A different proposal for XO upgrade.  C. Scott Ananian cscott at
laptop.org Mon Jun 25 17:51:19 EDT 2007  As food for discussion,
here's a counter proposal for an XO upgrade mechanism, focusing
on the network side. Design goals:  - minimize round-trips
necessary for successful upgrade  - minimize size of upgrades
Software versions are assigned sequential integers.  Version 1,
2, etc.  We assume that every machine should discover an
available upgrade within a day, on average.  The constants below
can be tweaked to adjust this interval.  Upgrades consist of a
number of messages U_0 through U_N with a maximum size Umax.
The maximum size is chosen either so that a) each U_i fits in a
single network packet, or b) each U_i can fits into a reserved
storage area on the XO (to allow it to be shared with
neighbors).  Upgrade messages are independent.  It may be
worthwhile for upgrade messages to be applied in any order, but
for simplicity we'll constrain the order of application to be
sequential.  U_i is authenticate