> I investigated this issue, and found that it only happens if the
> packet in question matched a traffic shaper rule. Explanation:

Thank you much for the comprehensive answer.
Did not know about the auto MSS on the fly patching in case of PPPoE
links, that's good.

goggling around, it resulted to be already pointed out:

http://unix.derkeiler.com/Mailing-Lists/FreeBSD/net/2004-02/0233.html



regards