Common operational misconceptions

Mon Feb 20 06:42:56 UTC 2012

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