[Tech] System architecture for the 5G System (5GS); 3GPP TS 23.501 V16.6.0 (2020-09)

5.6.10.2        Support of Ethernet PDU Session type

For a PDU Session set up with the Ethernet PDU Session type, the SMF and the UPF acting as PDU Session Anchor (PSA) can support specific behaviours related with the fact the PDU Session carries Ethernet frames.

Depending on operator configuration related with the DNN, different configurations for how Ethernet traffic is handled on N6 may apply, for example:

–    Configurations with a 1-1 relationship between a PDU Session and a N6 interface possibly corresponding to a dedicated tunnel established over N6. In this case the UPF acting as PSA transparently forwards Ethernet frames between the PDU Session and its corresponding N6 interface, and it does not need to be aware of MAC addresses used by the UE in order to route down-link traffic.

–    Configurations, where more than one PDU Session to the same DNN (e.g. for more than one UE) corresponds to the same N6 interface. In this case the UPF acting as PSA needs to be aware of MAC addresses used by the UE in the PDU Session in order to map down-link Ethernet frames received over N6 to the appropriate PDU Session. Forwarding behaviour of the UPF acting as PSA is managed by SMF as specified in clause 5.8.2.5.

NOTE 1:  The “MAC addresses used by the UE” correspond to any MAC address used by the UE or any device locally connected to the UE and using the PDU Session to communicate with the DN.

Based on operator configuration, the SMF may request the UPF acting as the PDU Session Anchor to respond to ARP/IPv6 Neighbour Solicitation requests based on local cache information, i.e. the mapping between the UE MAC address to the UE IP address, and the DN where the PDU Session is connected to, or to redirect the ARP traffic from the UPF to the SMF. Responding to ARP/IPv6 ND based on local cache information applies to ARP/IPv6 ND received in both UL and DL directions.

NOTE 2:  Responding to ARP/ND from a local cache assumes the UE or the devices behind the UE acquire their IP address via in-band mechanisms that the SMF/UPF can detect and by this link the IP address to the MAC address.

NOTE 3:  This mechanism is intended to avoid broadcasting or multicasting the ARP/IPv6 ND to every UE.

Ethernet Preamble and Start of Frame delimiter are not sent over 5GS:

–    For UL traffic the UE strips the preamble and frame check sequence (FCS) from the Ethernet frame.

–    For DL traffic the PDU Session Anchor strips the preamble and frame check sequence (FCS) from the Ethernet frame.

Neither a MAC nor an IP address is allocated by the 5GC to the UE for a PDU Session.

The PSA shall store the MAC addresses received from the UE, and associate those with the appropriate PDU Session.

The SMF may receive a list of allowed VLAN tags from DN-AAA (for a maximum of 16 VLAN tags) or may be locally configured with allowed VLAN tags values. The SMF may also be configured with instructions on VLAN handling (e.g., the VLAN tag to be inserted or removed, S-TAG to be inserted or removed). Taking this into account, the SMF determines the VLAN handling for the PDU Session, and instructs the UPF to accept or discard the UE traffic based on the allowed VLAN tags, as well as to handle VLAN tags (addition/removal) via PDR (Outer header removal) and FAR (UPF applying Outer header creation of a Forwarding policy). For example:

–    The UPF may insert (for uplink traffic) and remove (for downlink traffic) a S-TAG on N6 or N19 or internal interface (“5G VN internal”) for the traffic from and to the UE.

–    The UPF may insert (for uplink traffic) and remove (for downlink traffic) a VLAN tag on the N6 interface while there is no VLAN in the traffic to and from the UE.

–    The UPF may discard any UE traffic that does not contain any allowed VLAN tag when the UPF handles the UE uplink or downlink traffic.

NOTE  4: This can be used for traffic steering to N6-LAN but also for N6-based traffic forwarding related with 5G-VN service described in clause 5.29.4

Apart from specific conditions related to the support of PDU sessions over W-5GAN defined in TS 23.316 [84], the UPF shall not remove VLAN tags sent by the UE and the UPF shall not insert VLAN tags for the traffic sent to the UE.

PDU(s) containing a VLAN tag shall be switched only within the same VLAN by a PDU Session Anchor.

The UE may acquire from the SMF, at PDU Session Establishment, the MTU of the Ethernet frames’ payload that the UE shall consider, see clause 5.6.10.4.

NOTE 5:  The UE may operate in bridge mode with regard to a LAN it is connecting to the 5GS, thus different MAC addresses may be used as source address of different frames sent UL over a single PDU Session (and destination MAC address of different frames sent DL over the same PDU Session).

NOTE 6:  Entities on the LAN connected to the 5GS by the UE may have an IP address allocated by the DN but the IP layer is considered as an application layer which is not part of the Ethernet PDU Session.

NOTE 7:  In this Release of the specification, only the UE connected to the 5GS is authenticated, not the devices behind such UE.

NOTE 8:  5GS does not support the scenario where a MAC address or if VLAN applies a (MAC address, VLAN) combination is used on more than one PDU Session for the same DNN and S-NSSAI.

NOTE 9:  This Release of the specification does not guarantee that the Ethernet network remains loop-free. Deployments need to be verified on an individual basis that loops in the Ethernet network are avoided.

NOTE 10: This Release of the specification does not guarantee that the Ethernet network properly and quickly reacts to topology changes. Deployments need to be verified on an individual basis how they react to topology changes.

Different Frames exchanged on a PDU Session of Ethernet type may be served with different QoS over the 5GS. Thus, the SMF may provide to the UPF Ethernet Packet Filter Set and forwarding rule(s) based on the Ethernet frame structure and UE MAC address(es). The UPF detects and forwards Ethernet frames based on the Ethernet Packet Filter Set and forwarding rule(s) received from the SMF. This is further defined in clauses 5.7 and 5.8.2.

When a PDU Session of Ethernet PDU type is authorized by a DN as described in clause 5.6.6, the DN-AAA server may, as part of authorization data, provide the SMF with a list of allowed MAC addresses for this PDU Session; the list is limited to a maximum of 16 MAC addresses. When the list has been provided for a PDU Session, the SMF sets corresponding filtering rules in the UPF(s) acting as PDU Session Anchor for the PDU Session. The UPF discards any UL traffic that does not contain one of these MAC addresses as a source address if the list of allowed MAC addresses is provided.

In this Release of specification, the PDU Session of Ethernet PDU Session type is restricted to SSC mode 1 and SSC mode 2.

For a PDU Session established with the Ethernet PDU Session type, the SMF may, upon PCF request, need to ensure reporting to the PCF of all Ethernet MAC addresses used as UE address in a PDU Session. In this case, as defined in clause 5.8.2.12, the SMF controls the UPF to report the different MAC addresses used as source address of frames sent UL by the UE in the PDU Session.

NOTE 11: This relates to whether AF control on a per MAC address is allowed on the PDU Session as defined in TS 23.503 [45] clause 6.1.1.2.

The PCF may activate or deactivate the reporting of the UE MAC address using the “UE MAC address change” Policy Control Request Trigger as defined in Table 6.1.3.5-1 of TS 23.503 [45].

The SMF may relocate the UPF acting as the PDU Session Anchor for an Ethernet PDU Session as defined in clause 4.3.5.8 of TS 23.502 [3]. The relocation may be triggered by a mobility event such as a handover, or may be triggered independent of UE mobility, e.g. due to load balancing reasons. In order to relocate the PSA UPF, the reporting of the UE MAC addresses needs to be activated by the SMF.

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