WO2008051944A2

WO2008051944A2 - Method and apparatus for session initiation protocol registration

Info

Publication number: WO2008051944A2
Application number: PCT/US2007/082173
Authority: WO
Inventors: Sriram Sridhar; Kiran Kumar Das; Venkatesh J S
Original assignee: Motorola, Inc.
Priority date: 2006-10-26
Filing date: 2007-10-23
Publication date: 2008-05-02
Also published as: WO2008051944A3

Abstract

User equipment is registered with a network by passing a first REGISTER request from the user equipment to a Registrar of the network in accordance with a Session Initiation Protocol (SIP) and the Registrar passing an authorization challenge message to the user equipment in response to first REGISTER request. The user equipment responds to the authorization challenge message with a second REGISTER request that includes a response to the authorization challenge. The Registrar responds to the second REGISTER request with an OK message if the second REGISTER request is approved. The user equipment then responds to the OK message with an acknowledgment (ACK) message. Finally, the Registrar registers the user equipment if the ACK message is received by the registrar server. Messages and requests may be passed between the Registrar and the user equipment using a P-CSCF and one or more I-CSCF's.

Description

METHOD AND APPARATUS FOR SESSION INITIATION PROTOCOL REGISTRATION

Field of the Invention

[0001] The present invention relates generally to communication in mobile networks.

Background

[0002] Session Initiation Protocol (SIP) is a text-based application layer control and signaling protocol for creating, terminating and modifying sessions with one or more participants. It can be used for voice, video, instant messaging, gaming and other multimedia applications. SIP messages can be transported by a variety of access technologies including General Packet Radio Services (GPRS).

[0003] A GPRS Core Network (CN) provides wireless data services in mobile networks such as the GSM and UMTS networks. In particular, it provides mobility management, session management and transport for Internet Protocol packet services. The GPRS Core Network also provides support for other additional functions such as charging (the collection of data of network resource usage and services to enable billing of the subscriber) and lawful interception (wiretapping). From a GPRS charging point of view, SIP signaling is considered to be user data.

[0004] SIP provides a number of messages or methods such as INVITE, ACK, OPTIONS, CANCEL, BYE, REGISTER and SUBSCRIBE. SIP also provides a number of numerical responses, such as 100 Trying, 200 OK, 302 Moved Temporarily, 404 Not Found, 504 Server Timeout, and 603 Decline.

[0005] The REGISTER message is used to send a user's address to a server. When SIP signaling is used for multimedia applications in a mobile network, a problem can occur if the SIP 200 OK response to an SIP REGISTER message is lost. This can occur, for example, if the user equipment (the mobile device) has roamed out of coverage from the network. As a result, the network believes that the user equipment is successfully registered, whereas the user equipment is in an un-registered state since it has lost the SIP 200 OK message. The SIP does not contain any mechanisms for resolving this problem.

Brief Description of the Figures

[0006] The accompanying figures, in which like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.

[0007] FIG. 1 is an example of a network in accordance with some embodiments of the invention.

[0008] FIG. 2 is a call sequence diagram of the prior art.

[0009] FIG. 3 is a call sequence diagram in accordance with some embodiments of the invention.

[0010] FIG.'s 4-15 are listings of exemplary Session Initiation Protocol (SIP) messages.

[0011] FIG.'s 16-18 are listings of exemplary SIP acknowledgement messages in accordance with some embodiments of the invention. [0012] FIG. 19 is a further example of a network in accordance with some embodiments of the invention.

[0013] FIG. 20 is a call sequence diagram in accordance with some embodiments of the invention.

[0014] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. Detailed Description

[0015] Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to SIP registration in a mobile network. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.

[0016] In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "comprises ...a" does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.

[0017] It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of SIP registration in a mobile network described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as a method to perform SIP registration in a mobile network. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.

[0018] Session Initiation Protocol (SIP) is an application layer control and signaling protocol for creating, terminating and modifying network sessions with one or more participants. It can be used for voice, video, instant messaging, gaming and other multimedia applications, for example.

[0019] SIP has been selected as the signaling protocol for IP Multimedia Subsystems (IMS) of third generation (3G) networks, and as such will be used for call control and signaling of multimedia content to mobile handsets. In standards developed by The Third Generation Partnership Project (3GPP), the Call State Control Function (CSCF) in the network is the equivalent of a SIP server. The IMS network architecture has been defined to be access independent. This means that the access technology used to transport user SIP messages to the IMS network does not impact the functionality of the IMS network itself. Consequently, any access can be used, examples being DSL, Cable, WLAN, GPRS, etc.

[0020] The General Packet Radio Services (GPRS) Core Network provides wireless data services in mobile networks such as the GSM and UMTS networks. In particular, it provides mobility management, session management and transport for Internet Protocol packet services. The GPRS Core Network also provides support for other additional functions such as charging (the collection of data of network resource usage and services to enable billing of the subscriber) and lawful interception (wiretapping). From a GPRS charging point of view, SIP signaling is considered to be user data. [0021] Certain embodiments of the invention will be described below in the context of CSCF's, but it will be apparent to those of ordinary skill in the art that the invention has application in other network architectures where Session Initiation Protocol is used.

[0022] FIG. 1 is an example of a network 100 in accordance with some embodiments of the invention. Referring to FIG. 1, User Equipment (UE) 102 is a hardware device, such as a wireless telephone, that supports a SIP User Agent Client. The user equipment is operated by a user or subscriber. The Proxy CSCF (P-CSCF) 104 is the first point of contact in a visited network. Its function is to find the user's home network and provide some translation, security and authorization functions. The P- CSCF acts as an intermediary between a SIP user agent client and a SIP user agent server. The P-CSCF performs the functions of either a SIP user agent client or a SIP user agent server, depending upon the direction of the communication between client and server. The P-CSCF can simply forward the SIP request or modify it before sending it on.

[0023] The Serving CSCF (S-CSCF) 106 controls sessions, acts as a Registrar and triggers and executes services. The S-CSCF 106 can be located in the home network or in another network. It accesses the user's profile on the Home Subscriber Server (HSS) 108. The S-CSCF is also known as a registrar server. It receives REGISTER requests, which contain both the IP address and the SIP address (the Uniform Resource Locator (URL)) of the user agent. This allows the S-CSCF to keep track of the location of user agents from which the registrar server has received REGISTER requests. The S-CSCF provides the session control services for a mobile device, which enables the network operator to support the services. Functions include: the management of user equipment registrations, maintaining of the session, interaction with other services, charging and security.

[0024] The Home Subscriber Server (HSS) 108 is a database containing subscriber and service data. Parameters include user identity, allocated S-CSCF name, roaming profile, authentication parameters and service information.

[0025] The Interrogating CSCF (I-CSCF) 110 is the first point of contact in the home network. The I-CSCF provides a contact point within an operator's network, allowing subscribers of the network operator, or roaming subscribers to register. At registration, the I-CSCF queries the HSS 108 to retrieve location information and assigns the serving CSCF. It also forwards SIP requests. The I-CSCF checks that the user is allowed to register in the originating network and returns an S-CSCF name and capability if this is the case. The I-CSCF is then able to contact the S-CSCF with the register request. The I-CSCF function can be removed from the signaling path once it has been used to establish which S-CSCF is in use.

[0026] The network 100 may contain multiple Call State Control Functions.

[0027] Upon power up, the User Equipment 102 connects to the core network using GPRS procedures and acquires the necessary Internet Protocol (IP) address via activation of a packet data protocol (PDP) context, which includes, or is followed by, the P-CSCF discovery procedure, through which the P-CSCF 104 is selected. The User Equipment registers with the Registrar 106 (the S-CSCF) via the SIP registration procedure (described in the Internet Engineering Task Force (IETF) document IETF RFC-3261, for example) and can initiate an SIP call following the rules and procedure according to the SIP based signaling. The S-CSCF 106 is selected by the I-CSCF 110.

[0028] The only way a user can be contactable by the SIP core network is to have an IP address associated with the access point name (APN) providing GPRS connectivity to the SIP core network. In most of the networks, this IP address is dynamically allocated by the GPRS core network at the moment of activation of the "always-on" general purpose PDP context. This IP address is then bound with the services provided by the SIP core network when the user equipment includes both the user's public user ID and IP address in the SIP REGISTER message which registers the public user ID with the SIP core network.

[0029] Thus, the registration will lose its validity every time the UE changes or loses the IP address used to communicate with the SIP core network, since the SIP core network will no longer be able to contact the user at the previous address. This means that, in order to be able to continue to enjoy the services provided by/ through the SIP core network, the UE needs to perform a new registration with the SIP core network every time it changes its IP address. [0030] There are no PDP context re-try mechanisms provided by SIP level applications. Instead, the entity handling the user equipment's SIP registration state relies on the entity handling the "always-on" behavior of the general purpose PDP context to perform the appropriate PDP context activation re-tries and call back informing of the availability of the PDP context when successful. The entity handling the UE's SIP registration state then decides whether or not to (re-) register to the SIP core network.

[0031] FIG. 2 is a call sequence diagram of a network operation in accordance with the prior art where an SIP 200 OK response is lost. FIG. 2 shows time line 202 for the user equipment (UE) and time lines 204, 206 and 208, for the proxy CSCF (P- CSCF), Interrogating CSCF (I-CSCF) and serving CSCF (S-CSCF), respectively, of an SIP core network. At time Tl, the UE has sends an initial REGISTER message to the SIP core network in order to register its public user ID with the SIP CN. The message is passed via the P-CSCF and I-CSCF to the S-CSCF. At time T2 an authentication challenge is sent by the S-CSCF of the SIP core network in a '401 Unauthorized Response' message towards the UE. At time T3, the UE sends the authentication challenge response to the SIP core network in a second REGISTER request. At time T4, the SIP core network sends a 200 OK to the UE indicating that the registration was successful. Meanwhile, at time T5, the UE moves to a location where the service is not supported. Consequently, the '200 OK' message sent at time T4 does not reach the UE. At time T6, which is before the registration timer expiry, the UE moves back to a location where the service is supported. The PDP context gets re-activated and the UE tries to REGISTER with the network with the same Call- ID (for all registrations during a single boot cycle). At time T7, the SIP core network determines that the UE is trying to modify the bindings and so rejects the new REGISTER request with a '403 Forbidden' error response. Thus, the network connection has failed.

[0032] FIG. 3 is a call sequence diagram of a network operation in accordance with some embodiments of the present invention. FIG. 3 shows time line 302 for the user equipment (UE) and time lines 304, 306 and 308, for the proxy CSCF (P-CSCF), Interrogating CSCF (I-CSCF) and serving CSCF (S-CSCF), respectively, of an SIP core network. At time Tl, the UE has sends a REGISTER message to the SIP core network in order to register its public user ID with the SIP CN. The message is passed via the P-CSCF and I-CSCF to the S-CSCF. At time T2 an authentication challenge is sent by the S-CSCF of the SIP core network in a '401 Unauthorized Response' message towards the UE. At time T3, the UE sends the authentication challenge response to the SIP core network in the REGISTER request. At time T4, the SIP core network sends a '200 OK' message to the UE indicating that the registration was successful. If the UE is still is within the network service area at time T5, the '200 OK' message is received and an acknowledgement (ACK) response is sent to the core network. The ACK response is transmitted via the P-CSCF and the I- CSCF to the S-CSCF. If any part of the connection link fails, the S-CSCF will not receive the ACK message and can assume that the registration has failed. Thus, the core network will not assume that the user equipment is successfully registered until an SIP ACK message is received from the user equipment. The registration may be assumed to have failed if no ACK message is received from the user equipment within a specified time period.

[0033] If registration is not successful, the user equipment can re-register with the network at a later time without causing a '403' message to be issued by the S-CSCF.

[0034] FIG.'s 4-6 show exemplary REGISTER requests 310, 312 and 314.

[0035] FIG.'s 7-9 show exemplary '401 Unauthorized Response' messages 316, 318 and 320.

[0036] FIG.'s 10-12 show exemplary REGISTER requests 322, 324 and 326. [0037] FIG.'s 13-15 show exemplary 200 OK responses 328, 330 and 332. [0038] FIG.'s 16-18 show exemplary ACK messages 334, 336 and 338.

[0039] The process of acknowledging the '200 OK' response from the core network comprises a three-way handshake verifying the S-CSCF to I-CSCF link, the I-CSCF to P-CSCF link, and the P-CSCF to UE link.

[0040] FIG. 4 is a listing of an exemplary REGISTER request 310 from the UE to the P-CSCF. Referring to FIG. 4, the REGISTER request includes a number of general SIP headers such as 'Via:', 'From:', 'To:', 'Contact:', 'Call-ID:', and 'CSeq:', in addition to a request header, 'Max-Forwards:', and an entity header, 'Content- Length:'. These headers are followed by associated values. The headers are described in the Internet Engineering Task Force (IETF) document IETF RFC-3261, for example.

[0041] FIG. 5 is a listing of an exemplary REGISTER request 312 from the P-CSCF to the I-CSCF.

[0042] FIG. 6 is a listing of an exemplary REGISTER request 314 from the I-CSCF to the S-CSCF.

[0043] FIG. 7 is a listing of an exemplary '401 Unauthorized Response' message 316 from the S-CSCF to the I-CSCF

[0044] FIG. 8 is a listing of an exemplary '401 Unauthorized Response' message 318 from the I-CSCF to the P-CSCF.

[0045] FIG. 9 is a listing of an exemplary '401 Unauthorized Response' message 320 from the P-CSCF to the UE.

[0046] FIG. 10 is a listing of an exemplary REGISTER request 322 from the UE to the P-CSCF.

[0047] FIG. 11 is a listing of an exemplary REGISTER request 324 from the P-CSCF to the I-CSCF.

[0048] FIG. 12 is a listing of an exemplary REGISTER request 326 from the I-CSCF to the S-CSCF.

[0049] FIG. 13 is a listing of an exemplary '200 OK' message 328 from the S-CSCF to the I-CSCF.

[0050] FIG. 14 is a listing of an exemplary '200 OK' message 330 from the I-CSCF to the P-CSCF.

[0051] FIG. 15 is a listing of an exemplary '200 OK' message 332 from the P-CSCF to the UE.

[0052] FIG. 16 is a listing of an exemplary ACK message 324 from the UE to the P- CSCF.

[0053] FIG. 17 is a listing of an exemplary ACK message 326 from the P-CSCF to I- CSCF. [0054] FIG. 18 is a listing of an exemplary ACK message 328 from the I-CSCF to S- CSCF.

[0055] FIG. 19 is a further example of a network 1900 in accordance with some embodiments of the invention. In this example, the S-CSCF or serving node 106' is located in the visited network. The S-CSCF is selected during the registration process. The home I-CSCF 110 is contacted first to enable location information to be retrieved from the HSS 108 of the home network. Then the I-CSCF 110' of the visited network is contacted to retrieve the location of the S-CSCF 106' in the visited network from the HSS 108' of the visited network. The S-CSCF 106' of the visited network queries the HSS 108 of the home network to retrieve user profile information. A '200 OK' message is then passed back to the user equipment via the I-CSCF 's 110' and 110 and the P-CSCF 104. An ACK message is then passed from the user equipment to the S- CSCF 106' via the P-CSCF 104, the I-CSCF's 110 and the I-CSCF's 110'. Thus, a total of four SIP ACK messages are sent.

[0056] FIG. 20 is a call sequence diagram in accordance with some embodiments of the invention. The call sequence in FIG. 20 corresponds to an SIP registration procedure where the S-CSCF is located in a visited network, as shown in FIG. 19, for example. FIG. 20 is similar to FIG. 3, except that an additional I-CSCF, the visited I-CSCF, is included in the communication. The time line of the visited I-CSCF is denoted by 2002. Additional REGISTER requests 2004 and 2006, an additional '401 Unauthorized Response' message 2008, an additional '200 OK' message 2110 and an additional ACK message 2122 are passed between the home I-CSCF and the visited I- CSCF.

[0057] More generally, the REGISTER, '200 OK' and ACK messages may be passed only any path between the user equipment and the registrar. The registrar may in the home network, the visited network or another network.

[0058] The registration protocol described above may be implemented, for example, by a programmed processor executing program instructions stored on a computer readable medium, such as a computer memory.

[0059] In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.

Claims

What is claimed is:

1. A method for registering user equipment with a Serving Call State Control Function (S-CSCF) of a network, the method comprising: a Proxy Call State Control Function (P-CSCF) of the network receiving a

REGISTER request from the user equipment; the P-CSCF forwarding the REGISTER request to an Interrogating Call State Control Function (I-CSCF) of the network; the I-CSCF forwarding the REGISTER request to the S-CSCF; the S-CSCF passing an OK message to the I-CSCF in response to the

REGISTER request if the request is approved; the I-CSCF forwarding the OK message to the P-CSCF; the P-CSCF forwarding the OK message to the user equipment; the P-CSCF receiving an acknowledgment (ACK) message from the user equipment; the P-CSCF forwarding the ACK message to the I-CSCF; the I-CSCF forwarding ACK message to the S-CSCF; and the S-CSCF registering the user equipment in response to the ACK message from the I-CSCF.

2. A method in accordance with claim 1, wherein the network comprises an Internet Protocol Multimedia Subsystem (IMS).

3. A method in accordance with claim 1, wherein the REGISTER request comprises a challenge response REGISTER request that includes an authentication challenge response, the method further comprising: the P-CSCF receiving an initial REGISTER request from the user equipment; the P-CSCF forwarding the initial REGISTER request to the I-CSCF; the I-CSCF forwarding the initial REGISTER request to the S-CSCF; the S-CSCF generating an authentication challenge message in response to the initial REGISTER request and passing the authentication challenge message to the I- CSCF; the I-CSCF forwarding the authentication challenge message to the P-CSCF; and the P-CSCF forwarding the authentication challenge message to the user equipment, wherein the challenge response REGISTER request is generated by the user equipment in response to the authentication challenge message.

4. A method in accordance with claim 1 wherein the REGISTER request, the OK message and the ACK message are in accordance with a Session Initiation Protocol (SIP).

5. A method in accordance with claim 1, wherein the I-CSCF and the S-CSCF are located in a home network of the user equipment.

6. A method in accordance with claim 1, wherein the I-CSCF is located in a home network of the user equipment and the S-CSCF is located in a second network, wherein the I-CSCF of the home network forwarding the REGISTER request to the S- CSCF comprises the I-CSCF of the home network forwarding the REGISTER request via an I-CSCF of the second network, the S-CSCF passing an OK message to the I-CSCF of the home network comprises the S-CSCF passing the OK message via the I-CSCF of the second network; and the I-CSCF of the home network forwarding ACK message to the S-CSCF comprises the I-CSCF of the home network forwarding the ACK message via the I- CSCF of the second network.

7. A method in accordance with claim 1, further comprising the S-CSCF registering the user equipment only if an ACK message is received from the user equipment within a specified time period.

8. A method for registering user equipment with an Internet Protocol Multimedia Subsystem (IMS) of a network, the method comprising: passing a first REGISTER request from the user equipment to a Registrar of the IMS in accordance with a Session Initiation Protocol (SIP); the Registrar passing an authorization challenge message to the user equipment in response to the first REGISTER request; the user equipment responding to the authorization challenge message with a second REGISTER request that includes a response to the authorization challenge; the Registrar responding to the second REGISTER request with an OK message if the second REGISTER request is approved; the user equipment responding to the OK message with an acknowledgment (ACK) message; and the Registrar registering the user equipment if the ACK message is received by the registrar server.

9. A system for registering user equipment with a network, the system comprising: a Proxy Call State Control Function (P-CSCF) operable to receive a REGISTER request from the user equipment; an Interrogating Call State Control Function (I-CSCF) operable to receive a REGISTER request from the P-CSCF; and a Serving Call State Control Function (S-CSCF) operable to receive a REGISTER request from the I-CSCF and to pass an OK message to the I-CSCF if the user equipment is approved for registration with the network, wherein the I-CSCF is further operable to pass the OK message to the user equipment via the P-CSCF, and to pass an acknowledgment (ACK) message, generated by the user equipment and received via the P-CSCF, to the S-CSCF, and wherein the user equipment is registered with the network if the ACK message is received by the S- CSCF.

10. A system in accordance with claim 9, wherein the network comprises an Internet Protocol Multimedia Subsystem (IMS).

11. A system in accordance with claim 9, wherein the network wherein the REGISTER request comprises a challenge response REGISTER request that includes an authentication challenge response, wherein the I-CSCF is further operable to pass an initial REGISTER request, generated by the user equipment and received via the P-CSCF, to the S-CSCF, wherein the S-CSCF is further operable to generate an authentication challenge message in response to the initial REGISTER request and pass the authentication challenge message to the user equipment via the I-CSCF and the P-CSCF, and wherein the challenge response REGISTER request is generated by the user equipment in response to the authentication challenge message.

12. A system in accordance with claim 9 wherein the REGISTER request, the OK message and the ACK message are in accordance with a Session Initiation Protocol (SIP).

13. A system in accordance with claim 9, wherein the I-CSCF and the S-CSCF are located in a home network of the user equipment.

14. A system in accordance with claim 9, wherein the I-CSCF is located in a home network of the user equipment and the S-CSCF is located in a second network, the system further comprising: an I-CSCF located in the second network, wherein the I-CSCF of the home network is operable to communicate with the S-CSCF via the I-CSCF of the second network.

15. A system in accordance with claim 9, wherein the network includes a plurality of S-CSCF's, the further comprising: a Home Subscriber Server (HSS), wherein the I-CSCF is further operable to query the HSS to determine which of the plurality of S-CSCF's the user equipment is to be registered with.

16. A system in accordance with claim 15,wherein the S-CSCF is further operable to query the HSS to retrieve user equipment profile information.

17. A system for registering user equipment with a network, the system comprising: a proxy means for receiving a REGISTER request from the user equipment, the proxy means located in a visited network; an interrogating means for receiving a REGISTER request from the proxy means and a registrar means for receiving a REGISTER request from the interrogating means and passing an OK message to the interrogating means if the user equipment is approved for registration with the network, wherein the interrogating means is further operable to pass the OK message to the user equipment via the proxy means, and to pass an acknowledgment (ACK) message, generated by the user equipment and received via the proxy means, to the registrar means, and wherein the user equipment is registered with the network if the ACK message is received by the registrar means.

18. A system in accordance with claim 17, wherein the network comprises an Internet Protocol Multimedia Subsystem (IMS) and wherein the REGISTER request, the OK message and the ACK message are in accordance with a Session Initiation Protocol (SIP).

19. A system in accordance with claim 17, wherein the interrogating means is located in a home network of the user equipment and the registrar means is located in a second network, the system further comprising: an interrogating means located in the second network, wherein the interrogating means of the home network is operable to communicate with the registrar via the interrogating means of the second network.

20. A system in accordance with claim 17, wherein the network includes a plurality of registrar means, the further comprising: a Home Subscriber Server (HSS), wherein the interrogating means is further operable to query the HSS to determine which of the plurality of registrar means the user equipment is to be registered with.