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Path MTU discovery (PMTUD) is a technique in computing for determining the maximum transmission unit (MTU) size on the network path between two IP hosts with a view to avoiding IP fragmentation. Computing is usually defined like the activity of using and developing Computer technology Computer hardware and software. In Computer networking, the term Maximum Transmission Unit ( MTU) refers to the size (in Bytes of the largest packet or frame The Internet Protocol allows IP fragmentation so that Datagrams can be fragmented into pieces small enough to pass over a link with a smaller MTU than the

Path MTU discovery works by setting the DF (Don't Fragment) option bit in the IP headers of outgoing packets. Then, any device along the path whose MTU is smaller than the packet will drop it, and send back an ICMP "Fragmentation Needed" message containing its MTU, allowing the source host to reduce its assumed path MTU appropriately. The Internet Control Message Protocol ( ICMP) is one of the core protocols of the Internet Protocol Suite. The process repeats until the MTU is small enough to traverse the entire path without fragmentation.

If the path MTU changes after the connection is set up and is lower than the previously determined path MTU, the first large packet will cause an ICMP error and the new, lower path MTU will be found. Conversely, if PMTUD finds that the path allows a larger MTU than what is possible on the lower link, the OS will periodically reprobe to see if the path has changed and now allows larger packets. On Linux this timer is set by default to ten minutes. Linux (commonly pronounced ˈlɪnəks

Problems with PMTUD

Many "security" devices incorrectly block all ICMP messages, including the errors that are necessary for PMTUD to work. This can result in connections that complete the TCP three-way handshake correctly, but then hang when data is transferred. This state is referred to as a "black hole connection".

Some implementations of PMTUD now try to work around this by inferring that large payload packets have been dropped due to MTU rather than because of link congestion. However, in order for TCP to operate most efficiently, ICMP unreachables (type 3) should be permitted. A robust method for PMTUD that relies on TCP or some other Packetization Layer to probe the path with progressively larger packets has been standardized in RFC 4821 (Packetization Layer Path MTU Discovery, PLPMTUD).

Another workaround used by some routers is to change the maximum segment size of all connections passing through links with MTU lower than the Ethernet default of 1500. The maximum segment size ( MSS) is the largest amount of data specified in Bytes that a computer or communications device can handle in a single unfragmented Ethernet is a family of frame -based Computer networking technologies for Local area networks (LANs This is known as MSS clamping.

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