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rfc:rfc4165

Network Working Group T. George Request for Comments: 4165 B. Bidulock Category: Standards Track OpenSS7

                                                           R. Dantu
                                          University of North Texas
                                                    H. Schwarzbauer
                                                            Siemens
                                                       K. Morneault
                                                      Cisco Systems
                                                     September 2005
     Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) -
             User Peer-to-Peer Adaptation Layer (M2PA)

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 (2005).

Abstract

 This document defines a protocol supporting the transport of
 Signaling System Number 7 (SS7) Message Transfer Part (MTP) Level 3
 signaling messages over Internet Protocol (IP) using the services of
 the Stream Control Transmission Protocol (SCTP).  This protocol would
 be used between SS7 Signaling Points using the MTP Level 3 protocol.
 The SS7 Signaling Points may also use standard SS7 links using the
 SS7 MTP Level 2 to provide transport of MTP Level 3 signaling
 messages.  The protocol operates in a manner similar to MTP Level 2
 so as to provide peer-to-peer communication between SS7 endpoints.

George, et al. Standards Track [Page 1] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

Table of Contents

 1. Introduction ....................................................3
    1.1. Scope ......................................................3
    1.2. Terminology ................................................3
    1.3. Abbreviations ..............................................4
    1.4. Conventions ................................................5
    1.5. Signaling Transport Architecture ...........................5
    1.6. Services Provided by M2PA ..................................7
    1.7. Functions Provided by M2PA .................................9
    1.8. Definition of the M2PA Boundaries .........................10
    1.9. Differences Between M2PA and M2UA .........................10
 2. Protocol Elements ..............................................12
    2.1. Common Message Header .....................................12
    2.2. M2PA Header ...............................................13
    2.3. M2PA Messages .............................................14
 3. State Control ..................................................17
    3.1. SCTP Association State Control ............................17
    3.2. M2PA Link State Control ...................................18
 4. Procedures .....................................................19
    4.1. Procedures to Support MTP2 Features .......................19
    4.2. Procedures to Support the MTP3/MTP2 Interface .............30
    4.3. SCTP Considerations .......................................33
 5. Examples of M2PA Procedures ....................................34
    5.1. Link Initialization (Alignment) ...........................34
    5.2. Message Transmission and Reception ........................37
    5.3. Link Status Indication ....................................37
    5.4. Link Status Message (Processor Outage) ....................38
    5.5. Level 2 Flow Control ......................................42
    5.6. MTP3 Signaling Link Congestion ............................44
    5.7. Link Deactivation .........................................45
    5.8. Link Changeover ...........................................45
 6. Security Considerations ........................................47
 7. IANA Considerations ............................................47
    7.1. SCTP Payload Protocol Identifier ..........................47
    7.2. M2PA Protocol Extensions ..................................48
 8. Acknowledgements ...............................................49
 9. References .....................................................50
    9.1. Normative References ......................................50
    9.2. Informative References ....................................51

George, et al. Standards Track [Page 2] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

1. Introduction

1.1. Scope

 There is a need for Switched Circuit Network (SCN) signaling protocol
 delivery over an IP network.  This includes message transfer between
 the following:
  1. a Signaling Gateway (SG) and a Media Gateway Controller (MGC)

[RFC2719]

  1. a SG and an IP Signaling Point (IPSP)
  1. an IPSP and an IPSP
 This could allow for convergence of some signaling and data networks.
 SCN signaling nodes would have access to databases and other devices
 in the IP network domain that do not use SS7 signaling links.
 Likewise, IP telephony applications would have access to SS7
 services.  There may also be operational cost and performance
 advantages when traditional signaling links are replaced by IP
 network "connections".
 The delivery mechanism described in this document allows for full
 MTP3 message handling and network management capabilities between any
 two SS7 nodes communicating over an IP network.  An SS7 node equipped
 with an IP network connection is called an IP Signaling Point (IPSP).
 The IPSPs function as traditional SS7 nodes using the IP network
 instead of SS7 links.
 The delivery mechanism should:
  1. Support seamless operation of MTP3 protocol peers over an IP

network connection.

  1. Support the MTP Level 2 / MTP Level 3 interface boundary.
  1. Support management of SCTP transport associations and traffic

instead of MTP2 Links.

  1. Support asynchronous reporting of status changes to management.

1.2. Terminology

 MTP  - The Message Transfer Part of the SS7 protocol [Q.700] [Q.701]
 [Q.702] [Q.703] [Q.704] [Q.705] [T1.111].
 MTP2 - MTP Level 2, the MTP signaling link layer.

George, et al. Standards Track [Page 3] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 MTP3 - MTP Level 3, the MTP signaling network layer.
 MTP2-User - A protocol that normally uses the services of MTP Level
 2.  The only MTP2 user is MTP3.  The MTP2 user is equivalent to the
 M2PA user.
 Signaling End Point (SEP) - An SS7 Signaling Point that originates or
 terminates signaling messages.  One example is a central office
 switch.  [RFC2719]
 IP Signaling Point (IPSP) - An SS7 Signaling Point with an IP network
 connection used for SS7 over IP.
 Signaling Gateway (SG) - A signaling agent that receives/sends SCN
 native signaling at the edge of the IP network [RFC2719].  In this
 context, an SG is an SS7 Signaling Point that has both an IP network
 connection used for SS7 over IP, and a traditional (non-IP) link to
 an SS7 network.
 Signal Transfer Point (STP) - A Signal Transfer Point as defined by
 MTP standards, e.g., [Q.700].
 Signaling Point (STP) - A Signaling Point as defined by MTP
 standards, e.g., [Q.700].
 Association - An association refers to an SCTP association [RFC2960].
 The association provides the transport for MTP3 protocol data units
 and M2PA adaptation layer peer messages.
 Network Byte Order - Most significant byte first, also known as "Big
 Endian".  See [RFC791], Appendix B "Data Transmission Order".
 Stream - A stream refers to an SCTP stream [RFC2960].

1.3. Abbreviations

 BSNT   - Backward Sequence Number to be Transmitted
 FSNC   - Forward Sequence Number of last message accepted by remote
          level 2
 LI     - Length Indicator
 MSU    - Message Signal Unit
 SCCP   - Signaling Connection Control Part
 SCN    - Switched Circuit Network

George, et al. Standards Track [Page 4] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 SCTP   - Stream Control Transmission Protocol
 SIF    - Signaling Information Field
 SIO    - Service Information Octet
 SLC    - Signaling Link Code
 SS7    - Signaling System Number 7
 SSN    - Stream Sequence Number
 STP    - Signal Transfer Point

1.4. Conventions

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].

1.5. Signaling Transport Architecture

 The architecture that has been defined [RFC2719] for Switched Circuit
 Network (SCN) signaling transport over IP uses multiple components,
 including an IP transport protocol, the Stream Control Transmission
 Protocol (SCTP), and an adaptation module to support the services
 expected by a particular SCN signaling protocol from its underlying
 protocol layer.
 Within this framework architecture, this document defines an SCN
 adaptation module that is suitable for the transport of SS7 MTP3
 messages.  The adaptation layer, known as the MTP2 User Peer-to-peer
 Adaptation Layer (M2PA), provides MTP3 with an interface and services
 similar to MTP2.  In effect, MTP2 and lower layers of the traditional
 SS7 protocol stack are replaced by an IP equivalent.
 Figure 1 shows the seamless interworking at the MTP3 layer.  MTP3 is
 adapted to the SCTP layer using the MTP2 User Peer-to-peer Adaptation
 Layer (M2PA).  All the primitives between MTP3 and MTP2 are supported
 by M2PA.  The SCTP association acts as one SS7 link between the
 IPSPs.  An IPSP may have the Signaling Connection Control Part (SCCP)
 and other SS7 layers above MTP3.

George, et al. Standards Track [Page 5] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

  • * IP * IPSP *——–* IPSP * +——+ +——+ | TCAP | | TCAP | +——+ +——+ | SCCP | | SCCP | +——+ +——+ | MTP3 | | MTP3 | +——+ +——+ | M2PA | | M2PA | +——+ +——+ | SCTP | | SCTP | +——+ +——+ | IP | | IP | +——+ +——+ IP - Internet Protocol IPSP - IP Signaling Point SCTP - Stream Control Transmission Protocol [RFC2960] Figure 1. M2PA Symmetrical Peer-to-Peer Architecture Figure 2 shows an example of M2PA used in a Signaling Gateway (SG). The SG is an IPSP that is equipped with both traditional SS7 and IP network connections. The SEP and the SG communicate through a traditional SS7 link, which follows a protocol such as [Q.702]. The SG and the IPSP communicate through an IP link using the M2PA protocol. Messages sent from the SEP to the IPSP (and vice versa) are routed by the SG. Any of the nodes in the diagram could have SCCP or other SS7 layers above MTP3. The Signaling Gateway acts as a Signal Transfer Point (STP). Other STPs MAY be present in the SS7 path between the SEP and the SG. George, et al. Standards Track [Page 6] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005 SS7 * IP
  • SEP *——–* SG *——–* IPSP *
  • * *
     +------+                               +------+
     | TCAP |                               | TCAP |
     +------+                               +------+
     | SCCP |                               | SCCP |
     +------+        +-------------+        +------+
     | MTP3 |        |    MTP3     |        | MTP3 |
     +------+        +------+------+        +------+
     | MTP2 |        | MTP2 | M2PA |        | M2PA |
     |      |        |      +------+        +------+
     |      |        |      | SCTP |        | SCTP |
     +------+        +------+------+        +------+
     | MTP1 |        | MTP1 | IP   |        | IP   |
     +------+        +------+------+        +------+
      SEP   - SS7 Signaling Endpoint
          Figure 2.  M2PA in IP Signaling Gateway
 Figure 2 is only an example.  Other configurations are possible.  In
 short, M2PA uses the SCTP association as an SS7 link.  The
 M2PA/SCTP/IP stack can be used in place of an MTP2/MTP1 stack.

1.5.1. Point Code Representation

 MTP requires that each node with an MTP3 layer is identified by an
 SS7 point code.  In particular, each IPSP MUST have its own SS7 point
 code.

1.6. Services Provided by M2PA

 The SS7 MTP3/MTP2 (MTP2-User) interface is retained in the IPSP.  The
 M2PA protocol layer is required to provide a set of services to its
 user equivalent to that provided by MTP Level 2 to MTP Level 3.
 These services are described in the following subsections.

1.6.1. Support for MTP Level 2 / MTP Level 3 Interface Boundary

 This interface is the same as the MTP2/MTP3 interface described in
 the applicable SS7 standards [Q.703] [Q.704] [T1.111] [Q.2140], with
 the addition of support for the larger sequence numbers found in
 [T1.111] and [Q.2210].

George, et al. Standards Track [Page 7] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 M2PA receives the primitives sent from MTP3 to its lower layer.  M2PA
 processes these primitives or maps them to appropriate primitives at
 the M2PA/SCTP interface.  Likewise, M2PA sends primitives to MTP3
 similar to those used in the MTP3/MTP2 interface.
 Because M2PA uses larger sequence numbers than MTP2, the MTP3
 Changeover procedure MUST use the Extended Changeover Order and
 Extended Changeover Acknowledgement messages described in [Q.2210]
 and [T1.111].
 Also, the following MTP3/MTP2 primitives must use the larger sequence
 numbers:
  1. BSNT Confirmation
  1. Retrieval Request and FSNC

1.6.2. Support for Peer-to-Peer Communication

 In SS7, MTP Level 2 sends three types of messages, known as signal
 units: Message Signal Units (MSUs), Link Status Signal Units (LSSUs),
 and Fill-In Signal Units (FISUs).
 MSUs originate at a higher level than MTP2, and are destined for a
 peer at another node.  Likewise, M2PA passes these messages from MTP3
 to SCTP as data for transport across a link.  These are called User
 Data messages in M2PA.
 LSSUs allow peer MTP2 layers to exchange status information.
 Analogous messages are needed for M2PA.  The Link Status message
 serves this purpose.
 FISUs are transmitted continuously when no other signal units are
 waiting to be sent.  FISUs also carry acknowledgement of messages.
 Since an IP network is a shared resource, it would be undesirable to
 have a message type that is sent continuously as is the case with
 FISUs.  Furthermore, SCTP does not require its upper layer to
 continuously transmit messages.  Therefore, M2PA does not provide a
 protocol data unit like the FISU.  The M2PA User Data message is used
 to carry acknowledgement of messages.  If M2PA needs to acknowledge a
 message, and it has no MTP3 message of its own to send, an empty User
 Data message can be sent.

George, et al. Standards Track [Page 8] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

1.7. Functions Provided by M2PA

1.7.1. MTP2 Functionality

 M2PA provides MTP2 functionality that is not provided by SCTP; thus,
 together M2PA and SCTP provide functionality similar to that of MTP2.
 SCTP provides reliable, sequenced delivery of messages.
 M2PA functionality includes:
  1. Data retrieval to support the MTP3 changeover procedure
  1. Reporting of link status changes to MTP3
  1. Processor outage procedure
  1. Link alignment procedure

1.7.2. Mapping of SS7 and IP Entities

 The M2PA layer must maintain a map of each of its SS7 links to the
 corresponding SCTP association.

1.7.3. SCTP Association Management

 SCTP allows a user-specified number of streams to be opened during
 the initialization.  It is the responsibility of the M2PA layer to
 ensure proper management of the streams allowed within each
 association.
 M2PA uses two streams in each direction for each association.  Stream
 0 in each direction is designated for Link Status messages.  Stream 1
 is designated for User Data messages, as well as Link Status messages
 that must remain in sequence with the User Data messages.  Separating
 the Link Status and User Data messages into separate streams allows
 M2PA to prioritize the messages in a manner similar to MTP2.
 Notifications received from SCTP are processed by M2PA or translated
 into an appropriate notification to be sent to the upper layer MTP3.

1.7.4. Retention of MTP3 in the SS7 Network

 M2PA allows MTP3 to perform all of its Message Handling and Network
 Management functions with IPSPs as it does with other SS7 nodes.

George, et al. Standards Track [Page 9] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

1.8. Definition of the M2PA Boundaries

1.8.1. Definition of the M2PA / MTP Level 3 Boundary

 The upper layer primitives provided by M2PA are the same as those
 provided by MTP2 to MTP3.  These primitives are described in the
 applicable SS7 standards [Q.703] [Q.704] [T1.111] [Q.2140].

1.8.2. Definition of the Lower Layer Boundary between M2PA and SCTP

 The upper layer primitives provided by SCTP are described in
 [RFC2960] Section 10 "Interface with Upper Layer".

1.9. Differences Between M2PA and M2UA

 The MTP2 User Adaptation Layer (M2UA) [M2UA] also adapts the MTP3
 layer to the SCTP/IP stack.  It does so through a backhauling
 architecture [RFC2719].  This section is intended to clarify some of
 the differences between the M2PA and M2UA approaches.
 A possible M2PA architecture is shown in Figure 3.  Here the IPSP's
 MTP3 uses its underlying M2PA as a replacement for MTP2.
 Communication between the two layers MTP3/M2PA is defined by the same
 primitives as in SS7 MTP3/MTP2.  M2PA performs functions similar to
 MTP2.
  • * SS7 * IP
  • SEP *——–* SG *——–* IPSP *
  • * *
     +------+        +-------------+        +------+
     | SCCP |        |    SCCP     |        | SCCP |
     +------+        +-------------+        +------+
     | MTP3 |        |    MTP3     |        | MTP3 |
     +------+        +------+------+        +------+
     | MTP2 |        | MTP2 | M2PA |        | M2PA |
     |      |        |      +------+        +------+
     |      |        |      | SCTP |        | SCTP |
     +------+        +------+------+        +------+
     | MTP1 |        | MTP1 | IP   |        | IP   |
     +------+        +------+------+        +------+
                Figure 3.  M2PA in IP Signaling Gateway
 A comparable architecture for M2UA is shown in Figure 4.  In M2UA,
 the MGC's MTP3 uses the SG's MTP2 as its lower SS7 layer.  Likewise,
 the SG's MTP2 uses the MGC's MTP3 as its upper SS7 layer.  In SS7,

George, et al. Standards Track [Page 10] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 communication between the MTP3 and MTP2 layers is defined by
 primitives.  In M2UA, the MTP3/MTP2 communication is defined as M2UA
 messages and sent over the IP connection.
  • * SS7 * IP
  • SEP *——–* SG *——–* MGC *
  • * *
     +------+                               +------+
     | SCCP |                               | SCCP |
     +------+                               +------+
     | MTP3 |             (NIF)             | MTP3 |
     +------+        +------+------+        +------+
     | MTP2 |        | MTP2 | M2UA |        | M2UA |
     |      |        |      +------+        +------+
     |      |        |      | SCTP |        | SCTP |
     +------+        +------+------+        +------+
     | MTP1 |        | MTP1 | IP   |        | IP   |
     +------+        +------+------+        +------+
      NIF   - Nodal Interworking Function
                Figure 4.  M2UA in IP Signaling Gateway
 M2PA and M2UA are similar in that:
    a. Both transport MTP3 data messages.
    b. Both present an MTP2 upper interface to MTP3.
 Differences between M2PA and M2UA include:
    a. M2PA: IPSP processes MTP3/MTP2 primitives.
       M2UA: MGC transports MTP3/MTP2 primitives between the SG's MTP2
             and the MGC's MTP3 (via the NIF) for processing.
    b. M2PA: SG-IPSP connection is an SS7 link.
       M2UA: SG-MGC connection is not an SS7 link.  It is an
             extension of MTP to a remote entity.
    c. M2PA: SG is an SS7 node with a point code.
       M2UA: SG is not an SS7 node and has no point code.
    d. M2PA: SG can have upper SS7 layers, e.g., SCCP.
       M2UA: SG does not have upper SS7 layers since it has no MTP3.
    e. M2PA: relies on MTP3 for management procedures.
       M2UA: uses M2UA management procedures.

George, et al. Standards Track [Page 11] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 Potential users of M2PA and M2UA should be aware of these differences
 when deciding how to use them for SS7 signaling transport over IP
 networks.

2. Protocol Elements

 This section describes the format of various messages used in this
 protocol.
 All fields in an M2PA message must be transmitted in the network byte
 order, i.e., most significant byte first, unless otherwise stated.

2.1. Common Message Header

 The protocol messages for M2PA require a message header structure
 that contains a version, message class, message type, and message
 length.  The header structure is shown in Figure 5.
     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    |     Spare     | Message Class | Message Type  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Message Length                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                   Figure 5.  Common Message Header

2.1.1. Version

 The version field contains the version of M2PA.  The supported
 versions are:
          Value
        (decimal)  Version
        ---------  -------
            1      Release 1.0 of M2PA protocol

2.1.2. Spare

 The Spare field SHOULD be set to all zeroes (0's) by the sender and
 ignored by the receiver.  The Spare field SHOULD NOT be used for
 proprietary information.

George, et al. Standards Track [Page 12] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

2.1.3. Message Class

 The following List contains the valid Message Classes:
          Value
        (decimal)  Message Class
        ---------  -------------
           11      M2PA Messages
 Other values are invalid for M2PA.

2.1.4. Message Type

 The following list contains the message types for the defined
 messages.
          Value
        (decimal)  Message Type
        ---------  -------------
            1      User Data
            2      Link Status
 Other values are invalid.

2.1.5. Message Length

 The Message Length defines the length of the message in octets,
 including the Common Header.

2.2. M2PA Header

 All protocol messages for M2PA require an M2PA-specific header.  The
 header structure is shown in Figure 6.
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     unused    |                      BSN                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     unused    |                      FSN                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                Figure 6.  M2PA-specific Message Header

2.2.1. Backward Sequence Number (BSN)

 This is the FSN of the message last received from the peer.

George, et al. Standards Track [Page 13] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

2.2.2. Forward Sequence Number (FSN)

 This is the M2PA sequence number of the User Data message being sent.
 The FSN and BSN values range from 0 to 16,777,215.

2.3. M2PA Messages

 The following section defines the messages and parameter contents.
 An M2PA message consists of a Common Message Header and M2PA Header,
 followed by the data appropriate to the message.
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                     Common Message Header                     /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                  M2PA-specific Message Header                 /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                         Message Data                          /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The field "Message Data" contains either:
  1. a User Data message (Section 2.3.1), or
  2. a Link State message (Section 2.3.2)

2.3.1. User Data

 The User Data is the data sent from MTP3.  The User Data is an
 optional field.  It need not be included in an acknowledgement-only
 message.
 The format of the User Data 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 \                                                               \
 /                            Data                               /
 \                                                               \
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

George, et al. Standards Track [Page 14] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 The Data field contains the following fields of the MTP Message
 Signal Unit (MSU):
  1. the Message Priority field (PRI)
  2. Service Information Octet (SIO)
  3. Signaling Information Field (SIF)
 The MTP MSU is described in Q.703 [Q.703], Section 2.2, "Signal Unit
 Format", and T1.111.3 [T1.111], Section 2.2, "Signal Unit Format".
 The Japanese TTC standard uses the PRI field as an MTP3 Message
 Priority field [JT-Q703] [JT-Q704].  For versions of MTP that do not
 use these two bits, the entire first octet of the Data field is
 spare.
 The format of the first octet of the Data field is:
     0
     0 1 2 3 4 5 6 7
    +-+-+-+-+-+-+-+-+
    |PRI|   spare   | (followed by SIO, SIF)
    +-+-+-+-+-+-+-+-+
    PRI - Priority used only in national MTP defined in [JT-Q703] and
          [JT-Q704].  These bits are spare for other MTP versions.
 Note that the Data field SHALL NOT contain other components of the
 MTP MSU format:
  1. Flag
  2. Backward Sequence Number (BSN)
  3. Backward Indicator Bit (BIB)
  4. Forward Sequence Number (FSN)
  5. Forward Indicator Bit (FIB)
  6. Length Indicator (LI)
  7. Check bits (CK)
 The Data field SHALL be transmitted in the byte order as defined by
 MTP3.
 M2PA SHALL NOT add padding to the MTP3 message.
 Note: In the SS7 Recommendations, the format of the messages and
 fields within the messages are based on bit transmission order.  In
 these recommendations, the Least Significant Bit (LSB) of each field
 is positioned to the right.  The received SS7 fields are populated
 octet by octet as received into the 4-octet word, as shown below.

George, et al. Standards Track [Page 15] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 As an example, in the ANSI MTP protocol, the Data field format is
 shown below:
    |MSB---------------------------------------------------------LSB|
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |PRI|   spare   |      SIO      |   SIF octet   |      ...      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                               :                               /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      ...      |      ...      |      ...      |   SIF octet   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Within each octet, the Least Significant Bit (LSB) per the SS7
 Recommendations is to the right (e.g., bit 15 of SIO is the LSB).

2.3.2. Link Status

 The MTP2 Link Status message can be sent between M2PA peers to
 indicate link status.  This message performs a function similar to
 the Link Status Signal Unit in MTP2.  The format of the Link Status
 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            State                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 The valid values for State are shown in the following table.
          Value
        (decimal)  Description
        ---------  -----------
            1      Alignment
            2      Proving Normal
            3      Proving Emergency
            4      Ready
            5      Processor Outage
            6      Processor Recovered
            7      Busy
            8      Busy Ended
            9      Out of Service (OOS)

George, et al. Standards Track [Page 16] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

2.3.2.1. Link Status Proving

 The Link Status Proving message may optionally carry additional
 bytes.  If the optional bytes are used, the format of the 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            State                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    \                                                               \
    /                            filler                             /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 It is RECOMMENDED that the length of the Link Status Proving message
 be similar to the size of the User Data messages that will be carried
 on the link.
 It is RECOMMENDED that the filler field contain a number pattern that
 varies among the Link Status Proving messages, and that allows the
 SCTP checksum [RFC3309] to be used to verify the accuracy of
 transmission.

3. State Control

3.1. SCTP Association State Control

 Figure 7 illustrates state changes in the M2PA management of the SCTP
 association, together with the causing events.  Note that some of the
 error conditions are not shown in the state diagram.
 Following is a list of the M2PA Association States and a description
 of each.
 IDLE - State of the association during power-up initialization.
 ASSOCIATING - M2PA is attempting to establish an SCTP association.
 ESTABLISHED - SCTP association is established.

George, et al. Standards Track [Page 17] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

                       +-----------+
                       |   IDLE    |
                       +-----------+
                             |
                             | Associate
                             | (Issue SCTP associate)
                             |
                             |   +----------------------+
                             |   |         (Issue SCTP  |
                             V   V          associate)  |
                       +-------------+                  |
                       | ASSOCIATING |----------------->+
                       +-------------+  SCTP Comm Error |
                             |                          |
                             |                          |
                             | SCTP Comm Up             |
                             |                          |
                             V                          |
                       +-------------+                  |
                       | ESTABLISHED |----------------->+
                       +-------------+   SCTP Comm Error
                                      OR SCTP Comm Lost
         Figure 7.  M2PA Association State Transition Diagram

3.2. M2PA Link State Control

 The M2PA link moves from one state to another in response to various
 events.  The events that may result in a change of state include:
  1. MTP3 primitive requests
  1. Receipt of messages from the peer M2PA
  1. Expiration of timers
  1. SCTP notifications
 These events affect the M2PA link state in a manner similar to MTP2.

George, et al. Standards Track [Page 18] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

4. Procedures

 Because M2PA provides MTP3 with an interface and functionality like
 MTP2, its internal functioning is similar to that of MTP2.
 Except as modified in this document, M2PA SHOULD follow the
 requirements of the applicable MTP2 specification.  These may include
 [Q.703] or [T1.111].  The same standard MUST be followed on both ends
 of the M2PA link.
 In particular, the corresponding applicable timer value defaults and
 ranges specified for the applicable MTP2 standard should be used for
 the M2PA timers.
 When referring to MTP2 terminology in this document, the terminology
 of [Q.703] is used.  This does not imply that the requirements of
 [Q.703] are to be followed.

4.1. Procedures to Support MTP2 Features

4.1.1. Signal Unit Format, Delimitation, Acceptance

 Messages for transmission across the network must follow the format
 described in Section 2.
 SCTP provides reliable, in-sequence delivery of user messages.
 Therefore the related functionality of MTP2 is not needed.  SCTP does
 not provide functions related to Link State Control in MTP2.  These
 functions must be provided by M2PA.
 Since SCTP provides delivery of messages, there is no need for M2PA
 to delimit its messages with a flag, as is done in MTP2.
 Furthermore, M2PA does not need to perform zero bit insertion and
 deletion on its messages.
 Since SCTP uses a checksum to detect transmission errors, there is no
 need for an M2PA checksum, as is needed in MTP2.  This also
 eliminates the need for the error rate monitors of MTP2.
 Since SCTP provides reliable delivery and ordered delivery, M2PA does
 not perform retransmissions.  This eliminates the need for the
 forward and backward indicator bits in MTP2 signal units.
 Acceptance of a message is indicated by a successful receipt of the
 message from SCTP.

George, et al. Standards Track [Page 19] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

4.1.2. MTP and SCTP Entities

 This section describes how M2PA relates MTP and SCTP entities.
 Each MTP link corresponds to an SCTP association.  To prevent
 duplicate associations from being established, it is RECOMMENDED that
 each endpoint know the IP address (or IP addresses, if multi-homing
 is used) and port number of both endpoints.  SCTP prevents two
 associations with the same IP addresses and port numbers from being
 established.
 It is necessary for at least one of the endpoints to be listening on
 the port on which the other endpoint is trying to establish the
 association.  Therefore, at least one of the port numbers SHOULD be
 the M2PA registered port.
 If only one association is to be established between these two IP
 addresses, then the association SHOULD be established using the M2PA
 registered port at each endpoint.
 If it is desirable to create multiple associations (for multiple
 links) between the two IP addresses, different port numbers can be
 used for each association.  Nevertheless, the M2PA registered port
 number SHOULD be used at one end of each association.
 Each combination of IP address/port for the two endpoints (i.e., each
 association) MUST be mapped to the same Signaling Link Code (SLC) at
 each endpoint, so that each endpoint knows which link is being
 created at the time the SCTP association is established.  However,
 M2PA does not do any processing based on the SLC.
 Following are examples of the relationships between associations and
 links.  Note that a link is an SCTP association identified by two
 endpoints.  Each endpoint is identified by an IP address and port
 number.  Each association is mapped to an SLC.
 Figure 8 shows a case with two IPSPs, each with two IP addresses.
 Two associations are the links that connect the two IPSPs.  Since
 these links are in the same link set, they MUST have different SLCs.
 Table 1 shows the relationships in tabular form.  Table 1 is only
 conceptual.  The actual method for mapping the SCTP associations to
 the SLCs is implementation dependent.

George, et al. Standards Track [Page 20] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

                 IPSP X                        IPSP Y
             +-------------+               +-------------+
             |             |     SCTP      |             |
             |         IPA | association 1 | IPB         |
             |   port = PW +---------------+ port = PW   |
             |     SLC = a |               | SLC = a     |
             |             |               |             |
             |             |               |             |
             |             |     SCTP      |             |
             |         IPC | association 2 | IPD         |
             |   port = PW +---------------+ port = PW   |
             |     SLC = b |               | SLC = b     |
             |             |               |             |
             |             |               |             |
             +-------------+               +-------------+
          IPx = IP address
          PW  = Registered port number for M2PA
             Figure 8.  Two IPSPs with Two IP Addresses Each
      +-------------+---------------------------------------+-----+
      | Association |      IPSP X       |      IPSP Y       | SLC |
      |             +------------+------+------------+------+     |
      |             | IP address | Port | IP address | Port |     |
      +=============+============+======+============+======+=====+
      |      1      |    IPA     |  PW  |    IPB     |  PW  |  a  |
      +-------------+------------+------+------------+------+-----+
      |      2      |    IPC     |  PW  |    IPD     |  PW  |  b  |
      +-------------+------------+------+------------+------+-----+
            Table 1.  Two IPSPs with Two IP Addresses Each
 Figure 9 and Table 2 show an example with three IPSPs.  Note that in
 this example, the two links are in different link sets.  Therefore,
 it is possible that the SLC values a and b MAY be equal.

George, et al. Standards Track [Page 21] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

                 IPSP X                        IPSP Y
             +-------------+               +-------------+
             |             |     SCTP      |             |
             |         IPA | association 1 | IPB         |
             |   port = PW +---------------+ port = PW   |
             |     SLC = a |               | SLC = a     |
             |             |               |             |
             |             |               |             |
             |             |     SCTP      |             |
             |         IPC | association 2 |             |
             |   port = PW +-------+       |             |
             |     SLC = b |       |       |             |
             |             |       |       |             |
             |             |       |       |             |
             +-------------+       |       +-------------+
                                   |
                                   |
                                   |           IPSP Z
                                   |
                                   |       +-------------+
                                   |       |             |
                                   |       | IPD         |
                                   +-------+ port = PW   |
                                           | SLC = b     |
                                           |             |
                                           |             |
                                           |             |
                                           |             |
                                           |             |
                                           |             |
                                           |             |
                                           |             |
                                           +-------------+
          IPx = IP address
          PW  = Registered port number for M2PA
               Figure 9.  One IPSP Connected to Two IPSPs

George, et al. Standards Track [Page 22] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

      +-------------+---------------------------------------+-----+
      | Association |      IPSP X       |     IPSP Y/Z      | SLC |
      |             +------------+------+------------+------+     |
      |             | IP address | Port | IP address | Port |     |
      +=============+============+======+============+======+=====+
      |      1      |    IPA     |  PW  |    IPB     |  PW  |  a  |
      +-------------+------------+------+------------+------+-----+
      |      2      |    IPC     |  PW  |    IPD     |  PW  |  b  |
      +-------------+------------+------+------------+------+-----+
               Table 2.  One IPSP Connected to Two IPSPs
 Figure 10 and Table 3 show two associations between the same IP
 addresses.  This is accomplished by using different port numbers for
 each association at one endpoint.
                 IPSP X                        IPSP Y
             +-------------+               +-------------+
             |             |     SCTP      |             |
             |         IPA | association 1 | IPB         |
             |   port = P1 +---------------+ port = PW   |
             |     SLC = a |               | SLC = a     |
             |             |               |             |
             |             |               |             |
             |             |     SCTP      |             |
             |         IPA | association 2 | IPB         |
             |   port = PW +---------------+ port = PW   |
             |     SLC = b |               | SLC = b     |
             |             |               |             |
             |             |               |             |
             +-------------+               +-------------+
          IPx = IP address
          P1  = Pre-selected port number
          PW  = Registered port number for M2PA
           Figure 10.  Multiple Associations Between Two IP Addresses

George, et al. Standards Track [Page 23] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

      +-------------+---------------------------------------+-----+
      | Association |      IPSP X       |      IPSP Y       | SLC |
      |             +------------+------+------------+------+     |
      |             | IP address | Port | IP address | Port |     |
      +=============+============+======+============+======+=====+
      |      1      |    IPA     |  P1  |    IPB     |  PW  |  a  |
      +-------------+------------+------+------------+------+-----+
      |      2      |    IPA     |  PW  |    IPB     |  PW  |  b  |
      +-------------+------------+------+------------+------+-----+
         Table 3.  Multiple Associations Between Two IP Addresses
 The association SHALL contain two streams in each direction.  Stream
 0 is designated for Link Status messages.  Stream 1 is designated for
 User Data messages, as well as Link Status messages that must remain
 in sequence with the User Data messages.
 The following Link Status messages SHALL be sent on the Link Status
 stream (stream 0):
  1. Alignment
  2. Proving Normal
  3. Proving Emergency
  4. Ready (when sent during alignment)
  5. Busy
  6. Busy Ended
  7. Out of Service
 The following Link Status messages SHALL be sent on the User Data
 stream (stream 1):
  1. Processor Outage
  2. Processor Recovered
  3. Ready (when sent at the end of processor outage)

4.1.3. Link Alignment

 The purposes of the alignment procedure are:
    (1) To provide a handshaking procedure so that both endpoints are
        prepared to send SS7 traffic, and to prevent traffic from
        being sent before the other end is ready.
    (2) To verify that the SCTP association is suitable for use as an
        SS7 link.

George, et al. Standards Track [Page 24] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 Link alignment takes place after the association is established.  If
 SCTP fails to establish the association, and M2PA has received a
 Start Request from its MTP3, then M2PA SHALL report to MTP3 that the
 link is out of service.
 The Link Status Out of Service message replaces the SIOS message of
 MTP2.  Unlike MTP2, the message SHOULD NOT be transmitted
 continuously.  After the association is established, M2PA SHALL send
 a Link Status Out of Service message to its peer.  Prior to the
 beginning of alignment, M2PA MAY send additional Link Status Out of
 Service messages.
 The Link Status Alignment message replaces the SIO message of MTP2.
 This message is sent to signal the beginning of the alignment
 procedure.  The Link Status Alignment message SHOULD NOT be
 transmitted continuously.  M2PA MAY send additional Link Status
 Alignment until it receives Link Status Alignment, Link Status
 Proving Normal, or Link Status Proving Emergency from the peer.
 The Link Status Proving Normal message replaces the SIN message of
 MTP2.  The Link Status Proving Emergency message replaces the SIE
 message of MTP2.
 The proving period MAY be omitted if this is allowed by the
 applicable MTP2 standard (e.g., [Q.2140]).
 If proving is performed, then during the proving period (i.e., after
 M2PA starts the proving period timer T4), M2PA SHALL send Link Status
 Proving messages to its peer at an interval defined by the protocol
 parameter Proving_Interval.  It is RECOMMENDED that Proving_Interval
 be set so that the traffic load generated with the Link Status
 Proving messages during the proving period is comparable to the
 normal traffic load expected when the link is in service.
 The Link Status Ready message replaces the FISU of MTP2 that is sent
 at the end of the proving period.  The Link Status Ready message is
 used to verify that both ends have completed proving.  When M2PA
 starts timer T1, it SHALL send a Link Status Ready message to its
 peer in the case where MTP2 would send a FISU after proving is
 complete.  If the Link Status Ready message is sent, then M2PA MAY
 send additional Link Status Ready messages while timer T1 is running.
 These Link Status Ready messages are sent on the Link Status stream.
 In the case that MTP2 sends an MSU or SIPO message at the end of
 proving, M2PA SHALL send (respectively) a User Data or Link Status
 Processor Outage message.

George, et al. Standards Track [Page 25] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

4.1.4. Processor Outage

 The Link Status Processor Outage message replaces the SIPO message of
 MTP2.  Unlike MTP2, the message SHOULD NOT be transmitted
 continuously.  M2PA SHALL send a Link Status Processor Outage message
 to its peer at the beginning of a processor outage condition where
 MTP2 would send SIPO.  M2PA MAY send additional Link Status Processor
 Outage messages as long as that condition persists.  The Link Status
 Processor Outage message SHALL be sent on the User Data stream.
 While in a local processor outage (LPO) condition:
    (a) Any User Data messages received from the peer MUST NOT be
        acknowledged and MUST be buffered.
    (b) M2PA SHOULD continue to acknowledge User Data messages
        received and accepted by MTP3 before the local processor
        outage.
    (c) M2PA SHOULD continue to transmit messages that have been sent
        by its upper layer MTP3.
 While there is a remote processor outage (RPO) condition:
    (a) M2PA SHOULD continue to acknowledge User Data messages
        received and accepted by MTP3, regardless of the remote
        processor outage.
    (b) If any User Data messages received from the peer after the
        Link Status Processor Outage cannot be delivered to MTP3, then
        these messages MUST NOT be acknowledged and MUST be buffered.
 If M2PA receives a Flush command from MTP3,
    (a) M2PA SHALL discard any incoming messages that were queued and
        unacknowledged during the processor outage condition.
    (b) M2PA SHALL discard messages in the transmit and retransmit
        queues as required by MTP2.
 If M2PA receives a Continue command from MTP3, M2PA SHALL begin
 processing the incoming messages that were queued and unacknowledged
 during the processor outage condition.
 When the local processor outage condition ends, M2PA SHALL send a
 Link Status Processor Recovered message to its peer on the User Data
 stream.  This message is used to signal the end of the processor
 outage condition, instead of an MSU or FISU, as is used in MTP2.  The

George, et al. Standards Track [Page 26] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 BSN in the Link Status Processor Recovered message is set to the FSN
 of the last User Data message received (and not discarded) from the
 peer M2PA.  M2PA SHALL cease transmitting User Data messages after
 sending the Link Status Processor Recovered message, until it has
 received the Link Status Ready message (see below).
 Upon receiving the Link Status Processor Recovered message, the M2PA
 in RPO SHALL respond with a Link Status Ready message on the User
 Data stream.  The BSN in the Link Status Ready message is set to the
 FSN of the last User Data message received (and not discarded) from
 the peer M2PA.
 Upon receiving the Link Status Ready message, the M2PA formerly in
 LPO SHALL respond with a Link Status Ready message on the User Data
 stream.  The BSN in the Link Status Ready message is set to the FSN
 of the last User Data message received (and not discarded) from the
 peer M2PA.
 M2PA (at both the LPO and RPO ends) uses the BSN value in the
 received Link Status Ready message to resynchronize its sequence
 numbers, if this is required by MTP2.  M2PA SHALL NOT resume
 transmitting User Data messages until it has sent the Link Status
 Ready message.
 During resynchronization, M2PA SHALL NOT discard any received User
 Data messages that were sent after the processor outage ended.
 When M2PA experiences a local processor outage, it MAY put the link
 out of service by sending a Link Status Out of Service message, if
 this is allowed by the applicable MTP2 standard (e.g., [Q.2140]).
 In other respects, M2PA SHOULD follow the same procedures as MTP2 in
 processor outage.

4.1.5. Level 2 Flow Control

 The Link Status Busy message replaces the SIB message of MTP2.  The
 message SHOULD NOT be transmitted continuously.  M2PA SHALL send a
 Link Status Busy message to its peer at the beginning of a receive
 congestion condition where MTP2 would send SIB.  M2PA MAY send
 additional Link Status Busy messages as long as that condition
 persists.  When the condition ends, M2PA SHALL send a Link Status
 Busy Ended message to its peer.
 M2PA SHALL continue transmitting messages while it is in receive
 congestion, but MUST NOT acknowledge the message that triggered the
 sending of the Link Status Busy message, nor any messages received
 before the sending of Link Status Busy Ended.

George, et al. Standards Track [Page 27] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 When the peer M2PA receives the first Link Status Busy message, it
 SHALL start the Remote Congestion timer T6 if there are messages in
 the retransmission buffer awaiting acknowledgement (i.e., T7 is
 running).  M2PA SHALL stop the T7 timer if it is running.  Additional
 Link Status Busy messages received while T6 is running do not cause
 T6 to be reset and do not cause T7 to be started.  While T6 is
 running, T7 SHALL NOT be started.
 When the peer M2PA receives the Link Status Busy Ended message and T6
 has not expired, it SHALL stop T6 (if T6 is running) and start T7 (if
 there are messages awaiting acknowledgement in the retransmission
 buffer).
 The peer M2PA SHOULD continue receiving and acknowledging messages
 while the other end is busy, but MUST NOT send User Data messages
 after receiving Link Status Busy and before receiving Link Status
 Busy Ended.

4.1.6. Link Out of Service

 The Link Status Out of Service message replaces the SIOS message of
 MTP2.  Unlike MTP2, the message SHOULD NOT be transmitted
 continuously.  M2PA SHALL send a Link Status Out of Service message
 to its peer at the beginning of a condition where MTP2 would send
 SIOS.  M2PA MAY send additional Link Status Out of Service messages
 as long as that condition persists.
 When M2PA places a link in the OUT OF SERVICE state, M2PA SHOULD NOT
 terminate the SCTP association.

4.1.7. SCTP Association Problems

 The SCTP association for a link may become unusable, such as when one
 of the following occurs:
  1. SCTP sends a Send Failure notification to M2PA.
  1. SCTP sends a Communication Lost notification to M2PA.
  1. SCTP sends a Communication Error notification to M2PA.
  1. The SCTP association is lost.
 If the SCTP association for a link becomes unable to transmit or
 receive messages, M2PA SHALL report to MTP3 that the link is out of
 service and enter the OUT OF SERVICE state.

George, et al. Standards Track [Page 28] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

4.1.8. Transmission and Reception Priorities

 In MTP, Link Status messages have priority over User Data messages
 ([Q.703], Section 11.2).  To achieve this in M2PA, M2PA uses separate
 streams in its SCTP association for Link Status messages and User
 Data messages.
 M2PA SHALL send all messages using the ordered delivery option of
 SCTP.
 M2PA SHOULD give higher priority to messages sent on the Link Status
 stream than to messages sent on the User Data stream when sending
 messages to SCTP.
 M2PA SHOULD give higher priority to reading the Link Status stream
 than to reading the User Data stream.
 M2PA SHOULD give higher priority to receiving notifications from SCTP
 than to reading either the Link Status stream or the User Data
 stream.

4.1.9. M2PA Version Control

 A node upgraded to a newer version of M2PA SHOULD support the older
 versions used on other nodes with which it is communicating.  If that
 is the case, then alignment can proceed normally.
 In particular, it is recommended that for future modifications to
 this protocol:
  1. Any newer version SHOULD be able to process the messages from an

older version.

  1. A newer version of M2PA SHOULD refrain from sending messages to

an older version of M2PA messages that the older version cannot

      process.
  1. If an older version of M2PA receives a message that it cannot

process, it SHOULD discard the message.

  1. In cases where different processing is done in two versions for

the same format of a message, then the newer version SHOULD

      contain procedures to recognize and handle this appropriately.
 In case a newer version of M2PA is incompatible with an older
 version, the newer version SHOULD recognize this and prevent the
 alignment of the link.  If a Link Status Alignment message with an

George, et al. Standards Track [Page 29] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 unsupported version is received by the newer version, the receiving
 end's M2PA SHOULD reply with a Link Status Out of Service message and
 not complete the alignment procedure.

4.2. Procedures to Support the MTP3/MTP2 Interface

4.2.1. Sending and Receiving Messages

 When MTP3 sends a message for transmission to M2PA, M2PA passes the
 corresponding M2PA message to SCTP using the SEND primitive.
 User Data messages SHALL be sent via the User Data stream (stream 1)
 of the association.
 M2PA Link Status messages are passed to SCTP using the SEND
 primitive.
 The following Link Status messages SHALL be sent on the Link Status
 stream (stream 0):
  1. Alignment
  2. Proving Normal
  3. Proving Emergency
  4. Ready (when sent during alignment)
  5. Busy
  6. Busy Ended
  7. Out of Service
 The following Link Status messages SHALL be sent on the User Data
 stream (stream 1):
  1. Processor Outage
  2. Processor Recovered
  3. Ready (when sent at the end of processor outage)
 If M2PA receives a message from SCTP with an invalid Message Class or
 unsupported Message Type in the Common Message Header, M2PA SHALL
 discard the message.
 For message types other than User Data, the Forward Sequence Number
 is set to the FSN of the last User Data message sent.
 If M2PA receives a User Data message with an FSN that is out of
 order, M2PA SHALL discard the message.
 Note: In all calculations involving FSN and BSN, the programmer
 should be aware that the value wraps around to 0 after reaching its
 maximum value.

George, et al. Standards Track [Page 30] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 When there is a message to acknowledge, M2PA MUST acknowledge the
 message with the next User Data message sent.  If there is no User
 Data message available to be sent when there is a message to
 acknowledge, M2PA SHOULD generate and send a User Data message with
 no data payload, without delay.  (In other words, in the case where
 MTP2 would acknowledge a message with a FISU, M2PA SHOULD acknowledge
 the message with an empty User Data message.)  The FSN for this empty
 User Data message is not incremented.  It MUST contain the same FSN
 as the most recently sent User Data message that contains data.
 Delaying of acknowledgements can result in poor SS7 performance.
 If M2PA receives an empty User Data message, it SHALL NOT send an
 acknowledgement of that message.
 Note that there is no reason to place Link Status messages or empty
 User Data messages in the M2PA retransmit buffer, since these
 messages are not retrieved for changeover and timer T7 does not apply
 to them.
 Note that since SCTP provides reliable delivery and ordered delivery
 within the stream, M2PA does not perform retransmissions.
 Nevertheless, M2PA SHALL retain transmitted User Data messages in a
 retransmit queue until they are acknowledged.  These messages are
 needed in case MTP3 performs data retrieval as part of a changeover
 procedure.
 Because propagation delays in IP networks are more variable than in
 traditional SS7 networks, a single T7 timer (excessive delay of
 acknowledgement), as in MTP2, is inadequate.  If any message is
 unacknowledged after a period equal to the T7 value, the T7 timer
 SHALL expire.

4.2.2. MTP3 Signaling Link Congestion

 M2PA SHALL detect transmit congestion in its buffers according to the
 requirements for signaling link transmit congestion in MTP3, e.g.,
 Q.704 [Q.704], Section 3.8.

4.2.3. Changeover

 The objective of the changeover is to ensure that signaling traffic
 carried by the unavailable signaling link is diverted to the
 alternative signaling link(s) as quickly as possible while avoiding
 message loss, duplication, or mis-sequencing.  For this purpose, the
 changeover procedure includes data retrieval, which is performed
 before opening the alternative signaling links to the diverted
 traffic.  Data retrieval consists of these steps:

George, et al. Standards Track [Page 31] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

    (1) buffer updating, i.e., identifying all those User Data
        messages in the retransmission buffer of the unavailable
        signaling link which have not been received by the far end
        M2PA, as well as untransmitted messages, and
    (2) transferring those messages to the transmission buffers of the
        alternate links.
 Note that only User Data messages containing data are retrieved and
 transmitted over the alternate links.  Link Status messages and empty
 User Data messages SHALL NOT be retrieved and transmitted over the
 alternate links.
 M2PA's Sequence Numbers are 24 bits long.  MTP2's Forward and
 Backward Sequence Numbers are only seven bits long.  Hence, it is
 necessary for MTP3 to accommodate the larger sequence numbers.  This
 is done through the use of the Extended Changeover Order (XCO) and
 Extended Changeover Acknowledgement (XCA) messages instead of the
 Changeover Order (COO) and Changeover Acknowledgement (COA) messages.
 The XCO and XCA messages are specified in [Q.2210] Section 9.8.1 and
 T1.111.4 [T1.111], Section 15.4.  Only the XCO and XCA messages from
 [Q.2210] or [T1.111] are required.  The BSN is placed in the XCO/XCA
 message as explained in [Q.2210] and [T1.111].
 Also, the following MTP3/MTP2 primitives MUST use the larger sequence
 numbers:
  1. BSNT Confirmation
  1. Retrieval Request and FSNC
 If M2PA receives a Retrieval Request and FSNC request from MTP3, M2PA
 SHALL retrieve from its buffers and deliver to MTP3 in order:
    (a) any transmitted User Data messages beginning with the first
        unacknowledged message with FSN greater than FSNC.
    (b) any untransmitted User Data messages.
 For emergency changeover, MTP3 retrieves only the unsent messages for
 transmission on the alternate link(s).  If M2PA receives a Retrieval
 Request and FSNC request with no FSNC value, or with an invalid FSNC,
 then M2PA SHALL retrieve from its buffers and deliver to MTP3 in
 order:
    (a) any untransmitted User Data messages.

George, et al. Standards Track [Page 32] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 The Japanese TTC version of MTP defined in [JT-Q703] and [JT-Q704]
 has a Retrieval Request (as well as Retrieval Request and FSNC).  The
 Retrieval allows MTP3 to retrieve both unsent and unacknowledged
 messages for transmission on the alternate link(s).  In this version
 of MTP, if M2PA receives a Retrieval Request, then M2PA SHALL
 retrieve from its buffers and deliver to MTP3 in order:
    (a) any transmitted but unacknowledged User Data messages.
    (b) any untransmitted User Data messages.

4.2.3.1. Multiple User Data Streams and Changeover

 The changeover procedure makes it problematic for M2PA to have
 multiple User Data streams in one direction for a link.  Buffer
 updating would have to be done separately for each User Data stream
 to avoid duplication or loss of messages.  But MTP3 provides for only
 one XCO/XCA message for sending the last-received sequence number.
 Even with sequence numbering of User Data messages at the M2PA layer,
 it is necessary to perform buffer updating on each stream.  Since the
 M2PA messages would be delivered over multiple streams, there could
 be a gap in the M2PA sequence numbers at the receiving end when the
 changeover procedure begins.  If only the M2PA sequence number is
 used in the XCO/XCA message, there would be a possibility of losing
 the messages in the gap, or duplicating messages after the gap.
 M2PA links with multiple User Data streams would be possible if a
 multiple-BSNT XCO/XCA message is defined in MTP3, or if MTP3 allows
 multiple XCO/XCA messages (one for each User Data stream) to be sent
 during a changeover.  This is beyond the scope of this document.

4.3. SCTP Considerations

 Some M2PA procedures may be affected by the use of SCTP as a
 transport layer.  These considerations are discussed in this section.

4.3.1. SCTP Slow Start

 SCTP contains a slow start algorithm to control the amount of data
 being injected into the network.  The algorithm allows SCTP to probe
 the network to determine the available capacity.  The algorithm is
 invoked in these cases: when transmission begins on an association,
 after a sufficiently long idle period, or after repairing loss
 detected by the SCTP retransmission timer.

George, et al. Standards Track [Page 33] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 It is possible that transmission of M2PA messages MAY be delayed by
 SCTP slow start under certain conditions, including the following:
    (a) Link Alignment.  Link alignment takes place after an
        association is established.  SCTP invokes the slow start
        algorithm since transmission is beginning on the association.
    (b) Changeover.  Messages are retrieved from one link
        (association) and transferred to another for transmission.  If
        the second link had previously been idle, or is in the process
        of link alignment, SCTP may invoke the slow start algorithm.
    (c) Path failure (multi-homing).  If SCTP switches from a failed
        path to a new path, and the new path had previously been idle,
        SCTP may invoke the slow start algorithm.
    (d) Reduced traffic volume.  Any time that M2PA sends a low volume
        of traffic on a link and then the volume increases, SCTP may
        invoke the slow start algorithm.
 Programmers should be aware of this condition and how it may affect
 M2PA performance.  In some cases, it may be possible to avoid the
 negative effects of slow start.  For example, the Link Status Proving
 messages sent during the proving period may be used to complete slow
 start before the link is placed in service.

5. Examples of M2PA Procedures

 In general, messages passed between MTP3 and M2PA are the same as
 those passed between MTP3 and MTP2.  M2PA interprets messages from
 MTP3 and sends the appropriate message to SCTP.  Likewise, messages
 from SCTP are used to generate a meaningful message to MTP3.
 Note that throughout this section, the primitives between MTP3 and
 M2PA are named using the MTP terminology [Q.700] [Q.701] [Q.702]
 [Q.703] [Q.704] [Q.705].  Communications between M2PA and SCTP are
 named using SCTP terminology.

5.1. Link Initialization (Alignment)

 An example of the message flow used to bring an SS7 link in service
 is shown in Figures 11 and 12.  Alignment is done by both ends of the
 link.  To simplify the diagram, alignment is shown on one end only.
 Some messages from the remote end are not shown.  It is assumed in
 this example that SCTP has been initialized.

George, et al. Standards Track [Page 34] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      .           Associate   .           .           .           .
      .           ------------>           .           .           .
      .           .           .           .           .           .
      .           .           (SCTP Association       .           .
      .           .            procedure)             .           .
      .           .           .           .           .           .
      .           Communication Up        Communication Up        .
      .           <------------           ------------>           .
      .           .           .           .           .           .
      .           Link Status Out of Service          .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      Emergency OR            .           .           .           .
      Emergency Ceases        .           .           .           .
      ------------>           .           .           .           .
      .           .           .           .           .           .
      Start       .           .           .           .           .
      ------------>           .           .           .           .
      .           .           .           .           .           .
      .           .           .           .           .           .
      .           Link Status Alignment   .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      .           Start timer T2          .           .           .
      .           .           .           .           .           .
      .           .           .   Link Status Alignment           .
      .           <------------------------------------           .
      .           .           .           .           .           .
      .           Stop timer T2           .           .           .
      .           .           .           .           .           .
      Proving period begins.
         Figure 11.  Example: Link Initialization - Alignment

George, et al. Standards Track [Page 35] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      .           Start timer T3          .           .           .
      .           Link Status Proving     .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      .           .           .     Link Status Proving           .
      .           <------------------------------------           .
      .           .           .           .           .           .
      .           Stop timer T3           .           .           .
      .           .           .           .           .           .
      .           Start timer T4          .           .           .
      .           Link Status Proving     .           .           .
      .           ------------------------------------>           .
      .           ------------------------------------>           .
      .           ------------------------------------>           .
      .           ------------------------------------>           .
      .           ------------------------------------>           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      .           Timer T4 expires        .           .           .
      .           .           .           .           .           .
      Send Link Status Ready (one or more) and wait for the remote end
      to complete its proving period.
      .           .           .           .           .           .
      .           Start timer T1          .           .           .
      .           .           .           .           .           .
      .           Link Status Ready       .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      .           .           .           .           .           .
      .           .           .       Link Status Ready           .
      .           <------------------------------------           .
      .           .           .           .           .           .
      .           Stop timer T1           .           .           .
      .           .           .           .           .           .
      In Service              .           .           In Service
      <------------           .           .           ------------>
      .           .           .           .           .           .
      MTP3 MAY begin sending data messages.
          Figure 12.  Example: Link Initialization - Proving

George, et al. Standards Track [Page 36] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

5.2. Message Transmission and Reception

 Messages are transmitted using the Data Request primitive from MTP3
 to M2PA.  Figure 13 shows the case where the Link is In Service.  The
 message is passed from MTP3 of the source to MTP3 of the destination.
     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      Message for             .           .           .           .
      transmission            .           .           .           .
      ------------>           .           .           .           .
      .           .           .           .           .           .
      .           Send        .           .           .           .
      .           (Data Message)          .           .           .
      .           ------------>           .           .           .
      .           .           .           .           .           .
      .           .           (SCTP sends message)    .           .
      .           .           .           .           .           .
      .           .           .           Receive                 .
      .           .           .           ------------>           .
      .           .           .           .           .           .
      .           .           .           .        Received message
      .           .           .           .           ------------>
      .           .           .           .           .           .
          Figure 13.  Example: Link Initialization - In Service

5.3. Link Status Indication

 An example of a Link Status Indication is shown in Figure 14.  If
 SCTP sends a Communication Lost primitive to M2PA, M2PA notifies MTP3
 that the link is out of service.  MTP3 responds in its usual way.
     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      .           Communication Lost      .           .           .
      .           <------------           .           .           .
      .           .           .           .           .           .
      Out of Service          .           .           .           .
      <------------           .           .           .           .
      .           .           .           .           .           .
              Figure 14.  Example: Link Status Indication

George, et al. Standards Track [Page 37] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

5.4. Link Status Message (Processor Outage)

 Figure 15 shows how M2PA responds to a local processor outage.  M2PA
 sends a Link Status message to its peer.  The peer M2PA notifies MTP3
 of the outage.  MTP3 can then follow the processor outage procedures
 as in [Q.703].
     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      .       M2PA detects    .           .           .           .
      .       Local Processor .           .           .           .
      .       Outage          .           .           .           .
      .           .           .           .           .           .
      .           Link Status .           .           .           .
      .           Processor Outage        .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      .           .           .           .        Remote Processor
      .           .           .           .        Outage         .
      .           .           .           .           ------------>
      .           .           .           .           .           .
      .           Link Status             .           .           .
      .           Processor               .           .           .
      .           Recovered               .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      .           .           .           .        Remote Processor
      .           .           .           .        Outage Ceases
      .           .           .           .           ------------>
      .           .           .           .           .           .
      .           .           .       Link Status Ready           .
      .           <------------------------------------           .
      .           .           .           .           .           .
      .           Link Status Ready       .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      Message for             .           .           .           .
      transmission            .           .           .           .
      ------------>           .           .           .           .
      .           .           .           .           .           .
      .           User Data               .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      Figure 15.  Example: Link Status Message - Processor Outage

George, et al. Standards Track [Page 38] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 Figure 16 shows an example of processor outage in more detail.  All
 M2PA messages in this example are sent on the Data stream (stream 1).
                  A                                   B
     ----------------------------        ----------------------------
     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      6 Messages for          .           .           .           .
      transmission            .           .           .           .
      ------------>           .           .          6 Messages for
      .           .           .           .            transmission
      .           .           .           .           <------------
      .           User Data FSN=1         .           .           .
      .           ------------------------------------>           .
      .           User Data FSN=2         .           .           .
      .           ------------------------------------>           .
      .           User Data FSN=3         .           .           .
      .           ------------------------------------>           .
      .           .           .        User Data FSN=11           .
      .           <------------------------------------           .
      .           .           .        User Data FSN=12           .
      .           <------------------------------------           .
      .           .           .        User Data FSN=13           .
      .           <------------------------------------           .
 Side A detects LPO.
      .           .           .           .           .           .
      .           .           .  User Data FSN=14 BSN=3           .
      .           <------------------------------------           .
      .           .           .  User Data FSN=15 BSN=3           .
      .           <------------------------------------           .
      .           .           .  User Data FSN=16 BSN=3           .
      .           <------------------------------------           .
      .           LS PO FSN=3 BSN=11      .           .           .
      .           ------------------------------------>           .
      .           .           .           .        Remote Processor
      .           .           .           .        Outage         .
      .           .           .           .           ------------>
 While in LPO, A must buffer messages 14-16 without acknowledging
 them.  A may continue transmitting messages from MTP3, and
 acknowledging messages that were received before LPO.

George, et al. Standards Track [Page 39] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

      .           .           .           .           .           .
      .           User Data FSN=4 BSN=13  .           .           .
      .           ------------------------------------>           .
      .           User Data FSN=5 BSN=13  .           .           .
      .           ------------------------------------>           .
      .           User Data FSN=6 BSN=13  .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
  While in RPO, B may continue acknowledging messages.  Suppose that
  B receives message 4 and 5, but has not processed 6 yet.
       .           .           .           .           .           .
       .                  (empty) User Data FSN=16 BSN=4
       .           <------------------------------------           .
       .                  (empty) User Data FSN=16 BSN=5
       .           <------------------------------------           .
  LPO ends at A.  A flushes 14-16 (the messages that were buffered
  without acknowledgement).
       .           .           .           .           .           .
       .           LS PR FSN=6 BSN=13      .           .           .
       .           ------------------------------------>           .
       .           .           .           .        Remote Processor
       .           .           .           .        Outage Ceases
       .           .           .           .           ------------>
       .           .           .           .           .           .
  Suppose that B processed message 5, but never processed message 6.
  B flushes message 6 from its Receive Buffer.  B notifies A of this
  using the Link Status Ready message setting BSN=5, the last message
  that was processed at B.
       .           .           .           .           .           .
       .           .           .           .           .           .
       .           .           .   LS Ready FSN=13 BSN=5           .
       .           <------------------------------------           .
       .           .           .           .           .           .
  B has completed synchronization of sequence numbers and has sent
  an LS Ready, so it is able to resume sending data at this point
  with the new sequence numbers (starting with FSN=14).

George, et al. Standards Track [Page 40] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

       .           .           .           .           .           .
       .           .           .           .           . Message for
       .           .           .           .            transmission
       .           .           .           .           <------------
       .           .           .  User Data FSN=14 BSN=5           .
       .           <------------------------------------           .
       .           .           .           .           .           .
  A can use the Link Status Ready information to resynchronize its
  sequence numbers to begin with FSN=6 in the next User Data message.
       .           .           .           .           .           .
       .           LS Ready FSN=5 BSN=13   .           .           .
       .           ------------------------------------>           .
       .           .           .           .           .           .
  A has completed synchronization of sequence number and has both
  received and sent an LS Ready, so it is able to resume sending data
  at this point with the new sequence numbers and acknowledging data
  received after receiving LS Ready.
       .           .           .           .           .           .
       .           .           .           .           .           .
       .           User Data FSN=5 BSN=14 (empty)      .           .
       .           ------------------------------------>           .
       .           .           .           .           .           .
       Message for             .           .           . Message for
       transmission            .           .            transmission
       ------------>           .           .           <------------
       .           User Data FSN=6 BSN=14  .           .           .
       .           ------------------------------------>           .
       .           .           .  User Data FSN=15 BSN=5           .
       .           <------------------------------------           .
       .           .           .           .           .           .
       .           .      (empty) User Data FSN=15 BSN=6           .
       .           <------------------------------------           .
       .           User Data FSN=6 BSN=15 (empty)      .           .
       .           ------------------------------------>           .
       .           .           .           .           .           .
       .           .           .           .           .           .
       .           .           .           .           .           .
          Figure 16.  Example: Processor Outage and Recovery

George, et al. Standards Track [Page 41] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

5.5. Level 2 Flow Control

 Figures 17 and 18 illustrate the Level 2 Flow Control procedure.  In
 Figure 17, congestion ceases before timer T6 expires.  Figure 18
 shows the case where T6 expires.
     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      .           Implementation dependent            .           .
      .           determination of M2PA               .           .
      .           receive congestion onset            .           .
      .           .           .           .           .           .
      .           Link Status Busy        .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      .           .           .           .          Start        .
      .           .           .           .          Timer T6     .
      .           .           .           .           .           .
      .           Implementation dependent            .           .
      .           determination of M2PA               .           .
      .           receive congestion abatement        .           .
      .           .           .           .           .           .
      .           Link Status Busy Ended  .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      .           .           .           .          Stop         .
      .           .           .           .          Timer T6     .
      .           .           .           .           .           .
     Figure 17.  Example: Level 2 Flow Control - Congestion Ceases

George, et al. Standards Track [Page 42] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      .           Implementation dependent            .           .
      .           determination of M2PA               .           .
      .           receive congestion onset            .           .
      .           .           .           .           .           .
      .           Link Status Busy        .           .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      .           .           .           .          Start        .
      .           .           .           .          Timer T6     .
      .           .           .           .            :          .
      .           .           .           .            :          .
      .           .           .           .          Timer T6     .
      .           .           .           .          Expires      .
      .           .           .           .           .           .
      .           .          Link Status Out of Service           .
      .           <------------------------------------           .
      .           .           .           .           .           .
      .           .           .           .          Out of Service
      .           .           .           .           ------------>
      .           .           .           .           .           .
     Figure 18.  Example: Level 2 Flow Control - Timer T6 Expires

George, et al. Standards Track [Page 43] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

5.6. MTP3 Signaling Link Congestion

 In Figure 19, M2PA notifies MTP3 of congestion onset and abatement.
 The notification includes the congestion level, if there are levels
 of congestion defined.
     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      .           Implementation dependent            .           .
      .           determination of M2PA   .           .           .
      .           transmit congestion     .           .           .
      .           onset (level)           .           .           .
      .           .           .           .           .           .
      Congestion Indication   .           .           .           .
      (level)     .           .           .           .           .
      <------------           .           .           .           .
      .           .           .           .           .           .
      .           Implementation dependent            .           .
      .           determination of M2PA   .           .           .
      .           transmit congestion     .           .           .
      .           abatement (level)       .           .           .
      .           .           .           .           .           .
      Congestion Indication   .           .           .           .
      (level)     .           .           .           .           .
      <------------           .           .           .           .
      .           .           .           .           .           .
          Figure 19.  Example: MTP3 Signaling Link Congestion

George, et al. Standards Track [Page 44] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

5.7. Link Deactivation

 Figure 20 shows an example of link deactivation.  MTP3 can request
 that a link be taken out of service.
     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      Stop        .           .           .           .           .
      ------------>           .           .           .           .
      .           .           .           .           .           .
      .           Link Status Out of Service          .           .
      .           ------------------------------------>           .
      .           .           .           .           .           .
      Out of Service          .           .           .           .
      <------------           .           .           .           .
      .           .           .           .           .           .
                Figure 20.  Example: Link Deactivation

5.8. Link Changeover

 In Figure 21, MTP3 performs a changeover because the link went out of
 service.  MTP3 selects a different link to retransmit the
 unacknowledged and unsent messages.

George, et al. Standards Track [Page 45] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

     MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
     ----        ----        ----        ----        ----        ----
      .           .           .           .           .           .
      .           Communication Lost      .           .           .
      .           <------------           .           .           .
      .           .           .           .           .           .
      Out of Service          .           .           .           .
      <------------           .           .           .           .
      .           .           .           .           .           .
      Retrieve BSNT           .           .           .           .
      ------------>           .           .           .           .
      .           .           .           .           .           .
      BSNT Confirmation       .           .           .           .
      <------------           .           .           .           .
      .           .           .           .           .           .
      XCO (BSNT) on another link          .           .           .
      ------------------------------------------------------------>
      .           .           .           .           .           .
      .           .           .           .           Retrieve BSNT
      .           .           .           .           <------------
      .           .           .           .           .           .
      .           .           .           .       BSNT Confirmation
      .           .           .           .           ------------>
      .           .           .           .           .           .
      .           .           .           .           .  XCA (BSNT)
      <------------------------------------------------------------
      .           .           .           .           .           .
      Retrieval Request       .           .           .           .
      and FSNC    .           .           .           .           .
      ------------>           .           .           .           .
      .           .           .           .           .           .
      Retrieved Message       .           .           .           .
      <------------           .           .           .           .
      .  :        .           .           .           .           .
      .  :        .           .           .           .           .
      <------------           .           .           .           .
      .           .           .           .           .           .
      Retrieval Complete      .           .           .           .
      <------------           .           .           .           .
      .           .           .           .           .           .
      Send messages on another link.
                 Figure 21.  Example: Link Changeover

George, et al. Standards Track [Page 46] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

6. Security Considerations

 M2PA is designed to carry signaling messages for telephony services.
 As such, M2PA MUST involve the security needs of several parties:
  1. the end users of the services
  1. the network providers
  1. the applications involved
 Additional requirements MAY come from local regulation.
 While these parties may have some overlapping security needs, their
 needs may not be identical.  Any security solution SHOULD fulfill all
 of the different parties' needs.
 Consult [RFC3788] for a discussion of security requirements and for
 guidance on the use of security protocols.  Implementers of M2PA MUST
 follow the guidelines in [RFC3788].

7. IANA Considerations

7.1. SCTP Payload Protocol Identifier

 The SCTP Registered User Port Number Assignment for M2PA is 3565.
 The TCP Registered User Port Number 3565 is also assigned to M2PA, in
 case a specification for M2PA over TCP is created.
 The value assigned by IANA for the Payload Protocol Identifier in the
 SCTP Payload Data chunk is
      M2PA     5
 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.

George, et al. Standards Track [Page 47] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

7.2. M2PA Protocol Extensions

 This protocol may be extended through IANA in three ways:
  1. through definition of additional message classes,
  1. through definition of additional message types, and
  1. through definition of additional message parameters.
 The definition and use of new message classes, types, and parameters
 is an integral part of SIGTRAN adaptation layers.  Thus, these
 extensions are assigned by IANA through an IETF Consensus action as
 defined in [RFC2434].
 The proposed extension must in no way adversely affect the general
 working of the protocol.
 The defined values for the message classes, types, and parameters are
 listed in the Signaling User Adaptation Layer registry
 (sigtran-adapt).

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 the 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.

George, et al. Standards Track [Page 48] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 When an implementation receives a message type that it does not
 support, it MUST discard the message.

7.2.3. IETF-defined 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.
    (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.

7.2.4. Defined Values

 This section lists the values defined in this document that should be
 included in the Signaling User Adaptation Layer registry
 (sigtran-adapt).
 The following values for Message Class are defined in this document:
          Value
        (decimal)  Message Class
        ---------  -------------
           11      M2PA Messages
 The following values for Message Type are defined in this document:
          Value
        (decimal)  Message Type
        ---------  -------------
            1      User Data
            2      Link Status

8. Acknowledgements

 The authors would like to thank the following for their valuable
 comments and suggestions: Brian Tatum, Wayne Davis, Cliff Thomas,
 Jeff Copley, Monique Bernard, Malleswar Kalla, Ian Rytina, Greg
 Sidebottom, Al Varney, Jeff Craig, and Andrew Booth.

George, et al. Standards Track [Page 49] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

9. References

9.1. Normative References

 [JT-Q703]  TTC, "Message Transfer Part Signalling Link," TTC Standard
            JT-Q703, Telecommunication Technology Committee (TTC),
            version 3 (April 27, 1994).
 [JT-Q704]  TTC, "Message Transfer Part Signalling Network Functions,"
            TTC Standard JT-Q704, Telecommunication Technology
            Committee (TTC), version 4 (May 30, 2002).
 [Q.703]    ITU, "Signalling System No. 7 - Signalling Link," ITU-T
            Recommendation Q.703, ITU-T Telecommunication
            Standardization Sector of ITU (July 1996).
 [Q.704]    ITU, "Message Transfer Part - Signalling Network Functions
            and Messages," ITU-T Recommendation Q.704, ITU-T
            Telecommunication Standardization Sector of ITU (July
            1996).
 [Q.2140]   ITU, "B-ISDN ATM Adaptation Layer - Service Specific
            Coordination Function for Signalling at the Network Node
            Interface (SSCF at NNI)," ITU-T Recommendation Q.2140,
            ITU-T Telecommunication Standardization Sector of ITU
            (February 1995).
 [Q.2210]   ITU, "Message Transfer Part Level 3 Functions and Messages
            Using the Services of ITU-T Recommendation Q.2140," ITU-T
            Recommendation Q.2210, ITU-T Telecommunication
            Standardization Sector of ITU (July 1996).
 [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 2434,
            October 1998.
 [RFC2960]  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.

George, et al. Standards Track [Page 50] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

 [RFC3309]  Stone, J., Stewart, R., and D. Otis, "Stream Control
            Transmission Protocol (SCTP) Checksum Change", RFC 3309,
            September 2002.
 [RFC3788]  Loughney, J., Tuexen, M., and J. Pastor-Balbas, "Security
            Considerations for Signaling Transport (SIGTRAN)
            Protocols", RFC 3788, June 2004.
 [T1.111]   ANSI, "American National Standard for Telecommunications -
            Signaling System Number 7 (SS7) - Message Transfer Part
            (MTP)," ANSI T1.111-2001, American National Standards
            Institute (February 2001).

9.2. Informative References

 [M2UA]     K. Morneault, et. al., "Signaling System 7 (SS7) Message
            Transfer Part 2 (MTP2) - User Adaptation Layer," RFC 3331,
            Internet Engineering Task Force - Signalling Transport
            Working Group (September, 2002).
 [Q.700]    ITU, "Introduction to CCITT Signalling System No. 7,"
            ITU-T Recommendation Q.700, ITU-T Telecommunication
            Standardization Sector of ITU (March 1993).
 [Q.701]    ITU, "Functional Description of the Message Transfer Part
            (MTP) of Signalling System No. 7," ITU-T Recommendation
            Q.701, ITU-T Telecommunication Standardization Sector of
            ITU (March 1993).
 [Q.702]    ITU, "Signalling Data Link," ITU-T Recommendation Q.702,
            ITU-T Telecommunication Standardization Sector of ITU
            (November 1988).
 [Q.705]    ITU, "Signalling System No. 7 - Signalling Network
            Structure," ITU-T Recommendation Q.705, ITU-T
            Telecommunication Standardization Sector of ITU (March
            1993).
 [RFC2719]  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.

George, et al. Standards Track [Page 51] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

Authors' Addresses

 Tom George
 Plano, TX
 USA
 Phone: +1-972-985-4594
 EMail: tgeorge_tx@verizon.net
 Brian Bidulock
 OpenSS7 Corporation
 1469 Jeffreys Crescent
 Edmonton, AB  T6L 6T1
 Canada
 Phone: +1-780-490-1141
 EMail: bidulock@openss7.org
 Ram Dantu, Ph.D.
 Assistant Professor
 Department of Computer Science
 University of North Texas
 Denton, TX 76203
 USA
 Phone: +1-940-565-2822
 EMail: rdantu@unt.edu
 Hanns Juergen Schwarzbauer
 SIEMENS AG
 Hofmannstr. 51
 81359 Munich
 Germany
 Phone: +49-89-722-24236
 EMail: HannsJuergen.Schwarzbauer@Siemens.com
 Ken Morneault
 Cisco Systems Inc.
 13615 Dulles Technology Drive
 Herndon, VA 20171
 USA
 Phone: +1-703-484-3323
 EMail: kmorneau@cisco.com

George, et al. Standards Track [Page 52] RFC 4165 SS7 MTP2-User Peer-to-Peer Adaptation Layer September 2005

Full Copyright Statement

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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
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Acknowledgement

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 Internet Society.

George, et al. Standards Track [Page 53]

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