Follow Troubleshooting security software. Often, uninstalling third-party security software will resolve these errors.
There may be third-party software that modifies your default packet size in Windows by inserting a TcpWindowSize entry into your registry. Your default packet size being set incorrectly can cause these errors. Contact the manufacturer of the software that installed the packet size modification for assistance or follow this article by Microsoft: How to reset Internet Protocol (TCP/IP).
Verify that access to ports 80 and 443 are allowed on your network.
Verify that communication to albert.apple.com or phobos.apple.com is not blocked by a firewall, or other Internet security setting.
Discard the .ipsw file, open iTunes and attempt to download the update again. See the steps under Advanced Steps > Rename, move, or delete the iOS software file (.ipsw) below for file locations.
Restore your device while connected to a different network.
Restore using a different computer.
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SoloTurk's post in There was a problem downloading the software for the iPhone. Unknown error 9006 was marked as the answer