SIPCORE Working Group C. Holmberg
Internet-Draft Ericsson
Intended status: Standards Track February 1, 2011
Expires: August 5, 2011
Session Initiation Protocol (SIP) Response Code for Indication of
Terminated Dialog
draft-ietf-sipcore-199-05.txt
Abstract
This specification defines a new Session Initiation Protocol (SIP)
response code, 199 Early Dialog Terminated, that a SIP forking proxy
and a User Agent Server (UAS) can use to indicate towards upstream
SIP entities (including the User Agent Client (UAC)) that an early
dialog has been terminated, before a final response is sent towards
the SIP entities.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 5, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Applicability and Limitation . . . . . . . . . . . . . . . . . 4
4. User Agent Client behavior . . . . . . . . . . . . . . . . . . 5
5. User Agent Server behavior . . . . . . . . . . . . . . . . . . 6
6. Proxy behavior . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Backward compatibility . . . . . . . . . . . . . . . . . . . . 9
8. Usage with SDP offer/answer . . . . . . . . . . . . . . . . . 9
9. Message Flow Examples . . . . . . . . . . . . . . . . . . . . 10
9.1. Example with a forking proxy which generates 199 . . . . . 10
9.2. Example with a forking proxy which receives 200 OK . . . . 11
9.3. Example with two forking proxies, of which one
generates 199 . . . . . . . . . . . . . . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . . 12
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
11.1. IANA Registration of the 199 response code . . . . . . . . 13
11.2. IANA Registration of the 199 option-tag . . . . . . . . . 13
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
13. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 13
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
14.1. Normative References . . . . . . . . . . . . . . . . . . . 14
14.2. Informational References . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15
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1. Introduction
As defined in RFC 3261 [RFC3261], a Session Initiation Protocol (SIP)
early dialog is created when a non-100 provisional response is sent
to the initial dialog initiation request (e.g. INVITE, outside an
existing dialog). The dialog is considered to be in early state
until a final response is sent.
When a proxy receives an initial dialog initiation request, it can
forward the request towards multiple remote destinations. When the
proxy does that, it performs forking [RFC3261].
When a forking proxy receives a non-100 provisional response, or a
2xx final response, it forwards the response upstream towards the
sender of the associated request. After a forking proxy has
forwarded a 2xx final response, it normally generates and sends
CANCEL requests downstream towards all remote destinations where it
previously forked the request associated with the 2xx final response
and from which it has yet not received a final response. The CANCEL
requests are sent in order to terminate any outstanding early dialogs
associated with the request.
Upstream SIP entities might receive multiple 2xx final responses.
When a SIP entity receives the first 2xx final response, and it does
not intend to accept any subsequent 2xx final response, it will
automatically terminate any other outstanding early dialog associated
with the request. If the SIP entity receives a subsequent 2xx final
response, it will normally generate and send an ACK request, followed
with a BYE request, using the dialog identifier retrieved from the
2xx final response.
NOTE: A User Agent Client (UAC) can use the Request-Disposition
header field [RFC3841] to request that proxies do not generate and
send CANCEL requests downstream once they have received the first 2xx
final response.
When a forking proxy receives a non-2xx final response, it does not
always immediately forward the response upstream towards the sender
of the associated request. Instead, the proxy "stores" the response
and waits for subsequent final responses from other remote
destinations where the associated request was forked. At some point
the proxy uses a specified mechanism to determine the "best" final
response code, and forwards a final response using that response code
upstream towards the sender of the associated request. When an
upstream SIP entity receives the non-2xx final response it will
release resources associated with the session. The UAC will
terminate, or retry, the session setup.
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Since the forking proxy does not always immediately forward non-2xx
final responses, upstream SIP entities (including the UAC that
initiated the request) are not immediately informed that an early
dialog has been terminated, and will therefore maintain resources
associated with the early dialog reserved until a final response is
sent by the proxy, even if the early dialog has already been
terminated. A SIP entity could use the resources for other things,
e.g. to accept subsequent early dialogs that it otherwise would
reject.
This specification defines a new SIP response code, 199 Early Dialog
Terminated. A forking proxy can send a 199 provisional response to
inform upstream SIP entities that an early dialog has been
terminated. A UAS can send a 199 response code, prior to sending a
non-2xx final response, for the same purpose. SIP entities that
receive the 199 response can use it to trigger the release of
resources associated with the terminated early dialog. In addition,
SIP entities might also use the 199 response to make policy related
decisions related to early dialogs.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Applicability and Limitation
The 199 response code is an optimization, and it only optimizes how
quickly recipients might be informed about terminated early dialogs.
The achieved optimization is limited. Since the response is normally
not sent reliably by an UAS, and can not be sent reliably when
generated and sent by a proxy, it is possible that some or all of the
199 responses get lost before they reach the recipients. In such
cases, recipients will behave the same as if the 199 response code
were not used at all.
One example for which a UAC could use the 199 response, is that when
it receives a 199 response it releases resources associated with the
terminated early dialog. The UAC could also use the 199 response to
make policy related decisions related to early dialogs. For example,
if a UAC is playing media associated with an early dialog, and the it
receives a 199 response indicating the early dialog has been
terminated, it could start playing media associated with a different
early dialog.
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Applications designers utilizing the 199 response code MUST ensure
that the application's user experience is acceptable if all 199
responses are lost, and not delivered to the recipients.
4. User Agent Client behavior
When a UAC sends an initial dialog initiation request, and if it is
willing to receive 199 responses, it MUST insert an "199" option-tag
in the Supported header field [RFC3261] of the request. The option-
tag indicates that the UAC supports, and is willing to receive, 199
responses. A UAC SHOULD NOT insert an "199" option-tag in the
Require or the Proxy-Require header field [RFC3261] of the request,
since in many cases it would result in unnecessary session
establishment failures.
NOTE: The UAC always needs to insert an "199" option-tag in the
Supported header field, in order to indicate that it supports, and is
willing to receive, 199 responses, even if it also inserts the
option-tag in the Require or Proxy-Require header field.
It is RECOMMENDED that a UAC does not insert an "100rel" option-tag
[RFC3262] in the Require header field when it also indicates support
of 199 responses, unless the UAC also uses some other SIP extension
or procedure that mandates it to do so. The reason is that proxies
are not allowed to generate and send 199 responses when the UAC has
required provisional responses to be sent reliably.
When a UAC receives a 199 response, it might release resources
associated with the terminated early dialog. A UAC might also use
the 199 response to make policy related decisions related to early
dialogs.
NOTE: The 199 response indicates that the early dialog has been
terminated, so there is no need for the UAC to send a BYE request in
order to terminate the early dialog when it receives the 199
response.
NOTE: The 199 response does not affect other early dialogs associated
with the session establishment. For those the normal SIP rules,
regarding transaction timeout etc, still apply.
Once a UAC has received and accepted a 199 response, it MUST NOT send
Any media associated with the early dialog. In addition, if the UAC
is able to associate received media with early dialogs, it MUST NOT
process any received media associated with the early dialog that was
terminated.
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If multiple usages [RFC5057] are used within an early dialog, and it
is not clear which dialog usage the 199 response terminates, SIP
entities that keep dialog state SHALL NOT release resources
associated with the early dialog when they receive the 199 response.
If a UAC receives an unreliably sent 199 response on a dialog which
has not previously been established (this can happen if a 199
response reaches the client before the 18x response that would
establish the early dialog) it SHALL discard the 199 responses. If a
UAC receives a reliably sent 199 response on a dialog which has not
previously been created, it MUST acknowledge the 199 response, as
described in RFC 3262 [RFC3262].
If a UAC has received a 199 response for all early dialogs, and no
early dialog associated session establishment remains, it maintains
the "Proceeding" state [RFC3261] and waits for possible subsequent
early dialogs to be established, and eventually for a final response
to be received.
5. User Agent Server behavior
If a UAS receives an initial dialog initiation request, with a
Supported header field that contains a "199" option-tag, it SHOULD
NOT send a 199 response on an early dialog associated with the
request, before it sends a non-2xx final response, unless it e.g. has
been configured to do so due to lack of support of the 199 response
code by forking proxies or other intermediate SIP entities, or it is
used in an environment that specifies that it shall send a 199
response before sending a non-2xx response.
NOTE: If a UAS has created multiple early dialogs associated with an
initial dialog initiation request (the UAS is acting similar to a
forking proxy), it does not always intend to send a final response on
all of those early dialogs.
NOTE: If the Require header field of an initial dialog initiation
request contains a "100rel" option-tag, proxies will not be able to
generate and send 199 responses. In such cases the UAS might choose
to send a 199 response on an early dialog, before it sends a non-2xx
final response, even if it would not do so in other cases.
If the Supported header field of an initial dialog initiation request
does not contain an "199" option-tag, the UAC MUST NOT send a 199
response on any early dialog associated with the request.
When a UAS generates a 199 response, the response MUST contain a To
header field tag parameter [RFC3261], in order for other entities to
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identify the early dialog that has been terminated. The UAS MUST
also insert a Reason header field [RFC3326] that contains a response
code which describes the reason why the early dialog was terminated.
The UAS MUST NOT insert a "199" option-tag in the Supported, Require
or Proxy-Require header field of the 199 response.
If a UAS intends to send 199 responses, and if it supports the
procedures defined in RFC 3840 [RFC3840], it MAY during the
registration procedure use the sip.extensions feature tag [RFC3840]
to indicate support of the 199 response code.
A 199 response SHOULD NOT contain an SDP offer/answer message body,
unless required by the rules in RFC 3264 [RFC3264].
According to RFC 3264, if an INVITE request does not contain an SDP
offer, and the 199 response is the first reliably sent response
associated with the request, the 199 response is required to contain
an SDP offer. In this case the UAS SHOULD send the 199 response
unreliably, or send the 199 response reliably and include an SDP
offer with no m- lines in the response.
Since a 199 response is only used for information purpose, the UAS
SHOULD send it unreliably, unless the "100rel" option-tag is present
in the Require header field of the associated request.
Once a UAS has sent a 199 response, it MUST NOT send or process any
media associated with the terminated early dialog.
6. Proxy behavior
When a proxy receives a 199 response to an initial dialog initiation
request, it MUST process the response as any other non-100
provisional response. The proxy will forward the response upstream
towards the sender of the associated request. The proxy MAY release
resources it has reserved associated with the early dialog that is
terminated. If a proxy receives a 199 response out of dialog, it
MUST process it as other non-100 provisional responses received out
of dialog.
When a forking proxy receives a non-2xx final response to an initial
dialog initiation request, that it recognizes as terminating one or
more early dialogs associated with the request, it MUST generate and
send a 199 response upstream for each of the terminated early dialogs
that satisfy each of the following conditions:
- the forking proxy does not intend to forward the final response
immediately (in accordance with rules for a forking proxy)
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- the UAC has indicated support (by inserting the "199" option-tag in
a Supported header field) of the 199 response code in the associated
request
- the UAC has not required provisional responses to be sent reliably
(by inserting the "100rel" option-tag in a Require or Proxy-Require
header field) in the associated request
- the forking proxy has not already received and forwarded a 199
response for the early dialog
- the forking proxy has not already sent a final response for any of
the early dialogs
As a consequence, once a final response to an initial dialog
initiation request has been issued by the proxy, no further 199
responses associated with the request will be generated or forwarded
by the proxy.
When a forking proxy forks an initial dialog initiation request, it
generates a unique Via header branch parameter value for each forked
leg. A proxy can determine whether additional forking has occurred
downstream of the proxy by storing the top Via branch value from each
response which creates an early dialog. If the same top Via branch
value is received for multiple early dialogs, the proxy knows that
additional forking has occurred downstream of the proxy. A non-2xx
final response received for a specific early dialog also terminates
all other early dialog for which the same top Via branch value was
received in the responses which created those early dialogs.
Based on implementation policy, a forking proxy MAY wait before
sending the 199 response, e.g. if it expects to receive a 2xx final
response on another dialog shortly after it received the non-2xx
final response which triggered the 199 response.
When a forking proxy generates a 199 response, the response MUST
contain a To header field tag parameter, that identifies the
terminated early dialog. A proxy MUST also insert a Reason header
field that contains the SIP response code of the response that
triggered the 199 response. The SIP response code in the Reason
header field informs the receiver of the 199 response about the SIP
response code that was used by the UAS to terminate the early dialog,
and the receiver might use that information for triggering different
types of actions and procedures. The proxy MUST NOT insert a "199"
option-tag in the Supported, Require or Proxy-Require header field of
the 199 response.
A forking proxy that supports generating of 199 responses MUST keep
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track of early dialogs, in order to determine whether to generate a
199 response when the proxy receives a non-2xx final response. In
addition, a proxy MUST keep track on which early dialogs it has
received and forwarded 199 responses, in order to not generate
additional 199 responses for those early dialogs.
If a forking proxy receives a reliably sent 199 response for a
dialog, for which it has previously generated and sent a 199
response, it MUST forward the 199 response. If a proxy receives an
unreliably sent 199 response, for which it has previously generated
and sent a 199 response, it MAY forward the response, or it MAY
discard it.
When a forking proxy generates and sends a 199 response, the response
SHOULD NOT contain a Contact header field or a Record-Route header
field [RFC3261].
If the Require header field of an initial dialog initiation request
contains an "100rel" option-tag, a proxy MUST NOT generate and send
199 responses associated with that request. The reason is that a
proxy is not allowed to generate and send 199 responses reliably.
7. Backward compatibility
Since all SIP entities involved in a session setup do not necessarily
support the specific meaning of the 199 Early Dialog Terminated
provisional response, the sender of the response MUST be prepared to
receive SIP requests and responses associated with the dialog for
which the 199 response was sent (a proxy can receive SIP messages
from either direction). If such request is received by a UA, it MUST
act in the same way as if it had received the request after sending
the final non-2xx response to the INVITE request, as specified in RFC
3261. A UAC that receives a 199 response for an early dialog MUST
NOT send any further requests on that dialog, except for requests
which acknowledge reliable responses. A proxy MUST forward requests
according to RFC 3261, even if the proxy has knowledge that the early
dialog has been terminated.
A 199 response does not "replace" a final response. RFC 3261
specifies when a final response is sent.
8. Usage with SDP offer/answer
A 199 response SHOULD NOT contain an SDP offer/answer [RFC3264]
message body, unless required by the rules in RFC 3264.
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If an INVITE request does not contain an SDP offer, and the 199
response is the first reliably sent response, the 199 response is
required to contain an SDP offer. In this case the UAS SHOULD send
the 199 response unreliable, or include an SDP offer with no m- lines
in a reliable 199 response.
9. Message Flow Examples
9.1. Example with a forking proxy which generates 199
The figure shows an example, where a proxy (P1) forks an INVITE
received from UAC. The forked INVITE reaches UAS_2, UAS_3 and UAS_4,
which send 18x provisional responses in order to establish early
dialogs between themselves and the UAC. UAS_2 and UAS_3 reject the
INVITE by sending a 4xx error response each. When P1 receives the
4xx responses it immediately sends 199 responses towards the UAC, to
indicate that the early dialogs for which it received the 4xx
responses have been terminated. The early dialog leg is shown in
parenthesis.
UAC P1 UAS_2 UAS_3 UAS_4
| | | | |
|-- INVITE -->| | | |
| |--- INVITE (2) ->| | |
| |--- INVITE (3) --------->| |
| |--- INVITE (4) ----------------->|
| |<-- 18x (2) -----| | |
|<- 18x (2) --| | | |
| |<-- 18x (3) -------------| |
|<- 18x (3) --| | | |
| |<-- 18x (4) ---------------------|
|<- 18x (4) --| | | |
| |<-- 4xx (2) -----| | |
| |--- ACK (2) ---->| | |
|<- 199 (2) --| | | |
| |<-- 4xx (3) -------------| |
| |--- ACK (3) ------------>| |
|<- 199 (3) --| | | |
| |<-- 200 (4) ---------------------|
|<- 200 (4) --| | | |
|-- ACK (4) ->| | | |
| |--- ACK (4) -------------------->|
| | | | |
Figure 1: Example call flow
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9.2. Example with a forking proxy which receives 200 OK
The figure shows an example, where a proxy (P1) forks an INVITE
request received from UAC. The forked request reaches UAS_2, UAS_3
and UAS_4, that all send 18x provisional responses in order to
establish early dialogs between themselves and the UAC. Later UAS_4
accepts the session and sends a 200 OK final response. When P1
receives the 200 OK responses it immediately forwards it towards the
UAC. P1 does not send 199 responses for the early dialogs from UAS_2
and UAS_3, since P1 has yet not received any final responses on those
early dialogs (even if P1 sends CANCEL requests to UAS_2 and UAS_3 P1
may still receive 200 OK final response from UAS_2 or UAS_3, that P1
would have to forward towards the UAC. The early dialog leg is shown
in parenthesis.
UAC P1 UAS_2 UAS_3 UAS_4
| | | | |
|-- INVITE -->| | | |
| |--- INVITE (2) ->| | |
| |--- INVITE (3) --------->| |
| |--- INVITE (4) ----------------->|
| |<-- 18x (2) -----| | |
|<- 18x (2) --| | | |
| |<-- 18x (3) -------------| |
|<- 18x (3) --| | | |
| |<-- 18x (4) ---------------------|
|<- 18x (4) --| | | |
| |<-- 200 (4) ---------------------|
|<- 200 (4) --| | | |
|-- ACK (4) ->| | | |
| |--- ACK (4) -------------------->|
| | | | |
Figure 2: Example call flow
9.3. Example with two forking proxies, of which one generates 199
The figure shows an example, where a proxy (P1) forks an INVITE
request received from UAC. One of the forked requests reaches UAS_2.
The other requests reach another proxy (P2), that forks the request
to UAS_3 and UAS_4. UAS_3 and UAS_4 send 18x provisional responses
in order to establish early dialogs between themselves and UAC.
Later UAS_3 and UAS_4 reject the INVITE request by sending a 4xx
error response each. P2 does not support the 199 response code, and
forwards a single 4xx response. P1 supports the 199 response code,
and when it receives the 4xx response from P2, it also manages to
associate the early dialogs from both UAS_3 and UAS_4 with the
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response. Therefore it generates and sends two 199 responses to
indicate that the early dialogs from UAS_3 and UAS_4 have been
terminated. The early dialog leg is shown in parenthesis.
UAC P1 P2 UAS_2 UAS_3 UAS_4
| | | | | |
|-- INVITE -->| | | | |
| |-- INVITE (2) ------------------>| | |
| |-- INVITE ---->| | | |
| | |--- INVITE (3) --------->| |
| | |--- INVITE (4) ----------------->|
| | |<-- 18x (3) -------------| |
| |<- 18x (3) ----| | | |
|<- 18x (3) --| | | | |
| | |<-- 18x (4) ---------------------|
| |<- 18x (4) ----| | | |
|<- 18x (4) --| | | | |
| | |<-- 4xx (3) -------------| |
| | |--- ACK (3) ------------>| |
| | |<-- 4xx (4) ---------------------|
| | |--- ACK (4) -------------------->|
| |<- 4xx (3) ----| | | |
| |-- ACK (3) --->| | | |
|<- 199 (3) --| | | | |
|<- 199 (4) --| | | | |
| |<- 200 (2) ----------------------| | |
|<- 200 (2) --| | | | |
|-- ACK (2) ->| | | | |
| |-- ACK (2) --------------------->| | |
| | | | | |
Figure 3: Example call flow
10. Security Considerations
General security issues related to SIP responses are described in RFC
3261. Due to the nature of the 199 response, it may be attractive to
use it for launching attacks in order to terminate specific early
dialogs (other early dialogs will not be affected). In addition, if
a man-in-the-middle generates and sends a 199 response, which
terminates a specific dialog, towards the UAC, it can take a while
until the UAS finds out that the UAC, and possible stateful
intermediates, have terminated the dialog. SIP security mechanisms
(e.g. hop-to-hop TLS) can be used to minimize, or eliminate, the risk
for such attacks.
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11. IANA Considerations
This section registers a new SIP response code and a new option-tag,
according to the procedures of RFC 3261.
11.1. IANA Registration of the 199 response code
This section registers a new SIP response code, 199. The required
information for this registration, as specified in RFC 3261, is:
RFC Number: RFC XXXX [[NOTE TO IANA: Please replace XXXX with the
RFC number of this specification]]
Response Code Number: 199
Default Reason Phrase: Early Dialog Terminated
11.2. IANA Registration of the 199 option-tag
This section registers a new SIP option-tag, 199. The required
information for this registration, as specified in RFC 3261, is:
Name: 199
Description: This option-tag is for indicating support of the 199
Early Dialog Terminated provisional response code. When present
in a Supported header of a request, it indicates that the UAC
supports the 199 response code. When present in a Require or
Proxy-Require header field of a request, it indicates that the
UAS, or proxies, MUST support the 199 response code. It does
not require the UAS, or proxies, to actually send 199
responses.
12. Acknowledgements
Thanks to Paul Kyzivat, Dale Worley, Gilad Shaham, Francois Audet,
Attila Sipos, Robert Sparks, Brett Tate, Ian Elz, Hadriel Kaplan,
Timothy Dwight, Dean Willis, Serhad Doken, John Elwell, Gonzalo
Camarillo, Adam Roach, Bob Penfield, Tom Taylor, Ya Ching Tan, Keith
Drage, Hans Erik van Elburg and Cullen Jennings for their feedback
and suggestions.
13. Change Log
[RFC EDITOR NOTE: Please remove this section when publishing]
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Changes from draft-ietf-sipcore-199-04
o "Usage with 100rel" section removed based on comments from John
Elwell (31.01.2011)
o Editorial corrections based on comments from Paul Kyzivat
(31.01.2011)
Changes from draft-ietf-sipcore-199-03
o RFC 3262 update removed
o Functional modification: proxy must not send 199 in case of
Require:100rel
o Recommendation that UAC does not require reliable provisional
responses with 199
o Clarification that Require:199 does not mandate the UAS to send a
199 response
o Clarification that a UAC needs to insert the 199 option-tag in a
Supported header field, even if it also inserts the option-tag in
a Require or Proxy-Require header field
o Editorial corrections
Changes from draft-ietf-sipcore-199-02
o Usage example section rewritten and clarified
o Requirement has been removed
o SIP has been added to document title
o Acronyms expanded in the abstract and throughout the document
o Editorial fixes throughout the document
o Indication added that document is aimed for standards track
o Some references made informative
o Additional text added regarding the usage of the Reason header
o SBC latching text has been removed
o Usage of Require/Proxy-Require header removed
o Additional text added regarding sending SDP offer in 199
o Note added, which clarifies that 199 does not affect other early
dialogs
o References added to Security Considerations
o Clarification of local ringing tone
o Clarification that media must not be sent or processed after 199
o Text regarding sending media on terminated dialogs added to
security section
o Change: UAS must send 199 reliably in case of Require:100rel
o Change: Section 4 of RFC 3262 updated
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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Internet-Draft 199 February 2011
[RFC3261] 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.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3326] Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
Header Field for the Session Initiation Protocol (SIP)",
RFC 3326, December 2002.
[RFC3840] Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
"Indicating User Agent Capabilities in the Session
Initiation Protocol (SIP)", RFC 3840, August 2004.
14.2. Informational References
[RFC3841] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Caller
Preferences for the Session Initiation Protocol (SIP)",
RFC 3841, August 2004.
[RFC5057] Sparks, R., "Multiple Dialog Usages in the Session
Initiation Protocol", RFC 5057, November 2007.
[3GPP.24.182]
3GPP, "IP Multimedia Subsystem (IMS) Customized Alerting
Tones (CAT); Protocol specification", 3GPP TS 24.182.
[3GPP.24.628]
3GPP, "Common Basic Communication procedures using IP
Multimedia (IM)Core Network (CN) subsystem; Protocol
specification", 3GPP TS 24.628.
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Author's Address
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
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