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This Concept Map, created with IHMC CmapTools, has information related to: ch11 time sync, synchronizing physical clocks has Internal synchronization in a synchronous system One process p1 sends its local time t to process p2 in a message m, p2 could set its clock to t + Ttrans where Ttrans is the time to transmit m Ttrans is unknown but min ≤ Ttrans ≤ max uncertainty u = max-min. Set clock to t + (max - min)/2 then skew ≤ u/2, synchronizing physical clocks has a synchronous distributed system is one in which the following bounds are defined (ch. 2 p. 50): the time to execute each step of a process has known lower and upper bounds each message transmitted over a channel is received within a known bounded time each process has a local clock whose drift rate from real time has a known bound, synchronizing physical clocks has Berkeley algorithm Cristian’s algorithm - a single time server might fail, so they suggest the use of a group of synchronized servers it does not deal with faulty servers Berkeley algorithm (also 1989) An algorithm for internal synchronization of a group of computers A master polls to collect clock values from the others (slaves) The master uses round trip times to estimate the slaves’ clock values It takes an average (eliminating any above some average round trip time or with faulty clocks) It sends the required adjustment to the slaves (better than sending the time which depends on the round trip time) Measurements 15 computers, clock synchronization 20-25 millisecs drift rate < 2x10-5 If master fails, can elect a new master to take over (not in bounded time), synchronizing physical clocks has NTP - synchronisation of servers The synchronization subnet can reconfigure if failures occur, e.g. a primary that loses its UTC source can become a secondary a secondary that loses its primary can use another primary Modes of synchronization: Multicast A server within a high speed LAN multicasts time to others which set clocks assuming some delay (not very accurate) Procedure call A server accepts requests from other computers (like Cristian's algorithm). Higher accuracy. Useful if no hardware multicast. Symmetric Pairs of servers exchange messages containing time information Used where very high accuracies are needed (e.g. for higher levels), synchronizing physical clocks can have potential problems using a single time server, synchronizing physical clocks has A hardware clock, H is said to be correct if its drift rate is within a bound p > 0. (e.g. 10-6 secs/ sec) This means that the error in measuring the interval between real times t and t’ is bounded: Which forbids jumps in time readings of hardware clocks Weaker condition of monotonicity e.g. required by Unix make can achieve monotonicity with a hardware clock that runs fast by adjusting the values a faulty clock is one that does not obey its correctness condition crash failure - a clock stops ticking arbitrary failure - any other failure e.g. jumps in time, synchronizing physical clocks has External synchronization A computer’s clock Ci is synchronized with an external authoritative time source S, so that: |S(t) - Ci(t)| < D for i = 1, 2, … N over an interval, I of real time The clocks Ci are accurate to within the bound D. Internal synchronization The clocks of a pair of computers are synchronized with one another so that: | Ci(t) - Cj(t)| < D for i = 1, 2, … N over an interval, I of real time The clocks Ci and Cj agree within the bound D. Internally synchronized clocks are not necessarily externally synchronized, as they may drift collectively if the set of processes P is synchronized externally within a bound D, it is also internally synchronized within bound 2D, NTP - synchronisation of servers The synchronization subnet can reconfigure if failures occur, e.g. a primary that loses its UTC source can become a secondary a secondary that loses its primary can use another primary Modes of synchronization: Multicast A server within a high speed LAN multicasts time to others which set clocks assuming some delay (not very accurate) Procedure call A server accepts requests from other computers (like Cristian's algorithm). Higher accuracy. Useful if no hardware multicast. Symmetric Pairs of servers exchange messages containing time information Used where very high accuracies are needed (e.g. for higher levels) has Messages exchanged between a pair of NTP peers All modes use UDP Each message bears timestamps of recent events: Local times of Send and Receive of previous message Local times of Send of current message Recipient notes the time of receipt Ti ( we have Ti-3, Ti-2, Ti-1, Ti) In symmetric mode there can be a non-negligible delay between messages