D-Link NetDefend DFL-1660, User Manual

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Figure 10.9. A Server Load Balancing Configuration
Additional Benefits of SLB
Besides improving performance and scalability, SLB provides other benefits:
• SLB increases the reliability of network applications by actively monitoring the servers sharing
the load. NetDefendOS SLB can detect when a server fails or becomes congested and will not
direct any further requests to that server until it recovers or has less load.
• SLB can allow network administrators to perform maintenance tasks on servers or applications
without disrupting services. Individual servers can be restarted, upgraded, removed, or replaced,
and new servers and applications can be added or moved without affecting the rest of a server
farm, or taking down applications.
• The combination of network monitoring and distributed load sharing also provides an extra level
of protection against Denial Of Service (DoS) attacks.
SLB Deployment Considerations
The following issues should be considered when deploying SLB:
• Across which servers is the load is to be balanced.
• Which SLB algorithm will be used.
• Will "stickiness" be used.
• Which monitoring method will be used.
Each of these topics is discussed further in the sections that follow.
Identifying the Servers
An important first step in SLB deployment is to identify the servers across which the load is to be
balanced. This might be a server farm which is a cluster of servers set up to work as a single "virtual
server". The servers that are to be treated as a single virtual server by SLB must be specified.
10.4.2. SLB Distribution Algorithms
There are several ways to determine how a load is shared across a set of servers. NetDefendOS SLB
supports the following two algorithms for load distribution:
Round-robin The algorithm distributes new incoming connections to a list of servers on
a rotating basis. For the first connection, the algorithm picks a server
randomly, and assigns the connection to it. For subsequent connections, the
algorithm cycles through the server list and redirects the load to servers in
order. Regardless of each server's capability and other aspects, for instance,
the number of existing connections on a server or its response time, all the
available servers take turns in being assigned the next connection.
This algorithm ensures that all servers receive an equal number of requests,
therefore it is most suited to server farms where all servers have an equal
capacity and the processing loads of all requests are likely to be similar.
Connection-rate This algorithm considers the number of requests that each server has been
10.4.2. SLB Distribution Algorithms Chapter 10. Traffic Management
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