Internet Engineering Task Force SIP WG
Internet Draft J.Rosenberg
dynamicsoft
draft-ietf-sip-update-00.txt
February 16, 2002
Expires: August 2002
The SIP 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), or provide SIP level information (such as credentials), but
has no impact on the state of a dialog. This is very useful for
updating information about a call before the call has been accepted.
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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 where it 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,
before the INVITE itself is answered. It is important to 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 call, in addition to the session.
Another difficulty is when an INVITE is rejected, not because the
call is declined, but because some aspect of the request was not
acceptable, and a proxy on the call path was attempting to provide a
service that requires forking. That rejection won't be forwarded
upstream by the proxy unless all other branches reject the request.
That may never happen. The result is that the caller never has an
opportunity to update its request, so that it would then be
acceptable to the rejecting party. Even if the rejection is passed
upstream immediately, ther can be problems. The rejection of the
INVITE will trigger the caller to try again, resulting in a re-
invocation of the service at the proxy. That re-invocation will
frequently result in undesirable side-effects.
In both cases, a solution is needed that allows the caller to provide
updated information to the recipients of the request, before a
response to the initial 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 or
SIP information, without impacting the dialog state itself. This
specification also defines the 155 (Update Requested) response, which
can be used by the UAS to inform the UAC of a condition that prevents
it from processing the request, but which can be corrected with an
UPDATE.
2 Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
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and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
indicate requirement levels for compliant SIP implementations.
3 Definitions
Repairable Error Response: An error response in the 4xx class
which indicates that the request was unacceptable for some
reason which can be repaired by automated processing at the
UAC. For the 4xx response codes defined in RFC BBBB, this
includes the 401, 415, 420, 423 and 488 responses. However,
SIP extensions can define additional error codes with this
property, and those are covered by this specification.
4 Overview of Operation
When a UAC sends an INVITE, it includes a Supported tag in the
request that contains the option tag "update". This option tag
indicates that the UAC understands the mechanisms defined in this
specification. When the UAS gets that request, it performs the
processing defined in Section XX of [1]. This processing may
determine that the request is acceptable, and therefore the user is
alerted (or some other appropriate action is taken), or the
processing may determine that the request cannot be processed without
some kind of change.
OPEN ISSUE: do we need an option tag, or does Allow:
UPDATE suffice?
In the former case, the UAS generates a reliable provisional response
[3] that also contains the Supported header with the option tag
"update". This indicates to the UAC that the UAS supports this
extension, The provisional response also establishes an early dialog.
At any time thereafter, the UAC can generate an UPDATE method that
contains an SDP offer [4] for the purposes of updating the session.
The response to the UPDATE method contains the answer. Similarly, the
callee can send an UPDATE with an offer, and the caller places its
answer in the 2xx. The UPDATE method can also update other
information provided in previous requests, such as a new Call-Info or
Alert-Info header.
In the latter case, the request might be unacceptable because it does
not contain proper credentials, contains content not understood by
the UAS, or mandates an extension not supported by the UAS. Rather
than rejecting the request with a 401, 415, 420, 423 or 488 response,
respectively (these response codes are referred to as repairable
error responses), the UAS can choose to generate a 155 (Update
Requested) reliable provisional response. This response is formatted
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identically to the way the 401, 415, 420, 423 or 488 would be
formatted, with the exception of the status line. When this is
received by the UAC, the UAC generates an UPDATE request that
believes might be acceptable to the UAS. Such a request might include
credentials, omit bodies not acceptable to the UAS, or not mandate an
extension not supported by the UAS. When the UAS receives this
request, it processes it much like it would a re-INVITE, except that
it is answered immediately if acceptable, and the response to the
original INVITE is not sent until the call is explicitly answered or
rejected.
The usage of the 155, instead of rejecting the request, is not needed
unless there is a proxy upstream performing a forking application.
Therefore, in order to know whether to send a 155 or a final
response, the proxy can insert a Require: update header into the
request before proxying it. This informs the UAS that it should send
a 155 instead of a repairable error response.
5 UA Behavior
5.1 Initiating a Dialog
A UAC compliant to this specification SHOULD include a Supported
header in all requests it sends that can initiate a dialog, including
INVITE or SUBSCRIBE [5], with the option tag "update". This indicates
the ability to process the 155 response properly, and to receive
UPDATE requests on an early dialog.
The UAC MAY include a Require header in the request, with the option
tag "update", if it insists that the UAS generate a 155 response code
instead of an error response for all repairable error responses.
5.2 Generating a Provisional Response
When a UAS compliant to this specification receives a request, the
procedures in this section are followed.
The request is processed according to the rules in Section XXX of RFC
BBBB. If the request contains a Supported header with the option tag
of "update", and the guidelines in Section XXX would result in the
generation of a repairable error response, the processing in Section
5.3.1 is performed.
If, on the other hand, the processing of Section XXX of RFC BBBB
indicate that the request is generally acceptable, the procedures of
Section 5.2.2 are followed.
5.2.1 Requesting an Update
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Instead of generating the repairable error response, a UAS MAY
generate a 155 (Update Requested) response code. If the request
contains a Require header with the value of "update", the UAS MUST
generate the 155 (Update Requested) instead of the repairable error
response.
The 155 is generated exactly as the corresponding repairable error
response would be generated. The only difference is that the status
line has the response code of 155. The UAC will be able to infer the
particular reason for rejection from the headers in the response.
Alternatively, a Reason header [6] MAY be included in the response,
indicating the specific error condition.
OPEN ISSUE: This introduces a normative dependency on the
Reason header draft. I would rather soften the language, so
that there is no specific dependency on that draft. I think
the UAC can always use headers to determine what to do, so
that the response code is really not needed.
The 155 response MUST contain the Require header, with the value of
"update".
OPEN ISSUE: Require is not strictly needed in the response.
The 155 response MUST be sent reliably, using the reliability of
provisional responses extension to SIP [3].
Once the response is sent, the UAS starts a timer. The timer SHOULD
be equal to at least T1*64 seconds. This timer indicates how long the
UAS is willing to wait for the update before terminating the
transaction. During this time, the UAS MUST NOT perform application
layer processing of the request (alert the user, send media, etc.).
Processing of the request is effectively suspended. If the timer
fires before receipt of an UPDATE request on the dialog, the UAS MUST
generate the repairable error response. If an UPDATE is received
before the timer fires, processing continues in Section 6.2.
5.2.2 Continuing With the Dialog
If the request was acceptable, the UAS MAY generate a provisional
response. If that response contains a session description, that
response SHOULD contain a Supported header with the value of "update"
to indicate that this extension is understood.
5.3 Processing Provisional Responses
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If the UAC receives a provisional response with tags, and that
response contains a Supported header with the value of "update", or
the response is a 155, the UAC MUST create an early dialog.
If the response is a 155, the UAC follows the processing of Section
5.3.1. Otherwise, the response is processed normally. The requestor
can generate an UPDATE request within the dialog, as described in
Section 6.1.
5.3.1 Processing the 155
The 155 provisional response indicates to the UAC that the request it
generated was not acceptable, and that a modification of the request
in some way may make the request acceptable. That modification is
performed with an UPDATE request. Additional processing is applied as
described below.
If the 155 response contains a WWW-Authenticate header, the UAC
SHOULD include a Authorization header in the UPDATE with a response
to the challenge. The Authorization header is constructed as
described in Section XXX of RFC BBBB.
If the 155 response contains a Accept header, the UAC MAY include a
body in the UPDATE. That body MUST be with one of the types listed in
the Accept header of the 155. Similarly, if the 155 contains a
Accept-Encoding header, the UAC MAY apply an encoding to any bodies
in the request, but MUST only apply encodings listed in the Accept-
Encoding header of the 155.
If the 155 response contains a Unsupported header, the UAC MUST NOT
include a Require header in the UPDATE listing any option tags
present in the Unsupported header from the 155.
If the 155 response contains a description of media capabilities in
the body (for example, an SDP formatted according to Section XXX of
RFC OOOO [4]), the UAC MUST include a new offer in the UPDATE.
If the 155 response contains a Min-Expires header, the UAC MUST NOT
include a Expires header or parameter in the UPDATE with a value less
than the value in the Min-Expires header.
The UAC MAY add any other optional headers for the UPDATE request, as
defined in Table XXX.
The offer/answer rules in RFC BBBB apply to the UPDATE request.
Specifically, the UPDATE can only contain an offer if there are no
outstanding offers. Since the 155 effectively rejects the offer, the
UPDATE can contain a new offer.
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The UPDATE request is then sent according to the procedures of
Section XXX of RFC BBBB.
6 UPDATE Handling
6.1 Sending an UPDATE
The UPDATE request is constructed as would any other request within
an existing dialog, as described in Section XXX of RFC BBBB. It MAY
be sent for both early and confirmed dialogs.
6.1.1 Specifics for INVITE
In the case of INVITE, the UPDATE request can contain an offer, and
the response to the UPDATE would contain the answer. Of course, the
rules for inclusion of offers and answers in SIP messages as defined
in Section XXX of RFC BBBB still apply. Typically, 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 UAC 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
generated answers for any others 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 responses, and has not sent
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or received other 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 can contain an offer unless the callee has sent or
received other 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
which have not been answered.
6.1.2 Specifics for SUBSCRIBE
An UPDATE request MUST NOT be sent for SUBSCRIBE unless a SUBSCRIBE
request has generated a 155 response.
6.2 Receiving an UPDATE
If an UPDATE is received which does not match an existing dialog, it
MUST be rejected with a 487 response.
When a UAS receives an UPDATE request, it is being asked to update
aspects of the session or other application state, but not the dialog
state itself (which will be one of early, confirmed, or non-
existent). The UPDATE request is a target refresh request.
If the UPDATE matches an existing dialog, it is processed as would
any mid-dialog request, as per Section XXX of RFC BBBB. Furthermore,
its processing then follows the method-specific behavior as if the
UPDATE had the same method as the request which created the dialog.
However, the response to UPDATE does not result in a change to the
dialog state (i.e., it does not change if from early to confirmed).
Furthermore, in many cases, the session or other application state is
also confirmed when the dialog is established. The response to UPDATE
does not cause such confirmation either. In the case of INVITE, for
example, a 2xx response to an initial INVITE confirms the setup of
the call, but a 2xx response to UPDATE does not. As such, an UPDATE
MUST generate an immediate final response.
If the UPDATE is received after processing of the original request
was suspended because a 155 was generated, processing continues as if
the repairable error response had actually been sent, and, as above,
the UPDATE was the same method as that request. However, as described
above, the final response to UPDATE does not change the state of the
dialog, nor does it confirm any application state, and therefore,
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SHOULD generate a 2xx response immediately once it passes the
processing steps in Section XXX of RFC BBBB [processing mid-dialog
requests]. Once the application determines what the response to
request would have been, for example, acceptance of the call, the
original request is responded to with that response code.
OPEN ISSUE: The words above don't seem concise enough to
properly determine how to respond to UPDATE and the
original request. The difficulty is in the attempt to make
it method agnostic, and because we lack a concise split of
state between SIP itself, and the applications that make
use of it.
6.2.1 Specifics for INVITE
If the UPDATE is received by the UAS, and it had previously sent a
155, the UPDATE is processed like a re-INVITE. Assuming the steps of
Section XXX of RFC BBBB pass, the UPDATE should be responded to with
a 2xx response, and then the processing of the original request
continues, as if the original request were the same as the UPDATE
(excepting the method, of course). This means that the callee is
alerted, for example, or media can begin to be sent. The original
request can be rejected, of course, if a call screening application
decides that the caller is on the block list.
6.2.2 Specifics for SUBSCRIBE
If an UPDATE is received before the original SUBSCRIBE is responded
to (which should only occur if a 155 is sent), the UPDATE is
processed as if it were a new SUBSCRIBE refresh. Specifically, the
expiration, acceptable notification formats (from the Accept,
Accept-Encoding and Accept-Language headers), and body information
are used for the subscription. The UPDATE generates an immediate 2xx
response as long as those parameters are acceptable. Once the status
of the subscription itself is determined, the original SUBSCRIBE is
responded to. For example, if the status is pending, a 202 would be
sent to the SUBSCRIBE.
7 Proxy Behavior
If a proxy receives a request with a Supported header containing the
option tag "update", and the proxy knows it is going to fork the
request, either in parallel or in serial, it SHOULD add a Require
header to all proxied requests with the option tag "update" if such a
header is not already present.
For each request which generates a 420 response containing the
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Unsupported header with the value of "update", the proxy MUST
generate another branch to the same destination, this time, without
the Require header added in.
This behavior helps ensure that forking applications work properly,
but also provides for backwards compatibility with endpoints which do
not support this extension.
8 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
The following extends Table 2 of RFC BBBB for the UPDATE method:
The following extends Table 3 of RFC BBBB for UPDATE:
9 Definition of the 155 (Update Requested) Response
The semantics of the 155 (Update Requested) response code are defined
above. The default reason phrase is "Update Requested".
10 Example Call Flows
TODO
11 Security Considerations
This specification describes a condition under which a UAS would
receive a request that is unauthenticated, and maintain state for
processing that request, for a duration of 32 seconds. This
introduces a potential denial-of-service attack. Implementations
SHOULD restrict the number of transactions which are pending with a
155 to a small number. Subsequent unauthenticated requests SHOULD
then be rejected with a 401, even if a Require header is present with
the option tag "update".
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Header field where proxy UPDATE
____________________________________________
Accept R o
Accept 2xx o
Accept 415 o
Accept-Encoding R o
Accept-Encoding 2xx o
Accept-Encoding 415 o
Accept-Language R o
Accept-Language 2xx o
Accept-Language 415 o
Alert-Info R am o
Alert-Info 180 am o
Allow R o
Allow 2xx o
Allow r o
Allow 405 o
Authentication-Info 2xx o
Authorization R o
Call-ID c r m
Call-Info am o
Contact R o
Contact 1xx o
Contact 2xx o
Contact 3xx o
Contact 485 o
Content-Disposition o
Content-Encoding o
Content-Language o
Content-Length r t
Content-Type *
CSeq c r m
Date a o
Error-Info 300-699 o
Expires o
From c r m
In-Reply-To R o
Max-Forwards R amr m
Min-Expires 423 o
MIME-Version o
Organization am o
Table 1: Summary of header fields, A--O
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Header field where proxy UPDATE
____________________________________________________
Priority R a o
Proxy-Authenticate 407 m
Proxy-Authorization R r o
Proxy-Require R acr o
RAck R -
Record-Route R amr o
Record-Route 2xx,401,484 o
Reply-To o
Require acr o
Retry-After 404,413,480,486 o
500,503 o
600,603 o
Route R r c
RSeq 1xx o
Server r o
Subject R o
Supported R o
Supported 2xx o
Timestamp o
To c(1) r m
Unsupported 420 o
User-Agent o
Via c acmr m
Warning r o
WWW-Authenticate 401 m
Table 2: Summary of header fields, P--Z; (1): copied with possible
addition of tag
These guidelines imply that the usage of 155 for unauthenticated
requests is useful primarily for single user devices, which typically
have only a few transactions outstanding a time in general.
12 IANA Considerations
There are no IANA considerations associated with this specification.
13 Author's Addresses
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
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email: jdrosen@dynamicsoft.com
14 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," Request for Comments 2119, Internet Engineering Task Force,
Mar. 1997.
[3] J. Rosenberg and H. Schulzrinne, "Reliability of provisional
responses in SIP," Internet Draft, Internet Engineering Task Force,
Feb. 2002. Work in progress.
[4] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
SDP," Internet Draft, Internet Engineering Task Force, Jan. 2002.
Work in progress.
[5] A. Roach, "SIP-specific event notification," Internet Draft,
Internet Engineering Task Force, Nov. 2001. Work in progress.
[6] H. Schulzrinne, G. Camarillo, and D. Oran, "The reason header
field for the session initiation protocol," Internet Draft, Internet
Engineering Task Force, Dec. 2001. Work in progress.
15 Non-Normative References
[7] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments 2543, Internet
Engineering Task Force, Mar. 1999.
Full Copyright Statement
Copyright (c) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
J.Rosenberg [Page 13]
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Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
J.Rosenberg [Page 14]
Table of Contents
1 Introduction ........................................ 2
2 Terminology ......................................... 2
3 Definitions ......................................... 3
4 Overview of Operation ............................... 3
5 UA Behavior ......................................... 4
5.1 Initiating a Dialog ................................. 4
5.2 Generating a Provisional Response ................... 4
5.2.1 Requesting an Update ................................ 4
5.2.2 Continuing With the Dialog .......................... 5
5.3 Processing Provisional Responses .................... 5
5.3.1 Processing the 155 .................................. 6
6 UPDATE Handling ..................................... 7
6.1 Sending an UPDATE ................................... 7
6.1.1 Specifics for INVITE ................................ 7
6.1.2 Specifics for SUBSCRIBE ............................. 8
6.2 Receiving an UPDATE ................................. 8
6.2.1 Specifics for INVITE ................................ 9
6.2.2 Specifics for SUBSCRIBE ............................. 9
7 Proxy Behavior ...................................... 9
8 Definition of the UPDATE method ..................... 10
9 Definition of the 155 (Update Requested) Response
................................................................ 10
10 Example Call Flows .................................. 10
11 Security Considerations ............................. 10
12 IANA Considerations ................................. 12
13 Author's Addresses .................................. 12
14 Normative References ................................ 13
15 Non-Normative References ............................ 13
EOTOC
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