SIPPING G. Camarillo
Internet-Draft Ericsson
Intended status: Informational March 3, 2009
Expires: September 4, 2009
Re-INVITE Handling in the Session Initiation Protocol (SIP)
draft-camarillo-sipping-reinvite-00.txt
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Abstract
In this document, we clarify the handling of re-INVITEs in SIP. We
clarify in which situations a UAS (User Agent Server) should generate
a success response and in which situations a UAS should generate an
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error response to a re-INVITE. Additionally, we clarify issues
related to target refresh requests.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background on Re-INVITE Handling by UASs . . . . . . . . . . . 3
3. Clarifications on the Target Refresh Procedure . . . . . . . . 11
4. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
9. Normative References . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
As discussed in Section 14 of RFC 3261 [RFC3261], an INVITE request
sent within an existing dialog is known as a re-INVITE. A re-INVITE
is used to modify session parameters, dialog parameters, or both.
That is, a single re-INVITE can change both the parameters of its
associated session (e.g., changing the IP address where a media
stream is received) and the parameters of its associated dialog
(e.g., changing the remote target of the dialog). A re-INVITE can
change the remote target of a dialog because it is a target refresh
request, as defined in Section 6 of RFC 3261 [RFC3261].
There has been some confusion among implentors regarding how a UAS
(User Agent Server) should handle re-INVITEs. In particular,
implementors requested clarifications on which type of response a UAS
should generate in different situations. In this document, we
clarify these issues.
OPEN ISSUE: this document is written as an informational document but
could be turned into a normative document by adding a few normative
statements if needed.
2. Background on Re-INVITE Handling by UASs
A UAS receiving a re-INVITE will need to, eventually, generate a
response to it. Some re-INVITEs can be responded to immediately
because their handling does not require user interaction (e.g.,
changing the IP address where a media stream is received). The
handling of other re-INVITEs requires user interaction (e.g., adding
a video stream to an audio-only session). Therefore, these re-
INVITEs cannot be responded to immediately.
An error response to a re-INVITE has clear semantics. As specified
in Section 12.2.2 of RFC 3261 [RFC3261], if a re-INVITE is rejected,
none of the state changes requested by the re-INVITE are performed.
The example in Figure 1 illustrates this point.
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UAC UAS
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------------(2) 200 OK SDP2----------------|
| |
|------------------(3) ACK------------------>|
| |
| |
|-------------(4) INVITE SDP3--------------->|
| |
|<-----------------(5) 6xx-------------------|
| |
|------------------(6) ACK------------------>|
| |
Figure 1: Rejection of a re-INVITE
The UAs perform an offer/answer exchange to establish an audio-only
session:
SDP1:
m=audio 30000 RTP/AVP 0
SDP2:
m=audio 31000 RTP/AVP 0
At a later point, the UAC sends a re-INVITE (4) in order to add a
video stream to the session.
SDP3:
m=audio 30000 RTP/AVP 0
m=video 30002 RTP/AVP 31
The UAS is automatically configured to reject video streams.
Consequently, the UAS returns an error response (5). At that point,
the session parameters in use are still those resulting from the
initial offer/answer exchange, which are described by SDP1 and SDP2.
That is, the session and dialog states are the same as before the re-
INVITE was received.
In the previous example, the UAS wanted to reject all the changes
requested in the re-INVITE. That is why generating an error response
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was appropriate. However, there are situations where a UAS wants to
accept some but not all the changes requested in a re-INVITE. In
these cases, the UAS generates a 200 (OK) response with an SDP
indicating which changes were accepted and which were not. The
example in Figure 2 illustrates this point.
UAC UAS
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------------(2) 200 OK SDP2----------------|
| |
|------------------(3) ACK------------------>|
| |
| |
|-------------(4) INVITE SDP3--------------->|
| |
|<------------(5) 200 OK SDP4----------------|
| |
|------------------(6) ACK------------------>|
| |
Figure 2: Automatic rejection of a video stream
The UAs perform an offer/answer exchange to establish an audio only
session:
SDP1:
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.1
SDP2:
m=audio 31000 RTP/AVP 0
c=IN IP4 192.0.2.5
At a later point, the UAC moves to an access that provides a higher-
bandwidth. Therefore, the UAC sends a re-INVITE (4) in order to
change the target of the dialog to its new location, change the IP
address where it receives the audio stream to its new IP address, and
add a video stream to the session.
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SDP3:
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.2
m=video 30002 RTP/AVP 31
c=IN IP4 192.0.2.2
The UAS is automatically configured to reject video streams.
However, the UAS needs to accept the change of the remote dialog
target and the change of the audio stream's remote IP address.
Consequently, the UAS returns a 200 (OK) response and sets the port
of the video stream to zero in its SDP.
SDP4:
m=audio 31000 RTP/AVP 0
c=IN IP4 192.0.2.5
m=video 0 RTP/AVP 31
c=IN IP4 192.0.2.2
In the previous example, the UAS was configured to automatically
reject the addition of video streams. The example in Figure 3
assumes that the UAS requires its user's input in order to accept or
reject the addition of a video stream and uses reliable provisional
responses [RFC3262] (PRACK transactions are not shown for clarity).
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UAC UAS
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------------(2) 200 OK SDP2----------------|
| |
|------------------(3) ACK------------------>|
| |
| |
|-------------(4) INVITE SDP3--------------->|
| |
|<----(5) 183 Session Progress SDP4----------|
| |
| |
|<------------(6) UPDATE SDP5----------------|
| |
|-------------(7) 200 OK SDP6--------------->|
| |
|<---------------(8) 200 OK------------------|
| |
|------------------(9) ACK------------------>|
| |
Figure 3: Rejection of a video stream by the user
Everything up to (4) is identical to the previous example. In (5),
the UAS accepts the change of the audio stream's remote IP address
but does not accept the video stream yet (it provides a null IP
address instead of setting the stream to 'inactive' because inactive
streams still need to exchange RTCP traffic).
SDP4:
m=audio 31000 RTP/AVP 0
c=IN IP4 192.0.2.5
m=video 31002 RTP/AVP 31
c=IN IP4 0.0.0.0
At a later point, the UAS's user rejects the addition of the video
stream. Consequently, the UAS sends an UPDATE request setting the
port of the video stream to zero in its SDP.
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SDP5:
m=audio 31000 RTP/AVP 0
c=IN IP4 192.0.2.5
m=video 0 RTP/AVP 31
c=IN IP4 0.0.0.0
The UAS now returns a 200 (OK) response to the re-INVITE. Note that
if the UAS returned an error response at this point, the media state
would need to revert to the state before the re-INVITE. That is,
even if the change of the IP address of the audio stream had already
been accepted and executed within the re-INVITE, the audio stream
would need to go back to the old IP address. Using an error response
in this situation has a set of problems, which are discussed in
Section 4.
The example in Figure 4 also assumes that the UAS requires its user's
input in order to accept or reject the addition of a video stream and
uses reliable provisional responses [RFC3262] (PRACK transactions are
not shown for clarity).
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UAC UAS
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------------(2) 200 OK SDP2----------------|
| |
|------------------(3) ACK------------------>|
| |
| |
|-------------(4) INVITE SDP3--------------->|
| |
|<----(5) 183 Session Progress SDP4----------|
| |
|-------------(6) UPDATE SDP5--------------->|
| |
|<------------(7) 200 OK SDP6----------------|
| |
| |
|<------------(8) UPDATE SDP7----------------|
| |
|-------------(9) 200 OK SDP8--------------->|
| |
|<--------------(10) 200 OK------------------|
| |
|-----------------(11) ACK------------------>|
| |
Figure 4: Rejection of a video stream by the user
The UAs perform an offer/answer exchange to establish an audio only
session:
SDP1:
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.1
SDP2:
m=audio 31000 RTP/AVP 0
c=IN IP4 192.0.2.5
At a later point, the UAC sends a re-INVITE (4) in order to add a new
codec to the audio stream and to add a video stream to the session.
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SDP3:
m=audio 30000 RTP/AVP 0 3
c=IN IP4 192.0.2.1
m=video 30002 RTP/AVP 31
c=IN IP4 192.0.2.1
In (5), the UAS accepts the addition of the audio codec but does not
accept the video stream yet (it provides a null IP address instead of
setting the stream to 'inactive' because inactive streams still need
to exchange RTCP traffic).
SDP4:
m=audio 31000 RTP/AVP 0 3
c=IN IP4 192.0.2.5
m=video 31002 RTP/AVP 31
c=IN IP4 0.0.0.0
At a later point, the UAC sends an UPDATE request (6) to remove the
original audio codec from the audio stream (the UAC could have also
used the PRACK to (5) to request this change).
SDP5:
m=audio 30000 RTP/AVP 3
c=IN IP4 192.0.2.1
m=video 30002 RTP/AVP 31
c=IN IP4 192.0.2.1
SDP6:
m=audio 31000 RTP/AVP 3
c=IN IP4 192.0.2.5
m=video 31002 RTP/AVP 31
c=IN IP4 0.0.0.0
Yet at a later point, the UAS's user rejects the addition of the
video stream. Additionally, the UAS decides to revert to the
original audio codec. Consequently, the UAS sends an UPDATE request
(8) setting the port of the video stream to zero and offering the
original audio codec in its SDP.
SDP7:
m=audio 31000 RTP/AVP 0
c=IN IP4 192.0.2.5
m=video 0 RTP/AVP 31
c=IN IP4 0.0.0.0
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The UAC accepts the change in the audio codec in its 200 (OK)
response (9) to the UPDATE request.
SDP5:
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.1
m=video 0 RTP/AVP 31
c=IN IP4 192.0.2.1
The UAS now returns a 200 (OK) response (10) to the re-INVITE. Note
that the media state after this 200 (OK) response is the same as the
pre-re-INVITE media state. Section 4 discusses why the UAS uses an
UPDATE request (8) and a 200 (OK) response (10) to the re-INVITE
instead of an error response to the re-INVITE in order to go back to
the pre-re-INVITE media state.
3. Clarifications on the Target Refresh Procedure
Both re-INVITEs and UPDATE [RFC3311] requests are target refresh
requests for INVITE-initiated dialogs. There is some confusion
regarding how target refresh requests are handled within a re-INVITE.
Section 5.1 of [RFC3311] specifies how to handle UPDATE requests
within re-INVITEs but does not describe all possible situations in
detail. In this section, we clarify those situations that are more
likely to be unclear from the specification.
When a UAC sends a re-INVITE refreshing the target of the dialog, the
UAS needs to start using the new target immediately. For example, if
the UAS needs to send an UPDATE request within the re-INVITE, the UAS
will use the new target. When the UAS sends a request to the new
target, it is implicitly accepting the refresh of the target.
Therefore, as discussed in Section 4, the UAS will respond to the re-
INVITE with a 200 (OK) response, even if it needs to reject media
changes in the response's SDP.
A UAC can send a re-INVITE refreshing the target of the dialog and
later send an UPDATE request within the re-INVITE refreshing the
target again. In this case, a 200 (OK) response to the re-INVITE
would accept the latest target refresh within the re-INVITE. That
is, the target refresh performed by the UPDATE request.
4. UAS Behavior
UASs should only return an error response to a re-INVITE if no
changes to the session or to the dialog state have been performed
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since the re-INVITE was received. Such an error response indicates
that no changes have been made as a result of the re-INVITE or any
other transaction within it.
If changes requested by a re-INVITE or any transaction within it have
already been performed, the UAS should always return a 200 (OK)
response. Even if the UAS would like to revert to the pre-INVITE
state, it would still return a 200 (OK) to the INVITE request. In
order to undo changes that were already executed, the UAS uses a new
offer/answer exchange or a target refresh request. The reason for
not using an error response to undo already executed changes is that
an error response to a re-INVITE for which changes have already been
executed is effectively requesting a change in the session or the
dialog state. However, the UAC has no means to reject those changes
if it is unable to execute them. That is, if the UAC is unable to
revert to the pre-re-INVITE state, it will not be able to communicate
this fact to the UAS. On the other hand, if the UAS uses an offer or
a target refresh request to request changes in the session, the UAC
will able to reject the changes if it is unable to execute them.
Using an error response to undo already executed changes presents an
additional problem. SIP includes rules to avoid offer/answer
collisions in race conditions. Even when both UAs generate an offer
at the same time, there are rules to determine which one should be
processed first. However, there are no rules to avoid a collision
between an offer in an UPDATE request and an error response for a re-
INVITE. Since both the UPDATE request and the error response would
request changes, it would not be clear which changes would need to be
executed first. This is yet another reason why UASs should not use
error responses to undo already executed changes.
A change to the session state is considered to have been executed
when the new media parameters are being used. Therefore, a change to
a stream subject to preconditions [RFC4032] is considered to have
been executed when the new media parameters start being used, not
when the preconditions for the stream are met. See
[draft-camarillo-sipping-precons-00.txt] for a more-detailed
discussion on this issue.
5. UAC Behavior
The behavior of a UAC communicating with a well-behaved UAS (a UAS
that follows the guidelines in Section 4 is straight forward.
However, a UAC may face a legacy UAS that uses an error response to
undo already executed changes within a re-INVITE.
In order to cope with this type of UAS, a UAC that receives an error
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response to a re-INVITE but cannot revert to the media state before
the re-INVITE for some reason should perform a new offer/answer
exchange in order to set the parameters of the session and the dialog
so that the session can continue. This new offer/answer exchange
should contain the minimum set of changes needed to continue the
session in order to minimize the chances of the UAS rejecting it as
well.
If the error response reverted an already executed target refresh,
the UAC should use a new target refresh request in order to set the
target to an appropriate value.
6. Security Considerations
This document does not introduce any new security issue. Security
issues related to re-INVITEs and UPDATE requests are discussed in RFC
3261 [RFC3261] and RFC 3311 [RFC3311].
7. IANA Considerations
There are no IANA actions associated with this document.
8. Acknowledgements
Paul Kyzivat, Christer Holmberg, and Yang Gao provided useful ideas
on the topics discussed in this document.
9. Normative References
[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.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
[RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session
Initiation Protocol (SIP) Preconditions Framework",
RFC 4032, March 2005.
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Author's Address
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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