Maximum Transmission Unit
Each media type has a maximum frame size, called the maximum transmission unit (MTU), that cannot be exceeded. The link layer is responsible for discovering this MTU and reporting it to the protocols named earlier. NDIS drivers can 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, which optimizes packet sizes for each media automatically. For details, see the discussion of Path Maximum Transmission Unit (PMTU) discovery in "Internet Control Message Protocol" later in this chapter.
If a network adapter driver — such as an ATM driver — uses LAN emulation mode, it might report that it has an MTU that is higher than what is expected for that media type. For example, it might emulate Ethernet but report an MTU of 9180 bytes. Windows 2000 accepts and uses the MTU size reported by the adapter even when it exceeds the normal MTU for a particular 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 often reduces the MTU reported by 4 bytes due to larger data-link layer headers.