How a slow link is detected for processing user profiles and Group Policy

This article describes how a slow link is detected in Windows 2000 for processing user profiles and Group Policy.

Windows 2000 includes a new method of determining whether a client computer is gaining access to a domain controller over a slow link to apply Group Policy or download a roaming user profile. This takes the form of a sequence of TCP/IP ping requests to the destination server. There is more reliance on the detection of slow links in Windows 2000 because this is used to determine the types of Group Policy that are applied.

The new mechanism takes the form of measuring the response time from a sequence of TCP/IP pings from the client computer to the server to determine the average transfer rate in kilobits per second (kbps). The client pings the server three times with 0 bytes and three times with 2048 bytes. If the response time from any of the pings is less than 10 milliseconds (ms), the link is automatically considered fast. Otherwise, the average transfer rate is calculated by averaging the differences between the first (0 byte) and second (2048 byte) ping times. If the transfer rate is slower than the default or a value that is defined by the administrator, the connection is considered slow.

Windows 2000 uses the following formula: In this formula, ulTotal is the average of the differences between the first and second ping times.

Consider the following example.In this example, the difference between each time can be calculated in the following way:

First difference: 245-236=9
Second difference: 247-240=7
Third difference: 245-237=8

Therefore, ulTotal would be (9+7+8)/3 = 24/3 = 8. The estimated link speed would be 32000/ulTotal = 32000/8 = 4000.

Effects on Group Policy

When Group Policy is applied to the computer or to the user, the IP slow link detection mechanism is always used. By default, if a slow link is detected, some forms of Group Policy are not applied. For more information about which Group Policy components do not apply, and about how to modify this behavior, click the following article number to view the article in the Microsoft Knowledge Base: The administrator can specify the connection speed independently for the application of Group Policy to computers and users:

1. In Active Directory Users and Computers, right-click the site, domain, or organizational unit in which the Group Policy object (GPO) exists that will contain the policy for the connection speed, and then click Properties.
2. Click the Group Policy tab, click the appropriate GPO, and then click Edit.
3. To change the slow link detection speed for Windows 2000 when computer Group Policy is being applied, expand the Computer Configuration node and locate the Administrative Templates\System\Group Policy folder. Modify the Group Policy slow link detection policy.
   
   To change the slow link detection speed for Windows 2000 when user Group Policy is being applied, expand the User Configuration node and locate the Group Policy slow link detection policy.
   
   In either case, the value must be specified in kbps. To disable slow link detection, use 0 (zero) for this value. By default, the connection speed is set to 500 kbps.
4. Exit Active Directory Users and Computers.

Effects on roaming user profiles

Roaming user profiles are not required to be stored on a server on which TCP/IP is an installed protocol. However, when a user tries to download a roaming user profile, the IP slow link detection mechanism is used first. If Windows 2000 detects that the server on which the roaming user profile is located does not support TCP/IP, it uses the method that is used by Microsoft Windows NT 4.0 and earlier versions. In this method, the time that it takes for the server to respond to a request for the file attributes of the profile is timed and compared to the value determining a slow network.

The administrator can specify the connection speed that determines a slow network when user profiles are being downloaded:

1. In Active Directory Users and Computers, right-click the site, domain, or organizational unit in which the Group Policy object (GPO) exists that will contain the policy for the connection speed, and then click Properties.
2. Click the Group Policy tab, click the appropriate GPO, and then click Edit.
3. To change the slow link detection speed for Windows 2000 when a user's roaming user profile is being downloaded, expand the Computer Configuration node and locate the Administrative Templates\System\Logon folder. Modify the Slow network connection timeout for user profiles policy. This policy uses two values, one in kbps for the TCP/IP slow network detection mechanism and one in ms (milliseconds) for the time to contact server. To disable slow link detection, use 0 (zero) for these values. By default, the connection speed is set to 500 (kbps) and the time is set to 120 (milliseconds).
4. Exit Active Directory Users and Computers.

Viewing the slow network detection data in the debug log

If debug logging is enabled on a Windows 2000-based computer, the result of the slow network detection mechanism is recorded in the Userenv.log file. An example of the entries recorded when a client is on a fast link (where the TCP/IP pings resulted in times of less than 10 ms) might resemble the following: For more information about how to enable debug logging, click the following article number to view the article in the Microsoft Knowledge Base: