1. Network Time Protocol (NTP) General Overview
David L. Mills
University of Delaware
17-Sep-18

2. Introduction
Network Time Protocol (NTP) synchronizes clocks of hosts and routers in the Internet
Well over 100,000 NTP peers deployed in the Internet and its tributaries all over the world
Provides nominal accuracies of low tens of milliseconds on WANs, submilliseconds on LANs, and submicroseconds using a precision time source such as a cesium oscillator or GPS receiver
Unix NTP daemon ported to almost every workstation and server platform available today - from PCs to Crays - Unix, Windows, VMS and embedded systems
Following is a general overview of the NTP architecture, protocol and algorithms
Data are included from a survey of NTP clients and servers in the Internet of 1997
17-Sep-18

3. Needs for synchronized time
Stock market sale and buy orders and confirmation timestamps
Network fault isolation, reporting and restoral
Network monitoring, measurement and control
Distributed multimedia stream synchronization
RPC at-most-once transactions; replay defenses; sequence-number disambiguation
Research experiment setup, measurement and control
Cryptographic key management and lifetime control
17-Sep-18

4. NTP capsule summary
Primary (stratum 1) servers synchronize to national time standards via radio, satellite and modem
Secondary (stratum 2, ...) servers and clients synchronize to primary servers via hierarchical subnet
Clients and servers operate in master/slave, symmetric or multicast modes with or without cryptographic authentication
Reliability assured by redundant servers and diverse network paths
Engineered algorithms reduce jitter, mitigate multiple sources and avoid improperly operating servers
System clock is disciplined in time and frequency using an adaptive algorithm responsive to network time jitter and clock oscillator frequency wander
17-Sep-18

5. NTP configurations S3 S3 S3 S2 S2 S2 S2 * * S4 S3 S3
Workstation
(a)
Clients
(b) S1 S1 S1 S1 S1 S1 * * * S2 S2 S2
Clients
(c)
* to buddy (S2)
(a) Workstations use multicast mode with multiple department servers
(b) Department servers use client/server modes with multiple campus servers and symmetric modes with each other
(c) Campus servers use client/server modes with up to six different external primary servers and symmetric modes with each other and external secondary (buddy) servers
17-Sep-18

6. How NTP works
Peer 1
Filter 1
Intersection
and
Clustering
Algorithms
Combining
Algorithm
Peer 2
Filter 2
Loop Filter
P/F-Lock Loop
Peer 3
Filter 3
VFO
NTP Messages
Timestamps
Multiple synchronization peers provide redundancy and diversity
Clock filters select best from a window of eight clock offset samples
Intersection and clustering algorithms pick best subset of servers believed to be accurate and fault-free
Combining algorithm computes weighted average of offsets for best accuracy
Phase/frequency-lock feedback loop disciplines local clock time and frequency to maximize accuracy and stability
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7. NTP process decomposition (NTPv4)
Peer 1
Filter 1
Selection
and
Clustering
Algorithms
Combining
Algorithm
Peer 2
Filter 2
Loop Filter
Clock Adj. Proc.
Peer 3
Filter 3
System Process
VFO
Remote Servers
Peer Processes
Each peer process runs independently at poll intervals determined by the system process and remote server
System process runs at poll intervals determined by the measured network phase jitter and local clock oscillator frequency stability
Clock adjust process runs at 1-s intervals to discipline the VFO phase and frequency
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8. NTP dataflow analysis
Server 1
D, E
Peer 1
q, d, e, j
Selection
and
Combining
Algorithms
Server 2
D, E
Peer 2
q, d, e , j
System
Q, D, E, j
Server 3
D, E
Peer 3
q, d, e , j
Each server calculates server variables offset Q, delay D and dispersion E relative to the root of the synchronization subtree
At each NTP message arrival, the peer process updates peer offset q, delay d, dispersion e and filter error jr (NTPv4) from timestamps and clock filter algorithm
At system poll intervals, the clock selection and combining algorithms update system variables Q, D, E, and j
Dispersions e and E increase with time at a rate depending on specified frequency tolerance f
17-Sep-18

9. Clock filter algorithm
Server T3 x q0 T1
Client T4
The most accurate offset q0 is measured at the lowest delay d0 (apex of the wedge scattergram).
The correct time q must lie within the wedge q0 ± (d - d0)/2.
The d0 is estimated as the minimum of the last eight delay measurements and (d0 ,q0) becomes the offset and delay output.
Each output can be used only once and must be more recent than the previous output.
The distance metric l is based on delay, frequency tolerance and time since the last measurement.
17-Sep-18

10. Performance of clock filter algorithm
These plots show the absolute clock offset in semilog coordinates for a path between the US east and west coasts over six days
(left) Raw absolute data offset samples
(right) Data offset samples processed by the clock filter algorithm
The algorithm reduces offset errors by a factor of about ten
The algorithm is particularly effective at removing spikes
17-Sep-18

11. Intersection algorithmB
correctness interval = q - l £ q0 £ q + l
m = number of clocks
f = number of presumed falsetickers
A, B, C are truechimers
D is falseticker A D C
Correct DTS
Correct NTP
DTS correctness interval is the intersection which contains points from the largest number of correctness intervals
NTP algorithm requires the midpoint of the intervals to be in the intersection
Initially, set falsetickers f and counters c and d to zero
Scan from far left endpoint: add one to c for every lower endpoint, subtract one for every upper endpoint, add one to d for every midpoint
If c ³ m - f and d ³ m - f, declare success and exit procedure
Do the same starting from the far right endpoint
If success undeclared, increase f by one and try all over again
if f  m/2, declare failure
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12. Delete the survivor smax; reduce n by one
Clustering algorithm
Sort survivors of intersection algortihm by increasing synchronization distance. Let n be the number of survivors and nmin a lower limit.
For each survivor si, compute the select dispersion (weighted sum of clock difference squares) between si and all others.
Let smax be the survivor with maximum select dispersion (relative to all other survivors) and smin the survivor with minimum sample dispersion (clock differences relative to past samples of the same survivor).
smax £ smin or n £ nmin?
yes no
Delete the survivor smax; reduce n by one
The resulting survivors are processed by the combining algorithm to produce a weighted average used as the final offset adjustment
17-Sep-18

13. Error budget - notation
Constants (peers A and B) r maximum reading error f maximum frequency error w dispersion normalize: 0.5
Packet variables DB peer root delay EB peer root dispersion
Sample variables T1, T2, T3, T4 protocol timestamps x clock offset y roundtrip delay z dispersion t interval since last update
System variables Q clock offset D root delay E root dispersion js selection jitter j jitter t interval since last update
Peer variables q clock offset d roundtrip delay e dispersion jr filter jitter n filter stages: 8 t interval since last update
17-Sep-18

14. Error budget - calculations
Sample Variables
Peer Variables
System Variables S S
Peer A S
Peer B
NTP Version 4 Error Budget
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15. Clock discipline algorithm
qr+
NTP Vd Vs
Phase
Detector
Clock Filter
qc-
VFO
Loop Filter x Vc
Clock Adjust
Phase/Freq Prediction y
Vd is a function of the phase difference between NTP and the VFO
Vs depends on the stage chosen on the clock filter shift register
x and y are the phase update and frequency update, respectively, computed by the prediction functions
Clock adjust process runs once per second to compute Vc, which controls the frequency of the local clock oscillator
VFO phase is compared to NTP phase to close the feedback loop
17-Sep-18

16. NTP protocol header and timestamp formats
NTP Protocol Header Format (32 bits)
LI leap warning indicator
VN version number (4)
Strat stratum (0-15)
Poll poll interval (log2)
Prec precision (log2) LI VN
Mode
Strat
Poll
Prec
Root Delay
Root Dispersion
Reference Identifier
Reference Timestamp (64)
NTP Timestamp Format (64 bits)
Originate Timestamp (64)
Seconds (32)
Fraction (32)
Value is in seconds and fraction
since 0h 1 January 1900
Cryptosum
Receive Timestamp (64)
Transmit Timestamp (64)
NTPv4 Extension Field
Extension Field 1 (optional)
Field Length
Field Type
Extension Field
(padded to 32-bit boundary)
Extension Field 2… (optional)
Last field padded to 64-bit boundary
Key/Algorithm Identifier
NTP v3 and v4
Authenticator
(Optional)
Message Hash (64 or 128)
NTP v4 only
authentication only
Authenticator uses DES-CBC or MD5 cryptosum
of NTP header plus extension fields (NTPv4)
17-Sep-18

17. A day in the life of a busy NTP server
NTP primary (stratum 1) server rackety is a Sun IPC running SunOS and supporting 734 clients scattered all over the world
This machine supports NFS, NTP, RIP, IGMP and a mess of printers, radio clocks and an 8-port serial multiplexor
The mean input packat rate is 6.4 packets/second, which corresponds to a mean poll interval of 157 seconds for each client
Each input packet generates an average of 0.64 output packets and requires a total of 2.4 ms of CPU time for the input/output transaction
In total, the NTP service requires 1.54% of the available CPU time and generates 10.5, 608-bit packets per second, or 0.41% of a T1 line
The conclusion drawn is that even a slow machine can support substantial numbers of clients with no significant degradation on other network services
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18. Server population by stratum (from survey)
17-Sep-18

19. Client population by stratum (from survey)
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20. Reference clock sources
In a survey of 36,479 peers, found 1,733 primary and backup external reference sources
231 radio/satellite/modem primary sources
47 GPS satellite (worldwide), GOES satellite (western hemisphere)
57 WWVB radio (US)
17 WWV radio (US)
63 DCF77 radio (Europe)
6 MSF radio (UK)
5 CHU radio (Canada)
7 modem time service (NIST and USNO (US), PTB (Germany), NPL (UK))
25 other (precision PPS sources, etc.)
1,502 local clock backup sources (used only if all other sources fail)
For some reason or other, 88 of the 1,733 sources appeared down at the time of the survey
17-Sep-18

21. Current progress and status
NTP Version 4 protocol, architecture and algorithms
Backwards compatible protocol algorithm implemented and tested
Improved local clock model completed and tested
Nanokernel precision time kernel modifications simulated, implemented and tested with SPARC, Alpha and Intel architectures
IETF pulse-per-second application program interface implemented and tested for SPARC and Intel architectures
Autonomous configuration autoconfigure
Multicast discovery with propagation correction completed and tested
Manycast discovery largely completed
Distributed add/drop greedy heuristic designed and simulated
Span-limited, hierarchical multicast groups using NTP distributed mode and add/drop heuristics under study
Autonomous authentication autokey
Implemented and in test
17-Sep-18

22. Future plans
Complete autoconfigure and autokey implementation in NTP Version 4
Deploy, test and evaluate NTP Version 4 daemon in DARTnet II testbed, then at friendly sites in the US, Europe and Asia
Revise the NTP formal specification and launch on standards track
Participate in deployment strategies with NIST, USNO, others
Prosecute standards agenda in IETF, ANSI, ITU, POSIX
Develop scenarios for other applications such as web caching, DNS servers and other multicast services
17-Sep-18

23. NTP online resources NTP specification documents
Internet (Draft) NTP standard specification RFC-1305
Simple NTP (SNTP) RFC-2030
NTP Version 4 papers and reports at
Under consideration in ANSI, ITU, POSIX
NTP web page
NTP Version 3 and Version 4 software and HTML documentation
Utility programs for remote monitoring, control and performance evaluation
Ported to over two dozen architectures and operating systems
Supporting resources
List of public NTP time servers (primary and secondary)
NTP newsgroup and FAQ compendia
Tutorials, hints and bibliographies
Links to other NTP software
17-Sep-18

24. Further information Network Time Protocol (NTP): http://www.ntp.org/
Current NTP Version 3 and 4 software and documentation
FAQ and links to other sources and interesting places
David L. Mills:
Papers, reports and memoranda in PostScript and PDF formats
Briefings in HTML, PostScript, PowerPoint and PDF formats
Collaboration resources hardware, software and documentation
Songs, photo galleries and after-dinner speech scripts
FTP server ftp.udel.edu (pub/ntp directory)
Current NTP Version 3 and 4 software and documentation repository
Collaboration resources repository
Related project descriptions and briefings
See “Current Research Project Descriptions and Briefings” at
17-Sep-18