rfc2331.txt

<VC++网络游戏建摸与实现>源代码

Network Working Group                                           M. Maher
Request for Comments: 2331                                       USC/ISI
Category: Standards Track                                     April 1998


   ATM Signalling Support for IP over ATM - UNI Signalling 4.0 Update

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 (1998).  All Rights Reserved.

Abstract

   This memo describes how to efficiently use the ATM call control
   signalling procedures defined in UNI Signalling 4.0 [SIG40] to
   support IP over ATM environments as described in RFC 2225 [LAUB98]
   and in RFC 2332 [LUC98].  Among the new features found in UNI
   Signalling 4.0 are Available Bit Rate signalling and traffic
   parameter negotiation.  This memo highlights the features of UNI
   Signalling 4.0 that provide IP entities capabilities for requesting
   ATM service in sites with SVC support, whether it is private ATM or
   publicly provisioned ATM, in which case the SVC support is probably
   configured inside PVPs.

   This document is only relevant to IP when used as the well known
   "best effort" connectionless service. In particular, this means that
   this document does not pertain to IP in the presence of implemented
   IP Integrated Services.  The topic of IP with Integrated Services
   over ATM will be handled by a different specification or set of
   specifications being worked on in the ISSLL WG.

   This specification is a follow-on to RFC 1755, "ATM Signaling Support
   for IP over ATM", which is based on UNI 3.1 signalling [UNI95].
   Readers are assumed to be familiar with RFC 1755.









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RFC 2331        IP over ATM Signalling - SIG 4.0 Update       April 1998


Table of Contents

   1.  Conventions ...............................................   2
   2.  Overview ..................................................   2
   3.  Use of Protocol Procedures ................................   3
       3.1  VC Teardown...........................................   3
   4.  Overview of Call Establishment Message Content ............   3
   5.  Description of Information Elements .......................   4
       5.1  ATM Adaptation Layer Parameters ......................   4
       5.2  Broadband Low Layer Information  .....................   5
       5.3  Traffic Management Issues and Related IEs.............   5
            5.3.1  ATM Traffic Descriptor ........................   6
                   5.3.1.1  Tagging vs. Dropping .................   7
            5.3.2  Traffic Parameter Negotiation ..................  7
            5.3.3  Broadband Bearer Capability ....................  8
            5.3.4  QoS Parameter ..................................  8
                   5.3.4.1  Signalling of Individual QoS Parameters  8
       5.4  ATM Addressing Information ...........................   9
   6.  ABR Signalling In More Detail  ............................   9
   7.  Frame Discard Capability ..................................  10
   8.  Security Considerations ...................................  10
   9.  Acknowledgements...........................................  10
   10. References ................................................  10
   11. Author's Address ..........................................  12
   Appendix A  Sample Signalling Messages ........................  13
   Appendix B  ABR and nrt-VBR Signalling Guidelines for IP Routers 15
   Appendix C  Combinations of Traffic Related Parameters ........  18
   Full Copyright Statement ......................................  26

1.  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 RFC 2119 [BRA97].

2.  Overview

   UNI Signalling version 4.0 (SIG 4.0) is the ATM Forum follow-on
   specification to UNI 3.1 signalling (UNI 3.1). Among the new features
   in SIG 4.0, those of particular interest to IP over ATM environments
   are:

        o Available Bit Rate (ABR) Signalling for Point-to-Point Calls
        o Traffic Parameter Negotiation
        o Frame Discard Support
        o Leaf Initiated Join (LIJ) Capability
        o ATM Anycast Capability
        o Switched Virtual Path (VP) Service



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   This memo highlights the first three capabilities listed above. The
   last three capabilities are not discussed because models for their
   use in IP over ATM environments have not yet been defined.  The ION
   WG is considering the applicability of LIJ and Group Addressing to
   the RFC2022 problem space.  Furthermore, Anycast addressing is being
   explored as a technique for supporting server discovery in ATM
   networks.

3.  Use of Protocol Procedures

   Section 3 in RFC 1755 introduces requirements of virtual circuit (VC)
   management intended to prevent VC thrashing, excessive VC
   consumption, and other related problems. This section updates RFC
   1755's requirements related to VC teardown.

3.1.  VC Teardown

   In environments running layer 3 (L3) signalling protocols, such as
   RSVP [RSVP], over ATM, data VCs might correspond to L3 reserved flows
   (even if the VC is a 'best effort' VC). In such environments it is
   beneficial for VCs to be torn down only when the L3 reservation has
   expired. In other words, it is more efficient for the sender of a L3
   reserved flow to initiate VC tear-down when the receiver(s) has
   ceased refreshing the reservation.  To support such L3 behavior,
   systems implementing a Public ATM UNI interface and serving as the
   _called_ party of a VCC MUST NOT use an inactivity timer on such a
   VCC by default.  A system MAY use an inactivity timer on such a VCC
   if configured to do so.

4.  Overview of Call Establishment Message Content

   Signalling messages are structured to contain mandatory and optional
   variable length information elements (IEs).  A SETUP message which
   establishes an ATM connection to be used for IP and multiprotocol
   interconnection calls MUST contain the following IEs:

        AAL Parameters
        ATM Traffic Descriptor
        Broadband Bearer Capability
        Broadband Low Layer Information
        QoS Parameter
        Called Party Number
        Calling Party Number

   and MAY, under certain circumstance contain the following IEs:






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        Calling Party Subaddress
        Called Party Subaddress
        Transit Network Selection

        (New in SIG 4.0:)
        Minimum Acceptable ATM Traffic Descriptor
        Alternative ATM Traffic Descriptor
        ABR Setup Parameters
        ABR Additional Parameters
        Connection Scope Selection
        Extended QoS Parameters
        End-to-End Transit Delay

   In SIG 4.0, like UNI 3.1, the AAL Parameters and the Broadband Low
   Layer Information IEs are optional in a SETUP message.  However, in
   support of IP over ATM these two IEs MUST be included. Appendix A
   shows a sample setup message.

5.  Description of Information Elements

   This section describes the coding of, and procedures surrounding,
   information elements in SETUP and CONNECT messages. The first two IEs
   described, ATM Adaptation Layer Parameters and Broadband Low Layer
   Information, are categorized as having significance only to the end-
   points of an ATM call supporting IP. That is, the network does not
   process these IEs.

5.1.  ATM Adaptation Layer (AAL) Parameters

   The AAL Parameters IE carries information about the ATM adaptation
   layer to be used on the connection. The parameters specified in this
   IE are the same as specified in [PER95].


       Format and field values of AAL Parameters IE

       ----------------------------------------------------------
       | aal_parameters                                         |
       ----------------------------------------------------------
       |  aal_type                    5        (AAL 5)          |
       |  fwd_max_sdu_size_identifier 140                       |
       |  fwd_max_sdu_size            65,535   (desired IP MTU) |
       |  bkw_max_sdu_size_identifier 129                       |
       |  bkw_max_sdu_size            65,535   (desired IP MTU) |
       |  sscs_type identifier        132                       |
       |  sscs_type                   0        (null SSCS)      |
       ----------------------------------------------------------




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   This shows maximum size MTUs.  In practice, most sites have used 9180
   IP MTUs for ATM [RFC1626].

5.2.  Broadband Low Layer Information

   Selection of an encapsulation to support IP over an ATM VCC is done
   using the Broadband Low Layer Information (B-LLI) IE, along with the
   AAL Parameters IE, and the B-LLI negotiation procedure.  B-LLI
   negotiation is described in [PER95] in Appendix D. The procedures
   remain the same for this SIG 4.0 based specification.

       Format of B-LLI IE indicating LLC/SNAP encapsulation

       ----------------------------------------------------------
       | bb_low_layer_information                               |
       ----------------------------------------------------------
       |  layer_2_id                 2                          |
       |  user_information_layer     12  (lan_llc - ISO 8802/2) |
       ----------------------------------------------------------

5.3.  Traffic Management Issues and Related IEs

   The ATM Forum Traffic Management Sub-working group has completed
   version 4.0 of their specification [TMGT40]. This latest version
   focuses primarily on the definition of the ABR service category. As
   opposed to the Unspecified Bit Rate (UBR) traffic class, ABR uses a
   rate-based flow control mechanism to assure certain traffic
   guarantees (bandwidth and delay).  There has been much debate on
   whether IP benefits from ABR, and if so, how IP should use ABR. The
   IP Integrated Services (IIS) and RSVP models in IP add complexity to
   this issue because mapping IIS traffic classes to ATM traffic classes
   is not straightforward.

   This document attempts only to present the required IP to ATM
   signaling interface for IP over ATM systems that do not support IIS
   as yet.  It is an attempt to cause IP over ATM vendors to support
   enough options for signalling the traffic characteristics of VCs
   serving non-IIS IP datagrams. This specification also aims to give
   guidance to ATM system administrators so that they can configure
   their IP over ATM entities to conform to the varied services that
   their ATM provider may have sold to them.  By definition, IP without
   IIS cannot be expected to provide a signalling interface that is
   flexible and allows application specific traffic descriptors. The
   topic of IP over ATM signalling for IP _with_ IIS is to be presented
   in other specifications being produced by the ISSLL WG of the IETF.

   An IP over ATM interface may be configured to support all the defined
   ATM Service Categories (ASC). They are:



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    - CBR
    - CBR with CLR specified (loss-permitting CBR)
    - ABR
    - UBR
    - real time VBR
    - non-real time VBR

   The ATM Traffic Descriptor IE, Broadband Bearer Capability IE, and
   the QoS Parameter IE together define the signalling view of ATM
   traffic management. Additionally, the Extended QoS parameters IE and
   the End-to-end Transit Delay IE may be used to provide more specifics
   about traffic requirements, however this note does not provide
   explicit recommendations on their use.  Annex 9 of [SIG40] describes
   a set of allowable combinations of traffic and QoS related
   paramenters defined for SIG 4.0.  This set includes all forms of
   non-IIS IP signaling configurations that MUST be implemented in ATM
   endsystems to accommodate varied sites' needs. The principle is that
   IP over ATM service may be available in different sites by different
   types of procured ATM service; for one site, a CBR PVP might be
   cost-effective and then the SVCs that IP over ATM without IIS must
   establish must be CBR.  Similarly, VBR or ABR PVPs could be
   provisioned.  The intent of this document is to specify the use of
   the most sensible parameters within this non-IIS configuration.  For
   instance, for non-IIS VBR, the SCR value may need to be hand-
   configured for IP users, or for ABR, the PCR value may be link-rate
   with a 0 MCR.

   For the reader's convenience, we have replicated the tables found in
   Annex 9 of [SIG40] in Appendix C of this document. Ideally this
   document could recommend specific values for the various table
   parameters that would offer the most sensible IP over ATM service.
   Nevertheless, it is not possible to mandate specific values given the
   varied scenarios of procured ATM service.

5.3.1.  ATM Traffic Descriptor

   Even with the newly defined ABR ASC, the most convenient model for
   supporting IP still corresponds to the best effort capability, the
   UBR ASC. The rationale for this assertion stems from the fact that a
   non-IIS IP service has no notion of the performance requirements of
   the higher layers it supports. Therefore, if a site's configuration
   allows use of UBR, users SHOULD signal for it using the IE's and
   parameters pertaining to the UBR ATC.  See Appendix C for the list of
   those IE's and parameters.

   Although we consider the UBR ASC the most natural ASC for best-effort
   IP, ATM vendors that implement VBR and ABR services could possibly
   create hooks for convenient use of these services. If this is the



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   case, IP routers may perhaps have the most to gain from use of VBR or
   ABR services because of the large aggregated traffic volume they are
   required to forward. See Appendix B for detailed suggestions on VBR
   and ABR signalling for IP routers.  We simply note here that, in
   support of ABR service, two new subfields have been added in SIG 4.0
   to the Traffic Descriptor IE. These fields are the forward and
   backward 'Minimum Cell Rate' fields.

5.3.1.1.  Tagging vs. Dropping

   The Traffic Descriptor IE contains a 'tagging' subfield used for
   indicating whether the network is allowed to tag the source's data
   cells. Tagging in the network may occur during periods of congestion
   or when the source's traffic has violated the traffic contract for
   the connection. See Section 4 of [TMGT40] for an explanation of ATM
   connection conformance and the Usage Parameter Control (UPC)
   function.

   SIG 4.0 and TMGT 4.0 define two modes of UBR, UBR.1 which disables
   tagging and UBR.2 which enables tagging (see Appendix C).  In some
   network environments there is no potential for UBR traffic sources to
   violate the connection traffic contract because, either the user's
   terminal equipment supports traffic shaping, or the network does not