rfc3332.txt

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Network Working Group                                      G. Sidebottom
Request for Comments: 3332                         Signatus Technologies
Category: Standards Track                                   K. Morneault
                                                                   Cisco
                                                        J. Pastor-Balbas
                                                                Ericsson
                                                                 Editors
                                                          September 2002


       Signaling System 7 (SS7) Message Transfer Part 3 (MTP3) -
                      User Adaptation Layer (M3UA)

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 memo defines a protocol for supporting the transport of any SS7
   MTP3-User signalling (e.g., ISUP and SCCP messages) over IP using the
   services of the Stream Control Transmission Protocol.  Also,
   provision is made for protocol elements that enable a seamless
   operation of the MTP3-User peers in the SS7 and IP domains. This
   protocol would be used between a Signalling Gateway (SG) and a Media
   Gateway Controller (MGC) or  IP-resident Database, or between two
   IP-based applications.  It is assumed that the SG receives SS7
   signalling over a standard SS7 interface using the SS7 Message
   Transfer Part (MTP) to provide transport.

Table of Contents

   1.  Introduction..................................................3
   1.1 Scope.........................................................3
   1.2 Terminology...................................................4
   1.3 M3UA Overview.................................................6
   1.4 Functional Areas.............................................10
   1.5 Sample Configurations........................................18
   1.6 Definition of M3UA Boundaries................................21
   2.  Conventions..................................................25



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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   3.  M3UA Protocol Elements.......................................25
   3.1 Common Message Header........................................26
   3.2 Variable Length Parameter....................................29
   3.3 Transfer Messages............................................31
   3.4 SS7 Signalling Network Management (SSNM) Messages............35
   3.5 ASP State Maintenance (ASPSM) Messages.......................45
   3.6 Routing Key Management (RKM) Messages........................48
   3.7 ASP Traffic Maintenance (ASPTM) Messages.....................59
   3.8 Management (MGMT) Messages...................................63
   4.  Procedures...................................................69
   4.1 Procedures to Support the M3UA-User .........................69
   4.2 Procedures to Support the Management of SCTP Associations ...70
   4.3 AS and ASP State Maintenance.................................72
   4.4 Routing Key Management Procedures............................87
   4.5 Procedures to Support the Availability or Congestion Status
       of SS7 Destination...........................................89
   4.6 MTP3 Restart.................................................92
   5.  Examples of M3UA Procedures..................................93
   5.1 Establishment of Association and Traffic
       Between SGs and ASPs.........................................93
   5.2 ASP traffic Failover Examples................................99
   5.3 Normal Withdrawal of an ASP from an Application Server
       and Teardown of an Association..............................100
   5.4 M3UA/MTP3-User Boundary Examples............................101
   5.5 Examples of IPSP communication..............................105
   6.  Security Considerations.....................................108
   6.1 Introduction................................................108
   6.2 Threats.....................................................108
   6.3 Protecting Confidentiality..................................108
   7.  IANA Considerations.........................................109
   7.1 SCTP Payload Protocol Identifier............................109
   7.2 M3UA Port Number............................................109
   7.3 M3UA Protocol Extensions....................................109
   8. References...................................................111
   8.1 Normative References........................................111
   8.2 Informative References......................................111
   9. Acknowledgements.............................................113
   10. Document Contributors.......................................113
   Appendix A......................................................114
   A.1 Signalling Network Architecture.............................114
   A.2 Redundancy Models...........................................117
   Editors' Addresses..............................................119
   Full Copyright Statement........................................120








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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


1.  Introduction

   This memo defines a protocol for supporting the transport of any SS7
   MTP3-User signalling (e.g., ISUP and SCCP messages) over IP using the
   services of the Stream Control Transmission Protocol [17]. Also,
   provision is made for protocol elements that enable a seamless
   operation of the MTP3-User peers in the SS7 and IP domains.  This
   protocol would be used between a Signalling Gateway (SG) and a Media
   Gaway Controller (MGC) or  IP-resident Database [11], or between two
   IP-based applications.

1.1 Scope

   There is a need for Switched Circuit Network (SCN) signalling
   protocol delivery from an SS7 Signalling Gateway (SG) to a Media
   Gateway Controller (MGC) or IP-resident Database as described in the
   Framework Architecture for Signalling Transport [11].  The delivery
   mechanism should meet the following criteria:

   *  Support for the transfer of all SS7 MTP3-User Part messages (e.g.,
      ISUP [1,2,3], SCCP [4,5,6], TUP [12], etc.)
   *  Support for the seamless operation of MTP3-User protocol peers
   *  Support for the management of SCTP transport associations and
      traffic between an SG and one or more MGCs or IP-resident
      Databases
   *  Support for MGC or IP-resident Database process failover and load
      sharing
   *  Support for the asynchronous reporting of status changes to
      management

   In simplistic transport terms, the SG will terminate SS7 MTP2 and
   MTP3 protocol layers [7,8,9] and deliver ISUP, SCCP and/or any other
   MTP3-User protocol messages, as well as certain MTP network
   management events, over SCTP transport associations to MTP3-User
   peers in MGCs or IP-resident Databases.
















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1.2 Terminology

   Application Server (AS) - A logical entity serving a specific Routing
   Key.  An example of an Application Server is a virtual switch element
   handling all call processing for a unique range of PSTN trunks,
   identified by an SS7 SIO/DPC/OPC/CIC_range.  Another example is a
   virtual database element, handling all HLR transactions for a
   particular SS7 DPC/OPC/SCCP_SSN combination.  The AS contains a set
   of one or more unique Application Server Processes, of which one or
   more is normally actively processing traffic.  Note that there is a
   1:1 relationship between an AS and a Routing Key.

   Application Server Process (ASP) - A process instance of an
   Application Server. An Application Server Process serves as an active
   or backup process of an Application Server (e.g., part of a
   distributed virtual switch or database).  Examples of ASPs are
   processes (or process instances) of MGCs, IP SCPs or IP HLRs.  An ASP
   contains an SCTP endpoint and may be configured to process signalling
   traffic within more than one Application Server.

   Association - An association refers to an SCTP association.  The
   association provides the transport for the delivery of MTP3-User
   protocol data units and M3UA adaptation layer peer messages.

   IP Server Process (IPSP) - A process instance of an IP-based
   application.  An IPSP is essentially the same as an ASP, except that
   it uses M3UA in a point-to-point fashion.  Conceptually, an IPSP does
   not use the services of a Signalling Gateway node.

   Failover - The capability to reroute signalling traffic as required
   to an alternate Application Server Process, or group of ASPs, within
   an Application Server in the event of failure or unavailability of a
   currently used Application Server Process.  Failover also applies
   upon the return to service of a previously unavailable Application
   Server Process.

   Host - The computing platform that the process (SGP, ASP or IPSP) is
   running on.

   Layer Management - Layer Management is a nodal function that handles
   the inputs and outputs between the M3UA layer and a local management
   entity.









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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   Linkset - A number of signalling links that directly interconnect two
   signalling points, which are used as a module.

   MTP - The Message Transfer Part of the SS7 protocol.

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

   MTP3-User - Any protocol normally using the services of the SS7 MTP3
   (e.g., ISUP, SCCP, TUP, etc.).

   Network Appearance - The Network Appearance is a M3UA local reference
   shared by SG and AS (typically an integer) that together with an
   Signaling Point Code uniquely identifies an SS7 node by indicating
   the specific SS7 network it belongs to. It can be used to distinguish
   between signalling traffic associated with different networks being
   sent between the SG and the ASP over a common SCTP association. An
   example scenario is where an SG appears as an element in multiple
   separate national SS7 networks and the same Signaling Point Code
   value may be reused in different networks.

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

   Routing Key: A Routing Key describes a set of SS7 parameters and
   parameter values that uniquely define the range of signalling traffic
   to be handled by a particular Application Server. Parameters within
   the Routing Key cannot extend across more than a single Signalling
   Point Management Cluster.

   Routing Context - A value that uniquely identifies a Routing Key.
   Routing Context values are either configured using a configuration
   management interface, or by using the routing key management
   procedures defined in this document.

   Signalling Gateway Process (SGP) - A process instance of a Signalling
   Gateway.  It serves as an active, backup, load-sharing or broadcast
   process of a Signalling Gateway.

   Signalling Gateway - An SG is a signaling agent that receives/sends
   SCN native signaling at the edge of the IP network [11].  An SG
   appears to the SS7 network as an SS7 Signalling Point.  An SG
   contains a set of one or more unique Signalling Gateway Processes, of
   which one or more is normally actively processing traffic.  Where an
   SG contains more than one SGP, the SG is a logical entity and the
   contained SGPs are assumed to be coordinated into a single management
   view to the SS7 network and to the supported Application Servers.






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   Signalling Process - A process instance that uses M3UA to communicate
   with other signalling processes.  An ASP, an SGP and an IPSP are all
   signalling processes.

   Signalling Point Management Cluster (SPMC) - The complete set of
   Application Servers represented to the SS7 network under a single MTP
   entity (Signalling Point) in one specific Network Appearance.  SPMCs
   are used to aggregate the availability, congestion, and user part
   status of an MTP entity (Signalling Point) that is distributed in the
   IP domain, for the purpose of supporting MTP3 management procedures
   towards the SS7 network.  In some cases, the SG itself may also be a
   member of the SPMC.  In this case, the SG availability /congestion
   /User_Part status should also be taken into account when considering
   any supporting MTP3 management actions.

   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 M3UA Overview

1.3.1 Protocol Architecture

   The framework architecture that has been defined for SCN signalling
   transport over IP [11] uses multiple components, including a common
   signalling transport protocol and an adaptation module to support the
   services expected by a particular SCN signalling protocol from its
   underlying protocol layer.

   Within the framework architecture, this document defines an MTP3-User
   adaptation module suitable for supporting the transfer of messages of
   any protocol layer that is identified to the MTP Level 3 as an MTP
   User.  The list of these protocol layers includes, but is not limited
   to, ISDN User Part (ISUP) [1,2,3], Signalling Connection Control Part
   (SCCP) [4,5,6] and Telephone User Part (TUP) [12].  TCAP [13,14,15]
   or RANAP [16] messages are transferred transparently by the M3UA
   protocol as SCCP payload, as they are SCCP-User protocols.

   It is recommended that M3UA use the services of the Stream Control
   Transmission Protocol (SCTP) [17] as the underlying reliable common
   signalling transport protocol. This is to take advantage of various
   SCTP features such as: