Maximum Transfer Unit

Maximum Transfer Unit refers to the size of the largest chunk of data, within a single unit, that can be transmitted under a particular protocol. For example, the maximum transfer unit for Ethernet is 1,536 eight-bit bytes. The Maximum Transfer Unit for Fibre Channel, by comparison, is 2,112 eight-bit bytes

The Maximum Transmission Unit (MTU) is the largest size of IP datagram which may be transferred using a specific data link connection The MTU value is a design parameter of a LAN and is a mutually agreed value (i.e. both ends of a link agree to use the same specific value) for most WAN links.

The size of MTU may vary greatly between different links (e.g. typically from 128 B up to 10 kB).The prevalent Path MTU on the Internet is now 1500 bytes, the Ethernet MTU. There are some initiatives to support larger MTUs in networks (e.g. 8 KB), in particular on research networks. But their usability is hampered by last-mile deployment of Ethernet with an MTU of 1500 and lack of robustness of Path MTU Discovery.

Maximum Transmission Unit (MTU) and Datagram Fragmentation

The IP implementation of all devices on an IP internet needs to be aware of the capacity of the technology used by that implementation for its immediate data link layer connection to other devices. This limit is called the maximum transmission unit (MTU) of the network. This term is also sometimes seen as the maximum transfer unit.

If an IP layer receives a message to be sent across the internetwork, it looks at the size of the message and then computes how large the IP datagram would be after the addition of the 20 or more bytes needed for the IP header. If the total length is greater than the MTU of the underlying network, the IP layer will fragment the message into multiple IP fragments. So, if a host is connected using an Ethernet LAN to its local network, it may use an MTU of 1,500 for IP datagrams, and will fragment anything larger. Figure 88 shows an example of differing MTUs and fragmentation.

Key Concept: The size of the largest IP datagram that can be transmitted over a physical network is called that network’s maximum transmission unit (MTU). If a datagram is passed from a network with a high MTU to one with a low MTU, it must be fragmented to fit the network with the smaller MTU.

Since some physical networks on the path between devices may have a smaller MTU than others, it may be necessary to fragment more than once. For example, suppose the source device wants to send an IP message 12,000 bytes long. Its local connection has an MTU of 3,300 bytes. It will have to divide this message into four fragments for transmission: three that are about 3,300 bytes long and a fourth remnant about 2,100 bytes long. (I'm oversimplifying by ignoring the extra headers required; the next topic includes the full details of the fragmentation process.)

Figure 88: IP Maximum Transmission Unit (MTU) and Fragmentation

In this simple example, Device A is sending to Device B over a small internetwork consisting of one router and two physical links. The link from A to the router has an MTU of 3,300 bytes, but from the router to B it is only 1,300 bytes. Thus, any IP datagrams over 1,300 bytes will need to be fragmented.