Microsoft KB article 929868 describes a problem which will cause you to experience very slow browsing on certain websites when using Internet Explorer 7 on Windows Vista, and eventually the website might even drop the data completely. The cause is that by default, Windows Vista Enterprise fully supports RFC 1323 and supports a Windows Scaling factor of 8. This issue occurs when a Web site does not fully support RFC 1323 or when a Web site does not support the default scaling factor.

THE QUICK FIX

To work around the slow browsing issue change the Windows Scaling to a factor of 2. First open and Administrator command line prompt. Click Start –> All Programs –> Accessories, right click Command Prompt, then left click Run as administrator. As an alternative you can click Start then type into the Start Search box, or the Run box, cmd, then use the shortcut keys Ctrl-Shift-Enter. (NOTE: After you set a new autotuninglevel you will need to reboot in order for the changes to take effect.)

Type, or copy and paste, into the Administrator command prompt window:

netsh interface tcp set global autotuninglevel=highlyrestricted

This will set the autotuninglevel to "Allow the receive window to grow beyond its default value, but do so very conservatively". In this mode, Vista will by default use RWIN of 16,384 bytes with a scale factor of 2.

MORE AUTOTUNING OPTIONS

To restore the autotuninglevel back to the default setting type, or copy and paste into the Administrator command prompt window:

netsh interface tcp set global autotuninglevel=normal

There have been reports that disabling the autotuning feature will speed up threaded network downloads, mostly used with downloading agents and per-to-peer networking, along with great improvement to file copying on your network. Vista has issues where it will hang to a file copying task, but Service Pack 1, which is coming out in less than a month, is expected to fix this issue. There have been other reports that say disabling the autotuning of TCP can also fix other Vista specific issues related to slow file transfer between Outlook 2007, and Exchange server connections.

You can disable autotuning with the following command:

netsh int tcp set global autotuninglevel=disable

When autotuning is disabled, the RWIN receive window defaults to a value of 65536 bytes.

There are two more settings. First restricted:

netsh interface tcp set global autotuninglevel=restricted

Restricted allows for the receive window to grow beyond the default value, but limits such growth in some scenarios.

Second is experimental:

netsh interface tcp set global autotuninglevel=experimental

Allows for the receive window to grow to accommodate extreme scenarios. Note The experimental value can decrease performance in common scenarios. This value should be used only for research purposes, and is not suitable for most common uses like general web browsing.

NOTE: Vista may have autotuning disabled by default, unless you use Diagnose and repair for a network connection, which will cause Vista to enable autotuning using the "normal" autotuninglevel.

To read a more technical description about tuning the TCP receive window size (RWIN), click here.

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