How the round trip time(RTT) algorithm works.

While using more than one Forwarders, Infoblox just like BIND Name Servers uses a metric called Round Trip Time, or RTT, to choose among the Configured Servers to Forward to. Roundtrip time is a measurement of how long a remote Name Server takes to respond to Queries.

Each time a BIND Name Server sends a query to a remote server, it starts an internal stopwatch. When it receives a response, it stops the stopwatch and makes a note of how long that remote Server took to respond. When the Name Server must choose which of a group of Authoritative Name Servers / Forwarders to query, it simply chooses the one with the lowest Roundtrip time.

Before a BIND Name Server has queried a nameserver, it gives it a random Roundtrip time value lower than any real-world value. This ensures that the BIND Name Server queries all nameservers authoritative for a given zone in a random order before playing favorites.

On the whole, this simple but elegant algorithm allows BIND Name Server to “lock on” to the closest nameservers quickly and without the overhead of an out-of-band mechanism to measure performance.

To give you a clearer picture, here’s how it works

Initially, each forwarder’s RTT is seeded with a random, low value.

When the recursive name server needs to forward a query, it chooses the forwarder with the lowest RTT.

When it sends a query to the chosen forwarder, it starts an internal timer. When it receives a response, it stops the timer.

If all the recursive name server has is the seeded value for the chosen forwarder, it replaces that value with the value from the timer.

If the recursive name server has a real RTT based on previous responses, it updates the RTT based on the timer’s value: new RTT = (.7 * old RTT) + (.3 *timer).

Forwarders that aren’t selected have their RTT values “decayed” by multiplying them by .98. This enables all the configured Forwarders to eventually get their turn.

In short, No. Forwarders are not always used in the Order in which they are configured. It is actually based on RTT Values.