On 4/26/2017 1:30 PM, Brian E Carpenter wrote:
Individual packets and fragments can be smaller than the MTU, of course.
Nothing forces fragments to push up against any MTU limit at all. But I
would not describe that has a host changing its path MTU; it's just
sending packets.
I disagree, both on the definition and the action. You are correct in "how
the Path MTU is calculated". But the Path MTU, by definition, is the largest
packet that can be sent end to end under current routing conditions. It is
not, actually, an IP concept: it's a TCP concept if anything, or a transport
layer concept (if UDP ever decides to have one). I can imagine TCP probing
the Path MTU by trying packets that are larger than its current estimate to
see if the estimate is still accurate (1981 section 4), but I can't imagine
any reason that TCP would send packets larger than the "largest packet that
can be sent end to end under current routing conditions" in the normal case,
as those packets will by definition either be fragmented or not arrive.
istm that the robustness principle argues for what Fred is saying. Yes, any
path that doesn't transmit 1280 byte packets is breaking the law, but (given
that the protocol police do such a lousy job) if the objective fact is that
the path only transmits 1279 byte packets, what's best for the user?
IMO, the best thing is to loudly flag the path as no longer valid.
Either we have required minimums or not. If not, then we have a LOT of
new work to do.
Joe