SYMPTOMS

It takes a very long time to download an email message from a POP3 server in Microsoft Office Outlook 2007. For example, when you download a 1-megabyte (MB) message, the download process may take one hour.

Additionally, when you send email messages using the SMTP server of an Internet service provider (ISP), the connection may time out. When this occurs, you may receive the following error message:
 
Task 'Main - Sending and Receiving' reported error (0x800CCC0F) : 'The connection to the server was interrupted. If this problem continues, contact your server administrator or Internet service provider (ISP).'

CAUSE

This problem occurs if a network hardware device, such as a router, does not support TCP Window Scaling. TCP Window Scaling is a new Windows Vista feature.

RESOLUTION

To determine whether this problem is caused by a network hardware device that does not support TCP Window Scaling, follow these steps:

1.

Click Start Start button, click All Programs, and then click Accessories.

2.

Right-click Command Prompt, and then click Run as Administrator.

User Access Control permission If you are prompted for an administrator password or for confirmation, type the password, or click Continue.

3.

At the command prompt, type the following command, and then press ENTER:

netsh interface tcp set global autotuninglevel=disabled

4.

Start Outlook. If the problem is resolved, you can either keep the value for the autoTuningLevel parameter as disabled or isolate the hardware that does not support TCP Window Scaling. You can also set the autoTuningLevel parameter to other values as described in the "More Information" section.

MORE INFORMATION

To see the current configuration for the autoTuningLevel parameter, follow these steps:
1. Click Start Start button, click All Programs, and then click Accessories.
2. Right-click Command Prompt, and then click Run as Administrator.

User Access Control permission If you are prompted for an administrator password or for confirmation, type the password, or click Continue.
3. At the command prompt, type the following command, and then press ENTER:
netsh interface tcp show global

The value for the autoTuningLevel parameter is listed as Receive Window Auto-Tuning Level.

 

The following list includes all possible values for the autoTuningLevel parameter:
disabled
Fix the receive window at the default value.
highlyrestricted
Allow for the receive window to grow beyond the default value, but do so very conservatively.
restricted
Allow for the receive window to grow beyond the default value, but limit such growth in some scenarios.
normal
Allow for the receive window to grow to accommodate almost all scenarios.

experimental
Allow 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.

 

The TCP receive window size is the maximum amount of received data in bytes that can be buffered at the same time on the receiving side of a connection. The sending host can send only that amount of data before waiting for an acknowledgement and for a receive window update from the receiving host.

The TCP/IP stack in Windows Vista tunes itself in most environments by using larger default window sizes than in earlier versions of Microsoft Windows. Instead of using a hard-coded default receive window size, TCP automatically adjusts the window size to increase the percentage of full-sized TCP segments that are used during bulk data transmission. This helps limit the number of segments that are sent when large sets of data are transmitted.

When auto-tuning is enabled in Windows Vista, Receive Window Auto-Tuning continually determines the optimal true receive window size by measuring the bandwidth delay product and the application retrieve rate. Then, Receive Window Auto-Tuning adjusts the maximum receive window size based on the changing network conditions.

For more information about the Next Generation TCP/IP stack that includes Receive Window Auto-Tuning, refer to the following Microsoft article:
For more information about Windows TCP implementation features, refer to the following Microsoft article: For more information about TCP receive window size and about window scaling, refer to the following Microsoft article: For more information about RFC 1323, visit the Internet Engineering Task Force website: