PMTUD Black Hole Problem
koroush.saraf at lmco.com
Thu Jun 19 09:37:14 PDT 2003
I have the following network in the lap
WINXP <---mtu1500--->FREEBSD4.4<---mtu1280(gif tunnel--->FREEBSD4.4<---mtu1500--->WINXP
The BSD computers are setup as gateway routers. As you might see from diagram above, the MTU of the link between the two BSD computers is 1280Bytes because of a tunnel. When I try to transfer a file between the XP endpoints, the PMTUD goes off by sending a 1514B packet to the other end with the Don't fragment bit set. However this packet never generates the ICMP unreachable message back to the XP box during Path MTU discovery. So that's why I have concluded that the BSD router is the black hole.
Now I would like to know how to make my BSD router to participate in the MTU discovery. Changing the MTU of the windows computers is not an option for me. Also please note that the BSD computers only have one NIC interface, and the network is logically sperate but physically all connected to a switch, so playing with the MTU of the interface connecting the BSD computers is not an option either.
I hope that this is a known problem and easy to fix and thanks for taking the time to respond.
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