Slow Downloads with Sonicwall NSA 3500

So I was scratching my head today, I needed to download a 2GB file from MSDN via one of our Sonicwall firewalls (NSA 3500 running SonicOS Enhanced 5.6.5.1-33o) and I found that the initial download speed was fine but after a minute or two the speed dropped to almost nothing, however testing the Internet feed using http:\\speedtest.net and downloading the files from different locations all appeared fine with the source. I turned off all the security features one by one to see if that was the cause, no that was not it. After some digging I turned on “Enable enforcement of a limit on maximum allowed advertised tcp window with any DPI-based service enabled”.

You can enable this via a hidden menu by following the below steps.

1. Logon to your management webpage.
2. Replace main.html in the url to diag.html.
3. Select “Internal Settings”.
4. Scroll down to “Security Service Settings”.
5. Select “Enable enforcement of a limit on maximum allowed advertised TCP window with any DPI-based service enabled”.
6. Change “Set a limit on maximum allowed advertised TCP window with any DPI-based service enabled (KBytes)” from 64 to 256.
7. Scroll to the top of the page and select “Apply”.

 

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