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I'm having a strange problem on a connection behind a Cisco modem connecting to a DSL network : outbound connections hang after transferring exactly 1024 bytes. Inbound connections are just fine. This is 100% reproducible on all of the router ports and on all computers on the network.

Not being a network kind of guy, I can deduce the modem/router is at fault, but which setting could cause this behavior ?

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What's a packet capture look like? You can fit over 1KB into a single packet, so it's very strange that it'd stop on that border every time; what tools did you use to confirm? – Shane Madden Sep 7 '11 at 17:14
@François: what cisco modem? firmware? – petrus Sep 7 '11 at 17:49
@Shane : I used netcat to confirm, I'll get you a capture next time I go on site. – François Feugeas Sep 7 '11 at 20:18
@petrus, I'll need to go onsite to confirm f/w version as well, I'll edit my question with the additional information. – François Feugeas Sep 7 '11 at 20:19
up vote 1 down vote accepted

Try lowering the MTU to 1492 or even a bit lower. You could keep lowering it by 50 or 100 (until you get to 1000), to see if it will start working more reliably at some point. If you go too much lower than 1000, than this is not the issue.

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If this solves the problem, then the issue is likely that some administrator has blocked all ICMP despite that fact that ICMP is an Internet requirement. This administrator needs to be hit with a cluebat. – David Schwartz Sep 7 '11 at 17:48
This was a MTU issue. It was set to 1492, we tried to lower it by increments as suggested, with no luck. Out of despair we upped it to 1500, it solved the issue. – François Feugeas Oct 24 '11 at 13:18

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