rfc2492.txt

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Network Working Group                                       G. Armitage
Request for Comments: 2492                          Lucent Technologies
Category: Standards Track                                   P. Schulter
                                               BrightTiger Technologies
                                                                M. Jork
                                                 Digital Equipment GmbH
                                                           January 1999

                         IPv6 over ATM Networks

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 (1999).  All Rights Reserved.

Abstract

   This document is a companion to the ION working group's architecture
   document, "IPv6 over Non Broadcast Multiple Access (NBMA) networks".
   It provides specific details on how to apply the IPv6 over NBMA
   architecture to ATM networks. This architecture allows conventional
   host-side operation of the IPv6 Neighbor Discovery protocol, while
   also supporting the establishment of 'shortcut' ATM forwarding paths
   (when using SVCs).  Operation over administratively configured Point
   to Point PVCs is also supported.

1. Introduction.

   This document is an ATM-specific companion document to the ION
   working group's, "IPv6 over Non Broadcast Multiple Access (NBMA)
   networks" specification [1].  Terminology and architectural
   descriptions will not be repeated here.

   The use of ATM to provide point to point PVC service, or flexible
   point to point and point to multipoint SVC service, is covered by
   this document.

   A minimally conforming IPv6/ATM driver SHALL support the PVC mode of
   operation. An IPv6/ATM driver that supports the full SVC mode SHALL
   also support PVC mode of operation.




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RFC 2492                 IPv6 over ATM Networks             January 1999


2. Specification Terminology

   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 [16].

3. PVC Environments

   When the ATM network is used in PVC mode, each PVC will connect
   exactly two nodes and the use of Neighbor Discovery and other IPv6
   features is limited.  IPv6/ATM interfaces have only one neighbor on
   each Link. The MARS and NHRP protocols are NOT necessary, since
   multicast and broadcast operations collapse down to an ATM level
   unicast operation. Dynamically discovered shortcuts are not
   supported.

   The actual details of encapsulations, MTU, and link token generation
   are provided in the following sections.

   This use of PVC links does not mandate, nor does it prohibit the use
   of extensions to the Neighbor Discovery protocol which may be
   developed for either general use of for use in PVC connections (for
   example, Inverse Neighbor Discovery).

   Since ATM PVC links do not use link-layer addresses, the link-layer
   address options SHOULD not be included in any ND message [11].  If a
   link-layer address option is present in an ND message, then the
   option SHOULD be ignored.

   A minimally conforming IPv6/ATM driver SHALL support the PVC mode of
   operation.  PVC only implementations are not required to support any
   SVC mode of operation.

3.1 Default Packet Encapsulation

   Following the model in RFC 1483 [2], AAL5 SHALL be the default
   Adaptation Layer service, and (LLC/SNAP) encapsulation SHALL be
   default encapsulation used by unicast and multicast packets across
   pt-pt PVC links. As defined in [1], the default IPv6 packet
   encapsulation SHALL be:

         [0xAA-AA-03][0x00-00-00][0x86-DD][IPv6 packet]
             (LLC)       (OUI)     (PID)








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3.2 Optional null encapsulation

   IPv6/ATM drivers MAY also support null encapsulation as a
   configurable option. When null encapsulation is enabled, the IPv6
   packet is passed directly to the AAL5 layer. Both ends of the PVC
   MUST be configured to use null encapsulation. The PVC will not be
   available for use by protocols other than IPv6.

3.3 PPP encapsulation

   The concatentation of IPv6 over PPP with PPP over AAL5 PVCs is not
   covered by this specification.

3.4 MTU For PVC Environments

   The default IP MTU size for PVC links is 9180 bytes as specified in
   [7].  Other IP MTU values MAY be used.

3.5 Interface Token Formats in PVC Environments

   When the ATM network is used in PVC mode interface tokens SHALL be
   generated using one of the methods described in section 5. Interface
   tokens need only be unique between the two nodes on the PVC link.

4 SVC environments

4.1 SVC Specific Code Points

4.1.1 ATM Adaptation Layer encapsulation for SVC environments

   Following the model in RFC 1483 [2], AAL5 SHALL be the default
   Adaptation Layer service, and (LLC/SNAP) encapsulation SHALL be the
   default encapsulation used by unicast and multicast packets across
   SVC links.

4.1.2 Unicast Packet Encapsulation

   As defined in [1], the default IPv6 unicast packet encapsulation
   SHALL be:

         [0xAA-AA-03][0x00-00-00][0x86-DD][IPv6 packet]
             (LLC)       (OUI)     (PID)









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RFC 2492                 IPv6 over ATM Networks             January 1999


4.1.3 Multicast packet encapsulation

   As defined in [1], the default IPv6 multicast packet encapsulation
   SHALL be:

         [0xAA-AA-03][0x00-00-5E][0x00-01][pkt$cmi][0x86DD][IPv6
         packet]
             (LLC)       (OUI)     (PID)    (mars encaps)

         The IPv6/ATM driver's Cluster Member ID SHALL be copied into
         the 2 octet pkt$cmi field prior to transmission.

4.1.4 Optional null encapsulation

   IPv6/ATM drivers MAY also support null encapsulation as a
   configurable option. Null encapsulation SHALL only be used for
   passing IPv6 packets from one IPv6/ATM driver to another. Null
   encapsulation SHALL NOT be used on the pt-pt SVC between the IPv6/ATM
   driver and its local MARS.

   If null encapsulation is enabled, the IPv6 packet is passed directly
   to the AAL5 layer. Both ends of the SVC MUST agree to use null
   encapsulation during the call SETUP phase.  The SVC will not be
   available for use by protocols other than IPv6.

   If null encapsulation is enabled on data SVCs between routers,
   inter-router NHRP traffic SHALL utilize a separate, parallel SVC.

   Use of null encapsulation is not encouraged when IPv6/ATM is used
   with MARS/NHRP/ND as described in [1].

4.1.5 MARS control messages

   The encapsulation of MARS control messages (between MARS and MARS
   Clients) remains the same as shown in RFC 2022 [3]:

      [0xAA-AA-03][0x00-00-5E][0x00-03][MARS control message]
         (LLC)       (OUI)     (PID)

   The key control field values are:

      The mar$afn field remains 0x0F (ATM addresses)

      The mar$pro field SHALL be 0x86DD (IPv6)

      The mar$op.version field remains 0x00 (MARS)





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RFC 2492                 IPv6 over ATM Networks             January 1999


   The mar$spln and mar$tpln fields (where relevant) are either 0 (for
   null or non-existent information) or 16 (for the full IPv6 protocol
   address)

   The way in which ATM addresses are stored remains the same as shown
   in RFC 2022 [3]

4.1.6 NHRP control messages

   The encapsulation of NHRP control messages remains the same as shown
   in RFC 2332 [4]:

      [0xAA-AA-03][0x00-00-5E][0x00-03][NHRP control message]
         (LLC)       (OUI)     (PID)

   The key control field values are:

      The ar$afn field remains 0x0F (ATM addresses)

      The ar$pro field SHALL be 0x86DD (IPv6)

      The ar$op.version field remains 0x01 (NHRP)

   The ar$spln and ar$tpln fields (where relevant) are either 0 (for
   null or non-existent information) or 16 (for the full IPv6 protocol
   address)

   The way in which ATM addresses are stored remains the same as shown
   in RFC 2022 [3]

4.1.7 Neigbor Discovery control messages

   Section 5.2 of [1] describes the ND Link-layer address option.  For
   IPv6/ATM drivers, the subfields SHALL be encoded in the following
   manner:

      [NTL] defines the type and length of the ATM number immediately
      following the [STL] field. The format is as follows:

            7 6 5 4 3 2 1 0
            +-+-+-+-+-+-+-+-+
            |0|x|  length   |
            +-+-+-+-+-+-+-+-+








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RFC 2492                 IPv6 over ATM Networks             January 1999


      The most significant bit is reserved and MUST be set to zero.  The
      second most significant bit (x) is a flag indicating whether the
      ATM number is in:

          ATM Forum AESA format (x = 0).
          Native E.164 format (x = 1).

      The bottom 6 bits represent an unsigned integer value indicating
      the length of the associated ATM address field in octets.

   The [STL] format is the same as the [NTL] field. Defines the length
   of the subaddress field, if it exists. If it does not exist this
   entire octet field MUST be zero. If the subaddress exists it will be
   in AESA format, so flag x SHALL be zero.

   [NBMA Number] is a variable length field containing the ATM address
   of the Link layer target. It is always present.

   [NBMA Subaddress] is a variable length field containing the ATM
   subaddress of the Link layer target. It may or may not be present.
   When it is not, the option ends after the [NBMA Number] (or any
   additional padding for 8 byte alignment).

   The octet ordering of the [NBMA Number] and [NBMA Subaddress] fields
   SHALL be the same as that used in MARS and NHRP control messages.

4.2 UNI 3.0/3.1 signaling issues (SVC mode).

   When an IPv6 node places a call to another IPv6 node, it SHOULD
   follow the procedures in [6] and [7] for signalling UNI 3.0/3.1 SVCs
   [9] and negotiating MTU.  The default IP MTU size on a LL is 9180
   bytes as specified in [7].

   Note that while the procedures in [7] still apply to IPv6 over ATM,
   IPv6 Path MTU Discovery [8] is used by nodes and routers rather than
   IPv4 MTU discovery. Additionally, while IPv6 nodes are not required
   to implement Path MTU Discovery, IPv6/ATM nodes SHOULD implement it.
   Also, since IPv6 nodes will negotiate an appropriate MTU for each VC,
   Path MTU should never be triggered since neither node should ever
   receive a Packet Too Big message to trigger Path MTU Discovery.  When
   nodes are communicating via one or more routers Path MTU Discovery
   will be used just as it is for legacy networks.

5 Interface Tokens

   For both PVC and SVC modes of operation, one of the following methods
   SHALL be used to generate Interface Tokens as required by section 5.1
   of [1].



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