Domain Name System (DNS) load balancing is used for inbound link load balancing. Multiple host IP addresses are configured in DNS for a single host server name. DNS alternates the host IP address returned to a successive client host name resolution request. An advantage to this type of link load balancing is that it is a common DNS function.
Round robin DNS is a technique of load distribution, load balancing, or fault-tolerant provisioning of multiple, redundant Internet Protocol service hosts, such as web servers and FTP servers, by managing the DNS responses to address requests from client computers according to an appropriate statistical model.
In its simplest implementation Round-robin DNS works by responding to DNS requests not only with a single IP address, but with a list of IP addresses of several servers that host identical services. The order in which IP addresses from the list are returned is the basis for the term round robin. With each DNS response, the IP address sequence in the list is permuted. Usually, basic IP clients attempt connections with the first address returned from a DNS query so that on different connection attempts clients would receive service from different providers, thus distributing the overall load among servers.
There is no standard procedure for deciding which address will be used by the requesting application - a few resolvers attempt to re-order the list to give priority to numerically "closer" networks. Some desktop clients try alternate addresses after a connection timeout of 30-45 seconds.
Round robin DNS is often used for balancing the load of geographically-distributed web servers. For example, a company has one domain name and three identical websites residing on three servers with three different IP addresses. When one user accesses the home page it will be sent to the first IP address. The second user who accesses the home page will be sent to the next IP address, and the third user will be sent to the third IP address. In each case, once the IP address is given out, it goes to the end of the list. The fourth user, therefore, will be sent to the first IP address, and so on.
When PowerLink™ detects a link failure it automatically updates the DNS record for your domain so that the server requests are sent to the IP address of your alternate server or server cluster. PowerLink also provides for device failover through its active/passive failover capability. This eliminates the chance of the PowerLink being the single-point-of-failure.
Incoming bandwidth aggregation is accomplished by the PowerLink acting as the authoritative DNS server for the domain. The PowerLink advertises all available WAN links to the DNS cache servers which in turn resolve the domain names to queries in a round robin format. In this manner, all externally initiated sessions are load balanced over all available links. Since the PowerLink is resident at the domain site and is able to directly monitor the link status, failed links are removed from the DNS tables immediately upon failure. By setting the host name record Time-to-Live (TTL) to a short period, the DNS caching servers will flush their address tables and will update them from the PowerLink regularly, and thus be informed when a link fails.
FAQ terms to describe WAN optimization |
