DNS Name Resolution Efficiency Improvements: Caching and Local Resolution
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The basic resolution techniquesiterative and recursivecan be considered complete from an algorithmic standpoint. By starting at the top (root) and working our way down, we are guaranteed to always eventually arrive at the server that has the information we need. I put guaranteed in quotes only because as always, there are no real guarantees in networkingwe might have asked for a non-existent name, or a server might have bad data for example. But in the absence of such atypical problems, the process leads to the information eventually.
The problem is that last word: eventually. Both iterative and recursive resolution will get us to the right server, but they take a long time to do it, especially if the name we are trying to resolve is in a deep part of the DNS hierarchy (for example, F.E.D.C.B.A.). Since resolution is done so often, it is helpful to define changes to the basic resolution process that improve efficiency as much as possible.
A computer science principle called locality of reference describes two common phenomena related to how computers (and networks) are used. The first, sometimes called spatial locality of reference, observes that a resource is more likely to be referenced if it is near another resource that was recently referenced. The second, temporal locality of reference, says a resource is more likely to be accessed if it was recently accessed.
We can observe both of these phenomenon by using the example of browsing the Web. To observe spatial locality of reference, notice what happens when you visit a site such as http://www.tcpipguide.com. The initial request asks the server for the main index document of The TCP/IP Guide. However, that document contains links to several images and other items, all of which are also located at the domain tcpipguide.com. When your browser asks for the main document, it will shortly thereafter also ask for a number of graphics. Of course, as you navigate the site, you will click links to go to other Web pages. Again here, most of these will also be at the same domain, tcpipguide.com.
What this means is that if we resolve a particular domain name, it is likely that we will need to resolve it again very soon in the future. It would be silly to have to interrogate the same domain server dozens of times, each asking it to resolve the same name.
The second phenomenon, temporal locality of reference, is one you have probably noticed yourself. You are far more likely to access a resource you have used recently than one you have not looked at in a year. This means that maintaining information about recently-used resources can be inherently advantageous.
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