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I am experiencing a problem on the intranet of my institution. I am sitting in front of A and trying to connect to server B in another city.

My ssh sessions hang up quite often, especially when I have a lot of screen output or page up/down in $PAGER or change window in GNU screen. In ssh session, X11 forwarding is too slow to be used.

My vrdp sessions (with VBoxHeadless) also hang up a lot , usually when I drag a window around inside the remote guest OS.

I tried to use tcpdump to figure out why, and noticed that before the hang up, many packets with PSH flag from A is sent and after a while, many ACK packets from B all at once.

ping from A to B give a delay of 40ms, and traceroute gives 4 hops. wget gives the throughput to be on average ~50Mbps, but the speed is very unstable. All three routers along the way are with Cisco IOS 12.X (identified with nmap).

What seems to be the problem? How can I investigate more on such an issue? Any hints or ideas are appreciated.

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Try to enable keep-alives in SSH and see if the problem goes away. A firewall in the middle might have a small session duration value. – gtirloni Mar 8 '11 at 23:14
Sounds like a typical pix/asa feature. Do you have one of those en route? – 3molo Jun 4 '12 at 15:54

Could be an mtu (Maximum transfer unit) problem. If a lot of traffic is happening all of a sudden the connection hangs because one packet was to large. If path mtu discovery doesn't work, this can happen. Especially with DSL connections since the overhead of pppoe reduces the mtu. Usually one uses MSS Clamping on DSL links to mitigate the problem.

You can test the mtu with ping: use ping -s host to send a ping to the remote host. Start with size 100 and go up in hundreds to 1500 (the normal ethernet mtu)


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it is extremely unlikely that he has an MTU problem through the intranet of his company. – Mike Pennington Apr 30 '11 at 15:21

I agree, we have had to adjust the MTU size on numerous occasions to rectify the same problems. 800 is a good size for DSL connections.


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