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I have two Windows 10 machines (bld 2004) I am trying to send data to between two sites. The sites average about 50ms RTT with no packet loss. I am having an issue where the max transfer rate I can achieve is approximately 30Mbps (either direction). Site A is 100Mbps and B is 500Mbps. I have tested using different protocols such as SMB, FTP. IPerf tests confirm the same avg speed. These tests have been performed with the hosts directly attached to the ISP router.

I was told from Network Engineering, that my Window size is too small. Is there any way I can change it to help improve my transfer rate? I was thinking adjusting the TCP window scale size?

https://networkengineering.stackexchange.com/questions/73491/need-help-isolating-bandwidth-issues-between-sites

Ryan F
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  • Which TCP window? The congestion window is calculated and used by the sender, and the receive window is based on the receiver resources and sent to the sender with every ACK as dynamically changing number, based on the currently available resources. – Ron Maupin Apr 21 '21 at 16:10
  • "_These tests have been performed with the hosts directly attached to the ISP router._" That is very dangerous, akin to measuring how long it takes you to walk across a busy street by just stepping into the street and walking directly across, ignoring oncoming traffic. – Ron Maupin Apr 21 '21 at 16:14
  • @ronMaupin Yea, I made sure to check both ways to make sure I didn't hit any oncoming traffic. The difficult part was breaking in the colocation and finding the rack the router was in. The ISP equipment handoff is in the same rack our our endpoints. The point I was trying to make is our firewall or internal equipment was bypassed. – Ryan F Apr 21 '21 at 16:20
  • You are ignoring the traffic because you are bypassing your network firewall. The bad guys are always out there, pounding on your firewall, using all the tricks to try to break into a host on your network, and you dropped all your protection. – Ron Maupin Apr 21 '21 at 16:25

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Microsoft has some detailed information on this:

To set the receive window size to a specific value, add the TcpWindowSize value to the registry subkey specific to your version of Windows. To do so, follow these steps:

Select Start > Run, type Regedit, and then select OK.

Expand the registry subkey specific to your version of Windows:

For Windows 2000, expand the following subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces

For Windows Server 2003, expand the following subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

On the Edit menu, point to New, and then select DWORD Value.

Type TcpWindowSize in the New Value box, and then press Enter

Select Modify on the Edit menu.

Type the desired window size in the Value data box.

Ron Trunk
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  • Isn't the max tcp window size limited to 65535? If I set that in my registry, reboot and retry a copy, I don't see any improvement. If I view the initial TCP connection ACK in Wireshark, the calculated window size shows 262144 bytes. – Ryan F Apr 21 '21 at 18:56
  • Read the document for details about how to scale – Ron Trunk Apr 21 '21 at 19:57