Physical clock synchronization

1
Physical clock synchronization

• Question 1.
• Why is physical clock synchronization important?
• Question 2.
• With the price of atomic clocks or GPS coming
  down,
• should we care about physical clock
  synchronization?

2
Classification

• Types of Synchronization
• External Synchronization
• Internal Synchronization
• Phase Synchronization

• Types of clocks
• Unbounded 0, 1, 2, 3, . . .
• Bounded 0,1, 2, . . . M-1, 0, 1, . . .

Unbounded clocks are not realistic, but are
easier to deal with in the design of
algorithms. Real clocks are always bounded.
3
Terminologies

• What are these?
• Drift rate ?
• Clock skew ?
• Resynchronization interval R
• Max drift rate ? implies
• (1- ?) dC/dt lt (1 ?)
• Challenges
• (Drift is unavoidable)
• Accounting for propagation delay
• Accounting for processing delay
• Faulty clocks

4
Internal synchronization

• Step 1. Read every clock in the system.
• Step 2. Discard outliers and substitute them by
  the value of the local clock.
• Step 3. Update the clock using the average of
  these values.
• Resynchronization interval will depend on the
  drift rate.

• Berkeley Algorithm
• A simple averaging algorithm
• that guarantees mutual
• consistency c(i) - c(j) lt ?

5
Internal synchronization

• Lamport and Melliar-Smiths
• averaging algorithm handles
• byzantine clocks too

• Assume n clocks, at most t are faulty
• Step 1. Read every clock in the system.
• Step 2. Discard outliers and substitute them by
  the value of the local clock.
• Step 3. Update the clock using the average of
  these values.
• Synchronization is maintained if n gt 3t
• Why?

Bad clock
A faulty clocks exhibits 2-faced or byzantine
behavior
6
Internal synchronization

• Lamport Melliar-Smiths algorithm (continued)

• The maximum difference between
• the averages computed by two
• non-faulty nodes is (3td / n)
• To keep the clocks synchronized,
• 3td / n lt d
• So, 3t lt n

k
B a d c l o c k s
7
Cristians method
External Synchronization

• Client pulls data from a time server
• every R unit of time, where R lt ? / 2?.
• (why?)
• For accuracy, clients must compute
• the round trip time (RTT), and
• compensate for this delay
• while adjusting their own clocks.
• (Too large RTTs are rejected)

Time server
8
Network Time Protocol (NTP)

• Tiered architecture

• Broadcast mode
• - least accurate
• Procedure call
• - medium accuracy
• Peer-to-peer mode
• - upper level servers use this for max accuracy

Time server
Level 0
Level 1
Level 1
Level 1
Level 2
Level 2
Level 2
The tree can reconfigure itself if some node
fails.
9
P2P mode of NTP

• Let Qs time be ahead of Ps time by ?. Then
• T2 T1 TPQ ?
• T4 T3 TQP - ?
• y TPQ TQP T2 T4 -T1 -T3 (RTT)
• ? (T2 -T4 -T1 T3) / 2 - (TPQ - TQP) / 2

T2
T3
Q
P
T1
T4
x
Between y/2 and -y/2
So, x- y/2 ? x y/2
Ping several times, and obtain the smallest value
of y. Use it to calculate ?
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Problems with Clock adjustment
1. What problems can occur when a clock value
is Advanced from 171 to 174? 2. What problems
can occur when a clock value is Moved back from
180 to 175?
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Mutual Exclusion
CS
p0
CS
p1
CS
p2
CS
p3
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Why mutual exclusion?

• Some applications are
• Resource sharing
• Avoiding concurrent update on shared data
• Controlling the grain of atomicity
• Medium Access Control in Ethernet
• Collision avoidance in wireless broadcasts

13
Specifications

• ME1. At most one process in the CS. (Safety
  property)
• ME2. No deadlock. (Safety property)
• ME3. Every process trying to enter its CS must
  eventually succeed.
• This is called progress. (Liveness property)
• Progress is quantified by the criterion of
  bounded waiting. It measures
• a form of fairness by answering the question
• Between two consecutive CS trips by one process,
  how many times
• other processes can enter the CS?
• There are many solutions, both on the shared
  memory model and the
• message-passing model

14
Message passing solutionCentralized decision
making
Client do true ? send request wait until a
reply is received enter critical section
(CS) send release ltnon-CS activitiesgt od
server

busy boolean
queue
release
Server do request received and not busy ? send
reply busy true request received and busy
? enqueue sender release received and queue
is empty ? busy false release received and
queue not empty ? send reply to the head of the
queue od
req
reply
clients
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Comments

• - Centralized solution is simple.
• - But the server is a single point of failure.
  This is BAD.
• - ME1-ME3 is satisfied, but FIFO fairness is not
  guaranteed. Why?
• Can we do better? Yes!

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Decentralized solution 1Lamports algorithm

• Life of each process
• 1. Broadcast a timestamped request to all.
• Request received ? enqueue sender in local Q.
• Not in CS ? send ack
• In CS ? postpone sending ack (until exit from
  CS).
• 3. Enter CS, when
• (i) You are at the head of your own local Q
• (ii) You have received ack from all processes
• 4. To exit from the CS,
• (i) Delete the request from Q, and
• (ii) Broadcast a timestamped release
• 5. Release received ? remove sender from local Q.

Completely connected topology
Can you show that it satisfies all the
properties (i.e. ME1, ME2, ME3) of a correct
solution?