rfc2470.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                       M. Crawford
Request for Comments: 2470                                     Fermilab
Category: Standards Track                                     T. Narten
                                                                    IBM
                                                              S. Thomas
                                                             TransNexus
                                                          December 1998


         Transmission of IPv6 Packets over Token Ring 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 memo specifies the MTU and frame format for transmission of IPv6
   packets on Token Ring networks. It also specifies the method of
   forming IPv6 link-local addresses on Token Ring networks and the
   content of the Source/Target Link-layer Address option used the
   Router Solicitation, Router Advertisement, Redirect, Neighbor
   Solicitation and Neighbor Advertisement messages when those messages
   are transmitted on a Token Ring network.

   Implementors should be careful to note that Token Ring adaptors
   assume addresses are in non-canonical rather than canonical format,
   requiring that special care be taken to insure that addresses are
   processed correctly. See [CANON] for more details.

   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 [KWORD].

2.  Maximum Transmission Unit

   IEEE 802.5 networks have a maximum frame size based on the maximum
   time a node may hold the token. This time depends on many factors
   including the data signaling rate and the number of nodes on the
   ring. Because the maximum frame size varies, implementations must



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   rely on manual configuration or router advertisements [DISC] to
   determine actual MTU sizes. Common default values include
   approximately 2000, 4000, and 8000 octets.

   In the absence of any other information, an implementation should use
   a default MTU of 1500 octets. This size offers compatibility with all
   common 802.5 defaults, as well as with Ethernet LANs in an
   environment using transparent bridging.

   In an environment using source route bridging, the process of
   discovering the MAC-level path to a neighbor can yield the MTU for
   the path to that neighbor. The information is contained in the
   largest frame (LF) subfield of the routing information field. This
   field limits the size of the information field of frames to that
   destination, and that information field includes both the LLC [LLC]
   header and the IPv6 datagram. Since, for IPv6, the LLC header is
   always 8 octets in length, the IPv6 MTU can be found by subtracting 8
   from the maximum frame size defined by the LF subfield. If an
   implementation uses this information to determine MTU sizes, it must
   maintain separate MTU values for each neighbor.

   A detailed list of the LF values and the resulting maximum frame size
   can be found in [BRIDGE]. To illustrate the calculation of IPv6 MTU,
   the following table lists several common values. Note that some of
   the 802.1D LF values would result in an IP MTU less than 1280 bytes.
   This size is less than the IPv6 minimum, and communication across
   paths with those MTUs is generally not possible using IPv6.

           LF (base)  LF (extension)  MAC MTU  IP MTU
             001           000         1470     1462
             010           000         2052     2044
             011           000         4399     4391
             100           000         8130     8122
             101           000         11407    11399
             110           000         17749    17741
             111           000         41600    41592

   When presented with conflicting MTU values from several sources, an
   implementation should choose from those sources according to the
   following priorities:

      1.  Largest Frame values from source route bridging
           (only for specific, unicast destinations), but only if not
           greater than value from any router advertisements

      2.  Router advertisements, but only if not greater than any manual
           configuration (including DHCP)




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      3.  Manual configuration (including DHCP)

      4.  Default of 1500

3.   Frame Format

   IPv6 packets are transmitted in LLC/SNAP frames.  The data field
   contains the IPv6 header and payload. The following figure shows a
   complete 802.5 frame containing an IPv6 datagram.

      +-------+-------+-------+-------+
      |  SD   |  AC   |  FC   |       |
      +-----------------------+       |
      |      Destination Address      |
      |       +-----------------------+
      |       |     Source            |
      +-------+    Address    +-------+
      |                       | DSAP  |
      +-------+-------+-------+-------+
      | SSAP  |  CTL  |      OUI      |
      +-------+-------+-------+-------+
      |  OUI  |   EtherType   |       |
      +-------+---------------+       |
      |                               |
      ~  IPv6 header and payload...   ~
      |                               |
      +-------------------------------+
      |              FCS              |
      +-------+-------+---------------+
      |  ED   |  FS   |
      +-------+-------+

   Token Ring Header Fields

      SD:  Starting Delimiter

      AC:  Access Control

      FC:  Frame Control

      Destination Address: 48-bit IEEE address of destination
           station

      Source Address: 48-bit IEEE address of source station

      DSAP: Destination Service Access Point (for LLC/SNAP
           format, shall always contain the value 0xAA)




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      SSAP: Source Service Access Point (for LLC/SNAP format,
           shall always contain the value 0xAA)

      CTL: Control Field (for Unnumbered Information, shall
           always contain the value 0x03)

      OUI: Organizationally Unique Identifier (for EtherType
           encoding, shall always contain the value 0x000000)

      EtherType: Protocol type of encapsulated payload (for
           IPv6, shall always contain the value 0x86DD)

      FCS: Frame Check Sequence

      ED:  Ending Delimiter

      FS:  Frame Status

   In the presence of source route bridges, a routing information field
   (RIF) may appear immediately after the source address. A RIF is
   present in frames when the most significant bit of the source address
   is set to one. (This is the bit whose position corresponds to that of
   the Individual/Group bit in the Destination Address.)

   The RIF is a variable-length field that (when present) contains a
   two-octet Routing Control (RC) header, followed by zero or more two-
   octet Route Designator fields:

                             0                   1
                             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Routing Control:     |Bcast| Length  |D|  LF   |rsvd |
                            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Route Designator 1:  |    Segment 1          |Bridge1|
                            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                            ~              ...              ~
                            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Route Designator N:  |    Segment N          |BridgeN|
         (0 <= N <= 7)      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


       Route Designator Fields:

       Bcast:    Broadcast Indicator, Defined values:

                 10x: All Routes Explorer
                 11x: Spanning Tree Explorer
                 0xx: Specifically Routed Frame



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       Length:  Total length of RIF field in octets

       D:   Direction of source route. A value of 0 means that
            the left-to-right sequence of Route Designators
            provides the path from the sender to recipient. A
            value of 0 indicates the sequence goes from
            recipient to sender.

       LF:  Largest Frame

       rsvd: Reserved

   On transmission, the Route Designator fields give the sequence of
   (bridge, LAN segment) numbers the packet is to traverse. It is the
   responsibility of the sender to provide this sequence for
   Specifically Routed Frames, i.e., unicast IP datagrams.

4.  Stateless Autoconfiguration

   The Interface Identifier [AARCH] for a Token Ring interface is based
   on the EUI-64 identifier [EUI64] derived from the interface's built-
   in 48-bit IEEE 802 address. The OUI of the Token Ring 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 Token Ring
   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.  Complementing
   this bit will generally change a 0 value to a 1, since an interface's
   built-in address is expected to be from a universally administered
   address space and hence have a globally unique value.  A universally
   administered IEEE 802 address or an EUI-64 is signified by a 0 in the
   U/L bit position, while a globally unique IPv6 Interface Identifier
   is signified by a 1 in the corresponding position.  For further
   discussion on this point, see [AARCH].

   For example, the Interface Identifier for a Token Ring interface
   whose built-in address is, in hexadecimal and in canonical bit order,

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

   would be

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




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   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 of a
   Token Ring interface must have a length of 64 bits.

5.  Link-Local Address

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

     10 bits            54 bits                  64 bits
   +----------+-----------------------+----------------------------+
   |1111111010|         (zeros)       |    Interface Identifier    |
   +----------+-----------------------+----------------------------+

6.  Address Mapping -- Unicast

   The procedure for mapping unicast IPv6 addresses into Token Ring
   link-layer addresses is described in [DISC]. The Source/Target Link-
   layer Address option has the following form when the link layer is