The Session Initiation Protocol (SIP) UPDATE Method
RFC 3311

Document Type RFC - Proposed Standard (October 2002; No errata)
Last updated 2015-10-14
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Network Working Group                                       J. Rosenberg
Request for Comments: 3311                                   dynamicsoft
Category: Standards Track                                 September 2002

          The Session Initiation Protocol (SIP) UPDATE Method

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 specification defines the new UPDATE method for the Session
   Initiation Protocol (SIP).  UPDATE allows a client to update
   parameters of a session (such as the set of media streams and their
   codecs) but has no impact on the state of a dialog.  In that sense,
   it is like a re-INVITE, but unlike re-INVITE, it can be sent before
   the initial INVITE has been completed.  This makes it very useful for
   updating session parameters within early dialogs.

Rosenberg                   Standards Track                     [Page 1]
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                     [Page 2]
RFC 3311                   SIP UPDATE Method              September 2002

2 Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
   [2] and indicate requirement levels for compliant SIP
   implementations.

3 Overview of Operation

   Operation of this extension is straightforward.  The caller begins
   with an INVITE transaction, which proceeds normally.  Once a dialog
   is established, either early or confirmed, the caller can generate an
   UPDATE method that contains an SDP offer [3] for the purposes of
   updating the session.  The response to the UPDATE method contains the
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