Functions for Sites in Windows Server 2003

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

Windows Server 2003 uses site information for many purposes, including routing replication, client affinity, system volume replication, DFS, and service location.

Routing replication

Active Directory uses a multimaster, store-and-forward method of replication. A domain controller communicates directory changes to a second domain controller, which then communicates to a third, and so on, until all domain controllers have received the change. To achieve the best balance between reducing replication latency and reducing traffic, site topology controls Active Directory replication by distinguishing between replication that occurs within a site and replication that occurs between sites.

Within sites, replication is optimized for speed — data updates trigger replication and the data is sent without the overhead required by data compression. Conversely, replication between sites is compressed to minimize the cost of transmission over WAN links. When replication occurs between sites, a single domain controller per domain at each site collects and stores the directory changes and communicates them at a scheduled time to a domain controller in another site.

Client affinity

Domain controllers use site information to inform Active Directory clients about domain controllers present within the closest site as the client. For example, consider a client in the Seattle site that does not know its site affiliation and contacts a domain controller from the Atlanta site. Based on the IP address of the client, the domain controller in Atlanta determines which site the client is actually from and sends the site information back to the client. The domain controller also informs the client whether the chosen domain controller is the closest one to it. The client caches the site information provided by the domain controller in Atlanta and queries for the site-specific service (SRV) resource record (a DNS resource record used to locate domain controllers for Active Directory) and thereby finds a domain controller within the same site.

By finding a domain controller in the same site, the client avoids communications over WAN links. If no domain controllers are located at the client site, a domain controller that has the lowest cost connections relative to other connected sites advertises itself (registers a site-specific SRV resource record in DNS) in the site that does not have a domain controller. The domain controllers that are published in DNS are those from the closest site as defined by the site topology. This process ensures that every site has a preferred domain controller for authentication.

For more information about the process of locating a domain controller, see the Active Directory Collection of the Windows Server 2003 Technical Reference (or see the Active Directory Collection on the Web at https://www.microsoft.com/reskit).

SYSVOL replication

The system volume (SYSVOL) is a collection of folders in the file system that exists on each domain controller in a domain. The SYSVOL folders provide a default Active Directory location for files that must be replicated throughout a domain, including Group Policy objects (GPO), startup and shutdown scripts, and logon and logoff scripts. Windows Server 2003 uses the File Replication service (FRS) to replicate changes made to the SYSVOL folders from one domain controller to other domain controllers. FRS replicates these changes according to the schedule that you create during your site topology design.

DFS

DFS uses site information to direct a client to the server that is hosting the requested data within the site. If DFS does not find a copy of the data within the same site as the client, DFS uses the site information in Active Directory to determine which file server that has DFS shared data is closest to the client.

Service location

By publishing services such as file and print services in Active Directory, you allow Active Directory clients to locate the requested service within the same or nearest site. Print services use the location attribute stored in Active Directory to let users browse for printers by location without knowing their precise location. For information about enabling printer location tracking, see "Enable printer location tracking" in Help and Support Center for Windows Server 2003. For more information about designing and deploying print servers, see "Designing and Deploying Print Servers" in Planning Server Deployments of this kit.