George Bonser wrote:
It is seemingly working well means there is not much PMTU changes, which means we had better assumes some PMTU (1280B, for example) and use it without PMTUD.
It depends on the OS and the method being used. If you set the option to "2" on Linux, it will do MTU probing constantly and react to MTU changes.
It actually does nothing. Given the following statements in the RFC: An initial eff_pmtu of 1400 bytes might be a good compromise because it would be safe for nearly all tunnels over all common networking gear, and yet close to the optimal MTU for the majority of paths in the Internet today. and Each Packetization Layer MUST determine when probing has converged, that is, when the probe size range is small enough that further probing is no longer worth its cost. When probing has converged, a the hosts are keep assuming PMTU of 1400B and if local MTU is 1500B or less, no discovery is performed because "the probe size range is small enough".
Also, the MTU for a given path only "lives" for 5 minutes anyway (by default) and is "rediscovered" with Linux. (value in /proc/sys/net/ipv4/route/mtu_expires) but other operating systems may behave in other ways.
See above. Rediscovery with initial eff_pmtu of 1400B and search_high of 1500B immediately terminates without any probe packets sent. Masataka Ohta