Posted by Daniel Waldron on June 22nd, 2015
Network Time Protocol can operate according to four  different modes. If you’re an NTP beginner, discover the operating modes that define the NTP communication between NTP devices…
Network Time Protocol – Device Communication
NTP communication between two different devices consists of NTP time requests and NTP control queries. An NTP time request is a request from an NTP client for time synchronisation from an NTP server. NTP control queries are communication messages for configuration information.
The four principal operating modes of Network Time Protocol are:
1. Client mode.
2. Server mode.
3. Peer mode.
4. Broadcast/Multicast mode.
NTP client mode
An NTP client is a network device that’s configured to make an initial request for the time from an external NTP time server in order to synchronise its own clock.
Clients will receive the time once an NTP server broadcasts, if the clients are set up to ‘listen’ for broadcasts rather than making the initial request for the time.
NTP client mode devices do not supply synchronisation services to other devices on a network infrastructure.
NTP server mode
NTP servers are network devices operating an NTP service. These devices are configured to supply time information to NTP clients utilising Network Time Protocol.
NTP servers will only supply time information to authorised NTP clients and will not receive time synchronisation information from unauthorised devices.
The most common internet configuration for NTP is the client/server model. In this mode requests are sent by a client to a server, with the client expecting a response within milliseconds, unless the time source is unavailable or extremely busy.
The process sees a client distribute a network time protocol message to one or more servers and actions the replies as received. The server will then exchange addresses and ports, overwrite particular fields in the message, recalculate the checksum before returning the message straight away.
The returned message enables the client to calculate the server time, in respect of the local time, and alter a clock accordingly. Additionally, the message will contain information to calculate the expected timekeeping accuracy and reliability, plus choose the best server.
NTP peer mode
Peered devices act as back-up for each other and each peer operates with one or more primary references sources, but neither peer device has authority over the other.
Should one of the peers lose all time reference sources, or stop operating completely, the remaining peer device(s) automatically reconfigure to ensure that time values can flow from surviving peers to all others across the network.
In some instances this is often referred to as the ‘push-pull’ process, meaning that a peer either pulls or pushes the time value according to the configuration setup.
Perhaps the most self-explanatory, in broadcast/multicast mode NTP servers simply broadcasts/multicasts time synchronisation information to all network time protocol clients across a network.
In instances where time accuracy and reliability are not crucial, clients can be configured to utilise broadcast/multicast modes. Usually, these modes are not set up for servers with dependent clients.
The benefit of these modes is that clients do not need to be configured to a particular server, enabling all operating clients to utilise the same configuration file.
Broadcast mode does require a broadcast server on the same subnet. As broadcast messages are not circulated by routers, only broadcast servers on the same subnet are used.
Broadcast mode is purposed for configurations featuring one or a few servers, plus a potentially large client network. It’s advised, because a hacker could impersonate a broadcast server and provide false time values, that this mode should always be authenticated.
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