Application Delivery (ADX)

Server Load Balancing 101 with ServerIron

by pmorrissey on ‎05-21-2009 03:34 PM (242 Views)

Server Load Balancing with ServerIron

Feature Brief Intro

Server Load Balancing (SLB) maps one logical (virtual) connection to multiple physical (real) servers. This allows a single IP address (VIP, virtual server IP address) to serve as the connection point for application services such as web (HTTP), FTP, SIP, DNS, VOIP, streaming media delivery etc. whilst typically leveraging multiple servers behind the VIP in load balanced fashion to provide scale, performance and availability for that service

In ServerIron terminology, Server Load Balancing (SLB) is based on associations between real servers and virtual servers. The real servers are your application servers. The virtual servers have one or more Virtual IP addresses (VIPs). You associate a real server with a virtual server by binding TCP/UDP ports on the real servers with TCP/UDP ports on the virtual server. When a client sends a TCP/UDP request for a port on the virtual server, the ServerIron ADX sends the client’s request to the real server. The client is unaware of the real servers behind the virtual server but does experience enhanced throughput and availability for TCP/UDP services.

How it works


In Figure 2.27, a company establishes a Web site with the URL of The Web site is mapped to the virtual IP address, defined on a ServerIron ADX. All inquiries made to that Web site by users on the Internet or the company's Intranet use either the URL or virtual IP address to reach the company's Web site. Once these inquiries are received at the company site, the requests are handled by one of four separate physical (real) Web servers that the system administrator has mapped to the virtual IP address. The addresses of the four physical (real) Web servers are unknown and unseen to those users who send the inquiries. The only address the users ever see for the Web site is the virtual IP address.

A ServerIron ADX establishes a virtual server that acts as a front-end to physical servers, distributing user service requests among active real servers. SLB packet processing is based on the Network Address Translation (NAT) method. Packets received by the virtual server IP address are translated into

the real physical IP address based on the configured distribution metric (for example, “round robin”) and sent to a real server. Packets returned by the real server for the end user are translated by SLB so that the source address is that of the virtual server instead of the real server. NAT translation is performed for both directions of the traffic flow.    

The ServerIron uses a parameter called the predictor to determine how to balance the client load across group of like application servers. Persistence is used to ensure a client session remains on the same real server when appropriate for that application service.