    Network Working Group                                       G. Camarillo
    Request for Comments: 5002                                     G. Blanco
    Category: Informational                                         Ericsson
                                                                 August 2007

                     The Session Initiation Protocol (SIP)
                    P-Profile-Key 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.

    Abstract

       This document specifies the SIP P-Profile-Key P-header.  This header
       field is used in the 3rd-Generation Partnership Project (3GPP) IMS
       (IP Multimedia Subsystem) to provide SIP registrars and SIP proxy
       servers with the key of the profile corresponding to the destination
       SIP URI of a particular SIP request.

    Table of Contents

       1. Introduction ....................................................2
       2. Terminology .....................................................2
       3. Scenario ........................................................2
       4. Requirements ....................................................3
       5. P-Profile-Key Header Field Definition ...........................3
       6. Applicability ...................................................4
       7. IANA Considerations .............................................4
       8. Security Considerations .........................................5
       9. Acknowledgements ................................................5
       10. References .....................................................5
          10.1. Normative References ......................................5
          10.2. Informative References ....................................6

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    1.  Introduction

       The 3rd-Generation Partnership Project (3GPP) IMS (IP Multimedia
       Subsystem) uses SIP [RFC3261] as its main signalling protocol.  (For
       more information on the IMS, a detailed description can be found in
       3GPP TS 23.228 [3GPP.23.228] and 3GPP TS 24.229 [3GPP.24.229]).  3GPP
       has identified a set of requirements that can be met, according to
       the procedures in [RFC3427], by defining a new SIP P-header.

       The remainder of this document is organized as follows.  Section 3
       describes the scenario considered by 3GPP and Section 4 discusses the
       requirements derived from this scenario.  Section 5 defines the P-
       Profile-Key header field, which meets those requirements, and Section
       6 discusses the applicability and scope of this new header field.
       Section 7 registers the P-Profile-Key header field with the IANA and
       Section 8 discusses the security properties of the environment where
       this header field is intended to be used.

    2.  Terminology

       HSS:     Home Subscriber Server.

       I-CSCF:  Interrogating - Call/Session Control Function.

       Public Service Identity:
                A SIP URI that refers to a service instead of a user.

       S-CSCF:  Serving - Call/Session Control Function.

       Wildcarded Public Service Identity:
                A set of Public Service Identities that match a regular
                expression and share the same profile.

    3.  Scenario

       In the 3GPP IMS, there are scenarios where a set of proxies handling
       a request need to consult the same user database, as described in
       [RFC4457].  Those proxies typically use the destination SIP URI of
       the request as the key for their database queries.  Nevertheless,
       when a proxy handles a Wildcarded Public Service Identity, the key to
       be used in its database query is not the destination SIP URI of the
       request, but a regular expression instead.

       Public Service Identities are SIP URIs that refer to services instead
       of users.  That is, they address a specific application in an
       Application Server.  Wildcarded Public Service Identities are a set
       of Public Service Identities that match a regular expression and
       share the same profile.  For example, the Public Service Identities

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       'sip:chatroom-12@example.com' and 'sip:chatroom-657@example.com'
       would match the Wildcarded Public Service Identity
       'sip:chatroom-!.*!@example.com'.  For a description of Wildcarded
       Public Service Identities, see 3GPP TS 23.003 [3GPP.23.003].

       When a proxy queries the user database for a Public Service Identity
       for which there is no profile in the user database, the user database
       needs to find its matching Wildcarded Public Service Identity.  For
       example, if the user database receives a query for
       'sip:chatroom-657@example.com', the user database needs to go through
       all the Wildcarded Public Service Identity it has until it finds a
       matching one; in this case, 'sip:chatroom-!.*!@example.com'.  The
       process to find a matching Wildcarded Public Service Identity can be
       computationally expensive, time consuming, or both.

       When two proxies query the user database for the same Public Service
       Identity, which matches a Wildcarded Public Service Identity, the
       user database needs to perform the matching process twice.  Having to
       perform that process twice can be avoided by having the first proxy
       obtain the Wildcarded Public Service Identity from the user database
       and transfer it, piggy-backed in the SIP message, to the second
       proxy.  This way, the second proxy can query the user database using
       the Wildcarded Public Service Identity directly.

       An alternative, but undesirable, solution would consist of having the
       user database store every Public Service Identity and its matching
       Wildcarded Public Service Identity.  The scalability and
       manageability properties of this approach are considerably worse than
       those of the approach described earlier.

    4.  Requirements

       This section lists the requirements derived from the previous
       scenario:

       1.  It is necessary to optimize the response time for session
           establishment in the 3GPP IMS.

       2.  It is necessary to keep the user database's size and maintenance
           manageable (e.g., storing individual Public Service Identities
           matching a Wildcarded Public Service Identity in the user
           database is not believed to be an acceptable solution).

    5.  P-Profile-Key Header Field Definition

       This document defines the SIP P-Profile-Key P-header.  The P-
       Profile-Key P-header contains the key to be used by a proxy to query
       the user database for a given profile.

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       The augmented Backus-Naur Form (BNF) [RFC4234] syntax of the
       P-Profile-Key header field is the following:

          P-Profile-Key     = "P-Profile-Key" HCOLON (name-addr / addr-spec)
                               *( SEMI generic-param )

       The format of HCOLON, name-addr, addr-spec, and generic-param are
       defined in [RFC3261].  The format of Wildcarded Public Service
       Identities is defined in 3GPP TS 23.003 [3GPP.23.003].  They take the
       form of Extended Regular Expressions (ERE) as defined in Chapter 9 of
       IEEE 1003.1-2004 Part 1 [IEEE.1003.1-2004].

       The following is an example of a P-Profile-Key header field that
       contains a Wildcarded Public Service Identity:

          P-Profile-Key: <sip:chatroom-!.*!@example.com>

    6.  Applicability

       According to [RFC3427], P-headers have a limited applicability.
       Specifications of P-headers such as this RFC need to clearly document
       the useful scope of the proposal, and explain its limitations and why
       it is not suitable for the general use of SIP on the Internet.

       The P-Profile-Key header field is intended to be used in 3GPP IMS
       networks.  This header field carries the key of a service profile,
       that is stored in a user database referred to as HSS, between two
       proxies, which are referred to as I-CSCF and S-CSCF.  The I-CSCF and
       the S-CSCF belong to the same administrative domain and share a
       common frame of reference to the user database.  The I-CSCF inserts
       the P-Profile-Key header field into a SIP request and the S-CSCF
       removes it before routing the request further.  (For a description of
       how an I-CSCF and an S-CSCF query the same user database for a single
       request, see [RFC4457].)

       Typically, when SIP is used on the Internet, there are not multiple
       proxies with a trust relationship between them querying the same user
       database.  Consequently, the P-Profile-Key header field does not seem
       useful in a general Internet environment.

    7.  IANA Considerations

       This document defines a new SIP header field: P-Profile-Key.  This
       header field has been registered by the IANA in the SIP Parameters
       registry under the Header Fields subregistry.

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    8.  Security Considerations

       The P-Profile-Key defined in this document is to be used in an
       environment where elements are trusted and where attackers are not
       supposed to have access to the protocol messages between those
       elements.  Traffic protection between network elements is sometimes
       achieved by using IPsec and sometimes by physically protecting the
       network.  In any case, the environment where the P-Profile-Key header
       field will be used ensures the integrity and the confidentiality of
       the contents of this header field.  The P-Profile-Key header field
       MUST NOT be used in environments that do not have these
       characteristics.

       The P-Profile-Key header field needs to be integrity protected to
       keep attackers from modifying its contents.  An attacker able to
       modify the contents of this header field could make the network apply
       a different service than the one corresponding to the request
       carrying the P-Profile-Key header field.

       The contents of the P-Profile-Key field need to be kept confidential.
       An attacker able to access the contents of this header field would
       obtain certain knowledge about the way services are structured in a
       given domain.

    9.  Acknowledgements

       Alf Heidermark and Timo Forsman provided input to this document.
       Miguel Angel Garcia-Martin performed an expert review on this
       document on behalf of the SIPPING working group.  Jon Peterson
       provided comments on this document.

    10.  References

    10.1.  Normative References

       [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.

       [RFC3427]           Mankin, A., Bradner, S., Mahy, R., Willis, D.,
                           Ott, J., and B. Rosen, "Change Process for the
                           Session Initiation Protocol (SIP)", BCP 67, RFC
                           3427, December 2002.

       [RFC4234]           Crocker, D. and P. Overell, "Augmented BNF for
                           Syntax Specifications: ABNF", RFC 4234, October
                           2005.

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       [3GPP.23.003]       3GPP, "Numbering, addressing and identification",
                           3GPP TS 23.003 3.15.0, October 2006.

       [IEEE.1003.1-2004]  "Standard for information technology - portable
                           operating system interface (POSIX).  Base
                           definitions", IEEE  1003.1-2004, 2004.

    10.2.  Informative References

       [RFC4457]           Camarillo, G. and G. Blanco, "The Session
                           Initiation Protocol (SIP) P-User-Database
                           Private-Header (P-Header)", RFC 4457, April 2006.

       [3GPP.23.228]       3GPP, "IP Multimedia Subsystem (IMS); Stage 2",
                           3GPP TS 23.228 5.15.0, June 2006.

       [3GPP.24.229]       3GPP, "Internet Protocol (IP) multimedia call
                           control protocol based on Session Initiation
                           Protocol (SIP) and Session Description Protocol
                           (SDP); Stage 3", 3GPP TS 24.229 5.18.0, October
                           2006.

    Authors' Addresses

       Gonzalo Camarillo
       Ericsson
       Hirsalantie 11
       Jorvas  02420
       Finland

       EMail: Gonzalo.Camarillo@ericsson.com

       German Blanco
       Ericsson
       Via de los Poblados 13
       Madrid  28033
       Spain

       EMail: German.Blanco@ericsson.com

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