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A Caution On The Canonical Ordering Of Link-Layer Addresses (RFC2469)

IP.com Disclosure Number: IPCOM000003048D
Original Publication Date: 1998-Dec-01
Included in the Prior Art Database: 2019-Feb-11
Document File: 5 page(s) / 7K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

T. Narten: AUTHOR [+1]

Related Documents

10.17487/RFC2469: DOI

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. This memo provides information for the Internet community.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 35% of the total text.

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

Narten & Burton Informational [Page 1]

RFC 2469 Canonical Ordering Of Link-Layer Addresses December 1998

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 defini...

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