4.23.9 Branching Point or UL CL controlled by I-SMF

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

4.23.9.0 Overview

The procedures in this clause describe the Addition, Removal and Change of PDU Session Anchor (PSA2), Branching Point or UL CL controlled by I-SMF. They all rely on following principles:

1. When a (new) I-SMF is inserted (e.g. as described in clause 4.23.7 or clause 4.23.11), the I-SMF provides the DNAI list it supports to the SMF. This list is assumed to remain constant during the N16a association between the I-SMF and the SMF for a PDU Session.

2. Based on the DNAI list information received from I-SMF, the SMF may then at any time provide or update the list of DNAI(s) of interest for this PDU Session to I-SMF. This may take place e.g. when the I-SMF provides the DNAI list it supports or when new or updated or removed PCC rule(s) is/are received by the SMF as defined in clause 4.23.6. This list of DNAI(s) of interest for this PDU Session indicates to the I-SMF the list of DNAI(s) candidate for local traffic steering within the PDU Session.

An indication of whether Multi-homing is possible is also provided to the I-SMF and the I-SMF uses this information to decide whether multi-homing can be used for the PDU Session.

3. Whenever the I-SMF has inserted or removed or changed a local offload capability the I-SMF invokes a Nsmf_PDUSession_Update Request to indicate to the SMF the list of corresponding DNAI(s). Based on this indication the SMF invokes a Nsmf_PDUSession_Update Request to send the corresponding N4 information to the I-SMF.

4. Then SMF may then at any time invoke a Nsmf_PDUSession_Update Request to send N4 information to the I-SMF.

5. The I-SMF may at any time send a Nsmf_PDUSession_Update Request to forward to the SMF N4 events received from a local UPF; this may e.g. correspond to traffic reporting.

6. When source I-SMF is to be removed from a PDU Session (e.g. at I-SMF change or removal), the SMF issues the N4 information targeting the UL CL/BP (s) and L-PSA(s) controlled by this I-SMF, including requests to release the corresponding N4 Sessions.

If the N9 forwarding tunnel to support the EAS session continuity controlled by I-SMF(s) is not established between the source ULCL and the target UL CL, when the Source I-SMF receives a Nsmf_PDUSession_ReleaseSMContext Request from AMF, it initiates a data forwarding timer (if indirect data forwarding applies) before releasing the resources of the PDU Session. When the Source I-SMF has received N4 release from SMF, it releases the UL CL/BP (s) and L-PSA(s) resources either directly if no data forwarding timer is started, or at the expiry of the data forwarding timer. Otherwise if the N9 forwarding tunnel to support the EAS session continuity controlled by I-SMF(s) is established between the source UL CL and target UL CL, the source I-SMF releases the UL CL/BP (s) and L-PSA(s) after no active traffic over the N9 forwarding tunnel.

When IPv6 multi-homing is used and an I-SMF is removed, the SMF re-configure the UE to not use the original IP prefix @L-PSA(s).

4.23.9.1 Addition of PDU Session Anchor and Branching Point or UL CL controlled by I-SMF

This clause describes a procedure to add a PDU Session Anchor and Branching Point or UL CL controlled by I-SMF.

Figure 4.23.9.1-1: Addition of PDU Session Anchor and Branching Point or UL CL controlled by I-SMF

1. UE has an established PDU Session with a UPF including the PDU Session Anchor 1, which is controlled by SMF.The I-SMF and an I-UPF controlled by I-SMF have already been inserted for the PDU Session. Events described in item 1 and 2 of clause 4.23.9.0 have taken place.

2. At some point, using the list of DNAI(s) of interest for this PDU Session received from the SMF, the I-SMF decides to establish a new PDU Session Anchor e.g. due to UE mobility. The I-SMF selects a UPF and using N4 establishes the new PDU Session Anchor 2 (PSA2) of the PDU Session. During this step:

– (if needed) the PSA2 CN Tunnel Info of the local N9 termination on the PSA2 may be determined,

– In the case of IPv6 multi-homing applies to the PDU Session, a new IPv6 prefix corresponding to PSA2 is allocated by the I-SMF or by the UPF supporting the PSA2.

3. The I-SMF may select a UPF that will be acting as UL CL or Branching Point and replace the current I-UPF.

If a new UPF that will act as UL CL/Branching Point is selected (i.e. the existing I-UPF is replaced), the I-SMF uses N4 establishment to provide the 5G AN Tunnel Info, the PSA1 and (where applicable) PSA2 CN Tunnel Info to the new UPF.

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.

4. The I-SMF invokes Nsmf_PDUSession_Update Request (Indication of UL CL or Branching Point insertion, IPv6 prefix @PSA2, DNAI(s) supported by PSA2, DL Tunnel Info of the new UL CL/Branching Point, if any) to SMF. Whether the UL CL/Branching Point and PSA2 are supported by the same UPF is transparent to the SMF. Multiple local PSAs (i.e. PSA2) may be inserted at one time, each corresponds to a DNAI and/or an IPv6 prefix in the case of multi-homing.

The I-SMF informs the SMF that a UL CL or Branching Point is inserted, the I-SMF provides DNAI(s) supported by PSA2 to the SMF. The DL Tunnel Info of UL CL/Branching Point is provided to SMF if a new UPF is selected to replace I-UPF in step 3.

In the case of IPv6 multi-homing PDU Session, the IPv6 prefix @PSA2 is also provided to SMF.

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 also send a notification to the AF, as described in clause 4.3.6.3.

The DNAI(s) supported by PSA2 may be used by the SMF to determine which PCC rules are to be applied at UPF(s) controlled by the I-SMF. The SMF acknowledges the Nsmf_PDUSession_Update from the I‑SMF

5. If a new DL Tunnel Info of UL CL/ Branching Point has been provided in step 4, the SMF updates the PSA1 via N4 with the CN Tunnel Info for the downlink traffic. Now the downlink packets from PSA1 are sent to UE via the new UPF which will act as Branching Point/UL CL. The SMF may also update the forwarding rules in PSA1 if some traffic is to be moved to UPFs controlled by I-SMF.

6. The SMF provides I-SMF with N4 information for the PSA and for the UL CL with a SMF initiated Nsmf_PDUSession_Update Request (set of (N4 information, involved DNAI), Indication of no DNAI change, Indication of no local PSA change)). The SMF generates N4 information for local traffic handling based on PCC rules and CHF requests that will be enforced by UPFs controlled by I-SMF. The N4 information for local traffic handling corresponds to N4 rules (PDR, FAR, URR, QER, etc.) related with the support of a DNAI. This is described in clause 5.34.6 of TS 23.501 [2]. N4 information for local traffic handling may indicate information (as the 5G AN Tunnel Info) that the SMF does not know and that the I-SMF needs to determine itself to build actual rules sent to the UPF(s). If the rule is applied to the local PSA, the N4 information includes the associated DNAI.

If the "Indication of application relocation possibility" or "UE IP address preservation indication" attributes are included in the PCC rule, the SMF includes the corresponding Indication of no DNAI change and Indication no local PSA change respectively.

If the CN Tunnel Info at the PSA1 has changed, the SMF may also provide its new value.

The I-SMF uses N4 information for local traffic handling received from the SMF as well as 5G AN Tunnel Info received from the 5G AN via the AMF and local configuration to determine N4 rules to send to the UPF(s) it is controlling.

7. The I-SMF updates the PSA2 via N4 providing N4 rules determined in step 6. It also provides the Branching Point or UL CL CN Tunnel Info for down-link traffic if the PSA2 and the UL CL/Branching Point are supported by different UPF(s).

8. The I-SMF updates the Branching Point or UL CL via N4 providing N4 rules determined in step 6.

NOTE 2: If the Branching Point or UL CL and the PSA2 are co-located in a single UPF then step 7 and step 8 can be merged.

9. The I-SMF Issues a Nsmf_PDUSession_Update Response to SMF that may include N4 information received from the local UPF(s).

10. Steps 6-8 of clause 4.3.5.4 are performed. In the case of IPv6 multi-homing PDU Session, the SMF notifies the UE of the IPv6 prefix @PSA2 and updates the UE with IPv6 multi-homed routing rule via a PSA controlled by the SMF.

NOTE 3: Step 6 of clause 4.3.5.4 is skipped if the current I-UPF is selected to act as Branching Point or UL CL.

11. If a new UPF is selected to replace I-UPF in step 3, the I-SMF uses N4 Release to remove the I-UPF of the PDU Session. The I-UPF releases resources for the PDU Session.

4.23.9.2 Removal of PDU Session Anchor and Branching Point or UL CL controlled by I-SMF

This clause describes a procedure to remove a PDU Session Anchor and Branching Point or UL CL controlled by I-SMF.

Figure 4. 23.9.2-1: Removal of PDU Session Anchor and Branching Point or UL CL controlled by I-SMF

1. UE has an established PDU Session with a UPF including the PDU Session Anchor 1 (controlled by SMF) and the UL-CL/BP and the PDU Session Anchor 2 (controlled by I-SMF). Events described in item 1 and 2 of clause 4.23.9.0 have taken place.

At some point the I-SMF decides to remove the PDU Session Anchor 2 and UL-CL/BP function, e.g. due to UE mobility.

2. The I-SMF may select a new UPF acting as new I-UPF and replace the existing I-UPF which was acting as UL-CL/BP before.

If a new UPF acting as new I-UPF is selected, the I-SMF uses N4 establishment to provide the PSA1 CN Tunnel Info and (R)AN Tunnel Info to the new I-UPF.

3. The I-SMF invokes Nsmf_PDUSession_Update Request (Indication of Removal of traffic offload, Removal of IPv6 prefix @PSA2, DNAI associated with the PSA2, DL Tunnel Info of new I-UPF, if any) to SMF. Multiple local PSAs may be removed, in this case, the I-SMF provides for each local PSA to be removal, the associated DNAI and an IPv6 prefix in the case of multi-homing.

The I-SMF informs the SMF that local traffic offload is removed. In the case of IPv6 multi-homing, the I-SMF also notifies the SMF with the removal of the IPv6 prefix @PSA2.

The SMF issues a SM Policy Association Modification (clause 4.16.5) corresponding to the IP address allocation/release PCRT(Policy Control Request Trigger). The SMF may also send a notification to the AF, as described in clause 4.3.6.3.

4. If a new UPF that replaces existing I-UPF is selected in step 2, the SMF updates the PSA1 via N4. It provides the CN Tunnel Info of the new I-UPF for the downlink traffic. The SMF may update the packet handling rules in PSA1 as now all traffic is to be moved to PSA1.

5. In the case of IPv6 multi-homing, the SMF notifies the UE to stop using the IPv6 prefix corresponding to PSA2. Also the SMF sends IPv6 multi-homed routing rule along with the IPv6 prefix corresponding to PSA1 to the UE. Based on the information provided in the Router Advertisement, the UE starts using the IPv6 prefix (corresponding to PSA1) for corresponding traffic.

7. The SMF provides I-SMF with N4 information for the local UPF(s) with a SMF initiated Nsmf_PDUSession_Update Request; The N4 information indicates the removal of the traffic offload rules.

8. If a new UPF that replaces existing I-UPF is selected in step 2, the I-SMF releases the old I-UPF. Otherwise the I-SMF updates the existing I-UPF with new rules in order to remove the UL-CL/BP functionality from that I-UPF.

If a new UPF that replaces existing I-UPF is selected in step 2, the SMF updates the (R)AN with the new I-UPF CN Tunnel Info.

If the PSA2 is not collocated with UL-CL/BP function, the I-SMF releases it via N4.

9. The I-SMF answers to the SMF with a Nsmf_PDUSession_Update Response SMF that may include N4 information received from the local UPF(s).

4.23.9.3 Change of PDU Session Anchor for IPv6 multi-homing or UL CL controlled by I-SMF

This clause describes a change of UL-CL/BP function, e.g. addition of a new PDU Session Anchor (i.e. PSA2) and release of the existing additional PDU Session Anchor (i.e. PSA0), via modifying IPv6 multi-homing or UL CL rule in the same Branching Point or UL CL under controlled by the same I-SMF.

Figure 4.23.9.3-1: Change of PDU Session Anchor for Branching Point or UL CL controlled by I-SMF

1. The UE has an established PDU Session with a UPF including the PDU Session Anchor 1(controlled by SMF) and the PDU Session Anchor 0 (PSA0) and an I-UPF acting as UL CL or BP (controlled by I-SMF). Events described in item 1 and 2 of clause 4.23.9.0 have taken place.

2. At some point the I-SMF decides to establish a new PDU Session Anchor and release the existing PDU Session Anchor e.g. due to UE mobility. The I-SMF selects a UPF and using N4 establishes the new PDU Session Anchor 2 of the PDU Session.

In the case of IPv6 multi-homing PDU Session, the I-SMF ensures allocation of a new IPv6 prefix corresponding to PSA2.

3. The I-SMF invokes Nsmf_PDUSession_Update Request (Indication of Change of traffic offload, (new allocated IPv6 prefix @PSA2, DNAI(s) supported by PSA2), (Removal of IPv6 prefix @PSA0, DNAI(s) supported by PSA0)) to SMF.

The I-SMF informs the SMF that a change of traffic offload may occur. Multiple local PSAs may be changed. The I-SMF provides:

– for each local PSA to be added, the DNAI now reachable and in the case of multi-homing: the new allocated IPv6 prefix @PSA2;

– for each local PSA no more reachable, the DNAI no more reachable and in the case of multi-homing, the old IPv6 prefix @PSA0.

4. The SMF may issue a SM Policy Association Modification (clause 4.16.5) corresponding to the IP address allocation/release PCRT. The SMF may also send an "early" notification to the AF, as described in clause 4.23.6.3.

5. The SMF generates the N4 information based on DNAI(s) information received in step 3.The SMF provides I-SMF with N4 information for the PSA and for the UL CL with a SMF initiated Nsmf_PDUSession_Update Request (set of (N4 information, involved DNAI), Indication of no DNAI change, Indication of no local PSA change)). The information includes N4 information to remove the traffic offload related to the DNAI(s) that are no more reachable and to enable the traffic offload related to the DNAI(s) that are now reachable.

6-7. Same as step 7-8 of clause 4.23.9.1

8. The I-SMF releases via N4 the PSA0 if PSA0 is not collocated with UL CL/BP, or updates the UL CL/BP to remove corresponding rules if PSA0 is collocated with UL CL/BP.

9. The I-SMF issues a Nsmf_PDUSession_Update Response to SMF that may include N4 information received from the local UPF(s) including the PSA0.

10. Same as step 7-8 of clause 4.3.5.4 are performed.

4.23.9.4 Simultaneous change of Branching Point or UL CL and additional PSA controlled by I-SMF

This clause describes simultaneous change of UL-CL/BP function and additional PSA, e.g. addition of a new UL CL/BP and PDU Session Anchor (i.e. PSA2) and release of the existing UL CL/BP and additional PDU Session Anchor (i.e. PSA0), with target UPF(s) and source UPF(s) are all controlled by I-SMF.

This procedure may be triggered after N2 handover or Xn based handover procedure.

Figure 4.23.9.4-1: Simultaneous change of Branching Point or UL CL and additional PSA controlled by I-SMF

Comparing to the clause 4.23.9.3, this procedure in addition changes the source UL-CL/BP by a target UL-CL/BP.

1-2. These steps are the same steps 1-2 in clause 4.23.9.3.

3. The I-SMF selects a UPF and using N4 establishes the target UL CL or BP of the PDU Session.

4. The I-SMF invokes Nsmf_PDUSession_Update Request (Indication of Change of traffic offload, (new allocated IPv6 prefix @PSA2, DNAI(s) supported by PSA2), (Removal of IPv6 prefix @PSA0, DNAI(s) supported by PSA0), DL Tunnel Info of the new UL CL/Branching Point) to SMF.

The DL Tunnel Info of target UL CL/Branching Point is provided to SMF.

5. The SMF updates the remote PSA (PSA1) via N4 with the DL Tunnel Info of the Target UL CL/BP for the downlink traffic.

6-8. These steps are the same as steps 4-6 in clause 4.23.9.3.

If EAS session continuity upon UL CL relocation is required, in step 7 the SMF provides the I-SMF additionally with an indication that a N9 forwarding tunnel to support the EAS session continuity is required, UL traffic filter for N9 forwarding by the target UL CL and the value of the timer to detect the end of activity on the N9 forwarding tunnel to support the EAS session continuity. Based on the received information, the I-SMF uses N4 to establish between the source UL CL and target UL CL the N9 forwarding tunnel to support the EAS session continuity. The I-SMF configures the source UL CL to forward traffic received from source L-PSA related to that PDU session toward the target UL CL via the N9 forwarding tunnel.

9. Same as steps 6-8 of clause 4.3.5.4. The I-SMF updates (R)AN for uplink traffic.

10-11. If a N9 forwarding tunnel to support the EAS session continuity is not established between the source UL CL and target UL CL, the I-SMF releases via N4 the source UL-CL/BP as the source UL-CL/BP is replaced by the target UL-CL/BP. The I-SMF also releases via N4 the PSA0 if PSA0 is not collocated with source UL CL/BP ;

12. This step is the same as step 9 in clause 4.23.9.3.

12a. If a N9 forwarding tunnel to support the EAS session continuity is established between the source UL CL and target UL CL, the I-SMF releases the source UL-CL/BP and PSA0, and the N9 forwarding tunnel in target UL CL when detection of no active traffic over the N9 forwarding tunnel takes place per the value of the timer to detect the end of activity on the N9 forwarding tunnel to support the EAS session continuity received from SMF. The detection can be done by Source UL CL, which notifies the I-SMF of no active traffic over the N9 forwarding tunnel.

The I-SMF invokes Nsmf_PDUSession_Update Request to inform the SMF of the release of the resource in UL CL/BP and PSA0 for the PDU Session.

4.23.9.5 Simultaneous change of Branching Points or UL CLs controlled by different I-SMFs

This clause describes simultaneous change of UL-CL/BP function and additional PSA, e.g. addition of a new UL CL/BP and PDU Session Anchor (i.e. PSA2) and release of the existing UL CL/BP and PDU Session Anchor (i.e. PSA0), with target UPF(s) and source UPF(s) are all controlled by different I-SMF(s).

This procedure may be triggered after N2 handover or Xn based handover procedure.

Figure 4.23.9.5-1: Simultaneous change of Branching Point or UL CL and additional PSA controlled by different I-SMFs

1. UE has established PDU Session with Source Branching Point or UL CL and Source UPF (PSA0) controlled by source I-SMF and Remote PSA. The UE has mobility with I-SMF change, e.g. handed over from a source RAN to a target RAN. After mobility, the path between Target I-UPF and Remote PSA (PSA1) has been established.

2. This step is the same as steps 2 in clause 4.23.9.3.

3. Same as in step 3 of Figure 4.23.9.4-1.

4. Same as in step 4 of Figure 4.23.9.4-1.

5. Same as in step 5 of Figure 4.23.9.4-1..

6-8. These steps are the same as steps 4-6 in clause 4.23.9.3 with the following enhancement:

In step 7, to support EAS session continuity upon UL CL relocation the SMF provides the target I-SMF with an indication that a N9 forwarding tunnel to support the EAS session continuity is required, UL traffic filter for N9 forwarding by the target UL CL and the value of the timer to detect the end of activity on the N9 forwarding tunnel to support the EAS session continuity. Based on the received information, the target I-SMF installs corresponding N4 rules in the target UL CL for forwarding of uplink traffics via the N9 forwarding tunnel. The time interval for User Plane inactivity report is also provisioned to UL CL per the timer value received from SMF.

9. Same as in step 9 of Figure 4.23.9.4-1.

10. This step is the same as step 9 in clause 4.23.9.3.

The SMF triggers the procedure from step 11 onwards to support EAS session continuity upon UL CL relocation.

11. If N9 forwarding tunnel to support the EAS session continuity is required based on indication received from SMF in step 7, the target I-SMF request target UL CL to allocate N9 forwarding tunnel info via N4 session modification procedure.

12. The target I-SMF invokes Nsmf_PDUSession_UpdateSMContext Request (N9 forwarding tunnel required, target UL CL N9 forwarding tunnel info, value of the timer to detect the end of activity on the N9 forwarding tunnel to support the EAS session continuity) toward the source I-SMF.

13. The source I-SMF triggers N4 modification procedure to request the source UL CL to establish the N9 forwarding tunnel to support the EAS session continuity. The source I-SMF provides the target UL CL N9 forwarding tunnel info to the source UL CL and receives the source UL CL N9 forwarding tunnel info from the source UL CL. The source I-SMF configures the source UL CL to forward traffic received from source L-PSA related to that PDU session toward the target UL CL via the N9 forwarding tunnel.

The source I-SMF configures a time interval for the source ULCL for the detection of no active traffic over the N9 forwarding tunnel, which is based on the value of the timer to detect the end of activity on the N9 forwarding tunnel to support the EAS session continuity as received from target I-SMF.

After this step, the downlink data can be forwarded via N9 forwarding tunnel from the source L-PSA (PSA0).

14. The source I-SMF sends Nsmf_PDUSession_UpdateSMContext Response (source UL CL N9 forwarding to support the EAS session continuity tunnel info).

15. Based on the traffic filter information received from SMF at step 7 and source UL CL N9 forwarding tunnel to support the EAS session continuity info, the target I-SMF request the target UL CL to forward related UL traffic via N9 forwarding tunnel to support the EAS session continuity.

The target I-SMF configures a time interval for the target ULCL for the detection of no active traffic over the N9 forwarding tunnel, which is based on the value of the timer to detect the end of activity on the N9 forwarding tunnel to support the EAS session continuity received from SMF.

After this step, the uplink data can be forwarded via N9 forwarding tunnel toward the source L-PSA (PSA0).

16-17. Upon detection of no active traffic over the N9 forwarding tunnel to support the EAS session continuity, the source I-SMF releases via N4 the source UL-CL/BP The Source I-SMF also releases via N4 the PSA0 if PSA0 is not collocated with source UL CL/BP.

17a. The source I-SMF invokes Nsmf_PDUSession_Update Request to inform the SMF of the release of the resource in the UL CL/BP and the PSA0 for this PDU Session.

18. Upon detection of no active traffic over the N9 forwarding tunnel to support the EAS session continuity, the Target I-SMF releases the N9 forwarding tunnel in Target UL CL and remove the related filter.