free web hit counter

What is Network Time Protocol (NTP) in networking?

June 7, 2018
Print Friendly, PDF & Email

Network Time Protocol (NTP) is very important for modern networks. If the time across network devices is not accurate, this can have a direct impact on network security and network management, just to name two. The key to accurate time in the network is to have your devices receive the correct time from an atomic clock, and then synchronize this correct time across all of the devices. Network Time Protocol ensures this process is automated and secure. This article serves as your introduction to this critical network function.

NTP is a protocol designed to synchronize the clocks of computers over a network. Network Time Protocol (NTP) provides a mechanism to synchronize time throughout the network. An NTP device will form an association with NTP devices closer to the time source. NTP devices use a special measurement, called a stratum, to determine how far they are away from the time source.

Basic Overview of the NTP

NTP stands for Network Time Protocol, and it is an Internet protocol used to synchronize the clocks of computers to sometime reference. NTP is an Internet standard protocol originally developed by Professor David L. Mills at the University of Delaware.

The NTP system is based on Internet time servers, computers with access to atomic clocks such as those operated by the U.S. government. These NTP servers run a software service that provides the clock’s time of day to client computers over UDP port 123. NTP supports a hierarchy of multiple server levels to handle a large load of client requests. The protocol includes algorithms to accurately adjust the time of day being reported to account for Internet network transmission delays.

NTP uses a systematic, hierarchical level of clock sources for its reference. Each level is called a stratum and has a layer number that usually begins with zero. The stratum level serves as an indicator of the distance from the reference clock in order to avoid cyclic dependence in the hierarchy. However, the stratum does not represent the quality or reliability of time.

Network time protocol

Figure: Network Time Protocol

Some of the advantages of using NTP include:

  • NTP can be easily deployed on servers hosting different services.
  • NTP requires less resource overhead.
  • NTP has minimal bandwidth requirements.
  • NTP can handle hundreds of clients at a time with minimum CPU usage.

NTP support has now been extended to UNIX-like systems, and NTPv4 can be implemented on Windows NT, Windows 2000, XP, Vista and Windows 7.

NTP Modes of Operation

NTP synchronizes timekeeping between a group of distributed time servers and clients. This synchronization allows events to be correlated when system logs are created and other time-specific events occur. NTP operates in four different modes.

  • Server Modeis configured such that a device will synchronize NTP clients. Servers can be configured to synchronize all clients or only a specific group of clients. NTP servers, however, will not accept synchronization information from their clients. This restriction will not allow clients to update or manipulate a server’s time settings.
  • Client Modeis configured used to allow a device to set its clock by and synchronized by an external timeserver. NTP clients can be configured to use multiple servers to set their local time and can be configured to give preference to the most accurate time sources available to them. They will not, however, provide synchronization services to any other devices.
  • Peer Modeis when one NTP-enabled device does not have any authority over another. With the peering model, each device will share its time information with its peer. Additionally, each device can also provide time synchronization to the other.
  • Broadcast/Multicast Modeis a special server mode where the NTP server broadcasts its synchronization information to all clients. Broadcast mode requires that clients be on the same subnet as the server, and multicast mode requires that clients and servers have multicast capabilities configured.

Features of NTP

There exist several protocols to synchronize computer clocks, each having distinguished features. Here is a list of NTP’s features:

  • NTP needs some reference clock that defines the true time to operate. All clocks are set towards that true time.
  • NTP uses UTC as reference time.
  • NTP is a fault-tolerant protocol that will automatically select the best of several available time sources to synchronize to. Multiple candidates can be combined to minimize the accumulated error. Temporarily or permanently insane time sources will be detected and avoided.
  • NTP is highly scalable: A synchronization network may consist of several reference clocks. Each node of such a network can exchange time information either bidirectional or unidirectional. Propagating time from one node to another form a hierarchical graph with reference clocks at the top.
  • Having available several time sources, NTP can select the best candidates to build its estimate of the current time. The protocol is highly accurate, using a resolution of less than a nanosecond (about 2^-32 seconds).
  • Even when a network connection is temporarily unavailable, NTP can use measurements from the past to estimate current time and error.
  • For formal reasons NTP will also maintain estimates for the accuracy of the local time.


[1] “What is NTP?” available online at:

[2] “Network Time Protocol (NTP)”, available online at:

[3] Anthony Sequeira, “Introduction to Network Time Protocol (NTP)”, available online at:

[4] “NTP – Network Time Protocol”, available online at:

No Comments

Leave a Reply

Your email address will not be published. Required fields are marked *