  We are interested in the following problem we call global sum. Each processor
starts with a single real value. At each time step, every directed edge in the
graph can simultaneously be used to transmit a single (bounded) number between
the processors (vertices). How many time steps s are required to ensure that
every processor acquires the global sum? We know that s is bounded below by the
diameter and above by two times the diameter. We conjecture that for vertex
symmetric graphs, s is equal to the diameter. We show this is true if the
diameter is 2.
