MTU (networking)

In computer networking, the maximum transmission unit (MTU) is the size of the largest network layer that can be communicated in a single network transaction. Fixed MTU parameters usually appear in association with a communications interface or standard. Some systems may decide MTU at connect time. The MTU relates to, but is not identical with the maximum frame size that can be transported on the data link layer, e.g. Ethernet frame.

Larger MTU is associated with reduced overhead. Smaller values can reduce network delay. In many cases MTU is dependent on underlying network capabilities and must be or should be adjusted manually or automatically so as not to exceed these capabilities.

MTUs apply to and network layers. The MTU is specified in terms of bytes or octets of the largest that the layer can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). Standards (Ethernet, for example) can fix the size of an MTU; or systems (such as point-to-point serial links) may decide MTU at connect time.

Underlying data link and physical layers usually add overhead to the network layer data to be transported, so for a given maximum frame size of a medium one needs to subtract the amount of overhead to calculate that medium's MTU. For e.g. Ethernet, the maximum frame size is 1518 bytes, 18 bytes of which are overhead (header and FCS), resulting in an MTU of 1500 byte.

A larger MTU brings greater efficiency because each network packet carries more user data while protocol overheads, such as headers or underlying per-packet delays, remain fixed; the resulting higher efficiency means an improvement in bulk protocol throughput. A larger MTU also means processing of fewer packets for the same amount of data. In some systems, per-packet-processing can be a critical performance limitation.

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