Download presentation

Presentation is loading. Please wait.

Published byCiera Mattison Modified over 2 years ago

1
Clock Synchronization

2
Problem 5:33 5:57 5:20 4:53 6:01

3
Why is this hard? Hard to set the clocks simultaneously Hardware clocks have a small drift rate Network messages have random delay This delay is unbounded Various types of faults are possible: –Process failure –Communication failure

4
Making it Tractable Clocks have higher resolution than needed e.g. clocks measure microseconds, but we only care about milliseconds Drift rate ρ of clock H is known ρ is very small Small bounded error in measurement allowable: (1 - ρ)(t - t’) ≤ H(t) - H(t’) ≤ (1 + ρ)(t - t’) A message is on-time if sent and delivered within maxp time units Minimum transmission time min is known

5
Reading a Remote Clock 5:305:33 Real time P Q 5:31 P sends “Time = ?” to Q

6
Reading a Remote Clock 5:305:33 Real time P Q 5:31 5:355:41 5:37 Q sends “Time = 5:41” to P P sends “Time = ?” to Q Q receives “Time = ?” from P

7
Reading a Remote Clock 5:305:33 Real time P Q 5:31 5:355:40 5:37 P sends “Time = ?” to Q 5:425:46 5:44 P receives “Time = 5:41” from Q Q sends “Time = 5:41” to P Q receives “Time = ?” from P

8
Reading a Remote Clock 5:305:33 Real time P Q 5:31 5:355:40 5:37 P sends “Time = ?” to Q 5:425:46 5:44 P receives “Time = 5:41” from Q Q sends “Time = 5:41” to P Q receives “Time = ?” from P P measures the roundtrip delay to be 12 = 2D

9
What does P know about Q? P sends “Time = ?” to Q at P(t) P receives “Time = T” from Q at P(t’) Roundtrip delay = P(t’) – P(t) = 2D Drift rate ρ for P and Q Minimum message travel time min At P(t’), Q(t’) in interval [T + min(1 - ρ), T + 2D(1 + 2ρ) - min(1 + ρ)]

10
What does P know about Q? P sends “Time = ?” to Q at P(t) = 5:30 P receives “Time = 5:41” from Q at P(t’) = 5:42 Roundtrip delay = 12 minutes = 2D Drift rate ρ = 0.25 for P and Q Minimum message travel time min = 4 minutes At P(t’) = 5:42, Q(t’) in interval [T + min(1 - ρ), T + 2D(1 + 2ρ) - min(1 + ρ)] = ?

11
What does P know about Q? P sends “Time = ?” to Q at P(t) = 5:30 P receives “Time = 5:41” from Q at P(t’) = 5:42 Roundtrip delay = 12 minutes = 2D Drift rate ρ = 0.25 for P and Q Minimum message travel time min = 4 minutes At P(t’) = 5:42, Q(t’) in interval [5:41 + 4(1–.25), 5:41 + 12(1 +.5) - 4(1 +.25)] = [5:44, 5:54]

12
P’s Best Guess For Q(t’) At P(t’), Q(t’) in interval [T + min(1 - ρ), T + 2D(1 + 2ρ) - min(1 + ρ)] In example: [5:44, 5:54] To P, Q(t’) could be anywhere in the interval What should P guess that Q(t’) is equal to?

13
P’s Best Guess For Q(t’) At P(t’), Q(t’) in interval [T + min(1 - ρ), T + 2D(1 + 2ρ) - min(1 + ρ)] In example: [5:44, 5:54] To P, Q(t’) could be anywhere in the interval What should P guess that Q(t’) is equal to? The midpoint! Q(t’) = T + D(1 + 2ρ) - minρ Q(t’) = 5:41 + 6(1 +.5) – 4(.25) = 5:49

14
How Precise is the Measurement? Max error e = D(1 + 2ρ) - min P wants to know Q(t’) within error ε Only guaranteed if D ≤ U where U = (1 - 2ρ)(ε + min) Then P reaches rapport with Q Pick ε such that U > min(1+ ρ) If D > U then need to try again

15
Time Service 5:33 5:57 5:20 4:53 6:01 Q P2 P3P1 Master Slave P4 Slave

16
Time Service 5:33 5:57 5:20 4:53 6:01 Q P2 P3P1 Master Slave P4 Slave All slaves try to maintain rapport with Q

17
How Should P Change it’s Clock? At P(t’) = 5:42, P estimates Q(t’) = 5:49 What should P do?

18
How Should P Change it’s Clock? At P(t’) = 5:42, P estimates Q(t’) = 5:49 What should P do? Gradually adjust clock to eventually match Q Let P(t) = H(t) + A(t) H(t) is the value of the hardware clock A(t) = mH(t) + N is an adjustment to P’s clock Set m and N so P(t’ + x) = Q(t’ + x) x is the amortization parameter m = (Q(t’) - P(t’)) / x N = P(t’) - (1 + m)×H(t’)

19
Master-Slave Synchronization Slave tries up to k syncs with master Remember: rapport means D ≤ U Sync retry every W time units until rapport W > 2U At rapport, gradually adjust according to P(t) = H(t) + A(t) Slave knows it is out of sync after k tries

20
Importance Reliable Clock Synchronization required for: –Synchronous Atomic Broadcast http://www.cs.utexas.edu/~schrum2/cs386c/sab.php –Processor Group Membership Agreement Any distributed system needs accurate Clock Synchronization

Similar presentations

© 2017 SlidePlayer.com Inc.

All rights reserved.

Ads by Google

Ppt on earthquake for class 9 Microsoft office ppt online shopping A ppt on business plan Ppt on two point perspective photography Ppt on physical layer of osi model Free download ppt on solar system Topics for ppt on environment Ppt on solid dielectrics inc Ppt on social media strategy Free ppt on teachers day