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In computer networking, the maximum transmission unit (MTU) is the size of the largest protocol data unit (PDU) that can be communicated in a single network layer transaction. [ 1 ] : 25 The MTU relates to, but is not identical to the maximum frame size that can be transported on the data link layer , e.g., Ethernet frame .
Path MTU Discovery (PMTUD) is a standardized technique in computer networking for determining the maximum transmission unit (MTU) size on the network path between two Internet Protocol (IP) hosts, usually with the goal of avoiding IP fragmentation.
The frame size used by an end-to-end connection is typically limited by the lowest frame size in intermediate links. 802.5 Token Ring can support frames with a 4464-byte MTU, FDDI can transport 4352-byte, ATM 9180-byte and 802.11 can transport 7935-byte MTUs.
The table below shows the complete Ethernet packet and the frame inside, as transmitted, for the payload size up to the MTU of 1500 octets. [b] Some implementations of Gigabit Ethernet and other higher-speed variants of Ethernet support larger frames, known as jumbo frames.
The TCP should ask the IP for the Maximum Datagram Data Size (MDDS). This is the MTU minus the IP header length (MDDS = MTU − IPHdrLen). When opening a connection, TCP can send an MSS option with the value equal to: MDDS − TCPHdrLen. In other words, the MSS value to send is: MSS = MTU − TCPHdrLen − IPHdrLen
This added overhead can mean that a reduced maximum length limit (so-called ‘MTU’ or ‘MRU’) of 1500 − 8 = 1492 bytes is imposed on (for example) IP packets sent or received, as opposed to the usual 1500-byte Ethernet frame payload length limit which applies to standard Ethernet networks.
In contrast to IPv4, routers do not fragment IPv6 packets larger than the maximum transmission unit (MTU), it is the sole responsibility of the originating node. A minimum MTU of 1,280 octets is mandated by IPv6, but hosts are "strongly recommended" to use Path MTU Discovery to take advantage of MTUs greater than the minimum. [1]
An example of the fragmentation of a protocol data unit in a given layer into smaller fragments. IP fragmentation is an Internet Protocol (IP) process that breaks packets into smaller pieces (fragments), so that the resulting pieces can pass through a link with a smaller maximum transmission unit (MTU) than the original packet size.