rfc3057.txt

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Network Working Group                                       K. Morneault
Request for Comments: 3057                                 Cisco Systems
Category: Standards Track                                   S. Rengasami
                                                                M. Kalla
                                                  Telcordia Technologies
                                                           G. Sidebottom
                                                         Nortel Networks
                                                           February 2001


                    ISDN Q.921-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 (2001).  All Rights Reserved.

Abstract

   This document defines a protocol for backhauling of ISDN Q.921 User
   messages over IP using the Stream Control Transmission Protocol
   (SCTP).  This protocol would be used between a Signaling Gateway (SG)
   and Media Gateway Controller (MGC).  It is assumed that the SG
   receives ISDN signaling over a standard ISDN interface.




















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Table of Contents

   1.  Introduction.................................................  2
     1.1  Scope.....................................................  2
     1.2  Terminology...............................................  3
     1.3  IUA Overview..............................................  4
     1.4  Services Provided by the IUA Layer........................  9
     1.5  Functions Implemented by the IUA Layer.................... 12
     1.6  Definition of IUA Boundaries.............................. 14
   2.  Conventions.................................................. 16
   3.  Protocol Elements............................................ 17
     3.1  Common Message Header..................................... 17
     3.2  IUA Message Header........................................ 20
     3.3  Description of Messages................................... 22
   4.  Procedures................................................... 45
     4.1  Procedures to Support Service in Section 1.4.1............ 45
     4.2  Procedures to Support Service in Section 1.4.2............ 46
     4.3  Procedures to Support Service in Section 1.4.3............ 47
   5. Examples...................................................... 56
     5.1 Establishment of associations between SG and MGC examples.. 56
     5.2 ASP Traffic Fail-over Examples............................. 58
     5.3 Q.921/Q.931 primitives backhaul Examples................... 59
     5.4 Layer Management Communication Examples.................... 61
   6.  Security..................................................... 61
     6.1 Threats.................................................... 61
     6.2 Protecting Confidentiality ................................ 62
   7.  IANA Considerations.......................................... 62
     7.1 SCTP Payload Protocol Identifier........................... 62
     7.2 IUA Protocol Extensions.................................... 62
   8.  Acknowledgements............................................. 64
   9.  References................................................... 64
   10. Authors' Addresses........................................... 65
   11. Full Copyright Statement..................................... 66

1.  Introduction

   In this document, the term Q.921-User refers to an upper layer which
   uses the services of Q.921, not the user side of ISDN interface [1].
   Examples of the upper layer would be Q.931 and QSIG.

   This section describes the need for ISDN Q.921-User Adaptation (IUA)
   layer protocol as well as how this protocol shall be implemented.

1.1 Scope

   There is a need for Switched Circuit Network (SCN) signaling protocol
   delivery from an ISDN Signaling Gateway (SG) to a Media Gateway
   Controller (MGC) as described in the Framework Architecture for



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   Signaling Transport [4].  The delivery mechanism SHOULD meet the
   following criteria:

   *  Support for transport of the Q.921 / Q.931 boundary primitives
   *  Support for communication between Layer Management modules on SG
      and MGC
   *  Support for management of active associations between SG and MGC

   This document supports both ISDN Primary Rate Access (PRA) as well as
   Basic Rate Access (BRA) including the support for both point-to-point
   and point-to-multipoint modes of communication.  This support
   includes Facility Associated Signaling (FAS), Non-Facility Associated
   Signaling (NFAS) and NFAS with backup D channel.  QSIG adaptation
   layer requirements do not differ from Q.931 adaptation layer, hence;
   the procedures described in this document are also applicable for a
   QSIG adaptation layer.  For simplicity, only Q.931 will be mentioned
   in the rest of this document.

1.2 Terminology

   Interface - For the purposes of this document an interface supports
   the relevant ISDN signaling channel.  This signaling channel MAY be a
   16 kbps D channel for an ISDN BRA as well as 64 kbps primary or
   backup D channel for an ISDN PRA.  For QSIG, the signaling channel is
   a Qc channel.

   Q.921-User - Any protocol normally using the services of the ISDN
   Q.921 (e.g., Q.931, QSIG, etc.).

   Backhaul - A SG terminates the lower layers of an SCN protocol and
   backhauls the upper layer(s) to MGC for call processing.  For the
   purposes of this document the SG terminates Q.921 and backhauls Q.931
   to MGC.

   Association - An association refers to a SCTP association.  The
   association will provide the transport for the delivery of Q.921-User
   protocol data units and IUA adaptation layer peer messages.

   Stream - A stream refers to an SCTP stream; a uni-directional 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 un-ordered delivery service.

   Interface Identifier - The Interface Identifier identifies the
   physical interface at the SG for which the signaling messages are
   sent/received. The format of the Interface Identifier parameter can
   be text or integer, the values of which are assigned according to




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   network operator policy. The values used are of local significance
   only, coordinated between the SG and ASP.  Significance is not
   implied across SGs served by an AS.

   Application Server (AS) - A logical entity serving a specific
   application instance.  An example of an Application Server is a MGC
   handling the Q.931 and call processing for D channels terminated by
   the Signaling 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
   primary or backup MGC instances.

   Fail-over - The capability to re-route signaling traffic as required
   between related ASPs in the event of failure or unavailability of the
   currently used ASP (e.g., from primary MGC to back-up MGC).  Fail-
   over also applies upon the return to service of a previously
   unavailable process.

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

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

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

1.3 IUA Overview

   The architecture that has been defined [4] for SCN signaling
   transport over IP uses multiple components, including an IP transport
   protocol, a signaling common transport protocol and an adaptation
   module to support the services expected by a particular SCN signaling
   protocol from its underlying protocol layer.

   This document defines an adaptation module that is suitable for the
   transport of ISDN Q.921-User (e.g., Q.931) messages.

1.3.1  Example - SG to MGC

   In a Signaling Gateway, it is expected that the ISDN signaling is
   received over a standard ISDN network termination.  The SG then
   provides interworking of transport functions with IP Signaling
   Transport, in order to transport the Q.931 signaling messages to the
   MGC where the peer Q.931 protocol layer exists, as shown below:




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            ******   ISDN        ******      IP      *******
            * EP *---------------* SG *--------------* MGC *
            ******               ******              *******

            +-----+                                  +-----+
            |Q.931|              (NIF)               |Q.931|
            +-----+           +----------+           +-----+
            |     |           |     | IUA|           | IUA |
            |     |           |     +----+           +-----+
            |Q.921|           |Q.921|SCTP|           |SCTP |
            |     |           |     +----+           +-----+
            |     |           |     | IP |           | IP  |
            +-----+           +-----+----+           +-----+

            NIF  - Nodal Interworking Function
            EP   - ISDN End Point
            SCTP - Stream Control Transmission Protocol (Refer to [3])
            IUA  - ISDN User Adaptation Layer Protocol

   It is recommended that the IUA use the services of the Stream Control
   Transmission Protocol (SCTP) as the underlying reliable common
   signaling 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,
      -  network-level fault tolerance through support of multi-homing
         at either or both ends of an association,
      -  resistance to flooding and masquerade attacks, and
      -  data segmentation to conform to discovered path MTU size

   There are scenarios without redundancy requirements and scenarios in
   which redundancy is supported below the transport layer.  In these
   cases, the SCTP functions above MAY NOT be a requirement and TCP can
   be used as the underlying common transport protocol.

1.3.2  Support for the management of SCTP associations between the SG
       and ASPs

   The IUA layer at the SG maintains the availability state of all
   dynamically registered remote ASPs, in order to manage the SCTP
   Associations and the traffic between the SG and ASPs.  As well, the
   active/inactive state of remote ASP(s) are also maintained.  Active
   ASPs are those currently receiving traffic from the SG.




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   The IUA layer MAY be instructed by local management to establish an
   SCTP association to a peer IUA node.  This can be achieved using the
   M-SCTP ESTABLISH primitive to request, indicate and confirm the
   establishment of an SCTP association with a peer IUA node.

   The IUA layer MAY also need to inform local management of the status
   of the underlying SCTP associations using the M-SCTP STATUS request
   and indication primitive.  For example, the IUA MAY inform local
   management of the reason for the release of an SCTP association,
   determined either locally within the IUA layer or by a primitive from
   the SCTP.

1.3.3  Signaling Network Architecture

   A Signaling Gateway is used to support the transport of Q.921-User
   signaling traffic to one or more distributed ASPs (e.g., MGCs).
   Clearly, the IUA protocol is not designed to meet the performance and
   reliability requirements for such transport by itself.  However, the
   conjunction of distributed architecture and redundant networks does
   allow for a sufficiently reliable transport of signaling traffic over
   IP.  The IUA protocol is flexible enough to allow its operation and
   management in a variety of physical configurations, enabling Network
   Operators to meet their performance and reliability requirements.

   To meet the ISDN signaling reliability and performance requirements
   for carrier grade networks, Network Operators SHOULD ensure that
   there is no single point of failure provisioned in the end-to-end
   network architecture between an ISDN node and an IP ASP.

   Depending of course on the reliability of the SG and ASP functional
   elements, this can typically be met by the provision of redundant
   QOS-bounded IP network paths for SCTP Associations between SCTP End
   Points, and redundant Hosts, and redundant SGs.  The distribution of
   ASPs within the available Hosts is also important.  For a particular
   Application Server, the related ASPs SHOULD be distributed over at
   least two Hosts.

   An example logical network architecture relevant to carrier-grade
   operation in the IP network domain is shown in Figure 1 below:












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