Maximum transfer unit

The Maximum Transmission Unit (Maximum Transmission Unit - MTU) is a computer networking term that expresses the size in bytes of the largest unit of data that can be sent using a communications protocol.

Examples of MTUs for different protocols used on the Internet:

• Ethernet: 1500 bytes
• PPPoE: 1492 bytes
• ATM (AAL5): 9180 bytes
• FDDI: 4470 bytes
• PPP: 576 bytes

For IP, the maximum MTU value is 64 Kilobytes (2¹⁶ - 1). However, this is a theoretical maximum value, since, in practice, the IP entity will determine the maximum size of the IP datagrams depending on the network technology through which the datagram will be sent. By default, the IP datagram size is 576 bytes. Larger datagrams can only be sent if there is good knowledge that the network receiving the datagram can accept that size. In practice, since most machines are connected to Ethernet or branch networks, the datagram size that is sent is often 1500 bytes.

Datagrams can pass through several types of networks with different acceptable sizes before reaching their destination. Therefore, for a datagram to reach its destination without fragmentation, it must be less than or equal to the lowest MTU of all the networks through which it passes.

In the case of TCP/UDP, the maximum value is given by the MSS (Maximum Segment Size), and takes its value based on the maximum datagram size, since the MTU = MSS + IP headers + TCP/UDP headers udp. Specifically, the maximum segment size is equal to the maximum datagram size minus 40 (which is the minimum number of bytes that the IP and TCP/UDP headers will occupy in the datagram).

Potential problems

More and more networks block ICMP traffic (eg to prevent denial of service attacks), which prevents route MTU discovery from working. These hangs can often be detected if the connection is working with low data traffic but hangs when a host sends a large block of data at once. Also, in an IP network, the route from the source to the destination often changes dynamically for different reasons (load balancing, congestion, etc.); this can cause the MTU of the route to vary (sometimes repeatedly) during a transmission, which can cause subsequent packets to be dropped before the host finds a new reliable MTU for the route.