On Mon, 06 Aug 2007 17:21:49 -0000, John Levine said: > > >> Sounds like one of the global-scale load balancers - when you do a > >> (presumably) recursive DNS lookup of one of their hosts, they'll ping > >> the nameserver from several locations and see which one gets an > >> answer the fastest. > > Why would they ping rather than just sending the query to all of the > NS and see which one answers first? It's an IP round trip either way.
If you have sites in San Fran, London, and Tokyo, and you launch a ping from all 3 and see which one gets there first, you'll *know* the RTT from each site. If you just send DNS replies from all 3, you don't have a good way of telling which one got to the destination first. Your method works if *I* want to know which one of the 3 sites is closest (assuming I can identify an DNS server at the 3 sites). The problem of the owner of the 3 sites trying to identify which one I'm closest to isn't symmetric to it.
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