Maximum Transmission Unit (MTU)
Each media type has a maximum frame size that cannot be exceeded. The link layer is responsible for discovering this MTU and reporting it to the protocols above. NDIS drivers may be queried for the local MTU by the protocol stack. Knowledge of the MTU for an interface is used by upper layer protocols, such as TCP, that optimize packet sizes for each media automatically. For details, see the discussion of TCP path Maximum Transmission Unit (PMTU) discovery in the “Transmission Control Protocol (TCP)” section of this paper.
If a NIC driver—such as an ATM driver—uses LAN emulation mode, it may report that it has an MTU that is higher than what is expected for that media type. For example, it may emulate Ethernet but report an MTU of 9180 bytes. Windows Server 2003 TCP/IP accepts and uses the MTU size reported by the adapter, even when it exceeds the normal MTU for a given media type.
Sometimes the MTU reported to the protocol stack may be less than what would be expected for a given media type. For instance, use of the 802.1p standard for QoS over Ethernet often reduces the MTU reported by 4 bytes due to larger link-layer headers (this is hardware dependent).
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