2. Modes of Operation

An NTP implementation operates as a primary server, secondary server, or client. A primary server is synchronized to a reference clock directly traceable to UTC (e.g., GPS, Galileo, etc.). A client synchronizes to one or more upstream servers, but does not provide synchronization to dependent clients. A secondary server has one or more upstream servers and one or more downstream servers or clients. All servers and clients who are fully NTPv4-compliant MUST implement the entire suite of algorithms described in this document. In order to maintain stability in large NTP subnets, secondary servers SHOULD be fully NTPv4-compliant. Alternative algorithms MAY be used, but their output MUST be identical to the algorithms described in this specification.

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