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Network Working Group                                      J. van ElburgRequest for Comments: 5502                Ericsson Telecommunicatie B.V.Category: Informational                                       April 2009The SIP P-Served-User Private-Header (P-Header)for the 3GPP IP Multimedia (IM) Core Network (CN) SubsystemStatus 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) 2009 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents in effect on the date of   publication of this document (http://trustee.ietf.org/license-info).   Please review these documents carefully, as they describe your rights   and restrictions with respect to this document.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Abstract   This document specifies the SIP P-Served-User P-header.  This header   field addresses 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.  This header   field conveys the identity of the served user and the session case   that applies to this particular communication session and application   invocation.van Elburg                   Informational                      [Page 1]

RFC 5502               The P-Served-User P-Header             April 2009Table of Contents1. Introduction ....................................................32. Conventions .....................................................33. Definitions .....................................................3      3.1. Identity, Network Asserted Identity, Trust Domain,           and Spec(T) ................................................33.2. Served User ................................................34. Scenarios .......................................................44.1. General ....................................................4      4.2. Diversion: Continue on Terminating Leg, but Finish           Subsequent Terminating iFC First ...........................5      4.3. Diversion: Create New Originating Leg and Provide           Originating iFC Processing .................................6      4.4. Call Out of the Blue: on Behalf of User B, but           Service Profile of Service Identity C.......................85. Requirements ....................................................86. P-Served-User Header Field Definition ...........................97. Proxy Behavior ..................................................97.1. Generating the P-Served-User Header ........................97.2. Consuming the P-Served-User Header ........................108. Applicability ..................................................109. IANA Considerations ............................................1110. Security Considerations .......................................1111. Acknowledgments ...............................................1112. References ....................................................1212.1. Normative References .....................................1212.2. Informative References ...................................12Appendix A.  Why the History-Info Header Is Not Suitable to                Convey the Served User Information on the ISC                Interface ............................................13A.1.  Semantics  ................................................13A.2.  Additional Observations  ..................................13A.3.  Conclusion ................................................14van Elburg                   Informational                      [Page 2]

RFC 5502               The P-Served-User P-Header             April 20091.  Introduction   The 3rd Generation Partnership Project (3GPP) IMS (IP Multimedia   Subsystem) uses SIP (RFC 3261 [2]) as its main signaling protocol.   (For more information on the IMS, a detailed description can be found   in 3GPP TS 23.228 [9] and 3GPP TS 24.229 [11].) 3GPP has identified   issues with the linking in of a SIP application server that are most   appropriately resolved by defining a new SIP P-header, according to   the procedures inRFC 3427 [5].   The remainder of this document is organized as follows.Section 4   outlines the problem by using particular service scenarios, andSection 5 discusses the requirements derived from these scenarios.Section 6 defines the P-Served-User header field, which meets those   requirements,Section 7 specifies the proxy behavior for the new   header field, andSection 8 discusses the applicability and scope of   this new header field.Section 9 registers the P-Served-User header   field with the IANA, andSection 10 discusses the security properties   of the environment where this header field is intended to be used.2.  Conventions   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inBCP 14,RFC 2119 [1].3.  Definitions3.1.  Identity, Network Asserted Identity, Trust Domain, and Spec(T)   The terms Identity, Network Asserted Identity, Trust Domain, and   Spec(T) in this document are specified inRFC 3324 [3].3.2.  Served User   The served user to a proxy or AS (Application Server) is the user   whose service profile is accessed by that proxy or AS when an initial   request is received that is originated by, originated on behalf of,   or terminated to that user.  This profile in turn provides some   useful information (preferences or permissions) for processing at a   proxy and, potentially, at an AS.van Elburg                   Informational                      [Page 3]

RFC 5502               The P-Served-User P-Header             April 20094.  Scenarios4.1.  General   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) over the Cx interface, see 3GPP TS 29.228 [12].  This user   profile tells the S-CSCF whether the user is allowed to originate or   terminate calls or whether an AS needs to be linked in over the ISC   interface.  The user profile information that determines whether a   particular initial request needs to be sent to a particular AS is   called the initial Filter Criteria (iFC), see for example 3GPP TS   23.218 [8].   For an S-CSCF to be able to meet its responsibilities, it needs to   determine on which user's behalf it is performing its tasks and which   session case is applicable for the particular request.  (For a   definition of session case, see 3GPP TS 29.228 [12]).  The session   case distinguishes the originating and terminating call cases and   determines whether or not the particular user is registered.   When the S-CSCF determines that for an incoming initial request the   originating call case applies, it determines the served user by   looking at the P-Asserted-Identity header field (RFC 3325 [4]), which   carries the network asserted identity of the originating user.  When,   after processing the iFC for this initial request, the S-CSCF decides   to forward the request to an AS, the AS has to go through a similar   process of determining the session case and the served user.  Since   it should come to the same conclusion that this is an originating   session case, it also has to look at the P-Asserted-Identity header   field to determine the served user.   When the S-CSCF determines that for an incoming initial request the   terminating call case applies, it determines the served user by   looking at the Request-URI (RFC 3261 [2]), which carries the identity   of the intended terminating user.  When, after processing the iFC for   this initial request, the S-CSCF decides to forward the request to an   AS, the AS has to go through a similar process of determining the   session case and the served user.  Since it should come to the same   conclusion that this is a terminating session case, it also has to   look at the Request-URI to determine the served user.van Elburg                   Informational                      [Page 4]

RFC 5502               The P-Served-User P-Header             April 2009   In the originating case, it can be observed that while the   P-Asserted-Identity header field just represents the originating user   when it enters the S-CSCF, it is overloaded with another meaning when   it is sent to an AS over the ISC interface.  This other meaning is   that it serves as a representation of the served user.   In the terminating case, a similar overloading happens to the   Request-URI; while it first only represented the identity of the   intended terminating user, it is overloaded with another meaning when   it is sent to an AS over the ISC interface.  This other meaning is   that it serves as a representation of the served user.   In basic call scenarios, this does not show up as a problem, but once   more complicated service scenarios (notably forwarding services) need   to be realized, it poses severe limitations.  Such scenarios are   brought forward in the following subsections.4.2.  Diversion: Continue on Terminating Leg, but Finish Subsequent      Terminating iFC First   Imagine a service scenario where a user B has a terminating service   that diverts the call to a different destination but is required to   still execute subsequent terminating services for the same user.   This means that this particular user has multiple iFC configured that   are applicable for an incoming initial request.  When the S-CSCF   receives an initial INVITE request, it analyzes the request and   determines that the session case is for a terminating registered   user, then it determines the served user to be user B by looking at   the Request-URI.   Now the S-CSCF starts the iFC processing.  The first iFC that matches   the INVITE request causes the INVITE to be forwarded over the ISC   interface to an AS that hosts user B's diversion service by adding   the AS and S-CSCF's own hostnames to the Route header.  The S-CSCF   adds an Original Dialog Identifier (ODI) to the S-CSCF's own hostname   on the Route header.  This allows the S-CSCF to correlate an INVITE   coming from an AS over the ISC interface to the existing session that   forwarded the INVITE to the AS in the first place.   When the AS receives the initial INVITE request, it analyzes the   request and determines that the session case is for a terminating   registered user, then it determines the served user to be user B by   looking at the Request-URI.  Based on some criteria, the diversion   service concludes that the request needs to be diverted to another   user or application C.  It does this by changing the Request-URI to   C.  Optionally, it records the Request-URI history by using the   History- Info header field (RFC 4244 [7]).  Then the AS removesvan Elburg                   Informational                      [Page 5]

RFC 5502               The P-Served-User P-Header             April 2009   itself from the Route header and routes the INVITE request back to   the S-CSCF by using the topmost Route header field.   When the S-CSCF receives the INVITE over the ISC interface, it can   see that the Route header contains its own hostname and an ODI that   correlates to an existing terminating session for user B.  This can   be used by the S-CSCF to analyze whether there are still unexecuted   iFC.  (Note that the current behavior of the S-CSCF on receiving an   INVITE with a changed Request-URI is to terminate the iFC processing   and to route the request based on the new Request-URI value.)   The process repeats itself.  The INVITE is forwarded to the AS that   is associated with this particular iFC.  When the AS receives the   initial INVITE request, it analyzes the request and determines that   the session case is for a terminating registered user, then it   determines the served user to be user C by looking at the Request-   URI.  This is clearly wrong, as the user being served is still user   B.   This scenario clearly shows the problem that occurs when the Request-   URI is overloaded with the meanings "intended target identity" and   "served user" with the operation as described inSection 4.1.  And it   shows that this use case can not be realized without introducing a   mechanism that conveys information about the served user from the   S-CSCF to the AS.  Use of the History-Info element does not solve   this problem as it does not tell the AS which user is being served;   it just presents a history of diversions that might not be even   caused by the systems serving this particular user.  A more detailed   analysis on why the History-Info header field can't be used is   provided inAppendix A.4.3.  Diversion: Create New Originating Leg and Provide Originating iFC      Processing   Imagine a service scenario where a user B has a terminating service   that diverts the call to a different destination.  It is required   that a forwarded call leg is handled as an originating call leg and   that originating services for user B are executed.  This means that   this particular user has one or more iFC configured that are   applicable for an outgoing initial request.   When the S-CSCF receives an initial INVITE request, it analyzes the   request and determines that the session case is for a terminating   registered user, then it determines the served user to be user B by   looking at the Request-URI.van Elburg                   Informational                      [Page 6]

RFC 5502               The P-Served-User P-Header             April 2009   Now the S-CSCF starts the iFC processing.  The first iFC that matches   the INVITE request causes the INVITE to be forwarded over the ISC   interface to an AS that hosts user B's diversion service by adding   the AS and S-CSCF's own hostnames to the Route header.  The S-CSCF   adds an Original Dialog Identifier (ODI) to the S-CSCF's own hostname   on the Route header.  This allows the S-CSCF to correlate an INVITE   coming from an AS over the ISC interface to the existing session that   forwarded the INVITE to the AS in the first place.   When the AS receives the initial INVITE request, it analyzes the   request and determines that the session case is for a terminating   registered user, then it determines the served user to be user B by   looking at the Request-URI.  Based on some criteria, the diversion   service concludes that the request needs to be diverted to another   user or application C.  It does this by changing the Request-URI to   C.  Optionally, it records the Request-URI history by using the   History-Info header field (RFC 4244 [7]).  Then the AS removes itself   from the Route header.  To make sure that the request is handled as a   new originating call on behalf of user B, the AS adds the "orig"   parameter to the topmost route header.  Then it routes the INVITE   request back to the S-CSCF by using this topmost Route header field.   When the S-CSCF receives the INVITE over the ISC interface, it can   see that the topmost Route header contains its own hostname and an   "orig" parameter.  Because the topmost Route header contains the   "orig" parameter, the S-CSCF concludes that the INVITE should be   handled as if a call is originated by the served user.  The served   user is determined from the P-Asserted-Identity header to be user A.    This is clearly wrong, as the user being served is and should be   user B.   For the sake of discussion, let's assume that the S-CSCF can   determine that the served user is user B.  Then the procedure would   continue as follows: The S-CSCF starts the originating iFC   processing, the first iFC that matches the INVITE request causes the   INVITE to be forwarded over the ISC interface to an AS that hosts an   originating service of user B by adding the AS and S-CSCF's own   hostnames to the Route header.  The S-CSCF adds an Original Dialog   Identifier (ODI) to the S-CSCF's own hostname on the Route header.   The INVITE is forwarded to the AS that is associated with this   particular iFC.  When the AS receives the initial INVITE request, it   analyzes the request and determines that the session case is for an   originating registered user, then it determines the served user to be   user A by looking at the P-Asserted-Identity.  This is clearly wrong,   as the user being served is and should be user B.van Elburg                   Informational                      [Page 7]

RFC 5502               The P-Served-User P-Header             April 2009   This scenario clearly shows the problem that occurs when the   P-Asserted-Identity is overloaded with the meanings "call originator"   and "served user" with the operation as described inSection 4.1.   And it shows that this use case can not be realized without   introducing a mechanism that conveys information about the served   user from the S-CSCF to the AS and from the AS to the S-CSCF.  Use of   the History-Info element does not solve this problem as it does not   tell the AS which user is being served, but just presents a history   of diversions that might not be even caused by the systems serving   this particular user.  A more detailed analysis on why the History-   Info header field can't be used is provided inAppendix A.4.4.  Call Out of the Blue: on Behalf of User B, but Service Profile of      Service Identity C   There are services that need to be able to initiate a call, whereby   the call appears to be coming from a user B but the service profile   on behalf of service identity C needs to be executed in the S-CSCF.   When a call needs to appear as coming from user B, that means that   the P-Asserted-Identity needs to contain B's identity.  This is   because the Originating Identity Presentation (OIP) service as   defined in 3GPP TS 24.173 [10] uses the P-Asserted-Identity to   present the call originator.  This makes sense because that is the   main meaning expressed by the P-Asserted-Identity header field.   It is clear that no INVITE request can be constructed currently that   would achieve both requirements expressed in the first paragraph,   because the P-Asserted-Identity is overloaded with two meanings on   the ISC interface.  When the S-CSCF will receive this request, it   will determine that the served user is user B, which is not what we   want to achieve.5.  Requirements   This section lists the requirements derived from the previous   scenarios:   1.  To be able to offer real-world application services, it is       required that the identity of the served user can be conveyed on       the ISC interface (see 3GPP TS 23.218 [8]).   2.  To be able to offer appropriate services to the served user, it       is required that in addition to the served user identity the       session case is conveyed.van Elburg                   Informational                      [Page 8]

RFC 5502               The P-Served-User P-Header             April 20096.  P-Served-User Header Field Definition   This document defines the SIP P-Served-User P-header.  This header   field can be added to initial requests for a dialog or standalone   requests, which are routed between nodes in a Trust Domain for   P-Served-User.  The P-Served-User P-header contains an identity of   the user that represents the served user.  The "sescase" parameter   may be used to convey whether the initial request is originated by or   destined for the served user.  The "regstate" parameter may be used   to indicate whether the initial request is for a registered or   unregistered user.   The augmented Backus-Naur Form (BNF) (RFC 5234 [6]) syntax of the   P-Served-User header field is as follows:   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 inRFC 3261 [2].   The following is an example of a P-Served-User header field:   P-Served-User: <sip:user@example.com>; sescase=orig; regstate=reg7.  Proxy Behavior7.1.  Generating the P-Served-User Header   Proxies that support the header MUST only insert the header in   initial requests for a dialog or in standalone requests when the   following conditions hold:   o  The proxy has the capability to determine the served user for the      current request.   o  The next hop is part of the same Trust Domain for P-Served-User.   When the above conditions do not hold, the proxy MUST NOT insert the   header.van Elburg                   Informational                      [Page 9]

RFC 5502               The P-Served-User P-Header             April 20097.2.  Consuming the P-Served-User Header   A proxy that supports the header MUST, upon receiving from a trusted   node the P-Served-User header in initial requests for a dialog or in   standalone requests, take the value of the P-Served-User header to   represent the served user in operations that require such   information.   A proxy that supports the header MUST remove the header from requests   or responses when the header was received from a node outside the   Trust Domain for P-Served-User before further forwarding the message.   A proxy that supports the header MUST remove the header from requests   or responses when the next hop is a node outside the Trust Domain for   P-Served-User before further forwarding the message.8.  Applicability   According toRFC 3427 [5], 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 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 served user information lacks an indication of who or what   specifically determined the served user, and so it must be assumed   that the Trust Domain determined the served user.  Therefore, the   information is only meaningful when securely received from a node   known to be a member of the Trust Domain.   Because the served user typically only has validity in one   administrative domain, it is in general not suitable for inter-domain   use or use in the Internet at large.   Despite these limitations, there are sufficiently useful specialized   deployments that meet the assumptions described above, and that can   accept the limitations that result, to warrant informational   publication of this mechanism.  An example deployment would be a   closed network like 3GPP IMS.van Elburg                   Informational                     [Page 10]

RFC 5502               The P-Served-User P-Header             April 20099.  IANA Considerations   This document defines a new SIP header field: P-Served-User.  This   header field has been registered by the IANA in the SIP Parameters   registry under the Header Fields subregistry.10.  Security Considerations   The P-Served-User header field 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-Served-User header   field will be used ensures the integrity and the confidentiality of   the contents of this header field.   The Spec(T) that defines the Trust Domain for P-Served-User MUST   require that member nodes understand the P-Served-User header   extension.   There is a security risk if a P-Served-User header field is allowed   to propagate out of the Trust Domain where it was generated.  In that   case, user-sensitive information would be revealed.  To prevent such   a breach from happening, proxies MUST NOT insert the header when   forwarding requests to a hop that is located outside the Trust   Domain.  When forwarding the request to a node in the Trust Domain,   proxies MUST NOT insert the header unless they have sufficient   knowledge that the route set includes another proxy in the Trust   Domain that understands the header, such as the home proxy.  There is   no automatic mechanism to learn the support for this specification.   Proxies MUST remove the header when forwarding requests to nodes that   are not in the Trust Domain or when the proxy does not have knowledge   of any other proxy included in the route set that will remove it   before it is routed to any node that is not in the Trust Domain.11.  Acknowledgments   Alf Heidermark, Hubert Przybysz, and Erik Rolin for the discussion   that led to the solution written down in this document.  Spencer   Dawkins for performing the expert review.  Jon Peterson for   performing the AD review.  Gonzalo Camarillo, Paul Kyzivat, Nils   Haenstroem, Arunachalam Venkatraman, Mikael Forsberg, Miguel Garcia,   Jozsef Varga, Keith Drage, Tim Polk, and Cullen Jennings for   providing improvements.  Francis Dupont for performing the general   area review.  Sandy Murphy for performing the security area review.van Elburg                   Informational                     [Page 11]

RFC 5502               The P-Served-User P-Header             April 200912.  References12.1.  Normative References   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement         Levels",BCP 14,RFC 2119, March 1997.   [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]   Watson, M., "Short Term Requirements for Network Asserted         Identity",RFC 3324, November 2002.   [4]   Jennings, C., Peterson, J., and M. Watson, "Private Extensions         to the Session Initiation Protocol (SIP) for Asserted Identity         within Trusted Networks",RFC 3325, November 2002.   [5]   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.   [6]   Crocker, D. and P. Overell, "Augmented BNF for Syntax         Specifications: ABNF", STD 68,RFC 5234, January 2008.12.2.  Informative References   [7]   Barnes, M., "An Extension to the Session Initiation Protocol         (SIP) for Request History Information",RFC 4244,         November 2005.   [8]   3GPP, "IP Multimedia (IM) session handling; IM call model;         Stage 2", 3GPP TS 23.218 V7.   [9]   3GPP, "IP Multimedia Subsystem (IMS); Stage 2", 3GPP TS 23.228         V7.   [10]  3GPP, "IMS multimedia telephony communication service and         supplementary services; Stage 3", 3GPP TS 24.173 V7.   [11]  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 V7.   [12]  3GPP, "IP Multimedia (IM) Subsystem Cx and Dx interfaces;         Signalling flows and message contents", 3GPP TS 29.228 V7.van Elburg                   Informational                     [Page 12]

RFC 5502               The P-Served-User P-Header             April 2009Appendix A.  Why the History-Info Header Is Not Suitable to Convey the             Served User Information on the ISC InterfaceA.1.  Semantics   The History-Info (as specified inRFC 4244 [7]) holds a record of   subsequent Request-URI values that are put on an initial request   during its processing in the network.   If it would be possible at all to use the History-Info header for the   purpose of communicating the served user, then again the same   overloading would occur as the one that we are trying to get rid of   (Section 4.2).  In this case, we overload the particular History-Info   header field's hi-entry with the meaning "historic target identity"   and "served user".   Another reason that the History-Info header can not solve the   requirements as expressed in this document is that, in originating   session case scenarios, the served user is currently determined from   the P-Asserted-Identity, as that header field contains the asserted   originating user's identity.  The History-Info header, being a record   of Request-URIs, can never be a solution for this case.   Looking at the call-out-of-the-blue scenario (Section 4.4), it is   impossible to construct a History-Info header for an INVITE request   on behalf of user C that appears to come from user B and targets user   D that would express the served user C without violating the original   semantics of the History-Info header according to (RFC 4244 [7]).A.2.  Additional Observations   The purpose of the History-Info header is a header that has an end-   to-end application.  For the purpose of informing an AS on the ISC   interface, this is overkill.   At the moment that the AS receives an initial INVITE over the ISC   interface, this INVITE may have passed a vast number of proxies that   may or may not have added history information.  On top of that, the   request may have traversed several AS instances for the same served   user.  In case several subsequent iFC are active, all these AS   instances may perform a forwarding.  This means that it is not   possible to define an algorithm that points out which hi-entry of a   History-Info header should represent the served user.  In other   words, a History-Info header field with n entries expresses a branch   of depth n.  Any or none of these elements could be the served user   identity.van Elburg                   Informational                     [Page 13]

RFC 5502               The P-Served-User P-Header             April 2009   The History-Info header does not comply with the second requirement   as expressed inSection 5, as it does not have a means to express the   session case in a natural way.A.3.  Conclusion   Each observation in the previous subsections, alone, is enough to   disregard the History-Info header as an information element that is   suitable for transporting the served user information over the ISC   interface.   Note that this does not prohibit the use of the P-Served-User header   and the History-Info header in the same request.  In fact that will   be a quite likely scenario for network-based diversion services like,   for example, the Communication Diversion service as specified in   (3GPP TS 24.173 [10]).Author's Address   Hans Erik van Elburg   Ericsson Telecommunicatie B.V.   Ericssonstraat 2   Rijen  5121 ML   Netherlands   EMail: HansErik.van.Elburg@ericsson.comvan Elburg                   Informational                     [Page 14]