name: PSA
full_form: PDU Session Anchor
context: 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.
description: PDU Session Anchor (PSA) is a 5G Network Function that allows for Ethernet PDU Sessions to change the PDU Session Anchor while the session remains set up. It involves the Source UPF acting as the PSA, reporting Ethernet context information to the SMF, which determines whether and when a serving PDU Session anchor needs to be changed and selects the Target PSA UPF.
explanation: PDU Session Anchor is classified under Telecommunications Concept because it relates to the management of network sessions and the exchange of information between different nodes in the network, specifically in the context of 5G networks and Ethernet PDU Sessions.
section_title: 4.3.5.8 Ethernet PDU Session Anchor Relocation
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