Receiver limits TCP window size to 64,512

I've seen this as a driver-specific issue; in my case with QLogic network controllers that were attempting to use TCPChimney. This link describes the TCPChimney functionality added in Windows 2008 - but I'm pretty sure it still applies: https://support.microsoft.com/en-us/kb/951037

I would recommend testing the following, in order; after each test, reboot and see whether the receiver starts increasing the TCP RWIN as expected.

1) Load the latest versions of the drivers for the network adapter on the receiving computer. 1) Disable TCPChimney on the receiving computer 2) Disable all 'TCP Receive' offloading. This would be found in Advanced settings of the Network Adapter Properties (the same area where Speed & Duplex would be set) 3) Disable all 'TCP Send' offloading (also in the Network Adapter's Advanced properties)

( And contrary to the comment "And big TCP window sizes over 65k are bad for servers, as then the memory demand for connections increases. 65k alone might also not make you happy enough. – user303507 Aug 6 '15 at 11:30", large TCP Receive Windows are NOT inherently bad for the server. In the case of high-bandwidth, high-latency links (like Satellite relays), large RWIN values are necessary so that we have more TCP data "in the pipe". Imagine a 600 Mbps connection with 3000 ms latency; the high-bandwidth link would be limited to about 20 KBps ;as only 65 KB of un-acked TCP data could be "in the pipe" at a time. )