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full Contents for  TS 23.501  Word version:   16.4.0

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5.6.7  Application Function influence on traffic routingWord-p. 115
5.6.7.1  General [R16]
The content of this clause applies to non-roaming and to LBO deployments i.e. to cases where the involved entities (AF, PCF, SMF, UPF) belong to the Serving PLMN or AF belongs to a third party with which the Serving PLMN has an agreement. AF influence on traffic routing does not apply in the case of Home Routed deployments. PCF shall not apply AF requests to influence traffic routing to PDU Sessions established in Home Routed mode.
An AF may send requests to influence SMF routeing decisions for traffic of PDU Session. The AF requests may influence UPF (re)selection and allow routeing user traffic to a local access to a Data Network (identified by a DNAI)
The AF may issue requests on behalf of applications not owned by the PLMN serving the UE.
If the operator does not allow an AF to access the network directly, the AF shall use the NEF to interact with the 5GC, as described in clause 6.2.10.
The AF may be in charge of the (re)selection or relocation of the applications within the local DN. Such functionality is not defined. For this purpose, the AF may request to get notified about events related with PDU Sessions.
The AF requests are sent to the PCF via N5 (in the case of requests targeting specific on-going PDU Sessions of individual UE(s), for an AF allowed to interact directly with the 5GC NFs) or via the NEF. The AF requests that target existing or future PDU Sessions of multiple UE(s) or of any UE are sent via the NEF and may target multiple PCF(s), as described in clause 6.3.7.2. The PCF(s) transform(s) the AF requests into policies that apply to PDU Sessions. When the AF has subscribed to UP path management event notifications from SMF(s) (including notifications on how to reach a GPSI over N6), such notifications are sent either directly to the AF or via an NEF (without involving the PCF). For AF interacting with PCF directly or via NEF, the AF requests may contain the information as described in the Table 5.6.7-1:
Information Name
Applicable for PCF or NEF (NOTE 1)
Applicable for NEF only
Category

Traffic Description
Defines the target traffic to be influenced, represented by the combination of DNN and optionally S-NSSAI, and application identifier or traffic filtering information.
The target traffic can be represented by AF-Service-Identifier, instead of combination of DNN and optionally S-NSSAI.
Mandatory
Potential Locations of Applications
Indicates potential locations of applications, represented by a list of DNAI(s).
The potential locations of applications can be represented by AF-Service-Identifier.
Conditional (NOTE 2)
Target UE Identifier(s)
Indicates the UE(s) that the request is targeting, i.e. an individual UE, a group of UE represented by Internal Group Identifier (NOTE 3), or any UE accessing the combination of DNN, S-NSSAI and DNAI(s).
GPSI can be applied to identify the individual UE, or External Group Identifier can be applied to identify a group of UE.
Mandatory
Spatial Validity Condition
Indicates that the request applies only to the traffic of UE(s) located in the specified location, represented by areas of validity.
The specified location can be represented by a list of geographic zone identifier(s).
Optional
AF transaction identifier
The AF transaction identifier refers to the AF request.
N/A
Mandatory
Traffic Routing requirements
Routing profile ID and/or N6 traffic routing information corresponding to each DNAI and an optional indication of traffic correlation.
N/A
Optional
Application Relocation Possibility
Indicates whether an application can be relocated once a location of the application is selected by the 5GC.
N/A
Optional
UE IP address preservation indication
Indicates UE IP address should be preserved.
N/A
Optional
Temporal Validity Condition
Time interval(s) or duration(s).
N/A
Optional
Information on AF subscription to corresponding SMF events
Indicates whether the AF subscribes to change of UP path of the PDU Session and the parameters of this subscription.
N/A
Optional

NOTE 1:
When the AF request targets existing or future PDU Sessions of multiple UE(s) or of any UE and is sent via the NEF, as described in clause 6.3.7.2, the information is stored in the UDR by the NEF and notified to the PCF by the UDR.
NOTE 2:
The potential locations of applications and traffic routing requirements may be absent only if the request is for subscription to notifications about UP path management events only.
NOTE 3:
Internal Group ID can only be used by an AF controlled by the operator.

For each information element mentioned above in the AF request, the detailed description is as follows:
  1. Information to identify the traffic. The traffic can be identified in the AF request by
    • Either a DNN and possibly slicing information (S-NSSAI) or an AF-Service-Identifier
      • When the AF provides an AF-Service-Identifier i.e. an identifier of the service on behalf of which the AF is issuing the request, the 5G Core maps this identifier into a target DNN and slicing information (S-NSSAI)
      • When the NEF processes the AF request the AF-Service-Identifier may be used to authorize the AF request.
    • an application identifier or traffic filtering information (e.g. 5 Tuple). The application identifier refers to an application handling UP traffic and is used by the UPF to detect the traffic of the application
    When the AF request is for influencing SMF routing decisions, the information is to identify the traffic to be routed.
    When the AF request is for subscription to notifications about UP path management events, the information is to identify the traffic that the events relate to.
  2. Information about the N6 traffic routing requirements for traffic identified as defined in 1). This includes:
    • Information about the N6 traffic routing requirements that is provided per DNAI: for each DNAI, the N6 traffic routing requirements may contain a routing profile ID and/or N6 traffic routing information.
    • an optional indication of traffic correlation, when the information in 4) identifies a group of UEs. This implies the targeted PDU Sessions should be correlated by a common DNAI in the user plane for the traffic identified in 1). If this indication is provided by the AF, the 5GC should select a common DNAI for the target PDU Sessions from the list of DNAI(s) specified in 3).
    NOTE 1:
    The N6 traffic routing requirements are related to the mechanism enabling traffic steering in the local access to the DN. The routing profile ID refers to a pre-agreed policy between the AF and the 5GC. This policy may refer to different steering policy ID(s) sent to SMF and e.g. based on time of the day etc.
    NOTE 2:
    The mechanisms enabling traffic steering in the local access to the DN are not defined.
  3. Potential locations of applications towards which the traffic routing should apply. The potential location of application is expressed as a list of DNAI(s). If the AF interacts with the PCF via the NEF, the NEF may map the AF-Service-Identifier information to a list of DNAI(s). The DNAI(s) may be used for UPF (re)selection.
  4. Information on the UE(s). This may correspond to:
    • Individual UEs identified using GPSI, or an IP address/Prefix or a MAC address.
    • groups of UEs identified by an External Group Identifier as defined in TS 23.682 when the AF interacts via the NEF, or Internal-Group Identifier (see clause 5.9.7) when the AF interacts directly with the PCF.
    • any UE accessing the combination of DNN, S-NSSAI and DNAI(s).
    When the PDU Session type is IPv4 or IPv6 or IPv4v6, and the AF provides an IP address and/or an IP Prefix, or when the PDU Session type is Ethernet and the AF provides a MAC address, this allows the PCF to identify the PDU Session for which this request applies and the AF request applies only to that specific PDU Session of the UE. In this case, additional information such as the UE identity may also be provided to help the PCF to identify the correct PDU Session.
    Otherwise the request targets multiple UE(s) and shall apply to any existing or future PDU Sessions that match the parameters in the AF request.
    When the AF request targets an individual UE and GPSI is provided within the AF request, the GPSI is mapped to SUPI according to the subscription information received from UDM.
    When the AF request targets any UE or a group of UE, the AF request is likely to influence multiple PDU Sessions possibly served by multiple SMFs and PCFs.
    When the AF request targets a group of UE it provides one or several group identifiers in its request. The group identifiers provided by the AF are mapped to Internal-Group identifiers. Members of the group have this Group Identifier in their subscription. The Internal-Group Identifier is stored in UDM, retrieved by SMF from UDM and passed by SMF to PCF at PDU Session set-up. The PCF can then map the AF requests with user subscription and determine whether an AF request targeting a Group of users applies to a PDU Session.
    When the AF request is for influencing SMF routing decisions, the information is to identify UE(s) whose traffic is to be routed.
    When the AF request is for subscription to notifications about UP path management events, the information is to identify UE(s) whose traffic the events relate to.
    When the AF request is for traffic forwarding in a PDU Session serving for TSC, the MAC address used by the PDU Session is determined by the AF to identify UE whose traffic is to be routed according to the previously stored binding relationship of the 5GS Bridge and the port number of the traffic forwarding information received from TSN network.
  5. Indication of application relocation possibility. This indicates whether an application can be relocated once a location of the application is selected by the 5GC. If application relocation is not possible, the 5GC shall ensure that for the traffic related with an application, no DNAI change takes place once selected for this application.
  6. Temporal validity condition. This is provided in the form of time interval(s) or duration(s) during which the AF request is to be applied.
  7. When the AF request is for influencing SMF routing decisions, the temporal validity condition indicates when the traffic routing is to apply.
    When the AF request is for subscription to notifications about UP path management events, the temporal validity condition indicates when the notifications are to be generated.
  8. Spatial validity condition on the UE(s) location. This is provided in the form of validity area(s). If the AF interacts with the PCF via the NEF, it may provide a list of geographic zone identifier(s) and the NEF maps the information to areas of validity based on pre-configuration. The PCF in turn determines area(s) of interest based on validity area(s).
  9. When the AF request is for influencing SMF routing decisions, the spatial validity condition indicates that the request applies only to the traffic of UE(s) located in the specified location.
    When the AF request is for subscription to notifications about UP path management events, the spatial validity condition indicates that the subscription applies only to the traffic of UE(s) located in the specified location.
  10. Information on AF subscription to corresponding SMF events.
    • The AF may request to be subscribed to change of UP path associated with traffic identified in the bullet 1) above. The AF request contains:
    • A type of subscription (subscription for Early and/or Late notifications).
    • The AF subscription can be for Early notifications and/or Late notifications. In the case of a subscription for Early notifications, the SMF sends the notifications before the (new) UP path is configured. In the case of a subscription for Late notifications, the SMF sends the notification after the (new) UP path has been configured.
    • Optionally, an indication of "AF acknowledgment to be expected".
    • The indication implies that the AF will provide a response to the notifications of UP path management events to the 5GC. The SMF may, according to this indication, determine to wait for a response from the AF before the SMF configures in the case of early notification, or activates in the case of late notification, the new UP path as described in clause 5.6.7.2.
    The AF subscription can also request to receive information associating the GPSI of the UE with the IP address(es) of the UE and/or with actual N6 traffic routing to be used to reach the UE on the PDU Session; in this case the corresponding information is sent by the SMF regardless of whether a DNAI applies to the PDU Session.
  11. An AF transaction identifier referring to the AF request. This allows the AF to update or remove the AF request and to identify corresponding UP path management event notifications. The AF transaction identifier is generated by the AF.
  12. When the AF interacts with the PCF via the NEF, the NEF maps the AF transaction identifier to an AF transaction internal identifier, which is generated by the NEF and used within the 5GC to identify the information associated to the AF request. The NEF maintains the mapping between the AF transaction identifier and the AF transaction internal identifier. The relation between the two identifiers is implementation specific.
    When the AF interacts with the PCF directly, the AF transaction identifier provided by the AF is used as AF transaction internal identifier within the 5GC.
  13. Indication of UE IP address preservation. This indicates UE IP address related to the traffic identified in bullet 1) should be preserved. If this indication is provided by the AF, the 5GC should preserve the UE IP address by preventing reselection of PSA UPF for the identified traffic once the PSA UPF is selected.
An AF may send requests to influence SMF routeing decisions, for event subscription or for both.
The AF may request to be subscribed to notifications about UP path management events, i.e. a UP path change occurs for the PDU Session. The corresponding notification about a UP path change sent by the SMF to the AF may indicate the DNAI and /or the N6 traffic routing information that has changed as described in clause 4.3.6.3 of TS 23.502. It may include the AF transaction internal identifier, the type of notification (i.e. early notification or late notification), the Identity of the source and/or target DNAI, the IP address/prefix of the UE or the MAC address used by the UE, the GPSI and the N6 traffic routing information related to the 5GC UP.
NOTE 3:
The change from the UP path status where no DNAI applies to a status where a DNAI applies indicates the activation of this AF request; the change from the UP path status where a DNAI applies to a status where no DNAI applies indicates the de-activation of this AF request.
  • In the case of IP PDU Session Type, the IP address/prefix of the UE together with N6 traffic routing information indicates to the AF how to reach over the User Plane the UE identified by its GPSI. N6 traffic routing information indicates any tunnelling that may be used over N6. The nature of this information depends on the deployment.
NOTE 4:
N6 traffic routing information can e.g. correspond to the identifier of a VPN or to explicit tunnelling information such as a tunnelling protocol identifier together with a Tunnel identifier.
NOTE 5:
In the case of Unstructured PDU Session type the nature of the N6 traffic routing information related to the 5GC UP is described in clause 5.6.10.3.
  • In the case of Ethernet PDU Session Type, the MAC address of the UE together with N6 traffic routing information indicates to the AF how to reach over the User Plane the UE identified by its GPSI. The UE MAC address (es) is reported by the UPF as described in clause 5.8.2.12. The N6 traffic routing information can be, e.g. a VLAN ID or the identifier of a VPN or a tunnel identifier at the UPF.
  • In the case of PDU Session serving for TSC, the SMF informs AF of the 5GS Bridge user plane information as defined in clause 5.28.1, as well as the association between the MAC address used by the PDU Session, 5GS Bridge ID and port ID on DS-TT.
When notifications about UP path management events are sent to the AF via the NEF, if required, the NEF maps the UE identify information, e.g. SUPI, to the GPSI and the AF transaction internal identifier to the AF transaction identifier before sending the notifications to the AF.
The PCF authorizes the request received from the AF based on information received from the AF, operator's policy, etc. and determines for each DNAI, a traffic steering policy ID (derived from the routing profile ID provided by the AF) and/or the N6 traffic routing information (as provided by the AF) to be sent to the SMF as part of the PCC rules. The traffic steering policy IDs are configured in the SMF or in the UPF. The traffic steering policy IDs are related to the mechanism enabling traffic steering to the DN.
The DNAIs are related to the information considered by the SMF for UPF selection, e.g. for diverting (locally) some traffic matching traffic filters provided by the PCF.
The PCF acknowledges a request targeting an individual PDU Session to the AF or to the NEF.
For PDU Session that corresponds to the AF request, the PCF provides the SMF with a PCC rule that is generated based on the AF request, Local routing indication from the PDU Session policy control subscription information and taking into account UE location presence in area of interest (i.e. Presence Reporting Area). The PCC rule contains the information to identify the traffic, information about the DNAI(s) towards which the traffic routing should apply and optionally, an indication of traffic correlation and/or an indication of application relocation possibility and/or indication of UE IP address preservation. The PCC rule also contains per DNAI a traffic steering policy ID and/or N6 traffic routing information, if the N6 traffic routing information is explicitly provided in the AF request. The PCF may also provide in the PCC rule information to subscribe the AF (or the NEF) to SMF events (UP path changes) corresponding to the AF request in which case it provides the information on AF subscription to corresponding SMF events received in the AF request. This is done by providing policies at PDU Session set-up or by initiating a PDU Session Modification procedure. When initiating a PDU Session set-up or PDU Session Modification procedure, the PCF considers the latest known UE location to determine the PCC rules provided to the SMF. The PCF evaluates the temporal validity condition of the AF request and informs the SMF to activate or deactivate the corresponding PCC rules according to the evaluation result. When policies specific to the PDU Session and policies general to multiple PDU Sessions exist, the PCF gives precedence to the PDU Session specific policies over the general policies.
The spatial validity condition is resolved at the PCF. In order to do that, the PCF subscribes to the SMF to receive notifications about change of UE location in an area of interest (i.e. Presence Reporting Area). The subscribed area of interest may be the same as spatial validity condition, or may be a subset of the spatial validity condition (e.g. a list of TAs) based on the latest known UE location. When the SMF detects that UE entered the area of interest subscribed by the PCF, the SMF notifies the PCF and the PCF provides to the SMF the PCC rules described above by triggering a PDU Session Modification. When the SMF becomes aware that the UE left the area subscribed by the PCF, the SMF notifies the PCF and the PCF provides updated PCC rules by triggering a PDU Session Modification. SMF notifications to the PCF about UE location in or out of the subscribed area of interest are triggered by UE location change notifications received from the AMF or by UE location information received during a Service Request or Handover procedure.
When the PCC rules are activated, the SMF may, based on local policies, take the information in the PCC rules into account to:
  • (re)select UP paths (including DNAI(s)) for PDU Sessions. The SMF is responsible for handling the mapping between the UE location (TAI / Cell-Id) and DNAI(s) associated with UPF and applications and the selection of the UPF(s) that serve a PDU Session. This is described in clause 6.3.3. If the PDU Session is of IP type and if Indication of UE IP address preservation is included in the PCC rules, the SMF should preserve the UE IP address, by not reselecting the related PSA UPF once the PSA UPF is selected, for the traffic identified in the PCC rule. If the PCC rules are related to a 5G VN group served by the SMF and if the Information about the N6 traffic routing requirements includes an indication of traffic correlation, the SMF should select a common DNAI for the PDU Sessions of the 5G VN group.
  • configure traffic steering at UPF, including activating mechanisms for traffic multi-homing or enforcement of an UL Classifier (UL CL). Such mechanisms are defined in clause 5.6.4. This may include that the SMF is providing the UPF with packet handling instructions (i.e. PDRs and FARs) for steering traffic to the local access to the DN. The packet handling instructions are generated by the SMF using the traffic steering policy ID and/or the N6 traffic routing information in the PCC rules corresponding to the applied DNAI. In the case of UP path reselection, the SMF may configure the source UPF to forward traffic to the UL CL/BP so that the traffic is steered towards the target UPF.
  • if Information on AF subscription to corresponding SMF events has been provided in the PCC rule, inform the AF of the (re)selection of the UP path (UP path change). If the information includes an indication of "AF acknowledgment to be expected", the SMF may decide to wait for a response from the AF before it activates the new UP path, as described in clause 5.6.7.2.
When an I-SMF is inserted for a PDU Session, the I-SMF insertion, relocation or removal to a PDU session shall be transparent (i.e. not aware) to the PCF and to the AF. The processing of the AF influence on traffic routing is described in clause 5.34 and detail procedure is described in clause 4.23.6 of TS 23.502.
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5.6.7.2  Enhancement of UP path management based on the coordination with AFs [R16]Word-p. 120
In order to avoid or minimize service interruption during PSA relocation for a PDU session of SSC mode 3, or a PDU session with UL CL or branch point, according to the indication of "AF acknowledgment to be expected" on AF subscription to corresponding SMF events (DNAI change) in the PCC rules received from the PCF and local configuration (e.g. DN-related policies) the SMF may wait for a response from the AF after sending a notification (an early notification or a late notification) to the AF. In the case of late notification, based on the indication of "AF acknowledgment to be expected" on AF subscription, the SMF may send the notification before activating the UP path towards a new DNAI (possibly through a new PSA).
NOTE 1:
Before the UP path toward the new DNAI is activated, application traffic data (if any exists) is still routed toward the old DNAI.
The notification sent from the SMF to the AF indicates UP path management events (DNAI change) as described in clause 5.6.7.1. The AF can confirm the DNAI change indicated in the notification with the SMF by sending a positive response to the notification to the SMF or reject the DNAI change by sending a negative response.
NOTE 2:
The AF can determine whether application relocation is needed according to the notification of DNAI change. If not, the AF can send a positive response to the SMF immediately; otherwise, the AF sends the positive response after application relocation is completed or a negative response if the AF determines that the application relocation cannot be completed on time (e.g. due to temporary congestion). The AF decision and behaviours on application relocation are not defined. However, the new DNAI may be associated with a new AF. In such cases, the SMF and the old AF cancel earlier subscribed UP path management event notifications, and the new AF subscribes to receive UP path management event notifications from the SMF.
The AF can include N6 traffic routing information related to the target DNAI in a positive response sent to the SMF. The SMF configures the N6 traffic routing information from the AF response to the PSA on the UP path.
In the case of early notification, based on the indication of "AF acknowledgment to be expected" on AF subscription, the SMF does not configure the UP path towards the new DNAI until it receives a positive AF response as specified in clause 4.3.6.3 of TS 23.502.
In the case of late notification, based on the indication of "AF acknowledgment to be expected" on AF subscription, the SMF does not activate the UP path towards the new DNAI until it receives a positive AF response as specified in clause 4.3.5 of TS 23.502.
NOTE 3:
After the UP path toward the new DNAI is activated, data is routed toward the new DNAI.
If the SMF receives a negative response at any time, the SMF keeps using the original DNAI and may cancel related PSA relocation or addition. The SMF may perform DNAI reselection afterwards if needed.
The SMF can assume according to local policy a negative response if a response is expected and but not received from the AF within a certain time window.
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5.6.8  Selective activation and deactivation of UP connection of existing PDU SessionWord-p. 121
This clause applies to the case when a UE has established multiple PDU Sessions. The activation of a UP connection of an existing PDU Session causes the activation of its UE-CN User Plane connection (i.e. data radio bearer and N3 tunnel).
For the UE in the CM-IDLE state in 3GPP access, either UE or Network-Triggered Service Request procedure may support independent activation of UP connection of existing PDU Session. For the UE in the CM-IDLE state in non-3GPP access, UE-Triggered Service Request procedure allows the re-activation of UP connection of existing PDU Sessions, and may support independent activation of UP connection of existing PDU Session.
A UE in the CM-CONNECTED state invokes a Service Request (see TS 23.502, clause 4.2.3.2) procedure to request the independent activation of the UP connection of existing PDU Sessions.
Network Triggered re-activation of UP connection of existing PDU Sessions is handled as follows:
  • If the UE CM state in the AMF is already CM-CONNECTED on the access (3GPP, non-3GPP) associated to the PDU Session in the SMF, the network may re-activate the UP connection of a PDU Session using a Network Initiated Service Request procedure.
  • Otherwise:
  • If the UE is registered in both 3GPP and non-3GPP accesses and the UE CM state in the AMF is CM-IDLE in non-3GPP access, the UE can be paged or notified through the 3GPP access for a PDU Session associated in the SMF (i.e. last routed) to the non-3GPP access:
    • If the UE CM state in the AMF is CM-IDLE in 3GPP access, the paging message may include the access type associated with the PDU Session in the SMF. The UE, upon reception of the paging message containing an access type, shall reply to the 5GC via the 3GPP access using the NAS Service Request message, which shall contain the list of PDU Sessions associated with the received access type and whose UP connections can be re-activated over 3GPP (i.e. the list does not contain the PDU Sessions whose UP connections cannot be re-activated on 3GPP based on UE policies and whether the S-NSSAIs of these PDU Sessions are within the Allowed NSSAI for 3GPP access). If the PDU Session for which the UE has been paged is in the list of the PDU Sessions provided in the NAS Service Request and the paging was triggered by pending DL data, the 5GC shall re-activate the PDU Session UP connection over 3GPP access. If the paging was triggered by pending DL signalling, the Service Request succeeds without re-activating the PDU session UP connection over the 3GPP access and the pending DL signalling is delivered to the UE over the 3GPP access;
    • If the UE CM state in the AMF is CM-CONNECTED in 3GPP access, the notification message shall include the non-3GPP Access Type. The UE, upon reception of the notification message, shall reply to the 5GC via the 3GPP access using the NAS Service Request message, which shall contain the List of Allowed PDU Sessions that can be re-activated over 3GPP or an empty List of Allowed PDU Sessions if no PDU Sessions are allowed to be re-activated over 3GPP access.
    • NOTE:
      A UE that is in a coverage of a non-3GPP access and has PDU Session(s) that are associated in the UE (i.e. last routed) to non-3GPP access, is assumed to attempt to connect to it without the need to be paged.
  • If the UE is registered in both 3GPP and non-3GPP accesses served by the same AMF and the UE CM state in the AMF is CM-IDLE in 3GPP access and is in CM-CONNECTED in non 3GPP access, the UE can be notified through the non-3GPP for a PDU Session associated in the SMF (i.e. last routed) to the 3GPP access. The notification message shall include the 3GPP Access Type. Upon reception of the notification message, when 3GPP access is available, the UE shall reply to the 5GC via the 3GPP access using the NAS Service Request message.
In addition to the above, a PDU Session may be established as an always-on PDU Session as described in clause 5.6.13.
The deactivation of the UP connection of an existing PDU Session causes the corresponding data radio bearer and N3 tunnel to be deactivated. The UP connection of different PDU Sessions can be deactivated independently when a UE is in CM-CONNECTED state in 3GPP access or non-3GPP access. At the deactivation of the UP of a PDU Session using a N9 tunnel whose end-point is controlled by an I-SMF, the N9 tunnel is preserved. If a PDU Session is an always-on PDU Session, the SMF should not deactivate a UP connection of this PDU Session due to inactivity.
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5.6.9  Session and Service ContinuityWord-p. 122
5.6.9.1  General
The support for session and service continuity in 5G System architecture enables to address the various continuity requirements of different applications/services for the UE. The 5G System supports different session and service continuity (SSC) modes defined in this clause. The SSC mode associated with a PDU Session does not change during the lifetime of a PDU Session. The following three modes are specified with further details provided in the next clause:
  • With SSC mode 1, the network preserves the connectivity service provided to the UE. For the case of PDU Session of IPv4 or IPv6 or IPv4v6 type, the IP address is preserved.
  • With SSC mode 2, the network may release the connectivity service delivered to the UE and release the corresponding PDU Session(s). For the case of IPv4 or IPv6 or IPv4v6 type, the release of the PDU Session induces the release of IP address(es) that had been allocated to the UE.
  • With SSC mode 3, changes to the user plane can be visible to the UE, while the network ensures that the UE suffers no loss of connectivity. A connection through new PDU Session Anchor point is established before the previous connection is terminated in order to allow for better service continuity. For the case of IPv4 or IPv6 or IPv4v6 type, the IP address is not preserved in this mode when the PDU Session Anchor changes.
NOTE:
In this Release of the specification, the addition/removal procedure of additional PDU Session Anchor in a PDU Session for local access to a DN is independent from the SSC mode of the PDU Session.
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5.6.9.2  SSC mode
5.6.9.2.1  SSC Mode 1
For a PDU Session of SSC mode 1, the UPF acting as PDU Session Anchor at the establishment of the PDU Session is maintained regardless of the access technology (e.g. Access Type and cells) a UE is successively using to access the network.
In the case of a PDU Session of IPv4 or IPv6 or IPv4v6 type, IP continuity is supported regardless of UE mobility events.
In this Release of the specification, when IPv6 multihoming or UL CL applies to a PDU Session of in SSC mode 1, and the network allocates (based on local policies) additional PDU Session Anchors to such a PDU Session, these additional PDU Session Anchors may be released or allocated, and the UE does not expect that the additional IPv6 prefix is maintained during the lifetime of PDU Session.
SSC mode 1 may apply to any PDU Session type and to any access type.
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5.6.9.2.2  SSC Mode 2Word-p. 123
If a PDU Session of SSC mode 2 has a single PDU Session Anchor, the network may trigger the release of the PDU Session and instruct the UE to establish a new PDU Session to the same data network immediately. The trigger condition depends on operator policy e.g. request from Application Function, based on load status, etc. At establishment of the new PDU Session, a new UPF acting as PDU Session Anchor can be selected.
Otherwise, if a PDU Session of SSC mode 2 has multiple PDU Session Anchors (i.e. in the case of multi-homed PDU Sessions or in the case that UL CL applies to a PDU Session of SSC mode 2), the additional PDU Session Anchors may be released or allocated.
SSC mode 2 may apply to any PDU Session type and to any access type.
NOTE:
In UL CL mode, the UE is not involved in PDU Session Anchor re-allocation, so that the existence of multiple PDU Session Anchors is not visible to the UE.
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5.6.9.2.3  SSC Mode 3
For PDU Session of SSC mode 3, the network allows the establishment of UE connectivity via a new PDU Session Anchor to the same data network before connectivity between the UE and the previous PDU Session Anchor is released. When trigger conditions apply, the network decides whether to select a PDU Session Anchor UPF suitable for the UE's new conditions (e.g. point of attachment to the network).
In this Release of specification, SSC mode 3 only applies to IP PDU Session type and to any access type.
In the case of a PDU Session of IPv4 or IPv6 or IPv4v6 type, during the procedure of change of PDU Session Anchor, the following applies:
  1. For a PDU Session of IPv6 type, the new IP prefix anchored on the new PDU Session Anchor may be allocated within the same PDU Session (relying on IPv6 multi-homing specified in clause 5.6.4.3), or
  2. The new IP address and/or IP prefix may be allocated within a new PDU Session that the UE is triggered to establish.
After the new IP address/prefix has been allocated, the old IP address/prefix is maintained during some time indicated to the UE via NAS signalling (as described in TS 23.502, clause 4.3.5.2) or via Router Advertisement (as described in TS 23.502, clause 4.3.5.3) and then released.
If a PDU Session of SSC mode 3 has multiple PDU Session Anchors (i.e., in the case of multi-homed PDU Sessions or in the case that UL CL applies to a PDU Session of SSC mode 3), the additional PDU Session Anchors may be released or allocated.
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5.6.9.3  SSC mode selection
SSC mode selection is done by the SMF based on the allowed SSC modes -including the default SSC mode) in the user subscription as well as the PDU Session type and if present, the SSC mode requested by the UE.
The operator may provision a SSC mode selection policy (SSCMSP) to the UE as part of the URSP rule -see TS 23.503, clause 6.6.2). The UE shall use the SSCMSP to determine the type of session and service continuity mode associated with an application or group of applications for the UE as described in TS 23.503, clause 6.6.2.3. If the UE does not have SSCMSP, the UE can select a SSC mode based on UE Local Configuration as described in TS 23.503, if applicable. If the UE cannot select a SSC mode, the UE requests the PDU Session without providing the SSC mode.
NOTE:
The UE can use the SSC Mode Selection component of the URSP rule with match-all traffic descriptor if there is no SSC mode in the UE local configuration.
The SSC mode selection policy rules provided to the UE can be updated by the operator by updating the URSP rule.
The SMF receives from the UDM the list of allowed SSC modes and the default SSC mode per DNN per S-NSSAI as part of the subscription information.
If a UE provides an SSC mode when requesting a new PDU Session, the SMF selects the SSC mode by either accepting the requested SSC mode or rejecting the PDU Session Establishment Request message with the cause value and the SSC mode(s) allowed to be used back to UE based on the PDU Session type, subscription and/or local configuration. Based on that cause value and the SSC mode(s) allowed to be used, the UE may re-attempt to request the establishment of that PDU Session with the SSC mode allowed to be used or using another URSP rule.
If a UE does not provide an SSC mode when requesting a new PDU Session, then the SMF selects the default SSC mode for the data network listed in the subscription or applies local configuration to select the SSC mode.
SSC mode 1 shall be assigned to the PDU Session when static IP address/prefix is allocated to the PDU Session based on the static IP address/prefix subscription for the DNN and S-NSSAI. The SMF shall inform the UE of the selected SSC mode for a PDU Session.
The UE shall not request and the network shall not assign SSC mode 3 for the PDU Session of Unstructured type or Ethernet type.
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5.6.10  Specific aspects of different PDU Session typesWord-p. 124
5.6.10.1  Support of IP PDU Session type
The IP address allocation is defined in clause 5.8.1
The UE may acquire following configuration information from the SMF, during the lifetime of a PDU Session:
  • Address(es) of P-CSCF(s).
  • Address(es) of DNS server(s).
  • the GPSI of the UE.
The UE may acquire from the SMF, at PDU Session Establishment, the MTU that the UE shall consider, see clause 5.6.10.4.
The UE may provide following information to the SMF during the lifetime of a PDU Session:
    an indication of the support of P-CSCF re-selection based on procedures specified in TS 24.229 (clauses B.2.2.1C and L.2.2.1C).
  • PS data off status of the UE.
NOTE:
An operator can deploy NAT functionality in the network; the support of NAT is not specified in this release of the specification.
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5.6.10.2  Support of Ethernet PDU Session type
For a PDU Session set up with the Ethernet PDU Session type, the SMF and the UPF acting as PDU Session Anchor (PSA) can support specific behaviours related with the fact the PDU Session carries Ethernet frames.
Depending on operator configuration related with the DNN, different configurations for how Ethernet traffic is handled on N6 may apply, for example:
  • Configurations with a 1-1 relationship between a PDU Session and a N6 interface possibly corresponding to a dedicated tunnel established over N6. In this case the UPF acting as PSA transparently forwards Ethernet frames between the PDU Session and its corresponding N6 interface, and it does not need to be aware of MAC addresses used by the UE in order to route down-link traffic.
  • Configurations, where more than one PDU Session to the same DNN (e.g. for more than one UE) corresponds to the same N6 interface. In this case the UPF acting as PSA needs to be aware of MAC addresses used by the UE in the PDU Session in order to map down-link Ethernet frames received over N6 to the appropriate PDU Session. Forwarding behaviour of the UPF acting as PSA is managed by SMF as specified in clause 5.8.2.5.
NOTE 1:
The "MAC addresses used by the UE" correspond to any MAC address used by the UE or any device locally connected to the UE and using the PDU Session to communicate with the DN.
Based on operator configuration, the SMF may request the UPF acting as the PDU Session Anchor to respond to ARP/IPv6 Neighbour Solicitation requests based on local cache information, i.e. the mapping between the UE MAC address to the UE IP address, and the DN where the PDU Session is connected to, or to redirect the ARP traffic from the UPF to the SMF. Responding to ARP/IPv6 ND based on local cache information applies to ARP/IPv6 ND received in both UL and DL directions.
NOTE 2:
Responding to ARP/ND from a local cache assumes the UE or the devices behind the UE acquire their IP address via in-band mechanisms that the SMF/UPF can detect and by this link the IP address to the MAC address.
NOTE 3:
This mechanism is intended to avoid broadcasting or multicasting the ARP/IPv6 ND to every UE.
Ethernet Preamble and Start of Frame delimiter are not sent over 5GS:
  • For UL traffic the UE strips the preamble and frame check sequence (FCS) from the Ethernet frame.
  • For DL traffic the PDU Session Anchor strips the preamble and frame check sequence (FCS) from the Ethernet frame.
Neither a MAC nor an IP address is allocated by the 5GC to the UE for a PDU Session.
The PSA shall store the MAC addresses received from the UE, and associate those with the appropriate PDU Session.
The UPF handles VLAN tags (addition/removal) as instructed by the SMF via PDR (Outer header removal) and FAR (UPF applying Outer header creation of a Forwarding policy). For example:
  • The UPF may insert (for uplink traffic) and remove (for downlink traffic) a S-TAG on N6 interface on top of the C-TAG of the UE.
  • The UPF may insert (for uplink traffic) and remove (for downlink traffic) a VLAN tag on the N6 interface while there is no VLAN in the traffic to and from the UE.
NOTE 4:
This can be used for traffic steering to N6-LAN but also for N6-based traffic forwarding related with 5G-VN service described in clause 5.29.4
Apart from specific conditions related to the support of PDU sessions over W-5GAN defined in TS 23.316, the UPF shall not remove VLAN tags sent by the UE and the UPF shall not insert VLAN tags for the traffic sent to the UE.
PDU(s) containing a VLAN tag shall be switched only within the same VLAN by a PDU Session Anchor.
The UE may acquire from the SMF, at PDU Session Establishment, the MTU of the Ethernet frames' payload that the UE shall consider, see clause 5.6.10.4.
NOTE 5:
The UE may operate in bridge mode with regard to a LAN it is connecting to the 5GS, thus different MAC addresses may be used as source address of different frames sent UL over a single PDU Session (and destination MAC address of different frames sent DL over the same PDU Session).
NOTE 6:
Entities on the LAN connected to the 5GS by the UE may have an IP address allocated by the DN but the IP layer is considered as an application layer which is not part of the Ethernet PDU Session.
NOTE 7:
In this Release of the specification, only the UE connected to the 5GS is authenticated, not the devices behind such UE.
NOTE 8:
5GS does not support the scenario where a MAC address or if VLAN applies a (MAC address, VLAN) combination is used on more than one PDU Session for the same DNN and S-NSSAI.
NOTE 9:
This Release of the specification does not guarantee that the Ethernet network remains loop-free. Deployments need to be verified on an individual basis that loops in the Ethernet network are avoided.
NOTE 10:
This Release of the specification does not guarantee that the Ethernet network properly and quickly reacts to topology changes. Deployments need to be verified on an individual basis how they react to topology changes.
Different Frames exchanged on a PDU Session of Ethernet type may be served with different QoS over the 5GS. Thus, the SMF may provide to the UPF Ethernet Packet Filter Set and forwarding rule(s) based on the Ethernet frame structure and UE MAC address(es). The UPF detects and forwards Ethernet frames based on the Ethernet Packet Filter Set and forwarding rule(s) received from the SMF. This is further defined in clauses 5.7 and 5.8.2.
When a PDU Session of Ethernet PDU type is authorized by a DN as described in clause 5.6.6, the DN-AAA server may, as part of authorization data, provide the SMF with a list of allowed MAC addresses and/or a list of allowed VIDs for this PDU Session; the list is limited to a maximum of 16 MAC addresses and/or a maximum of 16 VIDs accordingly. When such list(s) have been provided for a PDU Session, the SMF sets corresponding filtering rules in the UPF(s) acting as PDU Session Anchor for the PDU Session. The UPF discards any UL traffic that does not contain one of these MAC addresses as a source address if the list of allowed MAC addresses is provided. The UPF discards any UL traffic that does not contain one of these VIDs if the list of allowed VIDs is provided.
In this Release of specification, the PDU Session of Ethernet PDU Session type is restricted to SSC mode 1 and SSC mode 2.
For a PDU Session established with the Ethernet PDU Session type, the SMF may, upon PCF request, need to ensure reporting to the PCF of all Ethernet MAC addresses used as UE address in a PDU Session. In this case, as defined in clause 5.8.2.12, the SMF controls the UPF to report the different MAC addresses used as source address of frames sent UL by the UE in the PDU Session.
NOTE 11:
This relates to whether AF control on a per MAC address is allowed on the PDU Session as defined in TS 23.503, clause 6.1.1.2.
The PCF may activate or deactivate the reporting of the UE MAC address using the "UE MAC address change" Policy Control Request Trigger as defined in Table 6.1.3.5-1 of TS 23.503.
The SMF may relocate the UPF acting as the PDU Session Anchor for an Ethernet PDU Session as defined in clause 4.3.5.8 of TS 23.502. The relocation may be triggered by a mobility event such as a handover, or may be triggered independent of UE mobility, e.g. due to load balancing reasons. In order to relocate the PSA UPF, the reporting of the UE MAC addresses needs to be activated by the SMF.
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5.6.10.3  Support of Unstructured PDU Session typeWord-p. 126
Different Point-to-Point (PtP) tunnelling techniques may be used to deliver Unstructured PDU Session type data to the destination (e.g. application server) in the Data Network via N6.
Point-to-point tunnelling based on UDP/IP encapsulation as described below may be used. Other techniques may be supported. Regardless of addressing scheme used from the UPF to the DN, the UPF shall be able to map the address used between the UPF and the DN to the PDU Session.
When Point-to-Point tunnelling based on UDP/IPv6 is used, the following considerations apply:
  • IPv6 prefix allocation for PDU Sessions are performed locally by the (H-)SMF without involving the UE.
  • The UPF(s) acts as a transparent forwarding node for the payload between the UE and the destination in the DN.
  • For uplink, the UPF forwards the received Unstructured PDU Session type data to the destination in the data network over the N6 PtP tunnel using UDP/IPv6 encapsulation.
  • For downlink, the destination in the data network sends the Unstructured PDU Session type data using UDP/IPv6 encapsulation with the IPv6 address of the PDU Session and the 3GPP defined UDP port for Unstructured PDU Session type data. The UPF acting as PDU Session Anchor decapsulates the received data (i.e. removes the UDP/IPv6 headers) and forwards the data identified by the IPv6 prefix of the PDU Session for delivery to the UE.
  • The (H-)SMF performs the IPv6 related operations but the IPv6 prefix is not provided to the UE, i.e. Router Advertisements and DHCPv6 are not performed. The SMF assigns an IPv6 Interface Identifier for the PDU Session. The allocated IPv6 prefix identifies the PDU Session of the UE.
  • For AF influence on traffic routing (described in clause 5.6.7), when the N6 PtP tunnelling is used over the DNAI and the AF provides, by value, information about N6 traffic routing requirements in the AF request, the AF provides N6 PtP tunnelling requirements (IPv6 address and UDP port of the tunnel end in the DN) as the N6 traffic routing information associated to the DNAI; when the SMF notifies the AF of UP path management events, it includes the N6 PtP tunnel information related to the UP (the IPv6 address and the 3GPP defined UDP port of the tunnel end at the UPF) as N6 traffic routing information in the notification.
In this Release of the specification there is support for maximum one 5G QoS Flow per PDU Session of Type Unstructured.
In this Release of specification, the PDU Session of Unstructured PDU Session type is restricted to SSC mode 1 and SSC mode 2.
The UE may acquire from the SMF, at PDU Session Establishment, the MTU that the UE shall consider, see clause 5.6.10.4.
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5.6.10.4  Maximum Transfer Unit size considerations [R16]Word-p. 127
In order to avoid data packet fragmentation between the UE and the UPF acting as PSA, the link MTU size in the UE should be set to the value provided by the network as part of the IP configuration. The link MTU size for IPv4 is sent to the UE by including it in the PCO (see TS 24.501). The link MTU size for IPv6 is sent to the UE by including it in the IPv6 Router Advertisement message (see RFC 4861 [54]).
NOTE 1:
Ideally the network configuration ensures that for PDU Session type IPv4v6 the link MTU values provided to the UE via PCO and in the IPv6 Router Advertisement message are the same. In cases where this condition cannot be met, the MTU size selected by the UE is unspecified.
When using a PDU Session type Unstructured, the maximum uplink packet size, and when using Ethernet, the Ethernet frames' payload, that the UE should use may be provided by the network as a part of the session management configuration by encoding it within the PCO (see TS 24.501).
When using a PDU Session type Unstructured, to provide a consistent environment for application developers, the network shall use a maximum packet size of at least 128 octets (this applies to both uplink and downlink).
When the MT and the TE are separated, the TE may either be pre-configured to use a specific default MTU size or the TE may use an MTU size provided by the network via the MT. Thus, it is not always possible to set the MTU value by means of information provided by the network.
NOTE 2:
In network deployments that have MTU size of 1500 octets in the transport network, providing a link MTU value of 1358 octets (as shown in Figure J-1) to the UE as part of the IP configuration information from the network will prevent the IP layer fragmentation within the transport network between the UE and the UPF. For network deployments that uniformly support transport with larger MTU size than 1500 octets (for example with ethernet jumbo frames of MTU size up to 9216 octets), providing a link MTU value of MTU minus 142 octets to the UE as part of the IP configuration information from the network will prevent the IP layer fragmentation within the transport network between the UE and the UPF. Link MTU considerations are discussed further in Annex J.
NOTE 3:
As the link MTU value is provided as a part of the session management configuration information, a link MTU value can be provided during each PDU Session establishment. In this release, dynamic adjustment of link MTU for scenarios where MTU is not uniform across transport are not addressed.
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5.6.11  UE presence in Area of Interest reporting usage by SMF
When a PDU Session is established or modified, or when the user plane path has been changed (e.g. UPF re-allocation/addition/removal), SMF may determine an Area of Interest, e.g. based on UPF Service Area, subscription by PCF for reporting UE presence in Presence Reporting Area, etc.
For 3GPP access, the Area of Interest constitutes:
  • a list of Tracking Areas and/or;
  • Presence Reporting Area ID(s) and optionally the elements for one or more of the Presence Reporting Areas, i.e. TAs and/or NG-RAN nodes and/or cells identifiers and/or;
  • LADN DNN.
For Non-3GPP access, the Area of Interest constitutes:
For UE location change into or out of an "area of interest", the SMF subscribes to "UE mobility event notification" service provided by AMF for reporting of UE presence in Area of Interest as described in clause 5.3.4.4. The AMF may send the UE location to the SMF along with the notification, e.g. for UPF selection. Upon reception of a notification from AMF, the SMF determines how to deal with the PDU Session, e.g. reallocate UPF.
In the case of LADN, the SMF provides the LADN DNN to the AMF to subscribe to "UE mobility event notification" for reporting UE presence in LADN service area. Upon reception of a notification from the AMF, the SMF determines how to deal with the PDU Session as described in clause 5.6.5.
For use cases related to policy control and charging decisions, the PCF may subscribe to event reporting from the SMF or the AMF, for UE presence in a Presence Reporting Area.
A Presence Reporting Area can be:
  • A "UE-dedicated Presence Reporting Area", defined in the subscriber profile and composed of a short list of TAs and/or NG-RAN nodes and/or cells identifiers in a PLMN; or derived from the Area of Interest provided by the Application Function to the PCF (see clause 5.6.7) and composed of a short list of TAs and/or NG-RAN nodes and/or cells identifiers in a PLMN; or
  • A "Core Network predefined Presence Reporting Area", predefined in the AMF and composed of a short list of TAs and/or NG-RAN nodes and/or cells identifiers in a PLMN.
In the case of Change of UE Presence in Presence Reporting Area, for core network predefined Presence Reporting Area, the AMF determines the "area of interest" corresponding to the Presence Reporting Area Identifier(s), provided by the PCF or the SMF, as a list of TAIs and/or cell identifiers and/or NG-RAN node identifiers based on local configuration. For UE-dedicated Presence Reporting Areas, the subscription for UE location change notification for an "area of interest" shall contain the PRA Identifier(s) and the list(s) of TAs, or NG-RAN Node identifier and/or cell identifiers composing the Presence Reporting Area(s). For Core Network predefined Presence Reporting Areas, the subscription for UE location change notification for an "area of interest" shall contain the PRA identifier(s).
NOTE 1:
If the Presence Reporting Area (PRA) and RAN Notification Area (RNA) are partially overlapping, the PCF will not get notified for the change of PRA when UE enters or leaves the PRA but remains in the RNA in CM-CONNECTED with RRC Inactive state, because AMF is not informed.
Each Core Network predefined Presence Reporting Area can be configured with a priority level in the AMF. In order to prevent overload, the AMF may set the reporting for one or more of the received Presence Reporting Area(s) to inactive under consideration of the priority configured for each of Core Network predefined Presence Reporting Area(s), while storing the reporting request for this Presence Reporting Area in the UE context.
NOTE 2:
Change of UE presence in Presence Reporting Area reporting does not apply to home routed roaming.
The AMF may be configured with a PRA identifier which refers to a Set of Core Network predefined Presence Reporting Areas. If the PCF subscribes to change of UE location for an area of interest for a Set of Presence reporting areas and provides a PRA identifier then the SMF may subscribe for event reporting for this Set of Presence Reporting Areas by only indicating this PRA Identifier in the area of interest. When the Presence Reporting Area(s) to be reported belong to a set of Core Network predefined Presence Reporting Areas in which the AMF is requested to report on change of UE presence, the AMF shall additionally add to the report the PRA Identifier of the Set of Core Network predefined Presence Reporting Areas.
Upon change of AMF, the PRA identifier(s) and if provided, the list(s) of Presence Reporting Area elements are transferred for all PDU sessions as part of MM Context information to the target AMF during the mobility procedure. If one or more Presence Reporting Area(s) was set to inactive, the target AMF may decide to reactivate one or more of the inactive Presence Reporting Area(s). The target AMF indicates per PDU session to the corresponding SMF/PCF the PRA identifier(s) and whether the UE is inside or outside the Presence Reporting Area(s) as well as the inactive Presence Reporting Area(s), if any.
NOTE 3:
The target AMF cannot set the Presence Reporting Area(s) received from the source serving node to inactive.
The subscription may be maintained during the life of PDU Session, regardless of the UP activation state of PDU Session (i.e. whether UP connection of the PDU Session is activated or not).
SMF may determine a new area of interest, and send a new subscription to the AMF with the new area of interest.
SMF un-subscribes to "UE mobility event notification" service when PDU Session is released.
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5.6.12  Use of Network InstanceWord-p. 129
The SMF may provide a Network Instance to the UPF in FAR and/or PDR via N4 Session Establishment or N4 Modification procedures.
NOTE 1:
a Network Instance can be defined e.g. to separate IP domains, e.g. when a UPF is connected to 5G-ANs in different IP domains, overlapping UE IP addresses assigned by multiple Data Networks, transport network isolation in the same PLMN, etc.
NOTE 2:
As the SMF can provide over N2 the Network Instance it has selected for the N3 CN Tunnel Info, the 5G AN does not need to provide Network Instance to the 5GC.
The SMF determines the Network Instance based on local configuration.
The SMF may determine the Network Instance for N3 and N9 interfaces, taking into account e.g. UE location, registered PLMN ID of UE, S-NSSAI of the PDU Session.
The SMF may determine the Network Instance for N6 interface taking into account e.g. (DNN, S-NSSAI) of the PDU Session.
The SMF may determine the Network Instance for N19 interface taking into account e.g. the (DNN, S-NSSAI) identifying a 5G VN group.
NOTE 3:
As an example, the UPF can use the Network Instance included in the FAR, together with other information such as Outer header creation (IP address part) and Destination interface in the FAR, to determine the interface in UPF (e.g. VPN or Layer 2 technology) for forwarding of the traffic.
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5.6.13  Always-on PDU session
An always-on PDU Session is a PDU Session for which User Plane resources have to be activated during every transition from CM-IDLE mode to CM-CONNECTED state.
Based on an indication from upper layers, a UE may request to establish a PDU Session as an always-on PDU Session. The SMF decides whether the PDU Session can be established as an always-on PDU Session. In Home Routed roaming case, based on local policies, the V-SMF shall be involved to determine whether the PDU Session can be established as an always-on PDU Session.
If the UE requests the 5GC to modify a PDU Session, which was established in EPS, to an always-on PDU Session after the first inter-system change from EPS to 5GS, the SMF decides whether the PDU Session can be established as an always-on PDU Session based on the procedure described above.
The UE shall request activation of User Plane resources for always-on PDU Sessions even if there are no pending uplink data for this PDU Session or when the Service Request is triggered for signalling only or when the Service Request is triggered for paging response only.
If the UE has one or more established PDU Sessions which are not accepted by the network as always-on PDU Sessions and the UE has no uplink user data pending to be sent for those PDU Sessions, the UE shall not request for activating User Plane resources for those PDU sessions.
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5.6.14  Support of Framed Routing
Framed Routing allows to support an IP network behind a UE, such that a range of IP addresses or IPv6 prefixes is reachable over a single PDU session, e.g. for enterprise connectivity. Frame Routes are IP routes behind the UE. The network does not send Framed Routing information to the UE: devices in the network(s) behind the UE get their IP address by mechanisms out of the scope of 3GPP specifications. See RFC 2865 [73], RFC 3162 [74].
A PDU Session may be associated with multiple Frame Routes. Framed Routing is defined only for PDU sessions of the IP type (IPv4, IPv6, IPv4v6).
Framed Routing information is provided by the SMF to the UPF (acting as PSA) as part of Packet Detection Rule (PDR, see clause 5.8.2.11.3) related with the network side (N6) of the UPF.
The actual ranges of IPv4 address / IPv6 Prefixes corresponding to a Framed Route may be provided to the SMF by the DN-AAA server as part of PDU Session Establishment authentication/authorization by a DN-AAA server (as defined in clause 5.6.6).
The IPv4 address / IPv6 Prefix allocated to the UE as part of the PDU Session establishment (e.g. delivered in NAS PDU Session Establishment Accept) may belong to one of the Framed Routes associated with the PDU Session or may be dynamically allocated outside of such Framed Routes.
The actual ranges of IPv4 address / IPv6 Prefixes corresponding to a Framed Route may be defined in the subscription data associated with a (DNN, S-NSSAI) received by the SMF from the UDM (Session Management Subscription data defined in Table 5.2.3.3.1-1 of TS 23.502)
If PCC applies to the PDU Session, at PDU Session establishment the SMF reports to the PCF the Framed Routes corresponding to the PDU Session. In this case, in order to support session binding, the PCF may further report to the BSF the Framed Routes corresponding to the PDU Session.
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