In network communications, the Maximum Transmission Unit (MTU) is the largest network-layer transmission unit that can fit into a Link Layer data frame. This can also be stated as being the largest data segment a network device is able to receive.
Per RFC2460; an MTU can be configurable via the Link Layer but must be at least 1280 octets and no more than 1500 octets. An “octet” is a series of 8 bits or, as it’s more commonly known, a byte. Network traffic is measured in bits, however, and therefore the octet mnemonic often takes precedence.
This is modeled by various protocol layer stacks such as OSI and/or TCP/IP protocol layer stacks. The MTU consists of application layer data + transport layer headers + network layer headers. While not completely defined by the maximum link-layer frame size, the MTU cannot exceed it.
When a packet size exceeds the MTU, it must be fragmented prior to network transmission which, in turn, necessitates re-assembly upon receipt. While this practice is possible, it is discouraged and applications are suggested to continually adjust packet sizing to meet calculated network MTU.
The MTU is also used as a relative size by which higher-layer protocols, such as TCP, use to create their structures. For example, TCP’s congestion control algorithm has a Slow Start state in which the congestion window is initially set to 1 x MSS (which is calculated to be 40 bits smaller, relative to the MTU).
Note: The MSS is 60 bits smaller during IPV6 usage