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RFC 4457
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
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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.
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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.
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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.
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>
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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.
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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.
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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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RFC 4457 The P-User-Database P-Header April 2006
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