Super Conduit

Due to the high latency of the lines between my works offices, file transfers can be slow. There are settings in Windows Vista+ systems that can allow the TCP window to grow, and allow much higher utilization on these lines. I call it Super Conduit. This may be possible on *nix systems, but the way this tweak works is that it tells the other side it will be doing this tweak. That means that both sides have to be at least Windows Vista Kernel, (Server 2008 works) that also means that linux file servers will not work because them seem to be linux machines with SMB. This should be done over wired connections, because the packet loss on wireless hurts these connections more than anything else.

With the “autotuninglevel” change, I have seen speed changes from a 1megabit a second line go to 150-200 megabits a second.

WARNING: Windows Vista/7 IP stack can not handle changing this setting and using normal connections, meaning once this is done usually the internet stops working until the setting is reversed. Windows 8+ seems to have no problems with this setting, and the internet; it just makes Win 8/8.1 more awesome than it already is, which is pretty awesome.

  1. Login under a administrator account to the Windows machine
  2. Open ‘cmd’ as a administrator
    1. Title bar should be “Administrator: C:\Windows\System32\cmd.exe”
  3. “netsh interface tcp show global” will show the current settings of your machine
    1. Command Line Status
  4. “netsh interface tcp set global autotuninglevel=experimental” enables the majority of what you need for faster transfers, all you will get back in response is “Ok.”
    1. Image2
  5. Another setting I have used in the past is “netsh interface tcp set global ecncapability=enabled” this adds a flag to the packs that tells routers “I dont care if I get slowed down, please dont drop me completely”. The problem you run into with large TCP windowing is one dropped lowers the TCP window size a lot and slows the connection making it a lot more spiky. This command doesnt always help, but setting it hasnt hurt in the past.
    1. Image3
  6. The “rss” receive-side scaling state should be set to enabled, that should be the default. This allows the receiver to do these types of conenctions.
  7. When you are done your transfer just run “netsh interface tcp set global autotuninglevel=normal”

 

Troubleshooting Notes:

Windows 7 seems to act oddly when starting to use this setting, so I would enable it then restart the machine. I believe that cached sessions already in progress do not take the new setting.

 

YAY MATH:

http://bradhedlund.com/2008/12/19/how-to-calculate-tcp-throughput-for-long-distance-links/

Default window size: 65536 bytes * 8 = 524288 bits

73ms latency between cross country offices, 524288 bits / 0.073 seconds = 7,182,027 Bits throughput, theoretically. 897,753 B/s, max.

This setting increases that window size to something larger, much larger, and thus gives better speeds. The only interesting downside is that since the TCP window is big, if a packet is then lost, TCP resizes the window to a much smaller setting; forcing the window to climb again.

That is a 1GB link going across the country.

That is a 1GB link going across the country.

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