A Proposal for Shared Dictionary Compression over HTTP

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Network Working Group                                          J. Butler
Intended status: Informational                                    W. Lee
Expires: May 1, 2017
                                                              B. McQuade

                                                               K. Mixter
                                                        October 28, 2016

         A Proposal for Shared Dictionary Compression over HTTP


   This paper proposes an HTTP/1.1-compatible extension that supports
   inter-response data compression by means of a reference dictionary
   shared between user agent and server.

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1.  Introduction

   In order to reduce payload size, HTTP/1.1 supports response
   compression via the Accept-Encoding and Content-Encoding headers.
   The most commonly used HTTP response compression encoding is gzip,
   which compresses data that is repeated within a given response.
   However, HTTP/1.1 does not provide a mechanism for compressing data
   that is repeated between responses.  A different class of encoding
   technique, known as delta encoding, has proven effective at
   compressing inter-response data.

   Previous efforts to extend HTTP/1.1 to support delta compression have
   focused on encoding an HTTP response as a delta of a previous version
   of that response.  One such approach is discussed in RFC 3229 "Delta
   encoding in HTTP" [RFC3229].  While RFC 3229 is effective at reducing
   payload size for many types of resources, it may not be suitable for
   certain classes of responses.

   Specifically, under RFC 3229, deltas can only be applied to responses
   originating from the same URL, and the means of identifying the
   instance to delta "from" is by a Last-Modified timestamp or entity-
   tag.  This makes RFC 3229 unsuitable for compressing dynamically
   generated responses to a given URL with varying query parameters
   (e.g. a search results page), since these types of responses are
   difficult to identify uniquely using entity tags or last modified
   timestamps.  Content hashes can be used, but false positives are
   possible.  Also, storing all previous responses on the server may not
   be practical.

2.  Proposal: Shared Dictionary Compression over HTTP

   Existing techniques compress each response in isolation, and so
   cannot take advantage of cross-payload redundancy.  For example,
   retrieving a set of HTML pages with the same header, footer, inlined
   JavaScript and CSS requires the retransmission of the same data
   multiple times.  This paper proposes a compression technique that
   leverages this cross-payload redundancy.

   In this proposal, a dictionary is a file downloaded by the user agent
   from the server that contains strings which are likely to appear in
   subsequent HTTP responses.  In the case described above, if the
   header, footer, JavaScript and CSS are stored in a dictionary
   possessed by both user agent and server, the server can substitute
   these elements with references to the dictionary, and the user agent
   can reconstruct the original page from these references.  By

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