rfc3331.txt

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Network Working Group                                       K. Morneault
Request for Comments: 3331                                 Cisco Systems
Category: Standards Track                                       R. Dantu
                                                                 NetRake
                                                           G. Sidebottom
                                                   Signatus Technologies
                                                             B. Bidulock
                                                                 OpenSS7
                                                                J. Heitz
                                                                  Lucent
                                                          September 2002


       Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) -
                         User Adaptation Layer

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines a protocol for the backhauling of Signaling
   System 7 Message Transfer Part 2 (SS7 MTP2) User signalling messages
   over IP using the Stream Control Transmission Protocol (SCTP).  This
   protocol would be used between a Signalling Gateway (SG) and Media
   Gateway Controller (MGC).  It is assumed that the SG receives SS7
   signalling over a standard SS7 interface using the SS7 Message
   Transfer Part (MTP) to provide transport.  The Signalling Gateway
   would act as a Signalling Link Terminal.













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Table of Contents

   1.  Introduction.............................................. 2
     1.1  Scope.................................................. 3
     1.2  Terminology............................................ 3
     1.3  M2UA Overview.......................................... 5
     1.4  Services Provided by the M2UA Adaptation Layer......... 7
     1.5  Functions Provided by the M2UA Layer................... 9
     1.6  Definition of the M2UA Boundaries..................... 12
   2.  Conventions.............................................. 16
   3.  Protocol Elements........................................ 16
     3.1  Common Message Header................................. 16
     3.2  M2UA Message Header................................... 22
     3.3  M2UA Messages......................................... 23
   4.  Procedures............................................... 58
     4.1  Procedures to Support the M2UA-User Layer............. 58
     4.2  Receipt of Primitives from the Layer Management....... 59
     4.3  AS and ASP State Maintenance.......................... 61
     4.4  Link Key Management Procedures........................ 73
   5.  Examples of MTP2 User Adaptation (M2UA) Procedures....... 75
     5.1  Establishment of associations between SGP and MGC..... 75
          examples
     5.2  ASP Traffic Fail-over Examples........................ 77
     5.3  SGP to MGC, MTP Level 2 to MTP Level 3 Boundary
          Procedures............................................ 78
   6.  Timer Values............................................. 85
   7.  Security Considerations.................................. 85
     7.1 Threats................................................ 85
     7.2 Protecting Confidentiality............................. 86
   8.  IANA Considerations...................................... 86
     8.1 SCTP Payload Protocol Identifier....................... 86
     8.2 M2UA Protocol Extensions............................... 86
   9.  Acknowledgements......................................... 87
   10. References............................................... 88
   Appendix A: Signalling Network Architecture.................. 90
   Authors' Addresses........................................... 92
   Full Copyright Statement..................................... 94

1.  Introduction

   This document defines a protocol for the backhauling of SS7 [1] MTP2
   User [2] [3] [4] (i.e. MTP3) signalling messages over IP using the
   Stream Control Transmission Protocol (SCTP) [8].  This protocol would
   be used between a Signalling Gateway (SG) and Media Gateway
   Controller (MGC).






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1.1 Scope

   There is a need for Switched Circuit Network (SCN) signalling
   protocol delivery from a Signalling Gateway (SG) to a Media Gateway
   Controller (MGC) [9].  The delivery mechanism addresses the following
   objectives:

   *  Support for MTP Level 2 / MTP Level 3 interface boundary
   *  Support for communication between Layer Management modules on SG
      and MGC
   *  Support for management of SCTP active associations between the SG
      and MGC

   The SG will terminate up to MTP Level 2 and the MGC will terminate
   MTP Level 3 and above.  In other words, the SG will transport MTP
   Level 3 messages over an IP network to a MGC.

1.2 Terminology

   Application Server (AS) - A logical entity serving a specific
   application instance.  An example of an Application Server is a MGC
   handling the MTP Level 3 and call processing for SS7 links terminated
   by the Signalling Gateways.  Practically speaking, an AS is modeled
   at the SG as an ordered list of one or more related Application
   Server Processes (e.g., primary, secondary, tertiary, ...).

   Application Server Process (ASP) - A process instance of an
   Application Server.  Examples of Application Server Processes are
   active or standby MGC instances.

   Association - An association refers to a SCTP association.  The
   association will provide the transport for the delivery of protocol
   data units for one or more interfaces.

   Backhaul - Refers to the transport of signalling from the point of
   interface for the associated data stream (i.e., SG function in the
   MGU) back to the point of call processing (i.e., the MGCU), if this
   is not local [9].

   Fail-over - The capability to reroute signalling traffic as required
   to an alternate Application Server Process within an Application
   Server in the event of failure or unavailability of a currently used
   Application Server Process.  Fail-back MAY apply upon the return to
   service of a previously unavailable Application Server Process.

   Host - The computing platform that the ASP process is running on.





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   Interface - For the purposes of this document, an interface is a SS7
   signalling link.

   Interface Identifier - The Interface Identifier identifies the
   physical interface at the SG for which the signalling messages are
   sent/received.  The format of the Interface Identifier parameter can
   be text or integer, the values of which are assigned according to
   network operator policy.  The values used are of local significance
   only, coordinated between the SG and ASP.

   Layer Management - Layer Management is a nodal function in an SG or
   ASP that handles the inputs and outputs between the M2UA layer and a
   local management entity.

   Link Key - The link key is a locally unique (between ASP and SG)
   value that identifies a registration request for a particular
   Signalling Data Link and Signalling Terminal pair.

   MTP - The Message Transfer Part of the SS7 protocol

   MTP2 - MTP Level 2, the signalling data link layer of SS7

   MTP3 - MTP Level 3, the signalling network layer of SS7

   MTP2-User - A protocol that uses the services of MTP Level 2 (i.e.
   MTP3).

   Network Byte Order: Most significant byte first, a.k.a Big Endian.

   Signalling Data Link - An SDL refers to a specific communications
   facility that connects two Signalling Link Terminals.

   Signalling Gateway (SG) - An SG is a signalling agent at the edge of
   the IP network.  An SG appears to the SS7 as one or more Signalling
   Link Terminals that are connected to one or more Signalling Data
   Links in the SS7 network.  An SG contains a set of one or more unique
   Signalling Gateway Processes, on which one or more is normally
   actively processing traffic.  Where an SG contains more than one SGP,
   the SG is a logical entity.

   Signalling Gateway Process (SGP) - A process instance that uses M2UA
   to communicate to and from a Signalling Link Terminal.  It serves as
   an active, backup or load-sharing process of a Signalling Gateway.

   Signalling Link Terminal (SLT) - Refers to the means of performing
   all of the functions defined at MTP level 2 regardless of their
   implementation [2,3].




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   Stream - A stream refers to an SCTP stream; a unidirectional logical
   channel established from one SCTP endpoint to another associated SCTP
   endpoint, within which all user messages are delivered in-sequence
   except for those submitted to the unordered delivery service.

1.3  M2UA Overview

   The framework architecture that has been defined for SCN signalling
   transport over IP [9] uses two components: a signalling common
   transport protocol and an adaptation module to support the services
   expected by a particular SCN signalling protocol from its underlying
   protocol layer.

   Within this framework architecture, this document defines a SCN
   adaptation module that is suitable for the transport of SS7 MTP2 User
   messages.  The only SS7 MTP2 User is MTP3.  The M2UA uses the
   services of the Stream Control Transmission Protocol [8] as the
   underlying reliable signalling common transport protocol.

   In a Signalling Gateway, it is expected that the SS7 MTP2-User
   signalling is transmitted and received from the PSTN over a standard
   SS7 network interface, using the SS7 Message Transfer Part Level 1
   and Level 2 [2,3,4] to provide reliable transport of the MTP3-User
   signalling messages to and from an SS7 Signalling End Point (SEP) or
   Signalling Transfer Point (STP).  The SG then provides an
   interworking of transport functions with the IP transport, in order
   to transfer the MTP2-User signalling messages to and from an
   Application Server Process where the peer MTP2-User protocol layer
   exists.






















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RFC 3331             SS7 MTP2 User Adaptation Layer       September 2002


1.3.1  Example - SG to MGC

   In a Signalling Gateway, it is expected that the SS7 signalling is
   received over a standard SS7 network termination, using the SS7
   Message Transfer Part (MTP) to provide transport of SS7 signalling
   messages to and from an SS7 Signalling End Point (SEP) or SS7
   Signalling Transfer Point (STP).  In other words, the SG acts as a
   Signalling Link Terminal (SLT) [2,3].  The SG then provides an
   interworking of transport functions with IP Signalling Transport, in
   order to transport the MTP3 signalling messages to the MGC where the
   peer MTP3 protocol layer exists, as shown below:

       ******    SS7    ******      IP     *******
       *SEP *-----------* SG *-------------* MGC *
       ******           ******             *******

       +----+                              +----+
       |S7UP|                              |S7UP|
       +----+                              +----+
       |MTP +                              |MTP |
       | L3 |            (NIF)             |L3  |
       +----+         +----+----+          +----+
       |MTP |         |MTP |M2UA|          |M2UA|
       |    |         |    +----+          +----+
       |L2  |         |L2  |SCTP|          |SCTP|
       |L1  |         |L1  +----+          +----+
       |    |         |    |IP  |          |IP  |
       +----+         +---------+          +----+

       NIF  - Nodal Interworking Function
       SEP  - SS7 Signalling Endpoint
       IP   - Internet Protocol
       SCTP - Stream Control Transmission Protocol (Reference [8])

           Figure 1  M2UA in the SG to MGC Application

   Note: STPs MAY be present in the SS7 path between the SEP and the SG.

   It is recommended that the M2UA use the services of the Stream
   Control Transmission Protocol (SCTP) [8] as the underlying reliable
   common signalling transport protocol.  The use of SCTP provides the
   following features:

   -  explicit packet-oriented delivery (not stream-oriented)
   -  sequenced delivery of user messages within multiple streams, with
      an option for order-of-arrival delivery of individual user
      messages,
   -  optional multiplexing of user messages into SCTP datagrams,



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