IP tunnel MTU

Mon Oct 29 23:02:33 UTC 2012

Hi Bill,

> Maybe something as simple as clearing the don't fragment flag and
> adding a TCP option to report receipt of a fragmented packet along
> with the fragment sizes back to the sender so he can adjust his mss to
> avoid fragmentation.

That is in fact what SEAL is doing, but there is no guarantee
that the size of the largest fragment is going to be an accurate
reflection of the true path MTU. RFC1812 made sure of that when
it more or less gave IPv4 routers permission to fragment packets
pretty much any way they want.

Thanks - Fred
fred.l.templin at boeing.com