Monday, May 14, 2018

NTP Time Synchronization―All Things Considered

In this article, we will cover the following topics:
  1. Importance of time synchronization
  2. Needs for NTP time synchronization
  3. How to synchronize time
  4. How to verify synchronized time

Importance of Time Synchronization

Time synchronization plays an important role in cloud computing:
As single systems have been replaced by multiple, loosely-coupled systems, this need has evolved into a requirement for both accurate and consistent clocks among these systems. The need for millisecond or microsecond timestamp accuracy derives from the need to execute transactions in a correct sequence, particularly if many transactions occur almost simultaneously. 
You can read [1] for more details.

Needs for NTP Time Synchronization

Each of your network of servers has its own clock.  Computer clocks are notorious for drifting. They are typically based on inexpensive oscillator circuits or battery backed quartz crystals that can easily drift seconds and minutes per day, accumulating significant errors over time. That is why most enterprise networks today rely on network time servers that acquire time from the Global Navigation Satellite System (GNSS) and distribute it to clients over a network through the Network Time Protocol (NTP).

Timing accuracy from NTP servers depends upon the accuracy of the source and the precision with which all operations and applications are synchronized.

NTP Time Synchronization via utpupdate

ntpdate can be run manually as necessary to set the host clock, or it can be run from the host startup script to set the clock at boot time.

This is useful in some cases to set the clock initially before starting the NTP daemon ntpd. It is also possible to run ntpdate from a cron script. However, it is important to note that ntpdate with contrived cron scripts is no substitute for the NTP daemon, which uses sophisticated algorithms to maximize accuracy and reliability while minimizing resource use. Finally, since ntpdate does not discipline the host clock frequency as does ntpd, the accuracy using ntpdate is limited.

# /usr/sbin/ntpdate -u
14 May 09:57:05 ntpdate[2890]: step time server offset 30.442100 sec

NTP Time Synchronization via NTP Daemon

Depending on your Linux systems, for example, below commands are used to update NTP server configuration file and restart ntpd in a local Oracle Linux system.

$ sudo bash

bash-3.2# vi /etc/ntp.conf
bash-3.2# /etc/init.d/ntpd restart
Shutting down ntpd:                                        [  OK  ]
ntpd: Synchronizing with time server:                      [  OK  ]
Starting ntpd:                                             [  OK  ]

Verification of Synchronized Time

There are multiple ways of verifying the results of NTP time synchronization:
  • ntpstat
    • ntpstat will report the synchronisation state of the NTP daemon running on the local machine.  
    • If the local system is found to be  synchronised to  a  reference  time source, ntpstat will report the approximate time accuracy.
  • date -R
    •  -R, --rfc-2822
      • output date and time in RFC 2822 format
  • /usr/sbin/ntpq -p
    • The  ntpq utility program is used to monitor NTP daemon ntpd operations and determine performance.
    • -p 
      • Print a list of the peers known to the server as well as a summary of their state. This is equivalent to the  peers  interactive command.


bash-3.2# ntpstat
  time server re-starting
   polling server every 64 s

bash-3.2# ntpstat
synchronised to NTP server ( at stratum 4
   time correct to within 31 ms
   polling server every 64 s


bash-4.1# date -R
Mon, 14 May 2018 08:17:33 -0700

/usr/sbin/ntpq -p

bash-4.1# /usr/sbin/ntpq -p

     remote           refid      st t when poll reach   delay   offset  jitter
*ucf-c2z3-opc-di       3 u   20   64  377    0.971    0.105   0.057
 LOCAL(0)        .LOCL.          10 l    -   64    0    0.000    0.000   0.000

remotehost name or IP number of peer
refidassociation ID or kiss code
stpeer status word
tu: unicast, b: broadcast, l: local
whensec/min/hr since last received packet
pollpoll interval (log2 s)
reachreach shift register (octal)
delayroundtrip delay


  1. The Importance of Network Time Synchronization for Enterprise Solutions Whitepaper
  2. Network Time Protocol (Version 3) Specification, Implementation and Analysis
  3. ntpq - standard NTP query program
  4. NTP Time Synchronization

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