Seeking Comcast Contact: need to troubleshoot packet loss and/or asymmetric routing issue between Comcast & Onvoy

Jim Shankland nanog at
Fri Aug 3 01:33:16 UTC 2007

Adrian Chadd wrote:
> On Thu, Aug 02, 2007, Jim Shankland wrote:
>> Linux has a nifty iptables option (clamp-mss-to-pmtu) to rewrite the
>> MSS in TCP SYN packets when forwarding a packet onto a link with
>> a lower MTU than the MSS in the packet.  Works like a charm.  If every
>> packet forwarding device on the Internet did this, PMTUD would not be
>> needed.  As is, PMTUD is simply broken, due to widespread firewall
>> misconfiguration.  As in so many other cases of Internet misbehavior,
>> you can avoid being part of the problem, but you can't be the solution.
> .. non-TCP traffic?

Hmm; I've never actually heard of anybody doing PMTUD on non-TCP
traffic, though it's possible.  Does anybody actually do it?

Jim Shankland

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