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Presentation NTP Noirin Plunkett


  • pg 1

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
 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
        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
 NTPv4 in use, not yet formalised
             Real World
 “Time is what prevents everything from
  happening at once.” - J.A. Wheeler
 Correlating logs of events
 Cryptographic expiries
 Air Traffic Control
 Networking
              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
             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
                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
         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
          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
  Precision vs Accuracy
                  Exp I - Imprecise,
                  Exp II - Imprecise,
                  Exp III - Precise,
                  Exp IV - Precise,
(image source)
               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
            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
           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
 Jitter - variability of latency over time
       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)
          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)
    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

 Client should peer with multiple servers
                  (image source)
              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
       Problems with Peering
 Loops
   NTP prevents loops, through spanning-tree

 Layers
   NTP prevents there being more than fourteen
    layers, by using 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
              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
 Hardware clock - quartz crystal
 Software clock - interrupt-driven timer
 Software clock more accurate for judging
  interval between two times
 Both need to be set to correct time
 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
 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.
              Phase Lock Loops
 Raises (or lowers) frequency of an oscillator until it
  matches a reference frequency
 Pauses oscillator as necessary to match ref. phase
              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
              NTP Traffic

 ntp.maths.tcd.ie

 Stratum 1 server, syncing from radio clock
                 Daily Peak

 Graph dates from IST - blip occurring at 1am local
             After IST Ends

 Peak remains - still at midnight GMT
       Daylight Savings Time

 NTP is time-zone agnostic
   Daylight Savings Time Ends

 2005-W43-7 - Daylight Savings Time ends
Strange Happenings
   RFC 1305
   http://www.ntp.isc.org
   http://www.eecis.udel.edu/~mills/
   ntp.maths.tcd.ie

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