microseconds, <em>i.e.</em> the time the caller
                   will wait before the request fails;
</li><li> A triple of heartbeat specifications:
     the <em>best</em> value, <em>i.e.</em> 
          the rate at which the caller 
                   prefers to send heartbeats,
     the <em>minimum</em> rate at which the caller 
                   can accept to send heartbeats,
     the <em>maximum</em> rate at which the caller 
                   can accept to send heartbeats.
     Note that all these 