Packet loss?

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Guest

I've got a strange problem with a Dell GX1. For a long time I've noticed
that transfering files between this and another machine took longer than
normal. Eventually I used Ethereal to do a sniff and found that there are
many "TCP Dup Ack" errors on this PC. So I tried installing a different
network card; no change. I booted using a DOS network boot disk and copied a
file to the other computer while doing a sniff. The other computer indicated
several retransmits. I hooked up a laptop to this same cable and got no
errors.

So I've eliminated the network hardware, the NIC, and the OS. What the heck
is the problem? Is the computer simply not keeping up with the network card?
Seems hard to imagine considering the card is built into the motherboard!
 
Need to know what sort of hub/switch you are plugged into, what the host
thinks the speed/duplex is and what the hub/switch thinks the speed/duplex
is. Often different OS's negotiate differently speed and duplex with the
same NIC so you have to be careful about that. If it is a managed switch
(cisco/foundry/procurve/extreme/etc) try setting the speed and duplex and
doing the same on the host in the OS. If that fixes the problem then it
mostly likely is a auto negotiate issue.
Post back what you have and we can go from there.
Regards,
Ed Horley
Microsoft MVP Server-Networking
 
Hi Ed,

The switch is a Linksys BEFSR41 running at 100Mbps Full Duplex. I've
already tried all the different settings on the network card (10Mbps and
100Mbps both full and half duplex) with no improvement. I believe I was
having this same problem a few months ago with a different switch as well.
Right now I have the card forced to 100Mbps FD. Same goes for a second
computer on the same switch which is not having any problems.
 
You might want to look at this:
http://www.ethereal.com/lists/ethereal-dev/200412/msg00024.html

It seems that "TCP Dup Ack" errors can be attributed to the OS changing tcp
window sizes. You might want to confirm that you are seeing the same
results as what is listed. If not, we can do some additional testing to
narrow things down.

You might want to tune your OS (XP pro/W2K3/W2K) so it performs better for
window size. There are a ton of sites that tell you have to do this, just
Google around. There are a lot of other parameters you can set also.

Some light reading on 2003 to get you started:
http://www.microsoft.com/technet/prodtechnol/windowsserver2003/technologies/networking/tcpip03.mspx

Regards,
Ed Horley
Microsoft MVP Server-Networking
 
I can try playing around with Windows registry settings next week. But since
I booted off a network boot disk, effectively bypassing Windows altogether,
and still saw retransmits on the network, I'm somewhat skeptical that it's a
Windows problem. Boot disk info is available at http://www.netbootdisk.com/.
 
Ah - I misunderstood that then. If that is the case you are down to
checking cables, trying a different port on the switch (ports do go bad) and
seeing if the interface has issues. Have you tried booting with Knoppix as
a second check of an OS?

Regards,
Ed Horley
Microsoft MVP Server-Networking
 
I booted with a laptop computer (running Win2k) connected to the same
port/cable and did not see the same problem. I have yet to boot into another
OS on the problem device to see if that changes anything. I'll look at
Knoppix.
 
Ok, I booted up Knoppix and did some testing. With Windows 2000 loaded I was
seeing over 1600 DUP ACK packets when running a line speed test. With
Knoppix on that PC, the number dropped to under 400. This is also true of an
XP system on the same LAN.

So it looks like it's Win2k that may have the problem, but I have no idea
why. It's definitely something with the basic TCP/IP interface and not a
Windows proprietary protocol.
 
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