Section: 5.6.10.4 Maximum Transfer Unit size considerations
Content: In order to avoid data packet fragmentation between the UE and the UPF acting as PSA, the link MTU size in the UE should be set to the value provided by the network as part of the IP configuration. The link MTU size for IPv4 is sent to the UE by including it in the PCO (see TS 24.501[ Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3 ]  [47]). The link MTU size for IPv6 is sent to the UE by including it in the IPv6 Router Advertisement message (see RFC 4861 [54]). NOTE 1: Ideally the network configuration ensures that for PDU Session type IPv4v6 the link MTU values provided to the UE via PCO and in the IPv6 Router Advertisement message are the same. In cases where this condition cannot be met, the MTU size selected by the UE is unspecified. When using a PDU Session type Unstructured, the maximum uplink packet size, and when using Ethernet, the Ethernet frames' payload, that the UE should use may be provided by the network as a part of the session management configuration by encoding it within the PCO (see TS 24.501[ Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3 ]  [47]). When using a PDU Session type Unstructured, to provide a consistent environment for application developers, the network shall use a maximum packet size of at least 128 octets (this applies to both uplink and downlink). When the MT and the TE are separated, the TE may either be pre-configured to use a specific default MTU size or the TE may use an MTU size provided by the network via the MT. Thus, it is not always possible to set the MTU value by means of information provided by the network. NOTE 2: In network deployments that have MTU size of 1500 octets in the transport network, providing a link MTU value of 1358 octets (as shown in Figure J-1) to the UE as part of the IP configuration information from the network will prevent the IP layer fragmentation within the transport network between the UE and the UPF. For network deployments that uniformly support transport with larger MTU size than 1500 octets (for example with ethernet jumbo frames of MTU size up to 9216 octets), providing a link MTU value of MTU minus 142 octets to the UE as part of the IP configuration information from the network will prevent the IP layer fragmentation within the transport network between the UE and the UPF. Link MTU considerations are discussed further in Annex J. NOTE 3: As the link MTU value is provided as a part of the session management configuration information, a link MTU value can be provided during each PDU Session establishment. In this release, dynamic adjustment of link MTU for scenarios where MTU is not uniform across transport are not addressed.