Re: PR 44508: UNREACH_NEEDFRAG ICMPs use the wrong mtu

[Dennis Ferguson <dennis.c.ferguson%gmail.com@localhost>]

On 15 Jun, 2012, at 02:02 , Mouse wrote:
>> It seems to me that it would clearly be a bug if the box reported a
>> different MTU in ICMP packets sent in response to packets with the DF
>> flag set than it used to fragment packets sent with the DF flag
>> clear.
> 
> I'm not certain it does that.  However, I _am_ sure that the behaviour
> I saw, the behaviour I've been calling a bug, leads PMTU-D-capable
> senders to repeatedly try packet sizes that do not actually get
> through.  In terms of the test setup sketched in the PR, host B tries
> generating packets based on an MTU 1500, but gets told the packets are
> too large and it should use-- 1500, the very MTU it did use.  In the
> case that led me to file the PR, host B believes this and tries MTU
> 1500 again, failing again, repeatedly, until it gets fed up and kills
> the connection with ETIMEDOUT (the cases I was working with were TCP
> connections).

Yes, that's an inconsistency, and a bug.  If it is dropping DF-set packets
based on the route MTU then the ICMP unreachable should be reporting the
route MTU.

Dennis Ferguson