Maxium Transmission Unit (MTU) is the largest packet size, measured in bytes, that can be transmitted over a network.
A packet is a basic unit of information that is transmitted over all modern computer networks, and in other telecommunications networks. Packages can be either fixed or variable, depending on the network and protocol (or unified format). Each packet, regardless of size, contains not only the data, but also the header that contains the source and destination IP addresses.
If larger than MTU, the message will be divided into smaller packets before sending. Splitting will slow down the transfer rate. Ideally, MTU should be the smallest MTU in all the connections between the computer and the destination computer to which the message is sent.
MTU varies by protocol and network. For example, Ethernet (most commonly used in LAN protocols) has a fixed MTU of 1500 bytes. ATM (Asynchronous Transfer Mode) has a fixed MTU of 48 bytes and a PPP (Point to Point Protocol) with MTUs usually in the range of 500 to 2000 bytes.
The MTU of an Internet transmission is the smallest MTU of any hop from the transmitting address to the receiving address, which is also the maximum MTU value that can be switched off through which the packet is not fragmented. Shortcut is a path or routing that crosses the path to the destination.
Path MTU Discovery is a technique to find paths between two IP addresses (ie two computers) to avoid packet fragmentation. This technique takes advantage of the feedback gained from sending a series of packets with incremental size.