Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP)
RFC 3960

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Network Working Group                                       G. Camarillo
Request for Comments: 3960                                      Ericsson
Category: Informational                                   H. Schulzrinne
                                                     Columbia University
                                                           December 2004

                Early Media and Ringing Tone Generation
                in the Session Initiation Protocol (SIP)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document describes how to manage early media in the Session
   Initiation Protocol (SIP) using two models: the gateway model and the
   application server model.  It also describes the inputs one needs to
   consider in defining local policies for ringing tone generation.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Session Establishment in SIP . . . . . . . . . . . . . . . . .  3
   3.  The Gateway Model. . . . . . . . . . . . . . . . . . . . . . .  4
       3.1.  Forking. . . . . . . . . . . . . . . . . . . . . . . . .  4
       3.2.  Ringing Tone Generation. . . . . . . . . . . . . . . . .  5
       3.3.  Absence of an Early Media Indicator. . . . . . . . . . .  7
       3.4.  Applicability of the Gateway Model . . . . . . . . . . .  8
   4.  The Application Server Model . . . . . . . . . . . . . . . . .  8
       4.1.  In-Band Versus Out-of-Band Session Progress Information.  9
   5.  Alert-Info Header Field. . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations. . . . . . . . . . . . . . . . . . . .  9
   7.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       8.1.  Normative References . . . . . . . . . . . . . . . . . . 11
       8.2.  Informative References . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 13

Camarillo & Schulzrinne      Informational                      [Page 1]
RFC 3960        Early Media and Ringing Tone Generation    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 [1]) only supports very simple
   early media mechanisms.  These simple mechanisms have a number of
   problems which relate to forking and security, and do not satisfy the
   requirements of most applications.  This document goes beyond the
   mechanisms defined in RFC 3261 [1] and describes two models of early
   media implementations using SIP: the gateway model and the
   application server model.

   Although both early media models described in this document are
   superior to the one specified in RFC 3261 [1], 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, which is specified in [2].

   The remainder of this document is organized as follows: Section 2
   describes the offer/answer model in the absence of early media, and
   Section 3 introduces the gateway model.  In this model, the early
   media session is established using the early dialog established by
   the original INVITE.  Sections 3.1, 3.2, and 3.4 describe the
   limitations of the gateway model and the scenarios where it is
   appropriate to use this model.  Section 4 introduces the application
   server model, which, as stated previously, resolves some of the
   issues present in the gateway model.  Section 5 discusses the
   interactions between the Alert-Info header field in both early media
   models.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", " NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this