In 5G (NR) systems, data is sent and received between the terminal and the network in Transfer Units (TU); the size of the MTU (Maximum Transmission Unit) is defined by 3GPP in TS23.501 as follows:
I. MTU Setting: To avoid packet fragmentation between the UE and the UPF acting as a PSA, the link MTU size in the UE should be set appropriately (based on the value provided by the network IP configuration). This is because:
II. Network Configuration: Ideally, the network configuration should ensure that for IPv4/v6 PDU sessions, the link MTU values sent to the UE via PCO and IPv6 router advertisement messages are the same. If this condition cannot be met, the MTU size selected by the UE is unspecified.
III. Unstructured PDU Sessions: When using unstructured PDU session types, the UE should use the maximum uplink packet size and, when using Ethernet, the payload of the Ethernet frame, which can be provided by the network as part of the session management configuration and encoded in the PCO (see TS 24.501 [47]). When using unstructured PDU session types, to provide a consistent environment for application developers, the network should use a minimum maximum packet size of 128 bytes (for both uplink and downlink).
IV. MT and TE: When the MT and TE are separated, the TE can be pre-configured to use a specific default MTU size, or the TE can use the MTU size provided by the network via the MT. Therefore, the MTU value is not always set by the information provided by the network.
V. Transport Network Settings: In network deployments where the transport network MTU size is 1500 bytes, providing a link MTU value of 1358 bytes to the UE (as shown in Figure J-1) as part of the network IP configuration information can prevent IP layer fragmentation in the transport network between the UE and the UPF. For deployments of transport networks that support MTU sizes greater than 1500 bytes (such as Ethernet jumbo frames with MTU sizes up to 9216 bytes), providing the UE with a link MTU value of MTU minus 142 bytes as part of the network IP configuration information can prevent IP layer fragmentation in the transport network between the UE and the UPF.
VI. Link Issues: Since the link MTU value is provided as part of the session management configuration information, it can be provided during each PDU session establishment. The dynamic adjustment of the link MTU in cases of inconsistent transport MTU is not discussed in Release 18.
