[Roll] numeric ranges in routing metrics

[edward.j.beroset at us.elster.com]

I mentioned it during the roll meeting
Tuesday, but I think it's probably worth documenting the thought in a little
more technical detail.  The routing metrics right now include 32-bit
floating point numbers for latency and throughput.  Since these numbers
are much more likely to be added or compared rather than multiplied, fixed
point rather than floating point is likely to be a better choice of representation.
 For example, for normalized positive 32-bit IEEE 754 numbers, the
maximum number is about 3.4e38 and the minimum is about 1.2e-38.  For
reference, the age of universe in milliseconds (taking the high end of
NASA's most recent estimate) is only 4.4e20 and the time it takes to travel
the distance across an electron (Lorentz diameter for you physics heads)
is about 1.9e-20 ms.  A negative latency would seem to imply that
packets appear before they're sent which is probably not practical.  If
instead, latency were a 32-bit unsigned integer in units of milliseconds,
we'd still be able to go from 0 or 1 milliseconds up to almost 1200 hours
if my math is right.  That should be plenty for any practical network.
 I haven't done so but I suspect that we could do the same thing with
throughput.

Ed Beroset