Specifications

Chapter 1: Overview
2 Equalizer Installation and Administration Guide
When you configure the servers in a virtual cluster, you assign a static weight between 20 and 200
for each server. When you select one of the adaptive load-balancing algorithms, Equalizer uses the
servers’ static weights as a starting point to determine the percentage of requests to route to each
server. Each server handles a percentage of the total load based on its fraction of the total weights in
the server cluster. Equalizer dynamically adjusts server weights according to real-time conditions to
ensure that Equalizer routes requests to the server that is best able to respond. A server with a
weight of zero (0) is considered down or unavailable: Equalizer does not route new requests to
servers in this state.
Real-Time Server Status Information
Equalizer can gather real-time information about a server’s status using Server Agents and Active
Content Verification (ACV).
You can install a server agent on each server to provide Equalizer with periodic performance
statistics. This enables Equalizer to adjust the dynamic weights of the servers in a cluster according
to their actual performance characteristics. If the server is overloaded and you have enabled
adaptive load balancing, Equalizer responds by reducing the server’s dynamic weight so that the
server receives fewer requests. Coyote Point provides APIs useful for creating these agents. For
more information see “Using Server Agents” on page 161.
Equalizer’s active content verification (ACV) provides a way to check the validity of a server’s
response using most network services that support a text-based request/response protocol, such as
HTTP. When you enable ACV for a cluster, Equalizer requests data from each server in the cluster
(using an ACV Probe string) and verifies the returned data (against an ACV Response string). If
Equalizer receives no response or the response string is not in the response, the verification fails and
Equalizer stops routing new requests to that server. (Note that you cannot use ACV with UDP-based
services.) For more information, see “Using Active Content Verification (ACV)” on page 83.
Network Address Translation and Spoofing
Equalizer’s Network Address Translation (NAT) subsystem distributes incoming Layer 4 or Layer 7
(with spoofing) client requests among the available servers. The NAT subsystem records the
existence of the request, selects the best available server, rewrites the TCP/UDP and IP headers of
the request packet, and then forwards the translated packet to the selected server. Because the
servers are configured to use Equalizer to gateway all packets, Equalizer performs the reverse
translation as the server response packets leave the cluster.
When IP spoofing is enabled, the servers see their client’s actual IP address. However any response
must be gatewayed through the Equalizer because clients will only recognize the Equalizer’s
address—they did not communicate directly with the server. (For more information about
configuring spoofing see “Adding a Virtual Cluster” on page 67.)
When Equalizer receives an incoming packet that is not destined for a virtual cluster address,
Equalizer passes the packet through unaltered. Similarly, when Equalizer receives an outgoing
packet that is not a response to an existing virtual cluster connection, Equalizer passes the packet
through to the external network.
Load Balancing UDP Services
You can configure Equalizer virtual clusters to provide load balancing and server failure detection
for many UDP (User Datagram Protocol) based services. UDP load balancing is ideal for stateless
protocols such as DNS and RADIUS, can load-balance WAP (Wireless Application Protocol)