rfc2497.txt

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Network Working Group                                       I. Souvatzis
Request for Comments: 2497                            The NetBSD Project
See Also: 1201                                              January 1999
Category: Standards Track


           Transmission of IPv6 Packets over ARCnet 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.

1. Introduction

   This memo specifies a frame format for transmission of IPv6 [IPV6]
   packets and the method of forming IPv6 link-local and statelessly
   autoconfigured addresses on ARCnet networks. It also specifies the
   content of the Source/Target Link-layer Address option used by the
   Router Solicitation, Router Advertisement, Neighbor Solicitation,
   Neighbor Advertisement and Redirect messages described in [DISC],
   when those messages are transmitted on an ARCnet.

      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
      [KWORD].

2. Frame Format

   IPv6 packets are link layer fragmented and reassembled according to
   [PHDS]. A brief but sufficient discussion of this fragmentation
   method can be found in [ARCIPV4].

   The protocol ID (System Code in ARCnet terminology) assigned to IPv6
   is C4 hexadecimal.








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RFC 2497                IPv6 Datagrams on ARCnet            January 1999


3. Maximum Transmission Unit

   The maximum IPv6 packet length possible using this encapsulation
   method is 60480 octets. Since this length is impractical because of
   its worst case transmission time of several seconds, all ARCnet
   implementations on a given ARCnet network should agree on a smaller
   value.

   The default MTU for IPv6 [IPV6] packets on an ARCnet is 9072 octets.

   In the presence of a router, this size MAY be changed by a Router
   Advertisement [DISC] containing an MTU option. If a Router
   Advertisement is received with an MTU option specifying an MTU larger
   than 60480, or larger than a manually configured value less than
   60480, that MTU option may be logged to system management but MUST be
   otherwise ignored.

   If no router is available, the local MTU MUST be left at 9072 or MUST
   be manually configured to the same different value on all connected
   stations.

   Implementations MAY accept arriving IPv6 datagrams which are larger
   than their configured maximum transmission unit.  They are not
   required to discard such datagrams. If they can not handle larger
   datagrams, they MAY log the event to the system administration, but
   MUST otherwise silently discard them.

4. Stateless Auto-configuration

   If a node has an EUI-64 which is not used to form the Interface
   Identifier for any other interface, it SHOULD use that EUI-64 to form
   the Interface Identifier for its ARCnet interface.  If that EUI-64 is
   in use for another interface attached to a different link, it MAY be
   used for the ARCnet interface as well.

   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.

   When a node has no EUI-64 available for forming its ARCnet Interface
   Identifer, it MUST form that identifier as specified in [AARCH],
   Appendix A, section "Links with Non-Global Identifier".  That is, the
   8 bit manually configured ARCnet address is appended to the 56 zero
   bits.

   For example, for an ARCnet interface with the configured address of
   49 hexadecimal this results in the following identifier:




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RFC 2497                IPv6 Datagrams on ARCnet            January 1999


   |0              1|1              3|3              4|4              6|
   |0              5|6              1|2              7|8              3|
   +----------------+----------------+----------------+----------------+
   |0000000000000000|0000000000000000|0000000000000000|0000000001001001|
   +----------------+----------------+----------------+----------------+

   Note that this results in the universal/local bit set to "0" to
   indicate local scope.

   An IPv6 address prefix used for stateless auto-configuration [ACONF]
   of an ARCnet interface MUST have a length of 64 bits.

5. Link-Local Addresses

   The IPv6 link-local address [AARCH] for an ARCnet interface is formed
   by appending the Interface Identifier, 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 IPv6 addresses into ARCnet link-layer
   addresses is described in [DISC]. The Source/Target link layer
   Address option has the following form when the link layer is ARCnet.

             0                   1
             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |     Type      |    Length     |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |ARCnet address |               |
            +---------------+              -+
            |                               |
            +-    5 octets of padding      -+
            |                               |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Option fields:

      Type      1 for Source Link-layer address.
                2 for Target Link-layer address.
      Length         1 (in units of 8 octets).

      ARCnet address The 8 bit ARCnet address, in canonical bit order.



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RFC 2497                IPv6 Datagrams on ARCnet            January 1999


7. Address Mapping -- Multicast

   As ARCnet only provides 1 multicast address (00 hexadecimal), all
   IPv6 multicast addresses MUST be mapped to this address.

8. Security Considerations

   The method of derivation of Interface Identifiers from ARCnet
   addresses is intended to preserve local uniqueness when possible.
   However, there is no protection from duplication through accident or
   forgery.

9. Acknowledgements

   Big parts of the new version of this memo are either based on
   [ETHIPV6] or on Matt Crawford's review of an earlier version.

10. References

   [AARCH]   Hinden, R. and S. Deering, "IP Version 6 Addressing
             Architecture", RFC 2373, July 1998.

   [ACONF]   Thomson, S. and T. Narten, "IPv6 Stateless Address
             Autoconfiguration", RFC 2462, December 1998.

   [ARCIPV4] Provan, D., "Transmitting IP Traffic over ARCNET Networks",
             RFC1201, Novell, Inc., February 1991.

   [DISC]    Narten, T., Nordmark, E. and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 2461, December
             1998.

   [ETHIPV6] Crawford, M., "Transmission of IPv6 Packets over Ethernet
             Networks", RFC 2464, December 1998.

   [EUI64]   "64-Bit Global Identifier Format Tutorial", http://stan