C.5 Split-UE Considerations for EAS (Re-)discovery

23.5483GPP5G System Enhancements for Edge ComputingRelease 17Stage 2TS

For the split-UE (i.e. the TE and MT are separated), information provided by the SMF in the NAS message during the PDU Session Establishment, Modification and Command is provided to the MT and MTs cannot provide the NAS provided IP parameters to the TE, i.e. the TE cannot receive that information from the MT because of separation between the TE and MT. Example of information are the DNS configuration or Rediscovery indication.

The TE gets LAN side IP parameters configuration from the MT, i.e. using DHCPv4 (for IPv4) or IPv6 Router Advertisement/DHCPv6 (for IPv6). MT hosts the DNS resolver for TE and its address can be obtained from MT using DHCP or IPv6 RA. When TE uses DNS resolver in MT, the MT in turn uses its configured network DNS resolver (e.g. EASDF, L-DNS) which is the expected DNS resolution chain and it results in the discovery of the correct EAS. An application in the TE that complies with EAS (re-)discovery described in this specification is not recommended to override operator-provided DNS settings as described in clause C.3.

For the split-UE and MTs cannot provide the NAS information requesting UE to redo DNS lookup received from the SMF to the TE or the TE OS. In such cases, the closest EAS is still reachable, for example, if anycast EAS address is used.

For the Split-UE in the option C case, if the new address of Local DNS Server cannot be provided to the TE or the TE OS from the MT, so the TE continues to use the old DNS Server to perform the EAS discovery and cannot receive the DNS Query/Response from the 5GC (e.g. the BP will route the DNS Query to the L-PSA). After no DNS Query response is received from the 5GC for serval times or an information indicating the old DNS Server unreachable (e.g. ICMP message of Host Not Reachable), the TE initiates a new DNS Server Discovery via a DHCP message to the 5GC, and the SMF may send the same new DNS Server IP address to the UE in the DHCP response message than sent via PCO in the PDU Session Command. After the UE gets the new DNS Server IP address, the UE uses the new DNS Server IP address to perform the EAS (re-)discovery.