How might the clocks in two computers that are linked by a local network be synchronized without reference to an external time source? What factors limit the accuracy of the procedure you have described? How could the clocks in a large number of computers connected by the Internet be synchronized? Discuss the accuracy of that procedure.
What will be an ideal response?
Several time synchronization protocols are described in Section 10.3. One of these is Cristian’s protocol.
Briefly, the round trip time t to send a message and a reply between computer A and computer B is measured
by repeated tests; then computer A sends its clock setting T to computer B. B sets its clock to T+t/2. The setting
can be refined by repetition. The procedure is subject to inaccuracy because of contention for the use of the
local network from other computers and delays in the processing the messages in the operating systems of A
and B. For a local network, the accuracy is probably within 1 ms.
For a large number of computers, one computer should be nominated to act as the time server and it
should carry out Cristian’s protocol with all of them. The protocol can be initiated by each in turn. Additional
inaccuracies arise in the Internet because messages are delayed as they pass through switches in wider area
networks. For a wide area network the accuracy is probably within 5-10 ms. These answers do not take into
account the need for fault-tolerance. See Chapter 10 for further details.
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