Referring to Multiple Resources in the Session Initiation Protocol (SIP)
draft-ietf-sip-multiple-refer-03

SIP Working Group                                           G. Camarillo
Internet-Draft                                                  Ericsson
Intended status: Standards Track                                A. Niemi
Expires: June 20, 2008                                        M. Isomaki
                                                                   Nokia
                                                        M. Garcia-Martin
                                                  Nokia Siemens Networks
                                                            H. Khartabil
                                                      Ericsson Australia
                                                       December 18, 2007

Referring to Multiple Resources in the Session Initiation Protocol (SIP)
                  draft-ietf-sip-multiple-refer-03.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document defines extensions to the SIP REFER method so that this
   method can be used to refer to multiple resources in a single

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   request.  These extensions include the use of pointers to Uniform
   Resource Identifier (URI)-lists in the Refer-To header field and the
   "multiple-refer" SIP option-tag.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Overview of operation  . . . . . . . . . . . . . . . . . . . .  4
   4.  The multiple-refer SIP Option-Tag  . . . . . . . . . . . . . .  5
   5.  Suppressing REFER's Implicit Subscription  . . . . . . . . . .  5
   6.  URI-List Format  . . . . . . . . . . . . . . . . . . . . . . .  6
   7.  Behavior of SIP REFER-Issuers  . . . . . . . . . . . . . . . .  7
   8.  Behavior of REFER-Recipients . . . . . . . . . . . . . . . . .  8
   9.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     12.1.  Normative References  . . . . . . . . . . . . . . . . . . 12
     12.2.  Informative References  . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 15

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1.  Introduction

   RFC 3261 (SIP) [RFC3261] is extended by RFC 3515 [RFC3515] with a
   REFER method that allows a user agent to request a second user agent
   to send a SIP request to a third party.  Still, a number of
   applications need to request this second user agent 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 the REFER method so that REFER requests can
   be used to refer other user agents (such as conference servers) to
   multiple destinations.  In addition, this mechanism uses the
   suppression of the REFER method implicit subscription specified in
   RFC 4488 [RFC4488] to suppress REFER's implicit subscription.

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 BCP 14, RFC 2119
   [RFC2119] and indicate requirement levels for compliant
   implementations.

   This document reuses the following terminology defined in RFC 3261
   [RFC3261]:

   o  User Agent (UA)
   o  User Agent Client (UAC)
   o  User Agent Server (UAS)

   This document defines the following new terms:

   REFER-Issuer:  a user agent issuing a REFER request.
   REFER-Recipient:  an entity receiving a REFER request and forwarding
      a SIP request to a number of REFER-Targets.  The REFER-Recipient
      is typically a network entity, such as a URI-List server, that
      acts as a UAS for REFER requests and as a UAC for other SIP
      requests.
   REFER-Target:  a UA of the intended final recipient of a SIP request
      generated by the REFER-Recipient.

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3.  Overview of operation

   This document describes an application of URI-List services
   [I-D.ietf-sipping-uri-services] that allows a URI-List service to
   receive a SIP REFER request containing a list of targets.  The URI-
   List service invokes the requested SIP method to each of the targets
   contained in the list.  This type of URI-List service is referred to
   as a REFER-Recipient throughout this document.

   This document defines an extension to the SIP REFER method specified
   in RFC 3515 [RFC3515] that allows a SIP UAC to include a URI-list as
   specified in the XML Format for Representing Resource Lists [RFC4826]
   of REFER-Targets in a REFER request and send it to a REFER-Recipient.
   The REFER-Recipient creates a new SIP request for each entry in the
   URI-list and sends it to each REFER-Recipient.

   The URI-list that contains the list of targets is used in conjunction
   with the XML Format Extension for Representing Copy Control
   Attributes in Resource Lists [I-D.ietf-sipping-capacity-attribute] to
   allow the sender indicate the role (e.g., 'to', 'cc', or anonymous)
   in which the REFER-Target is involved in the signalling.

   We represent multiple targets of a REFER request using a URI-list as
   specified in the XML Format for Representing Resource Lists
   [RFC4826].  A REFER-Issuer that wants to refer a REFER-Recipient 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 an option-tag in the Require header field: "multiple-refer".
   This option-tag indicates the requirement to support the
   functionality described in this specification.

   When the REFER-Recipient receives such request it creates a new
   request per REFER-Target and sends them, one to each REFER-Target.

   This document does not provide any mechanism for REFER-Issuers to
   find out about the results of a REFER request containing multiple
   REFER-Targets.  Furthermore, it does not provide support for the
   implicit subscription mechanism that is part of the SIP REFER method.
   The way REFER-Issuers are kept informed about the results of a REFER
   is service specific.  For example, a REFER-Issuer sending a REFER
   request to invite a set of participants to a conference can discover
   which participants were successfully brought into the conference by
   subscribing to the conference state event package specified in RFC
   4575 [RFC4575].

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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 field 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 implicitly a
   subscription to the refer event.  The REFER-Issuer is informed about
   the result of the transaction towards the REFER-Target through this
   implicit subscription.  As described in RFC 3515 [RFC3515], NOTIFY
   requests sent as a result of an implicit subscription created by a
   REFER request contain a body of type "message/sipfrag", RFC 3420
   [RFC3420], that describes the status of the transaction initiated by
   the REFER-Recipient.

   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 participants.

   Most of the applications using multiple REFER do not need its
   implicit subscription.  Consequently, a SIP REFER-Issuer generating a
   REFER request with multiple REFER-Targets SHOULD include the
   "norefersub" option-tag in a Require header field and SHOULD include
   a Refer-Sub header field set to "false" to indicate that no
   notifications about the requests should be sent to the REFER-Issuer.
   The REFER-Recipient SHOULD honor the suggestion and also include a
   Refer-Sub header field set to "false" in the 200 (OK) response.  The
   "norefersub" SIP option-tag and the Refer-Sub header field are
   specified in RFC 4488 [RFC4488].

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      RFC 4488 [RFC4488] indicates that a condition for the REFER-Issuer
      to include a Refer-Sub header is that the REFER-Issuer is sure
      that the REFER request will not fork.

   At the time of writing, there is no extension that allows to report
   the status of several transactions over the implicit subscription
   associated with a REFER dialog.  That is the motivation for this
   document to recommend the usage of the "norefersub" option-tag.  If
   in the future such an extension is defined, REFER-Issuers using it
   could refrain from using the "norefersub" option-tag and use the new
   extension instead.

6.  URI-List Format

   As described in the Framework and Security Considerations for SIP
   URI-List Services [I-D.ietf-sipping-uri-services], specifications of
   individual URI-list services need to specify a default format for
   'recipient-list' bodies used within the particular service.

   The default format for 'recipient-list' bodies for REFER-Issuers and
   REFER-Recipients is the XML Formats for Representing Resource Lists
   [RFC4826] extended with the XML Format Extension for Representing
   Copy Control Attributes in Resource Lists
   [I-D.ietf-sipping-capacity-attribute].  REFER-Recipients handling
   'recipient-list' bodies MUST support both of these formats.  Both
   REFER-Issuers and REFER-Recipients MAY support other formats.

   As described in the XML Format Extension for Representing Copy
   Control Attributes in Resource Lists
   [I-D.ietf-sipping-capacity-attribute], each URI can be tagged with a
   'copyControl' attribute set to either "to", "cc", or "bcc",
   indicating the role in which the target will get the referred SIP
   request.  However, depending on the target SIP method, a
   'copyControl' attribute lacks sense.  For example, while a
   'copyControl' attribute can be applied to INVITE requests, it does
   not make sense with mid-dialog requests such as BYE requests.

   In addition to the 'copyControl' attribute, URIs can be tagged with
   the 'anonymize' attribute, also specified in the XML Format Extension
   for Representing Copy Control Attributes in Resource Lists
   [I-D.ietf-sipping-capacity-attribute] to prevent that the REFER-
   Recipient discloses the target URI in a URI-list.

   Additionally, the XML Format Extension for Representing Copy Control
   Attributes in Resource Lists [I-D.ietf-sipping-capacity-attribute]
   defines a 'recipient-list-history' body that contains the list of
   targets.  The default format for 'recipient-list-history' bodies for

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   conference services is also the XML Formats for Representing Resource
   Lists [RFC4826] extended with the XML Format Extension for
   Representing Copy Control Attributes in Resource Lists
   [I-D.ietf-sipping-capacity-attribute].  REFER-Recipients supporting
   this specification MUST support both of these formats; REFER-Targets
   MAY support these formats.  Both REFER-Recipients and REFER-Targets
   MAY support other formats.

   Nevertheless, the XML Format for Representing Resource Lists
   [RFC4826] document provides features, such as hierarchical lists and
   the ability to include entries by reference relative to the XCAP root
   URI, that are not needed by the multiple REFER service defined in
   this document.

   Figure 1 shows an example of a flat list that follows the resource
   list document.

   <?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:bill@example.com" cp:copyControl="to"  />
       <entry uri="sip:joe@example.org" cp:copyControl="cc" />
       <entry uri="sip:ted@example.net" cp:copyControl="bcc" />
     </list>
   </resource-lists>

                            Figure 1: URI List

7.  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 "norefersub" option-tags in the Require header
   field and, if appropriate, a Refer-Sub header field set to "false".
   The REFER-Issuer includes the set of REFER-Targets in a recipient-
   list body whose disposition type is 'recipient-list', as defined in
   the Framework and Security Considerations for SIP URI-List Services
   [I-D.ietf-sipping-uri-services].  The URI-list body is further
   described in Section 6.

   The Refer-To header field of a REFER request with multiple REFER-
   Targets MUST contain a pointer (i.e., a Content-ID Uniform Resource
   Locator (URL) as per RFC 2392 [RFC2392]) that points to the body part
   that carries the URI-list.  The REFER-Issuer SHOULD NOT include any
   particular URI more than once in the URI-list.

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   The XML Format for Representing Resource Lists [RFC4826] document
   provides features, such as hierarchical lists and the ability to
   include entries by reference relative to the XCAP root URI.  However,
   these features are not needed by the multiple REFER service defined
   in this document.  Therefore, when using the default resource list
   document, SIP REFER-Issuers generating REFER requests with multiple
   REFER-Targets SHOULD use flat lists (i.e., no hierarchical lists) and
   SHOULD NOT use <entry-ref> elements.

8.  Behavior of REFER-Recipients

   The REFER-Recipient follows the rules in Section 2.4.2 of RFC 3515
   [RFC3515] to determine the status code of the response to the REFER.

   The REFER-Recipient SHOULD not create an implicit subscription, and
   SHOULD add a Refer-Sub header field set to "false" in the 200 OK
   response.

   The incoming REFER request typically contains a URI-list document or
   reference with the actual list of targets.  If this URI-list includes
   resources tagged with the 'copyControl' attribute set to a value of
   "to" or "cc", and if appropriate for the service, e.g., if it is non-
   mid dialog request, the REFER-Recipient SHOULD include a URI-list in
   each of the outgoing requests.  This list SHOULD be formatted
   according to the XML Format for Representing Resource Lists [RFC4826]
   and the XML Format Extension for Representing Copy Control Attributes
   in Resource Lists [I-D.ietf-sipping-capacity-attribute].  The REFER-
   Recipient MUST follow the procedures specified in XML Format for
   Representing Resource Lists [RFC4826] with respect handling of the
   'anonymize', 'count' and 'copyControl' attributes.

   Section 4 of the Framework and Security Considerations for SIP URI-
   List Services [I-D.ietf-sipping-uri-services] discusses cases when
   duplicated URIs are found in a URI-list.  In order to avoid
   duplicated requests, REFER-Recipients MUST take those actions
   specified in Framework and Security Considerations for SIP URI-List
   Services [I-D.ietf-sipping-uri-services] into account to avoid
   sending duplicated request to the same target.

   If the REFER-Recipient includes a URI-list in an outgoing request, it
   MUST include a Content-Disposition header field, specified in RFC
   2183 [RFC2183], with the value set to 'recipient-list-history' and a
   'handling' parameter, specified in RFC 3204 [RFC3204], set to
   "optional".

   Since the multiple REFER service does not use hierarchical lists nor
   lists that include entries by reference to the XCAP root URI, a

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   REFER-Recipient receiving a URI-list with more information than what
   has been described in Section 6 MAY discard all the extra
   information.

   The REFER-Recipient follows the rules in RFC 3515 [RFC3515] to
   generate the necessary requests towards the REFER-Targets, acting as
   if it had received a regular (no URI-list) REFER per each URI in the
   URI-list.

9.  Example

   Figure 2 shows an example flow where a REFER-Issuer sends a multiple-
   REFER request to the focus of a conference, which acts as the REFER-
   Recipient.  The REFER-Recipient generates a BYE request per REFER-
   Target.  Details for using REFER request to remove participants from
   a conference are specified in RFC 4579 [RFC4579].

   +--------+         +---------+    +--------+  +--------+  +--------+
   | REFER  |         |  REFER  |    | REFER  |  | REFER  |  | REFER  |
   | issuer |         |recipient|    |target 1|  |target 2|  |target 3|
   |        |         |         |    |        |  |        |  |        |
   | Carol  |         | (focus) |    |  Bill  |  |  Joe   |  |  Ted   |
   +--------+         +---------+    +--------+  +--------+  +--------+
        | 1. REFER         |             |           |           |
        | ---------------->|             |           |           |
        | 2. 202 Accepted  |             |           |           |
        |<---------------- |   3. BYE    |           |           |
        |                  | ----------->|           |           |
        |                  |   4. BYE    |           |           |
        |                  | ----------------------->|           |
        |                  |   5. BYE    |           |           |
        |                  | ----------------------------------->|
        |                  |   6. 200 OK |           |           |
        |                  |<----------- |           |           |
        |                  |   7. 200 OK |           |           |
        |                  |<----------------------- |           |
        |                  |   8. 200 OK |           |           |
        |                  |<----------------------------------- |
        |                  |             |           |           |
        |                  |             |           |           |
        |                  |             |           |           |

   Figure 2: Example flow of a REFER request containing multiple REFER-
                                  Targets

   The REFER request (1) contains a Refer-To header field that includes
   a pointer to the message body, which carries a list with the URIs of

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   the REFER-Targets.  In this example, the URI-list does not contain
   the copyControl attribute extension.  The REFER's Require header
   field carries the "multiple-refer" and "norefersub" option-tags.  The
   Request-URI is set to a Globally Routable User Agent URIs (GRUU)
   [I-D.ietf-sip-gruu] (as a guarantee that the REFER request will not
   fork).  The Refer-Sub header field is set to "false" to request the
   suppression of the implicit subscription.  Figure 3 shows an example
   of this REFER request.  The resource list document contains the list
   of REFER-Target URIs along with the method of the SIP request that
   the REFER-Recipient generates.

   REFER sip:conf-123@example.com;gruu;opaque=hha9s8d-999a  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>
   Refer-Sub: false
   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: recipient-list
   Content-Length: 362
   Content-ID: <cn35t8jf02@example.com>

   <?xml version="1.0" encoding="UTF-8"?>
   <resource-lists xmlns="urn:ietf:params:xml:ns: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 3: REFER request with multiple REFER-Targets

   Figure 4 shows an example of the BYE request (3) that the REFER-
   Recipient sends to the first REFER-Target.

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   BYE sip:bill@example.com SIP/2.0
   Via: SIP/2.0/TCP conference.example.com
           ;branch=z9hG4bKhjhs8assmm
   Max-Forwards: 70
   From: "Conference 123" <sip:conf-123@example.com>;tag=88734
   To: <sip:bill@example.com>;tag=29872
   Call-ID: d432fa84b4c34098s812
   CSeq: 34 BYE
   Content-Length: 0

                           Figure 4: BYE request

10.  Security Considerations

   The Framework and Security Considerations for SIP URI-List Services
   [I-D.ietf-sipping-uri-services] document discusses issues related to
   SIP URI-list services.  Given that a REFER-Recipient accepting REFER
   requests with multiple REFER-targets acts as a URI-list service,
   implementations of this type of server MUST follow the security-
   related rules in the Framework and Security Considerations for SIP
   URI-List Services [I-D.ietf-sipping-uri-services].  These rules
   include mandatory authentication and authorization of clients, and
   opt-in lists.

   Additionally, REFER-Recipients SHOULD only accept REFER requests
   within the context of an application that the REFER-Recipient
   understands (e.g., a conferencing application).  This implies that
   REFER-Recipients MUST NOT accept REFER requests for methods they do
   not understand.  The idea behind these two rules is that REFER-
   Recipients are not used as dumb servers whose only function is to
   fan-out random messages they do not understand.

11.  IANA Considerations

   This document defines a new SIP option-tag: "multiple-refer".  This
   option-tag should be registered in the SIP Parameters registry.

   The following row shall be added to the "Option Tags" section of the
   SIP Parameter Registry:

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   +-----------------+-------------------------------------+-----------+
   | Name            | Description                         | Reference |
   +-----------------+-------------------------------------+-----------+
   | multiple-refer  | This option tag indicates support   | [RFCXXXX] |
   |                 | for REFER requests that contain a   |           |
   |                 | resource list document describing   |           |
   |                 | multiple REFER targets.             |           |
   +-----------------+-------------------------------------+-----------+

      Table 1: Registration of the 'multiple-refer' Option-Tag in SIP

   Note to the RFC Editor: Please replace [RFCXXXX] with the RFC number
   of this specification.

12.  References

12.1.  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, "Communicating
              Presentation Information in Internet Messages: The
              Content-Disposition Header Field", RFC 2183, August 1997.

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, August 1998.

   [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.

   [RFC3420]  Sparks, R., "Internet Media Type message/sipfrag",
              RFC 3420, November 2002.

   [RFC3515]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
              Method", RFC 3515, April 2003.

   [RFC4488]  Levin, O., "Suppression of Session Initiation Protocol
              (SIP) REFER Method Implicit Subscription", RFC 4488,
              May 2006.

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

   [I-D.ietf-sipping-uri-services]
              Camarillo, G. and A. Roach, "Framework and Security
              Considerations for Session Initiation Protocol (SIP)
              Uniform Resource Identifier (URI)-List Services",
              draft-ietf-sipping-uri-services-07 (work in progress),
              November 2007.

   [I-D.ietf-sipping-capacity-attribute]
              Garcia-Martin, M. and G. Camarillo, "Extensible Markup
              Language (XML) Format Extension for Representing Copy
              Control Attributes in Resource Lists",
              draft-ietf-sipping-capacity-attribute-05 (work in
              progress), November 2007.

12.2.  Informative References

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

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

   [I-D.ietf-sip-gruu]
              Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent (UA) URIs (GRUU) in the  Session Initiation Protocol
              (SIP)", draft-ietf-sip-gruu-15 (work in progress),
              October 2007.

Authors' Addresses

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com

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   Aki Niemi
   Nokia
   P.O. Box 321
   NOKIA GROUP, FIN 00045
   Finland

   Email: Aki.Niemi@nokia.com

   Markus Isomaki
   Nokia
   P.O. Box 100
   NOKIA GROUP, FIN  00045
   Finland

   Email: markus.isomaki@nokia.com

   Miguel A. Garcia-Martin
   Nokia Siemens Networks
   P.O.Box 6
   Nokia Siemens Networks, FIN  02022
   Finland

   Email: miguel.garcia@nsn.com

   Hisham Khartabil
   Ericsson Australia

   Email: hisham.khartabil@gmail.com

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