TCP packet retransmission & packet loss

[imbrue]

I have a simple home network (NAT router through cable modem to my ISP) and I'm having packet retransmission and packet loss to certain servers and don't know where to go to identify the cause of the problem. The problems have occurred when using POP and also with FTP, but only to specific servers. If I connect through a VPN to a corporate network and then execute the same actions, I have no problems. This leads me to believe that it is my ISP that is the cause of the problem, but the only tech support I've been able to get from them goes something like: "if it works sometimes, the problem must be in your software" - so they are no help.

Any suggestions on how to identify the cause of this? I've used Ethereal to capture sessions. Is there anyway to determine if there is something configured on my side of the router that is causing these problems?

Thanks.

 

[mustaine737]

Are you able to reproduce the issue without using the router? By directly plugging your PC into the cable modem?

 

[imbrue]

Yes, it happens with or without the router. The interesting thing I've uncovered is that (at least with FTP) if I use a different user name to this server, I get through just fine. When I use a certain user name and issue a USER command via the FTP connection, I never get a response packet and my TCP stack goes into a retransmission mode. The same user name when issued over a VPN connection has no problems. A completely different user name issued in the FTP session works fine from either my straight connection or my VPN connection.

Weird if you ask me. All I can speculate is that some router at my ISP is dropping packets under certain conditions. But this is just a wild guess on my part. I'm pretty confused by it all.

Right now, I've found a work around... until the next strange instance of "What the heck is going on now?" occurs.