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Versions: 00 01 02                                                      
Internet Engineering Task Force                                   SIP WG
Internet Draft                                              G. Camarillo
                                                          H. Schulzrinne
                                                     Columbia University
February 10, 2003
Expires: August, 2003

                Early Media and Ringback Tone Generation
                   in the Session Initiation Protocol


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at

   To view the list Internet-Draft Shadow Directories, see


   This document describes how to manage early media in SIP using two
   models; the gateway model and the application server model. It also
   describes which inputs need to be taken into consideration to define
   local policies for ringback tone generation.

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                           Table of Contents

   1          Introduction ........................................    3
   2          The Gateway Model ...................................    3
   2.1        Media Clipping ......................................    4
   2.1.1      Forking .............................................    5
   2.2        Ringback Tone Generation ............................    6
   2.3        Applicability of the Gateway Model ..................    7
   3          The Application Server Model ........................    8
   4          Alert-Info Header Field .............................    8
   5          Acknowledgments .....................................    9
   6          Authors' Addresses ..................................    9
   7          Bibliography ........................................    9

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1 Introduction

   Early media refers to media (e.g., audio and/or video) that is
   exchanged before a particular session is accepted by the called user.
   Early media within a particular SIP dialog takes place from the
   moment the initial INVITE is sent until the UAS generates a final
   response. Early media can be unidirectional or bi-directional and can
   be generated by the caller or/and the callee. Typical examples of
   early media generated by the callee are ringback tone and
   announcements (e.g., queuing status). Early media generated by the
   caller typically consist of voice commands or DTMF tones to drive

   The basic SIP spec [1] only supports very simple early media. In
   order to support fully-featured early media, UAs need to implement
   some extensions in addition to the basic SIP spec. This document
   describes two models to implement early media and the extensions
   needed in each model.

   Section 2 introduces the gateway model. In this model, the early
   media session is established using the early dialog established by
   the original INVITE. Section 2.1, Section 2.2 and Section 2.3
   describe the limitation of the gateway model and the scenarios where
   it is appropriate to use this model. Section 3 introduces the
   application server model, which resolves some of the issues present
   in the gateway model. Section 4 discusses the interactions between
   the Alter-Info header field in both early media models.

2 The Gateway Model

   SIP [1] uses the offer/answer model [2] to negotiate session
   parameters. One of the user agents - the offerer - prepares a session
   description that is called the offer. The other user agent - the
   answerer - responds with another session description called the
   answer. This two-way handshake allows both user agents to agree upon
   the session parameters to be used to exchange media.

   The idea behind the offer/answer model is to decouple the
   offer/answer exchange from the mechanism used to transport the
   session descriptions. For example, the offer can be sent in an INVITE
   request and the answer can arrive in the 200 (OK) response for that
   INVITE. Or, alternatively, the offer can be sent in the 200 (OK) for
   an empty INVITE and the answer be sent in the ACK. When reliable
   provisional responses [3] and UPDATE requests [4] are used, there are
   many more possible ways to exchange offers and answers.

   An offer/answer exchange that takes place before a final response for
   the INVITE is sent establishes an "early" media session. Early media

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   sessions terminate when a final response for the INVITE is sent. If
   the final response is a 2xx, the early media session transitions to a
   regular media session. If the final response is a non-2xx final
   response, the early media session is simply terminated.

   Media exchanged within an early media session is, not surprisingly,
   referred to as early media. The gateway model consists of managing
   early media sessions using reliable provisional responses, PRACKs and

2.1 Media Clipping

   Media clipping occurs when the user (or the machine generating media)
   believes that the media session is already established but the
   establishment process has not finished yet. The user starts speaking
   (i.e., generating media) and the first few syllables or even the
   first few words are lost.

   Media clipping is closely related to the user's expectations. For
   example, in the PSTN, there usually isn't media clipping in the
   forward direction because callers are used to wait until the callee
   answers in order to start speaking. People do not typically start
   saying "Hello" while they are hearing a ringback tone.

   However, callees in the PSTN are used to pick up the phone and start
   speaking right away. That is why, in the PSTN, when the callee picks
   up the phone, the media path is already established. It avoids media
   clipping in the backward direction.

   Unlike in the PSTN, there are some situations involving SIP where the
   callee accepts a session invitation (e.g., picks up a SIP phone) but
   the media session has not been established yet. This happens, for
   instance, when the callee's receives an empty INVITE request and
   sends an offer in a 200 (OK) response. The UAS will not be able to
   send any media until it receives the answer in the ACK. Everything
   the callee says during that round trip time will get lost.

   However, the situation described above is a general SIP issue, not
   specifically related to early media. Therefore, it falls outside of
   the scope of this document. Section 2.1.1 focuses on the scenarios
   where the gateway model introduces media clipping.

   Another form of media clipping (not related to early media either)
   occurs in the caller->callee direction. If the callee picks up and
   starts speaking, the UAS will send a 2xx response with an answer and
   the first media packets in parallel. If the first media packets
   arrive to the UAC before the answer, and the caller starts speaking
   as well, the UAC will not be able to send media until the 2xx

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   response from the UAS arrives. Section 2.1.1 does not deal with this
   situation either, since it is not early media related.

2.1.1 Forking

   In the absence of forking, assuming that the initial INVITE contains
   an offer, the gateway model does not introduce media clipping.
   Following normal SIP procedures, the UAC is ready to play any
   incoming media as soon as it sends the initial offer in the INVITE.
   The UAS sends the answer in a reliable provisional response and
   starts sending media right away. Even if the first media packets
   arrive to the UAS before the 1xx response, the UAS will play them.

        Note that, in some situations, the UAC does need to receive
        the answer before being able to play any media. UAs in such
        a situation (e.g., QoS, media authorization or media
        encryption is required) use preconditions to avoid media

   However, if the INVITE forks, the gateway model may introduce media
   clipping. This happens when the UAC receives different answers to its
   offer in several provisional responses from different UASs. The UAC
   has to deal with bandwidth limitations and early media session

   If the UAC receives early media from different UASs, it needs to
   present it to the user. If the early media consists of audio, playing
   several audio streams to the user at the same time can be confusing.
   Other media types (e.g., video), on the other hand, can be presented
   to the user at the same time. The UAC can, for example, build a
   mosaic with the different inputs.

   However, even with media types that can be played at the same time to
   the user, if the UAC has limited bandwidth, it will not be able to
   receive early media from all the different UASs at the same time.
   Therefore, many times, the UAC needs to choose a single early media
   session and "mute" the rest of them sending UPDATE requests.

        It is difficult to decide which early media session carry
        more important information from the caller's perspective.
        Therefore, UACs typically pick up one early media session
        randomly and mute the rest.

   If one of the early media sessions that was muted transitions to a
   regular media session (i.e., the UAS sends a 2xx response), media
   clipping is likely to appear. The UAC typically sends an UPDATE with
   a new offer (upon reception of the 200 OK for the INVITE) to unmute
   the media session. The UAS cannot send any media until it receives

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   the offer from the UAC. Therefore, if the caller starts speaking
   before the offer from the UAC is received, his words will get lost.

        Having the UAS send the UPDATE to unmute the media session
        (instead of the UAC) does not avoid media clipping in the
        backward direction, and it causes possible race conditions.

2.2 Ringback Tone Generation

   In the PSTN, telephone switches typically play ringback tones to the
   caller to indicate that the callee is being alerted. When, where and
   how these ringback tones are generated has been standardized (i.e.,
   the local exchange of the callee generates a standardized ringback
   tone while the callee is being alterted). A standardized approach to
   provide this type of feedback for the user makes sense in a
   homogeneous environment such as the PSTN, where all the terminals
   have a similar user interface.

   This homogeneity is not found among SIP user agents. SIP user agents
   have different capabilities, different user interfaces and may be
   used to establish sessions that do not involve audio at all. Because
   of this, the way a SIP UA provides the user with information about
   the progress of session establishment is a matter of local policy.
   For example, a UA with a GUI may choose to display a message on the
   screen when the callee is being alerted while another UA may choose
   to show a picture of a phone ringing instead. Many SIP UAs choose to
   imitate the user interface of the PSTN phones. They provide a
   ringback tone to the caller when the callee is being alerted. Such a
   UAC is supposed to generate ringback tones locally for its user as
   long as no early media is received from the UAS. If the UAS generates
   early media (e.g., an announcement or a special ringback tone), the
   UAC is supposed to play it rather than generating the ringback tone

   The problem is that, sometimes, it is not an easy task for a UAC to
   know whether it should generate local ringback or it will be
   receiving early media. A UAS can send early media without using
   reliable provisional responses (very simple UASs do that) or it can
   send an answer in a reliable provisional response without any
   intention of sending early media (this is the case when preconditions
   are used). Therefore, by only looking at the SIP signalling, a UAC
   cannot be sure whether or not there will be early media for a
   particular session. The UAC needs to check if media packets are
   arriving at a given moment.

        An implementation could even choose to look at the contents
        of the media packets, since they could carry only silence
        or comfort noise.

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   With this in mind, a UAC should develop its local policy regarding
   local ringback generation. For example, a POTS-like SIP UA could
   implement the following local policy:

        1.   Unless a 180 (Ringing) response is received, never generate
             local ringback.

        2.   If a 180 (Ringing) has been received but there are no
             incoming media packets, generate local ringback.

        3.   If a 180 (Ringing) has been received and there are incoming
             media packets, play them and do not generate local

   Note, however, that implementing such a policy in a decomposed
   gateway (media gateway controller and media gateway) can be complex.
   The media gateway needs to inform the media gateway controller about
   the presence of incoming media, and based on that information, the
   media gateway controller needs to control the generation of local
   ringback in the media gateway. This type of gateway could choose to
   generate local ringback upon reception of a 180 (Ringing) response,
   and mix it with any incoming media that happens to arrive (if it does
   at all).

        Note that a 180 (Ringing) response means that the callee is
        being alerted, and a UAS should send such a response if the
        callee is being alerted, regardless of the status of the
        early media session.

   Note that while it is not desirable to standardize a common local
   policy to be followed by every SIP UA, a particular subset of more or
   less homogeneous SIP UAs could use the same local policy by
   convention. Examples of such subsets of SIP UAs may be "all the
   PSTN/SIP gateways" or "every 3G IMS terminal". However, defining the
   particular common policy that such groups of SIP devices may use is
   outside the scope of this document.

2.3 Applicability of the Gateway Model

   Section 2.1 and Section 2.2 described some of the limitations of the
   gateway model. It produces media clipping in forking scenarios and
   requires media detection to generate local ringback properly. These
   issues are addressed by the application server model, described in
   Section 3, which is the recommended way of generating early media
   that is not continuous with the regular media that will be generated
   during the session.

   The gateway model is, therefore, acceptable in situations where the

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   UA cannot distinguish between early media and regular media. A PSTN
   gateway is an example of this type of situation. The PSTN gateway
   receives media from the PSTN over a circuit, and sends it to the IP
   network. The gateway is not aware of the contents of the media, and
   it does not exactly know when the transition from early to regular
   media takes place. From the PSTN perspective, the circuit is a
   continuous source of media.

3 The Application Server Model

   The application server model consists of having the UAS behave as any
   other application server in the session [5]. The UAC includes a Join
   header field in the initial INVITE. In order to send early media, the
   UAS establishes a new dialog by sending a new INVITE to the URI in
   the Join header field.

   Sending early media using a different dialog than the one used for
   sending regular media helps avoid media clipping in case of forking.
   The UAC can reject or mute new invitations for early media without
   muting the sessions that will carry media when the original INVITE is
   accepted. The UAC can give priority to media received over the latter
   sessions. This way, the application server model achieves a smooth
   transition from early to regular media.

   Having a separate dialog for early media also helps UAs decide
   whether or not local ringback should be generated. If a new dialog to
   send early media is established, and that dialog contains at least an
   audio stream, the UAC can assume that there will be incoming early
   media and it can then avoid generating local ringback.

        An alternative model would consist of adding a new stream
        labeled as "early media" to the original session between
        the UAC and the UAS using an UPDATE, instead of
        establishing a new session. We have chosen to establish a
        new session to be coherent with the mechanism used by
        application servers that are NOT co-located with the UAS.
        This way, the UAS uses the same mechanism as any other
        application server in the network to interact with the UAC.

4 Alert-Info Header Field

   The Alert-Info header field allows specifying an alternative ringback
   tone to the UAC. This header field tells the UAC which tone should be
   played in case local ringback is generated, but it does not tell the
   UAC when to generate local ringback. A UAC should follow the rules
   described above for ringback tone generation in both models. If,
   after following those rules, the UAC decides to play local ringback,
   it can then use the Alert-Info header field to generate it.

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5 Acknowledgments

   Jon Peterson provided useful ideas on the separation between the
   gateway model and the application server model.

   Paul Kyzivat, Christer Holmberg, Bill Marshall, Francois Audet, John
   Hearty, Adam Roach and Rohan Mahy provided useful comments and

6 Authors' Addresses

   Gonzalo Camarillo
   Advanced Signalling Research Lab.
   FIN-02420 Jorvas
   electronic mail:  Gonzalo.Camarillo@ericsson.com

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University 1214 Amsterdam Avenue, MC 0401
   New York, NY 10027
   electronic mail:  schulzrinne@cs.columbia.edu

7 Bibliography

   [1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
   Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
   initiation protocol," RFC 3261, Internet Engineering Task Force, June

   [2] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
   session description protocol (SDP)," RFC 3264, Internet Engineering
   Task Force, June 2002.

   [3] J. Rosenberg and H. Schulzrinne, "Reliability of provisional
   responses in session initiation protocol (SIP)," RFC 3262, Internet
   Engineering Task Force, June 2002.

   [4] J. Rosenberg, "The session initiation protocol (SIP) UPDATE
   method," RFC 3311, Internet Engineering Task Force, Oct. 2002.

   [5] J. Rosenberg, "A framework and requirements for application
   interaction in SIP," internet draft, Internet Engineering Task Force,
   Nov. 2002.  Work in progress.

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