Network Working Group G. Camarillo

Network Working Group G. Camarillo
Request for Comments: 5366 Ericsson
Category: Standards Track A. Johnston
Avaya
October 2008

Conference Establishment Using Request-Contained Lists
in the Session Initiation Protocol (SIP)

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.

Abstract

This document describes how to create a conference using SIP URI-list
services. In particular, it describes a mechanism that allows a User
Agent Client to provide a conference server with the initial list of
participants using an INVITE-contained URI list.

Table of Contents

1. Introduction ....................................................2
2. Terminology .....................................................2
3. User Agent Client Procedures ....................................2
3.1. Response Handling ..........................................2
3.2. Re-INVITE Request Generation ...............................3
4. URI-List Document Format ........................................3
5. Conference Server Procedures ....................................5
5.1. Re-INVITE Request Handling .................................6
6. Example .........................................................6
7. Security Considerations ........................................10
8. IANA Considerations ............................................10
9. Acknowledgments ................................................11
10. References ....................................................11
10.1. Normative References .....................................11
10.2. Informative References ...................................12

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1. Introduction

Section 5.4 of [RFC4579] describes how to create a conference using
ad hoc SIP [RFC3261] methods. The client sends an INVITE request to
a conference factory URI and receives the actual conference URI,
which contains the "isfocus" feature tag, in the Contact header field
of a response -- typically a 200 (OK) response.

Once the UAC (User Agent Client) obtains the conference URI, it can
add participants to the newly created conference in several ways,
which are described in [RFC4579].

Some environments have tough requirements regarding conference
establishment time. They require the UAC to be able to request the
creation of an ad hoc conference and to provide the conference server
with the initial set of participants in a single operation. This
document describes how to meet this requirement using the mechanism
to transport URI lists in SIP messages described in [RFC5363].

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 [RFC2119].

3. User Agent Client Procedures

A UAC that wants to include the set of initial participants in its
initial INVITE request to create an ad hoc conference adds a body
whose disposition type is 'recipient-list', as defined in [RFC5363],
with a URI list that contains the participants that the UAC wants the
conference server to invite. Additionally, the UAC MUST include the
'recipient-list-invite' option-tag (which is registered with the IANA
in Section 8) in a Require header field. The UAC sends this INVITE
request to the conference factory URI.

The INVITE transaction is also part of an offer/answer exchange that
will establish a session between the UAC and the conference server,
as specified in [RFC4579]. Therefore, the INVITE request may need to
carry a multipart body: a session description and a URI list.

3.1. Response Handling

The status code in the response to the INVITE request does not
provide any information about whether or not the conference server
was able to bring the users in the URI list into the conference.
That is, a 200 (OK) response means that the conference was created
successfully, that the UAC that generated the INVITE request is in

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the conference, and that the server understood the URI list. If the
UAC wishes to obtain information about the status of other users in
the conference, it SHOULD use general conference mechanisms, such as
the conference package, which is defined in [RFC4575].

3.2. Re-INVITE Request Generation

The previous sections have specified how to include a URI list in an
initial INVITE request to a conference server. Once the INVITE-
initiated dialog between the UAC and the conference server has been
established, the UAC can send subsequent INVITE requests (typically
referred to as re-INVITE requests) to the conference server to, for
example, modify the characteristics of the media exchanged with the
server.

At this point, there are no semantics associated with 'recipient-
list' bodies in re-INVITE requests (although future extensions may
define them). Therefore, UACs SHOULD NOT include 'recipient-list'
bodies in re-INVITE requests sent to a conference server.

Note that a difference between an initial INVITE request and a
re-INVITE request is that while the initial INVITE request is sent
to the conference factory URI, the re-INVITE request is sent to
the URI provided by the server in a Contact header field when the
dialog was established. Therefore, from the UAC's point of view,
the resource identified by the former URI supports 'recipient-
list' bodies, while the resource identified by the latter does not
support them.

4. URI-List Document Format

As described in [RFC5363], specifications of individual URI-list
services, like the conferencing service described here, need to
specify a default format for 'recipient-list' bodies used within the
particular service.

The default format for 'recipient-list' bodies for conferencing UAs
(User Agents) is the XML resource list format (which is specified in
[RFC4826]) extended with the "Extensible Markup Language (XML) Format
Extension for Representing Copy Control Attributes in Resource Lists"
[RFC5364]. Consequently, conferencing UACs generating 'recipient-
list' bodies MUST support both of these formats and MAY support other
formats. Conferencing servers able to handle 'recipient-list' bodies
MUST support both of these formats and MAY support other formats.

As described in "Extensible Markup Language (XML) Format Extension
for Representing Copy Control Attributes in Resource Lists"
[RFC5364], each URI can be tagged with a 'copyControl' attribute set

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to either "to", "cc", or "bcc", indicating the role in which the
recipient will get the INVITE request. Additionally, URIs can be
tagged with the 'anonymize' attribute to prevent the conference
server from disclosing the target URI in a URI list.

In addition, "Extensible Markup Language (XML) Format Extension for
Representing Copy Control Attributes in Resource Lists" [RFC5364]
defines a 'recipient-list-history' body that contains the list of
recipients. The default format for 'recipient-list-history' bodies
for conferencing UAs is also the XML resource list document format
specified in [RFC4826] extended with "Extensible Markup Language
(XML) Format Extension for Representing Copy Control Attributes in
Resource Lists" [RFC5364]. Consequently, conferencing UACs able to
generate 'recipient-list-history' bodies MUST support these formats
and MAY support others. Conferencing UAs able to understand
'recipient-list-history' MUST support these formats and MAY support
others. Conferencing servers able to handle 'recipient-list-history'
bodies MUST support these formats and MAY support others.

Nevertheless, the XML resource list document specified in [RFC4826]
provides features, such as hierarchical lists and the ability to
include entries by reference relative to the XML Configuration Access
Protocol (XCAP) root URI, that are not needed by the conferencing
service defined in this document, which only needs to transfer a flat
list of URIs between a UA (User Agent) and the conference server.
Therefore, when using the default resource list document,
conferencing UAs SHOULD use flat lists (i.e., no hierarchical lists)
and SHOULD NOT use <entry-ref> elements. A conference factory
application receiving a URI list with more information than what has
just been described MAY discard all the extra information.

Figure 1 shows an example of a flat list that follows the XML
resource list document (specified in [RFC4826]) extended with
"Extensible Markup Language (XML) Format Extension for Representing
Copy Control Attributes in Resource Lists" [RFC5364].

<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
xmlns:cp="urn:ietf:params:xml:ns:copycontrol">
<list>
<entry uri="sip:[email protected]" cp:copyControl="to" />
<entry uri="sip:[email protected]" cp:copyControl="cc" />
<entry uri="sip:[email protected]" cp:copyControl="bcc" />
</list>
</resource-lists>

Figure 1: URI list

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5. Conference Server Procedures

Conference servers that are able to receive and process INVITE
requests with a 'recipient-list' body SHOULD include a 'recipient-
list-invite' option-tag in a Supported header field when responding
to OPTIONS requests.

On reception of an INVITE request containing a 'recipient-list' body
as described in Section 3, a conference server MUST follow the rules
described in [RFC4579] to create ad hoc conferences. Once the ad hoc
conference is created, the conference server SHOULD attempt to add
the participants in the URI list to the conference as if their
addition had been requested using any of the methods described in
[RFC4579].

The INVITE transaction is also part of an offer/answer exchange that
will establish a session between the UAC and the conference server,
as specified in [RFC4579]. Therefore, the INVITE request may carry a
multipart body: a session description and a URI list.

Once the conference server has created the ad hoc conference and has
attempted to add the initial set of participants, the conference
server behaves as a regular conference server and MUST follow the
rules in [RFC4579].

The incoming INVITE request will contain a URI-list body or reference
(as specified in [RFC5363]) with the actual list of recipients. If
this URI list includes resources tagged with the 'copyControl'
attribute set to a value of "to" or "cc", the conference server
SHOULD include a URI list in each of the outgoing INVITE requests.
This list SHOULD be formatted according to the XML format for
representing resource lists (specified in [RFC4826]) and the
copyControl extension specified in [RFC5364].

The URI-list service MUST follow the procedures specified in
[RFC5364] with respect to the handling of the 'anonymize', 'count',
and 'copyControl' attributes.

If the conference server includes a URI list in an outgoing INVITE
request, it MUST include a Content-Disposition header field (which is
specified in [RFC2183]) with the value set to 'recipient-list-
history' and a 'handling' parameter (as specified in [RFC3204]) set
to "optional".

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5.1. Re-INVITE Request Handling

At this point, there are no semantics associated with 'recipient-
list' bodies in re-INVITE requests (although future extensions may
define them). Therefore, a conference server receiving a re-INVITE
request with a 'recipient-list' body and, consequently, a
'recipient-list-invite' option-tag, following standard SIP
procedures, rejects it with a 420 (Bad Extension), which carries an
Unsupported header field listing the 'recipient-list-invite' option-
tag.

This is because the resource identified by the conference URI does
not actually support this extension. On the other hand, the
resource identified by the conference factory URI does support
this extension and, consequently, would include the 'recipient-
list-invite' option-tag in, for example, responses to OPTIONS
requests.

6. Example

Figure 2 shows an example of operation. A UAC sends an INVITE
request (F1) that contains an SDP body and a URI list to the
conference server. The conference server answers with a 200 (OK)
response and generates an INVITE request to each of the UASs (User
Agent Servers) identified by the URIs included in the URI list. The
conference server includes SDP and a manipulated URI list in each of
the outgoing INVITE requests.

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+--------+ +---------+ +--------+ +--------+ +--------+
|SIP UAC | | confer. | |SIP UAS | |SIP UAS | |SIP UAS |
| | | server | | 1 | | 2 | | n |
+--------+ +---------+ +--------+ +--------+ +--------+
| | | | |
| F1 INVITE | | | |
| ---------------->| | | |
| F2 200 OK | | | |
|<---------------- | F3 INVITE | | |
| | ------------->| | |
| | F4 INVITE | | |
| | ------------------------>| |
| | F5 INVITE | | |
| | ----------------------------------->|
| | F6 200 OK | | |
| |<------------- | | |
| | F7 200 OK | | |
| |<------------------------ | |
| | F8 200 OK | | |
| |<----------------------------------- |
| | | | |
| | | | |
| | | | |

Figure 2: Example of operation

Figure 3 shows an example of the INVITE request F1, which carries a
multipart/mixed body composed of two other bodies: an application/sdp
body that describes the session and an application/resource-lists+xml
body that contains the list of target URIs.

INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP atlanta.example.com
;branch=z9hG4bKhjhs8ass83
Max-Forwards: 70
To: "Conf Factory" <sip:[email protected]>
From: Alice <sip:[email protected]>;tag=32331
Call-ID: d432fa84b4c76e66710
CSeq: 1 INVITE
Contact: <sip:[email protected]>
Allow: INVITE, ACK, CANCEL, BYE, REFER
Allow-Events: dialog
Accept: application/sdp, message/sipfrag
Require: recipient-list-invite
Content-Type: multipart/mixed;boundary="boundary1"
Content-Length: 690

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--boundary1
Content-Type: application/sdp

v=0
o=alice 2890844526 2890842807 IN IP4 atlanta.example.com
s=-
c=IN IP4 192.0.2.1
t=0 0
m=audio 20000 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 20002 RTP/AVP 31
a=rtpmap:31 H261/90000

--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list

<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
xmlns:cp="urn:ietf:params:xml:ns:copyControl">
<list>
<entry uri="sip:[email protected]" cp:copyControl="to" />
<entry uri="sip:[email protected]" cp:copyControl="to"
cp:anonymize="true"/>
<entry uri="sip:[email protected]" cp:copyControl="to"
cp:anonymize="true"/>
<entry uri="sip:[email protected]" cp:copyControl="cc" />
<entry uri="sip:[email protected]" cp:copyControl="cc"
cp:anonymize="true"/>
<entry uri="sip:[email protected]" cp:copyControl="bcc" />
<entry uri="sip:[email protected]" cp:copyControl="bcc" />
</list>
</resource-lists>
--boundary1--

Figure 3: INVITE request received at the conference server

The INVITE requests F3, F4, and F5 are similar in nature. All those
INVITE requests contain a multipart/mixed body that is composed of
two other bodies: an application/sdp body describing the session and
an application/resource-lists+xml containing the list of recipients.
The application/resource-lists+xml bodies are not equal to the
application/resource-lists+xml included in the received INVITE
request F1, because the conference server has anonymized those URIs
tagged with the 'anonymize' attribute and has removed those URIs
tagged with a "bcc" 'copyControl' attribute. Figure 4 shows an
example of the message F3.

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INVITE sip:[email protected] SIP/2.0
Via: SIP/2.0/TCP conference.example.com
;branch=z9hG4bKhjhs8as454
Max-Forwards: 70
To: <sip:[email protected]>
From: Conference Server <sip:[email protected]>;tag=234332
Call-ID: 389sn189dasdf
CSeq: 1 INVITE
Contact: <sip:[email protected]>;isfocus
Allow: INVITE, ACK, CANCEL, BYE, REFER
Allow-Events: dialog, conference
Accept: application/sdp, message/sipfrag
Content-Type: multipart/mixed;boundary="boundary1"
Content-Length: 690

--boundary1
Content-Type: application/sdp

v=0
o=conf 2890844343 2890844343 IN IP4 conference.example.com
s=-
c=IN IP4 192.0.2.5
t=0 0
m=audio 40000 RTP/AVP 0
a=rtpmap:0 PCMU/8000
m=video 40002 RTP/AVP 31
a=rtpmap:31 H261/90000

--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list-history; handling=optional

<?xml version="1.0" encoding="UTF-8"?>
<resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
xmlns:cp="urn:ietf:params:xml:ns:copycontrol">
<list>
<entry uri="sip:[email protected]" cp:copyControl="to" />
<entry uri="sip:[email protected]" cp:copyControl="to"
cp:count="2"/>
<entry uri="sip:[email protected]" cp:copyControl="cc" />
<entry uri="sip:[email protected]" cp:copyControl="cc"
cp:count="1"/>
</list>
</resource-lists>
--boundary1--

Figure 4: INVITE request sent by the conference server

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7. Security Considerations

This document discusses setup of SIP conferences using a request-
contained URI list. Both conferencing and URI-list services have
specific security requirements, which are summarized here.
Conferences generally have authorization rules about who can or
cannot join a conference, what type of media can or cannot be used,
etc. This information is used by the focus to admit or deny
participation in a conference. It is RECOMMENDED that these types of
authorization rules be used to provide security for a SIP conference.

For this authorization information to be used, the focus needs to be
able to authenticate potential participants. Normal SIP mechanisms,
including Digest authentication and certificates, can be used. These
conference-specific security requirements are discussed further in
the requirements and framework documents -- [RFC4245] and [RFC4353].

For conference creation using a list, there are some additional
security considerations. "Framework and Security Considerations for
Session Initiation Protocol (SIP) URI-List Services" [RFC5363]
discusses issues related to SIP URI-list services. Given that a
conference server sending INVITE requests to a set of users acts as a
URI-list service, implementations of conference servers that handle
lists MUST follow the security-related rules in [RFC5363]. These
rules include opt-in lists and mandatory authentication and
authorization of clients.

8. IANA Considerations

This document defines the 'recipient-list-invite' SIP option-tag. It
has been registered in the Option Tags subregistry under the SIP
parameter registry. The following is the description used in the
registration:

+------------------------+------------------------------+-----------+
| Name | Description | Reference |
+------------------------+------------------------------+-----------+
| recipient-list-invite | The body contains a list of | [RFC5366] |
| | URIs that indicates the | |
| | recipients of the SIP INVITE | |
| | request | |
+------------------------+------------------------------+-----------+

Table 1: Registration of the 'recipient-list-invite' option-tag
in SIP

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9. Acknowledgments

Cullen Jennings, Hisham Khartabil, Jonathan Rosenberg, and Keith
Drage provided useful comments on this document. Miguel Garcia-
Martin assembled the dependencies to the 'copyControl' attribute
extension.

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2183] Troost, R., Dorner, S., and K. Moore, Ed., "Communicating
Presentation Information in Internet Messages: The
Content-Disposition Header Field", RFC 2183, August 1997.

[RFC3204] Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet,
F., Watson, M., and M. Zonoun, "MIME media types for ISUP
and QSIG Objects", RFC 3204, December 2001.

[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.

[RFC4579] Johnston, A. and O. Levin, "Session Initiation Protocol
(SIP) Call Control - Conferencing for User Agents", BCP
119, RFC 4579, August 2006.

[RFC4826] Rosenberg, J., "Extensible Markup Language (XML) Formats
for Representing Resource Lists", RFC 4826, May 2007.

[RFC5363] Camarillo, G. and A.B. Roach, "Framework and Security
Considerations for Session Initiation Protocol (SIP) URI-
List Services", RFC 5363, October 2008.

[RFC5364] Garcia-Martin, M. and G. Camarillo, "Extensible Markup
Language (XML) Format Extension for Representing Copy
Control Attributes in Resource Lists", RFC 5364, October
2008.

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10.2. Informative References

[RFC4245] Levin, O. and R. Even, "High-Level Requirements for
Tightly Coupled SIP Conferencing", RFC 4245, November
2005.

[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353, February
2006.

[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, Ed., "A
Session Initiation Protocol (SIP) Event Package for
Conference State", RFC 4575, August 2006.

Authors' Addresses

Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland

EMail: [email protected]

Alan Johnston
Avaya
St. Louis, MO 63124
USA

EMail: [email protected]

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