rfc2467.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 508 行 · 第 1/2 页

Network Working Group                                        M. Crawford
Request for Comments: 2467                                      Fermilab
Obsoletes: 2019                                            December 1998
Category: Standards Track


            Transmission of IPv6 Packets over FDDI 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 (1998).  All Rights Reserved.

1.  Introduction

   This document specifies the frame format for transmission of IPv6
   packets and the method of forming IPv6 link-local addresses and
   statelessly autoconfigured addresses on FDDI networks.  It also
   specifies the content of the Source/Target Link-layer Address option
   used in Router Solicitation, Router Advertisement, Neighbor
   Solicitation, Neighbor Advertisement and Redirect messages when those
   messages are transmitted on an FDDI network.

   This document replaces RFC 2019, "Transmission of IPv6 Packets Over
   FDDI", which will become historic.

   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].

2.  Maximum Transmission Unit

   FDDI permits a frame length of 4500 octets (9000 symbols), including
   at least 22 octets (44 symbols) of Data Link encapsulation when
   long-format addresses are used.  Subtracting 8 octets of LLC/SNAP
   header, this would, in principle, allow the IPv6 [IPV6] packet in the
   Information field to be up to 4470 octets.  However, it is desirable
   to allow for the variable sizes and possible future extensions of the
   MAC header and frame status fields.  The default MTU size for IPv6
   packets on an FDDI network is therefore 4352 octets.  This size may
   be reduced by a Router Advertisement [DISC] containing an MTU option



Crawford                    Standards Track                     [Page 1]

RFC 2467                     IPv6 over FDDI                December 1998


   which specifies a smaller MTU, or by manual configuration of each
   node.  If a Router Advertisement received on an FDDI interface has an
   MTU option specifying an MTU larger than 4352, or larger than a
   manually configured value, that MTU option may be logged to system
   management but must be otherwise ignored.

   For purposes of this document, information received from DHCP is
   considered "manually configured" and the term FDDI includes CDDI.

3.  Frame Format

   FDDI provides both synchronous and asynchronous transmission, with
   the latter class further subdivided by the use of restricted and
   unrestricted tokens.  Only asynchronous transmission with
   unrestricted tokens is required for FDDI interoperability.
   Accordingly, IPv6 packets shall be sent in asynchronous frames using
   unrestricted tokens.  The robustness principle dictates that nodes
   should be able to receive synchronous frames and asynchronous frames
   sent using restricted tokens.

   IPv6 packets are transmitted in LLC/SNAP frames, using long-format
   (48 bit) addresses.  The data field contains the IPv6 header and
   payload and is followed by the FDDI Frame Check Sequence, Ending
   Delimiter, and Frame Status symbols.



























Crawford                    Standards Track                     [Page 2]

RFC 2467                     IPv6 over FDDI                December 1998


                      0                   1
                      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                                     +-+-+-+-+-+-+-+-+
                                     |      FC       |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |          Destination          |
                     +-                             -+
                     |             FDDI              |
                     +-                             -+
                     |            Address            |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |            Source             |
                     +-                             -+
                     |             FDDI              |
                     +-                             -+
                     |            Address            |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |     DSAP      |     SSAP      |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |      CTL      |      OUI ...  |
                     +-+-+-+-+-+-+-+-+               +
                     |          ... OUI              |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |           Ethertype           |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |             IPv6              |
                     +-                             -+
                     |            header             |
                     +-                             -+
                     |             and               |
                     +-                             -+
                     /            payload ...        /
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    (Each tic mark represents one bit.)

   FDDI Header Fields:

   FC          The Frame Code must be in the range 50 to 57
               hexadecimal, inclusive, with the three low order bits
               indicating the frame priority.

   DSAP, SSAP  Both the DSAP and SSAP fields shall contain the value AA
               hexadecimal, indicating SNAP encapsulation.

   CTL         The Control field shall be set to 03 hexadecimal,
               indicating Unnumbered Information.




Crawford                    Standards Track                     [Page 3]

RFC 2467                     IPv6 over FDDI                December 1998


   OUI         The Organizationally Unique Identifier shall be set to
               000000 hexadecimal.

   Ethertype   The Ethernet protocol type ("ethertype") shall be set to
               the value 86DD hexadecimal.

4.  Interaction with Bridges

   802.1d MAC bridges which connect different media, for example
   Ethernet and FDDI, have become very widespread.  Some of them do IPv4
   packet fragmentation and/or support IPv4 Path MTU discovery [RFC
   1981], many others do not, or do so incorrectly.  Use of IPv6 in a
   bridged mixed-media environment must not depend on support from MAC
   bridges, unless those bridges are known to correctly implement IPv6
   Path MTU Discovery [RFC 1981, ICMPV6].

   For correct operation when mixed media are bridged together by
   bridges which do not support IPv6 Path MTU Discovery, the smallest
   MTU of all the media must be advertised by routers in an MTU option.
   If there are no routers present, this MTU must be manually configured
   in each node which is connected to a medium with a default MTU larger
   than the smallest MTU.

5.  Stateless Autoconfiguration

   The Interface Identifier [AARCH] for an FDDI interface is based on
   the EUI-64 identifier [EUI64] derived from the interface's built-in
   48-bit IEEE 802 address.  The EUI-64 is formed as follows.
   (Canonical bit order is assumed throughout.  See [CANON] for a
   caution on bit-order effects in LAN interfaces.)

   The OUI of the FDDI MAC address (the first three octets) becomes the
   company_id of the EUI-64 (the first three octets).  The fourth and
   fifth octets of the EUI are set to the fixed value FFFE hexadecimal.
   The last three octets of the FDDI MAC address become the last three
   octets of the EUI-64.

   The Interface Identifier is then formed from the EUI-64 by
   complementing the "Universal/Local" (U/L) bit, which is the next-to-
   lowest order bit of the first octet of the EUI-64.  For further
   discussion on this point, see [ETHER] and [AARCH].










Crawford                    Standards Track                     [Page 4]

RFC 2467                     IPv6 over FDDI                December 1998


   For example, the Interface Identifier for an FDDI interface whose
   built-in address is, in hexadecimal,

                             34-56-78-9A-BC-DE

   would be

                         36-56-78-FF-FE-9A-BC-DE.

   A different MAC address set manually or by software should not be
   used to derive the Interface Identifier.  If such a MAC address must
   be used, its global uniqueness property should be reflected in the
   value of the U/L bit.

   An IPv6 address prefix used for stateless autoconfiguration [ACONF]
   of an FDDI interface must have a length of 64 bits.

6.  Link-Local Addresses

   The IPv6 link-local address [AARCH] for an FDDI interface is formed
   by appending the Interface Identifier, as defined above, to the
   prefix FE80::/64.

     10 bits            54 bits                  64 bits