Network Working Group J. Mogul
Request for Comments: 3229 Compaq WRL
Category: Standards Track B. Krishnamurthy
F. Douglis
AT&T
A. Feldmann
Univ. of Saarbruecken
Y. Goland
A. van Hoff
Marimba
D. Hellerstein
ERS/USDA
January 2002
Delta encoding in HTTP
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes how delta encoding can be supported as a
compatible extension to HTTP/1.1.
Many HTTP (Hypertext Transport Protocol) requests cause the retrieval
of slightly modified instances of resources for which the client
already has a cache entry. Research has shown that such modifying
updates are frequent, and that the modifications are typically much
smaller than the actual entity. In such cases, HTTP would make more
efficient use of network bandwidth if it could transfer a minimal
description of the changes, rather than the entire new instance of
the resource. This is called "delta encoding."
Mogul, et al. Standards Track [Page 1]
RFC 3229 Delta encoding in HTTP January 2002
Table of Contents
1 Introduction.................................................... 3
1.1 Related research and proposals........................... 4
2 Goals........................................................... 5
3 Terminology..................................................... 6
4 The HTTP message-generation sequence............................ 8
4.1 Relationship between deltas and ranges................... 11
5 Basic mechanisms................................................ 13
5.1 Background: an overview of HTTP cache validation......... 13
5.2 Requesting the transmission of deltas.................... 14
5.3 Choice of delta algorithm and format..................... 16
5.4 Identification of delta-encoded responses................ 16
5.5 Guaranteeing cache safety................................ 17
5.6 Transmission of delta-encoded responses.................. 18
5.7 Examples of requests combining Range and delta encoding.. 19
6 Encoding algorithms and formats................................. 22
7 Management of base instances.................................... 23
7.1 Multiple entity tags in the If-None-Match header......... 24
7.2 Hints for managing the client cache...................... 25
8 Deltas and intermediate caches.................................. 27
9 Digests for data integrity...................................... 28
10 Specification.................................................. 28
10.1 Protocol parameter specifications....................... 28
10.2 IANA Considerations..................................... 30
10.3 Basic requirements for delta-encoded responses.......... 30
10.4 Status code specifications.............................. 30
10.4.1 226 IM Used...................................... 31
10.5 Header specifications................................... 31
10.5.1 Delta-Base....................................... 31
10.5.2 IM............................................... 32
10.5.3 A-IM............................................. 33
10.6 Caching rules for 226 responses......................... 35
10.7 Rules for deltas in the presence of content-codings..... 36
10.7.1 Rules for generating deltas in the presence of
content-codings.................................. 37
10.7.2 Rules for applying deltas in the presence of
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