Network Working Group                                       G. Camarillo
Request for Comments: 4457                                     G. Blanco
Category: Informational                                         Ericsson
                                                              April 2006


                 The Session Initiation Protocol (SIP)
               P-User-Database 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.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies the Session Initiation Protocol (SIP)
   P-User-Database Private-Header (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
   address of the database that contains the user profile of the user
   that generated a particular request.

Table of Contents

   1. Introduction ....................................................2
   2. Scenarios .......................................................2
      2.1. User Registering to the IMS ................................2
      2.2. Incoming Request for an Unregistered User ..................3
   3. Requirements ....................................................4
   4. P-User-Database Header Field Definition .........................4
   5. Applicability ...................................................5
   6. IANA Considerations .............................................5
   7. Security Considerations .........................................5
   8. Acknowledgements ................................................6
   9. References ......................................................6
      9.1. Normative References .......................................6
      9.2. Informative References .....................................6

1.  Introduction

   The 3rd-Generation Partnership Project (3GPP) IMS (IP Multimedia
   Subsystem) uses the Session Initiation Protocol (SIP) [2] as its main
   signalling protocol.  (For more information on the IMS, a detailed
   description can be found in 3GPP TS 23.228 [5] and 3GPP TS 24.229
   [6].) 3GPP has identified a set of requirements that can be met,
   according to the procedures in RFC 3427 [3], by defining a new SIP
   Private-Header (P-header).

   The remainder of this document is organized as follows.  Section 2
   describes the scenarios considered by 3GPP and Section 3 discusses
   the requirements derived from these scenarios.  Section 4 defines the
   P-User-Database header field, which meets those requirements, and
   Section 5 discusses the applicability and scope of this new header
   field.  Section 6 registers the P-User-Database header field with the
   IANA and Section 7 discusses the security properties of the
   environment where this header field is intended to be used.

2.  Scenarios

   In the 3GPP IMS, there are two scenarios where a set of proxies
   handling a request need to consult the same user database.  These
   scenarios consist of a user registering to the IMS network and an
   unregistered user receiving an incoming request that triggers a
   service (e.g., a voice mail service).

2.1.  User Registering to the IMS

   In the 3GPP IMS, SIP REGISTER requests generated by a User Agent (UA)
   traverse a set of SIP proxy servers before reaching the SIP
   registrar.  A REGISTER request sent by a UA is routed to the outbound
   proxy of the UA, which is referred to as the P-CSCF (Proxy-
   Call/Session Control Function).

   The P-CSCF routes the REGISTER request to another proxy, which is
   referred to as the I-CSCF (Interrogating-CSCF) and is always located
   in the home domain of the user.  The I-CSCF consults the user
   database of the domain, which is referred to as the Home Subscriber
   Server (HSS), in order to choose the registrar that will process the
   REGISTER request.

   With the information received from the HSS, the I-CSCF routes the
   REGISTER request to the appropriate registrar, which is referred to
   as the S-CSCF (Serving-CSCF).  At this point, the S-CSCF needs to
   contact the same HSS that was previously contacted by the I-CSCF in
   order to fetch the user profile of the user that generated the
   REGISTER request.

   The interface between the I-CSCF and the HSS and between the S-CSCF
   and the HSS is called Cx interface and is based on Diameter [4].

   When there is a single HSS (i.e., user database) handling all the
   users in the domain, both the I-CSCF and the S-CSCF can be configured
   with its address so that they contact it when necessary.  However,
   some domains have several HSSs, each of which handles a particular
   set of users.  When dealing with a REGISTER request, the I-CSCF and
   the S-CSCF need to discover which is the HSS that contains the
   profile of the user that generated the REGISTER request.

   In networks with more than one HSS, a Diameter redirect agent
   referred to as the Subscription Locator Function (SLF) is
   implemented.  The interface between the I-CSCF and the SLF and
   between the S-CSCF and the SLF is called Dx interface and, like the
   CX interface, is based on Diameter.  The SLF provides the I-CSCF and
   the S-CSCF with the address of the HSS that handles the user they are
   dealing with.

   Therefore, in a network with more than one HSS, the SLF is consulted
   twice per REGISTER request, first by the I-CSCF, and later by the
   S-CSCF.  If the I-CSCF could provide the S-CSCF with the address of
   the HSS handling the user that generated the REGISTER request, the
   S-CSCF could contact directly that HSS.  That is, the S-CSCF would
   not need to contact the SLF in order to obtain the address of the
   HSS.

2.2.  Incoming Request for an Unregistered User

   In the 3GPP IMS, incoming requests for a user traverse an I-CSCF in
   the home domain of the user.  This I-CSCF consults the HSS, using the
   Diameter-based Cx interface, in order to decide which S-CSCF should
   handle the request.  After consulting the HSS, the I-CSCF forwards
   the request to a S-CSCF, which is also located in the home domain of
   the user.

   If the user the request is addressed to is registered to the IMS
   network, the S-CSCF receiving the request knows which HSS handles the
   user.  The S-CSCF stored this information when the user registered.
   However, if the user is not registered, the S-CSCF needs to consult
   the SLF (assuming more than one HSS in the network) in order to
   discover the HSS handling the user.

   Therefore, like in the previous scenario, in a network with more than
   one HSS, the SLF is consulted twice per incoming request addresses to
   an unregistered user.  First by the I-CSCF, and later by the S-CSCF.
   If the I-CSCF could provide the S-CSCF with the address of the HSS
   handling the user that generated the request, the S-CSCF could
   contact directly that HSS.  That is, the S-CSCF would not need to
   contact the SLF in order to obtain the address of the HSS.

3.  Requirements

   This section lists the requirements derived from the previous
   scenarios:

   1.  It is necessary to optimize the registration process in the 3GPP
       IMS by reducing the time it takes for a UA to register to the IMS
       network.

   2.  It is necessary to optimize the handling of incoming requests to
       unregistered users in the 3GPP IMS by reducing the time it takes
       for a domain to handle these requests.

   3.  It is necessary to improve the scalability of SLFs in the 3GPP
       IMS by reducing the amount of traffic the SLF of a network needs
       to handle.

4.  P-User-Database Header Field Definition

   This document defines the SIP P-User-Database P-header.  This header
   field can be added to requests routed from an I-CSCF to an S-CSCF.
   The P-User-Database P-header contains the address of the HSS handling
   the user that generated the request.

   The augmented Backus-Naur Form (BNF) [1] syntax of the P-User-
   Database header field is the following:

   P-User-Database     = "P-User-Database" HCOLON database
                         *( SEMI generic-param )
   database            = LAQUOT DiameterURI RAQUOT

   DiameterURI is defined in RFC 3588 [4].  HCOLON, LAQUOT, RAQUOT, and
   generic-param are defined in RFC 3261 [2].

   The following is an example of a P-User-Database header field:

   P-User-Database: <aaa://host.example.com;transport=tcp>

5.  Applicability

   According to RFC 3427 [3], 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-User-Database header field is intended to be used in 3GPP IMS
   networks.  This header field carries the address of a user database,
   which is 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-User-Database header
   field into a SIP request and the S-CSCF removes it before routing the
   request further.

   When SIP is used on the Internet, there are typically no proxies
   querying a user database between the UA sending a REGISTER request
   and the registrar.  Consequently, the P-User-Database header field
   does not seem useful in a general Internet environment.

6.  IANA Considerations

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

7.  Security Considerations

   The P-User-Database 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 IP Security (IPsec) and sometimes by physically
   protecting the network.  In any case, the environment where the
   P-User-Database header field will be used ensures the integrity and
   the confidentiality of the contents of this header field.

   There is a slight security risk if a P-User-Database header field is
   allowed to propagate out of the administrative domain where it was
   generated.  No user-sensitive information would be revealed by such a
   breach, but this could result in disclosure of information about the
   topology of the operator network that goes beyond the level of
   disclosure explicit in SIP messages without this extension.
   Consequently, operators need to ensure that the P-User-Database
   header field is removed from requests before these are sent to
   another administrative domain.

8.  Acknowledgements

   Nuria Esteban, Stephen Terrill, and Jeroen van Bemmel provided
   comments on this document.  Dean Willis performed a thorough review
   of this document.

9.  References

9.1.  Normative References

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

   [2]  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.

   [3]  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.

   [4]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko,
        "Diameter Base Protocol", RFC 3588, September 2003.

9.2.  Informative References

   [5]  3GPP, "IP Multimedia Subsystem (IMS); Stage 2", 3GPP TS 23.228
        5.14.0, October 2005.

   [6]  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 5.14.0,
        October 2005.

Authors' Addresses

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   EMail: Gonzalo.Camarillo@ericsson.com


   German Blanco
   Ericsson
   Via de los Poblados 13
   Madrid  28035
   Spain

   EMail: german.blanco@ericsson.com

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