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A network time server is a computer or other system that sends time information to devices over a network so they can synchronize their clocks. NTP—the Network Time Protocol—is the widely used protocol for this exchange. The server is a role in a time-distribution system; it is not the same thing as the protocol or the clock that supplies the original reference time.
What does a network time server do?
A network time server responds to time requests from other networked systems, called clients. Those clients use the information to set or adjust their clocks. NIST describes NTP as a way to synchronize servers, workstations and other networked equipment across networks of different types and sizes (NIST Network Time Protocol glossary).
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The server does not make every device’s clock identical by itself. Each client has its own clock and implementation, and the time it obtains depends on the upstream source and the network path between client and server.
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NTP is the protocol—the rules and messages used to exchange timing information. A server is a system participating in that exchange in a server role. NTPv4 describes a hierarchy: primary servers obtain time from a reference source, secondary servers synchronize from upstream servers, and clients synchronize from servers above them in the hierarchy (RFC 5905, NTPv4: Protocol and Algorithms Specification).
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A primary server might use a GPS receiver, for example, but GPS is not required for every network time server. A primary source may instead be synchronized by another suitable reference, including wire or radio sources linked to national standards. The server distributes the time it has; its position in the hierarchy does not, on its own, establish how trustworthy or accurate that time is.
NTP and SNTP are related, but not identical
Simple Network Time Protocol (SNTP) uses the same message format as NTP but is intended for simpler arrangements. RFC 5905 describes SNTP clients using one upstream server and primary servers using one reference clock, without dependent clients. Fuller NTP is intended for more complex secondary servers and clients, and includes algorithms for handling multiple sources and mitigating network and server problems.
In practice, a device described as an NTP client may make only a single request rather than using the fuller set of NTP algorithms. The actual behavior depends on the device’s operating system and time-service implementation.
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What affects the time a client receives?
Accuracy is not a universal property of a server name or protocol. It depends on the reference source, the server and client implementations, the quality and diversity of upstream sources, and conditions on the network path. NIST notes that the stability and reciprocity of the connection usually have a large effect on the timestamp accuracy a user sees (NIST Internet Time Service).
NTP can measure and compensate for signaling delay, but that capability is not a guarantee of a particular precision for every device or network. Congestion, changing delays, server failure and client behavior can all affect the result. Some NTP deployments support dynamic server discovery, so a device may find a time server without a manually entered address; whether that happens depends on the operating system, implementation and network configuration.
Does a network time server need to be a physical appliance?
No. “Network time server” describes a system providing time to clients, not a required hardware form. It may be a general-purpose computer or a dedicated appliance. An appliance with a GPS reference can make sense where an organization needs a local primary source, but the NTP definition does not imply that every user needs one or that every server has GPS.
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How do authentication and trust work?
Ordinary NTP traffic is generally not encrypted, and a client may be vulnerable to forged time replies if it has no way to verify the sender. NIST’s authenticated NTP service uses prearranged symmetric keys and a message authentication code, allowing a client to check that a response came from someone holding the shared key and was not modified in transit (NIST Authenticated NTP Service).
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Quick Recap
In brief: server, protocol and reference source
| Term | Meaning |
|---|---|
| Network time server | A system that provides time information to networked clients. |
| NTP | The widely used protocol for exchanging time information and synchronizing clocks. |
| Reference source | The clock or time source from which a primary server obtains time, such as GPS in one example deployment. |
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