SMB networking issue in 10.5.2 - detailed analysis and cry for help

Question

I am a computer technician for a medium-sized company. Our setup in the company is mainly Windows-based, using Active Directory and Windows Server 2003 on our file servers, with a fully Gigabit network. We also have about a dozen Macs, mostly Mac Pros but some PowerMac G5s and a few MacBooks here and there.
In the past few weeks, we've started seeing an issue on some of the computers relating to transferring large files from the Macs to the file server. This generally only applies to files over 1GB in size, but sometimes it can even happen with files of a few hundred megabytes. What is happening is that when the Mac starts up, the first couple of times a user will transfer a file of this size to the server over SMB, it will act normally. Eventually (usually anywhere from the third to the seventh transfer of a file that size) a file transfer will stall out. It doesn't stop completely, but it begins transferring painfully slowly. We're talking maybe 100 or 200KBps, and it never speeds up again. If you cancel out that transfer, any subsequent transfers will do the same thing.
Here are the things we've determined through almost a week of testing this issue:
*It doesn't matter what file or what kind of file we're copying, what server we're copying it to, or what folder on the server it is being copied to
*The issue is relatively random as far as which file or transfer number will trigger it, but it is repeatable in that it inevitably will happen
*It doesn't appear to be a third-party product, hardware or software, that is causing this. It's happening on Mac Pros, G5s, a MacBook, and a MacBook Pro, none of which have very much overlap in terms of third-party things installed
*Using clean and basic installs, we've tested under controlled circumstances found that the problem does not manifest itself in Tiger or even Leopard 10.5.1
*The problem doesn't seem to be affected by the time of day at all, in that when it happens, it consistently does it whether morning, afternoon, evening, or overnight; subsequently, it does not appear to be affected by cross traffic on the network or peak network usage
*Using a quick AppleScript, we made a file transfer happen repeatedly over time; on Tiger and Leopard 10.5.1, the transfers were able to run uninterrupted for as much as 12 hours at a time without the problem manifesting itself, but with Leopard 10.5.2, the problem would occur under the same conditions within 10 to 15 minutes
*We have ruled out any faulty hardware by connecting the test machine directly to our main switch
*All the PCs on our network, in many different configurations, are performing the same types of transfers just fine
*Using Ethereal, we can see that when a transfer starts stalling out, you'll start seeing a more than usual number of duplicate ACKs on the connection
*We're not really sure what this means, but it is worth noting that doing the same types of transfers under the same conditions using curl never manifested the issue
*After the problem has manifested itself, rebooting the machine clears up the issue, but it will come back steadily
I think that's pretty much everything we've figured out, which leads us to believe that this is a problem that started happening in 10.5.2. We've eliminated as many variables as possible, so I feel very comfortable with that conclusion. The issue now, then, is finding a fix. Yes, we could just use 10.5.1, but there are so many good bug fixes in 10.5.2 that we'd like to not miss out on. We have tried several fixes on both our server side and on the Mac side, but nothing has helped.
I doubt that this is something that will get resolved, but I'm posting it here just hoping to see others having the same issue so maybe it will get noticed. Any suggestions, though, are very welcome.
Message was edited by: Josh R. Holloway

Answer 1

There have been some other posts suggesting that Apple might have messed up some default TCP configuration parameters in 10.5.2. It might be instructive to compare the result of:
sysctl -a | grep tcp
on otherwise similar 10.5.1 and 10.5.2 systems.