Load balancing for high availability

To protect against a complete site outage, Global Server Load Balancing (GSLB) can transparently direct customers to other Web sites. Further, GSLB helps provide customers with faster Web response times by directing them to the nearest site. Typically, GSLB works within the framework of Domain Name System (DNS) to direct customers to the best site. When a customer requests a page such as www.nwfusion.com, for example, the browser must first find the IP address for Network World's Web site. The browser goes to a local DNS server (provided by the ISP or network administrator), which finds the authoritative DNS server for Network World's Web site. A DNS server is considered authoritative for a particular zone if it is designated by that domain's network administrator.

When a DNS look-up request is received by the authoritative DNS server, it replies with one (or more) IP addresses for the requested Web site. Basic load balancing is accomplished by having the authoritative DNS server perform round robin on a list of Web site IP addresses. The authoritative DNS server has no knowledge of site availability or the load on sites. It does not understand whether a customer is closer to one site or another in terms of Internet response time.

At a high level, GSLB involves two tasks interoperating within the DNS framework to provide an intelligent DNS response, and selects the best site. A Web switch provides GSLB within a DNS framework in any of the following ways:

The Web switch acts as the authoritative DNS server for a specified domain.

The Web switch acts as a forward proxy to an existing DNS server. A virtual IP address on the Web switch is registered as the authoritative DNS server. The Web switch load balances all DNS traffic to the real DNS servers. The Web switch also modifies replies from the DNS servers, picks the best site and forwards it to the client - useful if you have one or more DNS servers and would like to load balance DNS traffic for scalability, yet provide GSLB at the same time.

The Web switch acts as a transparent DNS server proxy. When the Web switch is deployed in the DNS server traffic path, it intercepts responses from the authoritative DNS server to select the best site - useful if you don't want to disrupt to your DNS configuration or if the authoritative DNS server is not in your control.

Web switches use information about sites to determine the best site for each customer and provide an appropriate response. Web switches use several metrics to determine the best site. They:

Perform site health checks to determine if it is up and running.

Examine site load conditions to ensure it is capable of accommodating new customer requests.

Measure the proximity (Internet response time) from the customer to each site and direct customers to the site with the best response time.

Use static preferences to direct certain branch-office customers to a specific data center.

Because the local DNS server caches the response from the authoritative DNS server, Web switches also set the time-to-live parameter to a user-configurable value. This ensures the freshness of DNS server response data. To provide redundancy for the authoritative DNS server, multiple authoritative DNS servers can be deployed.

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Kopparapu is a product marketing manager at Foundry Networks, a maker of Web switches, Internet routers and Layer 3 switches located in San Jose. He can be reached at Chandra@foundrynet.com.

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