MTU Definition

The maximum transmission unit (MTU) is the largest packet size, measured in bytes, that can be transmitted over a network.

Packets are the fundamental unit of information transport in all modern computer networks, and increasingly in other communications networks as well. They can be a fixed size or variable sizes, depending on the type of network and protocol (i.e., agreed-upon format). Each packet, regardless of its size, contains, in addition to its data, a header, which includes its source and destination IP addresses.

Any messages larger than the MTU are divided into smaller packets before being sent. Breaking them up slows down transmission speeds. Ideally, the MTU should be the same as the smallest MTU of all the networks between the local computer and a message's final destination.

MTUs vary according to the type of protocol and network. For example, Ethernet (the dominant LAN protocol) has a fixed MTU of 1500 bytes, ATM (asynchronous transfer mode) has a fixed MTU of 48 bytes and PPP (point-to-point protocol) has a negotiated MTU that is usually between 500 and 2000 bytes.

The path MTU of an Internet transmission path is the smallest MTU of any of the hops of the path from the transmitting address to the destination address, which is the same as the largest MTU value that can traverse the path without packets being further fragmented. A hop is a link or router that is crossed en route to the destination.

Path MTU discovery is a technique for finding paths between two IP hosts (i.e., computers) that avoid packet fragmentation. This technique makes use of feedback obtained by sending a series of packets with gradually increasing sizes.

Created October 9, 2005.
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