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Updated on September 10, 2025
Active Directory’s reliability hinges on one fundamental requirement: every domain controller must maintain an identical copy of the directory database. When administrators make changes—whether adding a new user account, modifying group memberships, or updating organizational units—these modifications must propagate seamlessly across all domain controllers in the forest.
The Directory Replication Service (DRS) orchestrates this complex synchronization process. At its core lies a single, critical Remote Procedure Call (RPC) operation: DSGetNCChanges. This highly specialized command serves as the primary mechanism that domain controllers use to request and receive incremental changes from their replication partners.
DSGetNCChanges isn’t just another Active Directory component—it’s the workhorse that enables the multi-master replication model that makes Active Directory both scalable and fault-tolerant. Understanding this operation is essential for any administrator who needs to troubleshoot replication issues, optimize replication performance, or implement complex Active Directory topologies.
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Definition and Core Concepts
DSGetNCChanges is a Remote Procedure Call (RPC) defined by the Directory Replication Service Remote Protocol (MS-DRSR). It represents the primary operation used by an Active Directory domain controller to pull incremental changes for a specific directory partition from a replication partner.
The operation’s name reflects its function: “DS” stands for Directory Service, “Get” indicates it’s a request operation, “NC” refers to Naming Context (Microsoft’s term for directory partitions), and “Changes” specifies that it retrieves only modified data since the last replication cycle.
Multi-Master Replication Architecture
DSGetNCChanges enables Active Directory’s multi-master replication model. Unlike single-master systems where only one server accepts writes, Active Directory allows administrators to make changes on any writeable domain controller. The DSGetNCChanges operation ensures these distributed changes propagate efficiently to all replicas throughout the forest.
Each domain controller maintains its own copy of the directory database and can accept write operations independently. When changes occur, the modified domain controller uses change notifications and scheduled replication intervals to inform its partners. Those partners then use DSGetNCChanges to pull the specific updates they need.
Update Sequence Number (USN) Tracking
The Update Sequence Number serves as a 64-bit logical clock for each domain controller’s database. Every time a domain controller commits a change to its local database, it increments its USN counter and assigns that number to the modified object or attribute.
DSGetNCChanges requests rely heavily on USN values to determine which changes need replication. The requesting domain controller provides the highest USN it has received from the source domain controller. The source then returns only those changes with USN values greater than the provided threshold.
Directory Partition Scope
DSGetNCChanges operates on a per-partition basis. Active Directory organizes its database into several distinct partitions, and domain controllers must make separate DSGetNCChanges calls for each partition they need to synchronize:
- Domain Partition: Contains user accounts, computer objects, and organizational units specific to that domain
- Schema Partition: Defines the classes and attributes available throughout the forest
- Configuration Partition: Stores forest-wide configuration data including site topology and replication settings
- Application Partitions: Custom partitions that administrators can create for specific applications or services
How DSGetNCChanges Works
The DSGetNCChanges process follows a well-defined sequence that ensures reliable and efficient replication between domain controllers.
Request Initiation
A pulling domain controller initiates the DSGetNCChanges process when it determines that replication is necessary. This determination occurs through two primary triggers:
For intra-site replication, domain controllers use change notification. When a domain controller commits changes to its database, it notifies its replication partners within the same site immediately. These partners then initiate DSGetNCChanges requests to pull the new changes.
For inter-site replication, domain controllers follow scheduled intervals defined by the site link configuration. By default, inter-site replication occurs every 180 minutes, though administrators can modify this interval based on bandwidth and business requirements.
RPC Call Structure (IDL_DRSGetNCChangesRequest)
The pulling domain controller constructs an IDL_DRSGetNCChangesRequest packet containing several critical parameters:
The directory partition identifier specifies which naming context requires replication. This ensures the request targets the correct partition and prevents unnecessary data transfer.
The highWatermarkVector represents a data structure containing the last USN values the pulling domain controller received from each of its replication partners. This vector enables the source domain controller to identify exactly which changes the requester still needs.
Replication flags indicate the type of synchronization required. These flags specify whether the request represents an initial full synchronization, an incremental update, or a special operation like replicating urgent changes.
Additional parameters include security context information for authentication and authorization, compression preferences for inter-site replication, and filtering options for specialized scenarios like Read-Only Domain Controller replication.
Server-Side Processing
When the source domain controller receives the DSGetNCChanges request, it performs several operations to identify and package the required changes:
The server examines the highWatermarkVector to determine the last USN the requesting domain controller received. It then scans its local database for all objects and attributes in the specified directory partition that have USN values greater than this threshold.
The source domain controller also applies any filtering rules specified in the request flags. For Read-Only Domain Controllers, certain attributes are filtered out to maintain security boundaries. For normal replication, the server includes all changed attributes and objects.
The server packages these changes into a structured response, ensuring that dependent changes are ordered correctly. For example, if an organizational unit is created and then populated with user objects, the organizational unit creation must appear before the user object additions in the response.
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Response Structure (IDL_DRSGetNCChangesReply)
The source domain controller packages the identified changes into an IDL_DRSGetNCChangesReply packet. This response contains several key components:
The object data includes all new or modified objects along with their changed attributes. Each object includes its distinguished name, GUID, and the complete set of modified attributes with their new values.
The metadata for each change includes the originating domain controller, the originating USN, the originating timestamp, and version information. This metadata enables conflict resolution when the same object is modified on multiple domain controllers simultaneously.
The new high watermark provides the updated USN value that the pulling domain controller should store for future replication requests. This value represents the highest USN from the source domain controller included in the current response.
Key Features and Components
Incremental Synchronization Efficiency
DSGetNCChanges implements true incremental synchronization, transferring only the specific changes that have occurred since the last successful replication. This approach minimizes network bandwidth usage and reduces the time required for replication operations.
The incremental nature becomes particularly important in large Active Directory environments where full synchronization would be prohibitively expensive. A typical incremental replication might transfer only a few kilobytes of data, while a full synchronization could require transferring hundreds of megabytes or even gigabytes.
Compression for Bandwidth Conservation
When domain controllers communicate across site boundaries, DSGetNCChanges responses can be compressed to reduce bandwidth consumption. The compression algorithm optimizes the data payload while maintaining the integrity of the replication information.
Administrators can configure compression thresholds based on their network infrastructure. Sites connected by high-bandwidth links might disable compression to reduce CPU overhead, while sites connected by slower WAN links benefit significantly from compression.
Filtering for Security Boundaries
DSGetNCChanges supports sophisticated filtering mechanisms that enable specialized replication scenarios. Read-Only Domain Controllers use filtering to prevent sensitive attributes like passwords and security credentials from replicating to potentially less secure locations.
The filtering system operates at the attribute level, allowing fine-grained control over which information replicates to specific domain controllers. This capability supports branch office scenarios where security requirements differ from main office installations.
Troubleshooting and Diagnostic Considerations
Common Replication Failure Patterns
When Active Directory replication fails, the root cause almost always traces back to DSGetNCChanges RPC failures. Network connectivity issues represent the most frequent source of problems, particularly in complex network environments with firewalls, NAT devices, or unreliable WAN connections.
DNS configuration errors create another common failure pattern. DSGetNCChanges requires accurate DNS resolution to locate replication partners and establish RPC connections. Incorrect DNS records or resolution failures prevent domain controllers from initiating successful replication sessions.
Time synchronization problems can cause Kerberos authentication failures, which in turn prevent DSGetNCChanges operations from completing successfully. Active Directory requires time synchronization within five minutes by default, and larger time skews can cause authentication to fail entirely.
Diagnostic Tools and Techniques
The repadmin /showrepl command serves as the primary diagnostic tool for DSGetNCChanges troubleshooting. This command displays the replication status for each directory partition and identifies specific error codes when DSGetNCChanges calls fail.
The command output includes the last successful replication time, the number of consecutive failures, and detailed error codes that pinpoint the failure cause. Common error codes include RPC server unavailable, access denied, and name resolution failures.
Network packet analyzers provide deeper insight into DSGetNCChanges traffic patterns. Tools like Wireshark can capture and decode the actual RPC traffic, revealing whether requests reach the destination server and how the server responds to those requests.
Event log analysis complements these tools by providing detailed error descriptions and contextual information about replication failures. The Directory Service event log contains specific DSGetNCChanges error messages that help administrators identify root causes and develop remediation strategies.
Key Terms Reference
- DSGetNCChanges: The primary RPC operation that domain controllers use to request incremental changes from replication partners during Active Directory replication.
- MS-DRSR: The Directory Replication Service Remote Protocol specification that defines the DSGetNCChanges operation and related replication procedures.
- Update Sequence Number (USN): A 64-bit logical clock maintained by each domain controller to track the sequence of changes committed to its local database.
- Replication Partner: A domain controller configured to replicate directory information with another domain controller, forming a replication relationship.
- High Watermark: The highest USN value that a domain controller has received from a specific replication partner, used to determine which changes still need replication.
- Remote Procedure Call (RPC): The underlying communication mechanism that enables DSGetNCChanges operations between domain controllers over the network.
- Naming Context: Microsoft’s terminology for directory partitions, representing logically separated portions of the Active Directory database that replicate as units.
