The Early Session Disposition Type for the Session Initiation Protocol (SIP)
RFC 3959

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• 03

Network Working Group                                       G. Camarillo
Request for Comments: 3959                                      Ericsson
Category: Standards Track                                  December 2004

                The Early Session Disposition Type for
                 the Session Initiation Protocol (SIP)

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 (2004).

Abstract

   This document defines a new disposition type (early-session) for the
   Content-Disposition header field in the Session Initiation Protocol
   (SIP).  The treatment of "early-session" bodies is similar to the
   treatment of "session" bodies.  That is, they follow the offer/answer
   model.  Their only difference is that session descriptions whose
   disposition type is "early-session" are used to establish early media
   sessions within early dialogs, as opposed to regular sessions within
   regular dialogs.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.  Issues Related to Early Media Session Establishment  . . . . .  2
   4.  The Early Session Disposition Type . . . . . . . . . . . . . .  4
   5.  Preconditions  . . . . . . . . . . . . . . . . . . . . . . . .  4
   6.  Option tag . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   7.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
       11.1. Normative References . . . . . . . . . . . . . . . . . .  9
       11.2. Informational References . . . . . . . . . . . . . . . .  9
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 10
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 11

Camarillo                   Standards Track                     [Page 1]
RFC 3959             Early Session Disposition Type        December 2004

1.  Introduction

   Early media refers to media (e.g., audio and video) that is exchanged
   before a particular session is accepted by the called user.  Within a
   dialog, early media occurs from the moment the initial INVITE is sent
   until the User Agent Server (UAS) generates a final response.  It may
   be unidirectional or bidirectional, and can be generated by the
   caller, the callee, or both.  Typical examples of early media
   generated by the callee are ringing tone and announcements (e.g.,
   queuing status).  Early media generated by the caller typically
   consists of voice commands or dual tone multi-frequency (DTMF) tones
   to drive interactive voice response (IVR) systems.

   The basic SIP specification (RFC 3261 [2]) only supports very simple
   early media mechanisms.  These simple mechanisms have a number of
   problems related to forking and security, and do not satisfy the
   requirements of most applications.  RFC 3960 [8] goes beyond the
   mechanisms defined in RFC 3261 [2] and describes two models of early
   media using SIP: the gateway model and the application server model.

   Although both early media models described in RFC 3960 [8] are
   superior to the one specified in RFC 3261 [2], the gateway model
   still presents a set of issues.  In particular, the gateway model
   does not work well with forking.  Nevertheless, the gateway model is
   needed because some SIP entities (in particular, some gateways)
   cannot implement the application server model.

   The application server model addresses some of the issues present in
   the gateway model.  This model uses the early-session disposition
   type specified in this document.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [1] and indicate requirement levels for
   compliant implementations.

3.  Issues Related to Early Media Session Establishment

   Traditionally, early media sessions have been established in the same
   way as regular sessions.  That is, using an offer/answer exchange
   where the disposition type of the session descriptions is "session".
   Application servers perform an offer/answer exchange with the User
   Agent Client (UAC) to exchange early media exclusively, while UASs