Clock Synchronization

1
Clock Synchronization
2
Clock usage in distributed system

• Make
• Code versioning
• e-mail
• Accounting and auditing

3
Physical Clocks

• UTC Coordinated Universal Time
• Atomic time
• Seconds inserted to adjust to astronomical time
• Synchronize clocks over a network
• External synchronization
• Internal synchronization

4
Clock Correctness

• Clock never goes backwards (monotonicity)
• Clock has maximum and minimum skew
• Clock drift rate is bounded
• Ci(t) ?Hi(t) ?

Software clock
Manipulate to account for drift and skew
Hardware clock
5
Synchronous vs Asynchronous DS

• Synchronous distributed system
• Time execute step has known bounds
• Message transmission time has known bounds
• Local clock drift has known bound
• Asynchronous distributed system
• No bounds on process execution speeds
• Unknown message transmission delays
• Unknown clock drift rates

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Clock Synch in Synchronous DS
Synchronize, my time is t
Set clock to t transmission time
P1
P2
7
Cristians Algorithm
What is the current time?
Start timer
P1
Time Server
Tround
t
8
How much should we adjust ps clock?
9
Berkeley Algorithm
current time 300
current time 300
current time 300
10
Berkeley Algorithm
Difference 0
current time 300
300
current time 300
current time 300
Difference 25
Difference -10
325
250
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Berkeley Algorithm
Difference 0
(025-10)/35
Change clock 5
Change clock 15
Change clock -20
Difference 25
Difference -10
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Network Time Protocol

• Reliable
• Resynchronize frequently to offset drift rate
• Protection against interference

13
Synchronization Subnet
Primary server Connected to UTC source
1
Secondary servers Synch with primary server
2
2
3
3
3
14
Synchronization Modes

• Multicast
• LAN
• Assuming small delay
• Procedure-call
• Request server for current time
• Symmetric
• Synchronization between servers

15
Symmetric Mode
Time according to B
Time according to A
t
t
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Questions (1/2)

• A distributed file server is often implemented as
  a cluster of servers. Is such a system
  synchronous or asynchronous?
• Give three applications that need clocks to be
  synchronized in a distributed system.
• For which of these is internal synchronization
  sufficient?
• How does Cristians algorithm accomplish external
  sychronization?

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Questions (2/2)

• How should we compare two clock synchronization
  algorithms?
• How does NTP use layers to keep the clocks of
  processors in the network fairly synchronized?
• How do the time servers remain synchronized with
  respect to each other when using NTP.
• How do other processors in the system remain
  synchronized with respect to each other?