SIPPING Working Group G. Camarillo
Internet-Draft Ericsson
Expires: April 15, 2004 October 16, 2003
The Early Session Disposition Type for the Session Initiation
Protocol (SIP)
draft-camarillo-sipping-early-disposition-00.txt
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Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines a new disposition type (early-session) for the
Content-Disposition header field in 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Issues Related to Early Media Session Establishment . . . . . . 3
4. The Early Session Disposition Type . . . . . . . . . . . . . . . 4
5. Option tag . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
Normative References . . . . . . . . . . . . . . . . . . . . . . 7
Informational References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . 9
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1. Introduction
A SIP [2] user agent establishing an INVITE dialog may need to
exchange media with the destination user agent (or user agents if the
INVITE forks) or with application servers in the path before the
dialog is established. Media exchanged this way is referred to as
early media.
Section 3 describes the current approach to establish early media
sessions in SIP are discusses its problems. Section 4 defines the
"early-session" disposition type to resolve those problems.
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 UAC to
exchange early media exclusively, while UASs use the same offer/
answer exchange, first to exchange early media, and once the regular
dialog is established, to exchange regular media. There are issues
related to both, application servers and UASs using this mechanism.
Application servers may not be able to generate an answer for an
offer received in the INVITE. The UAC created the offer for the UAS,
and so, it may have applied end-to-end encryption or have included
information (e.g., related to key management) that the application
server is not supposed to use. Therefore, application servers need a
means to perform an offer/answer exchange with the UAC which is
independent from the offer/answer exchange between both UAs.
UASs using the offer/answer exchange that will carry regular media to
send and receive early media can cause media clipping, as described
in Section 2.1.1 of [5]. Some UACs cannot receive early media from
different UASs at the same time. So, when an INVITE forks and several
UASs start sending early media, the UAC mutes all the UASs but one
(which is usually randomly chosen). If the UAS that accepts the
INVITE (i.e., sends a 200 OK) was muted, a new offer/answer exchange
is needed to unmute it. This usually causes media clipping.
Therefore, UASs need a means to perform an offer/answer exchange with
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the UAC to exchange early media which is independent from the offer/
answer exchanged used to exchange regular media.
A potential solution to this need would be to establish a different
dialog using a globally routable URI to perform an independent offer/
answer exchange. This dialog would be labelled as a dialog for early
media and would be related to the original dialog somehow at the UAC.
However, performing all the offer/answer exchanges within the
original dialog has many advantages:
It is simpler.
It does not have synchronization problems, because all the early
dialogs are terminated when the session is accepted.
It does not require globally routable URIs.
It does not introduce service interaction issues related to
services that may be wrongly applied to the new dialog.
It makes firewall management easier.
4. The Early Session Disposition Type
We define a new disposition type for the Content-Disposition header
field: early-session. User agents MUST use early-session bodies to
establish early media sessions in the same way as they use session
bodies to establish regular sessions, as described in RFC 3261 [2]
and in RFC 3264 [3]. Particularly, early-session bodies MUST follow
the offer/answer model and MAY appear in the same messages as session
bodies do with the exceptions of 2xx responses for an INVITE and
ACKs.
If a UA needs to refuse an early-session offer, it MUST to so by
refusing all the media streams in it. When SDP [4] is used, this is
done by setting the port number of all the media streams to zero.
This is the same mechanism that UACs use to refuse regular offers
that arrive in a response to an empty INVITE.
An early media session established using early-session bodies MUST be
terminated when its corresponding early dialog is terminated or it
transitions to a regular dialog.
5. Option tag
We define an option tag to be used in Require and Supported header
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fields. Its name is early-session. A UA adding the early-session
option tag to a message indicates that it understands the
early-session disposition type.
6. Example
Figure 1 shows the message flow between two UAs. INVITE (1) has an
early-session option tag in its Supported header field and the body
shown in Figure 2. The UAS sends back a response with two body parts
,as shown in Figure 3; one of disposition type session and the other
early-session. The session body part is the answer to the offer in
the INVITE. The early-session body part is an offer to establish an
early media session. When the UAC receives the 183 (Session Progress)
response, it sends the answer to the early-session offer in a PRACK,
as shown in Figure 4. This early media session is terminated when the
early dialog transitions to a regular dialog. That is, when the UAS
sends the (5) 200 (OK) response for the INVITE.
A B
| |
|--------(1) INVITE-------->|
| session |
| |
|<--(2) Session Progress----|
| early-session |
| session |
| |
|---------(3) PRACK-------->|
| early-session |
| |
|<--------(4) 200 OK--------|
| |
| * * |
| ************************* |
|* Early Media *|
| ************************* |
| * * |
| |
|---------(5) 200 OK------->|
| |
|<---------(6) ACK----------|
| |
Figure 1
Content-Type: application/sdp
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Content-Disposition: session
v=0
o=alice 2890844730 2890844731 IN IP4 host.example.com
s=
c=IN IP4 192.0.0.1
t=0 0
m=audio 20000 RTP/AVP 0
Figure 2
Content-Type: multipart/mixed; boundary="--boundary1"
Content-Length: 399
--boundary1
Content-Type: application/sdp
Content-Disposition: session
v=0
o=Bob 2890844725 2890844725 IN IP4 host.example2.com
s=
c=IN IP4 192.0.0.2
t=0 0
m=audio 30000 RTP/AVP 0
--boundary1
Content-Type: application/sdp
Content-Disposition: early-session
v=0
o=Bob 2890844714 2890844714 IN IP4 host.example2.com
s=
c=IN IP4 192.0.0.2
t=0 0
m=audio 30002 RTP/AVP 0
--boundary1
Figure 3
Content-Type: application/sdp
Content-Disposition: early-session
v=0
o=alice 2890844717 2890844717 IN IP4 host.example.com
s=
c=IN IP4 192.0.0.1
t=0 0
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m=audio 20002 RTP/AVP 0
Figure 4
7. IANA Considerations
This document defines a new Content-Disposition header field
disposition type (early-session) in Section 4. This value should be
registered in the IANA registry for Content-Dispositions with the
following description:
early-session the body describes an early communications
session, for example, an RFC 2327 SDP body
This document defines a SIP option tag (early-session) in Section 5.
It should be registered in the SIP parameters registry (http://
www.iana.org/assignments/sip-parameters) under "Option Tags", with
the following description.
A UA adding the early-session option tag to a message indicates
that it understands the early-session content disposition.
Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
Informational References
[4] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[5] Camarillo, G. and H. Schulzrinne, "Early Media and Ringback Tone
Generation in the Session Initiation Protocol",
draft-camarillo-sipping-early-media-02 (work in progress), July
2003.
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Author's Address
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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