Three computers, A, B, and C communicate using a protocol that implements Lamport logical clocks (they include their clock time stamp in messages). At the beginning of time, all three computers begin with their
logical clock set to zero. Later, the following sequence of events occurs:
A sends message M1 to B
C sends message M2 to B
B receives M2 before M1
B replies message M3 to C first, and then message M4 to A
After receiving message M3, C sends message M5 to A
After receiving message M5, A sends message M6 to B
After receiving message M6, B sends message M7 to A
The last message A receive is M4
1. Indicate the time included with the messages as they are sent at each step.
Send(M1, _ )
Send(M2, _ )
Send(M3, _ )
Send(M4, _ )
Send(M5, _ )
Send(M6, _ )
Send(M7, _ )
2. After all of these messages have been sent and received, what time (Lamport logical clocks)
does each computer think it is?
3. Name any one pair of causally connected events correctly identified by Lamport scalar clocks.
4. Can B sending M4 and C sending M5 be identified as concurrent events using Lamport’s scalar
clock? Explain why.
a. Yes
b. No