job screening question

William Herrin bill at
Thu Jul 5 22:18:23 UTC 2012

On Thu, Jul 5, 2012 at 5:05 PM, Derek Andrew <Derek.Andrew at> wrote:
>> > You implement a firewall on which you block all ICMP packets. What
>> > part of the TCP protocol (not IP in general, TCP specifically)
>> > malfunctions as a result?
> Isn't MTU discovery on IP and not TCP?

If you want to overthink the question, the failure in the TCP protocol
is that it doesn't adjust the MSS to match the path MTU. It continues
to rely on the incorrect path MTU estimate, sending too-large packets
which will never arrive. This happens because TCP doesn't receive a
notification that the path MTU estimate has changed from the default
because the lower layer PMTUD algorithm never receives the expected
ICMP packet.

This is, incidentally, is a detail I'd love for one of the candidates
to offer in response to that question. Bonus points if you discuss MSS
clamping and RFC 4821.

The less precise answer, path MTU discovery breaks, is just fine.

Bill Herrin

William D. Herrin ................ herrin at  bill at
3005 Crane Dr. ...................... Web: <>
Falls Church, VA 22042-3004

More information about the NANOG mailing list