On Mon, 15 May 2006 21:49:31 -0400, Marshall Eubanks <tme@multicasttech.com> 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