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rfc:rfc4457

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

Camarillo & Blanco Informational [Page 1] RFC 4457 The P-User-Database P-Header April 2006

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.

Camarillo & Blanco Informational [Page 2] RFC 4457 The P-User-Database P-Header April 2006

 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.

Camarillo & Blanco Informational [Page 3] RFC 4457 The P-User-Database P-Header April 2006

 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>

Camarillo & Blanco Informational [Page 4] RFC 4457 The P-User-Database P-Header April 2006

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.

Camarillo & Blanco Informational [Page 5] RFC 4457 The P-User-Database P-Header April 2006

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.

Camarillo & Blanco Informational [Page 6] RFC 4457 The P-User-Database P-Header April 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  28035
 Spain
 EMail: german.blanco@ericsson.com

Camarillo & Blanco Informational [Page 7] RFC 4457 The P-User-Database P-Header April 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 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
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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 Administrative Support Activity (IASA).

Camarillo & Blanco Informational [Page 8]

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