Signaling Compression (SigComp) Corrections and Clarifications
RFC 4896

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Network Working Group                                         A. Surtees
Request for Comments: 4896                                       M. West
Updates: 3320, 3321, 3485                    Siemens/Roke Manor Research
Category: Standards Track                                     A.B. Roach
                                                        Estacado Systems
                                                               June 2007

     Signaling Compression (SigComp) Corrections and Clarifications

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 IETF Trust (2007).

Abstract

   This document describes common misinterpretations and some
   ambiguities in the Signaling Compression Protocol (SigComp), and
   offers guidance to developers to resolve any resultant problems.
   SigComp defines a scheme for compressing messages generated by
   application protocols such as the Session Initiation Protocol (SIP).
   This document updates the following RFCs: RFC 3320, RFC 3321, and RFC
   3485.

Surtees, et al.             Standards Track                     [Page 1]
RFC 4896         SigComp Corrections and Clarifications        June 2007

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Decompression Memory Size  . . . . . . . . . . . . . . . . . .  3
     2.1.  Bytecode within Decompression Memory Size  . . . . . . . .  3
     2.2.  Default Decompression Memory Size  . . . . . . . . . . . .  4
   3.  UDVM Instructions  . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Data Input Instructions  . . . . . . . . . . . . . . . . .  5
     3.2.  MULTILOAD  . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.3.  STATE-FREE . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.4.  Using the Stack  . . . . . . . . . . . . . . . . . . . . .  6
   4.  Byte Copying Rules . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Instructions That Use Byte Copying Rules . . . . . . . . .  9
   5.  State Retention Priority . . . . . . . . . . . . . . . . . . .  9
     5.1.  Priority Values  . . . . . . . . . . . . . . . . . . . . .  9
     5.2.  Multiple State Retention Priorities  . . . . . . . . . . . 10
     5.3.  Retention Priority 65535 (or -1) . . . . . . . . . . . . . 10
   6.  Duplicate State  . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  State Identifier Clashes . . . . . . . . . . . . . . . . . . . 14
   8.  Message Misordering  . . . . . . . . . . . . . . . . . . . . . 15
   9.  Requested Feedback . . . . . . . . . . . . . . . . . . . . . . 15
     9.1.  Feedback When SMS Is Zero  . . . . . . . . . . . . . . . . 15
     9.2.  Updating Feedback Requests . . . . . . . . . . . . . . . . 16
   10. Advertising Resources  . . . . . . . . . . . . . . . . . . . . 16
     10.1. The I-bit and Local State Items  . . . . . . . . . . . . . 16
     10.2. Dynamic Update of Resources  . . . . . . . . . . . . . . . 17
     10.3. Advertisement of Locally Available State Items . . . . . . 17
       10.3.1.  Basic SigComp . . . . . . . . . . . . . . . . . . . . 18
       10.3.2.  Dictionaries  . . . . . . . . . . . . . . . . . . . . 18
       10.3.3.  SigComp Extended Mechanisms . . . . . . . . . . . . . 19
   11. Uncompressed Bytecode  . . . . . . . . . . . . . . . . . . . . 19
   12. RFC 3485 SIP/SDP Static Dictionary . . . . . . . . . . . . . . 20
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     16.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Appendix A.  Dummy Application Protocol (DAP)  . . . . . . . . . . 24
     A.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 24
     A.2.  Processing a DAP Message . . . . . . . . . . . . . . . . . 24
     A.3.  DAP Message Format in ABNF . . . . . . . . . . . . . . . . 26
     A.4.  An Example of a DAP Message  . . . . . . . . . . . . . . . 26

Surtees, et al.             Standards Track                     [Page 2]
RFC 4896         SigComp Corrections and Clarifications        June 2007

1.  Introduction

   SigComp [1] defines the Universal Decompressor Virtual Machine (UDVM)
   for decompressing messages sent by a compliant compressor.  SigComp
   further describes mechanisms to deal with state handling, message