Network nodes and methods therein for enhanced network slice replacement

Abstract

The present disclosure provides a method (300) in a source Network Function, NF. The method (300) includes: transmitting (310), to a target NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a Packet Data Unit, PDU, session transferred from the second network slice to the first network slice.

Description

[0001] NETWORK NODES AND METHODS THEREIN FOR ENHANCED NETWORK SLICE REPLACEMENT TECHNICAL FIELD The present disclosure relates to communication technology, and more particularly, to network nodes and methods therein for enhanced network slice replacement. BACKGROUND Network Slice Replacement is described in the 3rd Generation Partnership Project (3GPP) Technical Specification (TS) 23.501, V18.3.0, which is incorporated herein by reference in its entirety. This feature is used to replace a network slice (or a Single Network Slice Selection Assistance Information (S-NSSAI)) with an alternative network slice (or alternative S-NSSAI) when the network slice becomes unavailable or congested in order to enable network slice service continuity. According to the 3GPP TS 23.501 and TS 24.501, V18.4.0, which is incorporated herein by reference in its entirety, an Access and Mobility Management Function (AMF) may determine that an S-NSSAI is to be replaced with an alternative S-NSSAI, based on a notification from a Network Slice Selection Function (NSSF), Policy Control Function (PCF), or Operation Administration and Maintenance (OAM), and then trigger Mobility Management as defined in Section 4.6.2.7 of TS 24.501. If the User Equipment (UE) and network support network slice replacement, and the AMF determines that an S-NSSAI included in the allowed NSSAI needs to be replaced with an alternative S-NSSAI, the AMF provides the alternative S-NSSAI in the allowed NSSAI (if not included yet) and in the configured NSSAI (if not included yet) and the mapping information between the S-NSSAI to be replaced and the alternative S-NSSAI to the UE during the UE configuration update procedure or during the registration procedure. Session Management as defined in Section 5.15.19 of TS 23.501 includes: o New Packet Data Unit (PDU) Session Establishment: If the UE has received together with the Allowed NSSAI a mapping of the S-NSSAI to an Alternative S-NSSAI, the UE shall provide both the Alternative S-NSSAI and the S-NSSAI in the PDU Session Establishment message. When the AMF receives the Alternative S-NSSAI and the S-NSSAI in the PDU Session Establishment message, the AMF includes both the Alternative S-NSSAI and the S-NSSAI to the Session Management Function (SMF) in Nsmf_PDUSession_CreateSMContext service operation. The SMF proceeds with the PDU Session establishment using the Alternative S-NSSAI. The SMF sends the Alternative S-NSSAI to Next Generation Radio Access Network (NG-RAN) in N2 Session Management (SM) information and to UE in PDU Session Establishment Accept message. o For an existing PDU Session associated with an S-NSSAI that is replaced with the Alternative S-NSSAI, after the AMF sends mapping of the S-NSSAI to the Alternative S-NSSAI to the supporting UE in UE Configuration Update message, the AMF sends updates to the SMF of the PDU Session, e.g. by triggering Nsmf_PDUSession_UpdateSMContext service operation, that the PDU Session is to be transferred to Alternative S-NSSAI and includes the Alternative S-NSSAI as follows: ● If the SMF determines that the PDU Session is to be retained (e.g., if the anchor User Plane Function (UPF) can be reused with the alternative S-NSSAI and Session and Service Continuity (SSC) mode 1), the SMF sends the Alternative S-NSSAI to the UPF in the N4 message, to the NG-RAN in N2 message and to the supporting UE in PDU Session Modification Command message. The S-NSSAI provided to the (R)AN and to the UPF is the Alternative S-NSSAI. ● If the SMF determines that the PDU Session is to be re-established, the SMF sends the Alternative S-NSSAI to the supporting UE either in PDU Session Modification Command if the PDU Session is of SSC mode 3, or in PDU Session Release if the PDU Session is of SSC mode 2 or SSC mode 1, to trigger the re-establishment of the PDU Session. The UE includes both, the S-NSSAI and the Alternative S-NSSAI in the PDU Session Establishment message. When the AMF is notified that the replaced S-NSSAI is available again (e.g., the congestion of the S-NSSAI has been mitigated), if the AMF has configured the supporting UE with the Alternative S-NSSAI, and the AMF determines for the UE to use the replaced S-NSSAI again, the AMF reconfigures the supporting UE (e.g., by using UE Configuration Update procedure or in the next registration procedure) to use the replaced S-NSSAI again by removing the mapping of the replaced S-NSSAI to Alternative S-NSSAI. If there is an existing PDU Session associated with the Alternative S-NSSAI, the AMF updates the SMF of the PDU Session, by using Nsmf_PDUSession_UpdateSMContext service operation, thereby causing the PDU Session to be transferred to the S-NSSAI. SUMMARY Currently, for example in a mobility scenario, only the Alternative S-NSSAI is signaled from a source (old) AMF to a target (new) AMF, or from a source (old) SMF (or Intermediate SMF, I-SMF, or Visited SMF, V-SMF) to a target (new) SMF (or I-SMF or V-SMF). Thus, the target AMF or (I- / V-)SMF does not know that network slice replacement had happened to the PDU Session. Accordingly, the target AMF or (I- / V-)SMF may not conduct the proper behavior when e.g., the replaced S-NSSAI becomes available again. Fig.1 shows an intra Public Land Mobile Network (PLMN) inter AMF mobility scenario, in which AMF1 is a source AMF and AMF2 is a target AMF. As shown, at Step 0, a UE registers in a 5th Generation System (5GS). At Step 1a (and 1b), an NSSF notifies AMF1 (and AMF2) that an S-NSSAI (denoted as S-NSSAI1) is to be replaced with an Alternative S-NSSAI (denoted as S-NSSAI2) using Nnssf_NSSAIAvailability_Notify. At Step 2, AMF1 initiates a UE Configuration Update procedure to provide the Alternative S-NSSAI (S-NSSAI2) and the mapping information between the S-NSSAI to be replaced (S-NSSAI1) and the alternative S-NSSAI (S-NSSAI2) to the UE. At Step 3, the UE initiates a PDU session establishment procedure, providing the replaced S-NSSAI (S-NSSAI1) and the alternative S-NSSAI (S-NSSAI2) to the AMF in a PDU Session Establishment message, and the AMF sends the replaced S-NSSAI (S-NSSAI1) and the alternative S-NSSAI (S-NSSAI2) to an SMF using an Nsmf_PDUSession_CreateSMContext service operation. The SMF proceeds with PDU Session establishment using the alternative S-NSSAI (S-NSSAI2). The SMF sends the alternative S-NSSAI (S-NSSAI2) to NG-RAN in N2 SM information and to UE in a PDU Session Establishment Accept message. At Step 4, the UE triggers a Mobility Registration Update procedure, and sends a Registration Request to AMF2. At Step 5, AMF2 sends an Namf_Communication_UEContextTransfer request to AMF1 to retrieve a UE Context. At Step 6, AMF1 replies with an Namf_Communication_UEContextTransfer response, including PduSessionContext containing S-NSSAI2. At Step 7, the steps 6-25 of Fig. 4.2.2.2.2-1 in TS 23.502, V18.3.0 (which is incorporated herein by reference in its entirety) are performed. At Step 8, the NSSF notifies AMF2 that S-NSSAI1 becomes available again using Nnssf_NSSAIAvailability_Notify. However, in this case, the target AMF, AMF2, is not aware of the network slice replacement that had happened to the UE. Accordingly, the target AMF cannot know whether the PDU session associated with S-NSSAI2 is a PDU session transferred from S-NSSAI1 to S-NSSAI2 as a result of network slice replacement or a PDU session established directly on S-NSSAI2. Therefore, when AMF2 is notified that S-NSSAI1 becomes available again at Step 8, it does not know what its proper behavior should be, e.g., whether to transfer the PDU session to S-NSSAI1. Fig.2 shows a V-SMF change scenario, in which V-SMF1 is a source V-SMF and V-SMF2 is a target V-SMF. As shown, at Step 0, a UE registers in a 5GS. Here, a network slice replacement is performed for a network slice of a serving PLMN (i.e., Visited PLMN (VPLMN)), with an S-NSSAI (S-NSSAI1) being replaced with an alternative S-NSSAI (S-NSSAI2). The UE then establishes a new PDU Session with the alternative S-NSSAI (S-NSSAI2) and the S-NSSAI (S-NSSAI1). At Step 1, the UE initiates a Service Request procedure, and sends a Service Request to an AMF. After receiving the Service Request, the AMF finds that V-SMF1 cannot serve the current tracking area where the UE is located, and thus V-SMF change (Steps 2-5) is needed. At Step 2, the AMF sends an Nsmf_PDUSession_CreateSMContext Request to V-SMF2. At Step 3, V-SMF2 sends an Nsmf_PDUSessionContext Request to V-SMF1 to retrieve an SM Context, and V-SMF1 replies with an Nsmf_PDUSessionContext Response containing S-NSSAI2. At Step 4, V-SMF2 sends an Nsmf_PDUSession_Update Request to an SMF in a Home PLMN (HPLMN), and the SMF replies with an Nsmf_PDUSession_Update Response. At Step 5, V-SMF2 sends an Nsmf_PDUSession_CreateSMContext Response to the AMF. At Step 6, the steps 17-26 of Fig.4.23.4.3-1 in TS 23.502 are performed. At Step 7, an NSSF notifies the AMF that S-NSSAI1 becomes available again using Nnssf_NSSAIAvailability_Notify. At Step 8, the AMF initiates a UE Configuration Update procedure to notify that S-NSSAI1 becomes available again. At Step 9, the AMF sends Nsmf_PDUSession_UpdateSMContext to V-SMF2, including S-NSSAI1 to notify that S-NSSAI1 becomes available again. However, in this case, the target V-SMF, V-SMF2, is not aware of the network slice replacement that had happened to the UE. Accordingly, the target V-SMF cannot know whether a PDU session associated with S-NSSAI2 is a PDU session transferred from S-NSSAI1 to S-NSSAI2 as a result of network slice replacement or a PDU session established directly on S-NSSAI2. Therefore, when V-SMF2 is notified that S-NSSAI1 becomes available again at Step 9, it does not know what its proper behavior should be, e.g., whether to transfer the PDU session to S-NSSAI1. In addition, since V-SMF2 is not aware of the network slice replacement, it may only include S-NSSAI2 in charging information to a charging function (not shown), which may cause inconsistent charging for the UE (e.g., when S-NSSAI1 and S-NSSAI2 have different charging rates). It is an object of the present disclosure to provide network nodes and methods therein, capable of solving or mitigating at least one of the above problems. According to a first aspect of the present disclosure, a method in a source Network Function (NF) is provided. The method includes transmitting, to a target NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a PDU session transferred from the second network slice to the first network slice. In an embodiment, the source NF may be a source AMF, and the target NF may be a target AMF, and the information on the first network slice and the information on the second network slice may be transmitted in PDU session context information. In an embodiment, the source NF may be a source SMF or I-SMF or V-SMF, and the target NF may be a target SMF or I-SMF or V-SMF, and the information on the first network slice and the information on the second network slice may be transmitted in SM context information. In an embodiment, the information on the first network slice may be carried in an Information Element (IE) indicating an S-NSSAI associated with the PDU session, and the information on the second network slice may be carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session. Alternatively, the information on the second network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the first network slice may be carried in an IE indicating an S-NSSAI that replaces the S-NSSAI associated with the PDU session. In an embodiment, the method may further include, prior to transmitting the information on the first network slice and the information on the second network slice: retrieving an NF profile of the target NF, the NF profile containing information indicating that the target NF supports network slice replacement. In an embodiment, the information on the first network slice and the information on the second network slice may be transmitted in a request. In an embodiment, the method may further include receiving, from the target NF, a request containing information indicating that the target NF supports network slice replacement. The information on the first network slice and the information on the second network slice may be transmitted in response to the request. According to a second aspect of the present disclosure, a method in a target NF is provided. The method includes receiving, from a source NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a PDU session transferred from the second network slice to the first network slice. The method further includes receiving a notification that the second network slice becomes available. The method further includes replacing the first network slice with the second network slice for the PDU session. In an embodiment, the source NF may be a source AMF and the target NF may be a target AMF, and the information on the first network slice and the information on the second network slice may be received in PDU session context information. In an embodiment, the operation of replacing may include transmitting, to a terminal device, a configuration update message excluding the information on the second network slice as a network slice to be replaced and the information on the first network slice as a corresponding alternative network slice. In an embodiment, the operation of replacing may further include transmitting, to an SMF, an update session management context message indicating that the PDU session is to be transferred to the second network slice. In an embodiment, the source NF may be a source SMF or I-SMF or V-SMF, and the target NF may be a target SMF or I-SMF or V-SMF, and the information on the first network slice and the information on the second network slice may be received in SM context information. In an embodiment, the operation of replacing may include transmitting, in response to determining that the PDU session is to be retained, the information on the second network slice to a UPF and an NG-RAN node associated with the PDU session; or transmitting, in response to determining that the PDU session is to be re-established, the information on the second network slice to a terminal device associated with the PDU session. In an embodiment, the method may further include transmitting, to a Charging Function (CHF), a charging data request for the PDU session, the charging data request including the information on the second network slice. In an embodiment, the information on the first network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the second network slice may be carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session. Alternatively, the information on the second network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the first network slice may be carried in an IE indicating an S-NSSAI that replaces the S-NSSAI associated with the PDU session. In an embodiment, the method may further include transmitting, to a Network Repository Function (NRF), a registration message containing information indicating that the target NF supports network slice replacement. In an embodiment, the information on the first network slice and the information on the second network slice may be received in a request. In an embodiment, the method may further include transmitting, to the source NF, a request containing information indicating that the target NF supports network slice replacement. In an embodiment of any of the first or second aspect, the PDU session may be established or may have been established for a terminal device which moves or has moved from the source NF to the target NF wherein serving or handling of the terminal device may be or may have been transferred from the source NF to the target NF. Therein, a PDU session context for the PDU session may be transferred from the source NF to the target NF. The terminal device may for example be a user equipment, UE. According to a third aspect of the present disclosure, a network node is provided. The network node includes a communication interface, a processor, and a memory. The memory contains instructions executable by the processor whereby the network node is operative to, when implementing a source NF, perform the method according to the above first aspect, or when implementing a target NF, perform the method according to the above second aspect. According to a fourth aspect of the present disclosure, a computer-readable storage medium is provided. The computer-readable storage medium has computer-readable instructions stored thereon. The computer-readable instructions, when executed by a processor of a network node, configure the network node to, when implementing a source NF, perform the method according to the above first aspect, or when implementing a target NF, perform the method according to the above second aspect. With the embodiments of the present disclosure, a target NF can be notified of information on a first network slice and information on a second network slice that has been replaced by the first network slice for a PDU session transferred from the second network slice to the first network slice. Accordingly, the target NF is aware that the PDU session is a PDU session transferred from the second network slice to the first network slice as a result of network slice replacement, rather than a PDU session established directly on the first network slice. In this way, when the target NF is later notified that the second network slice becomes available again, it can operate properly, e.g., transferring the PDU session back to the second network slice. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages will be more apparent from the following description of embodiments with reference to the figures, in which: Fig.1 is a schematic diagram showing an intra PLMN inter AMF mobility scenario; Fig.2 is a schematic diagram showing a V-SMF change scenario; Fig.3 is a flowchart illustrating a method in a source NF according to an embodiment of the present disclosure; Fig.4 is a flowchart illustrating a method in a target NF according to an embodiment of the present disclosure; Fig.5 is a sequence diagram showing a process of intra PLMN inter AMF mobility according to an embodiment of the present disclosure; Fig.6 is a schematic diagram showing a process of V-SMF change according to an embodiment of the present disclosure; and Fig.7 is a block diagram of a network node according to an embodiment of the present disclosure. DETAILED DESCRIPTION In the present disclosure, a network function, or NF, can be implemented either as a network element on a dedicated hardware, as a software instance running on a dedicated hardware, or as a virtualized function instantiated on an appropriate platform, e.g. on a cloud infrastructure. The term “network node” refers to any physical or virtual node configured to implement a network function. The term "terminal device" or “UE” refers to any end device that can access a wireless communication network and receive services therefrom. By way of example and not limitation, the terminal device refers to a mobile terminal, user equipment (UE), or other suitable devices. The UE may be, for example, a Subscriber Station (SS), a Portable Subscriber Station, a Mobile Station (MS), or an Access Terminal (AT). The terminal device may include, but not limited to, portable computers, desktop computers, image capture terminal devices such as digital cameras, gaming terminal devices, music storage and playback appliances, a mobile phone, a cellular phone, a smart phone, voice over IP (VoIP) phones, wireless local loop phones, tablets, personal digital assistants (PDAs), wearable terminal devices, vehicle-mounted wireless terminal devices, wireless endpoints, mobile stations, laptop-embedded equipment (LEE), laptop-mounted equipment (LME), USB dongles, smart devices, wireless customer-premises equipment (CPE) and the like. In the following description, the terms "terminal device", "terminal", "user equipment" and "UE" may be used interchangeably. As one example, a terminal device may represent a UE configured for communication in accordance with one or more communication standards promulgated by the 3rd Generation Partnership Project (3GPP), such as 3GPP's Global System for Mobile Communications (GSM), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE), and / or the 5th Generation (5G) standards. As used herein, a "user equipment" or "UE" may not necessarily have a "user" in the sense of a human user who owns and / or operates the relevant device. In some embodiments, a terminal device may be configured to transmit and / or receive information without direct human interaction. For instance, a terminal device may be designed to transmit information to a network on a predetermined schedule, when triggered by an internal or external event, or in response to requests from the wireless communication network. Instead, a UE may represent a device that is intended for sale to, or operation by, a human user but that may not initially be associated with a specific human user. References in the specification to "one embodiment," "an embodiment," "an example embodiment," and the like indicate that the embodiment described may include a particular feature, structure, or characteristic, but it is not necessary that every embodiment includes the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. It shall be understood that although the terms "first" and "second" etc. may be used herein to describe various elements not limiting these elements. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed terms. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. 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", "has", "having", "includes" and / or "including", when used herein, specify the presence of stated features, elements, and / or components etc., but do not preclude the presence or addition of one or more other features, elements, components and / or combinations thereof. In the following description and claims, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this disclosure belongs. Fig.3 is a flowchart illustrating a method 300 according to an embodiment of the present disclosure. The method 300 can be performed by a source NF, e.g., a source AMF or a source SMF (or I-SMF or V-SMF). At block 310, the source NF transmits, to a target NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a PDU session transferred from the second network slice to the first network slice. In an example, the source NF may be a source AMF and the target NF may be a target AMF, and the information on the first network slice and the information on the second network slice may be transmitted in PDU session context information (PduSessionContext). The information on the first network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the second network slice may be carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session. For example, PduSessionContext in the 3GPP TS 29.518 can be extended to include information on the replaced network slice, as shown in Table 1 below. Table 1 – PduSessionContext Attribute name Data type P Cardinality Description Applicability pduSessionId PduSessionId M 1 Indicates the identifier of the PDU Session. smContextRef Uri M 1 Indicates the resource URI of the SM context, including the apiRoot (see clause 6.1.3.3.2 of 3GPP TS 29.502). When present, it shall carry the URI of SM Context of: - I-SMF, for a PDU session with I-SMF; or - V-SMF, for HR PDU session; or - SMF, for non-roaming PDU session without I-SMF, or LBO roaming PDU session; sNssai Snssai M 1 Indicates the associated S-NSSAI for the PDU Session. It shall be the S-NSSAI in HPLMN in non-roaming, LBO roaming or HR roaming. additionalSnssai Snssai C 0..1 This IE shall be present in intra-VPLMN mobility of LBO roaming and HR roaming. When present, this IE shall indicate the associated S-NSSAI in VPLMN for the PDU Session. … … … … … replacedSnssai Snssai C 0..1 This IE shall be present if the network slice indicated in the sNssai IE replaced this network slice. It shall be the replaced S-NSSAI in HPLMN in non-roaming, LBO roaming, and HR roaming. replacedAddition Snssai C 0..1 This IE shall be present if the network alSnssai slice indicated in the additionalSnnsai IE replaced this network slice. When present, this IE shall indicate the replaced associated S-NSSAI in VPLMN for the PDU Session. In Table 1, a new Information Element (IE) “replacedSnssai” is introduced, indicating the S-NSSAI of the network slice replaced by the network slice indicated in “sNssai”. This IE is used to indicate the replaced S-NSSAI in HPLMN in non-roaming, Local Break Out (LBO) roaming, and Home Routed (HR) roaming. Another new IE “replacedAdditionalSnssai” indicates the S-NSSAI of the network slice replaced by the network slice indicated in “additionalSnnsai”, in VPLMN of the PDU session. It is to be noted that the terms “network slice” and “S-NSSAI” are used interchangeably in the context of the present disclosure. Alternatively, the information on the second network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the first network slice may be carried in an IE indicating an S-NSSAI that replaces the S-NSSAI associated with the PDU session. For example, as an alternative to Table 1, the IEs “sNssai” and “additionalSnnsai” in PduSessionContext in the 3GPP TS 29.518 can be used to indicate information on the replaced network slice, as shown in Table 2 below.

[0002] Table 2 – PduSessionContext Attribute name Data type P Cardinality Description Applicability pduSessionId PduSessionId M 1 Indicates the identifier of the PDU Session. smContextRef Uri M 1 Indicates the resource URI of the SM context, including the apiRoot (see clause 6.1.3.3.2 of 3GPP TS 29.502). When present, it shall carry the URI of SM Context of: - I-SMF, for a PDU session with I-SMF; or - V-SMF, for HR PDU session; or - SMF, for non-roaming PDU session without I-SMF, or LBO roaming PDU session; sNssai Snssai M 1 Indicates the associated S-NSSAI for the PDU Session. It shall be the S-NSSAI in HPLMN in non-roaming, LBO roaming or HR roaming. additionalSnssai Snssai C 0..1 This IE shall be present in intra-VPLMN mobility of LBO roaming and HR roaming. When present, this IE shall indicate the associated S-NSSAI in VPLMN for the PDU Session. … … … … … altSnssai Snssai C 0..1 This IE shall be present if network slice NSRP indicated in the sNssai IE is requested to be replaced. It shall be the Alternative S-NSSAI in HPLMN in non-roaming, LBO roaming, and HR roaming. altAdditionalSnss Snssai C 0..1 This IE shall be present if network slice NSRP ai indicated in the additionalSnssai IE is requested to be replaced. When present, this IE shall indicate the Alternative associated S-NSSAI in VPLMN for the PDU Session. In Table 2, the IE “sNssai” indicates the S-NSSAI of the network slice replaced by the network slice indicated in a new IE “altSnssai”, in HPLMN in non-roaming, LBO roaming, and Home Routed (HR) roaming. The IE “additionalSnnsai” indicates the S-NSSAI of the network slice replaced by the network slice indicated in a new IE “altAdditionalSnssai”, in VPLMN of the PDU session. For further details of PduSessionContext, reference can be made to the 3GPP TS 29.518, V18.3.0, which is incorporated herein by reference in its entirety. In another example, the source NF may be a source SMF (or I-SMF or V-SMF) and the target NF may be a target SMF (or I-SMF or V-SMF), and the information on the first network slice and the information on the second network slice may be transmitted in SM context information (SmContext). The information on the first network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the second network slice is carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session. For example, SmContext in the 3GPP TS 29.502 can be extended to include information on the replaced network slice, as shown in Table 3 below. Table 3 – SmContext Attribute name Data type P Cardi Description Applica nality bility pduSessionId PduSessionId M 1 This IE shall contain the PDU Session ID. dnn Dnn M 1 This IE shall contain the UE requested DNN of the PDU session. The DNN shall be the full DNN (i.e. with both the Network Identifier and Operator Identifier) for a HR PDU session, and it should be the full DNN in LBO and non-roaming scenarios. If the Operator Identifier is absent, the serving core network operator shall be assumed. selectedDnn Dnn C 0..1 This IE shall be present, if another DNN other than the UE requested DNN is selected for this PDU session. When present, it shall contain the selected DNN. The DNN shall be the full DNN (i.e. with both the Network Identifier and Operator Identifier) for a HR PDU session, and it should be the full DNN in LBO and non-roaming scenarios. If the Operator Identifier is absent, the serving core network operator shall be assumed. sNssai Snssai M 1 This IE shall contain the S-NSSAI for the serving PLMN. hplmnSnssai Snssai C 0..1 This IE shall be present for a HR PDU session. When present, it shall contain the S-NSSAI for the HPLMN. … … … … … replacedSnssai Snssai C 0..1 This IE shall be present if the network slice indicated NSRP in the sNssai IE replaced this network slice triggered by the network slice replacement. In this case, the NF service producer shall send the S-NSSAI and the alternative S-NSSAI. replacedHplmnS Snssai C 0..1 This IE shall be present for HR PDU session if the NSRP nssai network slice indicated in the hplmnSnssai IE replaced this network slice triggered by the network slice replacement. In this case, the NF service producer shall send the HPLMN S-NSSAI and the alternative HPLMN S-NSSAI. In Table 3, a new IE “replacedSnssai” is introduced, indicating the S-NSSAI of the network slice replaced by the network slice indicated in “sNssai”. Another new IE “replacedHplmnSnssai” indicates the S-NSSAI of the network slice replaced by the network slice indicated in “hplmnSnssai”. The information on the first network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the second network slice is carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session. As an alternative to Table 3, the IEs “sNssai” and “hplmnSnssai” in SmContext in the 3GPP TS 29.502 can be used to indicate information on the replaced network slice, as shown in Table 4 below. Table 4 – SmContext Attribute name Data type P Cardi Description Applica nality bility pduSessionId PduSessionId M 1 This IE shall contain the PDU Session ID. dnn Dnn M 1 This IE shall contain the UE requested DNN of the PDU session. The DNN shall be the full DNN (i.e. with both the Network Identifier and Operator Identifier) for a HR PDU session, and it should be the full DNN in LBO and non-roaming scenarios. If the Operator Identifier is absent, the serving core network operator shall be assumed. selectedDnn Dnn C 0..1 This IE shall be present, if another DNN other than the UE requested DNN is selected for this PDU session. When present, it shall contain the selected DNN. The DNN shall be the full DNN (i.e. with both the Network Identifier and Operator Identifier) for a HR PDU session, and it should be the full DNN in LBO and non-roaming scenarios. If the Operator Identifier is absent, the serving core network operator shall be assumed. sNssai Snssai M 1 This IE shall contain the S-NSSAI for the serving PLMN. hplmnSnssai Snssai C 0..1 This IE shall be present for a HR PDU session. When present, it shall contain the S-NSSAI for the HPLMN. … … … … … altSnssai Snssai C 0..1 This IE shall be present if the NF service consumer NSRP supports the network slice replacement, and the network slice indicated in the sNssai IE is requested to be replaced. In this case, the NF service producer shall send the S-NSSAI and the alternative S-NSSAI. altHplmnSnssai Snssai C 0..1 This IE shall be present for HR PDU session if the NF NSRP service consumer supports the network slice replacement, and the network slice indicated in the sNssai IE is requested to be replaced. In this case, the NF service producer shall send the HPLMN S-NSSAI and the alternative HPLMN S-NSSAI. In Table 4, the IE “sNssai” indicates the S-NSSAI of the network slice replaced by the network slice indicated in a new IE “altSnssai”. The IE “hplmnSnssai” indicates the S-NSSAI of the network slice replaced by the network slice indicated in a new IE “altHplmnSnssai”. For further details of SmContext, reference can be made to the 3GPP TS 29.502, V18.4.0, which is incorporated herein by reference in its entirety. In an example, e.g., before the block 310, the source NF may retrieve an NF profile of the target NF, e.g., from an NRF. The NF profile contains information indicating that the target NF supports network slice replacement. In the block 310, the information on the first network slice and the information on the second network slice may be transmitted in a request, e.g., in Namf_Communication_CreateUEContext Request in a handover procedure. Alternatively, the source NF may receive, from the target NF, a request containing information indicating that the target NF supports network slice replacement. The information on the first network slice and the information on the second network slice may be transmitted in the block 310 in response to the request. For example, when the target NF is a target AMF, a new feature may be introduced in Clause 6.1.8 of TS 29.518, as shown in Table 5 below. Table 5 – Supported Feature Feature Feature M / O Description Number … … … … xx NSRP O Network Slice Replacement An AMF that supports this feature shall support network slice replacement as specified in clause 5.15.19 of 3GPP TS 23.501. The feature can be included in e.g., Namf_Communication_UEContextTransfer request, or in an NF profile registered to the NRF for retrieval by the source AMF. When the target NF is a target SMF (or I-SMF or V-SMF), a feature “NSRP” is defined in Clause 6.1.8 of TS 29.502. The feature can be included in e.g., Nsmf_PDUSessionContext request, or in an NF profile registered to the NRF for retrieval by the source SMF (or I-SMF or V-SMF). In the above Tables 1~4, the presence of applicability “NSRP” means that the corresponding IE is used only when the feature “NSRP” is supported, whereas the absence of applicability “NSRP” means that the corresponding IE can be ignored when the feature “NSRP” is not supported. Fig.4 is a flowchart illustrating a method 400 according to an embodiment of the present disclosure. The method 400 can be performed by a target NF, e.g., a target AMF or a target SMF (or I-SMF or V-SMF). At block 410, the target NF receives, from a source NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a PDU session transferred from the second network slice to the first network slice. At block 420, the target NF receives a notification that the second network slice becomes available. At block 430, the target NF replaces the first network slice with the second network slice for the PDU session. In an example, the source NF may be a source AMF, and the target NF may be a target AMF, and the information on the first network slice and the information on the second network slice may be received in PDU session context information (PduSessionContext). The information on the first network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the second network slice is carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session, see Table 1 above for example. Alternatively, the information on the second network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the first network slice may be carried in an IE indicating an S-NSSAI that replaces the S-NSSAI associated with the PDU session, see Table 2 above for example. In response to receiving the notification, e.g., from an NSSF, in the block 420, the target AMF may, in the block 430, transmit, to a terminal device, a configuration update message excluding the information on the second network slice as a network slice to be replaced and the information on the first network slice as a corresponding alternative network slice, and transmit, to an SMF, an update session management context message indicating that the PDU session is to be transferred to the second network slice. In another example, the source NF may be a source SMF (or I-SMF or V-SMF), and the target NF may be a target SMF or I-SMF or V-SMF, and the information on the first network slice and the information on the second network slice may be received in SM context information (SmContext). The information on the first network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the second network slice is carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session, see Table 3 above for example. Alternatively, the information on the second network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the first network slice may be carried in an IE indicating an S-NSSAI that replaces the S-NSSAI associated with the PDU session, see Table 4 above for example. In response to receiving the notification, e.g., from an AMF, in the block 420, the target SMF (or I-SMF or V-SMF) may, in the block 430, transmit, in response to determining that the PDU session is to be retained, the information on the second network slice to a UPF and an NG-RAN node associated with the PDU session, e.g., in an N4 message and an N2 message, respectively. Alternatively, in the block 430, the target SMF (or I-SMF or V-SMF) may transmit, in response to determining that the PDU session is to be re-established, the information on the second network slice to a terminal device associated with the PDU session, e.g., in PDU Session Modification Command or PDU Session Release, to trigger the re-establishment of the PDU Session. Additionally, in response to receiving the notification, e.g., from an AMF, in the block 420, the target SMF (or I-SMF or V-SMF) may transmit, to a CHF, a charging data request for the PDU session. The charging data request includes the information on the second network slice. This allows the CHF to charge a terminal device for the PDU session consistently despite the network slice replacement, e.g., based on a charging rate corresponding to the second network slice. In an example, the target NF may transmit, to an NRF, a registration message containing information indicating that the target NF supports network slice replacement. In the block 410, the information on the first network slice and the information on the second network slice may be received in a request, e.g., in Namf_Communication_CreateUEContext Request in a handover procedure. Alternatively, the target NF may transmit, to the source NF, a request containing information indicating that the target NF supports network slice replacement. For details of this information, reference can be made to the feature “NSRP” as described above in connection with the method 300. The methods 300 and 400 will be further explained below with reference to Figs.5 and 6. Fig.5 shows an intra PLMN inter AMF mobility scenario according to an embodiment of the present disclosure. In this example, AMF1 is a source AMF and AMF2 is a target AMF. As shown, at Step 0, a UE registers in a 5GS. At Step 1a (and 1b), an NSSF notifies AMF1 (and AMF2) that an S-NSSAI (denoted as S-NSSAI1) is to be replaced with an Alternative S-NSSAI (denoted as S-NSSAI2) using Nnssf_NSSAIAvailability_Notify. At Step 2, AMF1 initiates a UE Configuration Update procedure to provide the Alternative S-NSSAI (S-NSSAI2) and the mapping information between the S-NSSAI to be replaced (S-NSSAI1) and the alternative S-NSSAI (S-NSSAI2) to the UE. At Step 3, the UE initiates a PDU session establishment procedure, providing the replaced S-NSSAI (S-NSSAI1) and the alternative S-NSSAI (S-NSSAI2) to the AMF in a PDU Session Establishment message, and the AMF sends the replaced S-NSSAI (S-NSSAI1) and the alternative S-NSSAI (S-NSSAI2) to an SMF using an Nsmf_PDUSession_CreateSMContext service operation. The SMF proceeds with PDU Session establishment using the alternative S-NSSAI (S-NSSAI2). The SMF sends the alternative S-NSSAI (S-NSSAI2) to NG-RAN in N2 SM information and to UE in a PDU Session Establishment Accept message. At Step 4, the UE triggers a Mobility Registration Update procedure, and sends a Registration Request to AMF2. At Step 5, AMF2 sends an Namf_Communication_UEContextTransfer request to AMF1 to retrieve a UE Context. At Step 6, AMF1 replies with an Namf_Communication_UEContextTransfer response, including PduSessionContext containing both S-NSSAI1 and S-NSSAI2 (see Table 1 or 2 above), such that AMF2 is now aware that the PDU session is a PDU session transferred from S-NSSAI1 to S-NSSAI2 as a result of network slice replacement. At Step 7, the steps 6-25 of Fig.4.2.2.2.2-1 in TS 23.502, V18.3.0 (which is incorporated herein by reference in its entirety) are performed. At Step 8, the NSSF notifies AMF2 that S-NSSAI1 becomes available again using Nnssf_NSSAIAvailability_Notify. At Step 9, AMF2 initiates a UE configuration update procedure to provide the updated Alternative NSSAI excluding S-NSSAI1 as an S-NSSAI to be replaced and the corresponding alternative S-NSSAI (S-NSSAI2) to the UE. At Step 10, AMF2 sends an Nsmf_PDUSession_UpdateSMContext request to the SMF, causing the PDU Session to be transferred to S-NSSAI1. At Step 11, the SMF sends an Nsmf_PDUSession_UpdateSMContext response to AMF2. At Step 12, the SMF triggers a PDU Session Modification or Release procedure, to transfer the PDU session back to S-NSSAI1, as described above and as defined in Section 5.15.19 of TS 23.501. Fig.6 shows a V-SMF change scenario according to an embodiment of the present disclosure. In this example, V-SMF1 is a source V-SMF and V-SMF2 is a target V-SMF. As shown, at Step 0, a UE registers in a 5GS. Here, a network slice replacement is performed for a network slice of a serving PLMN (i.e., Visited PLMN (VPLMN)), with an S-NSSAI (S-NSSAI1) being replaced with an alternative S-NSSAI (S-NSSAI2). The UE then establishes a new PDU Session with the alternative S-NSSAI (S-NSSAI2) and the S-NSSAI (S-NSSAI1). At Step 1, the UE initiates a Service Request procedure, and sends a Service Request to an AMF. After receiving the Service Request, the AMF finds that V-SMF1 cannot serve the current tracking area where the UE is located, and thus V-SMF change (Steps 2-5) is needed. At Step 2, the AMF sends an Nsmf_PDUSession_CreateSMContext Request to V-SMF2. At Step 3, V-SMF2 sends an Nsmf_PDUSessionContext Request to V-SMF1 to retrieve an SM Context, and V-SMF1 replies with an Nsmf_PDUSessionContext Response containing both S-NSSAI1 and S-NSSAI2 (see Table 3 or 4 above), such that V-SMF2 is now aware that the PDU session is a PDU session transferred from S-NSSAI1 to S-NSSAI2 as a result of network slice replacement. At Step 3a, V-SMF2 sends a Charging Data Request (containing S-NSSAI1) to a Visited Charging Function (V-CHF) to generate a new Charging Data Record (CDR). At Step 4, V-SMF2 sends an Nsmf_PDUSession_Update Request to an SMF in a Home PLMN (HPLMN), and the SMF replies with an Nsmf_PDUSession_Update Response. At Step 5, V-SMF2 sends an Nsmf_PDUSession_CreateSMContext Response to the AMF. At Step 6, the steps 17-26 of Fig.4.23.4.3-1 in TS 23.502 are performed. At Step 7, an NSSF notifies the AMF that S-NSSAI1 becomes available again using Nnssf_NSSAIAvailability_Notify. At Step 8, the AMF initiates a UE Configuration Update procedure to notify that S-NSSAI1 becomes available again. At Step 9, the AMF sends Nsmf_PDUSession_UpdateSMContext to V-SMF2, including S-NSSAI1 to notify that S-NSSAI1 becomes available again. At Step 10, V-SMF2 sends an Nsmf_PDUSession_UpdateSMContext response to the AMF. At Step 11, V-SMF2 triggers a PDU Session Modification or Release procedure, to transfer the PDU session back to S-NSSAI1, as described above and as defined in Section 5.15.19 of TS 23.501. It can be appreciated that the scenarios shown in Figs.5 and 6 are illustrative only. The present disclosure is applicable for the registration procedure with Unstructured Data Storage Function (UDSF) (e.g., in Nudsf_Unstructured Data Management_Query Response) or without UDSF(e.g., in Namf_Communication_UEContextTransfer Response), inter AMF N2 based handover procedure (e.g., in Namf_Communication_CreateUEContext Request), and other procedures with UE Context transfer, either explicit or implicit, between AMFs or Session Context Transfer (e.g. in Nsmf_PDUSession_ContextRequest Response) between SMFs. Fig.7 is a block diagram of a network node 700 according to an embodiment of the present disclosure. The network node 700 includes a communication interface 710, a processor 720 and a memory 730. The memory 730 may contain instructions executable by the processor 720 whereby the network node 700 is operative to, when implementing a source NF, perform the actions, e.g., of the procedure described earlier in conjunction with Fig.3. Particularly, the memory 730 may contain instructions executable by the processor 720 whereby the network node 700 is operative to, when implementing a source NF: transmit, to a target NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a PDU session transferred from the second network slice to the first network slice. In an embodiment, the source NF may be a source AMF, and the target NF may be a target AMF, and the information on the first network slice and the information on the second network slice may be transmitted in PDU session context information. In an embodiment, the source NF may be a source SMF or I-SMF or V-SMF, and the target NF may be a target SMF or I-SMF or V-SMF, and the information on the first network slice and the information on the second network slice may be transmitted in SM context information. In an embodiment, the information on the first network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the second network slice may be carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session. Alternatively, the information on the second network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the first network slice may be carried in an IE indicating an S-NSSAI that replaces the S-NSSAI associated with the PDU session. In an embodiment, the memory 730 may further contain instructions executable by the processor 720 whereby the network node 700 is operative to, when implementing the source NF, prior to transmitting the information on the first network slice and the information on the second network slice: retrieve an NF profile of the target NF, the NF profile containing information indicating that the target NF supports network slice replacement. In an embodiment, the information on the first network slice and the information on the second network slice may be transmitted in a request. In an embodiment, the memory 730 may further contain instructions executable by the processor 720 whereby the network node 700 is operative to, when implementing the source NF: receive, from the target NF, a request containing information indicating that the target NF supports network slice replacement. The information on the first network slice and the information on the second network slice may be transmitted in response to the request. Alternatively, the memory 730 may contain instructions executable by the processor 720 whereby the network node 700 is operative to, when implementing a target NF, perform the actions, e.g., of the procedure described earlier in conjunction with Fig.4. Particularly, the memory 730 may contain instructions executable by the processor 720 whereby the network node 700 is operative to, when implementing a target NF: receive, from a source NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a PDU session transferred from the second network slice to the first network slice; receive a notification that the second network slice becomes available; and replace the first network slice with the second network slice for the PDU session. In an embodiment, the source NF may be a source AMF and the target NF may be a target AMF, and the information on the first network slice and the information on the second network slice may be received in PDU session context information. In an embodiment, the operation of replacing may include transmitting, to a terminal device, a configuration update message excluding the information on the second network slice as a network slice to be replaced and the information on the first network slice as a corresponding alternative network slice. In an embodiment, the operation of replacing may further include transmitting, to an SMF, an update session management context message indicating that the PDU session is to be transferred to the second network slice. In an embodiment, the source NF may be a source SMF or I-SMF or V-SMF, and the target NF may be a target SMF or I-SMF or V-SMF, and the information on the first network slice and the information on the second network slice may be received in SM context information. In an embodiment, the operation of replacing may include transmitting, in response to determining that the PDU session is to be retained, the information on the second network slice to a UPF and an NG-RAN node associated with the PDU session; or transmitting, in response to determining that the PDU session is to be re-established, the information on the second network slice to a terminal device associated with the PDU session. In an embodiment, the memory 730 may further contain instructions executable by the processor 720 whereby the network node 700 is operative to, when implementing the target NF: transmit, to a CHF, a charging data request for the PDU session, the charging data request including the information on the second network slice. In an embodiment, the information on the first network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the second network slice may be carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session. Alternatively, the information on the second network slice may be carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the first network slice may be carried in an IE indicating an S-NSSAI that replaces the S-NSSAI associated with the PDU session. In an embodiment, the memory 730 may further contain instructions executable by the processor 720 whereby the network node 700 is operative to, when implementing the target NF: transmit, to an NRF, a registration message containing information indicating that the target NF supports network slice replacement. In an embodiment, the information on the first network slice and the information on the second network slice may be received in a request. In an embodiment, the memory 730 may further contain instructions executable by the processor 720 whereby the network node 700 is operative to, when implementing the target NF: transmit, to the source NF, a request containing information indicating that the target NF supports network slice replacement. The present disclosure also provides at least one computer program product in the form of a non-volatile or volatile memory, e.g., a non-transitory computer readable storage medium, an Electrically Erasable Programmable Read-Only Memory (EEPROM), a flash memory and a hard drive. The computer program product includes a computer program. The computer program includes: code / computer readable instructions, which when executed by the processor 720 causes the network node 700 to perform the actions, e.g., of the procedure described earlier in conjunction with Fig.3 or 4. The computer program product may be configured as a computer program code structured in computer program modules. The computer program modules could essentially perform the actions of the flow illustrated in Fig.3 or 4. The processor may be a single CPU (Central Processing Unit), but could also comprise two or more processing units. For example, the processor may include general purpose microprocessors; instruction set processors and / or related chips sets and / or special purpose microprocessors such as Application Specific Integrated Circuits (ASICs). The processor may also comprise board memory for caching purposes. The computer program may be carried in a computer program product connected to the processor. The computer program product may comprise a non-transitory computer readable storage medium on which the computer program is stored. For example, the computer program product may be a flash memory, a Random Access Memory (RAM), a Read-Only Memory (ROM), or an EEPROM, and the computer program modules described above could in alternative embodiments be distributed on different computer program products in the form of memories. The disclosure has been described above with reference to embodiments thereof. It should be understood that various modifications, alternations and additions can be made by those skilled in the art. Therefore, the scope of the disclosure is not limited to the above particular embodiments but only defined by the claims as attached. The present disclosure further includes changes to 3GPP TS 29.502 as described below in Appendix A and changes to 3GPP TS 29.518 as described below in Appendix B, where strikethroughs indicate deleted text and underlines indicate added text. Appendix A CR-Form-v12.2 CHANGE REQUEST 29.502 CR xxxx rev - Current version: 18.5.0 Proposed change affects: UICC apps ME Radio Access Network Core Network X Title: S-NSSAI Replacement in SM Context Source to TSG: CT4 Work item code: eNS_Ph3 Date: 2024-02-01 Category: B Release: Rel-18 Use one of the following categories: Use one of the following releases: F (correction) Rel-8 (Release 8) A (mirror corresponding to a change in an earlier Rel-9 (Release 9) Rel-10 (Release 10) release) Rel-11 (Release 11) B (addition of feature), … C (functional modification of feature) Rel-16 (Release 16) D (editorial modification) Rel-17 (Release 17) Detailed explanations of the above categories can Rel-18 (Release 18) be found in 3GPP TR 21.900. Rel-19 (Release 19) Reason for change: With slice replacement function, when the AMF was aware of the S-NSSAI of the PDU session is not available and an alternative S-NSSAI can be used, the AMF will inform the SMF both the S-NSSAI and the alternative S-NSSAI. The SMF can then behavior correctly, e.g. use the alternative S-NSSAI for resource reservation and use the replaced S-NSSAI for charging information. However, in SM context the alternative S-NSSAIs are not included. As a consequence, after SM context is relocated, e.g. during SM Context Transfer procedure, the new SMF cannot get the alternative S-NSSAIs from old SMF and may not beahvior correctly as expected. This CR propose to include the alternative S-NSSAIs in the SM Context. Summary of change: Add alternative S-NSSAIs (in serving PLMN and HPLMN for HR PDU session) Consequences if not Alternative S-NSSAIs are not passed during SM Context relocation, new approved: SMF may not behavior correctly. Clauses affected: 6.1.6.2.39 Y N Other specs X Other core specifications TS / TR ... CR ... affected: X Test specifications TS / TR ... CR ... (show related CRs) X O&M Specifications TS / TR ... CR ... Other comments: This CR introduce backward compatible new features to OpenAPI file of Nsmf_PDUSession API. * * * First Change * * * * 6.1.6.2.39 Type: SmContext Table 6.1.6.2.39-1: Definition of type SmContext Attribute name Data type P Card Description Applica inalit bility y pduSessionId PduSessionId M 1 This IE shall contain the PDU Session ID. dnn Dnn M 1 This IE shall contain the UE requested DNN of the PDU session. The DNN shall be the full DNN (i.e. with both the Network Identifier and Operator Identifier) for a HR PDU session, and it should be the full DNN in LBO and non-roaming scenarios. If the Operator Identifier is absent, the serving core network operator shall be assumed. selectedDnn Dnn C 0..1 This IE shall be present, if another DNN other than the UE requested DNN is selected for this PDU session. When present, it shall contain the selected DNN. The DNN shall be the full DNN (i.e. with both the Network Identifier and Operator Identifier) for a HR PDU session, and it should be the full DNN in LBO and non-roaming scenarios. If the Operator Identifier is absent, the serving core network operator shall be assumed. sNssai Snssai M 1 This IE shall contain the S-NSSAI for the serving PLMN. altSnssai Snssai C 0..1 This IE shall be present if the network slice for the NSRP serving PLMN (as indicated in the sNssai IE) was replaced by an alternative S-NSSAI. When present, this IE shall indicate the alternative S-NSSAI for the serving PLMN. See clause 5.15.19 of 3GPP TS 23.501 [2]. hplmnSnssai Snssai C 0..1 This IE shall be present for a HR PDU session. When present, it shall contain the S-NSSAI for the HPLMN. altHplmnSnssai Snssai C 0..1 This IE shall be present if the network slice for the NSRP HPLMN (as indicated in the hplmnSnssai IE) was replaced by an alternative S-NSSAI for the HPLMN. When present, this IE shall indicate the alternative S-NSSAI for the HPLMN. See clause 5.15.19 of 3GPP TS 23.501 [2]. pduSessionType PduSessionType M 1 This IE shall indicate the PDU session type. gpsi Gpsi C 0..1 This IE shall be present if it is available. When present, it shall contain the user's GPSI. hSmfUri Uri C 0..1 This IE shall be present in HR roaming scenarios. When present, it shall contain the API URI of the Nsmf_PDUSession service of the H-SMF. The API URI shall be formatted as specified in clause 6.1.1. smfUri Uri C 0..1 This IE shall be present for a PDU session with an I-SMF. When present, it shall contain the API URI of the Nsmf_PDUSession service of the SMF. The API URI shall be formatted as specified in clause 6.1.1. pduSessionRef Uri C 0..1 This IE shall be present for a HR PDU session or a PDU session with an I-SMF. When present, this IE shall include the absolute URI of the PDU Session in H-SMF or SMF, including apiRoot (see clause 6.1.3.6.2) interPlmnApiRoot Uri C 0..1 This IE shall be present, if available. When present, it shall contain the apiRoot of the PDU session context to be used in inter-PLMN signalling request targeting the PDU session context. (NOTE 2) intraPlmnApiRoot Uri C 0..1 This IE shall be present, if available. When present, it shall contain the apiRoot of the PDU session context to be used in intra-PLMN signalling request targeting the PDU session context. (NOTE 2) pcfId NfInstanceId O 0..1 When present, this IE shall contain the identifier of: - the H-PCF selected by the AMF (for UE Policy), for a HR PDU session; or - the V-PCF selected by the AMF (for Access and Mobility Policy), for a PDU session in LBO roaming scenarios; or - the PCF selected by the AMF (for Access and Mobility Policy and / or UE Policy), for a PDU session in non-roaming scenarios. pcfGroupId NfGroupId O 0..1 This IE may be present in non-roaming and HR roaming scenarios. When present, this IE shall contain the identity of the (home) PCF group serving the UE for Access and Mobility Policy and / or UE Policy. pcfSetId NfSetId O 0..1 This IE may be present if the pcfId IE is present. When present, it shall contain the NF Set ID of the PCF indicated by the pcfId IE. selMode DnnSelectionMode C 0..1 This IE shall be present if it is available. When present, it shall be set to: - "VERIFIED", if the requested DNN provided by UE or the selected DNN provided by the network corresponds to an explicitly subscribed DNN; or - "UE_DNN_NOT_VERIFIED", if the requested DNN provided by UE corresponds to the usage of a wildcard subscription; or - "NW_DNN_NOT_VERIFIED", if the selected DNN provided by network corresponds to the usage of a wildcard subscription. If both the requested DNN (i.e. dnn IE) and selected DNN (i.e. selected Dnn IE) are present, the selMode shall be related to the selected DNN. udmGroupId NfGroupId O 0..1 When present, it shall indicate the identity of the UDM group serving the UE. routingIndicator string O 0..1 When present, it shall indicate the Routing Indicator of the UE. hNwPubKeyId integer O 0..1 When present, it shall indicate the Home Network Public Key Identifier of the UE. (NOTE 1) sessionAmbr Ambr M 1 This IE shall contain the Session AMBR granted to the PDU session. qosFlowsList array(QosFlowSetupI M 1..N This IE shall contain the set of QoS flow(s) established tem) for the PDU session. It shall contain at least the Qos flow associated to the default Qos rule. The qosRules attribute of each QosFlowSetupItem shall be set to an empty string. hSmfInstanceId NfInstanceId C 0..1 This IE shall be present for a HR PDU session. When present, it shall contain the identifier of the home SMF. smfInstanceId NfInstanceId C 0..1 This IE shall be present for a PDU session with an I-SMF. When present, it shall contain the identifier of the SMF. pduSessionSmfSetId NfSetId C 0..1 This IE shall be present, if available. When present, this IE shall contain the NF Set ID of the home SMF as identified by hSmfInstanceId, or the SMF as identified by the smfInstanceId. pduSessionSmfServi NfServiceSetId C 0..1 This IE shall be present, if available. ceSetId When present, this IE shall contain the NF Service Set ID of the PDUSession service instance (for this PDU session) in the home SMF or the SMF. pduSessionSmfBindi SbiBindingLevel C 0..1 This IE shall be present, if available. ng When present, this IE shall contain the SBI binding level of the PDU session resource in the home SMF or the SMF. enablePauseChargin boolean C 0..1 This IE shall be present for a HR PDU session, if g available. When present, it shall indicate whether the use of Pause of Charging is enabled for the PDU session (see clause 4.4.4 of 3GPP TS 23.502 [3]). When present, it shall be set as follows: - true: enable Pause of Charging; - false (default): disable Pause of Charging. ueIpv4Address Ipv4Addr C 0..1 This IE shall be present if a UE IPv4 address to the PDU session. ueIpv6Prefix Ipv6Prefix C 0..1 This IE shall be present if a UE IPv6 prefix to the PDU session. epsPdnCnxInfo EpsPdnCnxInfo C 0..1 This IE shall be present if the PDU session may be moved to EPS during its lifetime. epsBearerInfo array(EpsBearerInfo) C 1..N This IE shall be present if the PDU session may be moved to EPS during its lifetime. maxIntegrityProtecte MaxIntegrityProtecte C 0..1 This IE shall be present if the upSecurity IE is present dDataRate dDataRate and indicates that integrity protection is preferred or required. When present, it shall indicate the maximum integrity protected data rate for uplink. If the maxIntegrityProtectedDataRateDl IE is absent, this IE applies to both uplink and downlink. maxIntegrityProtecte MaxIntegrityProtecte C 0..1 This IE may be present if the upSecurity IE is present dDataRateDl dDataRate and indicates that integrity protection is preferred or required. When present, it shall indicate the maximum integrity protected data rate for downlink. alwaysOnGranted boolean C 0..1 This IE shall be present if available. When present, it shall indicate whether this is an always On PDU session and it shall be set as follows: - true: always-on PDU session granted. - false (default): always-on PDU session not granted. upSecurity UpSecurity O 0..1 When present, this IE shall indicate the security policy for integrity protection and encryption for the user plane of the PDU session. hSmfServiceInstance string O 0..1 This IE may be present for a HR PDU session. Id When present, this IE shall contain the serviceInstanceId of the H-SMF service instance serving the PDU session. This IE may be used by the V-SMF to identify PDU sessions affected by a failure or restart of the H-SMF service (see clause 6.2 of 3GPP TS 23.527

[0024] ). smfServiceInstanceI string O 0..1 This IE may be present for a PDU session with an d I-SMF. When present, this IE shall contain the serviceInstanceId of the SMF service instance serving the PDU session. This IE may be used by the I-SMF to identify PDU sessions affected by a failure or restart of the SMF service (see clause 6.2 of 3GPP TS 23.527

[0024] ). recoveryTime DateTime O 0..1 This IE may be present if available. When present, this IE shall indicate the timestamp (in UTC) when the H-SMF or SMF service instance serving the PDU session was (re)started (see clause 6.3 of 3GPP TS 23.527

[0024] ). forwardingInd boolean C 0..1 This IE shall be present, when downlink data packets are buffered at I-UPF. The SMF or I-SMF shall use this IE to inform the NF service consumer that a forwarding tunnel is needed for receiving the buffered downlink data packets, as specified in clause 4.23.4 of 3GPP TS 23.502 [3]. When present, this IE shall be set as follows: - true: a forwarding tunnel is needed for sending buffered downlink data packets; - false (default): forwarding tunnel is not needed psaTunnelInfo TunnelInfo C 0..1 This IE shall be present if available. When present, this IE shall contain the N9 tunnel information of PDU Session Anchor UPF controlled by SMF or H-SMF. chargingId string C 0..1 This IE shall be present for a HR PDU session, in scenarios with a V-SMF insertion / change / removal. When present, it shall contain the Charging ID of the PDU session (see 3GPP TS 32.255

[0025] ). The string shall encode the Charging ID (32-bit unsigned integer value, with maximum value "4294967295") in decimal representation. Pattern: '^(0|([1-9]{1}[0-9]{0,9}))$' (NOTE 4) smfChargingId SmfChargingId C 0..1 This IE shall be present if available for a HR PDU SCID session, in scenarios with a V-SMF insertion / change / removal. When present, it shall contain the String based Charging ID of the PDU session (see 3GPP TS 32.255

[0025] ). chargingInfo ChargingInformation C 0..1 This IE shall be present for a HR PDU session, if available and if the NF Service Consumer requesting the SM Context pertains to the same PLMN (i.e. if the Retrieve SM Context Request does not contain the servingNetwork attribute set to a different PLMN ID). When present, it shall contain the addresses of the V-CHF used for the PDU session. roamingChargingPro RoamingChargingPr C 0..1 This IE shall be present for a HR PDU session, if file ofile available and if the NF Service Consumer requesting the SM Context pertains to the same PLMN (i.e. if the Retrieve SM Context Request does not contain the servingNetwork attribute set to a different PLMN ID). When present, it shall contain the Roaming Charging Profile selected by the HPLMN (see clauses 5.1.9.1, 5.2.1.7 and 5.2.2.12.2 of 3GPP TS 32.255

[0025] ). nefExtBufSupportInd boolean C 0..1 This IE shall be present with value "true", if the anchor NEF has indicated support of Extended Buffering for mobile terminated data during SMF-NEF connection establishment. When present, this IE shall be set as following: - true: Extended Buffering supported by NEF - false (default): Extended Buffering not supported by NEF ipv6Index IpIndex C 0..1 This IE shall be present during I-SMF change scenarios, if IPv6 Index has previously been received by old I-SMF. dnAaaAddress IpAddress O 0..1 When present, this IE shall contain the address of DN-AAA server for UE IP Address allocation previously received by old I-SMF. redundantPduSessio RedundantPduSessi C 0..1 This IE shall be present for a PDU session with an nInfo onInformation I-SMF, if this information has been received previously from the UE, the anchor SMF or the old I-SMF. ranTunnelInfo QosFlowTunnel C 0..1 This IE shall be present if the ranUnchangedInd IE is set to "true" in the SM context retrieve request. When present, this IE shall contain the N2 tunnel information of NG-RAN with associated QoS flows (see "DL QoS Flow per TNL Information" in clause 9.3.4.2 of 3GPP 38.413 [9]).addRanTunnelInfo array(QosFlowTunne C 1..N This IE shall be present if the ranUnchangedInd IE is l) set to "true" in the SM context retrieve request. When present, this IE shall contain the additional N2 tunnel information of NG-RAN together with associated QoS flows for split PDU session (see "Additional DL QoS Flow per TNL Information" in clause 9.3.4.2 of 3GPP 38.413 [9]). redRanTunnelInfo QosFlowTunnel C 0..1 This IE shall be present if the ranUnchangedInd IE is set to "true" in the SM context retrieve request. When present, this IE shall contain the additional N2 tunnel information of NG-RAN together with associated QoS flows for Redundant QoS Flow(s) (see "Redundant DL QoS Flow per TNL Information" in clause 9.3.4.2 of 3GPP 38.413 [9]). addRedRanTunnelIn array(QosFlowTunne C 1..N This IE shall be present if the ranUnchangedInd IE is fo l) set to "true" in the SM context retrieve request. When present, this IE shall contain the additional N2 tunnel information of NG-RAN together with associated QoS flows for Redundant QoS Flow(s) with split PDU session (see "Additional Redundant DL QoS Flow per TNL Information" in clause 9.3.4.2 of 3GPP 38.413 [9]). nspuSupportInd boolean C 0..1 This IE shall be present and set to "true" if the enablePauseCharging in the SmContext data type is set to "true" and if the (H-)SMF and PSA UPF support Notify Start Pause of Charging via user plane feature as specified in clause 5.30 of 3GPP TS 29.244

[0029] . When present, it shall be set as follows: - true: Notify Start Pause of Charging via user plane feature is supported. smfBindingInfo string C 0..1 This IE shall be present, if available. When present, this IE shall contain the Binding indications of the PDU session resource in the home SMF or the SMF and shall be set to the value of the 3gpp-Sbi-Binding header defined in clause 5.2.3.2.6 of 3GPP TS 29.500 [4], without the header name. satelliteBackhaulCat SatelliteBackhaulCat O 0..1 When present, this IE shall indicate the satellite egory backhaul category information last signalled towards the anchor SMF, if any. sscMode string C 0..1 This IE shall be present, if available. When present, this IE shall indicate the SSC mode applicable to the PDU session. When present, it shall be encoded as one character in hexadecimal representation, taking a value of "0" to "7", representing the 3 bits of the SSC mode value of the SSC mode IE specified in clause 9.11.4.16 of 3GPP TS 24.501 [7]. Pattern: "^[0-7]$" Example: SSC mode 3 shall be encoded as "3". dlsetSupportInd boolean C 0..1 This IE shall be present and set to "true" if the (H-)SMF supports the "DLSET" feature as specified in clause 6.1.8. When present, it shall be set as follows: - true: the (H-)SMF supports the "DLSET" feature. - false: the (H-)SMF does not support the "DLSET" feature n9fscSupportInd boolean C 0..1 This IE shall be present and set to "true" if the SMF supports the "N9FSC" feature as specified in clause 6.1.8. When present, it shall be set as follows: - true: "N9FSC" feature is supported. anchorSmfOauth2Re boolean O 0..1 This IE may be present when the NF consumer (i.e. quired new I-SMF or new V-SMF) and the NF producer (i.e. the old I-SMF, V-SMF or SMF) belong to the same PLMN. When present, this IE shall indicate whether the H-SMF or SMF for a PDU session with an I-SMF requires Oauth2-based authorization for accessing its Nsmf_PDUSession service. - true: OAuth2 based authorization is required. - false: OAuth2 based authorization is not required. The absence of this IE means that no indication is available about the usage of Oauth2 for authorization of the anchor SMF's Nsmf_PDUSession service. (NOTE 3) fullDnaiList array(Dnai) O 1..N This IE may be present to contain the full list of DNAIs DTSSA- of interest for PDU session, including DNAIs that may Ext1 not be supported by the (source) I-SMF and excluding the ones supported by the Anchor SMF. hrsboAuthResult boolean C 0..1 This IE shall be present for a HR PDU session, during HR-SBO a V-SMF change within the same PLMN (i.e. if the Retrieve SM Context Request does not contain the servingNetwork attribute set to a different PLMN ID), if HR-SBO was authorized by the H-SMF and the request indicates that the new V-SMF supports HR-SBO. When present, it shall Indicate whether HR-SBO request is authorized - true: authorized. - false: Not authorized. hDnsAddr IpAddress C 0..1 This IE shall be present for a HR PDU session, during HR-SBO a V-SMF change within the same PLMN (i.e. if the Retrieve SM Context Request does not contain the servingNetwork attribute set to a different PLMN ID), if available, HR-SBO was authorized by the H-SMF and the request indicates that the new V-SMF supports HR-SBO. When present, this IE shall contain the DNS server address of HPLMN. hPlmnAddr IpAddress C 0..1 This IE shall be present for a HR PDU session, during HR-SBO a V-SMF change within the same PLMN (i.e. if the Retrieve SM Context Request does not contain the servingNetwork attribute set to a different PLMN ID), if available, HR-SBO was authorized by the H-SMF and the request indicates that the new V-SMF supports HR-SBO. When present, this IE shall contain the HPLMN address information (e.g. H-UPF IP address on N6). The new V-SMF may configure the new V-EASDF to build EDNS Client Subnet option based on this HPLMN address information for target FQDN of DNS queries which are not authorized for HR-SBO. vplmnOffloadingInfoL array(VplmnOffloadin C 1..N This IE shall be present for a HR PDU session, during HR-SBO ist gInfo) a V-SMF change within the same PLMN (i.e. if the Retrieve SM Context Request does not contain the servingNetwork attribute set to a different PLMN ID), if available, HR-SBO was authorized by the H-SMF and the request indicates that the new V-SMF supports HR-SBO. When present, it shall contain the list of V-PLMN Offloading policies that apply to the PDU session and whose offload identifiers are not yet known by the target V-SMF. (NOTE 5) offloadIds array(OffloadIdentifie C 1...N This IE shall be present for a HR PDU session, during HR-SBO r) a V-SMF change within the same PLMN (i.e. if the Retrieve SM Context Request does not contain the servingNetwork attribute set to a different PLMN ID), if available, HR-SBO was authorized by the H-SMF and the request indicates that the new V-SMF supports HR-SBO, and if the offloadIds are part of the storedOffloadIds included in the the Retrieve SM Context Request. When present, this IE shall contain a list of offload identifiers that apply to the PDU session and that are already known by the target V-SMF. (NOTE 5) easInfoToRefresh EasInfoToRefresh C 0..1 This IE shall be present for a HR PDU session, during HR-SBO a V-SMF change within the same PLMN (i.e. if the Retrieve SM Context Request does not contain the servingNetwork attribute set to a different PLMN ID), if available, HR-SBO was authorized by the H-SMF and the request indicates that the new V-SMF supports HR-SBO. When present, it shall contain the EAS information to be refreshed for EAS re-discovery. easIpReplacementInf EasIpReplacementIn C 0..1 This IE shall be present, for a HR PDU session, if it is HR-SBO o fo received from H-SMF in AF during AF triggered EAS re-discovery and edge relocation via interacting with HPLMN (see clause 6.7.3.2 of 3GPP TS 23.548

[0074] ). When present, this IE shall contain the EAS IP replacement information. targetDnai Dnai C 0..1 This IE shall be present, for a HR PDU session, if it is HR-SBO received from H-SMF in AF triggered EAS re-discovery and edge relocation via interacting with HPLMN (see clause 6.7.3.2 of 3GPP TS 23.548

[0074] ). When present, this IE shall contain the target DNAI. pendingUpdateInfoLi array(PendingUpdat O 1..N This IE should be included by the old V-SMF / I-SMF if st eInfo) received from the (H-)SMF. When present, this IE shall indicate the list of information that are not required to be updated in real-time to the (H-)SMF, i.e. the change of the listed information (e.g. UE location or Timezone) may be piggybacked in a subsequent essential update (e.g. to exchange the N1 message from the UE) to the (H-)SMF. The NF service consumer (i.e. I-SMF / V-SMF) should not trigger an Update to the (H-)SMF including only the change(s) of the listed information. NOTE 1: If present, this attribute shall be used together with routingIndicator. This attribute is only used by the HPLMN in roaming scenarios. NOTE 2: See NOTE 7 of Table 6.1.6.2.10-1. NOTE 3: If the anchorSmfOauth2Required IE was received in SmContextCreateData from the AMF, this IE shall be ignored by the new I-SMF or V-SMF. NOTE 4: Usage of Charging ID with Uint32 value for roaming scenarios may lead to Charging ID collision between SMFs. NOTE 5: The same offloadId should not appear in both the vplmnOffloadingInfoList and offloadIds attributes. Both vplmnOffloadingInfoList and offloadIds attributes may be present when multiple offload identifers apply to the PDU session and some of them are already known by the target V-SMF while others are not yet known by the target V-SMF.

[0003] Appendix B CR-Form-v12.2 CHANGE REQUEST 29.518 CR xxxx rev - Current version: 18.4.0 Proposed change affects: UICC apps ME Radio Access Network Core Network X Title: Alternative S-NSSAIs for PDU Sessions in UE Context Source to TSG: CT4 Work item code: eNS_Ph3 Date: 2024-02-01 Category: B Release: Rel-18 Use one of the following categories: Use one of the following releases: F (correction) Rel-8 (Release 8) A (mirror corresponding to a change in an earlier Rel-9 (Release 9) Rel-10 (Release 10) release) Rel-11 (Release 11) B (addition of feature), … C (functional modification of feature) Rel-16 (Release 16) D (editorial modification) Rel-17 (Release 17) Detailed explanations of the above categories can Rel-18 (Release 18) be found in 3GPP TR 21.900. Rel-19 (Release 19) Reason for change: With slice replacement function, when the AMF was aware of the S-NSSAI of the PDU session is not available and an alternative S-NSSAI can be used, the AMF will inform the SMF both the S-NSSAI and the alternative S-NSSAI. Later when the AMF is aware the replaced S-NSSAI is available again, the AMF will inform the SMF to transfer the PDU session to the replaced S-NSSAI. When the UE is moved from one AMF to another AMF, the PDU session contexts are transferered within the UE context, include the S-NSSAIs of the PDU session. However, the alternative S-NSSAIs are not transferred as part of the PDU session context. This CR propose to include the alternative S-NSSAIs in the PDU session context. Summary of change: Add alternative S-NSSAIs (in serving PLMN and HPLMN for HR PDU session) in PDU session context within the UE context. Consequences if not Alternative S-NSSAIs are not passed during AMF changes, new AMF may approved: not behavior correctly, e.g., to ask SMF to transfer the PDU session to the replaced S-NSSAI aftger the replaced S-NSSAI is become available. Clauses affected: 6.1.6.2.37, A.2 Y N Other specs X Other core specifications TS / TR ... CR ... affected: X Test specifications TS / TR ... CR ... (show related CRs) X O&M Specifications TS / TR ... CR ... Other comments: This CR introduce backward compatible new features to OpenAPI file of Namf_Communication API. 6.1.6.2.37 Type: PduSessionContext Table 6.1.6.2.37-1: Definition of type PduSessionContext Attribute name Data type P Cardinality Description Applicability pduSessionId PduSessionId M 1 Indicates the identifier of the PDU Session. smContextRef Uri M 1 Indicates the resource URI of the SM context, including the apiRoot (see clause 6.1.3.3.2 of 3GPP TS 29.502

[0016] ). When present, it shall carry the URI of SM Context of: - I-SMF, for a PDU session with I-SMF; or - V-SMF, for HR PDU session; or - SMF, for non-roaming PDU session without I-SMF, or LBO roaming PDU session; sNssai Snssai M 1 Indicates the associated S-NSSAI for the PDU Session. It shall be the S-NSSAI in HPLMN in non-roaming, LBO roaming or HR roaming. altSnssai Snssai C 0..1 This IE shall be present if the S-NSSAI in the HPLMN (as indicated in the sNssai IE) was replaced by an alternative S-NSSAI in the HPLMN. When present, this IE shall indicate the alternative S-NSSAI in the HPLMN. See clause 5.15.19 of 3GPP TS 23.501 [2]. additionalSnssai Snssai C 0..1 This IE shall be present in intra-VPLMN mobility of LBO roaming and HR roaming. When present, this IE shall indicate the associated S-NSSAI in VPLMN for the PDU Session. altAdditionalSnssai Snssai C 0..1 This IE shall be present if S-NSSAI in the VPLMN (as indicated in the additionalSnssai IE) was replaced by an alternative S-NSSAI in the VPLMN. When present, this IE shall indicate the alternative S-NSSAI in the VPLMN. See clause 5.15.19 of 3GPP TS 23.501 [2]. Dnn Dnn M 1 This IE shall indicate the Data Network Name. The DNN shall be the full DNN (i.e. with both the Network Identifier and Operator Identifier) for a HR PDU session, and it should be the full DNN in LBO and non-roaming scenarios. If the Operator Identifier is absent, the serving core network operator shall be assumed. selectedDnn Dnn C 0..1 This IE shall be present, if another DNN other than the UE requested DNN is selected for this PDU session. When present, it shall contain the selected DNN. The DNN shall be the full DNN (i.e. with both the Network Identifier and Operator Identifier) for a HR PDU session, and it should be the full DNN in LBO and non-roaming scenarios. If the Operator Identifier is absent, the serving core network operator shall be assumed. accessType AccessType M 1 Indicates the access type of the PDU session. additionalAccessTyp AccessType C 0..1 Indicates the additional access type for e a MA PDU session, if the UE registers to both 3GPP access and Non-3GPP access. allocatedEbiList array(EbiArpMap C 1..N This IE shall be present when at least ping) one EBI is allocated to the PDU session. When present, this IE shall contain the EBIs currently allocated to the PDU session. hsmfId NfInstanceId C 0..1 This IE shall be present for non-roaming and home-routed PDU sessions. When present, it shall indicate the associated: - home SMF for HR PDU Session, or - SMF, for non-roaming PDU session, regardless of whether an I-SMF is involved or not. hsmfSetId NfSetId C 0..1 This IE shall be present, if available. When present, this IE shall contain the NF Set ID of the home SMF or the SMF indicated by hsmfId. hsmfServiceSetId NfServiceSetId C 0..1 This IE shall be present, if available. When present, this IE shall contain the NF Service Set ID of the selected PDUSession service instance of home SMF or the SMF indicated by hsmfId. smfBinding SbiBindingLevel C 0..1 This IE shall be present if available, for a non-roaming PDU session. When present, this IE shall contain the SBI binding level of the SMF's SM context resource. vsmfId NfInstanceId C 0..1 This IE shall be present for roaming PDU sessions. When present, it shall indicate the associated visited SMF for home-routed the PDU Session, or the SMF for the local-breakout PDU session (regardless of whether an I-SMF is involved or not). vsmfSetId NfSetId C 0..1 This IE shall be present, if available. When present, this IE shall contain the NF Set ID of the V-SMF. vsmfServiceSetId NfServiceSetId C 0..1 This IE shall be present, if available. When present, this IE shall contain the NF Service Set ID of the V-SMF's PDUSession service instance. vsmfBinding SbiBindingLevel C 0..1 This IE shall be present, if available. When present, this IE shall contain the SBI binding level of the V-SMF's SM context resource. ismfId NfInstanceId C 0..1 This IE shall be present if I-SMF is DTSSA involved in the PDU session. When present, it shall indicate the associated I-SMF for the PDU Session. ismfSetId NfSetId C 0..1 This IE shall be present, if available. DTSSA When present, this IE shall contain the NF Set ID of the I-SMF. ismfServiceSetId NfServiceSetId C 0..1 This IE shall be present, if available. DTSSA When present, this IE shall contain the NF Service Set ID of the I-SMF's PDUSession service instance. ismfBinding SbiBindingLevel C 0..1 This IE shall be present if available. DTSSA When present, this IE shall contain the SBI binding level of the I-SMF's SM Context resource. nsInstance NsiId C 0..1 This IE shall be present if available. When present, this IE shall Indicate Network Slice Instance for the PDU Session smfServiceInstanceI string O 0..1 When present, this IE shall contain the d serviceInstanceId of the SMF PDUSession service instance serving the SM Context, i.e. of: - the I-SMF, for a PDU session with I-SMF; - the V-SMF, for a HR PDU session; or - the SMF, for a non-roaming or an LBO roaming PDU session without I-SMF. This IE may be used by the AMF to identify PDU session contexts affected by a failure or restart of the SMF service instance (see clause 6.2 of 3GPP TS 23.527

[0033] ). maPduSession boolean C 0..1 This IE shall be present if available. When present, this IE shall indicate whether it is an MA PDU session. true: indicates the PDU session is MA PDU session; false (default): the PDU session is not MA PDU session. cnAssistedRanPara CnAssistedRanP C 0..1 This IE shall be present if available. ara When present, this IE shall contain the PDU Session specific parameters received from the SMF and used by the AMF to derive the Core Network assisted RAN parameters tuning. nrfManagementUri Uri C 0..1 If included, this IE shall contain the API URI of the NFManagement Service (see clause 6.1.1 of 3GPP TS 29.510

[0029] ) of the NRF or hNRF. It shall be present if it is returned from the NSSF or hNSSF (see clause 6.1.6.2.7 of 3GPP TS 29.531

[0018] ). nrfDiscoveryUri Uri C 0..1 If included, this IE shall contain the API URI of the NFDiscovery Service (see clause 6.2.1 of 3GPP TS 29.510

[0029] ) of the NRF or hNRF. It shall be present if it is returned from the NSSF or hNSSF (see clause 6.1.6.2.7 of 3GPP TS 29.531

[0018] ). nrfAccessTokenUri Uri C 0..1 If included, this IE shall contain the API URI of the Access Token Service (see clause 6.3.2 of 3GPP TS 29.510

[0029] ) of the NRF or hNRF. It shall be present if it is returned from the NSSF or hNSSF (see clause 6.1.6.2.7 of 3GPP TS 29.531

[0018] ). smfBindingInfo string C 0..1 This IE shall be present if available, for a non-roaming PDU session. When present, this IE shall contain the Binding indications of the SMF's SM context resource and shall be set to the value of the 3gpp-Sbi-Binding header defined in clause 5.2.3.2.6 of 3GPP TS 29.500 [4], without the header name. vsmfBindingInfo string C 0..1 This IE shall be present, if available. When present, this IE shall contain the Binding indications of the V-SMF's SM context resource and shall be set to the value of the 3gpp-Sbi-Binding header defined in clause 5.2.3.2.6 of 3GPP TS 29.500 [4], without the header name. ismfBindingInfo string C 0..1 This IE shall be present if available. DTSSA When present, this IE shall contain the Binding indications of the I-SMF's SM Context resource and shall be set to the value of the 3gpp-Sbi-Binding header defined in clause 5.2.3.2.6 of 3GPP TS 29.500 [4], without the header name. interPlmnApiRoot Uri C 0..1 This IE shall be present if this information is available. When present, it shall contain the apiRoot of the SM context to be used in inter-PLMN signalling request targeting the SM context. (NOTE) pgwFqdn Fqdn O 0..1 FQDN of the PGW in the PGW-C+SMF, to be included for interworking with EPS. pgwIpAddr IpAddress O 0..1 IP Address of the PGW in the PGW-C+SMF, to be included for interworking with EPS. plmnId PlmnId O 0..1 PLMN where the PGW-C+SMF is located. anchorSmfSupported SupportedFeatur O 0..1 When present, this IE shall include the Features es features of the Nsmf_PDUSession service (see clause 6.1.8 of 3GPP TS 29.502

[0016] ) that are supported by the H-SMF (or the SMF for a PDU sessions with I-SMF). (NOTE 2) anchorSmfOauth2Re boolean O 0..1 This IE may be present when new quired AMF and old AMF belong to the same PLMN. When present, this IE shall indicate whether the H-SMF (or the SMF for a PDU session with I-SMF) requires Oauth2-based authorization for accessing its Nsmf_PDUSession service - true: OAuth2 based authorization is required. - false: OAuth2 based authorization is not required. The absence of this IE means that no indication is available about the usage of Oauth2 for authorization of the anchor SMF's Nsmf_PDUSession service. hrsboAllowedInd boolean C 0..1 This IE shall be present during an intra-PLMN N2 handover, if this information is available (see clause 6.7.2.6 of 3GPP TS 23.548

[0058] ). When present, it shall indicate whether HR-SBO is allowed for the PDU session and it shall be set as follows: - true: HR-SBO is allowed - false: HR-SBO is not allowed The absence of this IE shall not be interpreted as meaning that HR-SBO is allowed or not allowed. NOTE 1: During an inter-PLMN mobility, the target AMF shall replace the apiRoot of the smContextRef with the interPlmnApiRoot if available and send the resulting smContextRef in the Create SM Context request towards the target V-SMF, I-SMF or anchor SMF. See 3GPP TS 29.502

[0016] . NOTE 2: The new AMF may use this IE to know the supported features of the H-SMF (or the SMF for a PDU session with I-SMF) and take action based on the supported features, e.g. the new AMF shall release the PDU session when V-SMF needs to be changed but the H-SMF does not support V-SMF change.

Claims

1. A method (300) in a source Network Function, NF, comprising: transmitting (310), to a target NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a Packet Data Unit, PDU, session transferred from the second network slice to the first network slice.

2. The method of claim 1, wherein the PDU session is established or has been established for a terminal device which moves or has moved from the source NF to the target NF wherein serving or handling of the terminal device is or has been transferred from the source NF to the target NF.

3. The method of claim 1 or 2, wherein a PDU session context for the PDU session is transferred from the source NF to the target NF.

4. The method (300) of any of claims 1 to 3, wherein the source NF is a source Access and Mobility Management Function, AMF, and the target NF is a target AMF, and wherein the information on the first network slice and the information on the second network slice are transmitted in PDU session context information.

5. The method (300) of any of claims 1 to 3, wherein the source NF is a source Session Management Function, SMF, or Intermediate SMF, I-SMF, or Visited SMF, V-SMF, and the target NF is a target SMF or I-SMF or V-SMF, and wherein the information on the first network slice and the information on the second network slice are transmitted in Session Management, SM, context information.

6. The method (300) of any of claims 1 to 5, wherein the information on the first network slice is carried in an Information Element, IE, indicating an Single Network Slice Selection Assistance Information, S-NSSAI, associated with the PDU session, and the information on the second network slice is carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session, or the information on the second network slice is carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the firstnetwork slice is carried in an IE an S-NSSAI that replaces the S-NSSAI associated with the PDU session.

7. The method (300) of any of claims 1 to 6, further comprising, prior to transmitting the information on the first network slice and the information on the second network slice: retrieving an NF profile of the target NF, the NF profile containing information indicating that the target NF supports network slice replacement.

8. The method (300) of any of claims 1 to 7, wherein the information on the first network slice and the information on the second network slice are transmitted in a request.

9. The method (300) of any of claims 1 to 6, further comprising: receiving, from the target NF, a request containing information indicating that the target NF supports network slice replacement, wherein the information on the first network slice and the information on the second network slice are transmitted in response to the request.

10. A method (400) in a target Network Function, NF, comprising: receiving (410), from a source NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a Packet Data Unit, PDU, session transferred from the second network slice to the first network slice; receiving (420) a notification that the second network slice becomes available; and replacing (430) the first network slice with the second network slice for the PDU session.

11. The method of claim 10, wherein the PDU session is established or has been established for a terminal device which moves or has moved from the source NF to the target NF wherein serving or handling of the terminal device is or has been transferred from the source NF to the target NF.

12. The method of claim 10 or 11, wherein a PDU session context for the PDU session is transferred from the source NF to the target NF.

13. The method (400) of any of claims 10 to 12, wherein the source NF is a source Access and Mobility Management Function, AMF, and the target NF is a target AMF, and wherein the information on the first network slice and the information on the second network slice are received in PDU session context information.

14. The method (400) of claim 13, wherein said replacing (430) comprises: transmitting, to a terminal device, a configuration update message excluding the information on the second network slice as a network slice to be replaced and the information on the first network slice as a corresponding alternative network slice.

15. The method (400) of claim 14, wherein said replacing (430) further comprises: transmitting, to a Session Management Function, SMF, an update session management context message indicating that the PDU session is to be transferred to the second network slice.

16. The method (400) of any of claims 10 to 15, wherein the source NF is a source Session Management Function, SMF, or Intermediate SMF, I-SMF, or Visited SMF, V-SMF, and the target NF is a target SMF or I-SMF or V-SMF, and wherein the information on the first network slice and the information on the second network slice are received in Session Management, SM, context information.

17. The method (400) of claim 16, wherein said replacing (430) comprises: transmitting, in response to determining that the PDU session is to be retained, the information on the second network slice to a User Plane Function, UPF, and a Next Generation Radio Access Network, NG-RAN, node associated with the PDU session; or transmitting, in response to determining that the PDU session is to be re-established, the information on the second network slice to a terminal device associated with the PDU session.

18. The method (400) of claim 16, further comprising: transmitting, to a Charging Function, CHF, a charging data request for the PDU session, the charging data request including the information on the second network slice.

19. The method (400) of any of claims 10 to 18, wherein the information on the first network slice is carried in an Information Element, IE, indicating an Single Network Slice Selection Assistance Information, S-NSSAI, associated with the PDU session, and the information on the second network slice is carried in an IE indicating an S-NSSAI that is replaced by the S-NSSAI associated with the PDU session, or the information on the second network slice is carried in an IE indicating an S-NSSAI associated with the PDU session, and the information on the first network slice is carried in an IE indicating an S-NSSAI that replaces the S-NSSAI associated with the PDU session.

20. The method (400) of any of claims 10 to 19, further comprising: transmitting, to a Network Repository Function, NRF, a registration message containing information indicating that the target NF supports network slice replacement.

21. The method (400) of any of claims 10 to 20, wherein the information on the first network slice and the information on the second network slice are received in a request.

22. The method (400) of any of claims 10 to 19, further comprising: transmitting, to the source NF, a request containing information indicating that the target NF supports network slice replacement.

23. A source network function, NF, configured to transmit (310), to a target NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a Packet Data Unit, PDU, session transferred from the second network slice to the first network slice.

24. The source network function of 23, being configured to perform the method of any of claims 2 to 9.

25. A network node (700), comprising a communication interface (710), a processor (720), and a memory (730), the memory (730) comprising instructions executable by the processor (720) whereby the network node (700) is operative to implement the source Network Function, NF, according to claim 23 or 24.

26. A target network function, NF, configured to receive (410), from a source NF, information on a first network slice and information on a second network slice that has been replaced by the first network slice, for a Packet Data Unit, PDU, session transferred from the second network slice to the first network slice; receive (420) a notification that the second network slice becomes available; and replace (430) the first network slice with the second network slice for the PDU session.

27. The target network function of claim 26, being configured to perform the method of any of claims 11 to 22.

28. A network node (700), comprising a communication interface (710), a processor (720), and a memory (730), the memory (730) comprising instructions executable by the processor (720) whereby the network node (700) is operative to implement the target Network Function, NF, according to claim 26 or 27.

29. A computer-readable storage medium having computer-readable instructions stored thereon, the computer-readable instructions, when executed by a processor of a network node, configure the network node to, when implementing a source Network Function, NF, perform the method according to any of claims 1-9, or when implementing a target NF, perform the method according to any of claims 10-22.

Images