WO2020121024A1

WO2020121024A1 - Dynamic proxy-call session control function re-assignment for ip multimedia subsystem session handling

Info

Publication number: WO2020121024A1
Application number: PCT/IB2018/059888
Authority: WO
Inventors: George Foti
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2018-12-11
Filing date: 2018-12-11
Publication date: 2020-06-18

Abstract

Apparatuses and methods for dynamic P-CSCF re-assignment are disclosed. In one embodiment, a method for a first Proxy-Call Session Control Function, P-CSCF, server associated with a first Internet Protocol, IP, Multimedia Subsystem, IMS, gateway includes determining whether to use a second IMS gateway for an IMS session associated with a user equipment, UE; and, as a result of a determination to use the second IMS gateway for the IMS session associated with the UE, identifying a second P-CSCF server associated with the second IMS gateway, the second IMS gateway being geographically closer to the UE than the first IMS gateway. In another embodiment, a method for a UE includes, while being registered in IMS with the first P-CSCF server, participating in an IMS session, the IMS session using a second P-CSCF server to control a second IMS gateway.

Description

DYNAMIC PROXY-CALL SESSION CONTROL FUNCTION REASSIGNMENT FOR IP MULTIMEDIA SUBSYSTEM SESSION HANDLING

TECHNICAL FIELD

Wireless communication and in particular, methods and apparatuses for dynamic Proxy-Call Session Control Function (P-CSCF) re-assignment for Internet Protocol (IP) Multimedia Subsystem (IMS) session handling.

BACKGROUND

An IP Multimedia Subsystem (hereinafter“IMS”) enables operators of a Public Land Mobile Network (hereinafter“PLMN”) to provide their subscribers with multimedia services based and built on Internet applications, services and protocols. Different services and applications can be offered on top of an IMS. Internet Protocol (IP) connectivity of a user equipment (UE) terminal to an IMS can be through an access network, also referred to as an IP-Connectivity Access Network (IP-CAN).

Before being authorized to make use of IMS services provided by IMS applications, the UE registers into the IMS using a P-CSCF server. The list of P- CSCFs to be used by the UE for IMS registration are typically sent to the UE at IMS packet data network (PDN) connection establishment. The UE may than be able to pick up any P-CSCF from the list and use it for IMS registration. Currently, P-CSCF restoration is generally the only way that the P-CSCF used by a UE can be changed and this is typically due to the P-CSCF being down, or not responsive. Thus, P-CSCF assignment/re-assignment is limited in this manner.

SUMMARY

Some embodiments advantageously provide methods and apparatuses for facilitating selecting and/or using an IMS gateway (e.g., IMS Access Gateway (AGW)) close to a user equipment (UE) location.

According to a first aspect of the present disclosure, a first Proxy-Call Session Control Function, P-CSCF, server associated with a first Internet Protocol, IP, Multimedia Subsystem, IMS, gateway is provided. The first P-CSCF server includes processing circuitry having a processor and a memory, the memory including instructions executable by the processor to configure the first P-CSCF server to determine whether to use a second IMS gateway for an IMS session associated with a user equipment, UE; and, as a result of a determination to use the second IMS gateway for the IMS session associated with the UE, identify a second P-CSCF server associated with the second IMS gateway, the second IMS gateway being

geographically closer to the UE than the first IMS gateway.

In some embodiments of this aspect, the second IMS gateway is different from the first IMS gateway and the second P-CSCF server is different from the first P- CSCF server. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the first P-CSCF server to determine whether to use the second IMS gateway based at least in part on a service associated with the IMS session. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the first P-CSCF server to determine a location of the UE, the identification of the second P-CSCF server associated with the second IMS gateway based at least in part on the determined location of the UE. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the first P-CSCF server to determine a coverage area associated with the second IMS gateway;

compare a location of the UE to the determined coverage area associated with the second IMS gateway; and select the second IMS gateway for the IMS session based at least in part on the comparison. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the first P-CSCF server to identify a service for the IMS session in a session description protocol, SDP, the determination of whether to use the second IMS gateway for the IMS session based at least in part on the identified service.

In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the first P-CSCF server to receive, from the UE, a session initiation protocol, SIP, INVITE message for the IMS session; select the second P-CSCF server associated with the second IMS gateway for the IMS session; and as a result of the selection of the second P-CSCF server, forward the SIP INVITE message to the second P-CSCF server. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the first P-CSCF server to select the second P-CSCF server associated with the second IMS gateway for the IMS session; and, as a result of the selection of the second P-CSCF server, communicate a SIP message to the second P-CSCF server, the SIP message including a role parameter, the role parameter indicating a support role of the second P-CSCF server for the IMS session associated with the UE. In some embodiments of this aspect, the support role includes at least controlling the second IMS gateway for the IMS session associated with the UE. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the first P-CSCF server to, forward a session initiation protocol, SIP, INVITE message for the IMS session to the second P-CSCF server associated with the second IMS gateway, the SIP INVITE message including a role parameter and a SIP ROUTE header entry, the role parameter indicating a support role of the second P-CSCF server for the IMS session associated with the UE and the SIP ROUTE header entry instructing the second P-CSCF server to re-route the SIP INVITE message back to the first P-CSCF server. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the first P-CSCF server to, communicate, to the UE, a redirection request message, the redirection request message including an address of the second P-CSCF server and a token, the address to be used by the UE to establish the IMS session with the second P-CSCF server and the token to be used for verification of the IMS session. In some embodiments of this aspect, the redirection request message is a 3XX redirection request message. In some embodiments of this aspect, the UE is IMS registered with the first P-CSCF server and not IMS registered with the second P-CSCF server.

According to a second aspect of this disclosure, a method for a first Proxy-Call Session Control Function, P-CSCF, server associated with a first Internet Protocol, IP, Multimedia Subsystem, IMS, gateway is provided. The method includes determining whether to use a second IMS gateway for an IMS session associated with a user equipment, UE; and, as a result of a determination to use the second IMS gateway for the IMS session associated with the UE, identifying a second P-CSCF server associated with the second IMS gateway, the second IMS gateway being

geographically closer to the UE than the first IMS gateway. In some embodiments of this aspect, the second IMS gateway is different from the first IMS gateway and the second P-CSCF server is different from the first P- CSCF server. In some embodiments of this aspect, the determining whether to use the second IMS gateway includes determining whether to use the second IMS gateway based at least in part on a service associated with the IMS session. In some embodiments of this aspect, the method further includes determining a location of the UE, the identification of the second P-CSCF server associated with the second IMS gateway based at least in part on the determined location of the UE. In some embodiments of this aspect, the method further includes determining a coverage area associated with the second IMS gateway; comparing a location of the UE to the determined coverage area associated with the second IMS gateway; and selecting the second IMS gateway for the IMS session based at least in part on the comparison. In some embodiments of this aspect, the method further includes identifying a service for the IMS session in a session description protocol, SDP, the determination of whether to use the second IMS gateway for the IMS session based at least in part on the identified service.

In some embodiments of this aspect, the method further includes receiving, from the UE, a session initiation protocol, SIP, INVITE message for the IMS session; selecting the second P-CSCF server associated with the second IMS gateway for the IMS session; and, as a result of the selection of the second P-CSCF server, forwarding the SIP INVITE message to the second P-CSCF server. In some embodiments of this aspect, the method further includes selecting the second P-CSCF server associated with the second IMS gateway for the IMS session; and, as a result of the selection of the second P-CSCF server, communicating a SIP message to the second P-CSCF server, the SIP message including a role parameter, the role parameter indicating a support role of the second P-CSCF server for the IMS session associated with the UE. In some embodiments of this aspect, the support role includes at least controlling the second IMS gateway for the IMS session associated with the UE. In some embodiments of this aspect, the method further includes forwarding a session initiation protocol, SIP, INVITE message for the IMS session to the second P-CSCF server associated with the second IMS gateway, the SIP INVITE message including a role parameter and a SIP ROUTE header entry, the role parameter indicating a support role of the second P-CSCF server for the IMS session associated with the UE and the SIP ROUTE header entry instructing the second P-CSCF server to re-route the SIP INVITE message back to the first P-CSCF server. In some embodiments of this aspect, the method further includes communicating, to the UE, a redirection request message, the redirection request message including an address of the second P-CSCF server and a token, the address to be used by the UE to establish the IMS session with the second P-CSCF server and the token to be used for verification of the IMS session. In some embodiments of this aspect, the redirection request message is a 3XX redirection request message. In some embodiments of this aspect, the UE is IMS registered with the first P-CSCF server and not IMS registered with the second P-CSCF server.

According to a third aspect of this disclosure, a user equipment, UE, is provided. The UE includes processing circuitry having a processor and a memory, the memory including instructions executable by the processor to configure the UE to register in Internet Protocol, IP, Multimedia Subsystem, IMS with a first Proxy-Call Session Control Function, P-CSCF, server, the first P-CSCF server being associated with a first IMS gateway; and, while being registered in IMS with the first P-CSCF server, participate in an Internet Protocol, IP, Multimedia Subsystem, IMS, session, the IMS session using a second P-CSCF server to control a second IMS gateway, the second IMS gateway being geographically closer to the UE than the first IMS gateway.

In some embodiments of this aspect, the second IMS gateway is different from the first IMS gateway and the second P-CSCF server is different from the first P- CSCF server. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the UE to communicate a session initiation protocol, SIP, message to the first P-CSCF server to initiate the IMS session, the SIP message identifying a requested service for the IMS session, wherein the use of the second P-CSCF server to control the second IMS gateway for the IMS session is based at least in part on the requested service. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the UE to receive, from the first P-CSCF server, a redirection request message, the redirection request message including an address of the second P-CSCF server. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the UE to receive, from the first P-CSCF server, a redirection request message, the redirection request message including a token, the token to be used for verification of the IMS session between the first and second P-CSCF servers. In some embodiments of this aspect, the memory includes further instructions executable by the processor to configure the UE to, as a result of the redirection request message, communicate a session initiation protocol, SIP, message to the second P-CSCF server, the SIP message including a role parameter, a SIP ROUTE header entry, and a token. In some embodiments of this aspect, the role parameter indicates a support role of the second P-CSCF server for the IMS session associated with the UE, the SIP ROUTE header entry includes an address of the first P-CSCF server and the token is configured to verify the IMS session with the first P- CSCF server. In some embodiments of this aspect, the UE is not registered with the second P-CSCF server.

According to a fourth aspect of this disclosure, a method for a user equipment, UE, is provided. The method includes registering in Internet Protocol, IP, Multimedia Subsystem, IMS with a first Proxy-Call Session Control Function, P-CSCF, server, the first P-CSCF server being associated with a first IMS gateway. The method further includes, while being registered in IMS with the first P-CSCF server, participating in an Internet Protocol, IP, Multimedia Subsystem, IMS, session, the IMS session using a second P-CSCF server to control a second IMS gateway, the second IMS gateway being geographically closer to the UE than the first IMS gateway.

In some embodiments of this aspect, the second IMS gateway is different from the first IMS gateway and the second P-CSCF server is different from the first P- CSCF server. In some embodiments of this aspect, the method further includes communicating a session initiation protocol, SIP, message to the first P-CSCF server to initiate the IMS session, the SIP message identifying a requested service for the IMS session, wherein use of the second P-CSCF server to control the second IMS gateway for the IMS session is based at least in part on the requested service. In some embodiments of this aspect, the method further includes receiving, from the first P- CSCF server, a redirection request message, the redirection request message including an address of the second P-CSCF server. In some embodiments of this aspect, the method further includes receiving, from the first P-CSCF server, a redirection request message, the redirection request message including a token, the token to be used for verification of the IMS session between the first and second P-CSCF servers. In some embodiments of this aspect, the method further includes, as a result of the redirection request message, communicating a session initiation protocol, SIP, message to the second P-CSCF server, the SIP message including a role parameter, a SIP ROUTE header entry, and a token. In some embodiments of this aspect, the role parameter indicates a support role of the second P-CSCF server for the IMS session associated with the UE, the SIP ROUTE header entry includes an address of the first P-CSCF server and the token is configured to verify the IMS session with the first P-CSCF server. In some embodiments of this aspect, the UE is not registered with the second P-CSCF server.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram of an exemplary network architecture according to some embodiments of the present disclosure;

FIG. 2 is a block diagram of an exemplary P-CSCF server and an exemplary user equipment (UE) according to some embodiments of the present disclosure;

FIG. 3 is a flow chart illustrating an exemplary method implemented in a P- CSCF server according to some embodiments of the present disclosure;

FIG. 4 is a flow chart illustrating an exemplary method implemented in a UE according to some embodiments of the present disclosure;

FIG. 5 is a flow diagram illustrating an example method according to one embodiment of the present disclosure;

FIG. 6 is a flow diagram continuing the example method of FIG. 5;

FIG. 7 is a flow diagram continuing the example method of FIGS. 5 and 6; FIG. 8 is a flow diagram illustrating another example method according to one embodiment of the present disclosure;

FIG. 9 is a flow diagram continuing the example method of FIG. 8;

FIG. 10 is a flow diagram continuing the example method of FIGS. 8 and 9;

FIG. 11 is a flow diagram illustrating an example method according to a second embodiment of the present disclosure;

FIG. 12 is a flow diagram continuing the example method of FIG. 11 ; and

FIG. 13 is a flow diagram continuing the example method of FIGS. 11 and 12.

DETAILED DESCRIPTION

There may exist use cases where the user plane for an IMS session should be closer to the UE location. This means that the IMS gateway (e.g., AGW) should be close to the UE. Hence, there is a need to be able to influence the P-CSCF used during an IMS session establishment if, for example, the UE requests or would benefit from having a close proximity to the user plane. However, the P-CSCF that the UE may be registered with may control an IMS AGW that is far from the UE location.

Unfortunately, in existing networks, P-CSCF restoration is generally the only way that the P-CSCF used by a UE is changed and this is typically due to the P-CSCF being down, or not responsive.

Thus, some embodiments of this disclosure provide techniques for allowing a re-assignment of the P-CSCF that controls an AGW close to the UE. Some examples of use cases where such re-assignment may be desirable include, for example, use cases in which the UE requests to stream video from a server identified by a Session Initiation Protocol (SIP) Public Service Identifier (PSI).

Some embodiments of this disclosure provide for at least two options to be able to dynamically select an IMS gateway (e.g., AGW) (and consequently a different P-CSCF than the P-CSCF that the UE is currently registered with, for e.g., purposes of providing services for a particular session) close to the UE location if, for example, that is requested by the UE, or otherwise indicated at session initiation. Thus, some of the techniques described in this disclosure may be implemented on a per-session basis. In a first embodiment, an anchor P-CSCF in which the UE is currently registered, and which receives the initial SIP INVITE (including a UE request or other indication for a user plane close to the UE location) locates an AGW close to the UE location. The UE location may be provided by the UE in, for example, a SIP header such as geo-location and can be verified by the anchor P-CSCF using the UE contact information and/or third-party databases as an option for the purpose of verifying the UE-provided information. Other options for performing the verification may be used in other embodiments.

In some embodiments, the anchor P-CSCF locates an IMS gateway (e.g., AGW) close to the UE location and locates a transit P-CSCF, which transit P-CSCF controls the close AGW. The information used by the anchor P-CSCF to locate the appropriate AGW, e.g., the geographical coverage of an AGW (and the P-CSF that controls such AGW) can be configured in each P-CSCF. In other embodiments, the location of appropriate AGW and the so-called“transit” P-CSCF that controls it may be discovered, as an alternative to the P-CSCF configuration.

Once the transit P-CSCF is selected for the ongoing session, the anchor P- CSCF may proxy the SIP INVITE to the transit P-CSCF with additional information included in the SIP Route header containing the transit P-CSCF address (next hop). This additional information inserted by the anchor P-CSCF may be used to indicate to the transit P-CSCF to only perform a proxy or slave role for the incoming session to control the AGW. The transit P-CSCF, based on the information in the Route header, may provide a proxy or slave role only, accordingly. In other words, the transit P- CSCF may proxy all SIP messages and responses transparently and/or the transit P- CSCF’s sole purpose for the session may be to control the AGW. Accordingly, the transit P-CSCF may open the SIP Requests/responses to fetch the information for that sole purpose. The transit P-CSCF may remain in the SIP signaling path until the session is complete and torn down, either gracefully or ungracefully. After the session is complete the UE may remain registered with the anchor P-CSCF.

In some embodiments, the Serving-Call Session Control Function (S-CSCF) to be contacted for this session is also included in a Route header in the SIP INVITE.

The SIP Route header including the S-CSCF address may also include an indication, inserted by the transit P-CSCF, to inform the S-CSCF of the“proxy” role of the transit P-CSCF in the chain according to the techniques disclosed herein, since the S- CSCF may expect the SIP INVITE to be received from the anchor P-CSCF. This may enable the S-CSCF to store the address of the transit P-CSCF and use such address for the duration of the session only.

In some embodiments, the anchor P-CSCF remains in charge of the session (excluding controlling the AGW), including communication with the Policy and Charging Rules Function (PCRF), until session termination. In addition, the UE may remain registered with the anchor P-CSCF after the session is completed.

In a second embodiment, similar to the first embodiment, the anchor P-CSCF locates the transit P-CSCF that controls a close AGW that can service the UE.

However, in the second embodiment, the anchor P-CSCF may send back to the UE the contact information of the transit P-CSCF in, for example, a 3XX response. The returned contact information may include the address of the anchor P-CSCF. This may enable the transit P-CSCF to contact the anchor P-CSCF and validate the received information later from the UE and therefore authenticate that the request is legitimate before servicing the UE request.

Again, as in the first embodiment, the S-CSCF to be contacted for this session may also be included in a Route header in the SIP INVITE. The SIP Route header including the S-CSCF address may also include an indication inserted by the transit P- CSCF to inform the S-CSCF of the“proxy” role of the transit P-CSCF in the chain since the S-CSCF expects the INVITE to be received from the anchor P-CSCF. This can enable the S-CSCF to store the address of the transit P-CSCF and use such address for the duration of the session.

In this second embodiment, the transit P-CSCF may become fully in charge of the session, including communication with the PCRF, until session termination. The UE remains registered with the anchor P-CSCF after the session is completed.

Before describing in detail exemplary embodiments, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to dynamic P-CSCF assignment/re-assignment for IMS session handling. Accordingly, components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments 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.

As used herein, relational terms, such as“first” and“second,”“top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the concepts described herein. As used herein, the singular forms“a”,“an” and“the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,”“comprising,”“includes” and/or“including” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

In embodiments described herein, the joining term,“in communication with” and the like, may be used to indicate electrical or data communication, which may be accomplished by physical contact, induction, electromagnetic radiation, radio signaling, infrared signaling or optical signaling, for example. One having ordinary skill in the art will appreciate that multiple components may interoperate and modifications and variations are possible of achieving the electrical and data communication.

In some embodiments described herein, the term“coupled,”“connected,” and the like, may be used herein to indicate a connection, although not necessarily directly, and may include wired and/or wireless connections.

The term“network node” used herein can be any kind of network node comprised in a radio network which may further comprise any of base station (BS), radio base station, base transceiver station (BTS), base station controller (BSC), radio network controller (RNC), g Node B (gNB), evolved Node B (eNB or eNodeB), Node B, multi-standard radio (MSR) radio node such as MSR BS, multi-cell/multicast coordination entity (MCE), relay node, donor node controlling relay, radio access point (AP), transmission points, transmission nodes, Remote Radio Unit (RRU) Remote Radio Head (RRH), a core network node (e.g., mobile management entity (MME), self-organizing network (SON) node, a coordinating node, positioning node, MDT node, etc.), an external node (e.g., 3rd party node, a node external to the current network), nodes in distributed antenna system (DAS), a spectrum access system (SAS) node, an element management system (EMS), etc. The network node may also comprise test equipment. The term“radio node” used herein may be used to also denote a wireless device (WD) such as a wireless device (WD) or a radio network node.

In some embodiments, the non-limiting terms wireless device (WD) or a user equipment (UE) are used interchangeably. The UE herein can be any type of wireless device capable of communicating with a network node or another UE over radio signals, such as wireless device (WD). The UE may also be a radio communication device, target device, device to device (D2D) UE, machine type UE or UE capable of machine to machine communication (M2M), low-cost and/or low-complexity UE, a sensor equipped with UE, Tablet, mobile terminals, smart phone, laptop embedded equipped (LEE), laptop mounted equipment (LME), USB dongles, Customer Premises Equipment (CPE), an Internet of Things (IoT) device, or a Narrowband IoT (NB-IOT) device etc. The UE herein can by any type of communication device capable of communicating with a cloud service provider and/or a network node and/or a server, such as, for example, personal computer (PC), a Tablet, a mobile terminal, via a wired connection and/or a wireless connection. The UE can, in some embodiments, be considered a client terminal, usable by a user to access an IMS, dual-register with the IMS, and communicate in an IMS session via one or more access networks, according to one or more of the techniques described herein.

Also, in some embodiments the generic term“radio network node” is used. It can be any kind of a radio network node which may comprise any of base station, radio base station, base transceiver station, base station controller, network controller, RNC, evolved Node B (eNB), Node B, gNB, Multi-cell/multicast Coordination Entity (MCE), relay node, access point, radio access point, Remote Radio Unit (RRU) Remote Radio Head (RRH).

Note that although terminology from one particular wireless system, such as, for example, 3GPP LTE, may be used in this disclosure, this should not be seen as limiting the scope of the disclosure to only the aforementioned system. The arrangements discussed herein may also be applied to 5G / New Radio (NR), and other technologies. Other wireless systems, including without limitation Wide Band Code Division Multiple Access (WCDMA), Worldwide Interoperability for

Microwave Access (WiMax), Ultra Mobile Broadband (UMB) and Global System for Mobile Communications (GSM), may also benefit from exploiting the ideas covered within this disclosure.

As used herein, the terms“anchor P-CSCF” and“transit P-CSCF” may be used. In some embodiments, anchor P-CSCF may indicate a P-CSCF that the UE, initiating a session, is IMS registered with. In some embodiments, the transit P-CSCF may indicate a P-CSCF that controls an IMS gateway that may be used for the session, although the transit P-CSCF is not the P-CSCF that the UE is IMS registered with.

Note further, that functions described herein as being performed by a UE or a P-CSCF may be distributed over a plurality of UEs and/or a plurality of P-CSCFs. In other words, it is contemplated that the functions of the P-CSCF and UE described herein are not limited to performance by a single physical device and, in fact, can be distributed among several physical devices.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Referring now to the drawings, in which like reference designators refer to like elements, there is shown in FIG. 1, an exemplary system, and its related components, constructed in accordance with the principles of the present disclosure and designated generally as“10.” Referring to FIG. 1, system 10 may include an IMS 12 and a first UE 14a and a second UE 14b (collectively referred to herein as UE 14), in addition to various support elements for supporting IMS communications, such as, a first P-CSCF server 16a and a second P-CSCF server 16b (collectively referred to herein as P-CSCF server 16). The UE 14 may be capable of accessing the IMS 12 via one or more access networks. The access network may be considered an IP- Connectivity Access Network (hereinafter IP-CAN or access network). An exemplary access network may be 4G, 5G, New Radio (NR), etc.

The P-CSCF server 16 may provide proxy communications between the UE 14 and the IMS 12 via the one or more access networks. The first and second P- CSCF servers 16a and 16b may each be associated with a respective first and second IMS gateway 18a and 18b (collectively referred to herein as IMS gateway 18). In some embodiments, the IMS gateway 18 may be an IMS Access Gateway (AGW). In some embodiments, the IMS gateway 18 is co-located with the P-CSCF that controls it. In some embodiments, the IMS gateway may be responsible for the user or media plane at the access point to the IMS network 12 and/or may provide for handling, forwarding and remote network address translation (NAT) traversal of media packets between the IP-CAN and the IMS core. Of note, although FIG. 1 shows two UEs 14, two P-CSCF servers 16 and two IMS-Gateways 18, it is understood that

implementations can have more or fewer than the two of each element shown in FIG.

1 and described herein.

In some embodiments, the first P-CSCF server 16a may be the anchor P-CSCF server and the second P-CSCF server 16b may be the transit P-CSCF server. In other embodiments, depending on e.g., the location of the UE 14 and the configuration of the system 10, the first P-CSCF server 16a may operate as the transit P-CSCF server for a particular session, while the second P-CSCF server 16b may operate as the anchor P-CSCF server for the session. In other words, in some embodiments, a P- CSCF server can be configured to operate as an anchor P-CSCF for some sessions and also operate as a transit P-CSCF for other sessions, according to the techniques provided in this disclosure.

The system 10 further includes the S-CSCF server 20, which may be a SIP Registrar and/or may manage the UE 14 session for the IMS 12. The system 10 may also include a Policy and Charging Rules Function/Policy Control Function

(PCRF PCF) server (hereinafter PCRF server) and a Home Subscriber Server/User Data Management (HSS/UDM) server (hereinafter HSS server). In some embodiments, the PCRF server may be interposed between the signalling and the bearer layers and responsible for triggering the installation of QoS-related rules towards a Policing and Charging Enforcement Function (PCEF, not shown) located in the traffic plane. In some embodiments, the HSS may be a database of subscriber information allowing users to be granted access to the IMS associated with the subscriber’s specific information.

As shown in FIG. 2, in one embodiment, a P-CSCF server 16 (e.g., 16a and/or 16b) includes a communication interface 30, processing circuitry 32, and memory 34. The communication interface 30 may be configured to communicate with the UE 14 and/or other elements in the system 10 to facilitate UE 14 access to the IMS 12 and associated services. In some embodiments, the communication interface 30 may be formed as or may include, for example, one or more radio frequency (RF) transmitters, one or more RF receivers, and/or one or more RF transceivers, and/or may be considered a radio interface. In some embodiments, the communication interface 30 may also include a wired interface.

The processing circuitry 32 may include one or more processors 36 and memory, such as, the memory 34. In particular, in addition to a traditional processor and memory, the processing circuitry 32 may comprise integrated circuitry for processing and/or control, e.g., one or more processors and/or processor cores and/or FPGAs (Field Programmable Gate Array) and/or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 36 may be configured to access (e.g., write to and/or read from) the memory 34, which may comprise any kind of volatile and/or nonvolatile memory, e.g., cache and/or buffer memory and/or RAM (Random Access Memory) and/or ROM (Read-Only Memory) and/or optical memory and/or EPROM (Erasable Programmable Read-Only

Memory).

Thus, the P-CSCF server 16 may further include software stored internally in, for example, memory 34, or stored in external memory (e.g., database) accessible by the P-CSCF server 16 via an external connection. The software may be executable by the processing circuitry 32. The processing circuitry 32 may be configured to control any of the methods and/or processes described herein and/or to cause such methods, and/or processes to be performed, e.g., by the P-CSCF server 16. The memory 34 is configured to store data, programmatic software code and/or other information described herein. In some embodiments, the software may include instructions that, when executed by the processor 36 and/or Gateway Controller 38, causes the processor 36 and/or Gateway Controller 38 to perform the processes described herein with respect to the P-CSCF server 16. In some embodiments, the first P-CSCF server 16a is associated with a first IMS gateway 18a and the first P-CSCF server 16a includes processing circuitry 32 having a processor 36 and a memory 34. The memory 34 may store instructions executable by the processor 36 and/or the Gateway Controller 38 to configure the first P-CSCF server 16a to determine whether to use a second IMS gateway 18b for an IMS session associated with a user equipment, UE 14. The memory 34 may store further instructions executable by the processor 36 and/or the Gateway Controller 38 to configure the first P-CSCF server 16a to, as a result of a determination to use the second IMS gateway 18b for the IMS session associated with the UE 14, identify a second P-CSCF server 16b associated with the second IMS gateway 18b, the second IMS gateway 18b being geographically closer to the UE 14 than the first IMS gateway 18a.

In some embodiments, the second IMS gateway 18b is different from the first IMS gateway 18a and the second P-CSCF server 16b is different from the first P- CSCF server 16a. In some embodiments, the memory 34 includes further instructions executable by the processor 36 to configure the first P-CSCF server 16a to determine whether to use the second IMS gateway 18b based at least in part on a service associated with the IMS session. In some embodiments, the memory 34 includes further instructions executable by the processor 36 to configure the first P-CSCF server 16a to determine a location of the UE 14, the identification of the second P- CSCF server 16b associated with the second IMS gateway 18b based at least in part on the determined location of the UE 14. In some embodiments, the memory 34 includes further instructions executable by the processor 36 to configure the first P- CSCF server 16a to determine a coverage area associated with the second IMS gateway 18b; compare a location of the UE 14 to the determined coverage area associated with the second IMS gateway 18b; and select the second IMS gateway 18b for the IMS session based at least in part on the comparison. In some embodiments, the memory 34 includes further instructions executable by the processor 36 to configure the first P-CSCF server 16a to identify a service for the IMS session in a session description protocol, SDP, the determination of whether to use the second IMS gateway 18b for the IMS session based at least in part on the identified service. In some embodiments, the memory 34 includes further instructions executable by the processor 36 to configure the first P-CSCF server 16a to receive, from the UE 14, a session initiation protocol, SIP, INVITE message for the IMS session; select the second P-CSCF server 16b associated with the second IMS gateway 18b for the IMS session; and as a result of the selection of the second P-CSCF server 16b, forward the SIP INVITE message to the second P-CSCF server 16b. In some embodiments, the memory 34 includes further instructions executable by the processor 36 to configure the first P-CSCF server 16a to select the second P-CSCF server 16b associated with the second IMS gateway 18b for the IMS session; and, as a result of the selection of the second P-CSCF server 16b, communicate a SIP message to the second P-CSCF server 16b, the SIP message including a role parameter, the role parameter indicating a support role of the second P-CSCF server 16b for the IMS session associated with the UE 14. In some embodiments, the support role includes at least controlling the second IMS gateway 18b for the IMS session associated with the UE 14. In some embodiments, the memory 34 includes further instructions executable by the processor 36 to configure the first P-CSCF server 16a to forward a session initiation protocol, SIP, INVITE message for the IMS session to the second P-CSCF server 16b associated with the second IMS gateway 18b, the SIP INVITE message including a role parameter and a SIP ROUTE header entry, the role parameter indicating a support role of the second P-CSCF server 16b for the IMS session associated with the UE 14 and the SIP ROUTE header entry instructing the second P-CSCF server 16b to re-route the SIP INVITE message back to the first P-CSCF server 16a. In some embodiments, the memory 34 includes further instructions executable by the processor 36 to configure the first P-CSCF server 16a to communicate, to the UE 14, a redirection request message, the redirection request message including an address of the second P-CSCF server 16b and a token, the address to be used by the UE 14 to establish the IMS session with the second P-CSCF server 16b and the token to be used for verification of the IMS session. In some embodiments, the redirection request message is a 3XX redirection request message. In some embodiments, the UE 14 is IMS registered with the first P-CSCF server 16a and not IMS registered with the second P-CSCF server 16b. As also shown in FIG. 2, in one embodiment, the UE 14 includes a communication interface 40, processing circuitry 42, and memory 44. The communication interface 40 may be configured to communicate with the P-CSCF server 16 and/or other elements in the system 10 to facilitate UE 14 access to the IMS 12 and associated services. In some embodiments, the communication interface 40 may be formed as or may include, for example, one or more radio frequency (RF) transmitters, one or more RF receivers, and/or one or more RF transceivers, and/or may be considered a radio interface. In some embodiments, the communication interface 40 may also include a wired interface.

The processing circuitry 42 may include one or more processors 46 and memory, such as, the memory 44. In particular, in addition to a traditional processor and memory, the processing circuitry 42 may comprise integrated circuitry for processing and/or control, e.g., one or more processors and/or processor cores and/or FPGAs (Field Programmable Gate Array) and/or ASICs (Application Specific Integrated Circuitry) adapted to execute instructions. The processor 46 may be configured to access (e.g., write to and/or read from) the memory 44, which may comprise any kind of volatile and/or nonvolatile memory, e.g., cache and/or buffer memory and/or RAM (Random Access Memory) and/or ROM (Read-Only Memory) and/or optical memory and/or EPROM (Erasable Programmable Read-Only Memory).

Thus, the UE 14 may further include software stored internally in, for example, memory 44, or stored in external memory (e.g., database) accessible by the UE 14 via an external connection. The software may be executable by the processing circuitry 42. The processing circuitry 42 may be configured to control any of the methods and/or processes described herein and/or to cause such methods, and/or processes to be performed, e.g., by the UE 14. The memory 44 is configured to store data, programmatic software code and/or other information described herein. In some embodiments, the software may include instructions that, when executed by the processor 46 and/or IMS Session Controller 48, causes the processor 46 and/or IMS Session Controller 48 to perform the processes described herein with respect to the UE 14. For example, the UE 14 may include the processing circuitry 42 having a processor 46 and a memory 44, the memory 44 including instructions executable by the processor 46 to configure the UE 14 to register in the IMS 12 with a first P-CSCF server 16a, the first P-CSCF server 16a being associated with a first IMS gateway 18a. The memory 44 includes further instructions executable by the processor 46 to configure the UE 14 to, while being registered in IMS 12 with the first P-CSCF server 16a, participate in an IMS session, the IMS session using a second P-CSCF server 16b to control a second IMS gateway 18b, the second IMS gateway 18b being geographically closer to the UE 14 than the first IMS gateway 18 a.

In some embodiments, the second IMS gateway 18b is different from the first IMS gateway 18a and the second P-CSCF server 16b is different from the first P- CSCF server 16a. In some embodiments, the memory 44 includes further instructions executable by the processor 46 to configure the UE 14 to communicate a session initiation protocol, SIP, message to the first P-CSCF server 16a to initiate the IMS session, the SIP message identifying a requested service for the IMS session, wherein the use of the second P-CSCF server 16b to control the second IMS gateway 18b for the IMS session is based at least in part on the requested service. In some embodiments, the memory 44 includes further instructions executable by the processor 46 to configure the UE 14 to receive, from the first P-CSCF server 16a, a redirection request message, the redirection request message including an address of the second P-CSCF server 16b. In some embodiments, the memory 44 includes further instructions executable by the processor 46 to configure the UE 14 to receive, from the first P-CSCF server 16a, a redirection request message, the redirection request message including a token, the token to be used for verification of the IMS session between the first and second P-CSCF servers 16a and 16b. In some embodiments, the memory 44 includes further instructions executable by the processor 46 to configure the UE 14 to, as a result of the redirection request message, communicate a session initiation protocol, SIP, message to the second P-CSCF server 16b, the SIP message including a role parameter, a SIP ROUTE header entry, and a token. In some embodiments, the role parameter indicates a support role of the second P-CSCF server 16b for the IMS session associated with the UE 14, the SIP ROUTE header entry includes an address of the first P-CSCF server 16a and the token is configured to verify the IMS session with the first P-CSCF server 16a. In some embodiments, the UE 14 is not registered with the second P-CSCF server 16b.

FIG. 3 is a flowchart illustrating an exemplary method that may be implemented in a P-CSCF server, such as, for example, the P-CSCF server 16. The exemplary method includes determining (block S100), such as via the processor 36 and/or Gateway Controller 38, whether to use a second IMS gateway 18b for an IMS session associated with a user equipment, UE 14; and, as a result of a determination to use the second IMS gateway 18b for the IMS session associated with the UE 14, identifying (block S102), such as via the processor 36 and/or Gateway Controller 38, a second P-CSCF server 16b associated with the second IMS gateway 18b, the second IMS gateway 18b being geographically closer to the UE 14 than the first IMS gateway 18a.

In some embodiments, the second IMS gateway 18b is different from the first IMS gateway 18a and the second P-CSCF server 16b is different from the first P- CSCF server 16a. In some embodiments, the determining whether to use the second IMS gateway 18b includes determining, such as via the processor 36 and/or Gateway Controller 38, whether to use the second IMS gateway 18b based at least in part on a service associated with the IMS session. In some embodiments, the method further includes determining, such as via the processor 36 and/or Gateway Controller 38, a location of the UE 14, the identification of the second P-CSCF server 16b associated with the second IMS gateway 18b based at least in part on the determined location of the UE 14. In some embodiments, the method further includes determining, such as via the processor 36 and/or Gateway Controller 38, a coverage area associated with the second IMS gateway 18b; comparing, such as via the processor 36 and/or Gateway Controller 38, a location of the UE 14 to the determined coverage area associated with the second IMS gateway 18b; and selecting, such as via the processor 36 and/or Gateway Controller 38, the second IMS gateway 18b for the IMS session based at least in part on the comparison. In some embodiments, the method further includes identifying, such as via the processor 36 and/or Gateway Controller 38, a service for the IMS session in a session description protocol, SDP, the determination of whether to use the second IMS gateway 18b for the IMS session based at least in part on the identified service. In some embodiments, the method further includes receiving, such as via the communication interface 30, from the UE 14, a session initiation protocol, SIP, INVITE message for the IMS session; selecting, such as via the processor 36 and/or Gateway Controller 38, the second P-CSCF server 16b associated with the second IMS gateway 18b for the IMS session; and, as a result of the selection of the second P-CSCF server 16b, forwarding the SIP INVITE message to the second P-CSCF server 16b. In some embodiments, the method further includes selecting, such as via the processor 36 and/or Gateway Controller 38, the second P-CSCF server 16b associated with the second IMS gateway 18b for the IMS session; and, as a result of the selection of the second P-CSCF server 16b, communicating a SIP message to the second P-CSCF server 16b, the SIP message including a role parameter, the role parameter indicating a support role of the second P-CSCF server 16b for the IMS session associated with the UE 14. In some embodiments, the support role includes at least controlling the second IMS gateway 18b for the IMS session associated with the UE 14. In some embodiments, the method further includes forwarding, such as via the communication interface 30, a session initiation protocol, SIP, INVITE message for the IMS session to the second P-CSCF server 16b associated with the second IMS gateway 18b, the SIP INVITE message including a role parameter and a SIP ROUTE header entry, the role parameter indicating a support role of the second P-CSCF server 16b for the IMS session associated with the UE 14 and the SIP ROUTE header entry instructing the second P-CSCF server 16b to re-route the SIP INVITE message back to the first P-CSCF server 16a. In some embodiments, the method further includes communicating, such as via the communication interface 30, to the UE 14, a redirection request message, the redirection request message including an address of the second P-CSCF server 16b and a token, the address to be used by the UE 14 to establish the IMS session with the second P-CSCF server 16b and the token to be used for verification of the IMS session. In some embodiments, the redirection request message is a 3XX redirection request message. In some embodiments, the UE 14 is IMS registered with the first P-CSCF server 16a and not IMS registered with the second P-CSCF server 16b.

FIG. 4 is a flowchart illustrating an exemplary method that may be implemented in a UE, such as, for example, the UE 14. The exemplary method includes registering (block S104), such as via the processor 46 and/or IMS Session Controller 48, in IMS 12 with a first P-CSCF server 16a, the first P-CSCF server 16a being associated with a first IMS gateway 18b. The exemplary method further includes, while being registered in IMS 12 with the first P-CSCF server 16a, participating (block S106), such as via the processor 46 and/or IMS Session

Controller 48, in an IMS session, the IMS session using a second P-CSCF server 16b to control a second IMS gateway 18b, the second IMS gateway 18b being geographically closer to the UE 14 than the first IMS gateway 18 a.

In some embodiments, the second IMS gateway 18b is different from the first IMS gateway 18a and the second P-CSCF server 16b is different from the first P- CSCF server 16a. In some embodiments, the method further includes

communicating, such as via the communication interface 40, a session initiation protocol, SIP, message to the first P-CSCF server 16a to initiate the IMS session, the SIP message identifying a requested service for the IMS session, wherein use of the second P-CSCF server 16b to control the second IMS gateway 18b for the IMS session is based at least in part on the requested service. In some embodiments, the method further includes receiving, such as via the communication interface 40, from the first P-CSCF server 16a, a redirection request message, the redirection request message including an address of the second P-CSCF server 16b. In some embodiments, the method further includes receiving, such as via the communication interface 40, from the first P-CSCF server 16a, a redirection request message, the redirection request message including a token, the token to be used for verification of the IMS session between the first and second P-CSCF servers 16a and 16b. The method further includes, as a result of the redirection request message,

communicating, such as via the communication interface 40, a session initiation protocol, SIP, message to the second P-CSCF server 16b, the SIP message including a role parameter, a SIP ROUTE header entry, and a token. In some embodiments, the role parameter indicates a support role of the second P-CSCF server 16b for the IMS session associated with the UE 14, the SIP ROUTE header entry includes an address of the first P-CSCF server 16a and the token is configured to verify the IMS session with the first P-CSCF server 16a. In some embodiments, the UE 14 is not registered with the second P-CSCF server 16b. Having generally described some embodiments of the present disclosure for dynamic P-CSCF re-assignment for IMS session handling, a more detailed description of some embodiments of the present disclosure will now be described with reference to the flow diagrams of FIGS. 5-13. The processes described with reference to these flow diagrams may be performed by any of the devices, servers, processing circuitry (memories and/or processors), communication interfaces, and the like discussed above, such as with reference to FIG. 2.

Some such embodiments of the present disclosure provide techniques for dynamically re-assigning a P-CSCF for IMS session handling, and, in some embodiments, for using a second P-CSCF associated with an IMS gateway that is closer to a UE than an IMS gateway associated with the P-CSCF that the UE is registered in IMS with. In some embodiments, the techniques provided in this disclosure provide for selection of an IMS gateway (e.g., AGW) close to the UE geographic location for e.g., certain types of sessions.

One example of a first embodiment is illustrated in the flow diagram of FIG.

5, as well as, FIGS. 6 and 7, which are continuations of the example process introduced in FIG. 5. In this first embodiment, the first P-CSCF server 16a (referred to as the anchor P-CSCF server 16a) may remain in control of the session e.g., until it is cleared. In some embodiments, the second P-CSCF server 16a may control the PCRF including subscriptions and notifications. In a second embodiment (discussed below with reference to FIGS.11-13), the second P-CSCF server 16b (referred to as the transit P-CSCF server 16b) may remain in control of the session e.g., until it is cleared, and may control the PCRF including subscriptions and notifications.

Returning again to this example of the first embodiment, as shown in FIG. 5, the UE 14a may register in IMS 12 with the anchor P-CSCF server 16a and the S- CSCF server 20 (S120). The UE 14a may initiate an IMS session, by, for example, sending a SIP INVITE message for UE 14b (S122). In some embodiments, upon examining the session description protocol (SDP), the anchor P-CSCF server 16a may determine that for the requested service, the UE 14a should use an IMS gateway 18 close to the user location. The anchor P-CSCF server 16a may, however, not control an IMS gateway 18 close to the user location, which user location may be included in e.g., the geolocation header in the SIP INVITE and which the anchor P-CSCF server 16a may validate with, for example, external databases. The anchor P-CSCF server 16a may be configured to recognize services that should use an IMS gateway 18 close to the user location and the ability to detect such services in, for example, the SIP INVITE message.

The anchor P-CSCF server 16a may determine that a local or close AGW should be used, but does not control such an AGW ; thus, the anchor P-CSCF server 16a may identify a transit P-CSCF server 16b that controls a local or close AGW (S124). In some embodiments, the anchor P-CSCF server 16a is configured with information to enable it to select another appropriate IMS gateway 18, controlled by a second P-CSCF server 16b, which may be referred to as a transit P-CSCF server 16b that can better service the requested service in this IMS session.

The anchor P-CSCF server 16a may forward the SIP INVITE message (“INVITE”) to the transit P-CSCF server 16b (S126). In some embodiments, since the transit P-CSCF server 16b has no information or any registration record on the UE 14a and does not expect such an INVITE from this UE 14a, the anchor P-CSCF server

16a may insert in the SIP ROUTE header entry that includes the transit P-CSCF server 16b address additional information that enables the transit P-CSCF server 16b to understand that it is to play a proxy role for this session. Such a parameter may be referred to herein as a“role parameter” in this document but may have other names in other embodiments. In some embodiments, the role parameter may be used because, otherwise the transit P-CSCF server 16b would have no knowledge or record for the UE 14a, and without the role parameter, would reject the SIP INVITE message.

The transit P-CSCF server 16b handles the first entry in the received ROUTE header and processes the role parameter. The transit P-CSCF server 16b may then inform the S-CSCF server 20 for this UE 14a that the transit P-CSCF server 16b will be the P-CSCF server for this session only. To this effect, the transit P-CSCF server 16b may insert additional information in the ROUTE header entry that includes the address of the S-CSCF server 20 to enable the S-CSCF server 20 to handle the IMS session accordingly. Thus, in some embodiments, the transit P-CSCF server 16b may include a special indication in the ROUTE header for the S-CSCF 20 to indicate that the transit P-CSCF 16b is acting as a proxy for the session (S128). The transit P- CSCF server 16b proxies the SIP INVITE to the S-CSCF server 20 including the information in the ROUTE header entry for the S-CSCF server 20 address (SI 30), as described above.

Referring now primarily to FIG. 6, which is a continuation of the example process of FIG. 5, the S-CSCF server 20 process the received information in the ROUTE header entry and processes the IMS session accordingly using the transit P- CSCF server 16b for that session (SI 32). It should be noted that in some

embodiments all CSCF servers may be configured to communicate via secure gateways or other secure means to ensure that all communication between them is protected and secure and that no rogue node can pretend to be a transit P-CSCF server 16b. The S-CSCF server 20 may forward the SIP INVITE message to UE 14b (SI 34). In some embodiments, the rest of the call flow establishes the session normally (SI 36) and session communications may then proceed via the close gateway 18 (e.g., the IMS gateway controlled by the transit P-CSCF 16b) (S138).

FIG. 7 illustrates session tear down for the session. For example, the UE 14a may initiate tear down by sending a BYE message for the UE 14b (S140). The BYE message may be forwarded from the anchor P-CSCF server 16a, to the transit P-CSCF server 16b, then onto the S-CSCF server 20 and then to the UE 14b, as shown in the flow diagram. The UE 14b may then send a 200 OK message to the S-CSCF server 20 (S142), which may then be forwarded to the transit P-CSCF server 16b, then to the anchor P-CSCF server 16a, and finally to the UE 14a. After session tear down, the transit P-CSCF 16b state is cleared (S144). Thus, the transit P-CSCF 16b is no longer used or otherwise associated with UE 14a communications in IMS. On the other hand, the UE 14a remains registered in the anchor P-CSCF 16a, even after the session is torn down (SI 46).

Yet another example of a first embodiment is illustrated in the flow diagram of FIG. 8, as well as, FIGS. 9 and 10, which are continuations of the example process introduced in FIG. 8. It should be noted that, in some embodiments, the anchor P- CSCF server 16a remains in control of the session until the session is cleared. The second P-CSCF 16b may control the PCRF including subscriptions and notifications. This example is similar to the example described above, with respect to FIGS. 5-7, except that the requests are routed via the anchor P-CSCF 16a. Stated another way, in this example, the transit P-CSCF server 16b routes all SIP requests for the session via the anchor P-CSCF server 16a to the S-CSCF server 20. Thus, the transit P-CSCF server 16b may not insert additional information in the ROUTE header to the S-CSCF server 20 since the transit P-CSCF server 16b may not visible in this example to the S-CSCF server 20.

As shown in FIG. 8, the UE 14a may register in IMS 12 with the anchor P- CSCF server 16a and the S-CSCF server 20 (SI 50), per normal IMS registration procedures. The UE 14a may initiate an IMS session, by, for example, sending a SIP INVITE message for UE 14b (S152). In some embodiments, upon examining the session description protocol (SDP), the anchor P-CSCF server 16a may determine that for the requested service, the UE 14a should use an IMS gateway 18 close to the user location. The anchor P-CSCF server 16a may, however, not control an IMS gateway 18 close to the user location, which user location may be included in e.g., the geolocation header in the SIP INVITE and which the anchor P-CSCF server 16a may validate with, for example, external databases. The anchor P-CSCF server 16a may be configured to recognize services that should use an IMS gateway 18 close to the user location and the ability to detect such services in, for example, the SIP INVITE message.

The anchor P-CSCF server 16a may determine that a local or close AGW should be used, but does not control such an AGW ; thus, the anchor P-CSCF server 16a may identify a transit P-CSCF server 16b that controls a local or close AGW (SI 54). In some embodiments, the anchor P-CSCF server 16a is configured with information to enable it to select another appropriate IMS gateway 18, controlled by a transit P-CSCF server 16b that can better service the requested service in this IMS session.

The anchor P-CSCF server 16a may forward the SIP INVITE message (“INVITE”) to the transit P-CSCF server 16b (SI 56). In some embodiments, since the transit P-CSCF server 16b has no information or any registration record on the UE 14a and does not expect such an INVITE from this UE 14a, the anchor P-CSCF server 16a may insert in the SIP ROUTE header entry that includes the transit P-CSCF server 16b address additional information that enables the transit P-CSCF server 16b to understand that it is to play a proxy role for this session. Such a parameter may be referred to herein as a“role parameter” in this document but may have other names in other embodiments. In some embodiments, the role parameter may be used because, otherwise the transit P-CSCF server 16b would have no knowledge or record for the UE 14a, and without the role parameter, would reject the SIP INVITE message.

In addition, in some embodiments, the anchor P-CSCF server 16a inserts, into the SIP INVITE message, a second ROUTE header to be used by the transit P-CSCF server 16b to route the request back to the anchor P-CSCF server 16a. Thus, in some embodiments, all the SIP requests received by the transit P-CSCF server 16b are re routed through the anchor P-CSCF server 16a (which may eliminate an additional step of informing the S-CSCF server 20 about the transit P-CSCF server 16b, as in the previous example, e.g., in block S128).

The transit P-CSCF server 16b handles the first entry in the received ROUTE header and processes the role parameter (S158). The transit P-CSCF server 16b then routes the INVITE request back to the anchor P-CSCF server 16a (SI 60), based on the second ROUTE header inserted by the anchor P-CSCF server 16a in S156. Thus, in this example, it is the anchor P-CSCF server 16a that proxies the SIP INVITE to the S-CSCF server 20 (SI 62).

Referring now primarily to FIG. 9, which is a continuation of the example process of FIG. 8, the S-CSCF server 20 forward the SIP INVITE to UE 14b (S164). In some embodiments, the rest of the call flow establishes the session normally (SI 66) and session communications may then proceed via the close gateway 18 (e.g., the IMS gateway controlled by the transit P-CSCF 16b) (S168). FIG. 10 illustrates session tear down for the session. For example, the UE 14a may initiate tear down by sending a BYE message for the UE 14b (S170). The BYE message may be forwarded from the anchor P-CSCF server 16a to the transit P-CSCF server 16b. In this example (different from the tear down shown in FIG. 7), the transit P-CSCF server 16b routes the BYE message back to the anchor P-CSCF server 16a, as shown in FIG. 10. The anchor P-CSCF server 16a then forwards the message to the S-CSCF server 20, which is then forwarded to the UE 14b. The UE 14b may then send a 200 OK message to the S-CSCF server 20 (S172), which may then be forwarded to the anchor P-CSCF server 16b. The anchor P-CSCF server 16a may then forward the 200 OK message to the transit P-CSCF server 16b and the transit P- CSCF server 16b routes the 200 OK message back to the anchor P-CSCF server 16a. The anchor P-CSCF server 16a then forwards the message to the UE 14a. After session tear down, the transit P-CSCF 16b state is cleared (S174). Thus, the transit P- CSCF 16b is no longer used or otherwise associated with UE 14a communications in IMS. On the other hand, the UE 14a remains registered in the anchor P-CSCF 16a, even after the session is torn down (S176).

One example of a second embodiment is illustrated in the flow diagram of FIG. 11, as well as, FIGS. 12 and 13, which are continuations of the example process introduced in FIG. 11. In the second embodiment, instead of the anchor P-CSCF, it is the transit P-CSCF server 16b that remains in control of the session e.g., until the session is cleared, and the transit P-CSCF server 16b controls the PCRF including subscriptions and notifications. It should be noted that in this example the anchor P- CSCF server 16a is not used at all in the session, and only the transit P-CSCF server 16b is in charge of the session.

As shown in FIG. 11, the UE 14a may register in IMS 12 with the anchor P- CSCF server 16a and the S-CSCF server 20 (S180). The UE 14a may initiate an IMS session, by, for example, sending a SIP INVITE message for UE 14b (S182). In some embodiments, upon examining the session description protocol (SDP), the anchor P- CSCF server 16a may determine that for the requested service, the UE 14a should use an IMS gateway 18 close to the user location. The anchor P-CSCF server 16a may, however, not control an IMS gateway 18 close to the user location, which user location may be included in e.g., the geolocation header in the SIP INVITE and which the anchor P-CSCF server 16a may validate with, for example, external databases.

The anchor P-CSCF server 16a may be configured to recognize services that should use an IMS gateway 18 close to the user location and the ability to detect such services in, for example, the SIP INVITE message.

The anchor P-CSCF server 16a may determine that a local or close AGW should be used, but does not control such an AGW ; thus, the anchor P-CSCF server 16a may identify a transit P-CSCF server 16b that controls a local or close AGW (SI 84). In some embodiments, the anchor P-CSCF server 16a is configured with information to enable it to select another appropriate IMS gateway 18, controlled by a second P-CSCF server 16b, which may be referred to as a transit P-CSCF server 16b that can better service the requested service in this IMS session.

The anchor P-CSCF server 16a sends a 3XX redirection request to the UE 14a (SI 86), which request includes in the contact information the address of the transit P- CSCF server 16b that the UE 14a should use to establish this IMS session. The anchor P-CSCF server 16a may also includes a token that the UE 14a should include in the ROUTE header entry for the transit P-CSCF server 16b and that the transit P-CSCF server 16b can use to confirm the session with the anchor P-CSCF server 16a for security reasons. The anchor P-CSCF server 16a may store the token with the UE 14a subscriber record for later verification.

The UE 14a sends back a 200 OK message for the session to the anchor P- CSCF server 16a (S188). It should be noted that the termination of this IMS session with the anchor P-CSCF server 16a is not shown here, however, the UE 14a and the anchor P-CSCF server 16a tear down this session.

The UE 14a forwards the SIP INVITE message to the transit P-CSCF server 16b (S190). Since the transit P-CSCF server 16b has no information or any registration record on the UE 14a and does not expect such an INVITE from this UE 14a, the UE 14a may insert in the SIP ROUTE header entry that includes the transit P- CSCF server 16b address additional information that enables the transit P-CSCF server 16b to understand that it will be handling this session for this UE 14a, according to the techniques described herein. Additionally, the ROUTE header entry may include other parameters that indicate the address of the anchor P-CSCF server 16a and the token received from the anchor P-CSCF server 16a in SI 86. The transit P-CSCF server 16b handles the first entry in the received ROUTE header and processes the above described parameters. The transit P-CSCF server 16b interacts with the anchor P-CSCF server 16a to validate the received token (SI 92) and may provide the UE 14a contact information for that purpose. In some embodiments, the interaction can be based on SIP using SIP MESSAGE for that purpose. In other embodiments, such verification may be performed in other ways.

Referring now primarily to FIG. 12, which is a continuation of the example process of FIG. 11, the transit P-CSCF server 16b prepares to inform the S-CSCF server 20 for this UE 14a that it will be the P-CSCF server for this session only (S194). To this effect, the transit P-CSCF server 16b may insert additional information in the ROUTE header entry that includes the address of the S-CSCF server 20 to enable the S-CSCF server 20 to handle the IMS session accordingly.

Thus, in some embodiments, the transit P-CSCF server 16b may include a special indication in the ROUTE header for the S-CSCF 20 to indicate that the transit P- CSCF 16b is acting as a proxy for the session. The transit P-CSCF server 16b sends the SIP INVITE to the S-CSCF server 20 including the information in the ROUTE header entry for the S-CSCF server 20 address (S196), as described above.

The S-CSCF server 20 processes the received information in the ROUTE header entry and processes the IMS session accordingly using the transit P-CSCF server 16b for that session (S198). The S-CSCF server 20 may forward the SIP INVITE message to UE 14b (S200). In some embodiments, the rest of the call flow establishes the session normally (S202) and session communications may then proceed via the close gateway 18 (e.g., the IMS gateway controlled by the transit P- CSCF 16b) (S204).

FIG. 13 is a flow diagram continuing the example process of FIGS. 11 and 12, and illustrates an example of a session tear down for the session. For example, the UE 14a may initiate tear down by sending a BYE message for the UE 14b (S206).

The BYE message may be forwarded from the transit P-CSCF server 16b to the S- CSCF server 20 and then to the UE 14b, as shown in the flow diagram. The UE 14b may then send a 200 OK message to the S-CSCF server 20 (S208), which may then be forwarded to the transit P-CSCF server 16b, then to the UE 14a. After session tear down, the transit P-CSCF 16b state is cleared (S210). Thus, the transit P-CSCF 16b is no longer used or otherwise associated with UE 14a communications in IMS. On the other hand, the UE 14a remains registered in the anchor P-CSCF 16a, even after the session is torn down (S212). In some embodiments, since the anchor P-CSCF server 16a does not know when the session terminates in this example, the anchor P-CSCF server 16a may keep the stored token until next UE 14a registration refresh where the token may be cleared.

Thus, some embodiments of this disclosure have been described for allowing a re-assignment of the P-CSCF that controls an AGW close to the UE. Some examples of use cases where such re-assignment may be desirable include, for example, use cases in which the UE requests to stream video from a server identified by a Session Initiation Protocol (SIP) Public Service Identifier (PSI).

In some embodiments, the anchor P-CSCF remains in charge of the session (excluding controlling the AGW), including communication with the Policy and Charging Rules Function (PCRF), until session termination.

In other embodiments, the transit P-CSCF may become fully in charge of the session, including communication with the PCRF, until session termination. The UE may remain registered with the anchor P-CSCF after the session is completed.

As will be appreciated by one of skill in the art, the concepts described herein may be embodied as a method, data processing system, and/or computer program product. Accordingly, the concepts described herein may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects all generally referred to herein as a “circuit” or“module.” Furthermore, the disclosure may take the form of a computer program product on a tangible computer usable storage medium having computer program code embodied in the medium that can be executed by a computer. Any suitable tangible computer readable medium may be utilized including hard disks, CD-ROMs, electronic storage devices, optical storage devices, or magnetic storage devices.

Some embodiments are described herein with reference to flowchart illustrations and/or block diagrams of methods, systems and computer program products. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable memory or storage medium that can direct a computer or other

programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

It is to be understood that the functions/acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that

communication may occur in the opposite direction to the depicted arrows.

Computer program code for carrying out operations of the concepts described herein may be written in an object oriented programming language such as Java® or C++. However, the computer program code for carrying out operations of the disclosure may also be written in conventional procedural programming languages, such as the "C" programming language. The program code may execute entirely on the user's computer, partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer. In the latter scenario, the remote computer may be connected to the user's computer through a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

Many different embodiments have been disclosed herein, in connection with the above description and the drawings. It will be understood that it would be unduly repetitious and obfuscating to literally describe and illustrate every combination and subcombination of these embodiments. Accordingly, all embodiments can be combined in any way and/or combination, and the present specification, including the drawings, shall be construed to constitute a complete written description of all combinations and subcombinations of the embodiments described herein, and of the manner and process of making and using them, and shall support claims to any such combination or subcombination.

It will be appreciated by persons skilled in the art that the embodiments described herein are not limited to what has been particularly shown and described herein above. In addition, unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. A variety of modifications and variations are possible in light of the above teachings without departing from the scope of the following claims.

Claims

What is claimed is:

1. A first Proxy-Call Session Control Function, P-CSCF, server (16a) associated with a first Internet Protocol, IP, Multimedia Subsystem, IMS, gateway (18a), the first P-CSCF server (16a) comprising processing circuitry (32) having a processor (36) and a memory (34), the memory (34) including instructions executable by the processor (36) to configure the first P-CSCF server (16a) to:

determine whether to use a second IMS gateway (18b) for an IMS session associated with a user equipment, UE (14); and

as a result of a determination to use the second IMS gateway (18b) for the

IMS session associated with the UE (14), identify a second P-CSCF server (16b) associated with the second IMS gateway (18b), the second IMS gateway (18b) being geographically closer to the UE (14) than the first IMS gateway (18a).

2. The first P-CSCF server (16a) of Claim 1, wherein the second IMS gateway

(18b) is different from the first IMS gateway (18a) and the second P-CSCF server (16b) is different from the first P-CSCF server (16a).

3. The first P-CSCF server (16a) of any one of Claims 1 and 2, wherein the memory (34) includes further instructions executable by the processor (36) to configure the first P-CSCF server (16a) to determine whether to use the second IMS gateway (18b) based at least in part on a service associated with the IMS session.

4. The first P-CSCF server (16a) of any one of Claims 1-3, wherein the memory (34) includes further instructions executable by the processor (36) to configure the first P-CSCF server (16a) to:

determine a location of the UE (14), the identification of the second P-CSCF server (16b) associated with the second IMS gateway (18b) based at least in part on the determined location of the UE (14).

5. The first P-CSCF server (16a) of any one of Claims 1-4, wherein the memory (34) includes further instructions executable by the processor (36) to configure the first P-CSCF server (16a) to:

determine a coverage area associated with the second IMS gateway (18b); compare a location of the UE (14) to the determined coverage area associated with the second IMS gateway (18b); and

select the second IMS gateway (18b) for the IMS session based at least in part on the comparison.

6. The first P-CSCF server (16a) of any one of Claims 1-5, wherein the memory (34) includes further instructions executable by the processor (36) to configure the first P-CSCF server (16a) to:

identify a service for the IMS session in a session description protocol, SDP, the determination of whether to use the second IMS gateway (18b) for the IMS session based at least in part on the identified service.

7. The first P-CSCF server (16a) of any one of Claims 1-6, wherein the memory (34) includes further instructions executable by the processor (36) to configure the first P-CSCF server (16a) to:

receive, from the UE (14), a session initiation protocol, SIP, INVITE message for the IMS session;

select the second P-CSCF server (16b) associated with the second IMS gateway (18b) for the IMS session; and

as a result of the selection of the second P-CSCF server (16b), forward the SIP INVITE message to the second P-CSCF server (16b).

8. The first P-CSCF server (16a) of any one of Claims 1-7, wherein the memory (34) includes further instructions executable by the processor (36) to configure the first P-CSCF server (16a) to:

select the second P-CSCF server (16b) associated with the second IMS gateway (18b) for the IMS session; and as a result of the selection of the second P-CSCF server (16b), communicate a SIP message to the second P-CSCF server (16b), the SIP message including a role parameter, the role parameter indicating a support role of the second P-CSCF server (16b) for the IMS session associated with the UE (14).

9. The first P-CSCF server (16a) of Claim 8, wherein the support role includes at least controlling the second IMS gateway (18b) for the IMS session associated with the UE (14).

10. The first P-CSCF server (16a) of any one of Claims 1-9, wherein the memory (34) includes further instructions executable by the processor (36) to configure the first P-CSCF server (16a) to:

forward a session initiation protocol, SIP, INVITE message for the IMS session to the second P-CSCF server (16b) associated with the second IMS gateway (18b), the SIP INVITE message including a role parameter and a SIP ROUTE header entry, the role parameter indicating a support role of the second P-CSCF server (16b) for the IMS session associated with the UE (14) and the SIP ROUTE header entry instructing the second P-CSCF server (16b) to re-route the SIP INVITE message back to the first P-CSCF server (16a).

11. The first P-CSCF server (16a) of any one of Claims 1-10, wherein the memory (34) includes further instructions executable by the processor (36) to configure the first P-CSCF server (16a) to:

communicate, to the UE (14), a redirection request message, the redirection request message including an address of the second P-CSCF server (16b) and a token, the address to be used by the UE (14) to establish the IMS session with the second P- CSCF server (16b) and the token to be used for verification of the IMS session.

12. The first P-CSCF server (16a) of Claim 11, wherein the redirection request message is a 3XX redirection request message.

13. The first P-CSCF server (16a) of any one of Claims 1-12, wherein the UE (14) is IMS registered with the first P-CSCF server (16a) and not IMS registered with the second P-CSCF server (16b).

14. A method for a first Proxy-Call Session Control Function, P-CSCF, server (16a) associated with a first Internet Protocol, IP, Multimedia Subsystem, IMS, gateway, the method comprising:

determining (SI 00) whether to use a second IMS gateway (18b) for an IMS session associated with a user equipment, UE (14); and

as a result of a determination to use the second IMS gateway (18b) for the IMS session associated with the UE (14), identifying (S102) a second P-CSCF server (16b) associated with the second IMS gateway (18b), the second IMS gateway (18b) being geographically closer to the UE (14) than the first IMS gateway (18a).

15. The method of Claim 14, wherein the second IMS gateway (18b) is different from the first IMS gateway (18a) and the second P-CSCF server (16b) is different from the first P-CSCF server (16a).

16. The method of any one of Claims 14 and 15, wherein the determining whether to use the second IMS gateway (18b) includes determining whether to use the second IMS gateway (18b) based at least in part on a service associated with the IMS session.

17. The method of any one of Claims 14-16, further comprising:

determining a location of the UE (14), the identification of the second P-CSCF server (16b) associated with the second IMS gateway (18b) based at least in part on the determined location of the UE (14).

18. The method of any one of Claims 14-17, further comprising:

determining a coverage area associated with the second IMS gateway (18b); comparing a location of the UE (14) to the determined coverage area associated with the second IMS gateway (18b); and selecting the second IMS gateway (18b) for the IMS session based at least in part on the comparison.

19. The method of any one of Claims 14-18, further comprising:

identifying a service for the IMS session in a session description protocol,

SDP, the determination of whether to use the second IMS gateway (18b) for the IMS session based at least in part on the identified service.

20. The method of any one of Claims 14-19, further comprising:

receiving, from the UE (14), a session initiation protocol, SIP, INVITE message for the IMS session;

selecting the second P-CSCF server (16b) associated with the second IMS gateway (18b) for the IMS session; and

as a result of the selection of the second P-CSCF server (16b), forwarding the SIP INVITE message to the second P-CSCF server (16b).

21. The method of any one of Claims 14-20, further comprising:

as a result of the selection of the second P-CSCF server (16b), communicating a SIP message to the second P-CSCF server (16b), the SIP message including a role parameter, the role parameter indicating a support role of the second P-CSCF server (16b) for the IMS session associated with the UE (14).

22. The method of Claim 21, wherein the support role includes at least controlling the second IMS gateway (18b) for the IMS session associated with the UE (14).

23. The method of any one of Claims 14-22, further comprising:

forwarding a session initiation protocol, SIP, INVITE message for the IMS session to the second P-CSCF server (16b) associated with the second IMS gateway (18b), the SIP INVITE message including a role parameter and a SIP ROUTE header entry, the role parameter indicating a support role of the second P-CSCF server (16b) for the IMS session associated with the UE (14) and the SIP ROUTE header entry instructing the second P-CSCF server (16b) to re-route the SIP INVITE message back to the first P-CSCF server (16a).

24. The method of any one of Claims 14-23, further comprising:

communicating, to the UE (14), a redirection request message, the redirection request message including an address of the second P-CSCF server (16b) and a token, the address to be used by the UE (14) to establish the IMS session with the second P- CSCF server (16b) and the token to be used for verification of the IMS session.

25. The method of Claim 24, wherein the redirection request message is a 3XX redirection request message.

26. The method of any one of Claims 14-25, wherein the UE (14) is IMS registered with the first P-CSCF server (16a) and not IMS registered with the second P-CSCF server (16b).

27. A user equipment, UE (14), comprising processing circuitry having a processor (46) and a memory (44), the memory (44) including instructions executable by the processor (46) to configure the UE (14) to:

register in Internet Protocol, IP, Multimedia Subsystem, IMS with a first Proxy-Call Session Control Function, P-CSCF, server (16a), the first P-CSCF server (16a) being associated with a first IMS gateway (18a); and

while being registered in IMS with the first P-CSCF server (16a), participate in an Internet Protocol, IP, Multimedia Subsystem, IMS, session, the IMS session using a second P-CSCF server (16b) to control a second IMS gateway (18b), the second IMS gateway (18b) being geographically closer to the UE (14) than the first IMS gateway (18a).

28. The UE (14) of Claim 27, wherein the second IMS gateway (18b) is different from the first IMS gateway (18a) and the second P-CSCF server (16b) is different from the first P-CSCF server (16a).

29. The UE (14) of any one of Claims 27 and 28, wherein the memory (44) includes further instructions executable by the processor (46) to configure the UE (14) to:

communicate a session initiation protocol, SIP, message to the first P-CSCF server (16a) to initiate the IMS session, the SIP message identifying a requested service for the IMS session, wherein the use of the second P-CSCF server (16b) to control the second IMS gateway (18b) for the IMS session is based at least in part on the requested service.

30. The UE (14) of any one of Claims 27-29, wherein the memory (44) includes further instructions executable by the processor (46) to configure the UE (14) to: receive, from the first P-CSCF server (16a), a redirection request message, the redirection request message including an address of the second P-CSCF server (16b).

31. The UE (14) of any one of Claims 27-30, wherein the memory (44) includes further instructions executable by the processor (46) to configure the UE (14) to: receive, from the first P-CSCF server (16a), a redirection request message, the redirection request message including a token, the token to be used for verification of the IMS session between the first and second P-CSCF servers (16a, 16b).

32. The UE (14) of any one of Claims 30 and 31, wherein the memory (44) includes further instructions executable by the processor (46) to configure the UE (14) to:

as a result of the redirection request message, communicate a session initiation protocol, SIP, message to the second P-CSCF server (16b), the SIP message including a role parameter, a SIP ROUTE header entry, and a token.

33. The UE (14) of Claim 32, wherein the role parameter indicates a support role of the second P-CSCF server (16b) for the IMS session associated with the UE (14), the SIP ROUTE header entry includes an address of the first P-CSCF server (16a) and the token is configured to verify the IMS session with the first P-CSCF server (16a).

34. The UE (14) of any one of Claims 27-33, wherein the UE (14) is not registered with the second P-CSCF server (16b).

35. A method for a user equipment, UE (14), the method comprising:

registering (SI 04) in Internet Protocol, IP, Multimedia Subsystem, IMS with a first Proxy-Call Session Control Function, P-CSCF, server (16a), the first P-CSCF server (16a) being associated with a first IMS gateway (18a); and

while being registered in IMS with the first P-CSCF server (16a), participating (S106) in an Internet Protocol, IP, Multimedia Subsystem, IMS, session, the IMS session using a second P-CSCF server (16b) to control a second IMS gateway (18b), the second IMS gateway (18b) being geographically closer to the UE (14) than the first IMS gateway (18a).

36. The method of Claim 35, wherein the second IMS gateway (18b) is different from the first IMS gateway (18a) and the second P-CSCF server (16b) is different from the first P-CSCF server (16a).

37. The method of any one of Claims 35 and 36, further comprising:

communicating a session initiation protocol, SIP, message to the first P-CSCF server (16a) to initiate the IMS session, the SIP message identifying a requested service for the IMS session, wherein use of the second P-CSCF server (16b) to control the second IMS gateway (18b) for the IMS session is based at least in part on the requested service.

38. The method of any one of Claims 35-37, further comprising:

receiving, from the first P-CSCF server (16a), a redirection request message, the redirection request message including an address of the second P-CSCF server (16b).

39. The method of any one of Claims 35-38, further comprising: receiving, from the first P-CSCF server (16a), a redirection request message, the redirection request message including a token, the token to be used for verification of the IMS session between the first and second P-CSCF servers (16a, 16b).

40. The method of any one of Claims 38 and 39, further comprising:

as a result of the redirection request message, communicating a session initiation protocol, SIP, message to the second P-CSCF server (16b), the SIP message including a role parameter, a SIP ROUTE header entry, and a token.

41. The method of Claim 40, wherein the role parameter indicates a support role of the second P-CSCF server (16b) for the IMS session associated with the UE (14), the SIP ROUTE header entry includes an address of the first P-CSCF server (16a) and the token is configured to verify the IMS session with the first P-CSCF server (16a).

42. The method of any one of Claims 35-41, wherein the UE (14) is not registered with the second P-CSCF server (16b).