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Computation of the Internet Checksum via Incremental Update (RFC1624)

IP.com Disclosure Number: IPCOM000002459D
Original Publication Date: 1994-May-01
Included in the Prior Art Database: 2000-Sep-12
Document File: 5 page(s) / 9K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

A. Rijsinghani: AUTHOR [+2]

Abstract

This memo describes an updated technique for incremental computation of the standard Internet checksum. It updates the method described in RFC 1141.

This text was extracted from a ASCII Text document.
This is the abbreviated version, containing approximately 33% of the total text.

Network Working Group A. Rijsinghani, Editor

Request for Comments: 1624 Digital Equipment Corporation

Updates: 1141 May 1994

Category: Informational

Computation of the Internet Checksum

via Incremental Update

Status of this Memo

This memo provides information for the Internet community. This memo

does not specify an Internet standard of any kind. Distribution of

this memo is unlimited.

Abstract

This memo describes an updated technique for incremental computation

of the standard Internet checksum. It updates the method described

in RFC 1141.

Table of Contents

1. Introduction .......................................... 1

2. Notation and Equations ................................ 2

3. Discussion ............................................ 2

4. Examples .............................................. 3

5. Checksum verification by end systems .................. 4

6. Historical Note ....................................... 4

7. Acknowledgments ....................................... 5

8. Security Considerations ............................... 5

9. Conclusions ........................................... 5

10. Author's Address ..................................... 5

11. References ........................................... 6

1. Introduction

Incremental checksum update is useful in speeding up several

types of operations routinely performed on IP packets, such as

TTL update, IP fragmentation, and source route update.

RFC 1071, on pages 4 and 5, describes a procedure to

incrementally update the standard Internet checksum. The

relevant discussion, though comprehensive, was not complete.

Therefore, RFC 1141 was published to replace this description

on Incremental Update. In particular, RFC 1141 provides a

more detailed exposure to the procedure described in RFC 1071.

However, it computes a result for certain cases that differs

from the one obtained from scratch (one's complement of one's

complement sum of the original fields).

For the sake of completeness, this memo briefly highlights key

points from RFCs 1071 and 1141. Based on these discussions,

an updated procedure to incrementally compute the standard

Internet checksum is developed and presented.

2. Notation and Equations

Given the following notation:

HC - old checksum in header

C - one's complement sum of old header

HC' - new checksum in header

C' - one's complement sum of new header

m - old value of a 16-bit field

m' - new value of a 16-bit field

RFC 1071 states that C' is:

C' = C + (-m) + m' -- [Eqn. 1]

= C + (m' - m)

As RFC 1141 points out, the equation above is not useful for direct

use in incremental updates since C and C' do not refer to the actual

checksum stored in the header. In addi...