Microsoft KB Archive/244382

= Information About Microsoft Windows NT 4.0 Security Account Manager Replication =

Article ID: 244382

Article Last Modified on 11/1/2006

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APPLIES TO


 * Microsoft Windows NT Server 4.0 Standard Edition
 * Microsoft Windows NT Server 4.0 Enterprise Edition

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This article was previously published under Q244382



SUMMARY
This article provides in-depth information about Netlogon functionality as it pertains to Security Account Manager (SAM) replication.



MORE INFORMATION
For each backup domain controller (BDC), the primary domain controller (PDC) records the serial number of the most recent change sent to that BDC, and then compares it with the serial number of the most recent change made to the SAM database. If this comparison shows that this BDC is up to date, the notification engine does not notify this BDC during this cycle. Also, the PDC may send a notification change to a BDC, but the BDC may not apply it because of a computer failure. However, the BDC makes a replication request to the PDC upon restarting, and this request includes the serial number of the most recent change successfully applied to the BDC. This information permits the PDC to keep the BDC up to date.

A comparison of the serial numbers between the PDC and its BDCs is made at two different times:
 * By the notification engine, whenever it considers sending a "pulse" to a BDC.
 * By the replication engine, whenever it receives a request for changes from a BDC.

How the PDC Requests BDC Notifications
The notification engine enumerates the BDCs using the capabilities of the SAM, and considers BDCs for notification in whatever order the SAM presents them. The order is consistent each time the notification engine runs, and there is no way for an administrator to control this order.

When a PDC starts, it reads the list of BDCs from the SAM, and then builds a circular list in memory. The SAM supplies the BDCs in reverse alphabetical order, and if a new BDC is added while the PDC is already running, this BDC appears in an unpredictable location in the circular list.

How a BDC Synchronizes After Notification
After a BDC receives a notification from the PDC, it requests changes from the PDC, including the serial number of the most recent change successfully applied to the BDC. Then, the PDC attempts to respond with more recent changes, and if the response does not fit into a single response, the PDC returns as much as it can, including a status indicating that the BDC should make another request. This process continues until the PDC is able to respond with the remaining changes.

If a PDC is updated during a replication cycle, BDCs that finish their replication before the update do not see the new value until the following replication cycle, while BDCs that finish their replication after the update see the new value in the current cycle.

How the PDC Controls Replication Concurrency
The PDC Netlogon service contains a notification engine that notifies BDCs that they should perform replication. Each time the notification engine starts, it runs a complete cycle. In one cycle, the engine considers each BDC and either notifies them by sending a "pulse" message, or not. At the end of a cycle, the engine either stops and waits for the configured number of "pulse" seconds to have elapsed since the last time the engine started its cycle, or immediately starts a new cycle if the configured number of "pulse" seconds have already elapsed.

The notification engine employs feedback to avoid overloading the PDC with replication requests from BDCs. The notification engine notifies BDCs in slots of size PulseConcurrency. For example, if the PulseConcurrency value is set 30, then 30 BDCs replicate with the PDC at a given time until all BDCs for this replication cycle are synchronized, in a sliding window format always containing 30 BDCs. When one BDC has finished replication, a new one is taken from the list. After notifying such a slot, the PDC waits for all the BDCs in the slot to complete their replication before starting the next slot of notifications.

Determining when a BDC is notified of a "pulse" event depends entirely on where the notification engine stopped at the end of the previous cycle. The notification engine considers stopping each time it examines a list entry and decides not to send a pulse. For example, if N is the number of "live" BDCs in the list, and the notification engine has just examined N "live" entries and not sent pulses to any of them, and it is not time to start a new cycle, the notification engine stops. Also, because BDCs may go up and down or become unresponsive, the stopping place is not predictable, and the first BDCs to get pulsed when the engine starts are not predictable.

NOTE: The PDC responds to all BDC replication requests, not just replication requests from BDCs in its current slot. There are several scenarios in which a BDC outside the current slot may request replication from the PDC:
 * The PDC notifies a BDC, but that BDC does not respond with a replication request within PulseTimeout1 seconds. The PDC then skips that BDC, and does not include it in the current slot. However, that BDC really did receive the notification, and eventually performs a replication request.
 * The PDC sends a replication response to a BDC, but the response does not bring that BDC fully up to date, and that BDC does not come back with a replication request within PulseTimeout2 seconds. The PDC then removes that BDC from the current slot. However, that BDC really did receive the replication response, and eventually performs a replication request.
 * A BDC failed, and then restarted. After restarting, the BDC makes a replication request to the PDC.
 * An administrator has manually issued a replication request at a BDC.

Additional information about this behavior is provided in the white paper "Domain Sizing and Capacity Planning for Windows NT Server 4.0", and the Microsoft Windows NT Resource Kit. Also, additional details can be gathered by installing a checked version of the Netlogon.dll file and examining the logs that are generated.

Keywords: kbinfo kbnetwork KB244382

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