Maximum Transmit Unit or Maximum Transmission Unit

A packet-size property of physical network interfaces. For example, for Ethernet the MTU is 1500 bytes. The MTU can also be specified for higher-level protocols such as TCP/IP and set to higher values. Furthermore, a network’s MTU has major performance implications. For example, in Microsoft Windows, the maximum packet size for the TCP protocol is specified in the Registry. If this value is set to too small a number, data will be fragmented into a relatively high number of smaller packets—with an overall negative impact on performance. On the other hand, if the maximum TCP packet size is set too high, it will exceed the physical layer’s MTU and, again, reduce performance. The reason for reduced performance under these circumstances is that each message on the TCP layer is split into at least two smaller ones—a process called fragmentation.

For owners of home PCs, setting an optimal TCP packet size can be a bit tricky. For LAN, leaving the MTU setting at 1500 bytes works well with Ethernet and is considered to be a wise bet. For communications over a dial-up connection to the Internet, the suggested MTU setting is 576 bytes. Finally, high-speed connections (including cable service, DSL, and home LANs) typically perform better at higher values.

See Also: Ethernet; Internet; Local Area Network (LAN); Network; Packet; Registry; TCP/IP or Transmission Control Protocol/Internet Protocol.