Group Policy and Slow Links

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Some Group Policy is not applied when the connection speed fall below specified thresholds. Therefore, when your Group Policy solution calls for applying policy over slow links or by using remote access, you need to consider policy settings for slow link detection.

Although slow links and remote access are related, there are distinctions, and Group Policy processing varies for each. Having a computer connected to a LAN does not necessarily imply a fast link, nor does a remote access connection imply a slow link. The default value for what Group Policy considers a slow link is any rate slower than 500 Kilobits per second (Kbps). You can change this threshold using Group Policy. Group policy uses the following algorithm to measure link speed.

  1. Ping the server with 0 bytes of data and time the number of milliseconds. This value is time#1. If it is less than 10 ms, exit (assume a fast link).

  2. Ping the server with 2 kilobytes (KB) of uncompressible data, and time the number of milliseconds. This value is time#2. The algorithm uses a compressed .jpg file to ping the server.

  3. DELTA = time#2 - time#1. This removes the overhead of session setup, with the result being equal to the time to move 2 KB of data.

  4. Calculate Delta three times, adding to TOTAL each DELTA value. Use the following calculations:

  5. TOTAL/3 = Average of DELTA in milliseconds.

  6. 2 * (2 KB) * (1000 ms/sec) / DELTA Average ms = X

  7. X = (4000 KB/sec) / DELTA Average

  8. Z Kbps = ((4000 KB) / DELTA Average) *(8 bits/byte)

  9. Z Kbps = 32000 kbps/Delta Average.

    Two KB of data have moved in each direction (this is represented by the leading factor two on the left side in step six) through each modem, Ethernet card, or other device in the loop once. The resulting Z value is evaluated against the policy setting. A default of less than 500 Kbps is considered a slow link; faster than 500 Kbps is a fast link.

Administrators can use a policy setting to define a slow link for the purpose of applying Group Policy, as described in the following sections.