Editing Maximum transmission unit (section)

===Path MTU Discovery===
{{Main|Path MTU Discovery}}
The Internet Protocol defines the ''path MTU'' of an Internet transmission path as the smallest MTU supported by any of the [[Hop (telecommunications)|hop]]s on the path between a source and destination. Put another way, the path MTU is the largest packet size that can traverse this path without suffering fragmentation.

''Path MTU Discovery'' is a technique for determining the path MTU between two IP hosts, defined for both [[IPv4]]{{Ref RFC|1191}} and [[IPv6]]{{Ref RFC|8201}}. It works by sending packets with the DF (don't fragment) option in the IP header set. Any device along the path whose MTU is smaller than the packet will drop such packets and send back an [[ICMP Destination Unreachable|ICMP Destination Unreachable (Datagram Too Big)]] message which indicates its MTU. This information allows the source host to reduce its assumed path MTU appropriately. The process repeats until the MTU becomes small enough to traverse the entire path without fragmentation.

Standard Ethernet supports an MTU of 1500 bytes and Ethernet implementation supporting jumbo frames, allow for an MTU up to 9000 bytes. However, border protocols like [[PPPoE]] will reduce this. Path MTU Discovery exposes the difference between the MTU seen by Ethernet end-nodes and the Path MTU.

Unfortunately, increasing numbers of networks [[Black hole (networking)|drop ICMP traffic]] (for example, to prevent [[denial-of-service attack]]s), which prevents path MTU discovery from working. ''Packetization Layer Path MTU Discovery''{{Ref RFC|4821}}{{Ref RFC|8899}} is a Path MTU Discovery technique which responds more robustly to ICMP filtering. In an IP network, the path from the source address to the destination address may change in response to various events ([[load balancing (computing)|load-balancing]], [[Network congestion|congestion]], [[Downtime|outages]], etc.) and this could result in the path MTU changing (sometimes repeatedly) during a transmission, which may introduce further packet drops before the host finds a new reliable MTU.

A failure of Path MTU Discovery carries the possible result of making some sites behind badly configured [[Firewall (networking)|firewall]]s unreachable. A connection with mismatched MTU may work for low-volume data but fail as soon as a host sends a large block of data. For example, with [[Internet Relay Chat]] a connecting client might see the initial messages up to and including the initial [[Ping (networking utility)|ping]] (sent by the server as an anti-spoofing measure), but get no response after that. This is because the large set of welcome messages sent at that point are packets that exceed the path MTU. One can possibly work around this, depending on which part of the network one controls; for example one can change the MSS ([[maximum segment size]]) in the initial packet that sets up the [[Transmission Control Protocol|TCP]] connection at one's firewall.