NTP Daemon Architecture

How NTP Works  Client A sends its time T1 to server B  B replies with its times T2, T3 and client's original T1  A computes offset θ and delay δ relative to B NTP Daemon Architecture  NTP daemon implements peer and poll processes for each server  Peer process handles time request replies and calculates time offset and delay    Poll process periodically sends time requests based on network conditions Offset and delay values are saved per association and groomed by clock filter Timestamp Representation    NTP uses 64-bit short format and 128-bit long format timestamps Short format has 32 bits for whole seconds since 1900 and 32 bits for fraction of second Long format has 64 bits for seconds and 64 bits for fraction - extremely high precision  Short format mapped from bits 32-63 of long format. Can represent unambiguous years 1900-2036  Long format can represent dates from the dawn of the universe to the far future  Computations use differences between timestamps to preserve precision Reckoning Time  Basic NTP time reckoning:  Offset and delay values filtered and processed by:  Selection algorithm discards falsetickers -  Clustering algorithm selects most accurate candidates  Combining averages survivors algorithm offsets of Result is best offset estimate relative to server population Clock Discipline  NTP adjusts system clock time and frequency to minimize offset  Hardware clock ticks regularly to advance software clock  But clocks have intrinsic frequency errors and temperature dependence  NTP discipline algorithm corrects for these errors about once per second  Can reduce errors to microseconds with kernel timekeeping support Association Modes  Client/server: Client stateless server polls  Symmetric: Peers sync each other  Broadcast: Server pushes time to many clients  Each NTP host can operate in any combination of modes  Modes determine time flow direction and configuration Highlights  NTP calculates offset/delay relative to servers with timestamp exchanges  Short and long timestamp formats represent time to high precision  Engineered algorithms filter noise to produce best offset estimate  Clock discipline compensates for intrinsic clock errors  Major modes define time flow and configuration methods