RFC 3311 (rfc3311) - Page 2 of 13

The Session Initiation Protocol (SIP) UPDATE Method

Alternative Format: Original Text Document

RFC 3311                   SIP UPDATE Method              September 2002


Table of Contents

   1    Introduction ..............................................    2
   2    Terminology ...............................................    3
   3    Overview of Operation .....................................    3
   4    Determining Support for this Extension ....................    3
   5    UPDATE Handling ...........................................    4
   5.1  Sending an UPDATE .........................................    4
   5.2  Receiving an UPDATE .......................................    5
   5.3  Processing the UPDATE Response ............................    6
   6    Proxy Behavior ............................................    7
   7    Definition of the UPDATE method ...........................    7
   8    Example Call Flow .........................................    7
   9    Security Considerations ...................................   11
   10   IANA Considerations .......................................   11
   11   Notice Regarding Intellectual Property Rights .............   11
   12   Normative References ......................................   11
   13   Acknowledgements ..........................................   12
   14   Author's Address ..........................................   12
   15   Full Copyright Statement ..................................   13

1 Introduction

   The Session Initiation Protocol (SIP) [1] defines the INVITE method
   for the initiation and modification of sessions.  However, this
   method actually affects two important pieces of state.  It impacts
   the session (the media streams SIP sets up) and also the dialog (the
   state that SIP itself defines).  While this is reasonable in many
   cases, there are important scenarios in which this coupling causes
   complications.

   The primary difficulty is when aspects of the session need to be
   modified before the initial INVITE has been answered.  An example of
   this situation is "early media", a condition where the session is
   established, for the purpose of conveying progress of the call, but
   before the INVITE itself is accepted.  It is important that either
   caller or callee be able to modify the characteristics of that
   session (putting the early media on hold, for example), before the
   call is answered.  However, a re-INVITE cannot be used for this
   purpose, because the re-INVITE has an impact on the state of the
   dialog, in addition to the session.

   As a result, a solution is needed that allows the caller or callee to
   provide updated session information before a final response to the
   initial INVITE request is generated.  The UPDATE method, defined
   here, fulfills that need.  It can be sent by a UA within a dialog
   (early or confirmed) to update session parameters without impacting
   the dialog state itself.



Rosenberg                   Standards Track
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