Transmission of IPv6 Packets over Token Ring Networks
RFC 2470

Document Type RFC - Proposed Standard (December 1998; No errata)
Updated by RFC 8064
Authors Matt Crawford  , Stephen Thomas  , Thomas Narten 
Last updated 2013-03-02
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Network Working Group                                       M. Crawford
Request for Comments: 2470                                     Fermilab
Category: Standards Track                                     T. Narten
                                                              S. Thomas
                                                          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",
   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

Crawford, et. al.           Standards Track                     [Page 1]
RFC 2470                  IPv6 over Token Ring             December 1998

   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)

Crawford, et. al.           Standards Track                     [Page 2]
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