>> A nice reponse. But what's the impact on PMTU discovery,
>> specifically in the case that path-PMTU increases?  Isn't the
>> required PMTU-probe behaviour in that case exactly the scenario
>> (remote peer sends "DF" segment with a lenght larger than the
>> current mtu) which you propse to ignore?

> No, you never get such messages from remote routers; they have no
> idea what size you could have sent, so they can't tell you to send
> something larger.  You're supposed to try larger MTUs after some
> interval.  See Sections 6.3 and 7.1 of RFC 1191.

No, you've missed the point.

Say we've settled on a path MTU of 1100 octets.  Then we send a probe
of (say) 1400 octets.  But the actual path MTU is now 1250; we get a
need-frag ICMP giving 1250.  But 1250 > 1100, so the algorithm would
have us ignore the ICMP.

The obvious fix, of course, is to add an exception that says "...unless
we've recently sent a PMTU-D MTU-growth probe no smaller than the size
in the packet".  Or, equivalently, when we send the probe, we raise our
local idea of the path MTU at the same time, optimistically, and let it
get cut back immediately if needed.

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