Session Initiation Protocol (SIP) Usage of the Offer/Answer Model
RFC 6337

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Internet Engineering Task Force (IETF)                        S. Okumura
Request for Comments: 6337                                     Softfront
Category: Informational                                        T. Sawada
ISSN: 2070-1721                                         KDDI Corporation
                                                              P. Kyzivat
                                                             August 2011

   Session Initiation Protocol (SIP) Usage of the Offer/Answer Model

Abstract

   The Session Initiation Protocol (SIP) utilizes the offer/answer model
   to establish and update multimedia sessions using the Session
   Description Protocol (SDP).  The description of the offer/answer
   model in SIP is dispersed across multiple RFCs.  This document
   summarizes all the current usages of the offer/answer model in SIP
   communication.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6337.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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RFC 6337           SIP Usage of the Offer/Answer Model       August 2011

   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Summary of SIP Usage of the Offer/Answer Model ..................3
      2.1. Terminology ................................................3
      2.2. Offer/Answer Exchange Pairs in SIP Messages ................4
      2.3. Rejection of an Offer ......................................5
      2.4. Session Description That Is Not an Offer or an Answer ......7
   3. Detailed Discussion of the Offer/Answer Model for SIP ...........8
      3.1. Offer/Answer for the INVITE method with 100rel Extension ...8
           3.1.1. INVITE Request with SDP .............................8
           3.1.2. INVITE Request without SDP .........................11
      3.2. Offer/Answer Exchange in Early Dialog .....................12
      3.3. Offer/Answer Exchange in an Established Dialog ............12
      3.4. Recovering from a Failed Re-INVITE ........................13
   4. Exceptional Case Handling ......................................13
      4.1. Message Crossing Case Handling ............................13
      4.2. Glare Case Handling .......................................18
      4.3. Interworking of UPDATE and Re-INVITE ......................21
   5. Content of Offers and Answers ..................................25
      5.1. General Principle for Constructing Offers and Answers .....26
      5.2. Choice of Media Types and Formats to Include and Exclude ..26
           5.2.1. Sending an Initial INVITE with Offer ...............26
           5.2.2. Responding with an Offer When the Initial
                  INVITE Has No Offer ................................27
           5.2.3. Answering an Initial INVITE with Offer .............27
           5.2.4. Answering When the Initial INVITE Had No Offer .....28
           5.2.5. Subsequent Offers and Answers ......................28
      5.3. Hold and Resume of Media ..................................29
      5.4. Behavior on Receiving SDP with c=0.0.0.0 ..................31
   6. Security Considerations ........................................31
   7. Acknowledgements ...............................................31
   8. References .....................................................32
      8.1. Normative References ......................................32
      8.2. Informative References ....................................33

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RFC 6337           SIP Usage of the Offer/Answer Model       August 2011