Network Time Protocol Version 4 Reference and Implementation Guide

In the 2006 report Network Time Protocol Version 4 Reference and Implementation Guide, David L. Mills the Network Time Protocol Version 4 (NTPv4), which is widely used to synchronize the time for Internet hosts, routers and ancillary devices to Coordinated Universal Time (UTC) as disseminated by national standards laboratories.

From the abstract:

It describes the core architecture, protocol, state machine, data structures and algorithms. It explains the fundamental on-wire protocol used to exchange time values between peers, servers and clients. It summarizes the clock offset, roundtrip delay and various other statistics used by the mitigation algorithms to calculate the maximum error and nominal error inherent in computing these values. It describes several changes from Version 3 of NTP (NTPv3) originally described in RFC 1305, including the introduction of a modified protocol header to accommodate Internet Protocol Version 6 and a new header extension field to support the Autokey public key authentication scheme.