Included with SP2 is the Scalable Networking Pack which was a redeisgn of some major networking components to offload some of the processing to onboard components of certain network cards. Unfortunately, this seems to still be in its infancy and we started noticing very strange problems right away. A colleauge found this post on the MS Exchange team’s blog which ultimately led us to the answer: http://msexchangeteam.com/archive/2007/07/18/446400.aspx.

The symptoms we noted on our side were specifically that OS X users behind certain NAT firewalls would timeout when trying to retrieve pages from our web servers. After weeks of troublehsooting, we finally narrowed it down to a problem where pages less than 1440 bytes worked fine, but larger pages simply timed out. We also noticed that some of our technicians had problems connecting via RDP to various servers. The initial connection would drop, but sub-sequent connections worked find. Most of our Dell systems have onboard Broadcom NIC’s. Supposedly, this new Networking Pack which is enabled by default works only with the latest drivers. However, we found that even using the latest drivers provided directly from Broadcom, we were still having problems.

Luckily, the fix does not require a reboot and is easily implemented. Simply disabling the tcp chimney offload solved all of our issues:

netsh int ip set chimney disabled

You can view the state of each of the connections to the server and their offload state by running the netstat -t command.