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Versions: 00 01 02 RFC 4457

SIPPING Working Group                                       G. Camarillo
Internet-Draft                                                 G. Blanco
Expires: August 12, 2005                                        Ericsson
                                                       February 11, 2005


  The Session Initiation Protocol (SIP) P-User-Database Private-Header
                               (P-Header)
              draft-camarillo-sipping-user-database-00.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies the SIP P-User-Database 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.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Scenarios  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1   User Registering to the IMS  . . . . . . . . . . . . . . .  3
     3.2   Incoming Request for an Unregistered User  . . . . . . . .  4
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  P-User-Database header field Definition  . . . . . . . . . . .  5
   6.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . .  5
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
   10.   References . . . . . . . . . . . . . . . . . . . . . . . . .  7
   10.1  Normative References . . . . . . . . . . . . . . . . . . . .  7
   10.2  Informational References . . . . . . . . . . . . . . . . . .  7
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  7
       Intellectual Property and Copyright Statements . . . . . . . .  8

































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1.  Introduction

   The 3rd-Generation Partnership Project (3GPP) IMS (IP Multimedia
   Subsystem) uses SIP [3] as its main signalling protocol.  3GPP has
   identified a set of requirements that can be met, according to the
   procedures in RFC 3427 [4], by defining a new SIP P-header.

   The remainder of this document is organized as follows.  Section 2
   defines the terminology used in this document.  Section 3 describes
   the scenarios considered by 3GPP and Section 4 discusses the
   requirements derived from these scenarios.  Section 5 defines the
   P-User-Database header field, which meets those requirements, and
   Section 6 discusses the applicability and scope of this new header
   field.  Section 7 registers the P-User-Database header field and a
   related parameter and Section 8 discusses the security properties of
   the environment where this header field is intended to be used.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [1] and indicate requirement levels for
   compliant implementations.

3.  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).

3.1  User Registering to the IMS

   In the 3GPP IMS, SIP REGISTER requests generated by a UA (User Agent)
   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
   always located in the home domain of the user, which is referred to
   as the I-CSCF (Interrogating-CSCF).  The I-CSCF consults the user
   database of the domain, which is referred to as the HSS (Home
   Subscriber Server), in order to choose the registrar that will
   process the REGISTER request.




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   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 it is based on Diameter [5].

   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 user that generated the REGISTER request.

   In networks with more than one HSS, a Diameter redirect agent
   referred to as SLF (Subscription Locator Function) 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.

3.2  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.



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   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.

4.  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.

5.  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 a 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) [2] syntax of the
   P-User-Database header field is the following (some of the
   definitions are taken from RFC 3261 [3]):


   P-User-Database     = "P-User-Database" HCOLON user-database-host
                         *( SEMI user-database-param )
   user-database-host  = host

   user-database-param = user-database-realm / generic-param
   user-database-realm = "realm" EQUAL hostname


6.  Applicability

   According to RFC 3427 [4], P-headers have a limited applicability.
   Specifications on P-headers such as this document need to clearly
   document the useful scope of the proposal, and explain its



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   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 HSS's host and, optionally,
   the HSS's realm between an I-CSCF and a S-CSCF.

   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.

7.  IANA Considerations

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

   This document defines a parameter for the P-User-Database header
   field: realm.  This header field parameter needs to be registered by
   the IANA in the "SIP/SIPS URI Parameters" subregistry, which is
   located under the "Session Initiation Protocol (SIP) Parameters"
   registry.  The reference should appear in single-brackets, as
   indicated in RFC 3969 [6].  The resulting entry should be:


   Parameter Name     Predefined Values     Reference
   --------------     -----------------     ---------
   realm              No                    [RFCxxxx]


8.  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 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.

9.  Acknowledgements

   Nuria Esteban and Stephen Terrill provided comments on this document.






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10.  References

10.1  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [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]  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.

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

   [6]  Camarillo, G., "The Internet Assigned Number Authority (IANA)
        Uniform Resource Identifier (URI) Parameter Registry for the
        Session Initiation Protocol (SIP)", BCP 99, RFC 3969, December
        2004.

10.2  Informational References


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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