Internet DRAFT - draft-ietf-sip-update
draft-ietf-sip-update
Internet Engineering Task Force SIP WG
Internet Draft J. Rosenberg
dynamicsoft
draft-ietf-sip-update-02.txt
April 30, 2002
Expires: October 2002
The Session Initiation Protocol UPDATE Method
STATUS OF THIS MEMO
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Abstract
This specification defines the new UPDATE method for the Session
Initiation Protocol (SIP). UPDATE allows a client to update
parameters of a session (such as the set of media streams and their
codecs) but has no impact on the state of a dialog. In that sense, it
is like a re-INVITE, but can be sent before the initial INVITE has
completed. This makes it very useful for updating session parameters
within early dialogs.
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Table of Contents
1 Introduction ........................................ 3
2 Terminology ......................................... 3
3 Overview of Operation ............................... 3
4 Determining Support for this Extension .............. 4
5 UPDATE Handling ..................................... 4
5.1 Sending an UPDATE ................................... 4
5.2 Receiving an UPDATE ................................. 6
5.3 Processing the UPDATE Response ...................... 6
6 Proxy Behavior ...................................... 7
7 Definition of the UPDATE method ..................... 7
8 Example Call Flow ................................... 7
9 Security Considerations ............................. 11
10 IANA Considerations ................................. 11
11 Acknowledgements .................................... 11
12 Author's Addresses .................................. 11
13 Normative References ................................ 11
14 Informative References .............................. 12
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1 Introduction
(Note to RFC Editor: replace all instances of BBBB in this
specification with the RFC number of draft-ietf-sip-rfc2543bis, all
all instances of OOOO with the RFC number of draft-ietf-mmusic-sdp-
offer-answer)
The Session Initiation Protocol (SIP) [1] defines the INVITE method
for the initiation and modification of sessions. However, this method
actually affects two important pieces of state. It impacts the
session (the media streams SIP sets up) and also the dialog (the
state that SIP itself defines). While this is reasonable in many
cases, there are important scenarios where this coupling causes
complications.
The primary difficulty is when aspects of the session need to be
modified before the initial INVITE has been answered. An example of
this situation is "early media", a condition where the session is
established, for the purpose of conveying progress of the call, but
before the INVITE itself is accepted. It is important that either
caller or callee be able to modify the characteristics of that
session (putting the early media on hold, for example), before the
call is answered. However, a re-INVITE cannot be used for this
purpose, because the re-INVITE has an impact on the state of the
dialog, in addition to the session.
As a result, a solution is needed that allows the caller or callee to
provide updated session information before a final response to the
initial INVITE request is generated. The UPDATE method, defined here,
fulfills that need. It can be sent by a UA within a dialog (early or
confirmed), to update either session parameters without impacting the
dialog state itself.
2 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
indicate requirement levels for compliant SIP implementations.
3 Overview of Operation
Operation of this extension is straigthforward. The caller begins
with an INVITE transaction, which proceeds normally. Once a dialog is
established, either early or confirmed, the caller can generate an
UPDATE method that contains an SDP offer [3] for the purposes of
updating the session. The response to the UPDATE method contains the
answer. Similarly, once a dialog is established, the callee can send
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an UPDATE with an offer, and the caller places its answer in the 2xx
to the UPDATE. The Allow header is used to indicate support for the
UPDATE method. There are additional constraints on when UPDATE can be
used, based on the restrictions of the offer/answer model.
4 Determining Support for this Extension
The initiation of a session operates as specified in RFC BBBB [1].
However, a UAC compliant to this specification SHOULD also include an
Allow header field in the request, listing the method UPDATE, to
indicate its ability to receive an UPDATE request.
When a UAS compliant to this specification receives an INVITE request
for a new dialog, and generates a reliable provisional response
containing SDP, that response SHOULD contain an Allow header field
that lists the UPDATE method. This informs the caller that the callee
is capable of receiving an UPDATE request at any time. An unreliable
provisional response MAY contain an Allow header listing the UPDATE
method, and a 2xx response SHOULD contain an Allow header listing the
UPDATE method.
Responses are processed normally as per RFC BBBB [1], and in the case
of reliable provisional responses, according to [4]. It is important
to note that a reliable provisional response will always create an
early dialog at the UAC. Creation of this dialog is necessary in
order to receive UPDATE creates from the callee.
If the response contains an Allow header containing the value
"UPDATE", the UAC knows that the callee supports UPDATE, and the UAC
is allowed to follow the procedures of Section 5.1.
5 UPDATE Handling
5.1 Sending an UPDATE
The UPDATE request is constructed as would any other request within
an existing dialog, as described in Section 12.2.1 of RFC BBBB. It
MAY be sent for both early and confirmed dialogs. Although UPDATE can
be used on confirmed dialogs, it is RECOMMENDED that a re-INVITE be
used instead. This is because an UPDATE needs to be answered
immediately, ruling out the possibility of user approval. Such
approval will frequently needed, and is possible with a re-INVITE.
The UAC MAY add optional headers for the UPDATE request, as defined
in Tables 1 and 2.
The rules for inclusion of offers and answers in SIP messages as
defined in Section 13.2.1 of RFC BBBB still apply. These rules exist
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to guarantee a consistent view of the session state. This means that,
for the caller:
o If the UPDATE is being sent before completion of the initial
INVITE transaction, and the initial INVITE contained an offer,
the UPDATE can contain an offer if the callee generated an
answer in a reliable provisional response, and the caller has
received answers to any other offers it sent in either PRACK
or UPDATE, and has generated answers for any offers it
received in an UPDATE from the callee.
o If the UPDATE is being sent before completion of the initial
INVITE transaction, and the initial INVITE did not contain an
offer, the UPDATE can contain an offer if the callee generated
an offer in a reliable provisional response, and the UAC
generated an answer in the corresponding PRACK. Of course, it
can't send an UPDATE if it has not received answers to any
other offers it sent in either PRACK or UPDATE, or has not
generated answers for any other offers it received in an
UPDATE from the callee.
o If the UPDATE is being sent after the completion of the
initial INVITE transaction, it cannot be sent if the caller
has generated or received offers in a re-INVITE or UPDATE
which have not been answered.
and for the callee:
o If the UPDATE is being sent before the completion of the
INVITE transaction, and the initial INVITE contained an offer,
the UPDATE cannot be sent unless the callee has generated an
answer in a reliable provisional response, has received a
PRACK for that reliable provisional response, has not received
any requests (PRACK or UPDATE) with offers that it has not
answered, and has not sent any UPDATE requests containing
offers that have not been answered.
o If the UPDATE is being sent before completion of the INVITE
transaction, and the initial INVITE did not contain an offer,
the UPDATE cannot be sent unless the callee has sent an offer
in a reliable provisional response, received an answer in a
PRACK, and has not received any UPDATE requests with offers
that it has not answered, and has not sent any UPDATE requests
containing offers that have not been answered.
o If the UPDATE is being sent after the completion of the
initial INVITE transaction, it cannot be sent if the callee
has generated or received offers in a re-INVITE or UPDATE
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which have not been answered.
5.2 Receiving an UPDATE
The UPDATE is processed as any other mid-dialog request, as described
in Section 12.2.2 of RFC BBBB [1]. If the request is generally
acceptable, processing continues as described below. This processing
is nearly identical to that of Section 14.2 of RFC BBBB [1],
generalized for the case of UPDATE.
A UAS that receives an UPDATE before it has generated a final
response to a previous UPDATE or INVITE on the same dialog MUST
return a 500 response to the new UPDATE, and MUST include a Retry-
After field with a Retry-After header field with a randomly chosen
value between 0 and 10 seconds.
If an UPDATE is received that contains an offer, and the UAS has
generated an offer (in an UPDATE, PRACK or INVITE) to which it has
not yet received an answer, the UAS MUST reject the UPDATE with a 491
response.
If a UA receives an UPDATE for an existing dialog, it MUST check any
version identifiers in the session description or, if there are no
version identifiers, the content of the session description to see if
it has changed. If the session description has changed, the UAS MUST
adjust the session parameters accordingly and generate an answer in
the 2xx response. However, unlike a re-INVITE, the UPDATE MUST be
responded to promptly, and therefore the user cannot generally be
prompted to approve the session changes. If the UAS cannot change the
session parameters without prompting the user, it SHOULD reject the
request with a 504 response. If the new session description is not
acceptable, the UAS can reject it by returning a 488 (Not Acceptable
Here) response for the UPDATE. This response SHOULD include a Warning
header field.
5.3 Processing the UPDATE Response
Processing of the UPDATE response at the UAC is nearly identical to
the rules in Section 14.1 of RFC BBBB [1], but generalized for
UPDATE.
If a UA receives a non-2xx final response to a UPDATE, the session
parameters MUST remain unchanged, as if no UPDATE had been issued.
Note that, as stated in Section 12.2.1 of RFC BBBB [1], if the non-
2xx final response is a 481 (Call/Transaction Does Not Exist), or a
408 (Request Timeout), or no response at all is received for the
UPDATE (that is, a timeout is returned by the UPDATE client
transaction), the UAC will terminate the dialog.
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If a UAC receives a 491 response to a UPDATE, it SHOULD start a timer
with a value T chosen as follows:
1. If the UAC is the owner of the Call-ID of the dialog ID
(meaning it generated the value), T has a randomly chosen
value between 2.1 and 4 seconds in units of 10 ms.
2. If the UAC is not the owner of the Call-ID of the dialog
ID, T has a randomly chosen value of between 0 and 2
seconds in units of 10 ms.
When the timer fires, the UAC SHOULD attempt the UPDATE once more, if
it still desires for that session modification to take place. For
example, if the call was already hung up with a BYE, the UPDATE would
not take place.
6 Proxy Behavior
Proxy processing of the UPDATE request is identical to any other
non-INVITE request.
7 Definition of the UPDATE method
The semantics of the UPDATE method are described in detail above.
This extension adds another value to the Method BNF described in RFC
BBBB:
UPDATEm = %x55.50.44.41.54.45 ; UPDATE in caps
Method = INVITEm / ACKm / OPTIONSm / BYEm
/ CANCELm / REGISTERm / UPDATEm
/ extension-method
Table 1 extends Table 2 of RFC BBBB for the UPDATE method.
Table 2 updates Table 3 of RFC BBBB for the UPDATE method.
8 Example Call Flow
This section presents an example call flow using the UPDATE method.
The flow is shown in Figure 1. The caller sends an initial INVITE (1)
which contains an offer. The callee generates a 180 response (2) with
an answer to that offer. With the completion of an offer/answer
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Header field where proxy UPDATE
____________________________________________
Accept R o
Accept 2xx o
Accept 415 c
Accept-Encoding R o
Accept-Encoding 2xx o
Accept-Encoding 415 c
Accept-Language R o
Accept-Language 2xx o
Accept-Language 415 c
Alert-Info -
Allow R o
Allow 2xx o
Allow r o
Allow 405 m
Allow-Events (1) -
Authentication-Info 2xx o
Authorization R o
Call-ID c r m
Call-Info ar o
Contact R m
Contact 1xx o
Contact 2xx m
Contact 3xx d o
Contact 485 o
Content-Disposition o
Content-Encoding o
Content-Language o
Content-Length ar t
Content-Type *
CSeq c r m
Date a o
Error-Info 300-699 a o
Event (1) -
Expires -
From c r m
In-Reply-To -
Max-Forwards R amr m
Min-Expires -
MIME-Version o
Organization ar o
Table 1: Summary of header fields, A--O ; (1) defined in [5],
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Header field where proxy UPDATE
____________________________________________________
Priority -
Proxy-Authenticate 407 ar m
Proxy-Authenticate 401 ar o
Proxy-Authorization R dr o
Proxy-Require R ar o
RAck R -
Record-Route R ar o
Record-Route 2xx,18x mr o
Reply-To -
Require ar c
Retry-After 404,413,480,486 o
500,503 o
600,603 o
Route R adr c
RSeq - -
Server r o
Subject - -
Subscription-State (1) -
Supported R o
Supported 2xx o
Timestamp o
To c r m
Unsupported 420 m
User-Agent o
Via R amr m
Via rc dr m
Warning r o
WWW-Authenticate 401 ar m
WWW-Authenticate 407 ar o
Table 2: Summary of header fields, P--Z
exchange, the session is established, although the dialog is still in
the early state. The caller generates a PRACK (3) to acknowledge the
180, and the PRACK is answered with a 200 OK (4). The caller decides
to update some aspect of the session - to put it on hold, for
example. So, they generate an UPDATE request (5) with a new offer.
This offer is answered in the 200 response to the UPDATE (6). Shortly
thereafter, the callee decides to update some aspect of the session,
so it generates an UPDATE request (7) with an offer, and the answer
is sent in the 200 response (8). Finally, the callee answers the
call, resulting in a 200 OK response to the INVITE (9), and then an
ACK (10). Neither the 200 OK to the INVITE, nor the ACK, will contain
SDP.
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Caller Callee
| |
| |
|(1) INVITE with offer 1 |
|---------------------------->|
| |
| |
|(2) 180 with answer 1 |
|<----------------------------|
| |
| |
|(3) PRACK |
|---------------------------->|
| |
| |
|(4) 200 PRACK |
|<----------------------------|
| |
| |
|(5) UPDATE with offer 2 |
|---------------------------->|
| |
| |
|(6) 200 UPDATE with answer 2 |
|<----------------------------|
| |
| |
|(7) UPDATE with offer 3 |
|<----------------------------|
| |
| |
|(8) 200 UPDATE with answer 3 |
|---------------------------->|
| |
| |
|(9) 200 INVITE |
|<----------------------------|
| |
| |
|(10) ACK |
|---------------------------->|
| |
| |
| |
| |
Figure 1: UPDATE Call Flow
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9 Security Considerations
The security considerations for UPDATE are identical to those for
re-INVITE. It is important that the UPDATE be integrity protected and
authenticated as coming from the same source as the entity on the
other end of the dialog. RFC BBBB [1] discusses security mechanisms
for achieving these functions.
10 IANA Considerations
As per Section 27.4 of RFC BBBB [1], this specification serves as a
registration for the SIP UPDATE request method. The information to be
added to the registry is:
RFC Number: This specification serves as the RFC for registering
the UPDATE request method.
Method Name: UPDATE
Reason Phrase: Not applicable.
11 Acknowledgements
The author would like to thank Jo Hornsby, Markus Isomaki, Rohan
Mahy, and Bob Penfield for their comments.
12 Author's Addresses
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com
13 Normative References
[1] J. Rosenberg, H. Schulzrinne, et al. , "SIP: Session initiation
protocol," Internet Draft, Internet Engineering Task Force, Feb.
2002. Work in progress.
[2] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," RFC 2119, Internet Engineering Task Force, Mar. 1997.
[3] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
J. Rosenberg [Page 11]
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SDP," Internet Draft, Internet Engineering Task Force, Feb. 2002.
Work in progress.
[4] J. Rosenberg and H. Schulzrinne, "Reliability of provisional
responses in SIP," Internet Draft, Internet Engineering Task Force,
Feb. 2002. Work in progress.
[5] A. Roach, "SIP-specific event notification," Internet Draft,
Internet Engineering Task Force, Mar. 2002. Work in progress.
14 Informative References
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