Slb Distribution Algorithms; Selecting Stickiness - D-Link NetDefendOS User Manual

• 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 receiving over a
certain time period. This time period is known as the Window Time. SLB sends the next
request to the server that has received the least number of connections during the last
Window Time number of seconds.
The Window Time is a setting that the administrator can change. The default value is 10
seconds.

10.4.3. Selecting Stickiness

In some scenarios, such as with SSL or TLS connections, it is important that the same server is
used for a series of connections from the same client. This is achieved by selecting the
appropriate stickiness option and this can be used with either the round-robin or connection-rate
algorithms. The options for stickiness are as follows:
• Per-state Distribution
This mode is the default and means that no stickiness is applied. Every new connection is
considered to be independent from other connections even if they come from the same IP
address or network. Consecutive connections from the same client may therefore be passed
to different servers.
This may not be acceptable if the same server must be used for a series of connections
809
Chapter 10: Traffic Management