NTP

Question 1

What is the purpose of NTP?

Answer 1

Enables clients across the Internet to be synchronised accurately to UTC

Question 2

What makes NTP reliable?

Answer 2

he use of redundant servers, redundant

paths between them

Question 3

How does NTP offset the drift rates found in most computers?

Answer 3

NTP enables clients to resynchronise sufficiently frequently

Question 4

What is the NTP logical hierarchy/subnet?

Answer 4

Network of servers located across the Internet

Primary are connected directly to a UTC source, secondary are synchronised with primary servers

Question 5

How can the Subnet deal with failures?

Answer 5

Subnet can reconfigure to deal with failures e.g. unreachability (e.g. if UTC source fails; if secondary server unreachable)

Question 6

Give an example synchronisation subnet in an NTP implementation diagram

Answer 6

1
   /   \
  2    2
 /\      /\
n  n  n  n

Question 7

State the three modes of synchronisation between NTP servers

Answer 7

1. Multicast: intended for use on a high-speed LAN
2. Procedure-call
3. Symmetric message exchange between servers

Question 8

Which of the three modes of synchronisation between NTP servers is most accurate?

Answer 8

Symmetric

Question 9

Describe the Multicast NTP mode of synchronisation and comment on accuracy

Answer 9

 Server periodically multicasts time to servers running in other computers which set their clocks assuming a small delay
 Can only achieve relatively low accuracies

Question 10

Describe the Procedure-call NTP mode of synchronisation and comment on accuracy

Answer 10

suitable for achieving higher accuracies ( Similar to Cristian’s alg.)
Server accepts requests from other computers; replies with its timestamp (current clock reading)

Question 11

Describe the Symmetricl NTP mode of synchronisation and comment on accuracy

Answer 11

Symmetric: message exchange between servers that supply time information in LANs and by higher levels of the synchronisation subnet.

most accurate mode

Pair of servers exchange messages bearing timing information
Timing data retained in order to improve accuracy of synchronisation over time

Question 12

Processes exchange pairs of (UDP) messages: each message carries three timestamps. Name them

Answer 12

♦ Transmission time of previous message to correspondent
♦ Receipt time of previous response from correspondent
♦ Transmission time of this message to correspondent