Active Directory Replication

Replication Transports

Replication transports provide the wire protocols that are required for data transfer. Windows 2000 provides three levels of connectivity for replication of Active Directory information:

Uniform high-speed, synchronous RPC over IP within a site.
Point-to-point, synchronous, low-speed RPC over IP between sites.
Low-speed, asynchronous SMTP between sites.

The following rules apply to the replication transports:

Replication within a site always uses RPC over IP.
Replication between sites can use either RPC over IP or SMTP over IP.
Replication between sites over SMTP is supported for only domain controllers of different domains. Domain controllers of the same domain must replicate by using the RPC over IP transport. Therefore, replication between sites over SMTP is supported for only schema, configuration, and Global Catalog replication, which means that domains can span sites only when point-to-point, synchronous RPC is available between sites.

The Inter-Site Transports container provides the means for mapping site links to the transport that the link uses. When you create a site link object, you create it in either the IP container (which associates the site link with the RPC over IP transport) or the SMTP container (which associates the site link with the SMTP transport).

Synchronous vs. Asynchronous Communication

In the context of Active Directory replication, synchronous communication implies that after the destination domain controller sends the request for data, it waits for the source domain controller to receive the request, construct the reply, and send the reply before it requests changes from any other domain controllers. Therefore, at any particular time, a domain controller has a maximum of one synchronous request outstanding.

In the case of asynchronous replication, the destination domain controller does not wait for the reply, and it can have multiple asynchronous requests outstanding at any particular time. In synchronous transmission, therefore, the reply is received in a short amount of time; in asynchronous transmission, the reply is not necessarily received in a short time.

Transport for Replication Within a Site

All replication within a site occurs over synchronous RPC over IP transport. The focus for replication within a site is the fast, uncompressed delivery of updates. Replication events occur more frequently within a site than between sites, and the overhead of compression would be inefficient over fast connections.

Note

Replication between sites is compressed.

By default, RPC-based replication uses dynamic port mapping. When connecting to an RPC endpoint during Active Directory replication, the RPC run time on the client contacts the RPC endpoint mapper on the server at a well-known port (port 135). The server queries the RPC locator on this port to determine what port has been assigned for Active Directory replication on the server. This query occurs whether the port assignment is dynamic (the default) or fixed. The client therefore never needs to know which port to use for Active Directory replication.

Note

An endpoint comprises the protocol, local address, and port address.

Active Directory registers the endpoint when it starts, and it uses either a dynamically assigned port or a specified port, whichever you have configured. To specify a fixed port for routed environments or where port filtering is employed, you can add or modify the TCP/IP Port entry in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NTDS\Parameters to configure Active Directory to register a specific port with the endpoint mapper. This value can be any valid TCP/IP port number.

To set the TCP/IP Port entry

In a registry editor, navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NTDS\Parameters.
Double-click the TCP/IP Port entry, and assign a valid port number.
Close the registry editor.

Caution

Editing the registry directly can have serious, unexpected consequences that can prevent the system from starting and require that you reinstall Windows 2000. There are programs available in Control Panel or Microsoft Management Console (MMC) for performing most administrative tasks. These programs provide safeguards that prevent you from entering conflicting settings or settings that are likely to degrade performance or damage your system. Registry editors bypass the standard safeguards that are provided by these administrative tools. Modifying the registry is recommended only when no administrative tool is available. Before you make changes to the registry, it is recommended that you back up any valuable data on the computer. For instructions about how to edit registry entries, see Help for the registry editor that you are using. For more information about the registry, see the Microsoft Windows 2000 Resource Kit Technical Reference to the Windows 2000 Registry (Regentry.chm).

Transports for Replication Between Sites

Windows 2000 supports two default transports for replication between sites:

RPC over TCP/IP (referred to as "IP" in administrative tools), which enables low-speed, point-to-point, synchronous replication between all directory partitions.
SMTP, which enables low-speed, asynchronous replication between the schema, configuration, and Global Catalog directory partitions, but not between domain directory partitions.

When sites are on opposite ends of a WAN link (or the Internet), it is not always desirable — or even possible — to perform synchronous, RPC-based directory replication. In some cases, the only method of communication between two sites is e-mail. To support such configurations, replication must be possible across asynchronous, store-and-forward transports such as SMTP.

SMTP replication substitutes mail messaging for the RPC transport. The message syntax is the same as for RPC-based replication. There is no change notification for SMTP-based replication, and scheduling information on the site link object is used as follows:

SMTP replication ignores the Replication Available and Replication Not Available settings on the site link schedule in Active Directory Sites and Services (the information that indicates when these sites are connected).
SMTP replication uses the replication interval to indicate how often the server requests changes. The interval (Replicate every ____ minutes) is set in hourly intervals on the General tab in site link Properties in Active Directory Sites and Services.

These settings combine to form the replication schedule on the connection object. The underlying SMTP messaging system is responsible for message routing between SMTP servers.

The Intersite Messaging (ISM) service allows for multiple transports to be used as add-ins to the ISM architecture. ISM provides services to the KCC in the form of querying the available replication paths. ISM enables messaging communication that can use SMTP servers other than those that are dedicated to processing e-mail applications such as Exchange Server.

Comparison of SMTP and RPC Replication

The following characteristics apply to both SMTP and RPC with respect to Active Directory replication:

For replication between sites, data replicated through both transports is compressed.
Active Directory can respond with only a fixed (maximum) number of changes per change request, on the basis of the size of the replication packet. The size of the replication packet is configurable. (For information about configuring the replication packet size, see "Replication Packet Size" later in this chapter.)
Active Directory can have only a single change request outstanding for a specific directory partition to a specific replication partner.
The response data (changes) are transported in one or many frames, based on the total number of changed or new values.
TCP transports the data portion by using the same algorithm for both SMTP and RPC.
If transmission of the data portion fails for either, complete retransmission is necessary.
If bandwidth is limited, the same TCP retransmission characteristics apply. (RPC time-out is much longer than TCP time-out.)

Because SMTP is not used for replication of domain directory partitions, Windows 2000 provides point-to-point synchronous RPC replication in addition to asynchronous SMTP replication between sites to allow the flexibility of having domains span multiple sites. RPC is best used between well-connected sites because it involves lower latency. SMTP is best used between sites where RPC over IP is not possible. For example, SMTP can be used by companies that have a network backbone that is not based on TCP/IP, such as companies that use an X.400 backbone.

Active Directory replication uses both transports to implement a request-response mechanism. Active Directory issues requests for changes and replies to requests for changes. RPC maps these requests into RPC requests and RPC replies. SMTP, on the other hand, actually uses long-lived TCP connections to deliver streams of mail in each direction. Thus, RPC transport expects a response to any request more or less immediately and can have a maximum of one active inbound RPC connection to a directory partition replica at a time. The SMTP transport expects much longer delays between a request and a response. As a result, multiple inbound SMTP connections to a directory partition replica can be active at the same time, provided the requests are all for a different source domain controller or directory partition.

Benefits of SMTP Replication Between Sites

Although SMTP replication usually is slower than RPC, there are conditions that call for SMTP replication exclusively, and others that favor it, such as the following:

RPC-based replication is currently synchronous, whereas SMTP is asynchronous. Where bandwidth is limited, it can be disadvantageous to force an entire transaction to complete before another can begin. With SMTP, several transactions can be processing simultaneously so that each transaction is receiving some attention most of the time, as opposed to no attention for prolonged periods, which can result in RPC time-outs.
SMTP traffic can be secured, monitored, and managed across a WAN environment.
Where end-to-end online IP connectivity is impossible — for example, A can communicate with B, and B can communicate with C, but A can never communicate with C directly — mail can be used and routed A to B, B to C, C to B, or B to A.

Replication Packet Size

By default, packet sizes are computed on the basis of memory size unless you have more than 1 gigabyte (GB) or less than 100 megabytes (MB) of memory. You can override these memory-based values in the registry.

To adjust the default size of the packets that transport Active Directory replication data, you can modify or add entries to the following registry path with the REG_DWORD data type: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\NTDS\Parameters. These entries determine the maximum number of objects per packet and maximum size of the packets.

For RPC replication within a site:

Replicator intra site packet size (objects)
Range: >=1
Replicator intra site packet size (bytes)
Range: >=10 KB

For RPC replication between sites:

Replicator inter site packet size (objects)
Range: >=1
Replicator inter site packet size (bytes)
Range: >=10 KB

For SMTP replication between sites:

Replicator async inter site packet size (objects)
Range: >=1
Replicator async inter site packet size (bytes)
Range: >=10 KB

If the preceding registry entries are not set, the system limits the packet size as follows:

The packet size in bytes is 1/100th the size of RAM with a minimum of 1 MB and a maximum of 10 MB.
The packet size in objects is 1/1,000,000th the size of RAM, with a minimum of 100 objects and a maximum of 1,000 objects.

There is one exception: the value of the Replicator async inter site packet size (bytes) entry is always 1 MB. Many mail systems limit the amount of data that can be sent in a mail message (2 MB to 4 MB is common), although most Windows-based mail systems can handle large 10-MB mail messages.

Caution