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Friday, February 5th, 2010

More ZyXEL Troubles

Categories: [ IT ]

Part 2 of the new ADSL modem story.

This is how it goes: reboot the modem, connect to its telnet server. The TCP window is 2048 bytes long. Everything works fine. Telnet again: the TCP window is now 1024. Actually, it shrunk to 1024 already on the FIN/ACK packet sent by the router when closing this first telnet session and stays like this for all the subsequent connections. During those connection, each ACK packet it sends is followed by a second ACK where it resets the window size to 1024 bytes. After some time (this is the tricky part, I have no idea how time or idleness affects it), when telneting again, some TCP packets are lost, the client side has to retransmit them. Insist a bit more, and there are more and more packets which don't get acknowledged. Eventually, the telnet server may become non-responsive and the whole device stops responding to pings. Reboot the modem.

Whether using the forwarded port from the WAN to the client machine on the LAN is involved or not in prooking this behaviour is not known, more investigation needs to be done.

I contacted ZyXEL, they asked me to return the device. Let's see if they are able to reproduce the problem.

[ Posted on February 5th, 2010 at 17:31 | 1 comment | ]

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