Each media type has a maximum frame size that cannot be exceeded. The link layer is responsible for discovering this maximum transfer unit (MTU) and reporting it to the protocols above. NDIS drivers can be queried for the local MTU by the protocol stack. The upper layer protocols, such as TCP, use the MTU to automatically optimize packet sizes for each media. See the section "Internet Control Message Protocol" later in this chapter for related information about path maximum transfer unit (PMTU) discovery.
If a NIC driver such as an ATM driver is using LAN-emulation mode, it may report that it has an MTU higher than what is expected for that media type. For example, it may emulate Ethernet but report an MTU of 9180 bytes. Windows NT accepts and uses the MTU size reported by the adapter even when it exceeds the normal MTU for a given media type.