Selecting an Intersite Transport

One of the last things to consider when designing a physical Active Directory topology is the transport that will be used to replicate information between sites. Your choices are Remote Procedure Calls (RPC) or Simple Mail Transfer Protocol (SMTP).

RPC

The default transport that can be used for intersite replication is RPC over TCP/IP. This transport can be used for intersite as well as intrasite replication. RPC uses synchronous transfer, so a direct connection must exist with the destination server before any information can be replicated. However, this poses a problem for WAN links that are unreliable because, if a connection cannot be established, replication cannot occur. Also, if the WAN link is slow or congested , RPC timeouts can occur, causing replication to fail. RPC is inadvisable for link speeds under 128Kbps. RPC timeouts and replication failures can also occur when using VPN connections, even at 128Kbps or higher. The reason for the timeouts is the unpredictable latency of VPN circuits, which depend on the Internet to transmit data from one location to another.

One of the main advantages of using RPC over TCP/IP is that it can support intersite replication traffic between all servers, including domain controllers from the same domain. RPC is also more efficient as an intersite transport.

Table 9.4 summarizes some of the advantages and disadvantages of using RPC over TCP/IP.

Table 9.4. Summary of the Features of the RPC Transport for Intersite Replication

Advantage	Disadvantage
Faster than SMTP	Must have a direct and reliable connection with a destination server to send information
Can be used for intersite replication between DCs in the same domain	Can be used only in a TCP/IP-based network
Uses the schedules set on the site links	Can have only one outstanding connection at one time

SMTP

SMTP sends information to be replicated between sites as email messages. Unlike RPC, it provides asynchronous data transfer, so a direct connection with the remote server is not required. It also uses the store-and-forward method of sending information. Therefore, if the destination host is not available, the message can be stored. This transport is an ideal choice if the link between two sites is unreliable. For example, when the link is not available, the message can be stored and sent when the destination server is available. Note, however, that the schedules set on a site link by an administrator are ignored when the SMTP protocol is used.

Even though this is an intersite transport, it cannot be used to replicate information between domain controllers in the same domain. In cases such as this, RPC would have to be used. The primary reason is that Sysvol replication, which is required to replicate part of a Group Policy Object (GPO), must use RPC. Even if no GPOs are configured, Windows 2000 will not replicate the domain naming context via SMTP.

Table 9.5 outlines some of the advantages and disadvantages associated with this transport.

Table 9.5. Advantages and Disadvantages of Using SMTP As an Intersite Transport

Advantage	Disadvantage
It uses store-and-forward messaging.	It is generally slower than RPC.
Multiple messages can be sent at one time.	The format of messages increases network traffic.
Because the information is in the form of an email message, it can be routed.	It can be used only for intersite replication.

Comparing the Protocols

When evaluating a site link to determine the proper protocol to use, consider the following points:

RPC requires a relatively fast, reliable WAN link. If links are slow (less than 128Kbps), unreliable, or congested, SMTP is a better choice.
If a domain spans sites, any links between these sites must use RPC.
SMTP replication uses the SMTP component of Internet Information Server (IIS) to communicate with other domain controllers.
SMTP replication ignores site link schedules.
SMTP replication generates slightly more network traffic than RPC ”typically about 5% “10% more.