Internet Engineering Task Force (IETF) G. Camarillo, Ed.
Request for Comments: 6141 C. Holmberg
Updates: 3261 Ericsson
Category: Standards Track Y. Gao
ISSN: 2070-1721 ZTE
March 2011
Re-INVITE and Target-Refresh Request Handling
in the Session Initiation Protocol (SIP)
Abstract
The procedures for handling SIP re-INVITEs are described in RFC 3261.
Implementation and deployment experience has uncovered a number of
issues with the original documentation, and this document provides
additional procedures that update the original specification to
address those issues. In particular, this document defines 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, this document defines further
details of procedures related to target-refresh requests.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6141.
Camarillo, et al. Standards Track [Page 1]
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Camarillo, et al. Standards Track [Page 2]
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Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................4
3. Changing the Session State during a Re-INVITE ...................5
3.1. Background on Re-INVITE Handling by UASs ...................5
3.2. Problems with Error Responses and Already Executed Changes .9
3.3. UAS Behavior ..............................................10
3.4. UAC Behavior ..............................................11
3.5. Glare Situations ..........................................11
3.6. Example of UAS Behavior ...................................12
3.7. Example of UAC Behavior ...................................14
3.8. Clarifications on Canceling Re-INVITEs ....................17
4. Refreshing a Dialog's Targets ..................................17
4.1. Background and Terminology on a Dialog's Targets ..........17
4.2. Background on Target-Refresh Requests .....................17
4.3. Clarification on the Atomicity of Target-Refresh Requests .18
4.4. UA Updating the Dialog's Local Target in a Request ........19
4.5. UA Updating the Dialog's Local Target in a Response .......19
4.6. A Request Updating the Dialog's Remote Target .............19
4.7. A Response Updating the Dialog's Remote Target ............20
4.8. Race Conditions and Target Refreshes ......................20
4.9. Early Dialogs .............................................21
5. A UA Losing Its Contact ........................................21
5.1. Background on Re-INVITE Transaction Routing ...............22
5.2. Problems with UAs Losing Their Contact ....................22
5.3. UAS Losing Its Contact: UAC Behavior ......................22
5.4. UAC Losing Its Contact: UAS Behavior ......................23
5.5. UAC Losing Its Contact: UAC Behavior ......................24
6. Security Considerations ........................................24
7. Acknowledgements ...............................................24
8. References .....................................................25
8.1. Normative References ......................................25
8.2. Informative References ....................................25
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].
Camarillo, et al. Standards Track [Page 3]
RFC 6141 Re-INVITE Handling in SIP March 2011
A re-INVITE transaction has an offer/answer [RFC3264] exchange
associated with 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 as the offerer includes an offer in the re-INVITE. The UAS
(User Agent Server) 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 implementors regarding how a UAS
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.
Indented passages such as this one are used in this document to
provide additional information and clarifying text. They do not
contain normative protocol behavior.
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.
UAS: User Agent Server.
Note that the terms UAC and UAS are used with respect to an INVITE
or re-INVITE transaction and do not necessarily reflect the role
of the UA concerned with respect to any other transaction, such as
an UPDATE transaction occurring within the INVITE transaction.
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3. Changing the Session State during a Re-INVITE
The following sub-sections discuss how to change the state of the
session during a re-INVITE transaction.
3.1. Background on Re-INVITE Handling by UASs
Eventually, a UAS receiving a re-INVITE will need to 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 (this section deals with
changes to the session state; target refreshes are discussed in
Section 4.2). That is, the session state is 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------------------>|
| |
Figure 1: Rejection of a re-INVITE
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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 configured to automatically 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 state is 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 a Session Description Protocol (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
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: Manual 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 RTP Control Protocol (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
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At a later point, the UAS's user rejects the addition of the video
stream. Consequently, the UAS sends an UPDATE request (6) setting
the port of the video stream to zero in its offer.
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 UAC returns a 200 (OK) response (7) to the UPDATE with the
following answer:
SDP6:
m=audio 30000 RTP/AVP 0
c=IN IP4 192.0.2.2
m=video 0 RTP/AVP 31
The UAS now returns a 200 (OK) response (8) to the re-INVITE.
In all the previous examples, the UAC of the re-INVITE transaction
was the offerer. 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 (Figures 2 and 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). Sections 3.3 and 3.4 contain the
normative statements needed to avoid these problems.
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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 (e.g., as a result of UPDATE transactions
or reliable provisional responses) is effectively requesting a change
in the session state. However, the UAC has no means to reject that
change 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 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, 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 UA has sent or received media using the
new parameters. Connection establishment messages (e.g., TCP SYN),
connectivity checks (e.g., when using Interactive Connectivity
Establishment (ICE) [RFC5245]), and any other messages used in the
process of meeting the preconditions for a stream are not considered
media.
Normally, a UA receiving media can easily detect when the new
parameters for the media stream are used (e.g., media is received
on a new port). However, in some scenarios, the UA will have to
process incoming media packets in order to detect whether they use
the old or new parameters.
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 to use them. During a session establishment, the UAS can
wait before 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
Camarillo, et al. Standards Track [Page 10]
RFC 6141 Re-INVITE Handling in SIP March 2011
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 indicates that the new parameters for the media stream
are already considered to be in 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 session (the UAC uses the criteria
in Section 3.3 in order to decide whether or not changes have been
executed for a particular 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, session parameters in
the offer/answer exchange SHOULD be as close to those in the
pre-re-INVITE state as possible.
3.5. Glare Situations
Section 4 of RFC 3264 [RFC3264] defines glare conditions as a user
agent receiving an offer after having sent one but before having
received an answer to it. That section specifies rules to avoid
glare situations in most cases. When, despite following those rules,
a glare condition occurs (as a result of a race condition), it is
handled as specified in Sections 14.1 and 14.2 of RFC 3261 [RFC3261].
The UAS returns a 491 (Request Pending) response and the UAC retries
the offer after a randomly selected time, which depends on which user
agent is the owner of the Call-ID of the dialog. The rules in RFC
3261 [RFC3261] not only cover collisions between re-INVITEs that
contain offers, they cover collisions between two re-INVITEs in
general, even if they do not contain offers. Sections 5.2 and 5.3 of
RFC 3311 [RFC3311] extend those rules to also cover collisions
between an UPDATE request carrying an offer and another message
(UPDATE, PRACK, or INVITE) also carrying an offer.
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The rules in RFC 3261 [RFC3261] do not cover collisions between an
UPDATE request and a non-2xx final response to a re-INVITE. Since
both the UPDATE request and the reliable response could be requesting
changes to the session state, it would not be clear which changes
would need to be executed first. However, the procedures discussed
in Section 3.4 already cover this type of situation. 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
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).
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
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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.
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
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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
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.
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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
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).
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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
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. UA1 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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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 Canceling 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.
4. Refreshing a Dialog's Targets
The following sections discuss how to refresh the targets of a
dialog.
4.1. Background and Terminology on a Dialog's Targets
As described in Section 12 of RFC 3261 [RFC3261], a UA involved in a
dialog keeps a record of the SIP or Session Initiation Protocol
Secure (SIPS) URI at which it can communicate with a specific
instance of its peer (this is called the "dialog's remote target URI"
and is equal to the URI contained in the Contact header of requests
and responses it receives from the peer). This document introduces
the complementary concept of the "dialog's local target URI", defined
as a UA's record of the SIP or SIPS URI at which the peer can
communicate with it (equal to the URI contained in the Contact header
of requests and responses it sends to the peer). These terms are
complementary because the "dialog's remote target URI" according to
one UA is the "dialog's local target URI" according to the other UA,
and vice versa.
4.2. 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.
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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 4.3 specifies new rules for the handling of target-
refresh requests. Note that the new rules apply to any target-
refresh request, not only to re-INVITEs.
4.3. Clarification on the Atomicity of Target-Refresh Requests
The local and remote targets of a dialog are special types of state
information because of their 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
received remote target to send SIP requests to the remote UA.
The dialog's local target is a piece of state information that is not
meant to be negotiated. When a UA changes its local target (i.e.,
the UA changes its IP address), the UA simply communicates its new
local target to the remote UA (e.g., the UA communicates its new IP
address to the remote UA in order to remain reachable by the remote
UA). UAs need to follow the behavior specified in Sections 4.4, 4.5,
4.6, and 4.7 of this specification instead of that specified in RFC
3261 [RFC3261], which was discussed in Section 4.2. 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 to with a final error response. This means that
target-refresh requests are not atomic.
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4.4. UA Updating the Dialog's Local Target in a Request
In order to update its local target, a UA can send a target-refresh
request. If the UA receives an error response to the target-refresh
request, the remote UA has not updated its remote target.
This allows UASs to authenticate target-refresh requests (see
Section 26.2 of RFC 3261 [RFC3261]).
If the UA receives a reliable provisional response or a 2xx response
to the target-refresh request, or the UA receives an in-dialog
request on the new local target, the remote UA has updated its remote
target. The UA 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.
A UA 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 remote UA (e.g., an IP address change).
Piggybacking a target refresh with more complicated session changes
would make it unnecessarily complicated for the remote UA to accept
the target refresh while rejecting the session changes. Only in case
the target refresh request is a re-INVITE and the UAS supports
reliable provisional response or UPDATE requests, the UAC MAY
piggyback session changes and a target refresh in the same re-INVITE.
4.5. UA Updating the Dialog's Local Target in a Response
A UA processing an incoming target refresh request can update its
local target by returning a reliable provisional response or a 2xx
response to the target-refresh request. The response needs to
contain the updated local target URI in its Contact header field. On
sending the response, the UA can consider the target refresh
operation completed.
4.6. A Request Updating the Dialog's Remote Target
Behavior of a UA after having received a target-refresh request
updating the remote target:
If the UA receives a target-refresh request that has been properly
authenticated (see Section 26.2 of RFC 3261 [RFC3261]), the UA 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 UA does not support reliable provisional responses and
needs user input before generating a final response), the UA SHOULD
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send an in-dialog request to the remote UA using the new remote
target (if the UA 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 UA 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]. 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. Note that
non-100 provisional responses are only applicable to INVITE
transactions [RFC4320].
If instead of sending a reliable provisional response or a 2xx
response to the target-refresh request, or a request to the new
target, the UA generates an error response to the target-refresh
request, the UA MUST NOT update its dialog's remote target.
4.7. A Response Updating the Dialog's Remote Target
If a UA receives a reliable provisional response or a 2xx response to
a target-refresh request, the UA MUST replace the dialog's remote
target URI with the URI from the Contact header field in that
response, if present.
If a UA receives an unreliable provisional response to a target-
refresh request, the UA MUST NOT refresh the dialog's remote target.
4.8. Race Conditions and Target Refreshes
SIP provides request ordering by using the Cseq header field. That
is, a UA 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.
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This document specifies the following rule to avoid the situation
just described. If the protocol allows a UA to use a target-refresh
request at the point in time that the UA wishes to refresh its local
target, the UA MUST use a target-refresh request instead of a
response to refresh its local target. This rule implies that a UA
only uses a response (i.e., a reliable provisional response or a 2xx
response to a target-refresh request) to refresh its local target if
the UA is unable to use a target-refresh request at that point in
time (e.g., the UAS of an ongoing re-INVITE without support for
UPDATE).
4.9. Early Dialogs
The rules given in this section about which messages can refresh the
target of a dialog also apply to early dialogs created by an initial
INVITE transaction. Additionally, as specified in Section 13.2.2.4
of RFC 3261 [RFC3261], on receiving a 2xx response to the initial
INVITE, the UAC recomputes the whole route set of the dialog, which
transitions from the "early" state to the "confirmed" state.
Section 12.1 of RFC 3261 allows unreliable provisional responses to
create early dialogs. However, per the rules given in this section,
unreliable provisional responses cannot refresh the target of a
dialog. Therefore, the UAC of an initial INVITE transaction will not
perform any target refresh as a result of the reception of an
unreliable provisional response with an updated Contact value on an
(already established) early dialog. Note also that a given UAS can
establish additional early dialogs, which can have different targets,
by returning additional unreliable provisional responses with
different To tags.
5. A UA Losing Its Contact
The following sections discuss the case where a UA loses its
transport address during an ongoing re-INVITE transaction. Such a UA
will refresh the dialog's local target so that it reflects its new
transport address. Note that target refreshes that do not involve
changes in the UA's transport address are outside of the scope of
this section. Also, UAs losing their transport address during a
non-re-INVITE transaction (e.g., a UA losing its transport address
right after having sent an UPDATE request before having received a
response to it) are out of scope as well.
The rules given in this section are also applicable to initial INVITE
requests that have established early dialogs.
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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 sent
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 4.3) 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.
The following sections describe the errors UAs face when they lose
their transport address during a re-INVITE. On detecting some of
these errors, UAs following the rules specified in RFC 3261 [RFC3261]
will terminate the dialog. When the dialog is terminated, the only
option for the UAs is to establish a new dialog. The following
sections change the requirements RFC 3261 [RFC3261] places on UAs
when certain errors occur so that the UAs can recover from those
errors. In short, the UAs generate a new re-INVITE transaction to
synchronize both UAs. Note that there are existing UA
implementations deployed that already implement this behavior.
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
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and, thus, generating the ACK request will either get a transport
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. Additionally, UAC SHOULD generate a new
re-INVITE in order to make sure that both UAs have a common view of
the state of the session.
It is possible that the errors ignored by the UAC were not related
to the target refresh operation. If that was the case, the second
re-INVITE would fail and the UAC would terminate the dialog
because, per the rules above, UACs only ignore errors when they
accept a target refresh within the re-INVITE.
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
measures. 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.
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.
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A UAS that accepts a target refresh within a re-INVITE and never
receives an ACK request after having sent a final 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.
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].
In particular, in order not to reduce the security level for a given
session, re-INVITEs and UPDATE requests SHOULD be secured using a
mechanism equivalent to or stronger than the initial INVITE request
that created the session. For example, if the initial INVITE request
was end-to-end integrity protected or encrypted, subsequent
re-INVITEs and UPDATE requests should also be so.
7. Acknowledgements
Paul Kyzivat provided useful ideas on the topics discussed in this
document.
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8. References
8.1. 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.
8.2. Informative References
[RFC4320] Sparks, R., "Actions Addressing Identified Issues with the
Session Initiation Protocol's (SIP) Non-INVITE
Transaction", RFC 4320, January 2006.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
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Authors' Addresses
Gonzalo Camarillo (editor)
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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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RFC, FYI, BCP