Internet Core Protocols: The Definitive Guide

Notes on Path MTU Discovery

Just as traceroute allows a system to discover the path taken between two devices, an algorithm called Path MTU Discovery allows the sender to find the most efficient packet size for the connection between two systems. This is particularly important when the two systems are going to exchange a significant amount of data, since sending large datagrams may cause fragmentation (which introduces delay and also increases the risk of errors), while sending small packets generates an excessive amount of traffic. Finding the largest possible MTU on an end-to-end connection allows the systems to send the most amount of data in the least number of packets, thereby keeping fragmentation from occurring.

Path MTU Discovery is described in detail by RFC 1191. Essentially, Path MTU Discovery works by having the sender decrement the size of the IP datagrams until they can be sent to the recipient without being fragmented.

The optimal MTU size is determined by the sender starting with the largest possible size (as determined by their local MTU), and then setting the Don’t Fragment flag in the datagram’s IP header. If an ICMP Destination Unreachable: Fragmentation Required but DF Bit Is Set error message comes back to the sender, then the datagram is too large to be sent across the network. The sender would then reduce the size of the datagram and try the operation again. This process is repeated until no errors are returned. At this point, the sender can use the last-known ...