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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: 2000-Sep-13
Document File: 4 page(s) / 9K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

T. Narten: AUTHOR [+2]

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 text was extracted from a ASCII Text document.
This is the abbreviated version, containing approximately 32% 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

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 a...