Network Working Group J. Rosenberg
Request for Comments: 3311 dynamicsoft
Category: Standards Track September 2002
The Session Initiation Protocol (SIP) UPDATE Method
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
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 unlike re-INVITE, it can be sent before
the initial INVITE has been completed. This makes it very useful for
updating session parameters within early dialogs.
Table of Contents
1 Introduction .............................................. 2
2 Terminology ............................................... 3
3 Overview of Operation ..................................... 3
4 Determining Support for this Extension .................... 3
5 UPDATE Handling ........................................... 4
5.1 Sending an UPDATE ......................................... 4
5.2 Receiving an UPDATE ....................................... 5
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 Notice Regarding Intellectual Property Rights ............. 11
12 Normative References ...................................... 11
13 Acknowledgements .......................................... 12
14 Author's Address .......................................... 12
15 Full Copyright Statement .................................. 13
1 Introduction
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 in which 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 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 BCP 14, RFC2119
[2] and indicate requirement levels for compliant SIP
implementations.
3 Overview of Operation
Operation of this extension is straightforward. 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
an UPDATE with an offer, and the caller places its answer in the 2xx
to the UPDATE. The Allow header field 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 RFC3261 [1].
However, a UAC compliant to this specification SHOULD also include an
Allow header field in the INVITE 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 field
listing the UPDATE method, and a 2xx response SHOULD contain an Allow
header field listing the UPDATE method.
Responses are processed normally as per RFC3261 [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 requests from the callee.
If the response contains an Allow header field 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 RFC3261. It
MAY be sent for both early and confirmed dialogs, and MAY be sent by
either caller or callee. 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 be
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.
UPDATE is a target refresh request. As specified in RFC3261 [1],
this means that it can update the remote target of a dialog. If a UA
uses an UPDATE request or response to modify the remote target while
an INVITE transaction is in progress, and it is a UAS for that INVITE
transaction, it MUST place the same value into the Contact header
field of the 2xx to the INVITE that it placed into the UPDATE request
or response.
The rules for inclusion of offers and answers in SIP messages as
defined in Section 13.2.1 of RFC3261 still apply. These rules exist
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 contain an offer 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 with an offer 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 with an offer 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 with an offer if the
callee has generated or received offers in a re-INVITE or
UPDATE which have not been answered.
5.2 Receiving an UPDATE
The UPDATE is processed as any other mid-dialog target refresh
request, as described in Section 12.2.2 of RFC3261 [1]. If the
request is generally acceptable, processing continues as described
below. This processing is nearly identical to that of Section 14.2
of RFC3261 [1], but generalized for the case of UPDATE.
A UAS that receives an UPDATE before it has generated a final
response to a previous UPDATE on the same dialog MUST return a 500
response to the new UPDATE, and MUST include 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. Similarly, if an UPDATE is received that contains an
offer, and the UAS has received an offer (in an UPDATE, PRACK, or
INVITE) to which it has not yet generated an answer, the UAS MUST
reject the UPDATE with a 500 response, and MUST include a Retry-After
header field with a randomly chosen value between 0 and 10 seconds.
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 follows the rules in
Section 12.2.1.2 of RFC3261 [1] for a target refresh request. Once
that processing is complete, it continues as specified below. This
processing is nearly identical to the processing of Section 14.1 of
RFC3261 [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 RFC3261 [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.
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 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
3261:
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 RFC3261 for the UPDATE method.
Table 2 updates Table 3 of RFC3261 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
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.
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].
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.
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
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. RFC3261 [1] discusses security mechanisms
for achieving these functions.
10 IANA Considerations
As per Section 27.4 of RFC3261 [1], this specification serves as a
registration for the SIP UPDATE request method. The information to
be added to the registry is:
RFC3311: This specification serves as the RFCfor registering
the UPDATE request method.
Method Name: UPDATE
Reason Phrase: Not applicable.
11 Notice Regarding Intellectual Property Rights
The IETF has been notified of intellectual property rights claimed
in regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
12 Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC3261, June 2002.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[3] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with the
Session Description Protocol (SDP)", RFC3264, June 2002.
[4] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional
Responses in the Session Initiation Protocol (SIP)", RFC3262,
June 2002.
[5] Roach, A.B., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC3265, June 2002.
13 Acknowledgements
The author would like to thank Jo Hornsby, Markus Isomaki, Rohan
Mahy, and Bob Penfield for their comments.
14 Author's Address
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
EMail: jdrosen@dynamicsoft.com
15 Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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