of ASP state.
 This mapping could even temporarily be invalid, for example, during
 fail-over of one ASP to another.  Therefore, the SG MUST maintain the
 states of AS/ASP and reference them during the routing of an messages
 to an AS/ASP.
 One example of the logical view of relationship between D channel,
 Interface Identifier, AS, and ASP in the SG is shown below:
        /---------------------------------------------------+
       /   /------------------------------------------------|--+
      /   /                                                 v  |
     /   /    +----+             act+-----+    +-------+ -+--+-|+--+-

D chan1——–>|IID |-+ +–>| ASP |—>| Assoc | v

       /      +----+ |  +----+  |   +-----+    +-------+ -+--+--+--+-
      /              +->| AS |--+                        Streams
     /        +----+ |  +----+   stb+-----+

D chan2——–>|IID |-+ | ASP |

              +----+                +-----+

Morneault, et al. Standards Track [Page 10] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 where IID = Interface Identifier
 Note that an ASP can be in more than one AS.

1.5.2. Status of ASPs

 The IUA layer on the SG MUST maintain the state of the ASPs it is
 supporting.  The state of an ASP changes because of reception of
 peer-to-peer messages (ASPM messages as described in Section 3.3.2)
 or reception of indications from the local SCTP association.  ASP
 state transition procedures are described in Section 4.3.1.
 At a SG, an Application Server list MAY contain active and inactive
 ASPs to support ASP load-sharing and fail-over procedures.  When, for
 example, both a primary and a backup ASP are available, IUA peer
 protocol is required to control which ASP is currently active.  The
 ordered list of ASPs within a logical Application Server is kept
 updated in the SG to reflect the active Application Server
 Process(es).
 Also the IUA layer MAY need to inform the local management of the
 change in status of an ASP or AS.  This can be achieved using the
 M-ASP STATUS or M-AS STATUS primitives.

1.5.3. SCTP Stream Management

 SCTP allows a user-specified number of streams to be opened during
 the initialization.  It is the responsibility of the IUA layer to
 ensure proper management of these streams.  Because of the
 unidirectional nature of streams, an IUA layer is not aware of the
 stream number to Interface Identifier mapping of its peer IUA layer.
 Instead, the Interface Identifier is in the IUA message header.
 The use of SCTP streams within IUA is recommended in order to
 minimize transmission and buffering delay, therefore improving the
 overall performance and reliability of the signaling elements.  It is
 recommended that a separate SCTP stream is used for each D channel.

1.5.4. Seamless Network Management Interworking

 The IUA layer on the SG SHOULD pass an indication of unavailability
 of the IUA-User (Q.931) to the local Layer Management, if the
 currently active ASP moves from the ACTIVE state.  The Layer
 Management could instruct Q.921 to take some action, if it deems
 appropriate.

Morneault, et al. Standards Track [Page 11] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 Likewise, if an SCTP association fails, the IUA layer on both the SG
 and ASP sides MAY generate Release primitives to take the data links
 out-of-service.

1.5.5. Congestion Management

 If the IUA layer becomes congested (implementation dependent), it MAY
 stop reading from the SCTP association to flow control from the peer
 IUA.

1.6. Definition of IUA Boundaries

1.6.1. Definition of IUA/Q.921 Boundary

 DL-ESTABLISH
 DL-RELEASE
 DL-DATA
 DL-UNIT DATA

1.6.2. Definition of IUA/Q.931 Boundary

 DL-ESTABLISH
 DL-RELEASE
 DL-DATA
 DL-UNIT DATA

1.6.3. Definition of SCTP/IUA Boundary

 An example of the upper layer primitives provided by SCTP are
 available in Section 10 of RFC 2960 [4].

1.6.4. Definition of IUA/Layer-Management Boundary

 M-SCTP ESTABLISH request
 Direction: LM -> IUA
 Purpose: LM requests ASP to establish an SCTP association with an SG.
 M-STCP ESTABLISH confirm
 Direction: IUA -> LM
 Purpose: ASP confirms to LM that it has established an SCTP
          association with an SG.
 M-SCTP ESTABLISH indication
 Direction: IUA -> LM
 Purpose: SG informs LM that an ASP has established an SCTP
          association.

Morneault, et al. Standards Track [Page 12] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 M-SCTP RELEASE request
 Direction: LM -> IUA
 Purpose: LM requests ASP to release an SCTP association with SG.
 M-SCTP RELEASE confirm
 Direction: IUA -> LM
 Purpose: ASP confirms to LM that it has released SCTP association
          with SG.
 M-SCTP RELEASE indication
 Direction: IUA -> LM
 Purpose: SG informs LM that ASP has released an SCTP association.
 M-SCTP STATUS request
 Direction: LM -> IUA
 Purpose: LM requests IUA to report status of SCTP association.
 M-SCTP STATUS indication
 Direction: IUA -> LM
 Purpose: IUA reports status of SCTP association.
 M-ASP STATUS request
 Direction: LM -> IUA
 Purpose: LM requests SG to report status of remote ASP.
 M-ASP STATUS indication
 Direction: IUA -> LM
 Purpose: SG reports status of remote ASP.
 M-AS-STATUS request
 Direction: LM -> IUA
 Purpose: LM requests SG to report status of AS.
 M-AS-STATUS indication
 Direction: IUA -> LM
 Purpose: SG reports status of AS.
 M-NOTIFY indication
 Direction: IUA -> LM
 Purpose: ASP reports that it has received a NOTIFY message
          from its peer.
 M-ERROR indication
 Direction: IUA -> LM
 Purpose: ASP or SG reports that it has received an ERROR
          message from its peer.

Morneault, et al. Standards Track [Page 13] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 M-ASP-UP request
 Direction: LM -> IUA
 Purpose: LM requests ASP to start its operation and send an ASP UP
          message to the SG.
 M-ASP-UP confirm
 Direction: IUA -> LM
 Purpose: ASP reports that is has received an ASP UP Acknowledgement
          message from the SG.
 M-ASP-DOWN request
 Direction: LM -> IUA
 Purpose: LM requests ASP to stop its operation and send an ASP DOWN
          message to the SG.
 M-ASP-DOWN confirm
 Direction: IUA -> LM
 Purpose: ASP reports that is has received an ASP DOWN
          Acknowledgement message from the SG.
 M-ASP-ACTIVE request
 Direction: LM -> IUA
 Purpose: LM requests ASP to send an ASP ACTIVE message to the SG.
 M-ASP-ACTIVE confirm
 Direction: IUA -> LM
 Purpose: ASP reports that is has received an ASP ACTIVE
          Acknowledgement message from the SG.
 M-ASP-INACTIVE request
 Direction: LM -> IUA
 Purpose: LM requests ASP to send an ASP INACTIVE message to the SG.
 M-ASP-INACTIVE confirm
 Direction: IUA -> LM
 Purpose: ASP reports that is has received an ASP INACTIVE
          Acknowledgement message from the SG.
 M-TEI STATUS request
 Direction: LM -> IUA
 Purpose: LM requests ASP to send a TEI status request to the SG.
 M-TEI STATUS indication
 Direction: IUA -> LM
 Purpose: ASP reports that is has received a TEI status indication
          from the SG.

Morneault, et al. Standards Track [Page 14] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 M-TEI STATUS confirm
 Direction: IUA -> LM
 Purpose: ASP reports that is has received a TEI status confirm from
          the SG.

2. Conventions

 The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
 SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
 they appear in this document, are to be interpreted as described in
 [6].

3. Protocol Elements

 This section describes the format of various messages used in this
 protocol.

3.1. Common Message Header

 The protocol messages for Q.921-User Adaptation require a message
 header that contains the adaptation layer version, the message type,
 and message length.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Version    |   Reserved    | Message Class | Message Type  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        Message Length                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 2.  Common Header Format
 All fields in an IUA message MUST be transmitted in the network byte
 order, unless otherwise stated.

3.1.1. Version

 The version field contains the version of the IUA adaptation layer.
 The supported versions are the following:
    Value    Version
    -----    -------
      1      Release 1.0

Morneault, et al. Standards Track [Page 15] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

3.1.2. Message Classes and Types

 The following list contains the valid Message Classes:
 Message Class: 8 bits (unsigned integer)
     0       Management (MGMT) Message
     1       Reserved for Other SIGTRAN Adaptation Layer
     2       Reserved for Other SIGTRAN Adaptation Layers
     3       ASP State Maintenance (ASPSM) Messages
     4       ASP Traffic Maintenance (ASPTM) Messages
     5       Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages
     6       Reserved for Other SIGTRAN Adaptation Layer
     7       Reserved for Other SIGTRAN Adaptation Layer
     8       Reserved for Other SIGTRAN Adaptation Layer
   9 to 127  Reserved by the IETF
 128 to 255  Reserved for IETF-Defined Message Class extensions
 The following list contains the message names for the defined
 messages.
  Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages
     0        Reserved
     1        Data Request Message
     2        Data Indication Message
     3        Unit Data Request Message
     4        Unit Data Indication Message
     5        Establish Request
     6        Establish Confirm
     7        Establish Indication
     8        Release Request
     9        Release Confirm
    10        Release Indication
  11 to 127   Reserved by the IETF
 128 to 255   Reserved for IETF-Defined QPTM extensions
  Application Server Process State Maintenance (ASPSM) messages
     0        Reserved
     1        ASP Up (UP)
     2        ASP Down (DOWN)
     3        Heartbeat (BEAT)
     4        ASP Up Ack (UP ACK)
     5        ASP Down Ack (DOWN ACK)
     6        Heatbeat Ack (BEAT ACK)
   7 to 127   Reserved by the IETF
 128 to 255   Reserved for IETF-Defined ASPSM extensions

Morneault, et al. Standards Track [Page 16] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

  Application Server Process Traffic Maintenance (ASPTM) messages
     0        Reserved
     1        ASP Active (ACTIVE)
     2        ASP Inactive (INACTIVE)
     3        ASP Active Ack (ACTIVE ACK)
     4        ASP Inactive Ack (INACTIVE ACK)
   5 to 127   Reserved by the IETF
 128 to 255   Reserved for IETF-Defined ASPTM extensions
  Management (MGMT) Messages
     0        Error (ERR)
     1        Notify (NTFY)
     2        TEI Status Request
     3        TEI Status Confirm
     4        TEI Status Indication
     5        TEI Query Request
   6 to 127   Reserved by the IETF
 128 to 255   Reserved for IETF-Defined MGMT extensions

3.1.3. Reserved

 The Reserved field is 8 bits.  It SHOULD be set to all '0's and
 ignored by the receiver.

3.1.4. Message Length

 The Message Length defines the length of the message in octets,
 including the Common Header.  The Message Length MUST include
 parameter padding bytes, if any.
 Note: A receiver SHOULD accept the message whether or not the final
 parameter padding is included in the message length.

3.1.5. Variable-Length Parameter Format

 IUA messages consist of a Common Header followed by zero or more
 variable-length parameters, as defined by the message type.  The
 variable-length parameters contained in a message are defined in a
 Type-Length-Value (TLV) format as shown below.

Morneault, et al. Standards Track [Page 17] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Parameter Tag        |       Parameter Length        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                       Parameter Value                         /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Mandatory parameters MUST be placed before optional parameters in a
 message.
 Parameter Tag: 16 bits (unsigned integer)
 The Tag field is a 16-bit identifier of the type of parameter.  It
 takes a value of 0 to 65534.  Common parameters used by adaptation
 layers are in the range of 0x00 to 0x3f.  The parameter Tags defined
 are as follows:
 Common Parameters.  These TLV parameters are common across the
 different adaptation layers:
 Parameter Name                     Parameter ID
 ==============                     ============
 Reserved                              0x0000
 Interface Identifier (integer)        0x0001
 Not Used in IUA                       0x0002
 Interface Identifier (text)           0x0003
 INFO String                           0x0004
 DLCI                                  0x0005
 Not Used in IUA                       0x0006
 Diagnostic Information                0x0007
 Interface Identifier Range            0x0008
 Heartbeat Data                        0x0009
 Not Used in IUA                       0x000a
 Traffic Mode Type                     0x000b
 Error Code                            0x000c
 Status                                0x000d
 Protocol Data                         0x000e
 Release Reason                        0x000f
 TEI Status                            0x0010
 ASP Identifier                        0x0011
 Not Used in IUA                       0x0012 - 0x003f
 The value of 65535 is reserved for IETF-defined extensions.  Values
 other than those defined in specific parameter description are
 reserved for use by the IETF.

Morneault, et al. Standards Track [Page 18] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 Parameter Length: 16 bits (unsigned integer)
 The Parameter Length field contains the size of the parameter in
 bytes, including the Parameter Tag, Parameter Length, and Parameter
 Value fields.  The Parameter Length does not include any padding
 bytes.
 Parameter Value: variable-length
 The Parameter Value field contains the actual information to be
 transferred in the parameter.
 The total length of a parameter (including Tag, Parameter Length, and
 Value fields) MUST be a multiple of 4 bytes.  If the length of the
 parameter is not a multiple of 4 bytes, the sender pads the Parameter
 at the end (i.e., after the Parameter Value field) with all zero
 bytes.  The length of the padding is NOT included in the Parameter
 Length field.  A sender SHOULD NEVER pad with more than 3 bytes.  The
 receiver MUST ignore the padding bytes.

3.2. IUA Message Header

 In addition to the common message header, there will be a specific
 message header for QPTM and the TEI Status MGMT messages.  The IUA
 message header will immediately follow the Common header in these
 messages.
 This message header will contain the Interface Identifier and Data
 Link Connection Identifier (DLCI).  The Interface Identifier
 identifies the physical interface terminating the signaling channel
 at the SG for which the signaling messages are sent/received.  The
 format of the Interface Identifier parameter can be text or integer.
 The Interface Identifiers are assigned according to network operator
 policy.  The integer values used are of local significance only,
 coordinated between the SG and ASP.
 The integer-formatted Interface Identifier MUST be supported.  The
 text-formatted Interface Identifier MAY optionally be supported.

Morneault, et al. Standards Track [Page 19] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Tag (0x1)           |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier (integer)                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Tag (0x5)           |             Length=8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            DLCI               |              Spare            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Figure 3.  IUA Message Header (Integer-based Interface Identifier)
 The Tag value for the Integer-based Interface Identifier is 0x1.  The
 length is always set to a value of 8.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Tag (0x3)           |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               /
 /                   Interface Identifier (text)                 \
 \                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Tag (0x5)           |             Length=8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |            DLCI               |             Spare             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 4.  IUA Message Header (Text-based Interface Identifier)
 The Tag value for the Text-based [2] Interface Identifier is 0x3.
 The length is variable.
 The DLCI format is shown below in Figure 5.
      most                                     least
   significant                              significant
       bit                                      bit
    +-----+-----+-----+-----+-----+-----+-----+-----+
    |            SAPI                   | SPR |  0  |
    +-----------------------------------------------+
    |            TEI                          |  1  |
    +-----------------------------------------------+
                        Figure 5.  DLCI Format

Morneault, et al. Standards Track [Page 20] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 SPR:  Spare 2nd bit in octet 1 (1 bit)
 SAPI: Service Access Point Identifier (6 bits)
 TEI:  Terminal Endpoint Identifier (7 bits)
 As an example, SAPI = 0, TEI = 64, SPR = 0 would be encoded as
 follows:
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Tag (0x5)           |             Length=8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      0x0      |      0x81     |               0x0             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The DLCI field (including the SAPI and TEI) is coded in accordance
 with Q.921.

3.3. IUA Messages

 The following section defines the messages and parameter contents.
 The IUA messages will use the common message header (Figure 2) and
 the IUA message header (Figure 3 and Figure 4).

3.3.1. Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages

3.3.1.1. Establish Messages (Request, Confirm, Indication)

 The Establish Messages are used to establish a data link on the
 signaling channel or to confirm that a data link on the signaling
 channel has been established.  The MGC controls the state of the D
 channel.  When the MGC desires the D channel to be in-service, it
 will send the Establish Request message.
 When the MGC sends an IUA Establish Request message, the MGC MAY
 start a timer.  This timer would be stopped upon receipt of an IUA
 Establish Confirm or Establish Indication.  If the timer expires, the
 MGC would resend the IUA Establish Request message and restart the
 timer.  In other words, the MGC MAY continue to request the
 establishment of the data link on a periodic basis until the desired
 state is achieved or take some other action (notify the Management
 Layer).
 When the SG receives an IUA Establish Request from the MGC, the SG
 shall send the Q.921 Establish Request primitive to the Q.921 entity.
 In addition, the SG shall map any response received from the Q.921
 entity to the appropriate message to the MGC.  For example, if the
 Q.921 entity responds with a Q.921 Establish Confirm primitive, the

Morneault, et al. Standards Track [Page 21] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 IUA layer shall map this to an IUA Establish Confirm message.  As
 another example, if the IUA Layer receives a Q.921 Release Confirm or
 Release Indication as an apparent response to the Q.921 Establish
 Request primitive, the IUA Layer shall map these to the corresponding
 IUA Release Confirm or Release Indication messages.
 The Establish messages contain the common message header followed by
 IUA message header.  It does not contain any additional parameters.

3.3.1.2. Release Messages (Request, Indication, Confirmation)

 The Release Request message is used to release the data link on the
 signaling channel.  The Release Confirm and Indication messages are
 used to indicate that the data link on the signaling channel has been
 released.
 If a response to the Release Request message is not received, the MGC
 MAY resend the Release Request message.  If no response is received,
 the MGC can consider the data link as being released.  In this case,
 signaling traffic on that D channel is not expected from the SG and
 signaling traffic will not be sent to the SG for that D channel.
 The Release messages contain the common message header followed by
 IUA message header.  The Release Confirm message is in response to a
 Release Request message and it does not contain any additional
 parameters.  The Release Request and Indication messages contain the
 following parameter:
    Reason
 The format for Release Message parameters is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Tag (0xf)           |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                              Reason                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Morneault, et al. Standards Track [Page 22] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The valid values for Reason are shown in the following table.
    Define     Value           Description
 RELEASE_MGMT   0x0     Management layer generated release.
 RELEASE_PHYS   0x1     Physical layer alarm generated release.
 RELEASE_DM     0x2     Specific to a request.  Indicates Layer 2
                        SHOULD release and deny all requests from
                        far end to establish a data link on the
                        signaling channel (i.e., if SABME is
                        received, send a DM)
 RELEASE_OTHER  0x3     Other reasons
 Note:  Only RELEASE_MGMT, RELEASE_DM, and RELEASE_OTHER are valid
 reason codes for a Release Request message.

3.3.1.3. Data Messages (Request, Indication)

 The Data message contains an ISDN Q.921-User Protocol Data Unit (PDU)
 corresponding to acknowledged information transfer service.
 The Data messages contain the common message header followed by IUA
 message header.  The Data message contains the following parameter:
    Protocol Data
 The format for Data Message parameters is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Tag (0xe)           |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                        Protocol Data                          /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The protocol data contains upper layer signaling message, e.g.,
 Q.931, QSIG.

3.3.1.4. Unit Data Messages (Request, Indication)

 The Unit Data message contains an ISDN Q.921-User Protocol Data Unit
 (PDU) corresponding to unacknowledged information transfer service.
 The Unit Data messages contain the common message header followed by
 IUA message header.  The Unit Data message contains the following
 parameter:

Morneault, et al. Standards Track [Page 23] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

     Protocol Data
 The format for Unit Data Message parameters is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           Tag (0xe)           |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                        Protocol Data                          /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.2. Application Server Process Maintenance (ASPM) Messages

 The ASPM messages will use only the common message header.

3.3.2.1. ASP Up (ASPUP)

 The ASP Up (ASPUP) message is sent by an ASP to indicate to an SG
 that it is ready to receive traffic or maintenance messages.
 The ASPUP message contains the following parameters:
   ASP Identifier           (Optional)
   INFO String              (Optional)
 The format for ASPUP Message parameters is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x0011          |           Length = 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         ASP Identifier                        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x0004          |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 ASP Identifier: 32-bit unsigned integer

Morneault, et al. Standards Track [Page 24] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The optional ASP Identifier parameter contains a unique value that is
 locally significant among the ASPs that support an AS.  The SG should
 save the ASP Identifier to be used, if necessary, with the Notify
 message (see Section 3.3.3.2).
 The optional INFO String parameter can carry any meaningful 8-bit
 ASCII [2] character string along with the message.  Length of the
 INFO String parameter is from 0 to 255 characters.  No procedures are
 presently identified for its use, but the INFO String MAY be used for
 debugging purposes.

3.3.2.2. ASP Up Ack

 The ASP Up Ack message is used to acknowledge an ASP Up message
 received from a remote IUA peer.
 The ASPUP Ack message contains the following parameters:
    INFO String (optional)
 The format for ASPUP Ack Message parameters is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x0004          |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The format and description of the optional INFO String parameter are
 the same as for the ASP Up message (see Section 3.3.2.1).

3.3.2.3. ASP Down (ASPDN)

 The ASP Down (ASPDN) message is sent by an ASP to indicate to an SG
 that it is NOT ready to receive traffic or maintenance messages.
 The ASPDN message contains the following parameters:
    INFO String (Optional)

Morneault, et al. Standards Track [Page 25] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the ASPDN message parameters is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x0004          |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The format and description of the optional INFO String parameter are
 the same as for the ASP Up message (see Section 3.3.2.1).

3.3.2.4. ASP Down Ack

 The ASP Down Ack message is used to acknowledge an ASP Down message
 received from a remote IUA peer.
 The ASP Down Ack message contains the following parameters:
    INFO String (Optional)
 The format for the ASP Down Ack message parameters is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x0004          |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The format and description of the optional INFO String parameter are
 the same as for the ASP Up message (see Section 3.3.2.1).

3.3.2.5. ASP Active (ASPAC)

 The ASPAC message is sent by an ASP to indicate to an SG that it is
 Active and ready to be used.

Morneault, et al. Standards Track [Page 26] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The ASPAC message contains the following parameters:
    Traffic Mode Type (Mandatory)
    Interface Identifiers (Optional)
       - Combination of integer and integer ranges, OR
       - string (text-formatted)
    INFO String (Optional)

Morneault, et al. Standards Track [Page 27] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the ASPAC message using integer-formatted Interface
 Identifiers is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x000b          |           Length = 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Traffic Mode Type                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Tag (0x1=integer)         |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Tag (0x8=integer range)    |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         .                                       .
         .                                       .
         .                                       .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier StartN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier StopN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /            Additional Interface Identifier Parameters         /
 \                  of Tag Type 0x1 or 0x8                       \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag (0x4)             |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Morneault, et al. Standards Track [Page 28] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the ASPAC message using text-formatted (string)
 Interface Identifiers is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x000b          |           Length = 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Traffic Mode Type                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Tag (0x3=string)        |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /           Additional Interface Identifier Parameters          /
 \                      of Tag Type 0x3                          \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag (0x4)             |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Traffic Mode Type parameter identifies the traffic mode of
 operation of the ASP within an AS.  The valid values for Type are
 shown in the following table:
   Value          Description
    0x1            Over-ride
    0x2            Load-share
 Within a particular AS, only one Traffic Mode Type can be used.  The
 Over-ride value indicates that the ASP is operating in Over-ride
 mode, where the ASP takes over all traffic in an Application Server
 (i.e., primary/backup operation), over-riding any currently active
 ASPs in the AS.  In Load-share mode, the ASP will share in the
 traffic distribution with any other currently active ASPs.
 The optional Interface Identifiers parameter contains a list of
 Interface Identifier integers (Type 0x1 or Type 0x8) or text strings
 (Type 0x3) indexing the Application Server traffic that the sending
 ASP is configured/registered to receive.  If integer-formatted

Morneault, et al. Standards Track [Page 29] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 Interface Identifiers are being used, the ASP can also send ranges of
 Interface Identifiers (Type 0x8).  Interface Identifier types Integer
 (0x1) and Integer Range (0x8) are allowed in the same message.
 Text-formatted Interface Identifiers (0x3) cannot be used with either
 Integer (0x1) or Integer Range (0x8) types.
 If no Interface Identifiers are included, the message is for all
 provisioned Interface Identifiers within the AS or ASes in which the
 ASP is provisioned.  If only a subset of Interface Identifiers is
 included, the ASP is noted as Active for all the Interface
 Identifiers provisioned for that AS.
 Note:  If the optional Interface Identifier parameter is present, the
 integer-formatted Interface Identifier MUST be supported, whereas the
 text-formatted Interface Identifier MAY be supported.
 The format and description of the optional INFO String parameter are
 the same as for the ASP Up message (see Section 3.3.2.1.).
 An SG that receives an ASPAC with an incorrect Traffic Mode Type for
 a particular Interface Identifier will respond with an Error Message
 (Cause: Unsupported Traffic Handling Mode).

3.3.2.6. ASP Active Ack

 The ASPAC Ack message is used to acknowledge an ASP Active message
 received from a remote IUA peer.
 The ASPAC Ack message contains the following parameters:
    Traffic Mode Type (Mandatory)
    Interface Identifier (Optional)
       - Combination of integer and integer ranges, OR
       - string (text formatted)
    INFO String (Optional)

Morneault, et al. Standards Track [Page 30] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the ASPAC Ack message with integer-formatted Interface
 Identifiers is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x000b          |           Length = 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Traffic Mode Type                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Tag (0x1=integer)         |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Tag (0x8=integer range)    |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         .                                       .
         .                                       .
         .                                       .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier StartN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier StopN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /            Additional Interface Identifier Parameters         /
 \                  of Tag Type 0x1 or 0x8                       \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag (0x4)             |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Morneault, et al. Standards Track [Page 31] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the ASP Active Ack message using text-formatted
 (string) Interface Identifiers is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x000b          |           Length = 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     Traffic Mode Type                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Tag (0x3=string)        |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /           Additional Interface Identifier Parameters          /
 \                      of Tag Type 0x3                          \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag (0x4)             |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The format of the Traffic Mode Type and Interface Identifier
 parameters is the same as for the ASP Active message (see Section
 3.3.2.5).
 The format and description of the optional INFO String parameter are
 the same as for the ASP Up message (see Section 3.3.2.1).

3.3.2.7. ASP Inactive (ASPIA)

 The ASPIA message is sent by an ASP to indicate to an SG that it is
 no longer an active ASP to be used from within a list of ASPs.  The
 SG will respond with an ASPIA Ack message and either discard incoming
 messages or buffer for a timed period and then discard.

Morneault, et al. Standards Track [Page 32] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The ASPIA message contains the following parameters:
    Interface Identifiers (Optional)
       - Combination of integer and integer ranges, OR
       - string (text formatted)
    INFO String (Optional)
 The format for the ASP Inactive message parameters using integer-
 formatted Interface Identifiers is as follows:

Morneault, et al. Standards Track [Page 33] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Tag (0x1=integer)         |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Tag (0x8=integer range)    |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         .                                       .
         .                                       .
         .                                       .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier StartN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier StopN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /            Additional Interface Identifier Parameters         /
 \                  of Tag Type 0x1 or 0x8                       \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag (0x4)             |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Morneault, et al. Standards Track [Page 34] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the ASP Inactive message using text-formatted (string)
 Interface Identifiers is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Tag (0x3=string)        |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /           Additional Interface Identifier Parameters          /
 \                      of Tag Type 0x3                          \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag (0x4)             |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The optional Interface Identifiers parameter contains a list of
 Interface Identifier integers or text strings indexing the
 Application Server traffic that the sending ASP is
 configured/registered to receive, but does not want to receive at
 this time.
 The format and description of the optional Interface Identifiers and
 INFO String parameters are the same as for the ASP Active message
 (see Section 3.3.2.5).

3.3.2.8. ASP Inactive Ack

 The ASP Inactive (ASPIA) Ack message is used to acknowledge an ASP
 Inactive message received from a remote IUA peer.
 The ASPIA Ack message contains the following parameters:
    Interface Identifiers (Optional)
       - Combination of integer and integer ranges, OR
       - string (text formatted)
    INFO String (Optional)

Morneault, et al. Standards Track [Page 35] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the ASP Inactive Ack message parameters using
 integer-formatted Interface Identifiers is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Tag (0x1=integer)         |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Tag (0x8=integer range)    |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         .                                       .
         .                                       .
         .                                       .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier StartN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier StopN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /            Additional Interface Identifier Parameters         /
 \                  of Tag Type 0x1 or 0x8                       \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag (0x4)             |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Morneault, et al. Standards Track [Page 36] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the ASP Inactive Ack message using text-formatted
 (string) Interface Identifiers is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Tag (0x3=string)        |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /           Additional Interface Identifier Parameters          /
 \                      of Tag Type 0x3                          \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag (0x4)             |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The format and description of the optional Interface Identifiers and
 INFO String parameters are the same as for the ASP Active message
 (see Section 3.3.2.5).

3.3.2.9. Heartbeat (BEAT)

 The Heartbeat message is optionally used to ensure that the IUA peers
 are still available to each other.  It is recommended for use when
 the IUA runs over a transport layer other than the SCTP, which has
 its own heartbeat.
 The BEAT message contains the following parameters:
    Heartbeat Data    (Optional)

Morneault, et al. Standards Track [Page 37] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the BEAT message is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x0009          |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                       Heartbeat Data                          /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Heartbeat Data parameter contents are defined by the sending
 node.  The Heartbeat Data could include, for example, a Heartbeat
 Sequence Number and/or Timestamp.  The receiver of a Heartbeat
 message does not process this field as it is only of significance to
 the sender.  The receiver MUST respond with a Heartbeat Ack message.

3.3.2.10. Heartbeat Ack (BEAT-Ack)

 The Heartbeat Ack message is sent in response to a received Heartbeat
 message.  It includes all the parameters of the received Heartbeat
 message, without any change.

3.3.3. Layer Management (MGMT) Messages

3.3.3.1. Error (ERR)

 The Error message is used to notify a peer of an error event
 associated with an incoming message.  For example, the message type
 might be unexpected given the current state, or a parameter value
 might be invalid.
 The Error message will have only the common message header.  The
 Error message contains the following parameters:
    Error Code
    Diagnostic Information (Optional)

Morneault, et al. Standards Track [Page 38] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Tag = 0x000c         |          Length = 8           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Error Code                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Tag = 0x0007         |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               /
 /                     Diagnostic Information                    \
 \                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The Error Code parameter indicates the reason for the Error message.
 The Error parameter value can be one of the following values:
    Invalid Version                               0x01
    Invalid Interface Identifier                  0x02
    Unsupported Message Class                     0x03
    Unsupported Message Type                      0x04
    Unsupported Traffic Handling Mode             0x05
    Unexpected Message                            0x06
    Protocol Error                                0x07
    Unsupported Interface Identifier Type         0x08
    Invalid Stream Identifier                     0x09
    Unassigned TEI                                0x0a
    Unrecognized SAPI                             0x0b
    Invalid TEI, SAPI combination                 0x0c
    Refused - Management Blocking                 0x0d
    ASP Identifier Required                       0x0e
    Invalid ASP Identifier                        0x0f
 The "Invalid Version" error would be sent if a message was received
 with an invalid or unsupported version.  The Error message would
 contain the supported version in the Common header.  The Error
 message could optionally provide the supported version in the
 Diagnostic Information area.
 The "Invalid Interface Identifier" error would be sent by an SG if an
 ASP sends a message with an invalid (unconfigured) Interface
 Identifier value.  For this error, the Diagnostic Information MUST
 contain enough of the offending message to identify the invalid
 Interface Identifier.  For example, in the case of QPTM and TEI
 Status management messages, the Common and IUA message headers of the
 offending message would be placed in the Diagnostic Information at a
 minimum.

Morneault, et al. Standards Track [Page 39] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The "Unsupported Traffic Handling Mode" error would be sent by an SG
 if an ASP sends an ASP Active with an unsupported Traffic Handling
 Mode.  An example would be a case in which the SG did not support
 load-sharing.
 The "Unexpected Message" error would be sent by an ASP if it received
 a QPTM message from an SG while it was in the Inactive state (the ASP
 could optionally drop the message and not send an error).  It would
 also be sent by an ASP if it received a defined and recognized
 message that the SG is not expected to send (e.g., if the MGC
 receives an IUA Establish Request message).
 The "Protocol Error" error would be sent for any protocol anomaly
 (i.e., a bogus message).
 The "Invalid Stream Identifier" error would be sent if a message was
 received on an unexpected SCTP stream (e.g., a MGMT message was
 received on a stream other than "0").
 The "Unsupported Interface Identifier Type" error would be sent by an
 SG if an ASP sends a text-formatted Interface Identifier and the SG
 only supports integer-formatted Interface Identifiers.  When the ASP
 receives this error, it will need to resend its message with an
 integer-formatted Interface Identifier.
 The "Unsupported Message Type" error would be sent if a message with
 an unexpected or unsupported Message Type is received.
 The "Unsupported Message Class" error would be sent if a message with
 an unexpected or unsupported Message Class is received.
 The "Unassigned TEI" error may be used when the SG receives an IUA
 message that includes a TEI that has not been assigned or recognized
 for use on the indicated ISDN D-channel.
 The "Unrecognized SAPI" error would handle the case of using an SAPI
 that is not recognized by the SG.  The "Invalid TEI, SAPI
 combination" error identifies errors where the TEI is assigned and
 the SAPI is recognized, but the combination is not valid for the
 interface (e.g., on a Basic Rate Interface (BRI), the MGC tries to
 send Q.921 Management messages via IUA when Layer Management at the
 SG SHOULD be performing this function).
 The "Refused - Management Blocking" error is sent when an ASP Up or
 ASP Active message is received and the request is refused for
 management reasons (e.g., management lockout).

Morneault, et al. Standards Track [Page 40] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The "ASP Identifier Required" is sent by an SG in response to an ASP
 Up message that does not contain an ASP Identifier parameter when the
 SG requires one.  The ASP SHOULD resend the ASP Up message with an
 ASP Identifier.
 The "Invalid ASP Identifier" is sent by a SG in response to an ASP Up
 message with an invalid (i.e., non-unique) ASP Identifier.
 Diagnostic Information: variable length
    When included, the optional Diagnostic information can be any
    information germane to the error condition, to assist in
    identification of the error condition.  The Diagnostic information
    SHOULD contain the offending message.
 Error messages MUST NOT be generated in response to other Error
 messages.

3.3.3.2. Notify (NTFY)

 The Notify message used to provide an autonomous indication of IUA
 events to an IUA peer.
 The Notify message will use only the common message header.  The
 Notify message contains the following parameters:
    Status                     (Mandatory)
    ASP Identifier             (Optional)
    Interface Identifiers      (Optional)
    INFO String                (Optional)
 The format for the Notify message with integer-formatted Interface
 Identifiers is as follows:

Morneault, et al. Standards Track [Page 41] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Tag = 0x000d           |           Length = 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Status Type            |    Status Identification      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Tag = 0x0011           |           Length = 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        ASP Identifier                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     Tag (0x1=integer)         |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    Tag (0x8=integer range)    |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop1*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier Start2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier Stop2*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         .                                       .
         .                                       .
         .                                       .
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                 Interface Identifier StartN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Interface Identifier StopN*                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /            Additional Interface Identifier Parameters         /
 \                  of Tag Type 0x1 or 0x8                       \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x0004          |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Morneault, et al. Standards Track [Page 42] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The format for the Notify message with text-formatted Interface
 Identifiers is as follows:
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Tag = 0x000d           |           Length = 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Status Type            |    Status Identification      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        Tag = 0x0011           |           Length = 8          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                        ASP Identifier                         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |       Tag (0x3=string)        |            Length             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Interface Identifiers                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /           Additional Interface Identifier Parameters          /
 \                      of Tag Type 0x3                          \
 /                                                               /
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         Tag = 0x0004          |             Length            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         INFO String                           /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  Status Type: 16 bits (unsigned integer)
    The Status Type parameter identifies the type of the Notify
    message.  The following are the valid Status Type values:
       1     Application Server State Change (AS-State_Change)
       2     Other
 Status Information: 16 bits (unsigned integer)
    The Status Information parameter contains more detailed
    information for the notification, based on the value of the Status
    Type.  If the Status Type is AS-State_Change, the following Status
    Information values are used:

Morneault, et al. Standards Track [Page 43] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

       1    reserved
       2    Application Server Inactive (AS-INACTIVE)
       3    Application Server Active (AS-ACTIVE)
       4    Application Server Pending (AS-PENDING)
 These notifications are sent from an SG to an ASP upon a change in
 status of a particular Application Server.  The value reflects the
 new state of the Application Server.
 If the Status Type is Other, then the following Status Information
 values are defined:
    Value          Description
      1    Insufficient ASP resources active in AS
      2    Alternate ASP Active
      3    ASP Failure
 These notifications are not based on the SG reporting the state
 change of an ASP or AS.  In the Insufficient ASP Resources case, the
 SG is indicating to an ASP-INACTIVE ASP(s) in the AS that another ASP
 is required in order to handle the load of the AS (Load-sharing
 mode).  For the Alternate ASP Active case, an ASP is informed when an
 alternate ASP transitions to the ASP-ACTIVE state in Over-ride mode.
 The ASP Identifier (if available) of the Alternate ASP MUST be placed
 in the message.  For the ASP Failure case, the SG is indicating to
 ASP(s) in the AS that one of the ASPs has transitioned to ASP-DOWN.
 The ASP Identifier (if available) of the failed ASP MUST be placed in
 the message.
 The format and description of the optional ASP Identifier are the
 same as for the ASP Up message (see Section 3.3.2.1).  The format and
 description of the optional Interface Identifiers and INFO String
 parameters are the same as for the ASP Active message (see Section
 3.3.2.5).

3.3.3.3. TEI Status Messages (Request, Confirm, and Indication)

 The TEI Status messages are exchanged between IUA layer peers to
 request, confirm, and indicate the status of a particular TEI.
 The TEI Status messages contain the common message header followed by
 IUA message header.  The TEI Status Request message does not contain
 any additional parameters.
 In the integrated ISDN Layer 2/3 model (e.g., in traditional ISDN
 switches), it is assumed that the Layer Management for the Q.921
 Layer and the Q.931 layer are co-located.  When backhauling ISDN,
 this assumption is not necessarily valid.  The TEI Status messages

Morneault, et al. Standards Track [Page 44] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 allow the two Layer Management entities to communicate the status of
 the TEI.  In addition, knowing that a TEI is in service allows the
 ASP to request the SG to establish the datalink to the terminal (via
 the IUA Establish message) for signaling if the ASP wants to be in
 control of data link establishment.  Another use of the TEI Status
 procedure is where the Layer Management at the ASP can prepare for
 send/receive signaling to/from a given TEI and confirm/verify the
 establishment of a datalink to that TEI.  For example, if a datalink
 is established for a TEI that the ASP did not know was assigned, the
 ASP can check to see whether it was assigned or whether there was an
 error in the signaling message.  Also, knowing that a TEI is out of
 service, the ASP need not request the SG to establish a datalink to
 that TEI.
 The TEI Status Indication and Confirm messages contain the following
 parameter:
   STATUS
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          Tag = 0x0010         |          Length = 8           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                              Status                           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The valid values for Status are shown in the following table.
    Define     Value           Description
 ASSIGNED       0x0        TEI is considered assigned by Q.921
 UNASSIGNED     0x1        TEI is considered unassigned by Q.921

3.3.3.4. TEI Query Message (Request)

 The TEI Query message is sent by the ASP to query the TEI(s).  This
 message consists of the common header and IUA header.  The DLCI in
 the IUA header MUST be ignored by the SG.  The SG will respond to
 this message with TEI Status Indication(s).

Morneault, et al. Standards Track [Page 45] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

4. Procedures

 The IUA layer needs to respond to various primitives it receives from
 other layers as well as messages it receives from the peer IUA layer.
 This section describes various procedures involved in response to
 these events.

4.1. Procedures to Support Service in Section 1.4.1

 These procedures achieve the IUA layer's "Transport of Q.921/Q.931
 boundary primitives" service.

4.1.1. Q.921 or Q.931 Primitives Procedures

 On receiving these primitives from the local layer, the IUA layer
 will send the corresponding QPTM message (Data, Unit Data, Establish,
 Release) to its peer.  While doing so, the IUA layer needs to fill
 various fields of the common and specific headers correctly.  In
 addition, the message needs to be sent on the SCTP stream that
 corresponds to the D channel (Interface Identifier).

4.1.2. QPTM Message Procedures

 On receiving QPTM messages from a peer IUA layer, the IUA layer on an
 SG or MGC needs to invoke the corresponding layer primitives
 (DL-ESTABLISH, DL-DATA, DL-UNIT DATA, DL-RELEASE) to the local Q.921
 or Q.931 layer.

4.2. Procedures to Support Service in Section 1.4.2

 These procedures achieve the IUA layer's "Support for Communication
 between Layer Managements" service.

4.2.1. Layer Management Primitives Procedures

 On receiving these primitives from the local Layer Management, the
 IUA layer will provide the appropriate response primitive across the
 internal local Layer Management interface.
 An M-SCTP ESTABLISH request from Layer Management will initiate the
 establishment of an SCTP association.  An M-SCTP ESTABLISH confirm
 will be sent to Layer Management when the initiated association setup
 is complete.  An M-SCTP ESTABLISH indication is sent to Layer
 Management upon successful completion of an incoming SCTP association
 setup from a peer IUA node.

Morneault, et al. Standards Track [Page 46] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 An M-SCTP RELEASE request from Layer Management will initiate the
 teardown of an SCTP association.  An M-SCTP RELEASE confirm will be
 sent by Layer Management when the association teardown is complete.
 An M-SCTP RELEASE indication is sent to Layer Management upon
 successful teardown of an SCTP association initiated by a peer IUA.
 M-SCTP STATUS request and indication support a Layer Management query
 of the local status of a particular SCTP association.
 M-NOTIFY indication and M-ERROR indication indicate to Layer
 Management the notification or error information contained in a
 received IUA Notify or Error message, respectively.  These
 indications can also be generated based on local IUA events.
 M-ASP STATUS request/indication and M-AS-STATUS request/indication
 support a Layer Management query of the local status of a particular
 ASP or AS.  No IUA peer protocol is invoked.
 M-ASP-UP request, M-ASP-DOWN request, M-ASP-INACTIVE request, and
 M-ASP-ACTIVE request allow Layer Management at an ASP to initiate
 state changes.  These requests result in outgoing IUA ASP UP, ASP
 DOWN, ASP INACTIVE, and ASP ACTIVE messages.
 M-ASP-UP confirmation, M-ASP-DOWN confirmation, M-ASP-INACTIVE
 confirmation, and M-ASP-ACTIVE confirmation indicate to Layer
 Management that the previous request has been confirmed.
 Upon receipt of an M-TEI Status primitive from Layer Management, the
 IUA will send the corresponding MGMT message (TEI Status) to its
 peer.  While doing so, the IUA layer needs to fill various fields of
 the common and specific headers correctly.
 All MGMT messages are sent on a sequenced stream to ensure ordering.
 SCTP stream '0' SHOULD be used.

4.2.2. Receipt of IUA Peer Management Messages

 Upon receipt of IUA Management messages, the IUA layer MUST invoke
 the corresponding Layer Management primitive indications (e.g., M-AS
 Status ind., M-ASP Status ind., M-ERROR ind., M-TEI STATUS) to the
 local layer management.
 M-NOTIFY indication and M-ERROR indication indicate to Layer
 Management the notification or error information contained in a
 received IUA Notify or Error message.  These indications can also be
 generated based on local IUA events.

Morneault, et al. Standards Track [Page 47] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 All MGMT messages are sent on a sequenced stream to ensure ordering.
 SCTP stream '0' SHOULD be used.

4.3. Procedures to Support Service in Section 1.4.3

 These procedures achieve the IUA layer's "Support for management of
 active associations between SG and MGC" service.

4.3.1. AS and ASP State Maintenance

 The IUA layer on the SG needs to maintain the states of each ASP as
 well as the state of the AS.

4.3.1.1. ASP States

 The state of the each ASP, in each AS that it is configured, is
 maintained in the IUA layer on the SG.  The state of an ASP changes
 due to the following type of events:
  • Reception of messages from peer IUA layer at that ASP
  • Reception of some messages from the peer IUA layer at other

ASPs in the AS

  • Reception of indications from SCTP layer
  • Local Management intervention
 The ASP state transition diagram is shown in Figure 6.  The possible
 states of an ASP are the following:
 ASP-DOWN: Application Server Process is unavailable and/or the
 related SCTP association is down.  Initially, all ASPs will be in
 this state.  An ASP in this state SHOULD NOT be sent any IUA
 messages.
 ASP-INACTIVE: The remote IUA peer at the ASP is available (and the
 related SCTP association is up) but application traffic is stopped.
 In this state, the ASP can be sent any non-QPTM IUA messages (except
 for TEI Status messages).
 ASP-ACTIVE: The remote IUA peer at the ASP is available and
 application traffic is active.

Morneault, et al. Standards Track [Page 48] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

                                  +--------------+
           +----------------------|              |
           |   Alternate  +-------|  ASP-ACTIVE  |
           |       ASP    |       +--------------+
           |    Takeover  |           ^     |
           |              |    ASP    |     | ASP Inactive /
           |              |    Active |     | ASP Up
           |              |           |     v
           |              |       +--------------+
           |              |       |              |
           |              +------>| ASP-INACTIVE |
           |                      +--------------+
           |                          ^    |
 ASP Down/ |                     ASP  |    | ASP Down /
 SCTP CDI/ |                     Up   |    | SCTP CDI /
 SCTP RI   |                          |    v SCTP RI
           |                      +--------------+
           +--------------------->|              |
                                  |   ASP-DOWN   |
                                  +--------------+
                Figure 6.  ASP State Transition Diagram
 SCTP CDI:  The local SCTP layer's Communication Down Indication to
 the Upper Layer Protocol (IUA) on an SG.  The local SCTP will send
 this indication when it detects the loss of connectivity to the ASP's
 peer SCTP layer.  SCTP CDI is understood as either a SHUTDOWN
 COMPLETE notification and COMMUNICATION LOST notification from the
 SCTP.
 SCTP RI: The local SCTP layer's Restart indication to the upper layer
 protocol (IUA) on an SG.  The local SCTP will send this indication
 when it detects a restart from the ASP's peer SCTP layer.

4.3.1.2. AS States

 The state of the AS is maintained in the IUA layer on the SG.
 The state of an AS changes due to events.  These events include the
 following:
  • ASP state transitions
  • Recovery timer triggers

Morneault, et al. Standards Track [Page 49] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The possible states of an AS are the following:
 AS-DOWN: The Application Server is unavailable.  This state implies
 that all related ASPs are in the ASP-DOWN state for this AS.
 Initially, the AS will be in this state.
 AS-INACTIVE: The Application Server is available but no application
 traffic is active (i.e., one or more related ASPs are in the
 ASP-INACTIVE state, but none in the ASP-ACTIVE state).  The recovery
 timer T(r) is not running or has expired.
 AS-ACTIVE: The Application Server is available and application
 traffic is active.  This state implies that at least one ASP is in
 the ASP-ACTIVE state.
 AS-PENDING: An active ASP has transitioned from active to inactive or
 down and it was the last remaining active ASP in the AS.  A recovery
 timer T(r) will be started and all incoming SCN messages will be
 queued by the SG.  If an ASP becomes active before T(r) expires, the
 AS will move to AS-ACTIVE state and all the queued messages will be
 sent to the active ASP.
 If T(r) expires before an ASP becomes active, the SG stops queuing
 messages and discards all previously queued messages.  The AS will
 move to AS-INACTIVE if at least one ASP is in ASP-INACTIVE state,
 otherwise it will move to AS-DOWN state.

Morneault, et al. Standards Track [Page 50] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

    +----------+ one ASP trans to ASP-ACTIVE +-------------+
    |    AS-   |---------------------------->|     AS-     |
    | INACTIVE |                             |   ACTIVE    |
    |          |<---                         |             |
    +----------+    \                        +-------------+
       ^   |         \ Tr Expiry,                ^    |
       |   |          \ at least one             |    |
       |   |           \ ASP in ASP-INACTIVE     |    |
       |   |            \                        |    |
       |   |             \                       |    |
       |   |              \                      |    |

one ASP | | all ASP \ one ASP | | Last ACTIVE trans | | trans to \ trans to | | ASP trans to to | | ASP-DOWN ——-\ ASP- | | ASP-INACTIVE ASP- | | \ ACTIVE | | or ASP-DOWN INACTIVE| | \ | | (start Tr)

       |   |                           \         |    |
       |   |                            \        |    |
       |   v                             \       |    v
    +----------+                          \  +-------------+
    |          |                           --|             |
    | AS-DOWN  |                             | AS-PENDING  |
    |          |                             |  (queueing) |
    |          |<----------------------------|             |
    +----------+    Tr Expiry and no ASP     +-------------+
                   in ASP-INACTIVE state
   Tr = Recovery Timer
               Figure 7: AS State Transition Diagram

4.3.2. ASPM Procedures for Primitives

 Before the establishment of an SCTP association, the ASP state at
 both the SG and ASP is assumed to be in the state ASP-DOWN.
 As the ASP is responsible for initiating the setup of an SCTP
 association to an SG, the IUA layer at an ASP receives an M-SCTP
 ESTABLISH request primitive from the Layer Management, the IUA layer
 will try to establish an SCTP association with the remote IUA peer at
 an SG.  Upon reception of an eventual SCTP-Communication Up confirm
 primitive from the SCTP, the IUA layer will invoke the primitive
 M-SCTP ESTABLISH confirm to the Layer Management.
 At the SG, the IUA layer will receive an SCTP Communication Up
 indication primitive from the SCTP.  The IUA layer will then invoke
 the primitive M-SCTP ESTABLISH indication to the Layer Management.

Morneault, et al. Standards Track [Page 51] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 Once the SCTP association is established and assuming that the local
 IUA-User is ready, the local ASP IUA Application Server Process
 Maintenance (ASPM) function will initiate the ASPM procedures, using
 the ASP Up/-Down/-Active/-Inactive messages to convey the ASP state
 to the SG (see Section 4.3.3).
 The Layer Management and the IUA layer on SG can communicate the
 status of the application server using the M-AS_STATUS primitives.
 The Layer Management and the IUA layer on both the SG and ASP can
 communicate the status of an SCTP association using the M-SCTP_STATUS
 primitives.
 If the Layer Management on SG or ASP wants to bring down an SCTP
 association for management reasons, it would send M-SCTP RELEASE
 request primitive to the local IUA layer.  The IUA layer would
 release the SCTP association and upon receiving the SCTP-
 COMMUNICATION_DOWN indication from the underlying SCTP layer, it
 would inform the local Layer Management using M-SCTP_RELEASE confirm
 primitive.
 If the IUA layer receives an SCTP-COMMUNICATION_DOWN indication from
 the underlying SCTP layer, it will inform the Layer Management by
 invoking the M-SCTP RELEASE indication primitive.  The state of the
 ASP will be moved to "Down" at both the SG and ASP.
 At an ASP, the Layer Management MAY try to reestablish the SCTP
 association using M-SCTP_ESTABLISH request primitive.
 In the case of an SCTP-RESTART indication at an ASP, the ASP is now
 considered by its IUA peer to be in the ASP-DOWN state.  The ASP, if
 it is to recover, must begin any recovery with the ASP Up procedure.

4.3.3. ASPM Procedures for Peer-to-Peer Messages

 All ASPM messages are sent on a sequenced stream to ensure ordering.
 SCTP stream '0' SHOULD be used.

4.3.3.1. ASP Up Procedures

 After an ASP has successfully established an SCTP association to an
 SG, the SG waits for the ASP to send an ASP Up message, indicating
 that the ASP IUA peer is available.  The ASP is always the initiator
 of the ASP Up message.  This action MAY be initiated at the ASP by an
 M-ASP_UP request primitive from Layer Management or MAY be initiated
 automatically by an IUA management function.

Morneault, et al. Standards Track [Page 52] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 When an ASP Up message is received at an SG and internally the remote
 ASP is in the ASP-DOWN state and not considered locked out for local
 management reasons, the SG marks the remote ASP in the state
 ASP-INACTIVE and informs Layer Management with an M-ASP_Up indication
 primitive.  If the SG is aware, via current configuration data, which
 Application Servers the ASP is configured to operate in, the SG
 updates the ASP state to ASP-INACTIVE in each AS that it is a member.
 Alternatively, the SG may move the ASP into a pool of Inactive ASPs
 available for future configuration within Application Server(s),
 determined in a subsequent ASP Active procedure.  If the ASP Up
 message contains an ASP Identifier, the SG should save the ASP
 Identifier for that ASP.  The SG MUST send an ASP Up Ack message in
 response to a received ASP Up message even if the ASP is already
 marked as ASP-INACTIVE at the SG.
 If for any local reason (e.g., management lockout) the SG cannot
 respond with an ASP Up Ack message, the SG responds to an ASP Up
 message with an Error message with reason "Refused - Management
 Blocking".
 At the ASP, the ASP Up Ack message received is not acknowledged.
 Layer Management is informed with an M-ASP_UP confirm primitive.
 When the ASP sends an ASP Up message, it starts timer T(ack).  If the
 ASP does not receive a response to an ASP Up message within T(ack),
 the ASP MAY restart T(ack) and resend ASP Up messages until it
 receives an ASP Up Ack message.  T(ack) is provisionable, with a
 default of 2 seconds.  Alternatively, retransmission of ASP Up
 messages MAY be put under control of Layer Management.  In this
 method, expiry of T(ack) results in an M-ASP_UP confirm primitive
 carrying a negative indication.
 The ASP must wait for the ASP Up Ack message before sending any other
 IUA messages (e.g., ASP Active).  If the SG receives any other IUA
 messages before an ASP Up message is received (other than ASP Down;
 see Section 4.3.3.2), the SG MAY discard them.
 If an ASP Up message is received and internally the remote ASP is in
 the ASP-ACTIVE state, an ASP Up Ack message is returned, as well as
 an Error message ("Unexpected Message"), and the remote ASP state is
 changed to ASP-INACTIVE in all relevant Application Servers.
 If an ASP Up message is received and internally the remote ASP is
 already in the ASP-INACTIVE state, an ASP Up Ack message is returned
 and no further action is taken.

Morneault, et al. Standards Track [Page 53] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

4.3.3.2. ASP Down Procedures

 The ASP will send an ASP Down message to an SG when the ASP wishes to
 be removed from the list of ASPs in all Application Servers that it
 is a member and no longer receive any IUA QPTM or ASPTM messages.
 This action MAY be initiated at the ASP by an M-ASP_DOWN request
 primitive from Layer Management or MAY be initiated automatically by
 an IUA management function.
 Whether the ASP is permanently removed from an AS is a function of
 configuration management.
 The SG marks the ASP as ASP-DOWN, informs Layer Management with an
 M-ASP_Down indication primitive, and returns an ASP Down Ack message
 to the ASP.
 The SG MUST send an ASP Down Ack message in response to a received
 ASP Down message from the ASP even if the ASP is already marked as
 ASP-DOWN at the SG.
 At the ASP, the ASP Down Ack message received is not acknowledged.
 Layer Management is informed with an M-ASP_DOWN confirm primitive.
 If the ASP receives an ASP Down Ack without having sent an ASP Down
 message, the ASP should now consider itself as in the ASP-DOWN state.
 If the ASP was previously in the ASP-ACTIVE or ASP-INACTIVE state,
 the ASP should then initiate procedures to return itself to its
 previous state.
 When the ASP sends an ASP Down message, it starts timer T(ack).  If
 the ASP does not receive a response to an ASP Down message within
 T(ack), the ASP MAY restart T(ack) and resend ASP Down messages until
 it receives an ASP Down Ack message.  T(ack) is provisionable, with a
 default of 2 seconds.  Alternatively, retransmission of ASP Down
 messages MAY be put under control of Layer Management.  In this
 method, expiry of T(ack) results in an M-ASP_DOWN confirm primitive
 carrying a negative indication.

4.3.3.3. IUA Version Control

 If a ASP Up message with an unsupported version is received, the
 receiving end responds with an Error message, indicating the version
 the receiving node supports and notifies Layer Management.
 This is useful when protocol version upgrades are being performed in
 a network.  A node upgraded to a newer version SHOULD support the
 older versions used on other nodes it is communicating with.  Because
 ASPs initiate the ASP Up procedure it is assumed that the Error
 message would normally come from the SG.

Morneault, et al. Standards Track [Page 54] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

4.3.3.4. ASP Active Procedures

 Any time after the ASP has received an ASP Up Ack from the SG, the
 ASP sends an ASP Active message to the SG indicating that the ASP is
 ready to start processing traffic.  This action MAY be initiated at
 the ASP by an M-ASP_ACTIVE request primitive from Layer Management or
 MAY be initiated automatically by an IUA management function.  In the
 case where an ASP is configured/registered to process the traffic for
 more than one Application Server across an SCTP association, the
 ASPAC contains one or more Interface Identifiers to indicate for
 which Application Servers the ASPAC applies.
 If the Application Server can be successfully activated, the SG
 responds to the ASP with an ASPAC Ack message acknowledging that the
 ASPAC message was received and starts sending traffic for the
 Application Server to that ASP.
 In the case where an "out-of-the-blue" ASP Active message is received
 (i.e., the ASP has not registered with the SG or the SG has no static
 configuration data for the ASP), the message MAY be silently
 discarded.
 The SG MUST send an ASP Active Ack message in response to a received
 ASP Active message from the ASP, if the ASP is already marked in the
 ASP-ACTIVE state at the SG.
 At the ASP, the ASP Active Ack message received is not acknowledged.
 Layer Management is informed with an M-ASP_ACTIVE confirm primitive.
 It is possible for the ASP to receive Data message(s) before the ASP
 Active Ack message as the ASP Active Ack and Data messages from an SG
 may be sent on different SCTP streams.  Message loss is possible as
 the ASP does not consider itself in the ASP-ACTIVE state until
 reception of the ASP Active Ack message.
 When the ASP sends an ASP Active message, it starts timer T(ack).  If
 the ASP does not receive a response to an ASP Active message within
 T(ack), the ASP MAY restart T(ack) and resend ASP Active messages
 until it receives an ASP Active Ack message.  T(ack) is
 provisionable, with a default of 2 seconds.  Alternatively,
 retransmission of ASP Active messages MAY be put under control of
 Layer Management.  In this method, expiry of T(ack) results in an M-
 ASP_ACTIVE confirm primitive carrying a negative indication.
 The ASP MUST wait for the ASP Active Ack message from the SG before
 sending any Data messages or it will risk message loss.  If the SG
 receives QPTM messages before an ASP Active is received, the SG
 SHOULD discard these messages.

Morneault, et al. Standards Track [Page 55] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 There are two modes of Application Server traffic handling in the SG
 IUA: Over-ride and Load-sharing.  The Type parameter in the ASPAC
 message indicates the mode used in a particular Application Server.
 If the SG determines that the mode indicates in an ASPAC is
 incompatible with the traffic handling mode currently used in the AS,
 the SG responds with an Error message indicating Unsupported Traffic
 Handling Mode.
 In the case of an Over-ride mode AS, reception of an ASPAC message at
 an SG causes the redirection of all traffic for the AS to the ASP
 that sent the ASPAC.  The SG responds to the ASPAC with an ASP Active
 Ack message to the ASP.  Any previously active ASP in the AS is now
 considered Inactive and will no longer receive traffic from the SG
 within the AS.  The SG sends a Notify (Alternate ASP Active) to the
 previously active ASP in the AS, after stopping all traffic to that
 ASP.
 In the case of a load-share mode AS, reception of an ASPAC message at
 an SG causes the direction of traffic to the ASP sending the ASPAC,
 in addition to all the other ASPs that are currently active in the
 AS.  The algorithm at the SG for load-sharing traffic within an AS to
 all the active ASPs is implementation dependent.  The algorithm
 could, for example, be round-robin or based on information in the
 Data message, such as Interface Identifier, depending on the
 requirements of the application and the call state handling
 assumptions of the collection of ASPs in the AS.  The SG responds to
 the ASPAC with an ASP Active Ack message to the ASP.

4.3.3.5. ASP Inactive Procedures

 When an ASP wishes to withdraw from receiving traffic within an AS,
 the ASP sends an ASP Inactive message to the SG.  This action MAY be
 initiated at the ASP by an M-ASP_INACTIVE request primitive from
 Layer Management or MAY be initiated automatically by an IUA
 management function.  In the case where an ASP is configured/
 registered to process the traffic for more than one Application
 Server across an SCTP association, the ASPIA contains one or more
 Interface Identifiers to indicate for which Application Servers the
 ASP Inactive message applies.
 There are two modes of Application Server traffic handling in the SG
 IUA when withdrawing an ASP from service: Over-ride and Load-sharing.
 In the case of an Over-ride mode AS, where normally another ASP has
 already taken over the traffic within the AS with an Over-ride ASPAC
 message, the ASP that sends the ASPIA message is already considered
 by the SG to be ASP-INACTIVE.  An ASPIA Ack message is sent to the
 ASP, after ensuring that all traffic is stopped to the ASP.

Morneault, et al. Standards Track [Page 56] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 In the case of a Load-share mode AS, the SG moves the ASP to the
 ASP-INACTIVE state and the AS traffic is re-allocated across the
 remaining ASP-ACTIVE ASPs per the load-sharing algorithm currently
 used within the AS.  An ASPIA Ack message is sent to the ASP after
 all traffic is halted to the ASP.  A Notify (Insufficient ASPs)
 message MAY be sent to all inactive ASPs, if required.
 When the ASP sends an ASP Inactive message it starts timer T(ack).
 If the ASP does not receive a response to an ASP Inactive message
 within T(ack), the ASP MAY restart T(ack) and resend ASP Inactive
 messages until it receives an ASP Inactive Ack message.  T(ack) is
 provisionable, with a default of 2 seconds.  Alternatively,
 retransmission of ASP Inactive messages MAY be put under control of
 Layer Management.  In this method, expiry of T(ack) results in a M-
 ASP_Inactive confirm primitive carrying a negative indication.
 If no other ASPs in the Application Server are in the state
 ASP-ACTIVE, the SG MUST send a Notify ("AS-Pending") message to all
 of the ASPs in the AS that are in the state ASP-INACTIVE.  The SG
 SHOULD start buffering the incoming messages for T(r) seconds, after
 which messages MAY be discarded.  T(r) is configurable by the network
 operator.  If the SG receives an ASP Active message from an ASP in
 the AS before expiry of T(r), the buffered traffic is directed to
 that ASP and the timer is cancelled.  If T(r) expires, the AS is
 moved to the AS-INACTIVE state.
 At the ASP, the ASP Inactive Ack message received is not
 acknowledged.  Layer Management is informed with an M-ASP_INACTIVE
 confirm primitive.  If the ASP receives an ASP Inactive Ack without
 having sent an ASP Inactive message, the ASP should now consider
 itself as in the ASP-INACTIVE state.  If the ASP was previously in
 the ASP-ACTIVE state, the ASP should then initiate procedures to
 return itself to its previous state.

4.3.3.6. Notify Procedures

 A Notify message reflecting a change in the AS state MUST be sent to
 all ASPs in the AS, except those in the ASP-DOWN state, with
 appropriate Status Information and any ASP Identifier of the failed
 ASP.  At the ASP, Layer Management is informed with an M-NOTIFY
 indication primitive.  The Notify message must be sent whether the AS
 state change was a result of an ASP failure or reception of an ASP
 State Management (ASPSM) / ASP Traffic Management (ASPTM) message.
 In the second case, the Notify message MUST be sent after any related
 acknowledgement messages  (e.g., ASP Up Ack, ASP Down Ack, ASP Active
 Ack, or ASP Inactive Ack).

Morneault, et al. Standards Track [Page 57] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 In the case where a Notify ("AS-Pending") message is sent by an SG
 that now has no ASPs active to service the traffic, or a NTFY
 ("Insufficient ASPs") is sent in the Load-share mode, the Notify does
 not explicitly compel the ASP(s) receiving the message to become
 active.  The ASPs remain in control of what (and when) action is
 taken.

4.3.3.7. Heartbeat

 The optional Heartbeat procedures MAY be used when operating over
 transport layers that do not have their own heartbeat mechanism for
 detecting loss of the transport association (i.e., other than the
 SCTP).
 Either IUA peer may optionally send Heartbeat messages periodically,
 subject to a provisionable timer T(beat).  Upon receiving a Heartbeat
 message, the IUA peer MUST respond with a Heartbeat Ack message.
 If no Heartbeat Ack message (or any other IUA message) is received
 from the IUA peer within 2*T(beat), the remote IUA peer is considered
 unavailable.  Transmission of Heartbeat messages is stopped and the
 signaling process SHOULD attempt to re-establish communication if it
 is configured as the client for the disconnected IUA peer.
 The BEAT message MAY optionally contain an opaque Heartbeat Data
 parameter that MUST be echoed back unchanged in the related Beat Ack
 message.  The ASP upon examining the contents of the returned BEAT
 Ack message MAY choose to consider the remote ASP as unavailable.
 The contents/format of the Heartbeat Data parameter is implementation
 dependent and only of local interest to the original sender.  The
 contents MAY be used, for example, to support a Heartbeat sequence
 algorithm (to detect missing Heartbeats), and/or a timestamp
 mechanism (to evaluate delays).
 Note:  Heartbeat-related events are not shown in Figure 6, "ASP State
 Transition Diagram".

5. Examples

5.1. Establishment of Association and Traffic between SGs and ASPs

5.1.1. Single ASP in an Application Server (1+0 sparing)

 This scenario shows the example IUA message flows for the
 establishment of traffic between an SG and an ASP, where only one ASP
 is configured within an AS (no backup).  It is assumed that the SCTP
 association is already setup.

Morneault, et al. Standards Track [Page 58] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

              SG                       ASP1
               |
               |<---------ASP Up----------|
               |--------ASP Up Ack------->|
               |                          |
               |-----NTFY(AS-INACTIVE)--->|
               |                          |
               |<-------ASP Active--------|
               |------ASP Active Ack----->|
               |                          |
               |------NTFY(AS-ACTIVE)---->|
               |                          |

5.1.2. Two ASPs in Application Server (1+1 sparing)

 This scenario shows the example IUA message flows for the
 establishment of traffic between an SG and two ASPs in the same
 Application Server, where ASP1 is configured to be Active and ASP2 a
 standby in the event of communication failure or the withdrawal from
 service of ASP1.  ASP2 MAY act as a hot, warm, or cold standby
 depending on the extent to which ASP1 and ASP2 share call state or
 can communicate call state under failure/withdrawal events.  The
 example message flow is the same whether the ASP Active messages are
 Over-ride or Load-share mode although typically this example would
 use an Over-ride mode.
        SG                        ASP1                        ASP2
         |                         |                          |
         |<--------ASP Up----------|                          |
         |-------ASP Up Ack------->|                          |
         |                         |                          |
         |----NTFY(AS-INACTIVE)--->|                          |
         |                         |                          |
         |<-----------------------------ASP Up----------------|
         |----------------------------ASP Up Ack------------->|
         |                         |                          |
         |                         |                          |
         |<-------ASP Active-------|                          |
         |-----ASP Active Ack----->|                          |
         |                         |                          |
         |-----NTFY(AS-ACTIVE)---->|                          |
         |----------------------NTFY(AS-ACTIVE)-------------->|

Morneault, et al. Standards Track [Page 59] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

5.1.3. Two ASPs in an Application Server (1+1 sparing, load-sharing

      case)
 This scenario shows a similar case to Section 5.1.2 but where the two
 ASPs are brought to active and load-share the traffic load.  In this
 case, one ASP is sufficient to handle the total traffic load.
        SG                       ASP1                       ASP2
         |                         |                          |
         |<---------ASP Up---------|                          |
         |--------ASP Up Ack------>|                          |
         |                         |                          |
         |----NTFY(AS-INACTIVE)--->|                          |
         |                         |                          |
         |<------------------------------ASP Up---------------|
         |-----------------------------ASP Up Ack------------>|
         |                         |                          |
         |                         |                          |
         |<--ASP Active (Ldshr)----|                          |
         |----ASP Active Ack------>|                          |
         |                         |                          |
         |-----NTFY(AS-ACTIVE)---->|                          |
         |----------------------NTFY(AS-ACTIVE)-------------->|
         |                         |                          |
         |<----------------------------ASP Active (Ldshr)-----|
         |-----------------------------ASP Active Ack-------->|
         |                         |                          |

5.1.4. Three ASPs in an Application Server (n+k sparing, load-sharing

      case)
 This scenario shows the example IUA message flows for the
 establishment of traffic between an SG and three ASPs in the same
 Application Server, where two of the ASPs are brought to active and
 share the load.  In this case, a minimum of two ASPs are required to
 handle the total traffic load (2+1 sparing).

Morneault, et al. Standards Track [Page 60] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

    SG                  ASP1                ASP2                ASP3
     |                    |                   |                   |
     |<------ASP Up-------|                   |                   |
     |-----ASP Up Ack---->|                   |                   |
     |                    |                   |                   |
     |-NTFY(AS-INACTIVE)->|                   |                   |
     |                    |                   |                   |
     |<--------------------------ASP Up-------|                   |
     |-----------------------ASP Up Ack------>|                   |
     |                    |                   |                   |
     |<---------------------------------------------ASP Up--------|
     |--------------------------------------------ASP Up Ack----->|
     |                    |                   |                   |
     |                    |                   |                   |
     |<-ASP Act (Ldshr)---|                   |                   |
     |----ASP Act Ack---->|                   |                   |
     |                    |                   |                   |
     |<---------------------ASP Act (Ldshr)---|                   |
     |----------------------ASP Act Ack------>|                   |
     |                    |                   |                   |
     |--NTFY(AS-ACTIVE)-->|                   |                   |
     |---------------NTFY(AS-ACTIVE)--------->|                   |
     |------------------------NTFY(AS-ACTIVE)-------------------->|

5.1.5. Interface Identifier Configuration Mismatch Example

 This scenario shows the example IUA message flows for the
 establishment of traffic between an SG and an ASP in which some of
 the Interface Identifiers have been misconfigured on the ASP side.
 The SG in this case has Interface Identifiers 1-5 configured for
 ASP1.
              SG                               ASP1
               |                                |
               |                                |
               |<----ASP Active (IIDs 1-10)-----|
               |---ASP Active Ack (IIDs 1-5)--->|
               |-------Error (IIDs 6)---------->|
               |-------Error (IIDs 7)---------->|
               |-------Error (IIDs 8)---------->|
               |-------Error (IIDs 9)---------->|
               |-------Error (IIDs 10)--------->|
               |                                |

Morneault, et al. Standards Track [Page 61] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

5.2. ASP Traffic Fail-over Examples

5.2.1. (1+1 Sparing, withdrawal of ASP, Backup Over-ride)

 The following example shows a case in which an ASP withdraws from
 service:
        SG                       ASP1                       ASP2
         |                         |                          |
         |<-----ASP Inactive-------|                          |
         |----ASP Inactive Ack---->|                          |
         |                         |                          |
         |----NTFY(AS-Pending)---->|                          |
         |-------------------NTFY(AS-Pending)---------------->|
         |                         |                          |
         |<------------------------------ ASP Active----------|
         |-----------------------------ASP Active Ack)------->|
         |                         |                          |
         |----NTFY(AS-ACTIVE)----->|                          |
         |-------------------NTFY(AS-ACTIVE)----------------->|
 In this case, the SG notifies ASP2 that the AS has moved to the Down
 state.  The SG could have also (optionally) sent a Notify message
 when the AS moved to the Pending state.
 Note:  If the SG detects loss of the IUA peer (IUA heartbeat loss or
 detection of SCTP failure), the initial SG-ASP1 ASP Inactive message
 exchange would not occur.

5.2.2. (1+1 Sparing, Backup Over-ride)

 The following example shows a case in which ASP2 wishes to override
 ASP1 and take over the traffic:
        SG                       ASP1                       ASP2
         |                         |                          |
         |<-------------------------------ASP Active----------|
         |-----------------------------ASP Active Ack-------->|
         |----NTFY( Alt ASP-Act)-->|
         |                         |                          |
 In this case, the SG notifies ASP1 that an alternative ASP has
 overridden it.

Morneault, et al. Standards Track [Page 62] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

5.2.3. (n+k Sparing, Load-sharing case, withdrawal of ASP)

 Following on from the example in Section 5.1.4, and ASP1 withdraws
 from service:
   SG                  ASP1                 ASP2                 ASP3
    |                    |                   |                   |
    |<----ASP Inact------|                   |                   |
    |---ASP Inact Ack--->|                   |                   |
    |                    |                   |                   |
    |---------------------------------NTFY(Ins. ASPs)----------->|
    |                    |                   |                   |
    |<-----------------------------------------ASP Act (Ldshr)---|
    |-------------------------------------------ASP Act (Ack)--->|
    |                    |                   |                   |
 In this case, the SG has knowledge of the minimum ASP resources
 required (implementation dependent), for example, if the SG knows
 that n+k = 2+1 for a load-share AS and n currently equals 1.
 Note:  If the SG detects loss of the ASP1 IUA peer (IUA heartbeat
 loss or detection of SCTP failure), the first SG-ASP1 ASP Inactive
 message exchange would not occur.

5.3. Q.921/Q.931 Primitives Backhaul Examples

 When the IUA layer on the ASP has a QPTM message to send to the SG,
 it will do the following:
  1. Determine the correct SG
  1. Find the SCTP association to the chosen SG
  1. Determine the correct stream in the SCTP association based on

the D channel

  1. Fill in the QPTM message, fill in IUA Message Header, fill in

Common Header

  1. Send the QPTM message to the remote IUA peer in the SG, over

the SCTP association

 When the IUA layer on the SG has a QPTM message to send to the ASP,
 it will do the following:
  1. Determine the AS for the Interface Identifier
  1. Determine the Active ASP (SCTP association) within the AS

Morneault, et al. Standards Track [Page 63] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

  1. Determine the correct stream in the SCTP association based on

the D channel

  1. Fill in the QPTM message, fill in IUA Message Header, fill in

Common Header

  1. Send the QPTM message to the remote IUA peer in the ASP, over

the SCTP association

 An example of the message flows for establishing a data link on a
 signaling channel, passing PDUs and releasing a data link on a
 signaling channel is shown below.  An active association between MGC
 and SG is established (Section 5.1) prior to the following message
 flows.
          SG                             ASP
                      <----------- Establish Request
    Establish Confirm  ---------->
                      <----------- Data Request
       Data Indication ----------->
                      <----------- Data Request
       Data Indication ----------->
                      <----------- Data Request
                      <----------- Data Request
       Data Indication ----------->
                      <----------- Release Request (RELEASE_MGMT)
      Release Confirm  ---------->
 An example of the message flows for a failed attempt to establish a
 data link on the signaling channel is shown below.  In this case, the
 gateway has a problem with its physical connection (e.g., Red Alarm),
 so it cannot establish a data link on the signaling channel.
          SG                             ASP
                      <----------- Establish Request (ESTABLISH_START)
    Release Indication ---------->
    (RELEASE_PHYS)

5.4. Layer Management Communication Examples

 An example of the message flows for communication between Layer
 Management modules between SG and ASP is shown below.  An active
 association between ASP and SG is established (Section 5.1) prior to
 the following message flows.

Morneault, et al. Standards Track [Page 64] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

                SG                       ASP
                      <----------- Data Request
      Error Indication ---------->
       (INVALID_TEI)
                      <----------- TEI Status Request
    TEI Status Confirm ---------->
         (Unassigned)

6. Security

 The security considerations discussed in "Security Considerations for
 SIGTRAN Protocols", RFC 3788 [3], apply to this document.

7. IANA Considerations

7.1. SCTP Payload Protocol Identifier

 The IANA has assigned an IUA value for the Payload Protocol
 Identifier in SCTP Payload Data chunk.  The following SCTP Payload
 Protocol Identifier has been registered:
       IUA    "1"
 The SCTP Payload Protocol Identifier is included in each SCTP Data
 chunk, to indicate which protocol the SCTP is carrying.  This Payload
 Protocol Identifier is not directly used by SCTP but MAY be used by
 certain network entities to identify the type of information being
 carried in a Data chunk.
 The User Adaptation peer MAY use the Payload Protocol Identifier as a
 way of determining additional information about the data being
 presented to it by SCTP.

7.2. IUA Protocol Extensions

 This protocol may also be extended through IANA in three ways:
  1. - through definition of additional message classes,
  2. - through definition of additional message types, and
  3. - through definition of additional message parameters.
 The definition and use of new message classes, types, and parameters
 are an integral part of SIGTRAN adaptation layers.  Thus, these
 extensions are assigned by IANA through an IETF Consensus action as
 defined in [7].

Morneault, et al. Standards Track [Page 65] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 The proposed extension must in no way adversely affect the general
 working of the protocol.

7.2.1. IETF-Defined Message Classes

 The documentation for a new message class MUST include the following
 information:
 (a) A long and short name for the message class.
 (b) A detailed description of the purpose of the message class.

7.2.2. IETF-Defined Message Types

 Documentation of the message type MUST contain the following
 information:
 (a) A long and short name for the new message type.
 (b) A detailed description of the structure of the message.
 (c) A detailed definition and description of intended use of each
     field within the message.
 (d) A detailed procedural description of the use of the new
     message type within the operation of the protocol.
 (e) A detailed description of error conditions when receiving this
     message type.
 When an implementation receives a message type that it does not
 support, it MUST respond with an Error (ERR) message with an Error
 Code of Unsupported Message Type.

7.2.3. IETF-Defined TLV Parameter Extension

 Documentation of the message parameter MUST contain the following
 information:
 (a) Name of the parameter type.
 (b) Detailed description of the structure of the parameter field.
     This structure MUST conform to the general type-length-value
     format described in Section 3.1.5.
 (c) Detailed definition of each component of the parameter value.
 (d) Detailed description of the intended use of this parameter type,
     and an indication of whether and under what circumstances
     multiple instances of this parameter type may be found within the
     same message type.

Morneault, et al. Standards Track [Page 66] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

8. Timer Values

 The following are suggestions for default timer values.
 T(r)                                    3-5 seconds
 T(ack)                                  2-5 seconds
 T(beat)   Heartbeat Timer               30 seconds

9. Acknowledgements

 The authors would like to thank Alex Audu, Maria Sonia Vazquez
 Arevalillo, Ming-te Chao, Keith Drage, Norm Glaude, Nikhil Jain,
 Bernard Kuc, Ming Lin, Stephen Lorusso, John Loughney, Barry
 Nagelberg, Neil Olson, Lyndon Ong, Heinz Prantner, Jose Luis Jimenez
 Ramirez, Ian Rytina, Michael Tuexen, and Hank Wang for their valuable
 comments and suggestions.

10. References

10.1. Normative References

 [1]  ITU-T Recommendation Q.920, 'Digital Subscriber signaling System
      No. 1 (DSS1) - ISDN User-Network Interface Data Link Layer -
      General Aspects'
 [2]  Coded Character Set--7-Bit American Standard Code for
      Information Interchange, ANSI X3.4-1986.
 [3]  Loughney, J., Tuexen, M., and J. Pastor-Balbas, "Security
      Considerations for Signaling Transport (SIGTRAN) Protocols", RFC
      3788, June 2004.

10.2. Informative References

 [4]  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
      H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V. Paxson,
      "Stream Control Transmission Protocol", RFC 2960, October 2000.
 [5]  Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L.,
      Lin, H., Juhasz, I., Holdrege, M., and C. Sharp, "Framework
      Architecture for Signaling Transport", RFC 2719, October 1999.
 [6]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [7]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
      Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

Morneault, et al. Standards Track [Page 67] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 [8]  Stone, J., Stewart, R., and D. Otis, "Stream Control
      Transmission Protocol (SCTP) Checksum Change", RFC 3309,
      September 2002.

11. Change Log

 Below is a list of the major changes between this document and RFC
 3057.
 1.  The TEI Query message was added.
 2.  An explanation of the DLCI format (shown in Figure 6) is
     provided.
 3.  Aligned the ASP and AS procedures in Section 4 with RFC3331 and
     RFC3332.
 4.  Alinged the format of the ASPSM and ASPTM messages with RFC3331
     and RFC3332.  These changes include removing the Reason field
     from the ASP Down and ASP Down Ack messages and the Traffic Mode
     Type field from the ASP Inactive and ASP Inactive Ack messages.
 5.  Sections 1.3.3 and 1.3.4 were moved to Appendix A.  A new section
     was added in place of Section 1.3.3.
 6.  The references have been split between Normative and Informative.
 7.  The new Sigtran security document is referenced and Section 6 has
     been updated appropriately.

Morneault, et al. Standards Track [Page 68] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

Appendix A

A.1. Signaling Network Architecture

 A Signaling Gateway is used to support the transport of Q.921-User
 signaling traffic to one or more distributed ASPs (e.g., MGCs).
 Clearly, the IUA protocol is not designed to meet the performance and
 reliability requirements for such transport by itself.  However, the
 conjunction of distributed architecture and redundant networks does
 allow for a sufficiently reliable transport of signaling traffic over
 IP.  The IUA protocol is flexible enough to allow its operation and
 management in a variety of physical configurations, enabling Network
 Operators to meet their performance and reliability requirements.
 To meet the ISDN signaling reliability and performance requirements
 for carrier grade networks, Network Operators SHOULD ensure that
 there is no single point of failure provisioned in the end-to-end
 network architecture between an ISDN node and an IP ASP.
 Depending of course on the reliability of the SG and ASP functional
 elements, this can typically be met by the provision of redundant
 Quality of Service (QoS)-bounded IP network paths for SCTP
 Associations between SCTP End Points, and redundant Hosts, and
 redundant SGs.  The distribution of ASPs within the available Hosts
 is also important.  For a particular Application Server, the related
 ASPs SHOULD be distributed over at least two Hosts.
 An example logical network architecture relevant to carrier-grade
 operation in the IP network domain is shown in Figure 8 below:

Morneault, et al. Standards Track [Page 69] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

                                                        Host1
   ********                                         **************
   *      *_________________________________________*  ********  *
   *      *                                _________*  * ASP1 *  *
   *  SG1 *   SCTP Associations           |         *  ********  *
   *      *_______________________        |         *            *
   ********                       |       |         **************
                                  |       |
   ********                       |       |
   *      *_______________________________|
   *      *                       |
   *  SG2 *    SCTP Associations  |
   *      *____________           |
   *      *            |          |                     Host2
   ********            |          |                 **************
                       |          |_________________*  ********  *
                       |____________________________*  * ASP1 *  *
                                                    *  ********  *
                                                    *            *
                                                    **************
                                                            .
                                                            .
                                                            .
                    Figure 8.  Logical Model Example
 For carrier-grade networks, the failure or isolation of a particular
 ASP SHOULD NOT cause stable calls to be dropped.  This implies that
 ASPs need, in some cases, to share the call state or be able to pass
 the call state between each other.  However, this sharing or
 communication of call state information is outside the scope of this
 document.

A.2. Application Server Process Redundancy

 To avoid a single point of failure, it is recommended that a minimum
 of two ASPs be in the list, resident in separate hosts and therefore
 available over different SCTP Associations.  For example, in the
 network shown in Figure 8, all messages from a particular D Channel
 (Interface Identifier) could be sent to ASP1 in Host1 or ASP1 in
 Host2.  The AS list at SG1 might look like the following:
    Interface Identifier(s) - Application Server #1
        ASP1/Host1  - State=Up, Active
        ASP1/Host2  - State=Up, Inactive

Morneault, et al. Standards Track [Page 70] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

 In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming
 message for the Interface Identifiers registered.  ASP1 in Host2
 would normally be brought to the active state upon failure of, or
 loss of connectivity to, ASP1/Host1.  In this example, both ASPs are
 Up, meaning that the related SCTP association and far-end IUA peer
 are ready.
 The AS List at SG1 might also be set up in load-share mode as shown
 below:
    Interface Identifier(s) - Application Server #1
        ASP1/Host1 - State=Up, Active
        ASP1/Host2 - State=Up, Active
 In this case, both the ASPs would be sent a portion of the traffic.
 In the process of fail-over, it is recommended that in the case of
 ASPs supporting call processing, stable calls do not get released.
 It is possible that calls in transition MAY fail, although measures
 of communication between the ASPs involved can be used to mitigate
 this problem.  For example, the two ASPs MAY share call state via
 shared memory, or MAY use an ASP-to-ASP protocol to pass call state
 information.  The ASP-to-ASP protocol is outside the scope of this
 document.

Morneault, et al. Standards Track [Page 71] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

Authors' Addresses

 Ken Morneault
 Cisco Systems Inc.
 13615 Dulles Technology Drive
 Herndon, VA. 20171
 USA
 Phone: +1-703-484-3323
 EMail: kmorneau@cisco.com
 Malleswar Kalla
 Telcordia Technologies
 PYA 2J-341
 3 Corporate Place
 Piscataway, NJ 08854
 USA
 Phone: +1-732-699-3728
 EMail: mkalla@telcordia.com
 Selvam Rengasami
 Tridea Works
 Phone: +1-732-512-0969
 EMail: selvam@trideaworks.com
 Greg Sidebottom
 Signatus Technologies
 Kanata, Ontario, Canada
 EMail: greg@signatustechnologies.com

Morneault, et al. Standards Track [Page 72] RFC 4233 ISDN Q.921-User Adaptation Layer January 2006

Full Copyright Statement

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Morneault, et al. Standards Track [Page 73]

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