In computer networking, the MTU (Maximum Transmission Unit) of a communications protocol of a layer is the size (in bytes) of the largest protocol data unit that the layer can pass onwards.
MTU parameters usually appear in association with a communications interface (NIC, serial port...). 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.
A larger MTU brings greater efficiency because each packet carries more user data while protocol overheads, such as headers or underlying per-packet delays, remain fixed.
However, this gain is not without a downside. Large packets occupy a slow link for more time than a smaller packet, causing greater delays to subsequent packets and increasing lag and minimum latency. Large packets are also problematic in the presence of communications errors. Corruption of a single bit in a packet requires that the entire packet be retransmitted.
