Content for  TS 23.501  Word version:  18.5.0

The SMF may configure the UPF(s) to apply different traffic forwarding methods to route traffic between PDU Sessions for a single 5G VN group. For example, depending on the destination address, some packet flows may be forwarded locally, while other packet flows are forwarded via N19 and other packet flows are forwarded to N6. If a single SMF serves the DNN/S-NSSAI of the 5G VN group, the UPF local switching, N6-based forwarding and N19-based forwarding methods described in clause 5.29.4 are coordinated by the SMF. If an SMF set serves the DNN/S-NSSAI of the 5G VN group, implementation based mechanisms can be used between SMF(s) that are part of the SMF set for controlling the connectivity between the PSA UPFs of the UE members of the 5G VN group. When a 5G VN group communication is extended in a wide area, bigger than the service area of any SMF set serving the DNN/S-NSSAI of the 5G VN group, multiple SMF sets might control the PDU Sessions of the UE members of the 5G VN group. In this case, N6/N19 connectivity between PSA UPFs of the UE members of the 5G VN group controlled by different SMF sets, is achieved via OAM configuration. As a deployment option, a subset of the UPFs controlled by an SMF Set may be configured with the N6/N19 connectivity to enable 5G VN group communication across SMF Sets. N6 connectivity between PSA UPFs via a DN may also exist. 5G VN group communication includes one to one communication and one to many communication. One to one communication supports forwarding of unicast traffic between two UEs within a 5G VN, or between a UE and a device on the DN. One to many communication supports forwarding of multicast traffic and broadcast traffic from one UE (or device on the DN) to many/all UEs within a 5G VN and devices on the DN. Traffic forwarding within the 5G VN group is realized by using a UPF internal interface ("5G VN internal") and a two-step detection and forwarding process. In the first step, the packets received from any 5G VN group member (via it's PDU Session, via N6 or via N19) are forwarded to the UPF internal interface (i.e. Destination Interface set to "5G VN internal"). In the second step, PDRs installed at the UPF internal interface (i.e. Source Interface set to "5G VN internal") detect the packet and forward it to the respective 5G VN group member (via it's PDU Session, via N6 or via N19). The details of the PDR and FAR configuration are described in the following clauses. For UEs belonging to the same 5G VN group and having PDU Sessions that correspond to N4 Sessions in the same PSA UPF, the following applies for traffic that is sent from one of these UEs to another one of these UEs using local switching: The incoming traffic for one PDU Session will match the corresponding N4 Session's PDR(s) of the source PDU Session (based on GTP-U header information). The traffic is then sent back to classification in that UPF (via the internal interface) and will match another N4 Session corresponding to the destination PDU Session (based on destination address in the PDU). The PDU is then forwarded to the target UE. If 5G VN group members' PDU Sessions are served by different PSA UPFs and N19-based forwarding is applied, the SMF creates a group-level N4 Session with each involved UPF to enable N19-based forwarding and N6-based forwarding. When the traffic is then sent back to classification in that UPF (via the internal interface) it may match group-level N4 Session corresponding to the 5G VN group (based on destination address in the PDU or a default PDR rule with match-all packet filter). The PDU is then forwarded to N6 or to the UPF indicated in the group-level N4 Session via corresponding N19 tunnel. This enables the PDU to be sent to the target group member in the other UPF or to the device in the DN. In the case of N19-based forwarding is not applied for a 5G VN group, group level N4 session is not required. If more than one 5G VN group has to be supported in the PLMN, the N4 rule attribute Network Instance is used in addition to the UPF internal interface and set to a value representing the 5G VN group. This keeps the traffic of different 5G VN groups separate from each other and thus enables isolation of the 5G VN group communication during the packet detection and forwarding process. The SMF shall provide the PDRs and FARs related to the UPF internal interface as follows whenever more than one 5G VN group has to be supported in the PLMN:

• The FAR with Destination Interface set to "5G VN internal" shall also contain the Network Instance set to the value representing the 5G VN group.
• The PDR with Source Interface set to "5G VN internal" shall also contain the Network Instance set to the value representing the 5G VN group.

Forwarding Ethernet unicast traffic towards the PDU Session corresponding to the Destination MAC address of an Ethernet frame may correspond:

• either to the SMF explicitly configuring DL PDR(s) with the MAC addresses detected by the UPF on PDU Sessions and reported to the SMF; this is further described in clause 5.8.2.13.1;
• or to the SMF relying on MAC address learning in UPF as defined in clause 5.8.2.5.3. To request this UPF behaviour the SMF sets the Ethernet PDU Session Information indication in the DL PDR of the "5G VN internal" interface related with a 5G VN group. This may apply in the case that all PDU Sessions related with this 5G VN group are served by the same PSA or by multiple PSAs not inter-connected via N19.

For Ethernet traffic on 5G-VN, in the former case above where SMF explicitly configures DL PDR with the MAC addresses detected on PDU Sessions supporting a 5G VN group, the SMF acts as a central controller which is responsible for setting up the forwarding rules in the UPFs so that it avoids forwarding loops. The SMF becomes aware of the MAC addresses in use within a 5G VN group by the UPF's reporting of the MAC addresses. The SMF is responsible to react to topology changes in the Ethernet network. Local switching without SMF involvement is not specified for a 5G-VN when different PDU Sessions related with this 5G VN group may be served by different PSA(s) connected over N19.

To enable unicast traffic forwarding in a UPF, the following applies:

• The SMF provides for each 5G VN group member's N4 Session (i.e. N4 Session corresponding to PDU Session) the following N4 rules that enable the processing of packets received from this UE.
  • in order to detect the traffic, a PDR containing Source Interface set to "access side", and CN Tunnel Information set to PDU Session tunnel header (i.e. N3 or N9 GTP-U F-TEID); and
  • in order to forward the traffic, a FAR containing Destination Interface set to "5G VN internal".
• The SMF provides for each 5G VN group member's N4 Session (i.e. N4 session corresponding to PDU Session) the following N4 rules that enable the processing of packets towards this UE.
  • in order to detect the traffic, a PDR containing Source Interface set to "5G VN internal", and Destination Address set to the IP/MAC address (es) of this 5G VN group member; and
  • in order to forward the traffic, a FAR containing Outer Header Creation indicating the N3/N9 tunnel information, and Destination Interface set "access side".
• If N19-based forwarding is applied, the SMF configures the group-level N4 Session for processing packets received from a N19 tunnel with the following N4 rules for each N19 tunnel.
  • in order to detect the traffic, a PDR containing Source Interface set to "core side", and CN Tunnel Information set to N19 tunnel header (i.e. N19 GTP-U F-TEID); and
  • in order to forward the traffic, a FAR containing Destination Interface set to "5G VN internal".
• If N19-based forwarding is applied, the SMF configures the group-level N4 Session for processing packets towards 5G VN group members anchored at other UPFs with the following N4 rules for each N19 tunnel.
  • in order to detect the traffic, a PDR containing Source Interface set to "5G VN internal", and Destination Address set to the IP/MAC address (es) of UEs anchored at the peer UPF of this N19 tunnel (e.g. based on the IP address range supported by the peer UPF); and
  • in order to forward the traffic to a 5G VN group member anchored at another UPF via the N19 tunnel, a FAR containing Outer Header Creation indicating the N19 tunnel information, Destination Interface set to "core side".
• The SMF configures the group-level N4 Session for processing packets received from a 5G VN group member connected via N6 with the following N4 rules.
  • in order to detect the traffic, a PDR containing Source Interface set to "core side", and Source Address set to the IP/MAC address (es) of this 5G VN group member; and
  • in order to forward the traffic, a FAR containing Destination Interface set to "5G VN internal".
• The SMF configures the group-level N4 Session for processing packets towards a 5G VN group member connected via N6 or packets towards a device residing in DN with the following N4 rules.
  • in order to detect the traffic, a PDR containing Source Interface set to "5G VN internal", and Destination Address set to the IP/MAC address (es) of this 5G VN group member; and
  • in order to forward the traffic to the 5G VN group member or device via N6, a FAR containing Destination Interface set to "core side".
• The SMF shall update N4 rules for group-level N4 Session to enable correct forwarding of packets towards UE who's PSA UPF has been reallocated and address is unchanged.
• The SMF may also configure the following N4 rules for the group-level N4 Session to process packets with an unknown destination address:
  • in order to detect the traffic, a PDR containing Source Interface set to "5G VN internal", a match-all Packet Filter, and a Precedence set to the lowest precedence value; and
  • in order to process the traffic, a FAR containing Destination Interface set to "core side" to route the traffic via N6 by default, or in the case of local SMF configuration that N6-based forwarding is not applied a FAR instructing the UPF to drop the traffic.

To enable the service continuity when the PSA UPF serving the UE changed, the following applies:

• Keep the UE address unchanged if N6-based forwarding is not used.
• Configure the UE's N4 Session with N4 rules (PDR, FAR) to detect and forward the traffic to this UE via its PDU Session tunnel(i.e. N3 tunnel) on the target PSA UPF.
• If N19-based forwarding is applied: To switch the traffic towards this UE from the source PSA UPF to the target PSA UPF for N19-based forwarding, the SMF deletes the N4 rule (PDR) that detects the traffic towards this UE in the group-level N4 Session at UPFs involved in the 5G VN group (except the source PSA UPF), then adds or updates the PDR that detects the traffic towards this UE with the FAR containing the N19 tunnel information of the target PSA UPF in the group-level N4 Session at UPFs involved in the 5G VN group (except the target PSA UPF).

5GC supports buffering of UE's downlink packets for deactivated PDU Sessions. Support for buffering in the UPF is mandatory and optional in the SMF. When the UP connection of a PDU Session is deactivated, buffering in UPF can be activated by the SMF. If the SMF supports buffering capability, the SMF can decide to activate buffering in SMF instead of buffering in UPF.

When the SMF decided to activate buffering in UPF, the SMF shall inform the UPF to start buffering packets for this PDU Session. The SMF provides instructions to the UPF for at least the following behaviour:

• buffer downlink packets with the following additional options:
  • reporting the arrival of first downlink packet (for a QoS Flow or a service data flow), and/or
  • reporting the first discarded downlink packet (for a service data flow), or
• drop downlink packets with the following additional options:
  • reporting the first discarded downlink packet (for a service data flow).
• buffer uplink packets.

When the SMF instructs the UPF for a service data flow to buffer downlink packets and to report the first discarded downlink packet, the SMF shall also instruct the UPF to report the arrival of the first downlink packet for this service data flow to enable the SMF check if this is also the first report for the QoS Flow (as described below). Buffering in the UPF may be configured based on timers or the amount of downlink data to be buffered. The SMF decides whether buffering timers or amount of downlink data are handled by the UPF or SMF. After starting buffering, when the first downlink packet (of a QoS Flow or a service data flow) arrives, UPF shall inform the SMF if it is setup to report. UPF sends a Downlink Data Report to the SMF via N4 unless specified otherwise and indicates the PDR by which the downlink packet was received. If the SMF receives a Downlink Data Report for a service data flow, the SMF shall also check if this is the first report for the QoS Flow corresponding to the PDR. If so, the SMF shall also proceed as described in clause 5.4.3.1. After starting buffering, when the first downlink packet (of a service data flow) in a configured period of time that has been buffered is discarded by the UPF because the configured buffering time or amount of downlink data to be buffered is exceeded, the UPF shall inform the SMF if it is setup to report. UPF sends a Downlink Data Report to the SMF via N4 and indicates the PDR by which the discarded downlink packet was received. A new report is sent if the SMF terminates and subsequently re-activates the buffering action at the UPF and the UPF again receives downlink packets. When the UP connection of the PDU Session is activated, the SMF updates the UPF of the change in buffering state. The buffered downlink packets, if any, are then forwarded to the (R)AN by the UPF. If the UP connection of the PDU Session has been deactivated for a long time, the SMF may indicate the UPF to stop buffering for this PDU Session. The SMF may indicate to the UPF to start or stop the buffering of uplink packets of an application associated to the PCC rule as described in clause 6.3.5 of TS 23.548. When the buffering of uplink packets is stopped the UPF shall forward all buffered uplink packets before it forwards any new uplink packets.

When the SMF supports buffering capability and the SMF decided to activate buffering in SMF for the PDU Session, the SMF shall inform the UPF to start forwarding the downlink packets towards the SMF. When the UP connection of the PDU Session is activated, if there are buffered downlink packets available and their buffering duration has not expired, the SMF shall forward those packets to the UPF to relay them to the UE. These packets are then forwarded by the UPF to the (R)AN.