|Title||The Early Session Disposition Type for the Session Initiation
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 (C) The Internet Society (2004).
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
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
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 ) 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  goes beyond the
mechanisms defined in RFC 3261  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  are
superior to the one specified in RFC 3261 , 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.
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  and indicate requirement levels for
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
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 establishing early media sessions is known as the
gateway model , which presents some issues related 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 that is
independent from the offer/answer exchange between both UAs.
UASs using the offer/answer exchange that will carry regular media
for sending and receiving early media can cause media clipping, as
described in Section 2.1.1 of . 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 chosen at random). 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 of performing an
offer/answer exchange with the UAC to exchange early media that is
independent from the offer/answer exchanged used to exchange regular
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 somehow related to the original dialog 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 . This model uses the
early-session disposition type defined in the following section.
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 RFCs 3261 
and 3264 . 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 that early offers in
INVITEs be included because they can fork, and the UAC could receive
multiple early answers establishing early media streams at roughly
the same time. Also, the use of the same transport address (IP
address plus port) in a session body and in an early-session body is
NOT RECOMMENDED. 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 User Agent (UA) needs to refuse an early-session offer, it MUST
do so by refusing all the media streams in it. When SDP  is used,
this is done by setting the port number of all the media streams to
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.
RFC 3312  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: early-session. A UA adding the early-session option tag to a
message indicates that it understands the early-session disposition
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.
| offer |
|<--(2) Session Progress----|
| early-offer |
| answer |
| early-answer |
|<--------(4) 200 OK--------|
| * * |
| ************************* |
|* Early Media *|
| ************************* |
| * * |
|<--------(5) 200 OK--------|
Figure 1: Message flow
o=alice 2890844730 2890844731 IN IP4 host.example.com
c=IN IP4 192.0.2.1
m=audio 20000 RTP/AVP 0
Figure 2: Offer
Content-Type: multipart/mixed; boundary="boundary1"
o=Bob 2890844725 2890844725 IN IP4 host.example.org
c=IN IP4 192.0.2.2
m=audio 30000 RTP/AVP 0
o=Bob 2890844714 2890844714 IN IP4 host.example.org
c=IN IP4 192.0.2.2
m=audio 30002 RTP/AVP 0
Figure 3: Early offer and answer
o=alice 2890844717 2890844717 IN IP4 host.example.com
c=IN IP4 192.0.2.1
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 when using session bodies; they are part of the
SIP uses the offer/answer model  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
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., Secure Realtime
Transport Protocol (SRTP)). In addition, UAs that wish to keep
their communications confidential SHOULD use media-level encryption
mechanisms (e.g, SRTP ).
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 description (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 Simple Traversal of the UDP
Protocol through NAT (STUN) in an end-to-end fashion, or by the
key exchange in SRTP .
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 Public Switched Telephone
Network (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
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 Personal Identification Number (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
9. IANA Considerations
This document defines a new Content-Disposition header field
disposition type (early-session) in Section 4. This value has been
registered in the IANA registry for Content-Dispositions with the
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 has been registered in the SIP parameters registry
(http://www.iana.org/assignments/sip-parameters) under "Option Tags",
with the following description.
early-session A UA adding the early-session option tag to a
message indicates that it understands the early-
session content disposition.
Francois Audet, Christer Holmberg, and Allison Mankin provided useful
comments on this document.
11.1. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 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.
 Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
 Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002.
 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.
 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
 Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
 Camarillo, G. and H. Schulzrinne, "Early Media and Ringing Tone
Generation in the Session Initiation Protocol (SIP)", RFC 3960,
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