This layer does not actually run any protocol. Its purpose is to provide the UGI interface over other low-level communication interfaces; in the example above, atm is a parameter to COM causing COM to connect itself to an ATM device interface. COM currently supports interfaces to IP, UDP (with or without broadcast), the Deering multicast extensions of IP and UDP, ATM, MACH messages, the x-kernel interfaces, and a network simulator which we developed. The default configuration runs over UDP. The COM layer also keeps track of byte-ordering and maintains some low-level message logging event-charts depicting which events happened where and in what order, both of which are useful for debugging and reproducing events that lead to a software failure. Note that COM may also run over reliable communication layers, in which case no additional protocols need be run.