NTP

1
NTP

• Network Time Protocol
• Nóirín Plunkett

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A Short French Lesson

• TAI Temps Atomique International
• International Atomic Time
• UTC Temps Universel Coordonné
• Universal Co-ordinated Time
• BIH Bureau International de lHeure
• International Time Bureau

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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

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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

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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

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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

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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

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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

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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

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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

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Precision vs Accuracy

• Exp I - Imprecise, inaccurate
• Exp II - Imprecise, accurate
• Exp III - Precise, inaccurate
• Exp IV - Precise, accurate

(image source)
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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

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NTP Timestamp

• 64-bit number
• First 32 bits represent seconds since 0000,
  January 1st, 1900
• Next 32 bits represent fractions of a second
• Sat, Nov 19 2005 192730.869
• c729fb22.de8afc9d
• 11000111 00101001 11111011 00100010 . 11011110
  10001010 11111100 10011101

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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

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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 -gt server (T2-T1)
• Latency server -gt client (T4-T3)

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Round Trip Times

• Actual RTT (T4-T1)
• Network RTT (T2-T1) (T4-T3)
• Latencies (T2-T1), (T4-T3)
• If latencies are symmetric (within reason),
  (T1T4) (T2T3)
• If (T1T4) ! (T2T3), our estimate of clock
  offset is (T1T4) - (T2T3)

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Truechimers Falsetickers

• Multiple servers providing time estimates
• If the majority of servers provide a consistent
  time - theyre probably right
• If there are one or two outliers - theyre
  probably wrong

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Peering

• Client should peer with multiple servers

(image source)
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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

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Problems with Peering

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

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Strata

• Radio clock - Stratum 0
• Computer running NTP syncing from radio clock -
  Stratum 1
• Computer syncing from Stratum n NTP server -
  Stratum n1
• Server unreachable - Stratum 16

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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

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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

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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!

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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 theyre told - regardless of
  what the admin intended
• If an admin tells the system to update its time,
  it updates. Things may break.

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Phase Lock Loops

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

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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

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NTP Traffic

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

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Daily Peak

• Graph dates from IST - blip occurring at 1am
  local time

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After IST Ends

• Peak remains - still at midnight GMT

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Daylight Savings Time

• NTP is time-zone agnostic

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Daylight Savings Time Ends

• 2005-W43-7 - Daylight Savings Time ends

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Strange Happenings
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References

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