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Lecture 10: Clocks and Time • Time service Overview – requirements and problems – sources of time • Clock synchronisation algorithms – clock skew & drift – Cristian algorithm – Berkeley algorithm – Network Time Protocol • Logical clocks – Lamport’s timestamps • Why needed? Time service – to measure delays between distributed components – to synchronise streams, e.g. sound and video – to establish event ordering • causal ordering (did A happen before B?) • ...