To eliminate fragmentation or minimize it, it is desirable to know what is the path MTU along the path from the source to destination. The path MTU is the minimum of the MTUs of each hop in the path. [INTERNET:14] describes a technique for dynamically discovering the maximum transmission unit (MTU) of an arbitrary internet path. For a path that passes through a router that does not support [INTERNET:14], this technique might not discover the correct Path MTU, but it will always choose a Path MTU as accurate as, and in many cases more accurate than, the Path MTU that would be chosen by older techniques or the current practice.
When a router is originating an IP datagram, it SHOULD use the scheme described in [INTERNET:14] to limit the datagram's size. If the router's route to the datagram's destination was learned from a routing protocol that provides Path MTU information, the scheme described in [INTERNET:14] is still used, but the Path MTU information from the routing protocol SHOULD be used as the initial guess as to the Path MTU and also as an upper bound on the Path MTU.