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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Geo location to IP mapping
On Mon, 15 May 2006 21:49:31 -0400, Marshall Eubanks <[email protected]> wrote: > > I seriously doubt this would work to better than the regional area. > > My zip code (20124) region is about 5 km across, which would be 15 > microseconds in vacuum, and > maybe at most 50 micro seconds in glass. So, you would need > accuracies at the 10's of microsecond level to specify zip codes. > > I can believe that you can measure transmission times down a fiber > and achieve repeatability at the microsecond level - in fact, I > remember a Michelson interferometer that they set up at JPL / > Goldstone that tested > the Sagnac effect in glass, which required substantially better > repeatibility than that. > > But do you really think that you can estimate the router delay on the > (for example) 9 hops between here and GMU > to better than 1 millisecond each ? (That would imply a 3 millisecond > rms error if these errors were random and Gaussian, or about 1000 km > in vacuum, and maybe 500 km error in glass.) > > So, I think that this would fail by at least 2 orders of magnitude for > zip codes in a real operational network. Which coast of the US, sure, > but not much better than that. I suspect you can do that; a bigger factor is the link type of the last hop. Cable modems, DSL, 802.11 -- they all have characteristic delays. The important insight is that you care about *minimum* time. You can lots of queueing delays and jitter most of the time, as long as you get one packet through unobstructed. Send enough probes and you'll make it. I did some similar work in 1992; see http://www.cs.columbia.edu/~smb/papers/netmeas.pdf for details. You couldn't repeat, today, exactly what I did then, because of the way pings are handled by modern routers, but I suspect one could find analogous schemes. To give one example of what I could tell -- and I was looking at the per-byte cost -- I was able to determine, from New Jersey, that a router outside Chicago was misconfigured; the site's backbone Ethernet should have been on the same card as the serial line (in the days of T-1 interfaces...), because copying the packet across the backplane introduced a noticeable per-byte delay. --Steven M. Bellovin, http://www.cs.columbia.edu/~smb
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