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Equalizer Overview

ICMP Probes

uses the Internet Control Message Protocol to send an "Echo request" to the server, and then wait

for the server to respond with an ICMP "Echo reply" message (like the Unix ping command). ICMP is a Layer 3

protocol. ICMP probes can be disabled via a global flag.

TCP Probes

establish (and tear down) a TCP connection between Equalizer and the server, in a typical Layer 4

exchange of TCP SYN, ACK, and FIN packets. If the connection cannot be completed, Equalizer considers the

server down and stops routing requests to it. TCP probes cannot be disabled.

Equalizer’s

Active Content Verification (ACV)

provides an optional method for checking the validity of a server’s

response using Layer 7 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 anACV 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 ACV is not supported for Layer 4 UDP clusters.) For more information, see "Active Content Verification

(ACV) Probes" on page 373.

Server AgentProbes

enable Equalizer to communicate with a user-written program (the

agent

) running on the

server. A server agent is written to open a server port and, when Equalizer connects to the port, the server agent

responds with an indication of the current server load and performance. This enables Equalizer to adjust the

dynamic weights of the server according to detailed performance measurements performed by the agent, based on

any metrics available on the server. If the server is overloaded and you have enabled server agent load balancing,

Equalizer reduces the server’s dynamic weight so that the server receives fewer requests. The interface between

Equalizer and server agents is simple and well-defined. Agents can be written in any language supported on the

server (e.g., perl, C, shell script, javascript, etc.). For more information see "Simple Health Checks and Load

Balancing Policies" on page 382.

For those who have one or more VMware ESX Servers,

Equalizer VLB

can be configured to use VMware’s status

reporting to determine server status, and can also be configured to automatically manage VMware servers based

on status information obtained from VMware.

Network Address Translation and Spoofing

The servers load balanced by Equalizer provide applications or services on specific IP addresses and ports, and

are organized into virtual clusters, each with its own IP address. Clients send requests to the cluster IP addresses

on Equalizer (instead of sending them to the IP addresses of the servers).

Central to the operation of any load balancer is the Network Address Translation (NAT) subsystem. On Equalizer,

NAT is used in the following ways:

1. When Equalizer receives a client packet, it

always

translates the destination IP (the cluster IP) to the IP

address of one of the server instances in a server pool. The server IP used is determined by the cluster’s

load balancing settings.

2. Depending on the setting of the cluster spoof option, Equalizer may also perform

Source NAT

, or

SNAT

When the spoof option is

enabled

, then SNAT is

disabled

: the NAT subsystem leaves the client IP address

as the source IP address in the packet it forwards to the server. For this reason, the servers in a cluster with

spoof enabled are usually configured to use Equalizer’s IP as their default gateway, to ensure that all

responses go through Equalizer (otherwise, the server would attempt to respond directly to the client IP).

When the spoof option is

disabled

, then SNAT is

enabled

. Equalizer translates the source IP (the client IP)

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