SIPCORE G. Camarillo, Ed.
Internet-Draft C. Holmberg
Updates: 3261 (if approved) Ericsson
Intended status: Standards Track Y. Gao
Expires: July 23, 2010 ZTE
January 19, 2010
Re-INVITE and Target-refresh Request Handling in the Session Initiation
Protocol (SIP)
draft-ietf-sipcore-reinvite-01.txt
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
error response to a re-INVITE. Additionally, we clarify issues
related to target-refresh requests.
Status of this Memo
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Re-INVITE Handling . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Background on Re-INVITE Handling by UASs . . . . . . . . . 4
3.2. Problems with Error Responses and Already-executed
Changes . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . 9
3.4. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . 10
3.5. Glare Situations . . . . . . . . . . . . . . . . . . . . . 10
3.6. Example of UAS Behavior . . . . . . . . . . . . . . . . . 11
3.7. Example of UAC Behavior . . . . . . . . . . . . . . . . . 14
3.8. Clarifications on Cancelling Re-INVITEs . . . . . . . . . 16
4. Target-refresh Handling . . . . . . . . . . . . . . . . . . . 17
4.1. Background on Target-refresh Requests . . . . . . . . . . 17
4.2. Clarification on the Atomicity of Target-Refresh
Requests . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.3. UAC Behavior . . . . . . . . . . . . . . . . . . . . . . . 18
4.4. UAS Behavior . . . . . . . . . . . . . . . . . . . . . . . 18
4.5. Race Conditions and Target Refreshes . . . . . . . . . . . 19
5. Re-INVITE Transaction Routing . . . . . . . . . . . . . . . . 20
5.1. Background on re-INVITE Transaction Routing . . . . . . . 20
5.2. Problems with UAs Losing their Contact . . . . . . . . . . 20
5.3. UAS Losing its Contact: UAC Behavior . . . . . . . . . . . 20
5.4. UAC Losing its Contact: UAS Behavior . . . . . . . . . . . 21
5.5. UAC Losing its Contact: UAC Behavior . . . . . . . . . . . 22
6. Security Considerations . . . . . . . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
9. Normative References . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23
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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].
A re-INVITE transaction has an offer/answer [RFC3264] exchange
associated to it. The UAC (User Agent Client) generating a given re-
INVITE can act as the offerer or as the answerer. A UAC willing to
act the offerer includes an offer in the re-INVITE. The UAS then
provides an answer in a response to the re-INVITE. A UAC willing to
act as answerer does not include an offer in the re-INVITE. The UAS
then provides an offer in a response to the re-INVITE becoming, thus,
the offerer.
Certain transactions within a re-INVITE (e.g., UPDATE [RFC3311]
transactions) can also have offer/answer exchanges associated to
them. A UA (User Agent) can act as the offerer or the answerer in
any of these transactions regardless of whether the UA was the
offerer or the answerer in the umbrella re-INVITE transaction.
There has been some confusion among implentors regarding how a UAS
(User Agent Server) should handle re-INVITEs. In particular,
implementors requested clarification on which type of response a UAS
should generate in different situations. In this document, we
clarify these issues.
Additionally, there has also been some confusion among implementors
regarding target refresh requests, which include but are not limited
to re-INVITEs. In this document, we also clarify the process by
which remote targets are refreshed.
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].
UA: User Agent.
UAC: User Agent Client.
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UAS: User Agent Server.
3. Re-INVITE Handling
The following sections discuss re-INVITE handling.
3.1. 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 the following semantics. As
specified in Section 12.2.2 of RFC 3261 [RFC3261], if a re-INVITE is
rejected, no state changes are performed. These state changes
include state changes associated to the re-INVITE transaction and all
other transactions within the re-INVITE (target refreshes, which are
discussed in Section 4.1, are an exception to this rule because in
certain cases they are performed even if the re-INVITE is rejected).
That is, the session and dialog states are the same as before the re-
INVITE was received. The example in Figure 1 illustrates this point.
UAC UAS
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------------(2) 200 OK SDP2----------------|
| |
|------------------(3) ACK------------------>|
| |
| |
|-------------(4) INVITE SDP3--------------->|
| |
|<-----------------(5) 4xx-------------------|
| |
|------------------(6) ACK------------------>|
| |
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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 rejected all the changes requested
in the re-INVITE by returning an error response. 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.
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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 IP address where it receives the audio stream to its new
IP address, and add a video stream to the session.
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 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.
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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).
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).
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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.
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.
In all the previous examples, the UAC was the offerer in the re-
INVITE transaction. Examples with UACs acting as the answerers would
be similar.
3.2. Problems with Error Responses and Already-executed Changes
Section 3.1 contains examples on how a UAS rejects all the changes
requested in a re-INVITE without executing any of them by returning
an error response (Figure 1), and how a UAS executes some of the
changes requested in a re-INVITE and rejects some of them by
returning a 2xx response (Figure 2 and Figure 3). A UAS can accept
and reject different sets of changes simultaneously (Figure 2) or at
different times (Figure 3).
The scenario that created confusion among implementors consists of a
UAS that receives a re-INVITE, executes some of the changes requested
in it, and then wants to reject all those already-executed changes
and revert to the pre-re-INVITE state. Such a UAS may consider
returning an error response to the re-INVITE (the message flow would
be similar to the one in Figure 1), or using an UPDATE request to
revert to the pre-re-INVITE state and then returning a 2xx response
to the re-INVITE (the message flow would be similar to the one in
Figure 3). This section explains the problems associated with
returning an error response in these circumstances. In order to
avoid these problems, the UAS should use the latter option (UPDATE
request plus a 2xx response). Section 3.3 and Section 3.4 contain
the normative statements needed to avoid these problems.
The reason for not using an error response to undo already executed
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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.
3.3. 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 executed
since the re-INVITE was received. Such an error response indicates
that no changes have been executed as a result of the re-INVITE or
any other transaction within it.
If any of the changes requested in a re-INVITE or in any transaction
within it have already been executed (with the exception of target
refreshes), the UAS SHOULD return a 2xx response.
A change to the session state is considered to have been executed if
an offer/answer without preconditions [RFC4032] for the stream has
completed successfully or the UAs have exchanged media using the new
parameters. Connection establishment messages (e.g., TCP SYN),
connectivity checks (e.g., when using ICE [I-D.ietf-mmusic-ice]), and
any other messages used in the process of meeting the preconditions
for a stream are not considered media.
The successful completion of an offer/answer exchange without
preconditions indicates that the new parameters for the media stream
are already considered to be in use. The successful completion of an
offer/answer exchange with preconditions means something different.
The fact that all mandatory preconditions for the stream are met
indicates that the new parameters for the media stream are ready to
be used. However, they will not actually be used until the UAS
decides so. During a session establishment, the UAS can wait for
using the media parameters until the callee starts being alerted or
until the callee accepts the session. During a session modification,
the UAS can wait until its user accepts the changes to the session.
When dealing with streams where the UAS sends media more or less
continuously, the UAC notices that the new parameters are in use
because the UAC receives media that uses the new parameters.
However, this mechanism does not work with other types of streams.
Therefore, it is RECOMMENDED that when a UAS decides to start using
the new parameters for a stream for which all mandatory preconditions
have been met, the UAS either sends media using the new parameters or
sends a new offer where the precondition-related attributes for the
stream have been removed. As indicated above, the successful
completion of an offer/answer exchange without preconditions
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indicates that the new parameters for the media stream are already
considered to be in use.
The point a change to the dialog state is considered to have been
executed depends on the particular dialog parameter being modified.
The specifications of different dialog parameters describe when the
new value of the parameter needs to be taken into use.
3.4. UAC Behavior
A UAC that receives an error response to a re-INVITE that undoes
already-executed changes within the re-INVITE may be facing a legacy
UAS that does not support this specification (i.e., a UAS that does
not follow the guidelines in Section 3.3). There are also certain
race condition situations that get both user agents out of
synchronization. In order to cope with these race condition
situations, a UAC that receives an error response to a re-INVITE for
which changes have been already executed SHOULD generate a new re-
INVITE or UPDATE request in order to make sure that both UAs have a
common view of the state of the dialog and the session (the UAC uses
the criteria in Section 3.3 in order to decide whether or not changes
have been executed for the stream). The purpose of this new offer/
answer exchange is to synchronize both UAs, not to request changes
that the UAS may choose to reject. Therefore, the dialog parameters
and the session parameters in the offer/answer exchange SHOULD be as
close as those in the pre-re-INVITE state as possible.
3.5. Glare Situations
Section 4 of RFC 3264 [RFC3264] specifies rules to avoid and detect
glare situations (i.e., to avoid offer/answer collisions in race
conditions). Section 14.1 of RFC 3261 [RFC3261] specifies general
rules to handle glare situations in SIP. Section 5.1 of RFC 3311
[RFC3311] specifies when UPDATE requests can be sent. The specified
rules include, among other things, procedures to cope with situations
where both UAs generate an offer at the same time. However, there
are no rules to avoid a collision between an offer in an UPDATE
request and an error response to a re-INVITE. Since both the UPDATE
request and the error response could be requesting changes, it would
not be clear which changes would need to be executed first. The
following rules avoid types of glare conditions that were not covered
by previous specifications.
When checking for glare situations, UAs MUST treat the exchange of a
non-2xx final response to a re-INVITE and its corresponding ACK
request as an offer/answer exchange. Consequently, the rules
regarding glare situations applicable to offer/answer exchanges are
also applicable to those exchanges. These rules imply that if the
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UAS of a re-INVITE transaction receives and UPDATE request with an
offer after having sent a non-2xx final response to the re-INVITE but
before having received the corresponding ACK request, the UA SHOULD
return a 491 (Request Pending) response to the UPDATE request. If
the UAC of a re-INVITE transaction sends an UPDATE request with an
offer, receives a non-2xx response to the re-INVITE, and then a 2xx
response to the UPDATE request, the UA SHOULD generate a new re-
INVITE or UPDATE request in order to make sure that both UAs have a
common view of the state of the session, as described in Section 3.4.
An UPDATE request without an offer can change dialog parameters. So
can a non-2xx final response to a re-INVITE request or a 2xx response
to an INVITE request (re-INVITE or initial INVITE). However, there
are no rules to avoid a collision between an offerless UPDATE request
and a final response to an INVITE request. The rules in Section 4.5,
which are given in the context of target refreshes, cover these types
of collisions as well. Therefore, there is no need to specify
further rules here.
3.6. Example of UAS Behavior
This section contains an example of a UAS that implements this
specification using an UPDATE request and a 2xx response to a re-
INVITE in order to revert to the pre-re-INVITE state. The example,
which is shown in Figure 4, 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.
SDP8:
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.
3.7. Example of UAC Behavior
Figure 5 shows an example of a race condition situation in which the
UAs end up with different views of the state of the session. The UAs
in Figure 5 are involved in a session that, just before the message
flows in the figures starts, includes a sendrecv audio stream and an
inactive video stream. UA1 sends a re-INVITE (1) requesting to make
the video stream sendrecv.
SDP1:
m=audio 20000 RTP/AVP 0
a=sendrecv
m=video 20002 RTP/AVP 31
a=sendrecv
UA2 is configured to automatically accept incoming video streams but
to ask for user input before generating an outgoing video stream.
Therefore, UAS2 makes the video stream recvonly by returning a 183
(Session Progress) response (2).
SDP2:
m=audio 30000 RTP/AVP 0
a=sendrecv
m=video 30002 RTP/AVP 31
a=recvonly
When asked for input, UA2's user chooses not to have either incoming
or outgoing video. In order to make the video stream inactive, UA2
returns a 4xx error response (5) to the re-INVITE. The ACK request
(6) for this error response is generated by the proxy between both
user agents. Note that this error response undoes already-executed
changes. So, UA2 is a legacy UA that does not support this
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specification.
The proxy relays the 4xx response (7) towards UA1. However, the 4xx
response (7) takes time to arrive to UA1 (e.g., the response may have
been sent over UDP and the first few retransmissions were lost). In
the meantime, UA2's user decides to put the audio stream on hold.
UA2 sends an UPDATE request (8) making the audio stream recvonly.
The video stream, which is inactive, is not modified and, thus,
continues being inactive.
SDP3:
m=audio 30000 RTP/AVP 0
a=recvonly
m=video 30002 RTP/AVP 31
a=inactive
The proxy relays the UPDATE request (9) to UA1. The UPDATE request
(9) arrives at UA1 before the 4xx response (7) that had been
previously sent. UA2 accepts the changes in the UPDATE request and
returns a 200 (OK) response (10) to it .
SDP4: m=audio 20000 RTP/AVP 0 a=sendonly m=video 30002 RTP/AVP 31
a=inactive
At a later point, the 4xx response (7) finally arrives at UA1. This
response makes the session return to its pre-re-INVITE state.
Therefore, for UA1, the audio stream is sendrecv and the video stream
is inactive. However, for UA2, the audio stream is recvonly (the
video stream is also inactive).
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a:sendrecv a:sendrecv
v:inactive v:inactive
UA1 Proxy UA2
| | |
|----(1) INVITE SDP1-->| |
| |----(2) INVITE SDP1-->|
| | |
| |<----(3) 183 SDP2-----| a:sendrecv
a:sendrecv |<----(4) 183 SDP2-----| | v:recvonly
v:sendonly | | |
| |<------(5) 4xx -------|
| |-------(6) ACK ------>| a:sendrecv
| +-(7) 4xx -| | v:inactive
| | |<---(8) UPDATE SDP3---|
|<---(9) UPDATE SDP3---| |
| | | |
a:sendonly |---(10) 200 OK SDP4-->| |
v:inactive | | |---(11) 200 OK SDP4-->| a:recvonly
|<-(7) 4xx -+ | | v:inactive
a:sendrecv |------(12) ACK ------>| |
v:inactive | | |
a: status of the audio stream
v: status of the video stream
Figure 5: Message flow with race condition
After the message flow in Figure 5, following the recommendations in
this section, when UA1 received an error response (7) that undid
already-executed changes, UA1 would generate an UPDATE request with
an SDP reflecting the pre-re-INVITE state (i.e., sendrecv audio and
inactive video). UA2 could then return a 200 (OK) response to the
UPDATE request making the audio stream recvonly, which is the state
UA2's user had requested. Such an UPDATE transaction would get the
UAs back into synchronization.
3.8. Clarifications on Cancelling Re-INVITEs
Section 9.2 of RFC 3261 [RFC3261] specifies the behavior of a UAS
responding to a CANCEL request. Such a UAS responds to the INVITE
request with a 487 (Request Terminated) at the 'should' level. Per
the rules specified in Section 3.3, if the INVITE request was a re-
INVITE and some of its requested changes had already been executed,
the UAS would return a 2xx response instead.
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4. Target-refresh Handling
The following sections discuss target-refresh request handling.
4.1. Background on Target-refresh Requests
A target-refresh request is defined as follows in Section 6 of RFC
3261 [RFC3261]:
"A target-refresh request sent within a dialog is defined as a
request that can modify the remote target of the dialog."
Additionally, 2xx responses to target-refresh requests can also
update the remote target of the dialog. As discussed in Section 12.2
of RFC 3261 [RFC3261], re-INVITEs are target-refresh requests.
RFC 3261 [RFC3261] specifies the behavior of UASs receiving target-
refresh requests and of UACs receiving a 2xx response for a target-
refresh request.
Section 12.2.2 of RFC 3261 [RFC3261] says:
"When a UAS receives a target-refresh request, it MUST replace the
dialog's remote target URI with the URI from the Contact header
field in that request, if present."
Section 12.2.1.2 of RFC 3261 [RFC3261] says:
"When a UAC receives a 2xx response to a target-refresh request,
it MUST replace the dialog's remote target URI with the URI from
the Contact header field in that response, if present."
The fact that re-INVITEs can be long-lived transactions and can have
other transactions within them makes it necessary to revise these
rules. Section 4.2 specifies new rules for the handing of target-
refresh requests. Note that the new rules apply to any target-
refresh request, not only to re-INVITEs.
4.2. Clarification on the Atomicity of Target-Refresh Requests
The remote target of a dialog is a special type of state information
because of its essential role in the exchange of SIP messages between
UAs in a dialog. A UA involved in a dialog receives the remote
target of the dialog from the remote UA. The UA uses the remote
target to send SIP requests to the remote UA.
The remote target is a piece of state information that is not meant
to be negotiated. When a UAC changes its address, the UAC simply
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communicates its new address to the UAS in order to remain reachable
by the UAS. UAs need to follow the behavior specified in Section 4.3
and Section 4.4 of this specification instead of that specified in
RFC 3261 [RFC3261], which was discussed in Section 4.1. The new
behavior regarding target-refresh requests implies that a target-
refresh request can, in some cases, update the remote target even if
the request is responded with a final error response. This means
that target-refresh requests are not atomic.
4.3. UAC Behavior
Behavior of a UAC after having sent a target-refresh request updating
the remote target:
If the UAC receives an error response to the target-refresh request,
the UAS has not updated its remote target.
This allows UASs to authenticate target-refresh requests.
If the UAC receives a reliable provisional response or a 2xx response
to the target-refresh request, or the UAC receives a request on the
new target, the UAS has updated its remote target. The UAC can
consider the target refresh operation completed.
Even if the target request was a re-INVITE and the final response
to the re-INVITE was an error response, the UAS would not revert
to the pre-re-INVITE remote target.
If the UAC receives a reliable provisional response or a 2xx response
to the target-refresh request, the UAC MUST replace the dialog's
remote target URI with the URI from the Contact header field in that
response, if present.
When interacting with a UACs that does not support reliable
provisional responses or UPDATE requests, a UAC SHOULD NOT use the
same target refresh request to refresh the target and to make session
changes unless the session changes can be trivially accepted by the
UAS (e.g., an IP address change). Piggybacking a target refresh with
more complicated session changes in this situation would make it
unnecessarily complicated for the UAS to accept the target refresh
while rejecting the session changes.
4.4. UAS Behavior
Behavior of a UAS after having received a target-refresh request
updating the remote target:
If the UAS receives a target-refresh request that has been properly
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authenticated, the UAS SHOULD generate a reliable provisional
response or a 2xx response to the target-refresh request. If
generating such responses is not possible (e.g., the UAS does not
support reliable provisional responses and needs user input before
generating a final response), the UAS SHOULD send a request to the
UAC using the new remote target (if the UAS does not need to send a
request for other reasons, the UAS can send an UPDATE request). On
sending a reliable provisional response or a 2xx response to the
target-refresh request, or a request to the new remote target, the
UAS MUST replace the dialog's remote target URI with the URI from the
Contact header field in the target-refresh request.
Reliable provisional responses in SIP are specified in RFC 3262
[RFC3262]. In this document, reliable provisional responses are
those that use the mechanism defined in RFC 3262 [RFC3262] on any
other SIP-based mechanism that may be specified in the future.
Other specifications may define ways to send provisional responses
reliably using non-SIP mechanisms (e.g., using media-level
messages to acknowledge the reception of the SIP response). For
the purposes of this document, provisional responses using those
non-SIP mechanisms are considered unreliable responses.
If instead sending a reliable provisional response or a 2xx response
to the target-refresh request, or a request to the new target, the
UAS generates an error response to the target-refresh request, the
UAS MUST NOT update its dialog's remote target.
4.5. Race Conditions and Target Refreshes
SIP provides request ordering by using the Cseq header field. That
is, a UAS that receives two requests at roughly the same time can
know which one is newer. However, SIP does not provide ordering
between responses and requests. For example, if a UA receives a 200
(OK) response to an UPDATE request and an UPDATE request at roughly
the same time, the UA cannot know which one was sent last. Since
both messages can refresh the remote target, the UA needs to know
which message was sent last in order to know which remote target
needs to be used.
Some of the procedures discussed in Section 3.5 could avoid these
types of situations. However, they are currently defined only for
SIP messages involved in offer/answer exchanges (e.g., the procedures
do not apply to an UPDATE request that does not carry an offer). The
following rules make those procedures applicable to the race
conditions described above so that UASs can cope with them.
When checking for glare situations, UAs MUST treat every UPDATE
request as if it contained an offer. Additionally, UAs MUST treat
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the exchange of a 2xx response to an INVITE request and its
corresponding ACK request as an offer/answer exchange. Consequently,
the rules regarding glare situations applicable to offer/answer
exchanges are also applicable to those exchanges.
5. Re-INVITE Transaction Routing
The following sections discuss re-INVITE transaction routing.
5.1. Background on re-INVITE Transaction Routing
Re-INVITEs are routed using the dialog's route set, which contains
all the proxy servers that need to be traversed by requests send
within the dialog. Responses to the re-INVITE are routed using the
Via entries in the re-INVITE.
ACK requests for 2xx responses and for non-2xx final responses are
generated in different ways. As specified in Sections 14.1 and
13.2.1 of RFC 3261 [RFC3261], ACK requests for 2xx responses are
generated by the UAC core and are routed using the dialog's route
set. As specified in Section 17.1.1.2 of RFC 3261 [RFC3261], ACK
requests for non-2xx final responses are generated by the INVITE
client transaction (i.e., they are generated in a hop-by-hop fashion
by the proxy servers in the path) and are sent to the same transport
address as the re-INVITE.
5.2. Problems with UAs Losing their Contact
Refreshing the dialog's remote target during a re-INVITE transaction
(see Section 4.2) presents some issues because of the fact that Re-
INVITE transactions can be long lived. As described in Section 5.1,
the way responses to the re-INVITE and ACKs for non-2xx final
responses are routed is fixed once the re-INVITE is sent. The
routing of this messages does not depend on the dialog's route set
and, thus, target refreshes within an ongoing re-INVITE do not affect
their routing. A UA that changes its location (i.e., performs a
target refresh) but is still reachable at its old location will be
able to receive those messages (which will be sent to the old
location). However, a UA that cannot be reachable at its old
location any longer will not be able to receive them.
5.3. UAS Losing its Contact: UAC Behavior
When a UAS that moves to a new contact and loses its old contact
generates a non-2xx final response to the re-INVITE, it will not be
able to receive the ACK request. The entity receiving the response
and, thus, generating the ACK request will either get a transport
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error or a timeout error, which, as described in Section 8.1.3.1 of
RFC 3261 [RFC3261], will be treated as a 503 (Service Unavailable)
response and as a 408 (Request Timeout) response, respectively. If
the sender of the ACK request is a proxy server, it will typically
ignore this error. If the sender of the ACK request is the UAC,
according to Section 12.2.1.2 of RFC 3261 [RFC3261], it is supposed
to (at the "should" level) terminate the dialog by sending a BYE
request. However, because of the special properties of ACK requests
for non-2xx final responses, most existing UACs do not terminate the
dialog when ACK request fails, which is fortunate.
A UAC that accepts a target refresh within a re-INVITE MUST ignore
transport and timeout errors when generating an ACK request for a
non-2xx final response if the UAC is communicating directly with the
UAS (i.e., there are no proxy servers in the dialog's route set).
5.4. UAC Losing its Contact: UAS Behavior
When a UAC moves to a new contact and loses its old contact, it will
not be able to receive responses to the re-INVITE. Consequently, it
will never generate an ACK request.
As described in Section 16.9 of RFC 3261 [RFC3261], a proxy server
that gets an error when forwarding a response does not take any
measurements. Consequently, proxy servers relaying responses will
effectively ignore the error.
If there are no proxy servers in the dialog's route set, the UAS will
get an error when sending a non-2xx final response. The UAS core
will be notified of the transaction failure, as described in Section
17.2.1 of RFC 3261 [RFC3261]. Most existing UASs do not terminate
the dialog on encountering this failure, which is fortunate.
A UAS that accepts a target refresh within a re-INVITE MUST ignore
transport and timeout errors when generating a non-2xx final response
to the re-INVITE if the UAS is communicating directly with the UAC
(i.e., there are no proxy servers in the dialog's route set).
Regardless of the presence or absence of proxy servers in the
dialog's route set, a UAS generating a 2xx response to the re-INVITE
will never receive an ACK request for it. According to Section 14.2
of RFC 3261 [RFC3261], such a UAS is supposed to (at the "should"
level) terminate the dialog by sending a BYE request.
A UAS that accepts a target refresh within a re-INVITE and never
receives an ACK request after having sent a 2xx response to the re-
INVITE SHOULD NOT terminate the dialog. If the UA has received a new
re-INVITE with a higher CSeq sequence number than the original one,
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the UA SHOULD just ignore the error. If the UA has not received such
a re-INVITE, UA SHOULD generate a new re-INVITE in order to make sure
that both UAs have a common view of the state of the session.
Note that the UA generates a re-INVITE and not an UPDATE request
because UPDATE requests can be sent within a re-INVITE. By
accepting the incoming re-INVITE, the remote UA indicates that the
old re-INVITE transaction has already been terminated.
A 500 (Server Internal Error) response to the new re-INVITE would
mean that the remote UA was still processing the original re-INVITE.
This may be because the remote UA is a legacy UA that does not
support this specification. In this situation, the UA SHOULD follow
the original recommendation in RFC 3261 [RFC3261] and terminate the
dialog.
5.5. UAC Losing its Contact: UAC Behavior
When a UAC moves to a new contact and loses its old contact, it will
not be able to receive responses to the re-INVITE. Consequently, it
will never generate an ACK request.
Such a UAC SHOULD generate a CANCEL request to cancel the re-INVITE
and cause the INVITE client transaction corresponding to the re-
INVITE to enter the "Terminated" state. The UAC SHOULD also send a
new re-INVITE in order to make sure that both UAs have a common view
of the state of the session.
Per Section 14.2 of RFC 3261 [RFC3261], the UAS will accept new
incoming re-INVITEs as soon as it has generated a final response
to the previous INVITE request, which had a lower CSeq sequence
number.
6. Security Considerations
This document does not introduce any new security issue. It just
clarifies how certain transactions should be handled in SIP.
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.
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8. Acknowledgements
Paul Kyzivat provided useful ideas on the topics discussed in this
document.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[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.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
Authors' Addresses
Gonzalo Camarillo (editor)
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Gonzalo.Camarillo@ericsson.com
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Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: Christer.Holmberg@ericsson.com
Yang Gao
ZTE
China
Email: gao.yang2@zte.com.cn
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