4.3.5 Session continuity, service continuity and UP path management

23.5023GPPProcedures for the 5G System (5GS)Release 18TS

4.3.5.1 Change of SSC mode 2 PDU Session Anchor with different PDU Sessions

The following procedure is triggered by SMF in order to change the PDU Session Anchor serving a PDU Session of SSC mode 2 for a UE when neither multi-homing nor UL CL applies to the PDU Session. This procedure releases the existing PDU Session associated with an old PDU Session Anchor (i.e. UPF1 in figure 4.3.5.1-1) and immediately establishes a new PDU Session with a new PDU Session Anchor (i.e. UPF2 in figure 4.3.5.1-1) to the same DN.

Figure 4.3.5.1-1: Change of SSC mode 2 PSA for a PDU Session

1. The SMF determines that the serving UPF needs to be changed due to events that may benefit from such change.

1a. If the UPF (PSA) cannot connect to the target DNAI that SMF received from SM-PCF, the SMF invokes Nsmf_PDUSession_SMContextStatusNotify Request (target DNAI information) service operation to the AMF. The SMF also indicates the SMF selection is expected.

The target DNAI information is used for SMF selection which can control UPF connecting to that DNAI at next PDU session establishment towards the same DNN and S-NSSAI. Due to it is for SMF selection, the AMF stores the target DNAI information received from SMF. The target DNAI information is not transferred outside, e.g. to support the UE context transfer between AMFs for AMF relocation.

2. The PDU Session Release procedure is initiated as described in clause 4.3.4. The SMF sends an N1 SM Information to the UE via the AMF by invoking Namf_Communication_N1N2MessageTransfer as described in Step 3b of clause 4.3.4.2. The PDU Session Release Command message in N1 SM Information contains the PDU Session ID and Cause indicating that a PDU Session re-establishment to the same DN is required.

3. Upon reception of PDU Session Release Command with Cause indicating that a PDU Session re-establishment to the same DN is required as sent in step 2, the UE generates a new PDU Session ID and initiates PDU Session Establishment procedure as described in clause 4.3.2.2.

Then, the AMF selects an SMF as described in clause 6.4.2 of TS 23.501 [2] and the SMF can select a new UPF (i.e. UPF2) for the re-established PDU Session of SSC mode 2.

If the AMF has received target DNAI information from old SMF (i.e. SMF1), for the PDU Session toward same DNN and S-NSSAI the AMF selects the SMF using the stored target DNAI information. The AMF includes the target DNAI in the Nsmf_PDUSession_CreateSMContext Request and deletes the stored target DNAI information. The SMF selects the new PDU Session Anchor using the target DNAI.

4.3.5.2 Change of SSC mode 3 PDU Session Anchor with multiple PDU Sessions

The following procedure is triggered by SMF in order to change the PDU Session Anchor serving a PDU Session of SSC mode 3 for a UE. This procedure releases the existing PDU Session associated with an old PDU Session Anchor (i.e. UPF1 in figure 4.3.5.2-1) after having established a new PDU Session to the same DN with a new PDU Session Anchor (i.e. UPF2 in figure 4.3.5.2-1), which is controlled by the same SMF. The SMF may determine that a new SMF needs to be reallocated.

Figure 4.3.5.2-1: Change of SSC mode 3 PDU Session Anchor with multiple PDU Sessions

1. The SMF determines that the serving UPF or the SMF needs to be changed. If the "Indication of application relocation possibility" attributes in the PCC rule indicates no DNAI change takes place once selected for this application, the SMF determines that the SMF can not be changed.

1a. If the UPF (PSA) cannot connect to the target DNAI(s) that SMF received from SM-PCF, the SMF invokes Nsmf_PDUSession_SMContextStatusNotify Request (target DNAI information) service operation to the AMF. The SMF also indicate the SMF selection is expected.

If the runtime coordination between 5GC and AF (Figure 4.3.6.3-1) is enabled, the SMF includes in the Nsmf_PDUSession_SMContextStatusNotify Request the SM Context ID as a reference to the SM Context that includes AF Coordination Information stored in the SMF.

The target DNAI information are used for SMF selection which can control UPF connecting to that DNAI at next PDU session establishment towards the same DNN and S-NSSAI. Due to it is for SMF selection, the AMF stores the target DNAI information received from SMF selection. The target DNAI information is not transferred outside, e.g. to support the UE context transfer between AMFs for AMF relocation.

2. If the SMF had sent an early notification to the AF and the runtime coordination between 5GC and AF is enabled based on local configuration as specified in clause 4.3.6.3, according to the indication of "AF acknowledgment to be expected" included in AF subscription to SMF events, the SMF waits for a notification response from the AF. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure. This is defined in Figure 4.3.6.3-1.

The SMF invokes the Namf_Communication_N1N2MessageTransfer (PDU Session ID, SMF Reallocation requested indication, N1 SM container (PDU Session Modification Command (Cause, PCO (PDU Session Address Lifetime value)))) where PDU Session ID indicates the existing PDU Session to be relocated and Cause indicates that a PDU Session re-establishment to the same DN is required.

The SMF Reallocation requested indication indicates whether the SMF is requested to be reallocated.

The PDU Session Address Lifetime value is delivered to the UE upper layers in PCO and indicates how long the network is willing to maintain the PDU Session. The SMF starts a PDU Session Release timer corresponding to the PDU Session Address Lifetime value.

3a. The AMF forwards the NAS message to the UE. The UE can provide the release timer value to the upper layers if received in the PDU Session Modification Command.

3b. The UE acknowledges the PDU Session Modification Command.

3c. The AMF forwards the N1 SM container (PDU Session Modification Command ACK) received from the (R)AN to the SMF1 via Nsmf_PDUSession_UpdateSMContext service operation.

3d. The SMF1 replies with a Nsmf_PDUSession_UpdateSMContext Response.

4. If the UE receives PDU Session Modification Command, the UE may decide to initiate the PDU Session Establishment procedure described in clause 4.3.2.2, to the same DN with the following differences:

In Step 1 of clause 4.3.2.2.1, according to the SSC mode, UE generates a new PDU Session ID and initiates the PDU Session Establishment Request using the new PDU Session ID. The new PDU Session ID is included as PDU Session ID in the NAS request message and the Old PDU Session ID which indicates the existing PDU Session to be released is also provided to AMF in the NAS request message.

In Step 2 of clause 4.3.2.2.1, if SMF reallocation was requested in Step 2 of this clause, the AMF selects a different SMF. Otherwise, the AMF sends the Nsmf_PDUSession_CreateSMContext Request to the same SMF serving the Old PDU Session ID.

If target DNAI information has been received from old SMF (i.e. SMF1), for the PDU Session toward same DNN and S-NSSAI the AMF selects the new SMF using the stored target DNAI information. The AMF includes the target DNAI in the Nsmf_PDUSession_CreateSMContext Request and deletes the stored target DNAI information. If the AMF has received the SM Context ID from the old SMF, the AMF includes the SM Context ID in the Nsmf_PDUSession_CreateSMContext Request.

In Step 3 of clause 4.3.2.2.1, if the SMF is not to be reallocated, the AMF include both PDU Session ID and Old PDU Session ID in Nsmf_PDUSession_CreateSMContext Request. The SMF detects that the PDU Session establishment request is related to the trigger in step 2 based on the presence of an Old PDU Session ID in the Nsmf_PDUSession_CreateSMContext Request.

In Step 3 of clause 4.3.2.2.1, the SMF stores the new PDU Session ID and selects a new PDU Session Anchor (i.e. UPF2) for the new PDU Session.

If the new SMF receives an SM Context ID in the Nsmf_PDUSession_CreateSMContext Request, the new SMF retrieves the AF Coordination Information by sending a Nsmf_PDUSession_ContextRequest to the old SMF and indicates that "AF Coordination Information" part of 5G SM Context is requested.

If the SMF receives a target DNAI in the Nsmf_PDUSession_CreateSMContext Request, the SMF selects the new PDU Session Anchor using the target DNAI.

If the AF Coordination Information in the Nsmf_PDUSession_ContextRequest Response includes a notification correlation id associated with an "uplink buffering" indication, the SMF may also indicate PSA2 to buffer the uplink data associated with the same notification correlation id in the PCC Rules.

If the PCC Rules received from the PCF as in step 7b or step 9 in clause 4.3.2.2.1 indicate that a late notification is requested by the AF (directly or via NEF), the SMF sends a late notification to the AF before step 11 of clause 4.3.2.2.1 in Nsmf_EventExposure_Notify service operation, as in step 4a or step 4c in Figure 4.3.6.3-1 (directly or via NEF, respectively). The late notification contains the Source DNAI and the UE IP address in the Source DNAI included in the AF Coordination Information as received in the Nsmf_PDUSession_CreateSMContext Response from the old SMF.

If the SMF received a negative notification response from the AF, the SMF may stop the procedure. This is defined in Figure 4.3.6.3-1. Otherwise the SMF continue the following procedures to activate the UP path of the new PDU Session. The SMF may also indicate PSA2 to stop buffering and start forwarding uplink data.

5. After the new PDU Session is established the UE starts using the IP address/prefix associated with the new PDU Session for all new traffic and may also proactively move existing traffic flow (where possible) from the old PDU Session to the new PDU Session.

NOTE: The mechanisms used by the UE to proactively move existing traffic flows from one IP address/prefix to another are outside the scope of 3GPP specifications.

6. The old PDU Session is released as described in clause 4.3.4 either by the UE before the timer provided in step 3 expires (e.g. once the UE has consolidated all traffic on new PDU Session or if the session is no more needed) or by the SMF upon expiry of this timer.

4.3.5.3 Change of SSC mode 3 PDU Session Anchor with IPv6 Multi-homed PDU Session

Clause 4.3.5.3 describes a procedure for service continuity with SSC mode 3 that uses the multi-homed PDU Session described in clause 5.6.4.3 of TS 23.501 [2]. In this case the SMF prepares a new PDU Session Anchor first and then notifies the UE of the existence of a new IP prefix, as depicted in figure 4.3.5.3-1. This procedure is applicable only to PDU Sessions of IPv6 type.

Figure 4.3.5.3-1: Change of PDU Session Anchor with IPv6 Multi homed PDU Session

The UE has an established PDU Session with the PDU Session Anchor (i.e. UPF1 in Figure 4.3.5.3-1). The PDU Session’s User Plane involves at least the (R)AN and the PDU Session Anchor.

1. At some point the SMF decides to allocate the PDU Session with a new PDU Session Anchor.

2. The SMF selects a new UPF and using N4 configures the UPF as a new PDU Session Anchor (i.e. UPF2 in Figure 4.3.5.3-1) of the multi-homed PDU Session. In the process a new IPv6 prefix (IP@2) is allocated for the PDU Session. If the PCF has subscribed to the IP allocation/release event, the SMF performs a Session Management Policy Modification procedure as defined in clause 4.16.5 to provide the new allocated IPv6 prefix to the PCF. The PCF invokes Nbsf_Management_Update service operation to register the tuple (IPv6 prefix, PCF id) for the PDU session identified by (SUPI, DNN, S-NSSAI) in the BSF.

If the runtime coordination between 5GC and AF is enabled based on local configuration, according to the indication of "AF acknowledgment to be expected" included in AF subscription to SMF events, the SMF sends an early notification to the AF after the new UPF (new PSA) is selected and waits for a notification response from the AF. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure. This is further defined in Figure 4.3.6.3-1.

3. The SMF selects a Branching Point (BP) UPF as described in Clause of 6.3.3 of TS 23.501 [2]. The selection of BP UPF may consider the location of UPF1 and UPF2 to ensure a suitable location of the BP UPF relative to the UPF1 and the UPF2.

NOTE 1: If BP UPF is co-located with one of PDU Session Anchors, steps between SMF and BP UPF can be skipped.

4. The SMF configures via N4 the UPF selected in step 3 (BP UPF in Figure 4.3.5.3-1) as a Branching Point for the multi-homed PDU Session. It provides the Branching Point with the necessary UL traffic forwarding rules (related with the prefix of the IPv6 source address of UL traffic). Also, the SMF provides AN Tunnel Info for N3 tunnel setup and CN Tunnel Info for N9 tunnel setup to the BP UPF and obtains CN Tunnel Info from the BP UPF.

5-6. The SMF performs N4 Session Modification procedure with PSAs. During this procedure, the SMF provides CN Tunnel Info received from the BP UPF to set up an N9 tunnel between BP and PSAs. The SMF may also indicate local PSA2 to buffer the uplink data.

7. The SMF invokes the Namf_Communication_N1N2MessageTransfer service operation containing N2 SM Information with CN Tunnel Info for the N3 tunnel setup.

8. The AMF sends an N2 Request including N2 SM Information received from the SMF to the (R)AN. The (R)AN acknowledges to the AMF with an N2 Response.

9a. The AMF carries the N2 Response sent by the (R)AN to the SMF by invoking the Nsmf_PDUSession_UpdateSMContext service operation.

9b. The SMF responds to Nsmf_PDUSession_UpdateSMContext service operation from the AMF.

10-11. If the runtime coordination between 5GC and AF is enabled based on local configuration as specified in clause 4.3.6.3, according to the indication of "AF acknowledgment to be expected" is included in AF subscription to SMF events, the SMF sends a late notification to the AF and waits for a notification response from the AF. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure. This is further defined in Figure 4.3.6.3-1.

The SMF notifies the UE of the availability of the new IP prefix. This is performed using an IPv6 Router Advertisement message (RFC 4861 [6]). The SMF sends a Router Advertisement to the UE via the new PSA with a new prefix (IP@2) and sends another Router Advertisement to the UE via the old PSA with the old prefix (IP@1) and zero value in the preferred lifetime field and a value in the valid lifetime field according to RFC 4862 [8]. The UE shall update the valid lifetime of the old prefix (IP@1) to the signalled value regardless of the remaining lifetime. The valid lifetime value indicates the time how long the SMF is willing to keep the old prefix. The valid lifetime value may be decided by SMF based on local configuration.

The UE starts using IP@2 for all new traffic and may also proactively move existing traffic flow (where possible) from IP@1 to IP@2.

NOTE 2: The mechanisms used by the UE to proactively move existing traffic flows from one IP prefix to another are outside the scope of 3GPP specifications.

12. After the timer expires, the SMF releases the UE’s old IPv6 prefix (IP@1). At this point the UE implicitly releases the old IP prefix. The SMF sends an N4 Session Modification Request to the BP to release UP resource for N9 tunnel between the BP and old PSA.

13. The SMF releases the old PDU Session context with the old PDU Session Anchor (UPF1 in Figure 4.3.5.3-1). If the PCF has subscribed to the IP allocation/release event, the SMF performs a Session Management Policy Modification procedure as defined in clause 4.16.5 to notify the PCF of the IPv6 prefix release. The PCF shall invoke Nbsf_Management_Update service operation to remove the tuple (IPv6prefix, PCF id) for the PDU session identified by (SUPI, DNN,S-NSSAI) in BSF.

14-18. The SMF may optionally release the Branching Point from the User Plane path. In step 14, the SMF may also indicate PSA2 to stop buffering and start forwarding uplink data.

4.3.5.4 Addition of additional PDU Session Anchor and Branching Point or UL CL

Clause 4.3.5.4 describes a procedure to add a PDU Session Anchor and a Branching Point or UL CL for an established PDU Session.

Figure 4.3.5.4-1: Addition of additional PDU Session Anchor and Branching Point or UL CL

1. UE has an established PDU Session with a UPF including the PDU Session Anchor 1 (PSA1 in Figure 4.3.5.4-1). The PDU Session User Plane involves at least the (R)AN and the PDU Session Anchor 1.

2. At some point the SMF decides to establish a new PDU Session Anchor e.g. due to UE mobility, new flow detection. The SMF selects a UPF and using N4 establish the new PDU Session Anchor 2 (PSA2 in Figure 4.3.5.4-1) of the PDU Session. In the case of IPv6 multi-homing PDU Session, the SMF also ensures that a new IPv6 prefix corresponding to PSA2 is allocated and if the PCF has subscribed to the IP allocation/release event, the SMF performs the Session Management Policy Modification procedure as defined in clause 4.16.5 to provide the new allocated IPv6 prefix to the PCF.

If the runtime coordination between 5GC and AF is enabled based on local configuration, according to the indication of "AF acknowledgment to be expected" is included in AF subscription to SMF events, the SMF sends an early notification to the AF after new PSA (PSA2 in Figure 4.3.5.4-1) is selected and waits for a notification response from the AF before configuring the new PSA. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure.

3. The SMF selects a UPF and using N4 establish the Branching Point (in the case of IPv6 multi-homing) or a UL CL for the PDU Session. It provides the necessary uplink forwarding rules towards PSA1 and PSA2 including the PSA1 CN Tunnel Info and the PSA2 CN Tunnel Info. In addition, the AN Tunnel Info is provided for downlink forwarding. In the case of IPv6 multi-homing, the SMF also provides traffic filters for the IPv6 prefixes corresponding to PSA1 and PSA2 indicating what traffic shall be forwarded towards PSA1 and PSA2 respectively.

In the case of UL CL, the SMF provides traffic filters indicating what traffic shall be forwarded towards PSA1 and PSA2 respectively. If the runtime coordination between 5GC and AF is enabled based on local configuration, according to the indication of "AF acknowledgment to be expected" is included in AF subscription to SMF events, the SMF sends a late notification to the AF and waits for a notification response from the AF before configuring the UL CL. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure.

NOTE 1: If the Branching Point or UL CL and the PSA2 are co-located in a single UPF then steps 2 and 3 can be merged. If a Branching Point is already allocated, step 3 is skipped.

4. The SMF updates the PSA1 via N4. It provides the Branching Point or UL CL CN Tunnel Info for the downlink traffic.

NOTE 2: If the Branching Point or UL CL and the PSA1 are co-located in a single UPF then steps 3 and 4 can be merged.

5. The SMF updates PSA2 via N4. It provides the Branching Point or UL CL CN Tunnel Info for down-link traffic.

NOTE 3: If the Branching Point or UL CL and the PSA2 are co-located in a single UPF then step 5 is not needed.

6. The SMF updates (R)AN via N2 SM information over N11. It provides the new CN Tunnel Info corresponding to the UPF (Branching Point or UL CL). In the case of UL CL, if there is an existing UPF between the (R)AN and new inserted UL CL, the SMF updates the existing UPF via N4 instead of updating the (R)AN.

7. In the case of IPv6 multi-homing, the SMF notifies the UE of the availability of the new IP prefix @ PSA2. This is performed using an IPv6 Router Advertisement message (RFC 4861 [6]). Also, the SMF sends IPv6 multi-homed routing rule along with the IPv6 prefix to the UE using an IPv6 Router Advertisement message (RFC 4191 [21]) as described in clause 5.8.2.2.2 of TS 23.501 [2].

If the runtime coordination between 5GC and AF is enabled based on local configuration, according to the indication of "AF acknowledgment to be expected" included in AF subscription to SMF events, the SMF sends a late notification to the AF and waits for a notification response from the AF before sending the new IP prefix to the UE. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure.

8. In the case of IPv6 multi-homing, the SMF may re-configure the UE for the original IP prefix @ PSA1,i.e. SMF sends IPv6 multi-homed routing rule along with the IPv6 prefix to the UE using an IPv6 Router Advertisement message (RFC 4191 [21]) as described in clause 5.8.2.2.2 of TS 23.501 [2].

4.3.5.5 Removal of additional PDU Session Anchor and Branching Point or UL CL

Clause 4.3.5.5 describes a procedure to remove a PDU Session Anchor and (optionally) remove Branching Point or UL CL for an established PDU Session.

Figure 4.3.5.5-1: Removal of additional PDU Session Anchor and Branching Point or UL CL

1. UE has an established PDU Session with a UPF including the Branching Point or UL CL, the PDU Session Anchor 1 (PSA1 in Figure 4.3.5.5-1) and the PDU Session Anchor 2 (PSA2 in Figure 4.3.5.5-1).

At some point the SMF decides to remove the PDU Session Anchor 1 e.g. due to UE mobility, flow terminated.

2. In the case of IPv6 multi-homing, the SMF notifies the UE to stop using the IPv6 prefix corresponding to PSA1. This is performed by IPv6 Router Advertisement message (RFC 4861 [6] and RFC 4862 [8]). Also, the SMF sends IPv6 multi-homed routing rule along with the IPv6 prefix corresponding to PSA2 to the UE as described in clause 5.8.2.2.2 of TS 23.501 [2]. Based on the information provided in the Router Advertisement, the UE starts using the IPv6 prefix (corresponding to PSA2) for all the traffic.

4. If the Branching Point or UL CL is to be released, the SMF updates the (R)AN with the PSA2 CN Tunnel Info. In the case of UL CL, if there is an existing UPF between the (R)AN and the UL CL to be removed, the SMF updates the existing UPF via N4 instead of updating the (R)AN.

5. If the Branching Point or UL CL is to be released, the SMF updates via N4 the PSA2 providing the AN Tunnel Info. In the case of UL CL, if there is an existing UPF between the (R)AN and the UL CL to be removed, the SMF updates the PSA2 providing the UPF CN tunnel Info.

6. The SMF releases via N4 the PSA1. In the case of IPv6 multi-homing, the SMF also releases the corresponding IPv6 prefix and if the PCF has subscribed to the IP allocation/release event, the SMF performs the Session Management Policy Modification procedure as defined in clause 4.16.5 to notify the PCF of the IPv6 prefix release.

7. If steps 4 and 5 were executed, the SMF releases the Branching Point / UL CL.

4.3.5.6 Change of additional PDU Session Anchor for IPv6 multi-homing or UL CL

The following procedure is triggered by an SMF when the SMF needs to modify IPv6 multi-homing or UL CL rule (i.e. traffic filter in the Branching Point or the UL CL) in order to move the some or whole traffic flows of the existing additional PDU Session Anchor which was established by the IPv6 multi-homing or the UL CL operations (i.e. PSA1 in figure 4.3.5.6-1) to a new additional PDU Session Anchor (i.e. PSA2 in figure 4.3.5.6-1) which is established under the same Branching Point or UL CL for a UE where the UE already has a PDU Session Anchor which was established before the event of Branching Point or UL CL insertion (i.e. PSA0 in figure 4.3.5.6-1). This procedure establishes a new additional PDU Session Anchor (i.e. PSA2) and conditionally releases the existing additional PDU Session Anchor (i.e. PSA1), while modifying IPv6 multi-homing or UL CL rule in the same Branching Point or UL CL under controlled by the same SMF.

Figure 4.3.5.6-1: Change of additional PSA for a PDU Session in IPv6 multi-homing or UL CL case

1. The SMF decides to change one additional PSA of a PDU Session with IPv6 multi-homing or UL CL, due to events that may benefit from such change or upon request from an Application Function.

2. If the runtime coordination between 5GC and AF is enabled based on local configuration, according to the indication of "AF acknowledgment to be expected" included in AF subscription to SMF events, the SMF sends an early notification to the AF after PSA2 is selected and waits for a notification response from the AF before configuring the PSA2. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure. This is defined in Figure 4.3.6.3-1.

The SMF sends an N4 Session Establishment Request to PSA2 and provides the tunnel ID of Branching Point or UL CL, Packet detection, enforcement and reporting rules to be installed on the PSA2 for this PDU Session. The SMF may also indicate local PSA2 to buffer the uplink data.

The PSA2 acknowledges by sending an N4 Session Establishment Response. The tunnel ID of PSA2 is provided to the SMF in this step.

In the case of IPv6 multi-homing PDU Session, a new IPv6 prefix corresponding to PSA2 is allocated (by the SMF or by the UPF depending on the deployment) and if the PCF has subscribed to the IP allocation/release event, the SMF performs the Session Management Policy Modification Procedure as defined in clause 4.16.5 to provide the new allocated IPv6 prefix to the PCF.

3a. In the case of PDU session with UL CL, if the runtime coordination between 5GC and AF is enabled based on local configuration, according to the indication of "AF acknowledgment to be expected" included in AF subscription to SMF events, the SMF sends a late notification to the AF and waits for a notification response from the AF. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure. This is further defined in Figure 4.3.6.3-1.

The SMF sends an N4 Session Modification Request to the Branching Point or UL CL to update the UL traffic filter according to new allocated IPv6 prefix allocated to PSA 2 or the UL CL rules regarding to the traffic flows that the SMF tries to move from PSA1 to PSA2. The N4 Session Modification Request message contains the identifications of traffic filter that needs to be updated and the tunnel ID of PSA2.

NOTE: The identification of a traffic filter can be either the index of the traffic filter, or a single value of the information field in traffic filter (e.g. the tunnel ID of next hop), or a combination value of some information field in the traffic filter (e.g. the tunnel ID of next hop with source port number).

3b. The Branching Point or the UL CL acknowledges by N4 Session Modification Response the Branching Point or when the UL CL successfully updates all the traffic filters that the SMF requests to modify.

3c. The SMF may also indicate PSA2 to stop buffering and start forwarding uplink data.

4. In the case of IPv6 multi-homing PDU Session, if the runtime coordination between 5GC and AF is enabled based on local configuration, according to the indication of "AF acknowledgment to be expected" included in AF subscription to SMF events, the SMF sends a late notification to the AF and waits for a notification response from the AF. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure. This is further defined in Figure 4.3.6.3-1.

In the case of IPv6 multi-homing PDU Session, The SMF notifies the UE of the availability of the new IP prefix @ PSA2. This is performed using an IPv6 Router Advertisement message (RFC 4861 [6]). Also, the SMF sends IPv6 multi-homed routing rule along with the IPv6 prefix to the UE using an IPv6 Router Advertisement message (RFC 4191 [21]) as described in clause 5.8.2.2.2 of TS 23.501 [2]. The SMF may also indicate PSA2 to stop buffering and start forwarding uplink data.

5. In the case of IPv6 multi-homing PDU Session, The SMF may re-configure the UE for the original IP prefix @ PSA0,i.e. SMF sends IPv6 multi-homed routing rule along with the IPv6 prefix to the UE using an IPv6 Router Advertisement message (RFC 4191 [21]) as described in clause 5.8.2.2.2 of TS 23.501 [2].

6. Step 6 occurs only if the Branching Point or UL CL does not have any traffic filter on the PDU Session which forwards a traffic flow to PSA1.

6a. The SMF sends an N4 Session Release Request with N4 session ID to PSA1. The PSA1 shall release all tunnel resources and contexts associated with the N4 session.

6b. PSA1 sends an N4 Session Release Response with N4 session ID to the SMF at the same moment that PSA1 successfully releases all tunnel resources and contexts associated with the N4 session.

4.3.5.7 Simultaneous change of Branching Point or UL CL and additional PSA for a PDU Session

Simultaneous change of UL CL or Branching Point and additional PSA can be performed after Xn based handover, N2 based handover and Service Request procedures.

The following procedure is triggered by SMF in order to change the Branching Point or the UL CL and additional PSA serving a PDU Session for a UE.

Figure 4.3.5.7-1: Simultaneous change of Branching Point or UL CL and additional PSA for a PDU Session

UE has an established PDU Session with a UPF including the PDU Session Anchor (Remote UPF). The PDU Session user plane involves at least the Source (R)AN, Source Branching Point or Source UL CL, local Source UPF (PSA2) and the Remote UPF (PDU Session Anchor, PSA1), where Source Branching Point or Source UL CL and PSA2 can be co-located.

1. At some point SMF decides to change the Branching Point or the UL CL due to UE mobility.

2. The SMF selects a local Target UPF (PSA3) and using N4 establishes the local Target UPF for the PDU Session.

In the case of IPv6 multi-homing PDU Session, a new IPv6 prefix corresponding to PSA3 is allocated (by the SMF or by the UPF depending on the deployment) and if the PCF has subscribed to the IP allocation/release event, the SMF performs the Session Management Policy Modification procedure as defined in clause 4.16.5 to provide the new allocated IPv6 prefix to the PCF.

The SMF may send an Early notification to the AF after PSA3 is selected. If the runtime coordination between 5GC and AF is enabled based on local configuration as specified in clause 4.3.6.3, according to the indication of "AF acknowledgment to be expected" included in AF subscription to SMF events, the SMF waits for a notification response from the AF before configuring the PSA3. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure. This is defined in Figure 4.3.6.3-1.

3. The SMF selects a UPF and using N4 establishes the Target Branching Point or Target UL CL for the PDU Session. SMF provides the necessary uplink forwarding rules towards the PSA3 and PSA1 including the Tunnel Info for each UPF. If session continuity upon UL CL relocation is used, the SMF also uses N4 to establish an N9 forwarding tunnel between the Source UL CL and Target UL CL, including the Tunnel Info for each UPF. In addition, the AN Tunnel Info to target (R)AN is provided for downlink forwarding. In the case of UL CL, the SMF provides traffic filters indicating what traffic shall be forwarded towards PSA3, PSA1 and Source UL CL, respectively. In the case of IPv6 multi-homing, the SMF also provides traffic filters for the IPv6 prefixes corresponding to PSA3 and PSA1 indicating what traffic shall be forwarded towards PSA3 and PSA1 respectively. Target Branching Point or Target UL CL provides the CN Tunnel Info for downlink traffic.

NOTE 1: If the Target Branching Point or Target UL CL and the PSA3 are co-located in a single UPF then steps 2 and 3 can be merged.

NOTE 2: When session continuity upon UL CL relocation is used, the downlink traffic at this point goes through Source UL CL, Target UL CL and Target (R)AN.

4. The SMF updates the PSA1 via N4. It provides the PDU Session CN Tunnel Info for the downlink traffic.

5. The SMF updates the PSA3 via N4. It provides the CN Tunnel Info for downlink traffic. The SMF may also indicate PSA3 to buffer uplink data.

NOTE 3: If the Target Branching Point or the Target UL CL and the PSA3 are co-located in a single UPF then step 5 is not needed.

6. In the case of PDU session with UL CL, if the runtime coordination between 5GC and AF is enabled based on local configuration, according to the indication of "AF acknowledgment to be expected" included in AF subscription to SMF events, the SMF sends a late notification to the AF and waits for a notification response from the AF as described in step 9. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure and remove the Target Branching Point or Target UL CL and PSA3. This is further defined in Figure 4.3.6.3-1.

The SMF updates (R)AN via N2 SM information over N11. It provides the new CN Tunnel Info corresponding to the Target Branching Point or the Target UL CL. If there is an existing UPF between the Target (R)AN and Target Branching Point or Target UL CL, the SMF updates the existing UPF via N4 instead of updating the (R)AN.

NOTE 4: When session continuity upon UL CL relocation is used, the uplink traffic destined to PSA2 at this point goes through Target (R)AN, Target UL CL and Source UL CL.

7. In the case of IPv6 multi-homing PDU Session, if the runtime coordination between 5GC and AF is enabled based on local configuration, according to the indication of "AF acknowledgment to be expected" included in AF subscription to SMF events, the SMF sends a late notification to the AF and waits for a notification response from the AF as described in step 9. If the SMF receives a negative notification response from the AF, the SMF may stop the procedure. This is further defined in Figure 4.3.6.3-1.

In the case of IPv6 multi-homing, the SMF notifies the UE of the availability of the new IP prefix @ PSA3. This is performed using an IPv6 Router Advertisement message (RFC 4861 [6]). Also, the SMF sends IPv6 multi-homed routing rule along with the IPv6 prefix to the UE using an IPv6 Router Advertisement message (RFC 4191 [21]) as described in clause 5.8.2.2.2 of TS 23.501 [2].

8. In the case of IPv6 multi-homing, the SMF may re-configure the UE for the original IP prefix @ PSA1, i.e. SMF sends IPv6 multi-homed routing rule along with the IPv6 prefix to the UE using an IPv6 Router Advertisement message (RFC 4191 [21]) as described in clause 5.8.2.2.2 of TS 23.501 [2].

9. The SMF sends a Late Notification to the AF indicating a change of DNAI as described in clause 4.3.6.3. In cases where target local DN is associated with another AF instance, SMF also sends notification to target AF as described in 4.3.6.3 and cancels any future notification message to source AF as it is no longer involved. The SMF may also indicate PSA3 to stop buffering and start forwarding uplink data based on the positive response for the Late Notification.

NOTE 5: The message can include routing information to the application located in the target local DN. Alternatively the routing information to the application located in the target local DN can be determined by the AF based on the new DNAI, in which case the AF can invoke the AF triggered influence on traffic routing procedure targeting single UE as described in clause 4.3.6.4, which assists the SMF in generation of the routing rule on the Target UL CL towards PSA3 (i.e. towards the application located in the target local DN). It is up to network configuration whether the routing information to the application located in the target local DN is configured in the SMF or in the AF.

NOTE 6: When session continuity upon UL CL relocation is used the AF can also trigger mechanisms that are out of the scope of this specification (e.g. IP-level or HTTP-level redirection) by which the traffic is redirected towards the application in the target local DN. Based on this redirection the UE starts using a new destination IP address which leads the Target UL CL to force the traffic towards PSA3.

10. When session continuity upon UL CL relocation is used, detection of no active traffic over the N9 forwarding tunnel is performed during a time interval provisioned by SMF for User Plane inactivity report in order to release the N9 forwarding tunnel. The detection can be done by Source UL CL, which notifies the SMF of no active traffic over the N9 forwarding tunnel.

NOTE 7: It is up to network configuration whether the detection of no active traffic is performed by the Source UL CL or the Target UL CL. As an alternative to the detection of no active traffic, the AF can send an explicit notification to the SMF when traffic to/from this UE ceases to exist, leading the SMF to release the Source UL CL and the Source UPF (PSA2).

11. The SMF releases via N4 the PSA2.

12. The SMF releases the Source Branching Point or the Source UL CL.

NOTE 8: If the Source Branching Point or UL CL and the PSA2 are co-located in a single UPF then steps 11 and 12 can be merged.

4.3.5.8 Ethernet PDU Session Anchor Relocation

This procedure allows for Ethernet PDU Sessions to change the PDU Session Anchor (PSA) while the session remains set up. Originally the Ethernet PDU Session goes via the Source UPF acting as the PSA. The Ethernet context which contains all Ethernet specific information including the MAC address of the UE and possibly its VLAN tag(s) is reported from the Source PSA UPF to the SMF. It is possible to report multiple MAC addresses (with their VLAN tag(s)) if these are reachable via the UE.

The SMF determines whether and when a serving PDU Session anchor for an Ethernet PDU Session needs to be changed and selects the Target PSA UPF, establishes the N4 session at the Target PSA UPF. The information within the Ethernet context is sent to the Target UPF. Based on the information in the Ethernet context, the Target PSA UPF may take action to update the Ethernet forwarding in the data network.

Figure 4.3.5.8-1: Ethernet PDU Session Anchor Relocation

Initially, the Ethernet PDU Session is established with the user data going via the Source UPF. The Source UPF acts as the PSA.

1. The Source UPF reports the Ethernet context which contains all Ethernet specific information including the MAC address of the UE and possibly its VLAN tag(s) that the UPF has learned from the UE side to the SMF. In the case of any changes in the Ethernet context, the change is updated to the SMF so that the SMF maintains an up-to-date state of the Ethernet context. This signalling is realized based on N4 reporting.

2. The UPF’s report is acknowledged.

3-5. In the case of handover, the RAN handover preparation and execution is followed by path switch signalling to the AMF and corresponding signalling to SMF, as defined in clause 4.9.1.2.

6. The SMF decides that the PSA is to be changed for the Ethernet PDU Session and selects the Target UPF that will act as the new PSA. The decision may be triggered by a mobility event, such as receiving the message in step 5.

7. The Target UPF N4 session is established. The SMF provides the N3 tunnel endpoint used by RAN.

8. The establishment of the new N4 session is acknowledged from the Target UPF to the SMF. The UPF provides its N3 tunnel endpoint.

9-10. In the case of handover, the SMF signals to the RAN via the AMF to provide the Path Switch Request Ack, which includes the update of the uplink N3 tunnel endpoint to the target UPF. This signalling is defined in clause 4.9.1.2. In steps 9-10, an indication is sent from the SMF via the AMF to the RAN node indicating that the RAN node should not expect to receive an end marker packet. The RAN node may skip trying to reorder the downlink packets.

11-12. If there is no handover and the UE is in connected mode, the SMF sends PDU Session Modify Request message to the RAN via the AMF, which includes the update of the uplink N3 tunnel endpoint to the target UPF. The RAN acknowledges the message. The RAN node does not need to to reorder the downlink packets.

NOTE 1: Due to the change in the end to end path, packet re-ordering can occur both for the handover and no handover cases. If necessary, upper layer protocols can ensure in sequence delivery.

After steps 9-10 or 11-12, uplink Ethernet frames pass via the target UPF. Downlink Ethernet frames may continue to be delivered from the source UPF to the RAN node. In the case of handover, the Ethernet frames are forwarded from the source RAN node to the target RAN node.

13-14. The SMF sends an N4 Session Modification Request to the Target UPF which includes the information in Ethernet context (i.e. MAC address and VLAN tag(s)) and a trigger for updating the Ethernet forwarding (next step). The Target UPF acknowledges by an N4 Session Modification Response.

15. The Target UPF may assist in the update of Ethernet forwarding tables of Ethernet switches in the DN via a variety of mechanisms, the use of which are specific to the DN and the specification of which are out-of-scope for 3GPP.

NOTE 2: The UPF acting as a switch in the DN, can for example issue a Gratuitous ARP (GARP) containing the MAC address(es) of the UE that has switched to the new anchor, or the UPF can send an unsolicited Neighbor Discovery Protocol (NDP) Neighbor Advertisement message indicating the UE MAC addresses, or the UPF can generate a uplink Ethernet frame with the UE’s MAC addresses as source MAC addresses (and possibly its VLAN tag(s)) and configurable payload which will be dropped by endhosts, or the UPF can send another message or Ethernet frame compatible with DN protocols.

In the case of a central controller in the Ethernet network which sets the forwarding tables, the central controller can be instructed that the given MAC address is reachable at the new location.

When multiple MAC addresses are present in the Ethernet context, the update of the Ethernet forwarding is performed for each MAC address.

16. The N4 session is released at the Source UPF. The source UPF may wait for a configurable period before it stops delivering downlink Ethernet frames for the given PDU Session.

17. The N4 session release is acknowledged from the Source UPF to the SMF.