Logical Clock or Lamport's Clock is a basic protocol that gives a notion of virutal time to all processes. Lamport's clock can be used to totally order all events in a distributed system. This is done by assigning timestamps to each event in a way that is consistent with the happened-before relation. Therefore, the following property should always hold.


Where C(i) is the timestamp for event i.
Note the implication is one way.

The protocol is defined as follows:

For process Pi
* On executing an interval event:
    Ci := Ci + 1

* On sending a message m:
    Ci := Ci + 1
    piggyback Ci on m

* On receive a message m:
    let tm be the timestamp piggybacked on m
    Ci := max{Ci, tm} + 1

Note that logical clocks cannot be used to determine two events are concurrent. 

e || f does not imply C(e) = C(f)