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I connect to my gigabit LAN with my RTL8111C-based NIC. It's on a Gigabyte GA-P43-DS3. In the last 3 weeks I noticed that my transfer rate dropped from the usual 40-60 MB/s (using a NAS over SMB) to about 2-10 MB/s. I thought maybe the server did some bandwidth limiting, but from any other PC transfer rates were OK.
I changed the cable of the PC and tried different ports of the switch. When I checked the traffic with Wireshark, I noticed that there were TCP errors, like duplicate ACKs. Later, I checked the transfer rate with iperf and it was just as low as when I copied files over SMB, so the problem must be with the network.
When I did a test over UDP it showed that about 10-20% of the packages were dropped. The funny thing is, that if I use a lower numbered port on my switch, the error rate is much higher. (E.g. port#1: 21% vs. port#8: 11%)
I think the transfer rate dropped because packets were lost during the transfer, so TCP slowed down the process. I also noticed that these error rates only apply to receiving packets. The computer can send packets with nearly 0% losses.
I checked the network with different cables and computers and every combination was working OK. The drop rate was about 1 vs. 460,000 packets (nearly 0%).
Is it common to NICs stop receiving packets? Why does it matter which port I use on my switch?
Update: I use a Linksys SD2008 switch. I tried all of its ports with known-good PC and cables. No matter what combination I used, there was no problem, just with the affected PC. I'm pretty sure that its NIC is failing. I just wonder why does it matter which port I use. The difference between the ports can be measured, but only with the affected PC.
Update 2: I use Windows 7 x64, but I checked with Linux (Fedora 12 x64) and got similar results, so it's unlikely that's a driver issue. The SD2008 ports are divided to 2 4-port groups. The issue is present no matter which group I use. However, I still doesn't understand how is it possible that lower numbered ports have higher error rate...
It would help to isolate the problem if you clarify a few things - what happens when you connect the affected PC to the same port as an unaffected one with a cable that you know is good? What happens when you connect a known good PC and cable to the affected port(s)? And what sort of switch are you using? Switches and switch ports can fail and often groups of ports share circuitry so failures may happen that affect some ports but not all. Of course it might be your nic too but if you carry out the above checks it will help to isolate the problem. – Helvick – 2010-01-17T18:30:10.553
@Helvick: I updated my question to include checks you mentioned. – KovBal – 2010-01-17T19:36:50.897