requests may not have reached the processor in question for a couple of rounds in any event this condition represents a failure threshold triggering non exemption of time stamps from guaranteed delivery under these circumstances guaranteed delivery retries are applied to request and response time stamps messages excluding synchronization time stamp messages from retries frees system bus sbs and the processors from the overhead associated with guaranteed delivery since retries are not permitted those time stamps that are received are not subject to the delays associated with retries this makes transit time determinations more certain in addition non guaranteed messages can be communicated under circumstances in which guaranteed messages cannot this again makes transit times more determinable however where the failure rate is high synchronization time stamps can be subject to guaranteed delivery retires since imprecise synchronization is better than no synchronization the present invention provides for many variations to the illustrated embodiment in general the invention provides for systems with much greater numbers of processors where the number of processors is very large it may not be practical for each master to communicate with all other processors in such systems the invention provides for grouping processors and providing synchronization within groups and having a separate procedure for synchronizing across groups also the groups can overlap so the intra group synchronization procedure addresses all processors for example each master can have a unique group of slaves assigned to it in these larger systems request messages can be multi cast rather than broadcast in addition the invention provides for groups of processors that have commonly driven clocks for example all the processors on a multi processor integrated circuit can have clocks.