Hello is a routing protocol which was designed and
implemented in a experimental software router called a
"Fuzzball" which runs on a PDP-11. It does not have
wide usage, but is the routing protocol currently used
on the NSFnet backbone. The data transferred between
nodes is similar to RIP (a list of networks and their
metrics). The metric, however, is milliseconds of delay.
This allows Hello to be used over nets of various link
speeds and performs better in congestive situations.
One of the most interesting side effects of Hello based
networks is their great timekeeping ability. If you
consider the problem of measuring delay on a link for
the metric, you find that it is not an easy thing to
do. You cannot measure round trip time since the
return link may be more congested, of a different
speed, or even not there. It is not really feasible
for each node on the network to have a builtin WWV
(nationwide radio time standard) receiver. So, you
must design an algorithm to pass around time between
nodes over the network links where the delay in
transmission can only be approximated. Hello routers
do this and in a nationwide network maintain synchronized
time within milliseconds.