P-Served-User Header Field Parameter for Originating CDIV session case in Session Initiation Protocol (SIP)

The P-Served-User header field was defined in to address an issue that was found in the 3rd Generation Partnership Project (3GPP) IMS (IP Multimedia Subsystem) between an S-CSCF (Serving Call Session Control Function) and an AS (Application Server) on the ISC (IMS Service Control) interface. For more information on the IMS, a detailed description can be found in .This header field conveys the identity of the served user and the session case that applies to this particular communication session and application invocation. defines the originating and terminating session cases for a registered or unregistered user. This document extends the P-Served-User header field to include the session case for a forwarded leg when a call diversion service (CDIV) has been invoked. The generic-param of the P-Served-User is extended by the "orig-cdiv-param" created as a new parameter for this Originating_CDIV session case. The following section defines the orig-cdiv-param P-Served-User header field parameter usage, Section 2 specifies the proxy behavior for the new header field parameter handling, and Section 3 discusses the applicability and scope of this new header field parameter. Section 4 descibes the Syntax, Section 5 registers the orig-cdiv header field parameter with the IANA, and Section 6 discusses the security properties of the environment where this new header field parameter is intended to be used.

In the 3GPP IMS (IP Multimedia Subsystem), the S-CSCF (Serving CSCF) is a SIP proxy that serves as a registrar and handles originating and terminating session states for users allocated to it. This means that any call that is originated by a specific user or any call that is terminated to that specific user will pass through the S-CSCF that is allocated to that user. At the moment that an S-CSCF is allocated for a specific user, a user profile is downloaded to the S-CSCF from the HSS (Home Subscriber Server), see .To be able to determine which responsibilities the S-CSCF and the Application Server haves to perform and on which user’s behalf, it is necessary to know in which situation is the session and who is the current served user. defines the originating and terminating session cases.In the case of a call diversion service, the received request is first considered as a terminating session case and the terminating iFC configured in the S-CSCF are performed. Receiving the call initiation request, the Application Server is able to determine the served user and the session case (here "term") from the received P-Served-User header field content and to execute terminating services. When the call diversion service is executed, the Application Server changes the target (Request-URI) of the session and a new call leg is created. This new call leg could be considered as an originating call leg but this is not the case. Indeed, the originating user remains the same and the diverting user’s originating services do not have to be triggered as if it was an originating call. For instance, the originating user identity should not be hidden because the diverting user has a privacy service for his/her own identity. In the same manner, some specific services may be triggered when performing a call diversion that would not be for a normal originating call. For this particular use case, this document created a new parameter to be embedded in the P-Served-User header field.

The orig-cdiv-param header filed parameter can be used inside a trust domain by proxies that are processing call diversion services. The following section illustrates how this header field parameter can be used in a 3GPP network.For a terminating call, when receiving the initial INVITE request, the S-CSCF will determines that the session case is for a terminating user as described in , then it determines the served user by looking at the Request-URI and saves this Request-URI. Then, the S-CSCF starts the iFC processing and triggers the served user Application Server for the terminating services to be executed.Based on some criteria, the diversion service concludes that the request needs to be diverted to another user or application and the received Request-URI is replaced with the new diverted-to address. The Application Server stores the Request-URI(s) by adding one or two new History-Info header field entry(ies) . The served user address is tagged thanks to the mp-param header field parameter added in the History-Info entry associated to the diverted-to address created. Then the AS forwards the INVITE request back to the S-CSCF. When the S-CSCF receives the INVITE, it can see that the topmost Route header field contains its own hostname but the Request-URI does not match the saved Request-URI. In that case, the S-CSCF updates the P-Served-User header field content by removing both "sescase" and "regstate" header field parameters and inserting the "orig-cdiv" header field parameter.Then the procedure would continue forwarding the INVITE request over to an AS that hosts the originating services of the served user that specifically need to be executed on the forwarded leg after a call diversion service. When the AS receives the INVITE request, it determines that the session case is for "orig-cdiv" session case and will perform the originating services to be executed after retargeting for the served user.

The use of the P-Served-User header field extensions is only applicable inside a Trust Domain for served user. Nodes in such a Trust Domain explicitly trust each other to convey the served user and to be responsible for withholding that information outside of the Trust Domain. The means by which the network determines the served user and the policies that are executed for a specific served user is outside the scope of this document.

The orig-cdiv-param parameter is defined as an instance of generic-param from the current served-user-param component of P-Served-User header field.Following is a reminder of the P-Served-User syntax as defined in :

P-Served-User = "P-Served-User" HCOLON PServedUser-value *(SEMI served-user-param) served-user-param = sessioncase-param / registration-state-param / generic-param PServedUser-value = name-addr / addr-spec sessioncase-param = "sescase" EQUAL "orig" / "term" registration-state-param = "regstate" EQUAL "unreg" / "reg" "EQUAL", "HCOLON", "SEMI", "name-addr", "addr-spec", and "generic-param" are defined in .

The augmented Backus-Naur Form (ABNF) syntax of the P-Served-User header field is described in .The existing P-Served-User header field ABNF syntax is extended as follows:

served-user-param =/ orig-cdiv-param orig-cdiv-param = "orig-cdiv" The following is an example of a P-Served-User header field: P-Served-User: <sip:user@example.com>; orig-cdiv; regstate=reg

This specification defines a new P-Served-User header field parameter called orig-cdiv-param in the "Header Field Parameters and Parameter Values" sub-registry as per the registry created by .The syntax is defined in Section 4. The required information is:

Header Name References -------------- ------------------ P-Served-User [RFC5502][RFCXXXX] Note to RFC Editor: Please replace XXXX with the RFC number of this document.

The security considerations in apply.As the orig-cdiv-param P-Served-User header field parameter can be used to trigger applications, it is important to ensure that the parameter has not been added to the SIP message by an unauthorized SIP entity.

TBD