Signaling Compression (SigComp) - Extended Operations
RFC 3321

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Network Working Group                                           H. Hannu
Request for Comments: 3321                            J. Christoffersson
Category: Informational                                         Ericsson
                                                             S. Forsgren
                                                             K.-C. Leung
                                                   Texas Tech University
                                                                  Z. Liu
                                                                   Nokia
                                                                R. Price
                                                      Siemens/Roke Manor
                                                            January 2003

         Signaling Compression (SigComp) - Extended Operations

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 (2003).  All Rights Reserved.

Abstract

   This document describes how to implement certain mechanisms in
   Signaling Compression (SigComp), RFC 3320, which can significantly
   improve the compression efficiency compared to using simple per-
   message compression.

   SigComp uses a Universal Decompressor Virtual Machine (UDVM) for
   decompression, and the mechanisms described in this document are
   possible to implement using the UDVM instructions defined in RFC
   3320.

Hannu, et. al.               Informational                      [Page 1]
RFC 3321             SigComp - Extended Operations          January 2003

Table of Contents

   1.  Introduction..................................................2
   2.  Terminology...................................................3
   3.  Architectural View of Feedback................................4
   4.  State Reference Model.........................................5
   5.  Extended Mechanisms...........................................6
   6.  Implications on SigComp......................................13
   7.  Security Considerations......................................17
   8.  IANA Considerations..........................................17
   9.  Acknowledgements.............................................17
   10. Intellectual Property Right Considerations...................17
   11. References...................................................17
   12. Authors' Addresses...........................................18
   13. Full Copyright Statement.....................................19

1.  Introduction

   This document describes how to implement mechanisms with [SIGCOMP] to
   significantly improve the compression efficiency compared to per-
   message compression.

   One such mechanism is to use previously sent messages in the SigComp
   compression process, referred to as dynamic compression.  In order to
   utilize information from previously sent messages, it is necessary
   for a compressor to gain knowledge about the reception of these
   messages.  For a reliable transport, such as TCP, this is guaranteed.
   For an unreliable transport however, the SigComp protocol can be used
   to provide such a functionality itself.  That functionality is
   described in this document and is referred to as explicit
   acknowledgement.

   Another mechanism that will improve the compression efficiency of
   SigComp, especially when SigComp is applied to protocols that are of
   request/response type, is shared compression.  This involves using
   received messages in the SigComp compression process.  In particular
   the compression of the first few messages will gain from shared
   compression.  Shared compression is described in this document.

   For better understanding of this document the reader should be
   familiar with the concept of [SIGCOMP].

Hannu, et. al.               Informational                      [Page 2]
RFC 3321             SigComp - Extended Operations          January 2003

2.  Terminology

   The reader should consult [SIGCOMP] for definitions of terminology,
   since this document uses the same terminology.  Further terminology
   is defined below.

     Compressor

       Entity that encodes application messages using a certain
       compression algorithm and keeps track of state that can be used
       for compression.  The compressor is responsible for ensuring that
       the messages it generates can be decompressed by the remote UDVM.

     Decompressor

       The decompressor is responsible for converting a SigComp message
       into uncompressed data.  Decompression functionality is provided
       by the UDVM.

     Dynamic compression

       Compression relative to messages sent prior to the current
       compressed message.

     Explicit acknowledgement

       Acknowledgement for a state.  The acknowledgment is explicitly