The Ethernet adapter then does the TCP segmentation offload to segment the data into the MTU sized packets, calculates TCP and IP checksum. When large_send is enabled, TCP can send up to 64K bytes of data with one call down the stack, which reduces host processing and reduces CPU usage.

To understand the reason for this behavior, lets understand large_send, large_receive and which layer iptrace is capturing the packet. In this trace, packet loss starts at the first hop in the network (10.27.136.1), so the link on the window's machine's ethernet card / router is a possible cause congestion at the first hop is another possible cause.When iptrace and tcpdump collected on aix/vios host is read using wireshark, it marks TCP checksum and IP checksum fields as incorrect even though communication is working fine. When you see a hop where packet loss starts, and continues on all hops afterwards, that is the place to start looking for issues. Your goal is to baseline consistent packet loss for at least 10 or 15 minutes sometimes I will let it run for hours to catch intermittent packet loss at a hop. Run this against destination(s) where you experience throughput problems. Please download winmtr and install it on your Windows 2008 server this is a free utility that will track packet loss per hop.

Many TCP performance problems come down to very simple issues with consistent packet loss even loss rates that appear small (0.5%) can cause significant problems for TCP. Interesting thing is in one of our IP's in our server we are not receiving this error, and the web sites using that IP is rapid enough. when we disable 'Checksum offload' from our network adapter, then we began to receive lots of 'suspected transmission' and server is still slow. We cehcked the frames in detail and saw that majority of the errors are 'IP checksum offload errors'. The client demands the same packets several times, please check below. We began testing with wireshark and saw that our packets are lost during transmission. Our web sites (iis 7.5) are loading very slowly and sometimes its freezing.

Our 2008 server began to work very slowly within 2 weeks.

Wireshark bad checksum