SIPPING Working Group G. Camarillo
Internet-Draft Ericsson
Expires: December 15, 2004 June 16, 2004
The Early Session Disposition Type for the Session Initiation
Protocol (SIP)
draft-ietf-sipping-early-disposition-03.txt
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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. Preconditions . . . . . . . . . . . . . . . . . . . . . . . 5
6. Option tag . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1 Normative References . . . . . . . . . . . . . . . . . . . 10
11.2 Informational References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . 11
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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 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 DTMF
tones to drive IVRs.
The basic SIP specification (RFC 3261 [2]) 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. RFC xxxx [8] goes beyond the
mechanisms defined in RFC 3261 [2] and describes two models to
implement early media using SIP: the gateway model and the
application server model.
Although both early media models described in RFC xxxx [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, which is 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 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. This way of
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establishing early media sessions is known as the gateway model [8],
which presents some issues which relate to forking and security.
These issues exist when this model is used by either an application
server or by a UAS.
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 [8]. 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
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:
o It is simpler.
o It does not have synchronization problems, because all the early
dialogs are terminated when the session is accepted.
o It does not require globally routable URIs.
o It does not introduce service interaction issues related to
services that may be wrongly applied to the new dialog.
o It makes firewall management easier.
This way of performing offer/answer exchanges for early media is
referred to as the application server model [8]. This model uses the
early-session disposition type, which we define in the following
section.
4. The Early Session Disposition Type
We define a new disposition type for the Content-Disposition header
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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. Nevertheless, it is NOT RECOMMENDED to include early offers in
INVITEs because they can fork, and the UAC could receive multiple
early answers establishing early media streams at roughly the same
time. It is also NOT RECOMMENDED to use the same transport address
(IP address plus port) in a session body and in an early-session
body. Using different transport addresses (e.g., different ports) to
receive early and regular media makes it easy to detect the start of
the regular media.
If a UA needs to refuse an early-session offer, it MUST to so by
refusing all the media streams in it. When SDP [7] 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.
It is RECOMMENDED that UAs generating regular and early session
descriptions use, as long as it is possible, the same codecs in both.
This way, the remote UA does not need to change codecs when the early
session transitions to a regular session.
5. Preconditions
RFC 3312 [4] defines a framework for preconditions for SDP.
Early-sessions MAY contain preconditions, which are treated in the
same way as preconditions in regular sessions. That is, the UAs do
not exchange media and the called user is not alerted until the
preconditions are met.
6. Option tag
We define an option tag to be used in Require and Supported header
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.
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7. 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-------->|
| offer |
| |
|<--(2) Session Progress----|
| early-offer |
| answer |
| |
|---------(3) PRACK-------->|
| early-answer |
| |
|<--------(4) 200 OK--------|
| |
| * * |
| ************************* |
|* Early Media *|
| ************************* |
| * * |
| |
|<--------(5) 200 OK--------|
| |
|----------(6) ACK--------->|
| |
Figure 1: Message flow
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Content-Type: application/sdp
Content-Disposition: session
v=0
o=alice 2890844730 2890844731 IN IP4 host.example.com
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 20000 RTP/AVP 0
Figure 2: Offer
Content-Type: multipart/mixed; boundary="boundary1"
Content-Length: 401
--boundary1
Content-Type: application/sdp
Content-Disposition: session
v=0
o=Bob 2890844725 2890844725 IN IP4 host.example.org
s=
c=IN IP4 192.0.2.2
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.example.org
s=
c=IN IP4 192.0.2.2
t=0 0
m=audio 30002 RTP/AVP 0
--boundary1--
Figure 3: Early offer and answer
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Content-Type: application/sdp
Content-Disposition: early-session
v=0
o=alice 2890844717 2890844717 IN IP4 host.example.com
s=
c=IN IP4 192.0.2.1
t=0 0
m=audio 20002 RTP/AVP 0
Figure 4: Early answer
8. Security Considerations
The security implications of using early-session bodies in SIP are
the same as the ones of using session bodies; they are part of the
offer/answer model.
SIP uses the offer/answer model [3] to establish early sessions in
both the gateway and the application server models. User Agents (UAs)
generate a session description, which contains the transport address
(i.e., IP address plus port) where they want to receive media, and
send it to their peer in a SIP message. When media packets arrive at
this transport address, the UA assumes that they come from the
receiver of the SIP message carrying the session description.
Nevertheless, attackers may attempt to gain access to the contents of
the SIP message and send packets to the transport address contained
in the session description. To prevent this situation, UAs SHOULD
encrypt their session descriptions (e.g., using S/MIME).
Still, even if a UA encrypts its session descriptions, an attacker
may try to guess the transport address used by the UA and send media
packets to that address. Guessing such a transport address is
sometimes easier than it may seem because many UAs always pick up the
same initial media port. To prevent this situation, UAs SHOULD use
media-level authentication mechanisms (e.g., SRTP [6]). In addition,
UAs that wish to keep their communications confidential SHOULD use
media-level encryption mechanisms (e.g, SRTP [6]).
Attackers may attempt to make a UA send media to a victim as part of
a DoS attack. This can be done by sending a session description with
the victim's transport address to the UA. To prevent this attack,
the UA SHOULD engage in a handshake with the owner of the transport
address received in a session descriptions (just verifying
willingness to receive media) before sending a large amount of data
to the transport address. This check can be performed by using a
connection oriented transport protocol, by using STUN [5] in an
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end-to-end fashion, or by the key exchange in SRTP [6].
In any event, note that the previous security considerations are not
early media specific, but apply to the usage of the offer/answer
model in SIP to establish sessions in general.
Additionally, an early media-specific risk (roughly speaking, an
equivalent to forms of "toll fraud" in the PSTN) attempts to exploit
the different charging policies some operators apply to early and to
regular media. When UAs are allowed to exchange early media for free,
but are required to pay for regular media sessions, rogue UAs may try
to establish a bidirectional early media session and never send a 2xx
response for the INVITE.
On the other hand, some application servers (e.g., Interactive Voice
Response systems) use bidirectional early media to obtain information
from the callers (e.g., the PIN code of a calling card). So, we do
not recommend that operators disallow bidirectional early media.
Instead, operators should consider a remedy of charging early media
exchanges that last too long, or stopping them at the media level
(according to the operator's policy).
9. 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 6.
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.
10. Acknowledgements
Francois Audet, Christer Holmberg, and Allison Mankin provided useful
comments on this document.
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11. References
11.1 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.
[4] Camarillo, G., Marshall, W. and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002.
[5] Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
Simple Traversal of User Datagram Protocol (UDP) Through Network
Address Translators (NATs)", RFC 3489, March 2003.
[6] Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
3711, March 2004.
11.2 Informational References
[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[8] 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.
Author's Address
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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