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Network Working Group                                          T. NartenRequest for Comments: 2469                                     C. BurtonCategory: Informational                                              IBM                                                           December 1998A Caution On The Canonical Ordering Of Link-Layer AddressesStatus 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 Contents1.  Introduction.............................................22.  Canonical Form...........................................23.  Implementors Beware: Potential Trouble Spots.............33.1.  Neighbor Discovery in IPv6..........................33.2.  IPv4 and ARP........................................34.  Security Considerations..................................35.  References...............................................46.  Authors' Addresses.......................................47.  Full Copyright Statement.................................5Narten & Burton              Informational                      [Page 1]

RFC 2469       Canonical Ordering Of Link-Layer Addresses  December 19981.  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       3DNarten & Burton              Informational                      [Page 2]

RFC 2469       Canonical Ordering Of Link-Layer Addresses  December 1998   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 Spots3.1.  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.Narten & Burton              Informational                      [Page 3]

RFC 2469       Canonical Ordering Of Link-Layer Addresses  December 19985.  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: narten@raleigh.ibm.com   Charles F. Burton, III   IBM Corporation   3039 Cornwallis Ave.   PO Box 12195   Research Triangle Park, NC 27709-2195   Phone: 919-254-4355   EMail: burton@rtp.vnet.ibm.comNarten & Burton              Informational                      [Page 4]

RFC 2469       Canonical Ordering Of Link-Layer Addresses  December 19987.  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.Narten & Burton              Informational                      [Page 5]