The Distribution Algorithm; Connections From Three Clients - D-Link NetDefend DFL-210 User Manual

10.4.4. The Distribution Algorithm

Network Stickiness
10.4.4. The Distribution Algorithm
There are several ways to determine how a load is shared across a server farm. NetDefendOS SLB
supports the following algorithms:
Round Robin
Connection Rate
The Connection Rate Algorithm and Stickiness
If the Connection Rate algorithm is used without stickiness, it will behave as a Round Robin
algorithm that allocates new connections to servers in an orderly fashion. It will also behave like the
Round Robin algorithm if there are always clients with a new IP address that make one connection.
The real benefit of using the Connection Rate algorithm together with stickiness is when clients
make multiple connections. Connection Rate will then ensure that the distribution of new
connections is as even as possible among servers. Before the interval reaches the specified Idle
Timeout of stickiness, new incoming connections from the same IP address as a previous connection
are assigned to the same server. The connection with a new address will be redirected to a server
with the lowest connection rate. The algorithm aims to minimize the new connection load for a
server, but the distribution may get uneven if a client from a single IP is sending lots of new
connections in a short time and the other servers do not get as many new connections.
In the management interface, the time window is variable for counting the number of seconds back
in time to summarize the number of new connections for the connection-rate algorithm. As the
default value, 10 is used so that the number of new connections which were made to each server in
the last 10 seconds will be remembered.
An Example Connection Scenario
An example scenario is illustrated in the figure below. In this example, the NetDefend Firewall is
responsible for balancing connections from 3 clients with different addresses to 2 servers. Stickiness
is enabled.
Figure 10.9. Connections from Three Clients
to the same host.
This mode is similar to IP stickiness except that by using a
subnet mask, a range of hosts in a subnet can be specified.
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.
This algorithm considers the number of requests that each server has
received over a certain timeframe. SLB sends the next request to the server
that has received the lowest number of connections in that time. The
administrator is able to specify the timeframe to use with this algorithm.
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Chapter 10. Traffic Management