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Network Working Group G. Camarillo Request for Comments: 5368 Ericsson Category: Standards Track A. Niemi

                                                            M. Isomaki
                                                      M. Garcia-Martin
                                                          H. Khartabil
                                                    Ericsson Australia
                                                          October 2008

Referring to Multiple Resources 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.


 This document defines extensions to the SIP REFER method so that it
 can be used to refer to multiple resources in a single 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 . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  2
 3.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  3
 4.  The multiple-refer SIP Option-Tag  . . . . . . . . . . . . . .  4
 5.  Suppressing REFER's Implicit Subscription  . . . . . . . . . .  4
 6.  URI-List Format  . . . . . . . . . . . . . . . . . . . . . . .  5
 7.  Behavior of SIP REFER-Issuers  . . . . . . . . . . . . . . . .  6
 8.  Behavior of REFER-Recipients . . . . . . . . . . . . . . . . .  6
 9.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
 10. Security Considerations  . . . . . . . . . . . . . . . . . . . 10
 11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   12.1.  Normative References  . . . . . . . . . . . . . . . . . . 10
   12.2.  Informative References  . . . . . . . . . . . . . . . . . 11

Camarillo, et al. Standards Track [Page 1] RFC 5368 SIP Multiple REFER October 2008

1. Introduction

 RFC 3261 (SIP) [RFC3261] is extended by RFC 3515 [RFC3515] with a
 REFER method that allows a user agent (UA) to request a second UA to
 send a SIP request to a third party.  For example, if Alice is in a
 call with Bob, and decides Bob needs to talk to Carol, Alice can
 instruct her SIP UA to send a REFER request to Bob's UA providing
 Carol's SIP Contact information.  Assuming Bob has given it
 permission, Bob's UA will attempt to call Carol using that contact.
 That is, it will send an INVITE request to that contact.
 A number of applications need to request this second UA 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
 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].

2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in BCP 14, RFC 2119
 [RFC2119] and indicate requirement levels for compliant
 This document reuses the following terminology defined in RFC 3261
 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

Camarillo, et al. Standards Track [Page 2] RFC 5368 SIP Multiple REFER October 2008

 REFER-Target:  a UA of the intended final recipient of a SIP request
    generated by the REFER-Recipient.

3. Overview of Operation

 This document describes an application of URI-list services [RFC5363]
 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 RFC 4826 [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
 The URI list that contains the list of targets is used in conjunction
 with RFC 5364 [RFC5364] 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 RFC 4826 [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 a 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].

Camarillo, et al. Standards Track [Page 3] RFC 5368 SIP Multiple REFER October 2008

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 packages 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 the multiple REFER technology
 described in this memo 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].
    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.

Camarillo, et al. Standards Track [Page 4] RFC 5368 SIP Multiple REFER October 2008

 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 RFC 5363 [RFC5363], 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 RFC 4826 [RFC4826] extended with RFC 5364
 [RFC5364].  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 RFC 5364 [RFC5364], 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 RFC 5364 [RFC5364]) to
 prevent that the REFER-Recipient discloses the target URI in a URI
 Additionally, RFC 5364 [RFC5364] defines a 'recipient-list-history'
 body that contains the list of targets.  The default format for
 'recipient-list-history' bodies for conference services is also RFC
 4826 [RFC4826] extended with RFC 5364 [RFC5364].  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, RFC 4826 [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 multiple REFER service defined in this

Camarillo, et al. Standards Track [Page 5] RFC 5368 SIP Multiple REFER October 2008

 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"
     <entry uri="" cp:copyControl="to"  />
     <entry uri="" cp:copyControl="cc" />
     <entry uri="" cp:copyControl="bcc" />
                          Figure 1: URI list

7. Behavior of SIP REFER-Issuers

 As indicated in Sections 4 and 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 RFC 5363
 [RFC5363].  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.
 RFC 4826 [RFC4826] 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

Camarillo, et al. Standards Track [Page 6] RFC 5368 SIP Multiple REFER October 2008

 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 the request is appropriate for the service,
 e.g., it is not received mid-dialog, the REFER-Recipient SHOULD
 include a URI list in each of the outgoing requests.  This list
 SHOULD be formatted according to RFC 4826 [RFC4826] and RFC 5364
 [RFC5364].  The REFER-Recipient MUST follow the procedures specified
 in RFC 4826 [RFC4826] with respect to handling of the 'anonymize',
 'count', and 'copyControl' attributes.
 Section 4 of RFC 5363 [RFC5363] 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 RFC 5363
 [RFC5363] into account to avoid sending a 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
 Since the multiple REFER service does not use hierarchical lists nor
 lists that include entries by reference to the XCAP root URI, a
 REFER-Recipient receiving a URI list with more information than what
 has been described in Section 6 MAY discard all the extra
 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].

Camarillo, et al. Standards Track [Page 7] RFC 5368 SIP Multiple REFER October 2008

 +--------+         +---------+    +--------+  +--------+  +--------+
 | 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
 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 URI (GRUU)
 [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.

Camarillo, et al. Standards Track [Page 8] RFC 5368 SIP Multiple REFER October 2008

 REFER;gruu;opaque=hha9s8d-999a  SIP/2.0
 Via: SIP/2.0/TCP
 Max-Forwards: 70
 To: "Conference 123" <>
 From: Carol <>;tag=32331
 Call-ID: d432fa84b4c76e66710
 CSeq: 2 REFER
 Contact: <>
 Refer-To: <>
 Refer-Sub: false
 Require: multiple-refer, norefersub
 Allow-Events: dialog
 Accept: application/sdp, message/sipfrag
 Content-Type: application/resource-lists+xml
 Content-Disposition: recipient-list
 Content-Length: 362
 Content-ID: <>
 <?xml version="1.0" encoding="UTF-8"?>
 <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists"
     <entry uri="" />
     <entry uri="" />
     <entry uri="" />
          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.
 BYE SIP/2.0
 Via: SIP/2.0/TCP
 Max-Forwards: 70
 From: "Conference 123" <>;tag=88734
 To: <>;tag=29872
 Call-ID: d432fa84b4c34098s812
 CSeq: 34 BYE
 Content-Length: 0
                         Figure 4: BYE request

Camarillo, et al. Standards Track [Page 9] RFC 5368 SIP Multiple REFER October 2008

10. Security Considerations

 RFC 5363 [RFC5363] 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 RFC 5363
 [RFC5363].  These rules include opt-in lists and mandatory
 authentication and authorization of clients.
 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 has been registered in the SIP Parameters registry.
 The following row has been added to the "Option Tags" section of the
 SIP Parameter Registry:
 | Name            | Description                         | Reference |
 | multiple-refer  | This option tag indicates support   | [RFC5368] |
 |                 | for REFER requests that contain a   |           |
 |                 | resource list document describing   |           |
 |                 | multiple REFER targets.             |           |
    Table 1: Registration of the 'multiple-refer' option-tag in SIP

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.

Camarillo, et al. Standards Track [Page 10] RFC 5368 SIP Multiple REFER October 2008

 [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.
 [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.

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.
 [SIP-GRUU]  Rosenberg, J., "Obtaining and Using Globally Routable
             User Agent (UA) URIs (GRUU) in the Session Initiation
             Protocol (SIP)", Work in Progress, October 2007.

Camarillo, et al. Standards Track [Page 11] RFC 5368 SIP Multiple REFER October 2008

Authors' Addresses

 Gonzalo Camarillo
 Hirsalantie 11
 Jorvas  02420
 Aki Niemi
 P.O. Box 321
 Markus Isomaki
 P.O. Box 100
 Miguel A. Garcia-Martin
 Via de los Poblados 13
 Madrid  28033
 Hisham Khartabil
 Ericsson Australia

Camarillo, et al. Standards Track [Page 12] RFC 5368 SIP Multiple REFER October 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an

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 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
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Camarillo, et al. Standards Track [Page 13]

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