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NTP

NTP. Network Time Protocol Nóirín Plunkett. A Short French Lesson. TAI – Temps Atomique International International Atomic Time UTC – Temps Universel Coordonné Universal Co-ordinated Time BIH – Bureau International de l’Heure International Time Bureau. Second. SI Unit

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NTP

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  1. NTP Network Time Protocol Nóirín Plunkett

  2. A Short French Lesson • TAI – Temps Atomique International • International Atomic Time • UTC – Temps Universel Coordonné • Universal Co-ordinated Time • BIH – Bureau International de l’Heure • International Time Bureau

  3. Second • SI Unit • 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom

  4. Brief History of Time • Second defined in 1967 • UTC started on 1st January 1972 • RFC 778, 1981 - Internet Clock Service • RFC 958, 1985 - description of NTP • RFC 1059, 1988 - NTPv1, protocol & algorithms • RFC 1305, 1992 - NTPv3, formal correctness principles • NTPv4 in use, not yet formalised

  5. Real World • “Time is what prevents everything from happening at once.” - J.A. Wheeler • Correlating logs of events • Cryptographic expiries • Air Traffic Control • GPS • Networking

  6. Atomic Time • 260 atomic clocks in 40 labs contribute to the international time standards contribute to TAI • Radio broadcasts • DCF77, Physikalisch-Technische Bundesanstalt, Braunschweig (77.5kHz) • GPS based on US Naval Observatory, DC

  7. NTP and UTC • NTP is based on UTC • NTP has no memory • Every leap-second, NTP ‘resets’ itself to the current UTC value • Using a clock synchronised to UTC in 2005 to calculate the time of an event in early 1972 would result in 22 seconds difference

  8. What NTP Does • Provide most accurate time possible, based on ‘reference time’ - not just syncing to a common time • Keep in sync with leap seconds • Ignore ‘falsetickers’ - clocks it could use for reference, but which provide an apparently wrong time • Use previous figures to estimate current difference between system time and reference time, in the absence of a network connection

  9. What NTP Does Not • Convert NTP timestamps into system time format • Set the hardware clock • Handle time-zones/summer time • Recognise when the system clock is far off and accept apparent ‘falsetickers’ as true

  10. Some Definitions • Reference clock - a device which gives a known accurate time • Accuracy - the difference between the value of a measurement and the actual measurand • Precision - how close two measurements of the same value are

  11. Precision vs Accuracy • Exp I - Imprecise, inaccurate • Exp II - Imprecise, accurate • Exp III - Precise, inaccurate • Exp IV - Precise, accurate (image source)

  12. NTP Packet • UDP Packet • Request • Time of client system clock at sending • Response • Time of client system clock at sending • Time of receipt at server • Time of server system clock at sending

  13. NTP Timestamp • 64-bit number • First 32 bits represent seconds since 00:00, January 1st, 1900 • Next 32 bits represent fractions of a second • Sat, Nov 19 2005 19:27:30.869 • c729fb22.de8afc9d • 11000111 00101001 11111011 00100010 . 11011110 10001010 11111100 10011101

  14. More Definitions • Latency - the time taken for a packet to reach its destination • Round trip time - the time taken between the client sending out a packet, and receiving a response to that packet from the server • Jitter - variability of latency over time

  15. Timestamps in Packets • Client sends packet - T1 • Server receives packet, adds receipt timestamp - T2 • Server prepares packet to send to client, adds sending timestamp - T3 • Client receives packet - T4 • Latency client -> server = (T2-T1) • Latency server -> client = (T4-T3)

  16. Round Trip Times • Actual RTT = (T4-T1) • Network RTT = (T2-T1) + (T4-T3) • Latencies = (T2-T1), (T4-T3) • If latencies are symmetric (within reason), (T1+T4) = (T2+T3) • If (T1+T4) != (T2+T3), our estimate of clock offset is (T1+T4) - (T2+T3)

  17. Truechimers & Falsetickers • Multiple servers providing time estimates • If the majority of servers provide a consistent time - they’re probably right • If there are one or two outliers - they’re probably wrong

  18. Peering • Client should peer with multiple servers (image source)

  19. Law of Averages • Peering with multiple servers makes for more reliable results • Falsetickers will be easier to identify • Truechimers will be more useful (because more servers means we can dismiss borderline true/false) • Assumptions become statistically more reliable • Latency symmetry • Clock regularity

  20. Problems with Peering • Loops • NTP prevents loops, through spanning-tree mechanism • Layers • NTP prevents there being more than fourteen layers, by using strata

  21. Strata • Radio clock - Stratum 0 • Computer running NTP syncing from radio clock - Stratum 1 • Computer syncing from Stratum n NTP server - Stratum n+1 • Server unreachable - Stratum 16

  22. Lies, Damn Lies • NTP checks that values are consistent with previous measurements • Deals with sudden changes in network/system load • Allows NTP to ‘guess’, in case of network failure • If values are very different from previously, but remain consistent, NTP accepts that local clock may be falseticker

  23. Clocks • Hardware clock - quartz crystal • Software clock - interrupt-driven timer chips • Software clock more accurate for judging interval between two times • Both need to be set to correct time

  24. Problems • Inaccurate time needs to be corrected • Massive time changes are undesirable • Time travel is undesirable • Skipping seconds is bad • Going backwards is not allowed • Best way of balancing all this is to slow down/speed up time • This only works with computers, and in the movies!

  25. Exceptions • On system boot-up, time can be set, regardless of offset from previous time • init scripts are aware of this, bootup expects it, nothing should break • Computers do what they’re told - regardless of what the admin intended • If an admin tells the system to update its time, it updates. Things may break.

  26. Phase Lock Loops • Raises (or lowers) frequency of an oscillator until it matches a reference frequency • Pauses oscillator as necessary to match ref. phase

  27. More on PLL • Seconds can be speeded up or slowed down, until they happen as often as reference clock - frequency is matched • NTP can also use this speed change to match the phase - some PLLs will stop the oscillator momentarily to do this

  28. NTP Traffic • ntp.maths.tcd.ie • Stratum 1 server, syncing from radio clock

  29. Daily Peak • Graph dates from IST - blip occurring at 1am local time

  30. After IST Ends • Peak remains - still at midnight GMT

  31. Daylight Savings Time • NTP is time-zone agnostic

  32. Daylight Savings Time Ends • 2005-W43-7 - Daylight Savings Time ends

  33. Strange Happenings

  34. References • RFC 1305 • http://www.ntp.isc.org • http://www.eecis.udel.edu/~mills/ • ntp.maths.tcd.ie

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