Network Working Group                                          T. Narten
Request for Comments: 2469                                     C. Burton
Category: Informational                                              IBM
                                                          December 1998


     A Caution On The Canonical Ordering Of Link-Layer Addresses

Status of this Memo

  This memo provides information for the Internet community.  It does
  not specify an Internet standard of any kind.  Distribution of this
  memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

  Protocols such as ARP and Neighbor Discovery have data fields that
  contain link-layer addresses. In order to interoperate properly, a
  sender setting such a field must insure that the receiver extracts
  those bits and interprets them correctly.  In most cases, such fields
  must be in "canonical form".  Unfortunately, not all LAN adaptors are
  consistent in their use of canonical form, and implementations may
  need to explicitly bit swap individual bytes in order to obtain the
  correct format.  This document provides information to implementors
  to help them avoid the pitfall of using non-canonical forms when
  canonical forms are required.

Table of Contents

  1.  Introduction.............................................    2
  2.  Canonical Form...........................................    2
  3.  Implementors Beware: Potential Trouble Spots.............    3
     3.1.  Neighbor Discovery in IPv6..........................    3
     3.2.  IPv4 and ARP........................................    3
  4.  Security Considerations..................................    3
  5.  References...............................................    4
  6.  Authors' Addresses.......................................    4
  7.  Full Copyright Statement.................................    5









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1.  Introduction

  Protocols such as ARP [ARP] and ND [DISCOVERY] have data fields that
  contain link-layer addresses.  In order to interoperate properly, a
  sender setting such a field must insure that the receiver extracts
  those bits and interprets them correctly.  In most cases, such fields
  must be in "canonical form".  Unfortunately, not all LAN adaptors are
  consistent in their use of canonical form, and implementations may
  need to explicitly bit swap individual bytes in order to obtain the
  correct format.

2.  Canonical Form

  Canonical form (also known as "LSB format" and "Ethernet format") is
  the name given to the format of a LAN adapter address as it should be
  presented to the user according to the 802 LAN standard.  It is best
  defined as how the bit order of an adapter address on the LAN media
  maps to the bit order of an adapter address in memory: The first bit
  of each byte that appears on the LAN maps to the least significant
  (i.e., right-most) bit of each byte in memory (the figure below
  illustrates this).  This puts the group address indicator (i.e., the
  bit that defines whether an address is unicast or multicast) in the
  least significant bit of the first byte.  Ethernet and 802.3 hardware
  behave consistently with this definition.

  Unfortunately, Token Ring (and some FDDI) hardware does not behave
  consistently with this definition; it maps the first bit of each byte
  of the adapter address to the most significant (i.e., left-most) bit
  of each byte in memory, which puts the group address indicator in the
  most significant bit of the first byte.  This mapping is variously
  called "MSB format", "IBM format", "Token-Ring format", and "non-
  canonical form".  The figure below illustrates the difference between
  canonical and non-canonical form using the canonical form address
  12-34-56-78-9A-BC as an example:

  In memory,      12       34       56       78       9A       BC
  canonical:   00010010 00110100 01010110 01111000 10011010 10111100

               1st bit appearing on LAN (group address indicator)
               |
  On LAN:      01001000 00101100 01101010 00011110 01011001 00111101

  In memory,
  MSB format:  01001000 00101100 01101010 00011110 01011001 00111101
                  48       2C       6A       1E       59       3D






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  The implication of this inconsistency is that addresses extracted
  from adaptors, assigned to adaptors, or extracted from link-layer
  packet headers obtained from adaptors may need to be bit-swapped to
  put them into canonical form. Likewise, addresses in canonical form
  that are handed to adaptors (e.g., to set an address, to specify a
  destination address in a link-layer header, etc.) may need to be
  bit-swapped in order for the adaptor to process the request as
  expected.

3.  Implementors Beware: Potential Trouble Spots

3.1.  Neighbor Discovery in IPv6

  All of the IPv6 over specific link layers documents specify that
  link-layer addresses must be transmitted in canonical order [IPv6-
  ETHER, IPv6-FDDI, IPv6-TOKEN].  As far as the authors can tell, all
  Ethernet LAN adaptors use canonical order and no special processing
  by implementations is needed. In contrast, some FDDI and all Token
  Ring adaptors appear to use non-canonical format.  Implementors must
  insure that any addresses that appear in link-layer address options
  of Neighbor Discovery [DISCOVERY] messages are sent in canonical
  order and that any link-layer addresses extracted from ND packets are
  interpreted correctly on the local machine and its adaptors.

3.2.  IPv4 and ARP

  Ethernet addresses that appear in ARP packets are in canonical order.
  In contrast, when running ARP over Token Ring, the de facto practice
  is to transmit addresses in non-canonical order. Because all Token
  Ring adaptors assume non-canonical ordering, no interoperability
  problems result between communicating nodes attached to the same
  Token Ring.

  In some environments, however, Token Rings and Ethernets are
  connected via a bridge. When a node on the Token Ring attempts to
  communicate with a node on the Ethernet, communication would normally
  fail, since the Ethernet will misinterpret the Token Ring address
  (and vice versa). To get around this problem, bridges that forward
  packets between dissimilar network types perform bit swaps of the
  addresses in the address fields of ARP packets that are forwarded
  from a network of one type to one of the other.

4.  Security Considerations

  There are no known security issues raised by this document.






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5.  References

  [ARP]        Plummer, D., "An Ethernet Address Resolution Protocol",
               STD 37, RFC 826, November 1982.

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

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

  [IPv6-FDDI]  Crawford, M., "Transmission of IPv6 Packets over FDDI
               Networks", RFC 2467, December 1998.

  [IPv6-TOKEN] Crawford, M., Narten, T. and S. Thomas, "Transmission of
               IPv6 Packets over Token Ring Networks", RFC 2470,
               December 1998.

6.  Authors' Addresses

  Thomas Narten
  IBM Corporation
  3039 Cornwallis Ave.
  PO Box 12195
  Research Triangle Park, NC 27709-2195

  Phone: 919-254-7798
  EMail: [email protected]

  Charles F. Burton, III
  IBM Corporation
  3039 Cornwallis Ave.
  PO Box 12195
  Research Triangle Park, NC 27709-2195

  Phone: 919-254-4355
  EMail: [email protected]













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7.  Full Copyright Statement

  Copyright (C) The Internet Society (1998).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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