The Session Initiation Protocol (SIP) P-Refused-URI-List Private-Header (P-Header)

Network Working Group                                      J. Hautakorpi
Request for Comments: 5318                                  G. Camarillo
Category: Informational                                         Ericsson
                                                           December 2008


                 The Session Initiation Protocol (SIP)
              P-Refused-URI-List Private-Header (P-Header)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

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Abstract

   This document specifies the Session Initiation Protocol (SIP)
   P-Refused-URI-List Private-Header (P-Header).  This P-Header is used
   in the Open Mobile Alliance's (OMA) Push to talk over Cellular (PoC)
   system.  It enables URI-list servers to refuse the handling of
   incoming URI lists that have embedded URI lists.  This P-Header also
   makes it possible for the URI-list server to inform the client about
   the embedded URI list that caused the rejection and the individual
   URIs that form such a URI list.

Table of Contents

1. Introduction ....................................................2
2. Terminology .....................................................2
3. Usage Scenario ..................................................3
4. Overview of Operation ...........................................6
5. Syntax of P-Refused-URI-List Header Field .......................6
6. Response Generation .............................................7
7. Message Sequence Example ........................................7
8. Applicability ...................................................9
9. Security Considerations ........................................10
10. IANA Considerations ...........................................11
11. Acknowledgements ..............................................11
12. References ....................................................11
   12.1. Normative References .....................................11
   12.2. Informative References ...................................12

1.  Introduction

   The Open Mobile Alliance (OMA) has specified the Push to talk over
   Cellular (PoC) service, which uses the Session Initiation Protocol
   (SIP) [3] and Uniform Resource Identifier (URI)-list services [5]
   (more information about OMA PoC can be found at [8]).

   OMA PoC needs a mechanism for servers to refuse the handling of
   incoming URI lists when these have embedded URI lists.  Such a
   mechanism is intended to be used to establish a particular type of
   PoC session called an ad-hoc PoC group session.

   The remainder of this document is organized as follows.  Section 3
   describes the scenario where the mechanism will be used.  Section 4
   provides an overview of the mechanism, which includes a new P-Header
   called P-Refused-URI-List.  Section 5 defines the syntax of this new
   P-Header.  Section 6 specifies how to use the P-Header.  Section 7
   provides a usage example.  Section 8 describes the applicability of
   the P-Header.  Security considerations are discussed in Section 9
   and, finally, the IANA considerations are stated in Section 10.

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 RFC 2119 [1].

3.  Usage Scenario

   An ad-hoc PoC group session is a type of multi-party PoC session.
   The originator of a particular ad-hoc PoC group session chooses in an
   ad-hoc manner (e.g., selecting from an address book) the set of
   desired participants.  In order to establish the ad-hoc PoC group
   session, the originator sends an INVITE request with a URI list that
   contains the URIs of those participants.

   The PoC network, following the procedures defined in [6], receives
   such an INVITE request and generates an individual INVITE request
   towards each of the URIs in the URI list.

   In previous versions of the OMA PoC service, the originator of an
   ad-hoc PoC group session was only allowed to populate the initial URI
   list with URIs identifying individual PoC users.  Later versions of
   the service allow the originator to also include URI lists whose
   entries represent URI lists.  That is, the initial URI list contains
   entries that are URI lists themselves.  The expected service behavior
   then is that the members of the embedded URI lists are invited to
   join the ad-hoc PoC group session.

   Figure 1 illustrates the desired behavior.  The originator (not
   shown) places the URI list friends@example.org, along with the URI
   alice@example.com, in the initial URI list.  The PoC network resolves
   friends@example.org into its members, bob@example.org and
   carol@example.net, and sends INVITE requests to all the recipients.

                                   2. INVITE
                               +------------------>
                               |   alice@example.com
                               |
                               |
                        +-------------+
                        |             |
       1. INVITE        |             | 3. INVITE
     ------------------>| PoC Network |---------------->
    alice@example.com   |             | bob@example.org
    friends@example.org |             |
                        +-------------+
                               |
                               |
                               |
                               |   4. INVITE
                               +------------------>
                                   carol@example.net

                      Figure 1: PoC Expected Behavior

   The PoC network in Figure 1 consists of PoC servers, which are SIP
   entities that can behave as proxies or B2BUAs (Back-to-Back User
   Agents).  There are two types of logical PoC servers: controlling and
   participating.

   In an ad-hoc PoC group session, there is always exactly one
   controlling PoC server.  The controlling PoC server of an ad-hoc PoC
   group session resolves an incoming URI list and sends INVITEs to the
   members of the list.  The controlling PoC server also functions as
   the focus of the session.  Every participant in an ad-hoc PoC group
   has an associated participating PoC server, which resides in the home
   domain of the participant.

   Figure 2 shows how the PoC servers of the PoC network behave in the
   scenario shown in Figure 1.  An originating PoC user agent sends an
   INVITE request (1) with a URI list to its participating PoC server.
   The participating PoC server of the originator receives the INVITE
   request, assumes the role of controlling PoC server for the ad-hoc
   PoC group session, and sends an INVITE request to each of the URIs in
   the URI list.

                                              +-------------+
                              2. INVITE       | Particip.   |
                          +------------------>| PoC server  |->
                          | alice@example.com | example.com |
                          |                   +-------------+
                          |
                   +-------------+ 3. INVITE           +-------------+
                   |             |-------------------->|             |
     1. INVITE     | Controlling | friends@example.org | Particip.   |
  ---------------->| PoC server  |                     | PoC server  |->
alice@example.com  |             | 4. 403 Forbidden    | example.org |
friends@example.org|             |<--------------------|             |
                   +-------------+  bob@example.org    +-------------+
                      |      |      carol@example.net         ^
                      |      |                                |
                      |      |     5. INVITE                  |
                      |      +--------------------------------+
                      |             bob@example.org
                      |
                      |                   +------------+
                      |   6. INVITE       | Particip.  |
                      +------------------>| PoC server |->
                        carol@example.net | example.net|
                                          +------------+

                      Figure 2: PoC Network Behavior

   The first URI of the list, alice@example.com, identifies a single
   user.  The second URI of the URI list, friends@example.org,
   identifies a URI list.  In PoC terminology, friends@example.com
   identifies a pre-arranged PoC group.  The PoC server at example.org,
   which knows the membership of friends@example.com, cannot send INVITE
   requests to the members of friends@example.org because that PoC
   server does not act as a controlling PoC server for the ad-hoc PoC
   group session being established.  Instead, it informs the controlling
   PoC server that friends@example.org is a list whose members are
   bob@example.org and carol@example.net.  Upon receiving this
   information, the controlling PoC server generates INVITE requests
   towards bob@example.org and carol@example.net.

   Although not shown in the above example, the participating PoC server
   (example.org) can include -- based on policy, presence of the
   members, etc. -- just a partial list of URIs of the URI list.
   Furthermore, a URI that the participating PoC server returns can be a
   URI list.

   At present, there is not a mechanism for a participating PoC server
   to inform a controlling PoC server that a URI identifies a list and
   the members of that list, nor is there a mechanism to indicate the
   URIs contained in the list.  This document defines such a mechanism:
   the P-Refused-URI-List P-Header.

4.  Overview of Operation

   When a URI-list server receives an INVITE request with a URI list
   containing entries that are URI lists themselves, and the server
   cannot handle the request, it returns a 403 (Forbidden) response with
   a P-Refused-URI-List P-Header, as shown in Figure 3.  The P-Refused-
   URI-List P-Header contains the members of the URI list or lists that
   caused the rejection of the request.  This way, the client can send
   requests directly to those member URIs.

           +---------+        INVITE request         +----------+
           |         |------------------------------>|          |
           |         |   [URI list in a URI list]    | URI-list |
           | Client  |                               |  server  |
           |         |        403 Forbidden          |          |
           |         |<------------------------------|          |
           |         | [Content of refused URI list] |          |
           +---------+                               +----------+

                      Figure 3: Operational Overview

5.  Syntax of P-Refused-URI-List Header Field

   The following is the augmented Backus-Naur Form (BNF) [4] syntax of
   the P-Refused-URI-List P-Header:

       P-Refused-URI-List = "P-Refused-URI-List" HCOLON
                                 uri-list-entry
                                 *(COMMA uri-list-entry)
       uri-list-entry     = ( name-addr / addr-spec )
                                 *( SEMI refused-param )
       refused-param      = members-param / generic-param
       members-param      = "members" EQUAL
                                 LDQUOT *( qdtext / quoted-pair ) RDQUOT

   The members P-Header parameter MUST contain a cid-url, which is
   defined in RFC 2392 [2].

   The HCOLON, SEMI, EQUAL, LDQUOT, RDQUOT, and generic-param are
   defined in [3].

6.  Response Generation

   A 403 (Forbidden) response can contain more than one P-Refused-URI-
   List entries.  The P-Refused-URI-List header field MUST NOT be used
   with any other response.  The P-Refused-URI-List P-Header contains
   one or more URIs, which were present in the URI list in the incoming
   request and could not be handled by the server.  Additionally, the
   P-Refused-URI-List can optionally carry some or all of the members of
   the URI lists identified by those URIs.

   The 403 (Forbidden) response MAY contain body parts which contain URI
   lists.  Those body parts can be referenced by the P-Refused-URI-List
   entries through their Content-IDs [2].  If there is a Content-ID
   defined in the P-Refused-URI-List, one of the body parts MUST have an
   equivalent Content-ID.  The format of a URI list is service specific.

   This kind of message structure enables clients to determine which URI
   relates to which URI list, if the URI-list server is willing to
   disclose that information.  Furthermore, the information enclosed in
   the URI lists enable clients to take further actions to remedy the
   rejection situation (e.g., send individual requests to the members of
   the URI list).

7.  Message Sequence Example

   In the following message sequence example, a controlling PoC server
   sends an INVITE request to a participating PoC server.  The
   participating PoC server rejects the request with a 403 (Forbidden)
   response.  The 403 response has a P-Refused-URI-List P-Header that
   carries the members of the rejected URI lists that the participating
   PoC server determines to disclose to this controlling PoC server in
   the body of the message.

           Controlling PoC server           Participating PoC server
               example.com                      example.net

                    |                                 |
                    |             INVITE              |
                    |-------------------------------->|
                    |                                 |
                    |          403 Forbidden          |
                    |<--------------------------------|
                    |                                 |

                    Figure 4: Message Sequence Example

   The INVITE request shown in Figure 4 is as follows (Via header fields
   are not shown for simplicity):

      INVITE sip:poc-service@example.net SIP/2.0
      Max-Forwards: 70
      From: PoC service <sip:poc-service@example.com>;tag=4fxaed73sl
      To: PoC service <sip:poc-service@example.net>
      Call-ID: 7xTn9vxNit65XU7p4@example.com
      CSeq: 1 INVITE
      Contact: <sip:poc-service@poc-as.example.com>
      Require: recipient-list-invite
      Content-Type: multipart/mixed;boundary="boundary1"
      Content-Length: 538

      --boundary1
      Content-Type: application/sdp

      (SDP not shown)

      --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">
        <list>
          <entry uri="sip:bob@example.net"/>
          <entry uri="sip:friends-list@example.net"/>
          <entry uri="sip:colleagues-list@example.net"/>
        </list>
      </resource-lists>
      --boundary1--


   The URIs sip:friends-list@example.net and
   sip:colleagues-list@example.net in the example above are actually
   references to URI lists (i.e., pre-arranged PoC groups).  In the
   following response, the URI lists are in the XML resource list format
   [7].

   The content of the 403 (Forbidden) response in Figure 4 is as follows
   (Via header fields are not shown for simplicity):

      SIP/2.0 403 Forbidden
      From: PoC service <sip:poc-service@example.com>;tag=4fxaed73sl
      To: PoC service <sip:poc-service@example.net>;tag=814254
      Call-ID: 7xTn9vxNit65XU7p4@example.com
      CSeq: 1 INVITE
      P-Refused-URI-List: sip:friends-list@example.net;
        members=<cid:an3bt8jf03@example.net>
      P-Refused-URI-List: sip:colleagues-list@example.net;

        members=<cid:bn35n8jf04@example.net>
      Content-Type: multipart/mixed;boundary="boundary1"
      Content-Length: 745

      --boundary1
      Content-Type: application/resource-lists+xml
      Content-Disposition: recipient-list
      Content-ID: <an3bt8jf03@example.net>

      <?xml version="1.0" encoding="UTF-8"?>
      <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
        <list>
          <entry uri="sip:bill@example.org"/>
          <entry uri="sip:randy@example.com"/>
          <entry uri="sip:eddy@example.com"/>
        </list>
      </resource-lists>

      --boundary1
      Content-Type: application/resource-lists+xml
      Content-Disposition: recipient-list
      Content-ID: <bn35n8jf04@example.net>

      <?xml version="1.0" encoding="UTF-8"?>
      <resource-lists xmlns="urn:ietf:params:xml:ns:resource-lists">
        <list>
          <entry uri="sip:joe@example.org"/>
          <entry uri="sip:carol@example.com"/>
        </list>
      </resource-lists>
      --boundary1--

   Using the message body of the 403 (Forbidden) response above, the
   controlling PoC server can determine the members of
   sip:friend-list@example.net and sip:colleagues-list@example.net that
   the participating PoC server determines to disclose to this
   controlling PoC server.  Furthermore, the controlling PoC server can
   deduce that the participating PoC server has not sent any outgoing
   requests, per regular URI-list server procedures.

8.  Applicability

   The P-Refused-URI-List header field is intended to be used in OMA PoC
   networks.  This header field is used between PoC servers and carries
   information about those URI lists that were rejected by the server
   receiving the request.

   The OMA PoC services is designed so that, in a given session, only
   one PoC server can resolve incoming URI lists and send INVITEs to
   members of these lists.  This restriction is not present on services
   developed to be used on the public Internet.  Therefore, the
   P-Refused-URI-List P-Header does not seem to have general
   applicability outside the OMA PoC service.

   Additionally, the use of the P-Refused-URI-List P-Header requires
   special trust relationships between servers that do not typically
   exist on the public Internet.

   It is important to note that the P-Refused-URI-List is optional and
   does not change the basic behavior of a SIP URI-list service.  The
   P-Refused-URI-List only provides clients with additional information
   about the refusal of the request.

9.  Security Considerations

   It is assumed that the network elements handling the P-Refused-URI-
   List P-Header are trusted.  Also, attackers are not supposed to have
   access to the protocol messages between those elements.  This is
   because the P-Refused-URI-List is intended to be used in the OMA PoC
   environment, which is implemented in the operators' core network; for
   more on OMA PoC security assumptions, see [9].  Traffic protection
   between network elements is achieved by using IP Security (IPsec) and
   sometimes by physically protecting the network.

   However, implementors and administrators should be aware of two
   special security considerations related to the use of P-Refused-URI-
   List:

   Eavesdropping:  403 (Forbidden) responses may contain information
      about the members of a given URI list.  Eavesdroppers can acquire
      this information if the 403 (Forbidden) responses are not
      encrypted.  Therefore, it is RECOMMENDED that either hop-by-hop or
      end-to-end encryption (e.g., using TLS or S/MIME, respectively) is
      used.

   Disclosing information:  A rogue entity may be able to acquire
      information about the members of a given URI list if the URI-list
      server sends information about those URI lists to unauthorized
      users.  Therefore, it is RECOMMENDED that a URI-list server
      discloses the content of that URI-list only to authorized clients.

10.  IANA Considerations

   The IANA has made two additions to the Session Initiation Protocol
   (SIP) Parameters registry.  The following header field has been added
   to the Header Fields sub-registry.

     Header Name        compact    Reference
     -----------------  -------    ---------
     P-Refused-URI-List            [RFC5318]

   The following header field parameter has been added to the Header
   Field Parameters and Parameter Values sub-registry.

                                                  Predefined
   Header Field                  Parameter Name     Values     Reference
   ----------------------------  ---------------   ---------   ---------
   P-Refused-URI-List            members              No       [RFC5318]

11.  Acknowledgements

   Authors would like to thank Tom Hiller who did a thorough, dedicated
   review for this document.

12.  References

12.1.  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]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", STD 68, RFC 5234, January 2008.

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

   [6]  Camarillo, G. and A. Johnston, "Conference Establishment Using
        Request-Contained Lists in the Session Initiation Protocol
        (SIP)", RFC 5366, October 2008.

12.2.  Informative References

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

   [8]  Open Mobile Alliance, "OMA PoC System Description: Draft Version
        2.0", April 2007.

   [9]  Open Mobile Alliance, "Push to talk over Cellular (PoC) -
        Architecture: Draft Version 2.0", April 2007.

Authors' Addresses

   Jani Hautakorpi
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Jani.Hautakorpi@ericsson.com


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com