What is a Maximum Transmission Unit?

The Maximum Transmission Unit (MTU) is used to notify the other party of the maximum size of the data service unit that can be accepted, and it indicates the size of the payload that the sender can accept. ^[1]

The configuration response of the remote node will include the actual MTU size used on the channel, the direction of the channel is to the local node, and the MTU value is the minimum of the MTU in configReq and the output MTU capability of the remote node. This MTU can only be used for this channel, not for channels in the opposite direction. ^[4]

The MTU is an important factor in network regulation because the amount of overhead in the packet is quite high. A high MTU reduces the number of bytes of wasted header information. It is especially important for large data transmissions, but has no effect on transmissions smaller than the MTU. Therefore, pay attention to configuring the MTU on servers (such as file servers and FTPH &. Servers) that transmit a large number of data streams. ^[5]

Different data links have different Maximum Transmission Units (MTUs). Since the IP protocol is the upper layer of the data link, it must be able to be used without being affected by the MTU size of the data link. When the IP datagram is too large, it is necessary to use fragmentation technology to ensure that the data frame is not larger than the MTU of the network to be passed. ^[3]

In addition to the routing addressing function of the IP protocol, another important function is the fragmentation processing of IP datagrams. The maximum length of each frame that can be determined by each data link layer is called the maximum transmission unit. In Ethernet, the MTU is 1500 bytes; in FDDI, the MTU is 4352 bytes; in IP over AMT, the MTU is 9180 bytes. ^[3]

If the IP datagram to be sent is larger than the MTU of the data link layer, the datagram cannot be sent. For the IP protocol from the upper layer, when the IP datagram required to be sent is larger than the MTU of the data link layer, the datagram must be divided into multiple IP datagrams before it can be sent. In addition, there is a data link with a different MTU between the hosts that communicate; during the transmission process, the MTU shrinks. When the above situation occurs, there must be a router capable of fragment processing in the sending process. ^[3]

The receiving host must restore the IP datagram that has been fragmented. In the relay router, although the router performs fragmentation processing, it does not perform restoration processing. In addition, the fragmented IP datagram can only be restored to the original IP datagram after the restoration process, and the data can be transmitted to the upper module. ^[3]