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PROPOSED STANDARD
Errata Exist
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.













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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











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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:

      - a Signaling Gateway (SG) and a Media Gateway Controller (MGC)
        [RFC2719]

      - a SG and an IP Signaling Point (IPSP)

      - 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:

      - Support seamless operation of MTP3 protocol peers over an IP
        network connection.

      - Support the MTP Level 2 / MTP Level 3 interface boundary.

      - Support management of SCTP transport associations and traffic
        instead of MTP2 Links.

      - 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.



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   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



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   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.









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               ********   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.














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       ********  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].





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   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:

      - BSNT Confirmation

      - 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.









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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:

      - Data retrieval to support the MTP3 changeover procedure

      - Reporting of link status changes to MTP3

      - Processor outage procedure

      - 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.






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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,




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   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.



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   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.








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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.




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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:

      - a User Data message (Section 2.3.1), or
      - 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                               /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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   The Data field contains the following fields of the MTP Message
   Signal Unit (MSU):

      - the Message Priority field (PRI)
      - Service Information Octet (SIO)
      - 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:

      - Flag
      - Backward Sequence Number (BSN)
      - Backward Indicator Bit (BIB)
      - Forward Sequence Number (FSN)
      - Forward Indicator Bit (FIB)
      - Length Indicator (LI)
      - 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.





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   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)





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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.










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                         +-----------+
                         |   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:

      - MTP3 primitive requests

      - Receipt of messages from the peer M2PA

      - Expiration of timers

      - SCTP notifications

   These events affect the M2PA link state in a manner similar to MTP2.












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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.






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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.








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                   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.













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                   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












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        +-------------+---------------------------------------+-----+
        | 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













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        +-------------+---------------------------------------+-----+
        | 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):

      - Alignment
      - Proving Normal
      - Proving Emergency
      - Ready (when sent during alignment)
      - Busy
      - Busy Ended
      - Out of Service

   The following Link Status messages SHALL be sent on the User Data
   stream (stream 1):

      - Processor Outage
      - Processor Recovered
      - 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.






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   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.





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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



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   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.



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   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:

      - SCTP sends a Send Failure notification to M2PA.

      - SCTP sends a Communication Lost notification to M2PA.

      - SCTP sends a Communication Error notification to M2PA.

      - 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.





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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:

      - Any newer version SHOULD be able to process the messages from an
        older version.

      - A newer version of M2PA SHOULD refrain from sending messages to
        an older version of M2PA messages that the older version cannot
        process.

      - If an older version of M2PA receives a message that it cannot
        process, it SHOULD discard the message.

      - 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




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   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):

      - Alignment
      - Proving Normal
      - Proving Emergency
      - Ready (when sent during alignment)
      - Busy
      - Busy Ended
      - Out of Service

   The following Link Status messages SHALL be sent on the User Data
   stream (stream 1):

      - Processor Outage
      - Processor Recovered
      - 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.



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   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:




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      (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:

      - BSNT Confirmation

      - 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.





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   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.






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   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.






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       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














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       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





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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






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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





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   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.






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        .           .           .           .           .           .
        .           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).








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         .           .           .           .           .           .
         .           .           .           .           . 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







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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




















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       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


























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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






















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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.


























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       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








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6.  Security Considerations

   M2PA is designed to carry signaling messages for telephony services.
   As such, M2PA MUST involve the security needs of several parties:

      - the end users of the services

      - the network providers

      - 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.








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7.2.  M2PA Protocol Extensions

   This protocol may be extended through IANA in three ways:

      - through definition of additional message classes,

      - through definition of additional message types, and

      - 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.



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   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.





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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.





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   [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.









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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






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Full Copyright Statement

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   This document is subject to the rights, licenses and restrictions
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   The IETF invites any interested party to bring to its attention any
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.







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