Patent Description:
Network slicing utilizes virtualization technology (e.g., Software Defined Networking (SDN) and Network Functions Virtualization (NFV)) to allow multiple virtual (i.e., logical) networks to be created on top of a common shared physical infrastructure. These virtual networks are referred to as network slices. The network slices can then be customized to meet needs of different use cases.

Network slicing has been proposed in the Third Generation Partnership Project (3GPP) for the Fifth Generation System (5GS). Further, the concept of Internet Protocol (IP) Multimedia Subsystem (IMS) slicing has been proposed in Patent Cooperation Treaty (PCT) Patent Application Publication No. <CIT>, entitled NETWORK SLICING IN IMS. When using network slicing in the 5GS and IMS slicing, a single User Equipment (UE) may having two or more IMS registrations using the same IMPI/IMPU using two or more respective IMS Protocol Data Unit (PDU) sessions over two or more respective network slices in the 5GS. While this has great benefits, it also creates new challenges that need to be addressed within the 5GS and IMS.

Document <CIT> discloses a method for providing multimedia Services to subscriber user equipment (UE) within an IP multimedia Subsystem network (IMS). The method may include configuring the IMS to enable a single UE to fork register and cooperate with multiple serving-call session control functions (S-CSCFs). After obtaining IP Connectivity, the single UE signals to register with the IMS and the IMS determines whether the UE is configured to fork register with multiple S-CSCFs. If the UE is configured, the IMS determines which S-CSCFs are eligible for the UE registration and fork registers the UE to multiple S-CSCFs of the eligible S-CSCFs. Consequently, incoming and outgoing calls to/from the UE are routed by the IMS to any of the multiple registered S-CSCFs.

Document 3GPP TR <NUM> V9. <NUM> discloses changes required in the 3GPP IMS specifications so that a consistent state is restored in the IMS Core Network, after, or during a planned, or unplanned stop of a network element.

Document <CIT> discloses a method for use by an Interrogating Call/Session Control Function (I-CSCF) of an IP Multimedia Subsystem (IMS). The method comprises, in response to receipt of a Session Initiation Protocol (SIP) message, requesting capabilities information relating to a user from a Home Subscriber Server (HSS) of the IMS. On receipt of the capabilities information from the HSS, a primary and a secondary Serving Call/Session Control Function (S-CSCF) of the IMS are selected to provide Services to the user. The I-CSCF attempts to forward the SIP message to the primary S-CSCF with information relating to the secondary S-CSCF, such that the information can be provided subsequently to the HSS. If the attempt is determined to have failed, the SIP message can be forwarded to the secondary S-CSCF instead.

Document <NPL>, discusses possible enhancements for IMS to support devices and networks with multiple slices and different IMS services per <NUM> core network slice.

Document S1-<NUM>, entitled "FS_enIMSTR22. <NUM>: Use case on network slicing of IMS independent to 5GC slice", published on <NUM>-<NUM>-<NUM>, discloses a proposal to introduce network slicing capability into an IMS network.

Further details are defined in the dependent claims.

Systems and methods are disclosed herein that relate to supporting Internet Protocol (IP) Multimedia Subsystem (IMS) routing with multiple IMS Protocol Data Unit (PDU) sessions over different core network slices.

According to the present disclosure, methods, an S-CSCF, a system and a computer-readable medium according to the independent claims are provided. Developments are set forth in the dependent claims.

According to a first aspect of the present disclosure, there is provided method comprising at an Interrogating Call Session Control Function, I-CSCF: receiving a Session Initiation Protocol, SIP, invite message for an incoming session, the SIP invite message comprising an Internet Protocol, IP, Multimedia Public Identity, IMPU, of a target User Equipment, UE; sending, to a Home Subscriber Server, HSS, a query for Serving Call Session Control Functions, S-CSCFs, having registrations for the IMPU of the target UE; receiving, from the HSS, information that indicates two or more S-CSCFs having registrations for the IMPU of the target UE; and forwarding the SIP invite message to at least one of the two or more S-CSCFs; at the HSS: receiving the query from the I-CSCF; obtaining the information that indicates the two or more S-CSCFs having registrations for the IMPU of the target UE; and sending, to the I-CSCF,
the information that indicates the two or more S-CSCFs having registrations for the IMPU of the target UE; and at each S-CSCF of the at least one of the two or more S-CSCFs: receiving the SIP invite message from the I-CSCF; determining whether to accept the incoming session based on the SIP invite message; if determined to reject the incoming session, sending a message to the I-CSCF that indicates an error or rejection; if determined to accept the incoming session, forwarding the SIP invite message to another node.

According to a second aspect of the present disclosure, there is provided a method performed by a Serving Call Session Control Function, S-CSCF, for an Internet Protocol, IP, Multimedia Subsystem, IMS. The method comprises receiving, from an Interrogating Call Session Control Function, I-CSCF, a Session Initiation Protocol, SIP, invite message for an incoming session for a target User Equipment, UE; determining whether to accept the incoming session based on the SIP invite message; if determined to reject the incoming session, sending a message to the I-CSCF that indicates an error or rejection; and if determined to accept the incoming session, forwarding the SIP invite message to another node.

According to a third aspect of the present disclosure, there is provided a Serving Call Session Control Function, S-CSCF, for an Internet Protocol, IP, Multimedia Subsystem, IMS, the S-CSCF adapted to perform the method of the second aspect.

According to a fourth aspect of the present disclosure, there is provided a system comprising an Interrogating Call Session Control Function, I-CSCF, adapted to: receive a Session Initiation Protocol, SIP, invite message for an incoming session, the SIP INVITE message comprising an Internet Protocol, IP, Multimedia Public Identity, IMPU, of a target User Equipment, UE; send, to a Home Subscriber Server, HSS, a query for Serving Call Session Control Functions, S-CSCFs, having registrations for the IMPU of the target UE; receive, from the HSS, information that indicates two or more S-CSCFs having registrations for the IMPU of the target UE; and forward the SIP invite message to at least one of the two or more S-CSCFs; the HSS, wherein the HSS is adapted to: receive the query from the I-CSCF; obtain the information that indicates the two or more S-CSCFs having registrations for the IMPU of the target UE; and send, to the I-CSCF, the information that indicates the two or more S-CSCFs having registrations for the IMPU of the target UE; and the two or more S-CSCFs, wherein each S-CSCF of the at least one of the two or more S-CSCFs is adapted to: receive the SIP invite message from the I-CSCF; determine whether to accept the incoming session based on the SIP invite message; if determined to reject the incoming session, send a message to the I-CSCF that indicates an error or rejection; and if determined to accept the incoming session, forward the SIP invite message to another node.

According to a fifth aspect of the present disclosure, there is provided a computer-readable medium comprising code portions which, when executed on a processor of a Serving Call Session Control Function, S-CSCF, configure the processor to perform the method according to the second aspect.

Whenever in the following disclosure any of the above-stated aspects (corresponding to the independent claims) is disclosed as "optional" (e.g., due to usage of conjunctive terms, such as "can", "may", "should", etc.), it is nevertheless to be read as "mandatory".

Hereinabove and in the following, "examples" pertain to principles underlying the claimed subject-matter and/or being useful for understanding the claimed subject-matter, while "embodiments" pertain to the claimed subject-matter within the claim scope.

Whenever in this description an "embodiment" is described, reference is to be made to the above figure list to determine whether this is to be read as "embodiment" or "example".

Core Network Node: As used herein, a "core network node" is any type of node in a core network or any node that implements a core network function. Some examples of a core network node include, e.g., a Mobility Management Entity (MME), a Packet Data Network Gateway (P-GW), a Service Capability Exposure Function (SCEF), a Home Subscriber Server (HSS), or the like. Some other examples of a core network node include a node implementing a Access and Mobility Function (AMF), a UPF, a Session Management Function (SMF), an Authentication Server Function (AUSF), a Network Slice Selection Function (NSSF), a Network Exposure Function (NEF), a
Network Function (NF) Repository Function (NRF), a Policy Control Function (PCF), a Unified Data Management (UDM), or the like.

Internet Protocol (IP) Multimedia System (IMS) Node: As used herein, an "IMS node" is any type of node in an IMS. The IMS node may be a virtualized node in an IMS network slice.

The description provided herein is based on the following assumptions:.

With the above assumptions, a UE can establish two IMS Protocol Data Unit (PDU) sessions using different 5GC network slices using the same IP Multimedia Private Identity (IPMI) / IP Multimedia Public Identity (IMPU). For example, a UE can have two IMS registrations over the two IMS PDU sessions - one for regular Multimedia Telephony (MMTEL) service using an enhanced Mobile Broadband (eMBB) 5GC slice and the other one for Mission Critical (MC) IMS services using a MC 5GC slice. Each IMS registration can have a different Proxy Call Session Control Function (P-CSCF) and can have a different S-CSCF or the same S-CSCF.

The UE can receive regular incoming IMS sessions over the IMS PDU session associated with the eMBB network slice, while the same UE can receive incoming MC IMS sessions over the IMS PDU session associated with the MC 5GC slice. However, IMS nodes that receive an incoming session need to decide which UE contact to use for the incoming session. This implies selecting the correct S-CSCF, the correct P-CSCF, and the correct UE contact.

If the same S-CSCF is used based on current limitations in IMS, then this can be resolved by existing legacy solutions. However, if different S-CSCFs are used, the existing solutions do not work.

<FIG> illustrates this problem that the embodiments of the solutions described herein are solving. The Interrogating Call Session Control Function (I-CSCF) needs to ensure that the correct S-CSCF/P-CSCF and UE contact receive the incoming IMS session according to the 5GC slice over which the session arrived. <FIG> implies that a terminating MC call must use S-CSCF2 to end up selecting the MC slice, while a terminating regular Voice of Long Term Evolution (VoLTE) session must use S-CSCF1 to end up selecting the eMBB slice for the target UE. The I-CSCF needs to make the correct decision as described above.

Certain aspects of the present disclosure and their embodiments may provide solutions to the aforementioned or other challenges. Embodiments of the present disclosure ensure that the I-CSCF and other IMS nodes identify the 5GC slice used for an incoming session to enable locating the proper IMS nodes for terminating the session to the intended target. Embodiments of the present disclosure ensure that the I-CSCF and other IMS nodes identify IMS nodes (e.g., S-CSCF/P-CSCF) for terminating an incoming IMS session to an intended UE via a respective 5GC slice when the UE having a single MSISDN has two or more IMS sessions established over two or more 5GC slices using the same IMPI/IMPU.

Two solutions are disclosed herein, namely, an IMS solution and non-IMS solution. For the IMS-solution, additional behavior is provided in the I-CSCF, HSS, and S-CSCF to support the correct routing. These are the IMS nodes involved in a terminating session. Details are described below.

For the non-IMS solution, a UE centric solution and a network-centric solution are disclosed. In the UE centric solution, the UE provides, at IMS registration, information that indicates the 5GC slice associated with the IMS PDU session associated with this registration. This information that indicates the 5GC slice associated with the IMS PDU session associated with this registration is stored in (or otherwise made available to) all IMS nodes that need it for proper routing of a terminating IMS session and session initiation. In the network centric solution, the network derives the 5GC slice associated with the incoming IMS registration and returns information that indicates the 5GC slice to the UE in the SIP <NUM> OK response to the IMS registration. IMS nodes that need this information will now store this information (or this information is otherwise made available to those IMS nodes). The network locates the Subscription Permanent Identifier (SUPI) that maps to the International Mobile Subscriber Identity (IMSI) / MSISDN for the registering IMS UE (also referred to herein as IMS user), then locates the PDU session associated with the IMS PDU session over which the IMS registration arrives, and then locates the 5GC slice used for that PDU session.

Certain embodiments may provide one or more of the following technical advantage(s). Embodiments disclosed herein provide a simple way of allowing an IMS user to establish multiple PDU sessions for different purposes and allowing different S-CSCFs to be used for supporting these services using the same identity (IMPI/IMPU) while ensuring proper routing for terminating sessions.

Before describing embodiments of the present disclosure, a brief description of network slicing in the 5GC (i.e., 5GC slicing) is beneficial. In this regard, <FIG> illustrates an example of a system <NUM> including a <NUM> Radio Access Network (RAN) <NUM>, a <NUM> Core Network (5GC) <NUM>, and an Internet Protocol (IP) Multimedia Subsystem (IMS) <NUM>. Together, the <NUM> RAN <NUM> and the 5GC <NUM> are referred to as a 5GS. As illustrated, the <NUM> RAN <NUM> includes a number of radio access nodes <NUM> (referred to as New Radio (NR) base stations (gNBs)). The 5GC <NUM> includes a number of core network nodes, or core network functions, such as, e.g., an Access and Mobility Function(s) (AMF(s)) <NUM>, a User Plane Function(s) (UPF(s)) <NUM>, a Policy Control Function(s) (PCF(s)) <NUM>, etc. The IMS <NUM> includes a number of IMS nodes such as, e.g., a Serving Call Session Control Function(s) (S-CSCF(s)) <NUM>, a Proxy Call Session Control Function(s) (P-CSCF(s)) <NUM>, I-CSCF(s) <NUM>, etc..

<FIG> illustrates the concept of network slicing with respect to the 5GS. As shown in <FIG>, the 5GS supports multiple services. In this example, there are four services labelled as Service <NUM>, Service <NUM>, Service <NUM>, and Service <NUM>. Example services include, but are not limited to, enhanced Mobile Broadband (eMBB), Ultra Reliable Low Latency Communication (URLLC), massive Machine Type Communication (mMTC) or massive Internet of Things (IoT), emergency services, and/or the like. As shown, each of the services is mapped to a 5GS network slice that is customized to meet the needs of that service. Note that more than one service may be mapped to the same 5GS network slice. As also illustrated in <FIG>, the 5GS network slices are implemented on the same physical infrastructure.

Embodiments of the present disclosure relate to network slicing in the IMS. In this regard, <FIG> illustrates one example of a wireless communication system <NUM> in which embodiments of the present disclosure may be implemented. In the embodiments described herein, the wireless communication system <NUM> is a 5GS including a <NUM> RAN <NUM> and a 5GC <NUM>. The wireless communication system <NUM> also includes an IMS <NUM>. As illustrated, the <NUM> RAN <NUM> includes a number of base stations <NUM>, which are referred to as gNBs in 3GPP <NUM>, serving a number of wireless devices <NUM>, which are also referred to herein as UEs <NUM>. The base stations <NUM> are connected to the 5GC <NUM>. The 5GC <NUM> is connected to the IMS <NUM>. Notably, 5GC slicing is implemented in the 5GC <NUM>, e.g., as described above with respect to <FIG>.

In the embodiments described herein, the IMS <NUM> includes a number of IMS network slices <NUM>-<NUM> through <NUM>-N. The IMS network slices <NUM>-<NUM> through <NUM>-N include respective S-CSCFs <NUM>-<NUM> through <NUM>-N. The IMS network slices <NUM>-<NUM> through <NUM>-N are generally referred to herein as IMS network slices <NUM>, and the S-CSCFs <NUM>-<NUM> through <NUM>-N are generally referred to herein as S-CSCFs <NUM>. The S-CSCFs <NUM> are virtual nodes (e.g., IMS entities that are implemented by a (physical) network node(s) (e.g., as a virtual entity such as, e.g., a virtual machine).

The IMS <NUM> also includes one or more P-CSCFs <NUM>, one or more Interrogating Call Session Control Functions (I-CSCFs) <NUM>, and one or more HSSs <NUM>. As discussed above, in some embodiments, different P-CSCFs <NUM> can be associated with different IMS network slices <NUM>. For example, each P-CSCF <NUM> may be associated with a different one of the IMS network slices <NUM> or each P-CSCF <NUM> may be associated with one or more of the IMS network slices <NUM>.

Now, a description of some example embodiments of the present disclosure is provided.

<FIG> and <FIG> illustrate a call flow for an IMS-based solution in accordance with some embodiments of the present disclosure. The steps of this call flow are as follows:.

In the process of <FIG> and <FIG>, the I-CSCF <NUM> uses "serial forking" to try the S-CSCFs <NUM> in the list returned by the HSS <NUM> (in step <NUM> of <FIG>) for the particular subscriber one at a time. The HSS <NUM> also has new functionality in that it returns the list of S-CSCFs. The S-CSCF <NUM> (e.g., S-CSCF1 <NUM>-<NUM> and S-CSCF2 <NUM>-<NUM> of <FIG> and <FIG>) each also has additional behavior since it has to check the incoming SIP INVITE to sees if the S-CSCF <NUM> has the correct contact to handle the SIP INVITE. In conventional IMS systems, the conventional S-CSCF selects the most probable UE contact for the SIP INVITE; however, in the embodiment described above, the S-CSCF <NUM> is looking for an exact match between the UE contact and the requested service from the SIP INVITE.

<FIG> and <FIG> illustrate a call flow for another embodiment in which parallel forking is utilized by the I-CSCF <NUM>. The steps of this call flow are as follows:.

In the process of <FIG> and <FIG>, the I-CSCF <NUM> attempts to reach all S-CSCFS <NUM> in the list received for the subscriber (in step <NUM> of <FIG>) at the same time (steps 610a and 610b). Only one S-CSCF <NUM> (S-CSCF <NUM> in the example of <FIG> and <FIG>) will accept the session.

Note that the embodiments described above with respect to <FIG> and <FIG> and <FIG> and <FIG> are SIP based solutions that do not require any interaction with the 5GC, nor do they have any impacts on the UE. These solutions rely on existing SIP capabilities to locate the proper route for a terminating IMS session. Hence, they are cheap to implement and deploy.

<FIG> and <FIG> illustrate a call flow that shows both a UE centric solution (option <NUM>) and a network centric solution (option <NUM>) in accordance with some embodiments of the present disclosure.

For option <NUM> (UE centric solution), it can be seen in the call flow that the UE <NUM> includes information that indicates the 5GC slice used for the IMS PDU session associated with the IMS PDN connection in IMS registration. The P-CSCS, S-CSCF, and HSS stores this information in the record associated with the registering IMPI/IMPU pair.

For option <NUM> (network centric solution), when the Registration Request arrives at the HSS <NUM>, the HSS <NUM> locates the <NUM> record based on the SUPI associated with the IMS subscription IMSI/MSISDN, then fetches the PDU session associated with the Registration IP address information in the contact, extracts the 5GS slice used for this PDU session, and stores it in the HSS <NUM>. This information is returned in the SIP <NUM> OK to IMS nodes as shown in the call flow.

So, both option <NUM> and option <NUM>, ends up with the same outcome.

The call flows of <FIG>, <FIG>, and <FIG> detail the routing for incoming sessions using the proper IMS registration information, given that the called target is registered in IMS over two different 5GC slices. The goal as described above is that the I-CSCF <NUM> selects the proper S-CSCF <NUM> for the incoming IMS session associated with the correct service. Several terminating session scenarios are described.

<FIG> illustrates a terminating session scenario in which an indication of the 5GC slice is included in the SIP invite message for the incoming IMS session in accordance with some embodiments of the present disclosure.

As can be seen in <FIG>, in Scenario <NUM>, an indication of the 5GC slice used by the UE originating the session is included in the incoming SIP INVITE to the terminating UE/network. This enables the HSS <NUM> to locate the correct S-CSCF <NUM> (i.e., the S-CSCF <NUM> associated with the 5GC slice for the target UE that is registered over the IMS PDU session that matches the 5GC slice for the incoming INVITE). The incoming 5GC slice is carried over to the S-CSCF <NUM> so the S-CSCF <NUM> can choose the appropriate P-CSCF <NUM>. The HSS <NUM> has stored this information at IMS registration, as described above.

<FIG> illustrates a terminating session scenario in which an indication of the 5GC slice is not included in the SIP invite message for the incoming IMS session in accordance with some embodiments of the present disclosure.

In this case, the used 5GC by the UE originating the session slice is NOT included in the incoming SIP INVITE from the external network. For this scenario, HSS <NUM> assumes the 5GC slice to be a particular 5GC slice (e.g., the eMBB slice), the 5GC slice used for regular VoLTE sessions and which this scenario assumes and returns information that indicates the S-CSCF <NUM> for the target UE <NUM> that is registered over the IMS PDU session that matches assumed 5GC slice (e.g., the eMBB slice). Information that indicates the assumed 5GC slice (e.g., eMBB slice) is also returned to the I-CSCF <NUM> in the response. This information is included in the subsequent SIP INVITE to the S-CSCF <NUM> so it can choose the appropriate P-CSCF <NUM>.

<FIG> illustrates a termination session scenario in which the incoming IMS session is an incoming IMS session that originated from the operator's own network and does not include an indication of the used 5GC slice, in accordance with some embodiments of the present disclosure.

In this case, the incoming session is an incoming session that originated from the operator's own network, and does not include the 5GC slice used by the UE originating the IMS session. For this scenario, HSS locates the incoming HSS record associated with the IP contact of the incoming INVITE. HSS then locates the PDU session associated with that contact (since the operator has both the IMS and 5GC record of the originating subscriber), determines the 5GC slice used for that PDU session, and then proceeds as in scenario <NUM>.

Note that embodiments of the present disclosure may also ensure that the originating SIP INVITE will have the correct 5GC slice used since the 5GC slice inserted by the originating UE can be verified by the P-CSCF <NUM> if included. If the UE originating the session did not include the indication of the 5GC slice used, the P-CSCF can include it if it has one stored.

<FIG> is a schematic block diagram of an IMS physical infrastructure node <NUM>. The IMS network slices <NUM> are logical or virtual networks that are implemented using virtualization technology on a number of IMS physical infrastructure nodes such as the IMS physical infrastructure node <NUM>. In this regard, the S-CSCFs <NUM> and, in some embodiments, the P-CSCF(s) <NUM>, the I-CSCF(s) <NUM>, and/or the HSS <NUM> are implemented as virtual nodes operating on a number of IMS physical infrastructure nodes <NUM>. Note, however, that some of the IMS nodes (e.g., the P-CSCF(s) <NUM>, the I-CSCF(s) <NUM>, and/or the HSS <NUM>) may alternatively be implemented as physical nodes (i.e., as physical infrastructure nodes).

In this regard, as illustrated in <FIG>, the IMS physical infrastructure node <NUM> includes one or more processors <NUM> (e.g., Central Processing Units (CPUs), Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), and/or the like), memory <NUM>, and a network interface(s) <NUM>. In some embodiments, using virtualization, the IMS nodes (the S-CSCFs <NUM> and, in some embodiments, the P-CSCF(s) <NUM>, the I-CSCF(s) <NUM>, and/or the HSS <NUM>) are implemented as virtual nodes that utilize physical resources (e.g., the processor(s) <NUM>, the memory <NUM>, and the network interface(s) <NUM>) of one or more of the IMS physical infrastructure nodes <NUM>.

In some embodiments, a computer program including instructions which, when executed by at least one processor, causes the at least one processor to carry out the functionality of an IMS node according to any of the embodiments described herein is provided.

<FIG> is a schematic block diagram of the IMS physical infrastructure node <NUM> according to some other embodiments of the present disclosure. The IMS physical infrastructure node <NUM> includes one or more modules <NUM>, each of which is implemented in software. The module(s) <NUM> provide the functionality of one or more of the IMS nodes described herein.

<FIG> is a schematic block diagram of the UE <NUM> according to some embodiments of the present disclosure. As illustrated, the UE <NUM> includes one or more processors <NUM> (e.g., CPUs, ASICs, FPGAs, and/or the like), memory <NUM>, and one or more transceivers <NUM> each including one or more transmitters <NUM> and one or more receivers <NUM> coupled to one or more antennas <NUM>. The transceiver(s) <NUM> includes radio-front end circuitry connected to the antenna(s) <NUM> that is configured to condition signals communicated between the antenna(s) <NUM> and the processor(s) <NUM>, as will be appreciated by on of ordinary skill in the art. The processors <NUM> are also referred to herein as processing circuitry. The transceivers <NUM> are also referred to herein as radio circuitry. In some embodiments, the functionality of the UE <NUM> described above may be fully or partially implemented in software that is, e.g., stored in the memory <NUM> and executed by the processor(s) <NUM>. Note that the UE <NUM> may include additional components not illustrated in <FIG> such as, e.g., one or more user interface components (e.g., an input/output interface including a display, buttons, a touch screen, a microphone, a speaker(s), and/or the like and/or any other components for allowing input of information into the UE <NUM> and/or allowing output of information from the UE <NUM>), a power supply (e.g., a battery and associated power circuitry), etc..

Claim 1:
A method for Internet Protocol, IP, Multimedia Subsystem, IMS, routing, the method comprising:
• at an Interrogating Call Session Control Function, I-CSCF, (<NUM>, <NUM>):
∘ receiving (<NUM>; <NUM>) a Session Initiation Protocol, SIP, invite message for an incoming session, the SIP invite message comprising an Internet Protocol, IP, Multimedia Public Identity, IMPU, of a target User Equipment, UE;
∘ sending (<NUM>; <NUM>), to a Home Subscriber Server, HSS, (<NUM>), a query for Serving Call Session Control Functions, S-CSCFs, (<NUM>, <NUM>) having registrations for the IMPU of the target UE;
∘ receiving (<NUM>; <NUM>), from the HSS (<NUM>), information that indicates two or more S-CSCFs (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>) having registrations for the IMPU of the target UE; and
∘ forwarding (<NUM> or <NUM>; 610a-610b) the SIP invite message to at least one of the two or more S-CSCFs (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>);
• at the HSS (<NUM>):
∘ receiving (<NUM>; <NUM>) the query from the I-CSCF (<NUM>, <NUM>);
∘ obtaining (<NUM>; <NUM>) the information that indicates the two or more S-CSCFs (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>) having registrations for the IMPU of the target UE; and
∘ sending (<NUM>; <NUM>), to the I-CSCF (<NUM>, <NUM>), the information that indicates the two or more S-CSCFs (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>) having registrations for the IMPU of the target UE; and
• at each S-CSCF (<NUM>, <NUM>) of the at least one of the two or more S-CSCFs (<NUM>-<NUM>, <NUM>-<NUM>, <NUM>), wherein each S-CSCF (<NUM>, <NUM>) is included in an IMS network slice (<NUM>-<NUM>, ..., <NUM>-N):
o receiving (<NUM> or <NUM>; 610a-610b) the SIP invite message from the I-CSCF (<NUM>, <NUM>) for the incoming session for the target UE, wherein the target UE has two or more IMS registrations over two or more IMS Protocol Data Unit, PDU, sessions established using different core network slices using a same IP Multimedia Private Identity, IMPI, and IMPU, and each of the two or more IMS registrations is associated to one of two or more S-CSCFs;
o determining (<NUM> or <NUM>; 612a or 612b) whether to accept the incoming session based on the SIP invite message;
∘ if determined to reject the incoming session, sending (<NUM>; 614a) a message to the I-CSCF (<NUM>, <NUM>) that indicates an error or rejection; and
∘ if determined to accept the incoming session, forwarding (<NUM>; 614b) the SIP invite message to another node (<NUM>).