What is NTP server? How it works? What are NTP server recommendations, limitations and considerations?

1. NTP ServerHow it works?

2. Time Reference:
• Cesium clock
• GPS (Global Pointing System)
• DCF77
• MSF
• WWV
• Local Time of Machine
Stratum:
• Quality level of time reference.
• Maximum is 16 level – Maximum Stratum number is 15 and
Stratum increase at each level.
• Stratum 0 is a machine with highest possible quality.
• Stratum 1 is the best available NTP Server.
Cesium
Clock
*GPS Cesium
Clock
Stratum 0
Stratum 1

3. NTP Client NTP Server
UDP Protocol
Port 123
Receive Timestamp
Send Timestamp
Client Connection Server
Timestamp 1 Timestamp 2
Timestamp 4 Timestamp 3
1
2
Offset = ((T2 – T1) + (T3 – T4)) / 2
Delay = (T4 – T1) – (T3 – T2)

4. Poll Time From Reference Clock
Default Between 64s to 1024s
Poll Interval Dynamically Detected By
NTP (Minimal Noise)
MinPoll Will be Selected as Poll
Interval After Service Restart
Client Send Request (Packet)
Client Stores Own Time into Packet
(Originate Timestamp)
Server Receive Request (Packet)
Server Stores Own Time into Packet
(Receive Timestamp)
Reference Clock
Provide Current Time
Server Send The Response (Packet)
Server Store Own Time into Packet
(Transmit Timestamp)
Client Receive The Response (Packet)
Estimating Delay (The Total Delay
Minus Remote Processing Time) and
Offset (Time Difference)
Client and Server Exchange Packets
Several Times
Estimating Time By NTP
Compute Quality of The Time
Using offset (or phase), jitter (or
dispersion), frequency error, and
stability
Agreeing Servers Marking as Valid
Reference Time
(True-Chimers)
Other Servers Marking as Invalid
(False-Tickers)

5. Considerations
Network
NTP Client NTP Server
Poor Network
• Poor Network Connection => Large Delay => Increase Offset >
128ms is not acceptable
• Offset > 1s is not acceptable – Client will mark NTP server
as invalid peer and use local time as source.
• Offset > 128ms => Slewed (adjusted slowly) Or Stepped (set
anew)
• Clock will continue with the last corrections effective.
• Huge Offset Rejection => Terminate NTPD
• NTP Server will not be accessible by all clients because of
bandwidth limitation.
Unexpected Clock Changes
NTP Client NTP Server
Clock Synchronization
• NTP has no special method to handle this situation

6. Considerations
Deployment
• Never Use Poor Devices with Poor Network Connection
• At Least 4 NTP Servers (Best Practices)
• N+1 Servers (Recommended)
• More servers get help to clients to find “False-Tickers” and “True-
Chimers”, also improve time accuracy.
• Never Use Virtual Machine
• Put NTP Servers and NTP Clients in Same Subnet.
• Less Hope Between NTP Servers and NTP Clients (Recommended)
NTP Client
NTP Server NTP Server NTP Server NTP Server