Content for  TS 23.501  Word version:  18.0.0

These parameters are used by SMF to control the functionality of the UPF as well as to inform SMF about events occurring at the UPF. The N4 session management procedures defined in clause 4.4.1 of TS 23.502 will use the relevant parameters in the same way for all N4 reference points: the N4 Session Establishment procedure as well as the N4 Session Modification procedure provide the control parameters to the UPF, the N4 Session Release procedure removes all control parameters related to an N4 session, and the N4 Session Level Reporting procedure informs the SMF about events related to the PDU Session that are detected by the UPF. The parameters over N4 reference point provided from SMF to UPF comprises an N4 Session ID and may also contain:

• Packet Detection Rules (PDR) that contain information to classify traffic (PDU(s)) arriving at the UPF;
• Forwarding Action Rules (FAR) that contain information on whether forwarding, dropping or buffering is to be applied to a traffic identified by PDR(s);
• Multi-Access Rules (MAR) that contain information on how to handle traffic steering, switching and splitting for a MA PDU Session;
• Usage Reporting Rules (URR) contains information that defines how traffic identified by PDR(s) shall be accounted as well as how a certain measurement shall be reported;
• QoS Enforcement Rules (QER), that contain information related to QoS enforcement of traffic identified by PDR(s);
• Session Reporting Rules (SRR) that contain information to request the UP function to detect and report events for a PDU session that are not related to specific PDRs of the PDU session or that are not related to traffic usage measurement.
• Trace Requirements;
• Port Management Information Container in 5GS;
• Bridge Information.

The N4 Session ID is assigned by the SMF and uniquely identifies an N4 session. If the UPF indicated support of Trace, the SMF may activate a trace session during a N4 Session Establishment or a N4 Session Modification procedure. In that case it provides Trace Requirements to the UPF. The SMF may deactivate an on-going trace session using a N4 Session Modification procedure. There shall be at most one trace session activated per N4 Session at a time. For the MA PDU Session, the SMF may add an additional access tunnel information during an N4 Session Modification procedure by updating MAR with addition of an FAR ID which refers to an FAR containing the additional access tunnel information for the MA PDU session for traffic steering in the UPF. For the MA PDU Session, the SMF may request Access Availability report per N4 Session, during N4 Session Establishment procedure or N4 Session Modification procedure. A N4 Session may be used to control both UPF and NW-TT behaviour in the UPF. A N4 session support and enable exchange of TSN bridge configuration between the SMF and the UPF:

• Information that the SMF needs for bridge management (clause 5.8.2.11.9);
• Information that 5GS transparently relays between the TSN AF or NEF and the NW-TT: transparent Port Management Information Container along with the associated NW-TT port number.
• Information that 5GS transparently relays between the TSN AF or NEF and the NW-TT: transparent user plane node Management Information Container (clause 5.8.2.11.14).

When a N4 Session related with bridge management is established, the UPF allocates a dedicated port number for the DS-TT side of the PDU Session. The UPF then provides to the SMF following configuration parameters for the N4 Session:

• DS-TT port number.
• user-plane node ID.

To support TSN, the user-plane node ID is Bridge ID. The User Plane Node ID/Bridge ID may be pre-configured in the UPF based on deployment. After the N4 session has been established, the SMF and UPF may at any time exchange transparent user plane node and Port Management Information Container over a N4 session.

N4 Session Context is identified by an N4 Session ID. An N4 Session Context is generated by SMF and UPF respectively to store the parameters related to an N4 session, including the N4 session ID and following information (see TS 29.244 for an exhaustive list):

1. general session related parameters such as S-NSSAI, PDU Session Type, Trace Information, APN/DNN, ATSSS Control Information;
2. the PDRs, URRs, QERs, BAR(s), FARs, MARs used for this N4 session;
3. parameters sent to support UPF statistics.

The UPF may use parameters listed above in bullets 1) (e.g. S-NSSAI) and 2) (e.g. Network Instance in PDR/FAR(s)) for determining internal UPF resources.

The following Table describes the Packet Detection Rule (PDR) containing information required to classify a packet arriving at the UPF. Every PDR is used to detect packets in a certain transmission direction, e.g. UL direction or DL direction.

The following Table describes the QoS Enforcement Rule (QER) that defines how a packet shall be treated in terms of bit rate limitation and packet marking for QoS purposes. All Packet Detection Rules that refer to the same QER share the same QoS resources, e.g. MFBR.

The following Table describes the Usage Reporting Rule (URR) that defines how a packet shall be accounted as well as when and how to report the measurements.

The following Table describes the Forwarding Action Rule (FAR) that defines how a packet shall be buffered, dropped or forwarded, including packet encapsulation/decapsulation and forwarding destination.

The UPF sends the Usage Report to inform the SMF about the measurement of an active URR or about the detection of application traffic of an active Packet Detection Rule. For each URR, the Usage Report may be generated repeatedly, i.e. as long as any one of the valid event triggers applies. A final Usage Report is sent for a URR when it is no longer active, i.e. either the URR is removed or all the references to this URR in any of the Packet Detection Rules belonging to the N4 session. The following attributes can be included:

The following Table describes the Multi-Access Rule (MAR) that includes the association to the two FARs for both 3GPP access and non-3GPP access in the case of supporting ATSSS.

The following Table describes the User plane node Information (UI) that includes the information required to configure a 5GS logical bridge for TSC PDU Sessions.

The following Table describes the Session Reporting Rule (SRR) that defines the detection and reporting events that the UPF shall report, that are not related to specific PDRs of the PDU Session, as follows:

• Per QoS Flow per UE QoS Monitoring Report, as specified in clause 5.33.3.2.
• Change of 3GPP or non-3GPP access availability, for an MA PDU session.

The UPF sends the session report to inform the SMF the detected events for a PDU Session that are related to an SRR. The UPF may support notification to the AF possibly via local NEF as described in clause 6.4 of TS 23.548.

The following Table describes the TSC Management Information Container (TSC MIC) that includes UMIC, PMIC and the associated NW-TT port number.

The UPF sends the Downlink Data Report to inform the SMF about the events related to receiving or discarding of downlink packets. The SMF controls this type of UPF report by providing instructions in the Buffer Action Rule of a FAR. Following attributes can be included:

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. The UPF local switching, N6-based forwarding and N19-based forwarding methods require that a common SMF is controlling the PSA UPFs for the 5G VN group. 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; 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).