Ethernet Slower with Leopard?

While we do not have fully investigated the issue, we want to publish 2 independent reports about this problem as it seems to be not isolated.

After several for installing NetInstall on Mac OS X Server 10.5.1 and as well as controls on several Mac clients running 10.5.1, I noticed that transfer speed of large files were 3 to 4 times slower with 10.5.1 than with 10.4.
AS an example, backups performed with Superduper take 28 minutes with 10.5 while it was only requiring 10 minutes when running 10.4 for the same amount of data. If Leopard is a nice OS, there is a problem with ethernet transfer speed...

A report from Philippe provides some hints.

I also noticed that ethernet was faster with Tiger than with Leopard, but it is not only a transfer speed issue, but also loading issue. I have servers with Zope and MySQL. When using Ethernet from my Mac running Tiger, I did not encounter any issue, but after migrating to Leopard, it does not support simultaneous connexions for large volumes. I then simply lose the network, the connexion is still active, but there is not data flow anymore.
I quickly identified the problem: loading of ethernet requests saturates SynFlood type web attack, and the TCP/IP pile of my Mac simply drops. To fix it, you simply need to inactivate/activate the connexion, as it resets the pile, however, it is not a viable solution as avery new request will fill up the pile.
This issue was not in Tiger as the system was much more robust. I do not know what Apple modified in Leopard, but it affects the ethernet behavior, functionality and speed.

We will keep posting information about this issue as soon as we obtain additional information. Users requiring fast ethernet connections should be the first one to be affected by this issue.