rfc2719.txt

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Network Working Group                                              L. Ong
Request for Comments: 2719                                Nortel Networks
Category: Informational                                         I. Rytina
                                                                M. Garcia
                                                                 Ericsson
                                                          H. Schwarzbauer
                                                                 L. Coene
                                                                  Siemens
                                                                   H. Lin
                                                                Telcordia
                                                                I. Juhasz
                                                                    Telia
                                                              M. Holdrege
                                                                   Lucent
                                                                 C. Sharp
                                                            Cisco Systems
                                                             October 1999


             Framework Architecture for Signaling Transport

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   This document defines an architecture framework and functional
   requirements for transport of signaling information over IP.  The
   framework describes relationships between functional and physical
   entities exchanging signaling information, such as Signaling Gateways
   and Media Gateway Controllers.  It identifies interfaces where
   signaling transport may be used and the functional and performance
   requirements that apply from existing Switched Circuit Network (SCN)
   signaling protocols.










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RFC 2719     Framework Architecture for Signaling Transport October 1999


Table of Contents

   1. Introduction..................................................2
   1.1 Overview.....................................................2
   1.2 Terminology..................................................3
   1.3  Scope.......................................................5
   2.  Signaling Transport Architecture.............................5
   2.1  Gateway Component Functions.................................5
   2.2  SS7 Interworking for Connection Control.....................6
   2.3  ISDN Interworking for Connection Control....................8
   2.4  Architecture for Database Access............................9
   3. Protocol Architecture........................................10
   3.1 Signaling Transport Components..............................10
   3.2 SS7 access for Media Gateway Control........................11
   3.3 Q.931 Access to MGC.........................................12
   3.4 SS7 Access to IP/SCP........................................12
   3.5 SG to SG....................................................14
   4. Functional Requirements......................................15
   4.1 Transport of SCN Signaling Protocols........................15
   4.2 Performance of SCN Signaling Protocols......................17
   4.2.1 SS7 MTP Requirements......................................17
   4.2.2 SS7 MTP Level 3 Requirements..............................17
   4.2.3 SS7 User Part Requirements................................18
   4.2.4 ISDN Signaling Requirements...............................18
   5. Management...................................................19
   6. Security Considerations......................................19
   6.1 Security Requirements.......................................19
   6.2 Security Mechanisms Currently Available in IP Networks......20
   7. Abbreviations................................................21
   8. Acknowledgements.............................................21
   9. References...................................................21
   Authors' Addresses..............................................22
   Full Copyright Statement........................................24

1. Introduction

1.1 Overview

   This document defines an architecture framework for transport of
   message-based signaling protocols over IP networks.  The scope of
   this work includes definition of encapsulation methods, end-to-end
   protocol mechanisms and use of existing IP capabilities to support
   the functional and performance requirements for signaling transport.

   The framework portion describes the relationships between functional
   and physical entities used in signaling transport, including the
   framework for control of Media Gateways, and other scenarios where
   signaling transport may be required.



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RFC 2719     Framework Architecture for Signaling Transport October 1999


   The requirements portion describes functional and performance
   requirements for signaling transport such as flow control, in-
   sequence delivery and other functions that may be required for
   specific SCN signaling protocols.

1.2 Terminology

   The following are general terms are used in this document:

   Backhaul:

   Backhaul refers to the transport of signaling 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.

   Signaling Transport (SIG):

   SIG refers to a protocol stack for transport of SCN signaling
   protocols over an IP network. It will support standard primitives to
   interface with an unmodified SCN signaling application being
   transported, and supplements a standard IP transport protocol
   underneath with functions designed to meet transport requirements for
   SCN signaling.

   Switched Circuit Network (SCN):

   The term SCN is used to refer to a network that carries traffic
   within channelized bearers of pre-defined sizes.  Examples include
   Public Switched Telephone Networks (PSTNs) and Public Land Mobile
   Networks (PLMNs).  Examples of signaling protocols used in SCN
   include Q.931, SS7 MTP Level 3 and SS7 Application/User parts.

   The following are terms for functional entities relating to signaling
   transport in a distributed gateway model.

   Media Gateway (MG):

   A MG terminates SCN media streams, packetizes the media data,, if it
   is not already packetized, and delivers packetized traffic  to the
   packet network.  It performs these functions in reverse order for
   media streams flowing from the packet network to the SCN.









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RFC 2719     Framework Architecture for Signaling Transport October 1999


   Media Gateway Controller (MGC):

   An MGC handles the registration and management of resources at the
   MG. The MGC may have the ability to authorize resource usage based on
   local policy.  For signaling transport purposes, the MGC serves as a
   possible termination and origination point for SCN application
   protocols, such as SS7 ISDN User Part and Q.931/DSS1.

   Signaling Gateway (SG):

   An SG is a signaling agent that receives/sends SCN native signaling
   at the edge of the IP network. The SG function may relay, translate
   or terminate SS7 signaling in an SS7-Internet Gateway. The SG
   function may also be co-resident with the MG function to process SCN
   signaling associated with line or trunk terminations controlled by
   the MG (e.g., signaling backhaul).

   The following are terms for physical entities relating to signaling
   transport in a distributed gateway model:

   Media Gateway Unit (MGU)

   An MG-Unit is a physical entity that contains the MG function.  It
   may contain other functions, esp. an SG function for handling
   facility-associated signaling.

   Media Gateway Control Unit (MGCU)

   An MGC-Unit is a physical entity containing the MGC function.

   Signaling Gateway Unit (SGU)

   An SG-Unit is a physical entity containing the SG function.

   Signaling End Point (SEP):

   This is a node in an SS7 network that originates or terminates
   signaling messages.  One example is a central office switch.

   Signal Transfer Point (STP):

   This is a node in an SS7 network that routes signaling messages based
   on their destination point code in the SS7 network.








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RFC 2719     Framework Architecture for Signaling Transport October 1999


1.3  Scope

   Signaling transport provides transparent transport of message-based
   signaling protocols over IP networks.   The scope of this work
   includes definition of encapsulation methods, end-to-end protocol
   mechanisms and use of IP capabilities to support the functional and
   performance requirements for signaling.

   Signaling transport shall be used for transporting SCN signaling
   between a Signaling Gateway Unit and Media Gateway Controller Unit.
   Signaling transport may also be used for transport of message-based
   signaling between a Media Gateway Unit and Media Gateway Controller
   Unit, between dispersed Media Gateway Controller Units, and between
   two Signaling Gateway Units connecting signaling endpoints or signal
   transfer points in the SCN.

   Signaling transport will be defined in such a way as to support
   encapsulation and carriage of a variety of SCN protocols.  It is
   defined in such a way as to be independent of any SCN protocol
   translation functions taking place at the endpoints of the signaling
   transport, since its function is limited to the transport of the SCN
   protocol.

   Since the function being provided is transparent transport, the
   following areas are considered outside the scope of the signaling
   transport work:

   -  definition of the SCN protocols themselves.
   -  signaling interworking such as conversion from Channel Associated
      Signaling (CAS) to message signaling protocols.
   -  specification of the functions taking place within the SGU or MGU
   -  in particular, this work does not address whether the SGU provides
      mediation/interworking, as this is transparent to the transport
      function.
   -  similarly, some management and addressing functions taking place
      within the SGU or MGU are also considered out of scope, such as
      determination of the destination IP address for signaling, or
      specific procedures for assessing the performance of the transport
      session (i.e., testing and proving functions).

2.  Signaling Transport Architecture

2.1  Gateway Component Functions

   Figure 1 defines a commonly defined functional model that separates
   out the functions of SG, MGC and MG.  This model may be implemented
   in a number of ways, with functions implemented in separate devices
   or combined in single physical units.



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RFC 2719     Framework Architecture for Signaling Transport October 1999


   Where physical separation exists between functional entities,
   Signaling Transport can be applied to ensure that SCN signaling
   information is transported between entities with the required
   functionality and performance.

        +---------------+                      +--------------+
        |               |                      |              |
  SCN<-------->[SG]  <--+---------O------------+--> [SG]  <------> SCN
 signal |       |       |                      |     |        |   signal
        +-------|-------+                      +-----|--------+
       Signaling|gateway                    Signaling|gateway (opt)
                O                                    O
                |                                    |
        +-------|-------+                      +-----|--------+
        |       |       |                      |     |        |
        |      [MGC] <--+--------O-------------+--> [MGC]     |
        |       |       |                      |     |        |
        |       |       |                      |     |        |
        +-------|-------+                      +-----|--------+
        Gateway | controller                 Gateway | controller (opt)
                O                                    O
                |                                    |
        +-------|-------+                      +-----|--------+
  Media |       |       |                      |     |        | Media
 <------+---->[MG]  <---+-----RTP stream-------+-> [MG]  <----+-------->
  stream|               |                      |              | stream
        +---------------+                      +--------------+
        Media gateway                           Media gateway


                   Figure 1: Sigtran Functional Model

   As discussed above, the interfaces pertaining to signaling transport
   include SG to MGC, SG to SG.  Signaling transport may potentially be
   applied to the MGC to MGC or MG to MGC interfaces as well, depending
   on requirements for transport of the associated signaling protocol.

2.2  SS7 Interworking for Connection Control

   Figure 2 below shows some example implementations of these functions
   in physical entities as used for interworking of SS7 and IP networks
   for Voice over IP, Voice over ATM, Network Access Servers, etc.  No
   recommendation is made as to functional distribution and many other
   examples are possible but are not shown to be concise.  The use of
   signaling transport is independent of the implementation.