Delta encoding in HTTP
RFC 3229

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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
                    content-codings.................................. 37
             10.7.3 Examples for using A-IM, IM, and content-codings. 38
        10.8 New Cache-Control directives............................ 40