SIPPING Working Group G. Camarillo
Internet-Draft Ericsson
Expires: January 5, 2005 A. Niemi
H. Khartabil
M. Isomaki
M. Garcia-Martin
Nokia
July 7, 2004
Refering to Multiple Resources in the Session Initiation Protocol
(SIP)
draft-ietf-sipping-multiple-refer-00.txt
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines extensions to the SIP REFER method so that this
method can be used to refer servers to multiple resources. These
extensions include the use of pointers to URI-lists in the Refer-To
header field and the multiple-refer SIP option-tag.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of operation . . . . . . . . . . . . . . . . . . . . 3
4. The multiple-refer SIP Option-Tag . . . . . . . . . . . . . . 4
5. Suppressing REFER's Implicit Subscription . . . . . . . . . . 4
6. Behavior of SIP User Agents . . . . . . . . . . . . . . . . . 5
6.1 Behavior of SIP REFER-Issuers . . . . . . . . . . . . . . 5
6.2 Behavior of REFER-Recipients . . . . . . . . . . . . . . . 5
7. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1 Normative References . . . . . . . . . . . . . . . . . . . . 7
10.2 Informational References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . 10
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1. Introduction
The SIP REFER method [5] allows a user agent to request a server to
send a request to a third party. Still, a number of applications need
to request a server to initiate transactions towards a set of
destinations. In one example, the moderator of a conference may want
the conference server to send BYE requests to a group of
participants. In another example, the same moderator may want the
conference server to INVITE a set of new participants.
We define an extension to REFER so that REFER can be used to refer
servers to multiple destinations. In addition, we use the REFER
extension defined in [7] which suppresses REFER's implicit
subscription.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [1] and indicate requirement levels for
compliant implementations.
We define the following three new terms:
REFER-Issuer: the user agent issuing the REFER request.
REFER-Recipient: the user agent receiving the REFER request.
REFER-Target: the user agent designated in the Refer-To URI.
3. Overview of operation
This document defines an extension to the SIP REFER method [5] that
allows a SIP UAC to include a list of REFER-Targets in a REFER
request and send it to a server. The server will create a new request
for each entry in the list of REFER-Target URIs.
We represent the multiple REFER-Targets of a REFER using the URI-list
format specified in [8]. A UAC (User Agent Client) that wants to
refer a server to a set of destinations creates a SIP REFER request.
The Refer-To header contains a pointer to a URI-list, which is
included in a body part, and two option-tags in the Required header
field: "multiple-refer" and "norefersub". The former indicates the
requirement to support the functionality described in this
specification and the latter removes the implicit subscription
associated to REFER requests by default.
When the server receives such request it creates a new request per
destination and sends them.
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This document does not provide any mechanism for UACs to find out
about the results of a REFER with multiple REFER-Targets.
Furthermore, we do not provide support for the implicit subscription
mechanism that is part of the SIP REFER method. The way UACs are kept
informed about the results of a REFER is service specific. For
example, a UAC sending a REFER to INVITE a set of participants to a
conference may discover which participants were successfully brought
in into the conference by using the conference package [10]
4. The multiple-refer SIP Option-Tag
We define a new SIP option-tag for the Require and Supported header
fields: "multiple-refer".
A user agent including the "multiple-refer" option-tag in a Supported
header indicates compliance with this specification.
A user agent generating a REFER with a pointer to a URI-list in its
Refer-To header field MUST include the "multiple-refer" option-tag in
the Require header field of the REFER.
5. Suppressing REFER's Implicit Subscription
REFER requests with a single REFER-Target establish a subscription
implicitly. The REFER-Issuer is informed about the result of the
transaction towards the REFER-Target through this implicit
subscription.
In the case of a REFER-Issuer that generates a REFER with multiple
REFER-targets, the REFER-Issuer is typically already subscribed to
other event package that can provide the information about the result
of the transactions towards the REFER-Targets. For example, a
moderator instructing a conference server to send a BYE request to a
set of participants is usually subscribed to the conference state
event package for the conference. Notifications to this event package
will keep the moderator and the rest of the subscribers informed of
the current list of conference participant.
Consequently, we have decided to remove the implicit subscription
from a multiple REFER request. So, a SIP REFER-Issuer generating a
REFER request with multiple REFER-Targets MUST include the
'norefersub' option-tag in a Require header field to indicate that no
notifications about the requests should be sent to the REFER-Issue.
The 'norefersub' SIP option-tag is defined in [7] and suppresses the
REFER's implicit subscription.
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6. Behavior of SIP User Agents
Implementations of this specification MUST suppot the transfer
mechanism for URI-lists defined in [8].
6.1 Behavior of SIP REFER-Issuers
As indicated in Section 4 and Section 5 a SIP REFER-Issuer that
creates a REFER request with multiple REFER-Targets includes a
"multiple-refer" and a "norefersub" option-tags in the Require header
field.
The Refer-To header field of a REFER request with multiple
REFER-Targets MUST contain a pointer (i.e., a Content-ID URL [2])
that points to the body part (whose disposition type is "uri-list")
that carries the URI-list.
As described in [8], the default format for URI-lists in SIP is the
XCAP resource list format [6]. Still, specific services need to
describe which information clients should include in their URI lists,
as described in [8].
SIP REFER-Issuers generating REFERs with multiple REFER-Targets
SHOULD use flat lists (i.e., no hierarchical lists), SHOULD NOT use
any entry's attributes but "uri", and SHOULD NOT include any elements
inside entries but "display-name" elements.
6.2 Behavior of REFER-Recipients
A REFER-Recipient receiving a URI-list with more information than
what we have described in Section 6.1 SHOULD discard all the extra
information.
The REFER-Recipient follows the rules in Section 2.4.2 of RFC 3515
[5] to determine the status code of the response to the REFER.
7. Example
The following is an example of a REFER request with multiple
REFER-Targets. The REFER's Refer-To header field carries a pointer to
the message body, which carries a list with the URIs of the
REFER-Targets. The REFER's Require header field carries both the
"multiple-refer" and the "norefersub" option-tags.
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REFER sip:conf-123@example.com
SIP/2.0 Via: SIP/2.0/TCP client.chicago.example.com
;branch=z9hG4bKhjhs8ass83
Max-Forwards: 70
To: Conference 123 <sip:conf-123@example.com>
From: Carol <sip:carol@chicago.example.com>;tag=32331
Call-ID: d432fa84b4c76e66710
CSeq: 2 REFER
Contact: <sip:carol@client.chicago.example.com>
Refer-To: <cid:cn35t8jf02@example.com>
Require: multiple-refer, norefersub
Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY
Allow-Events: dialog
Accept: application/sdp, message/sipfrag
Content-Type: application/resource-lists+xml
Content-Disposition: uri-list
Content-Length: 307
Content-ID: <cn35t8jf02@example.com>
<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<list>
<entry uri="sip:bill@example.com?method=BYE" />
<entry uri="sip:joe@example.org?method=BYE" />
<entry uri="sip:ted@example.net?method=BYE" />
</list>
</resource-lists>
Figure 1: REFER request with multiple REFER-Targets
8. Security Considerations
The Security Considerations Section of the Requirements and Framework
for SIP URI-List Services [9] discusses issues related to SIP
URI-list services. Given that a server accepting REFERs with multiple
REFER-targets acts as an URI-list service, implementations of this
type of server MUST follow the security-related rules in [9]. These
rules include mandatory authentication and authorization of clients,
and opt-in lists.
Additionally, servers SHOULD only accept REFER requests within the
context of an application the server understands (e.g., a
conferencing application). This implies that servers MUST NOT accept
REFERs for methods they do not understand. The idea behind these two
rules is that servers are not used as dumb servers whose only
function is to fan-out random messages they do not understand.
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9. IANA Considerations
This document defines a SIP option-tag (multiple-refer) in Section 4.
This option-tag should be registered in the SIP parameter registry
(http://www.iana.org/assignments/sip-parameters).
SIP user agents that place the multiple-refer option-tag in a
Supported header field understand REFER requests with multiple
REFER-Targets.
10. References
10.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[3] 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.
[4] Sparks, R., "Internet Media Type message/sipfrag", RFC 3420,
November 2002.
[5] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[6] Rosenberg, J., "An Extensible Markup Language (XML)
Configuration Access Protocol (XCAP) Usage for Presence Lists",
draft-ietf-simple-xcap-list-usage-02 (work in progress),
February 2004.
[7] Olson, S., "Extended-REFER framework and other REFER
extensions", draft-olson-sipping-refer-extensions-01 (work in
progress), February 2004.
[8] Camarillo, G., "Providing a Session Initiation Protocol (SIP)
Application Server with a List of URIs",
draft-camarillo-sipping-uri-list-01 (work in progress), February
2004.
[9] Camarillo, G., "Requirements for Session Initiation Protocol
(SIP) Exploder Invocation", draft-camarillo-sipping-exploders-02
(work in progress), February 2004.
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10.2 Informational References
[10] Rosenberg, J. and H. Schulzrinne, "A Session Initiation
Protocol (SIP) Event Package for Conference State",
draft-ietf-sipping-conference-package-03 (work in progress),
February 2004.
[11] Camarillo, G., "A Transaction Event Package for the Session
Initiation Protocol (SIP)",
draft-camarillo-sipping-transac-package-00 (work in progress),
February 2004.
Authors' Addresses
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
Aki Niemi
Nokia
P.O. Box 321
NOKIA GROUP, FIN 00045
Finland
EMail: Aki.Niemi@nokia.com
Hisham Khartabil
Nokia
P.O. Box 321
NOKIA GROUP, FIN 00045
Finland
EMail: Hisham.Khartabil@nokia.com
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Markus Isomaki
Nokia
Itamerenkatu 11-13
Helsinki 00180
Finland
EMail: Markus.Isomaki@nokia.com
Miguel A. Garcia-Martin
Nokia
P.O.Box 407
NOKIA GROUP, FIN 00045
Finland
EMail: miguel.an.garcia@nokia.com
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