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To: Jim Shankland <jas@flyingfox.com>
cc: bugs@freebsd.org, freebsd-isp@freebsd.org, bvt@mp.aha.ru,
        freebsd-questions@freebsd.org, robert@nanguo.chalmers.com.au
Subject: Re: I give up! no ideas left. 
In-reply-to: Your message of "Sat, 08 Feb 97 22:52:01 PST."
             <199702090652.WAA07862@saguaro.flyingfox.com> 
Date: Sun, 9 Feb 1997 12:57:05 PST
From: Bill Fenner <fenner@parc.xerox.com>
Message-Id: <97Feb9.125709pst.177476@crevenia.parc.xerox.com>
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In message <199702090652.WAA07862@saguaro.flyingfox.com> you write:
>I receive the *second* data packet from you (covering bytes
>1440:2049, or something like that), but I never get the first
>(bytes 1:1440).  Of course, my end immediately does an ACK 1
>to signal that it got an out-of-sequence packet; but to no
>avail.  That packet simply never arrives.

So it looks like there's a router in the middle that drops big packets
but doesn't return ICMP packet-too-big errors.  This router is in
violation of RFC1812 (but that never stops anyone).

This is a problem with Path MTU Discovery as specified; it doesn't
allow for a hop that simply discards packets with no notification.

You can probably find this hop by using traceroute; "traceroute <host>
1500" will just start timing out at the hop that is not returning ICMP
errors; then "traceroute <host>" and see what router that hop is.
Contact the owner of the router and get them to configure it (or
upgrade it) so that it replies properly when dropping a packet with DF
set.

  Bill