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Network Working Group                                       G. CamarilloRequest for Comments: 4457                                     G. BlancoCategory: Informational                                         Ericsson                                                              April 2006The 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 Contents1. Introduction ....................................................22. Scenarios .......................................................22.1. User Registering to the IMS ................................22.2. Incoming Request for an Unregistered User ..................33. Requirements ....................................................44. P-User-Database Header Field Definition .........................45. Applicability ...................................................56. IANA Considerations .............................................57. Security Considerations .........................................58. Acknowledgements ................................................69. References ......................................................69.1. Normative References .......................................69.2. Informative References .....................................6Camarillo & Blanco           Informational                      [Page 1]

RFC 4457              The P-User-Database P-Header            April 20061.  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 inRFC 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 andSection 3 discusses   the requirements derived from these scenarios.Section 4 defines the   P-User-Database header field, which meets those requirements, andSection 5 discusses the applicability and scope of this new header   field.Section 6 registers the P-User-Database header field with the   IANA andSection 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 inRFC 3588 [4].  HCOLON, LAQUOT, RAQUOT, and   generic-param are defined inRFC 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 20065.  Applicability   According toRFC 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 20068.  Acknowledgements   Nuria Esteban, Stephen Terrill, and Jeroen van Bemmel provided   comments on this document.  Dean Willis performed a thorough review   of this document.9.  References9.1.  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 2006Authors' 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.comCamarillo & Blanco           Informational                      [Page 7]

RFC 4457              The P-User-Database P-Header            April 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Camarillo & Blanco           Informational                      [Page 8]