1. NTP Procedure Descriptions and Flow Diagrams
David L. Mills
University of Delaware
20-Sep-18

2. Process decomposition
Server 1
Peer/Poll 1
Selection
and
Clustering
Algorithms
System Process
Clock Discipline
Process
Combining
Algorithm
Server 2
Peer/Poll 2
Loop Filter
Server 3
Peer/Poll 3
VFO
Remote Servers
Peer/Poll Processes
Clock Adjust
Process
Peer process runs when a packet is received.
Poll process sends packets at intervals determined by the clock discipline process and remote server.
System process runs when a new peer process update is received.
Clock discipline process runs at intervals determined by the measured network phase jitter and clock oscillator (VFO) frequency wander.
Clock adjust process runs at intervals of one second.
20-Sep-18

3. Time values and computations
Packet Variables
Peer Variables
System Variables S S
Client S
Server
Packet variables are computed directly from the packet header.
Peer variables are groomed by the clock filter.
System variables are groomed from the available peers.
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4. NTP packet header format
Packet header Variables Description LI VN
Mode
Strat
Poll
Prec
Root Delay
leap leap indicator (LI) version version number (VN) mode protocol mode stratum stratum t poll interval (log2 s) r clock reading precision (log2 s) D root delay E root dispersion refid reference ID reftime reference timestamp T1 originate timestamp T2 receive timestamp T3 transmit timestamp T4 destination timestamp* MAC MD5 message hash (optional)
Root Dispersion
Reference Identifier
Reference Timestamp (64)
Originate Timestamp (64)
Receive Timestamp (64)
Transmit Timestamp (64)
MAC (optional 160)
* Strictly speaking, T4 is not a packet variable; it is the value of the system clock upon arrival.
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5. NTP peer protocol Packet header includes T1, T2 and T3 timestamps.
Peer state variables org, rec and xmt record the transmit and receive times of the most recent packet received.
When a packet is transmitted
Copy org to T1 and rec to T2.
Copy the current time to xmt and to T3.
When a packet received
If T3 is the same as xmt, this is a duplicate packet.
If T1 is not the same as org, this is a bogus packet.
Otherwise, copy T3 to pkt and copy the current time to rec.
Note that the protocol is symmetric and allows time values to flow both ways and is resistant to replays and drops.
Note the special conditions when either or both peers first start up.
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6. NTP peer protocol examplet1 t3
t5.
Packet Variables
Peer B
State Variables T2 t2 t4 t6 T3 t1
t3 = clock
t5.
t7 = clock T4
t2 = clock
t6 = clock
State Variables Name Description
org t1
t5? t5
rec t2 t2 t6 t6
org originate timestamp rec receive timestamp xmt transmit timestamp
xmt t3
t3? t7 t2 t3 t6 t7
Packet Header Variables Name Description t1 t4 t5 t8 t1 t4 t5 t8
tn originate timestamp tn+1 receive timestamp tn+2 transmit timestamp tn+3 destination timestamp T1 t1
t3. t5
Packet Variables
Peer A
State Variables T2 t2 t4 t6 T3
t1 = clock
t3.
t5 = clock t7 T4
t4 = clock
t8 = clock
org 0.
t3? t3
t7?
rec t4 t4 t8
xmt t1
t1? t5
t5?
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7. peer process receive procedure crypto_recv procedure packet procedure
Verify integrity, authenticity and consistency of packet data.
Match association by destination IP address and NTP protocol mode.
crypto_recv procedure
Implement Autokey proventication protocol described elsewhere.
packet procedure
Process receive and transmit timestamps and symmetric protocol.
Compute clock offset, roundtrip delay and dispersion sample.
Copy packet header data to peer state variables
clock_filter procedure
Select the best from among the past eight samples.
Calculate filter dispersion, jitter and related values.
Implement popcorn spike suppressor. 7
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8. Packet sanity tests
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9. Peer_process: variables and procedures
peer process Variables Process Description
peer process Procedures Process Called by
peer clock offset d peer roundtrip delay e peer dispersion j peer jitter t peer update epoch T peer last update epoch m peer time since last update rR peer server precision DR peer server root delay ER peer server root dispersion org peer originate timestamp rec peer receive timestamp xmt peer transmit timestamp leap peer leap indicator version peer version mode peer mode stratum peer stratum ppoll peer peer poll interval hpoll poll host poll interval min poll min poll interval max poll max poll interval clock system system clock F constant tolerance (15 PPM) lmax constant max distance (16 s) MAXSTRAT constant max stratum (15)
receive peer peer packet peer peer clock_filter peer packet, poll poll_update poll packet, clock, poll transmit poll receive clock_select system clock filter, poll crypto_recv Autokey peer
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10. peer process: receive procedure
access OK? no
access deny
peer
yes
format OK? no
format error
wait for pkt
yes
auth=MD5
receive
find assoc
Peer Process
go to (association mode, packet mode) found in transition matrix
receive procedure
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11. Transition matrix Packet Mode Mode ACTIVE PASSIVE CLIENT SERVER BCAST
NO_PEER
NEWPS
FXMIT
NEWMC
NEWBC
PROC
ERROR
BCLIENT
Association Mode
The default (empty box) behavior is to discard the packet without comment.
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12. Transition functions 20-Sep-18 NEWBC NEWPS NEWMC auth? auth? auth? yesno
yes no
yes no
mobilize BCLIENT association
mobilize PASSIVE association
exit
mobilize CLIENT association
exit
crypto_recv
PROC
PROC
exit
ERROR
FXMIT
reset
transmit
exit
exit
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13. peer process: timestamp processing
xmt = 0?
yes
invalid no
xmt = T3?
yes
duplicate pkt no
mode = BCST no
T1andT2= 0? no
T1orT2= 0?
yes
invalid
yes no
yes no
org = T1?
yes
auth?
auth?
auth?
bad
reset ok
bad
bad
yes ok
org=T1 rec=T2 xmt=T3
auth error
auth error
exit
org=T1 rec=T2 xmt=T3
crypto_recv
bad
crypto error ok
packet
exit
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14. peer process: packet procedure
Packet header related values Packet Process Peer
packet
leap peer leap version peer version mode peer mode stratum peer stratum poll peer ppoll rR peer rR DR peer DR ER peer ER refid peer refid reftime peer reftime
header?
bad
header error ok
copy header
poll_update
reach |= 1
clock_filter
(q, d, e, t)
exit
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15. peer process: clock filter procedure
(q, d, e, t)
Shift sample (q, d, e, t) in clock filter; adjust dispersion e for old samples
Create (xi, yi, zi, ti) from each sample in clock filter; save in temporary array; sort array by increasing delay y
tmp = q
clock filter Variables Process Description
peer clock offset d peer roundtrip delay e peer filter dispersion j peer filter jitter t peer current epoch tP peer epoch of last update n peer samples (n ≤ 8) burst poll burst counter (xi, yi, zi, ti) local temp sample tmp local temp
|q – tmp| > 3j and t0 – t < 2t?
yes no
t0 > tP no
yes
tP = t0
clock select
yes
burst = 0? no
exit
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16. system process clock select procedure valid function select algorithm
Select and groom servers to discipline the system clock.
valid function
Return 1 if selected server is acceptable and root distance less than 1s.
select algorithm
Classify available servers as truechimers or falsetickers.
cluster algorithm
Find and discard outlyers until no more than three survivors remain.
mitigation procedure
Select survivors as required by the minigation rules.
acceptable function
Return 1 if server passes certain sanity checks.
rootdist function
Return synchronization distance to the primary reference source.
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17. system process: variables and procedures
system process Variables Process Description
system process Procedures Process Called by
Q system clock offset J system jitter jS system select jitter TS system epoch of last update sys system system peer q peer clock offset j peer filter jitter jC clock clock jitter m clock time since last update type peer type prefer peer prefer enable
clock select system clock filter, poll valid system clock select mitigation system clock select acceptable system valid, poll rootdist system valid, mitigation clock update clock clock select select algorithm clock select cluster algorithm clock select
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18. system process: clock select procedure
scan
candidates
valid? NO
YES
add peer
select algorithm
survivors? no
yes
add modem
yes
modem? no
add local
yes
local? no
cluster algorithm
exit (0)
mitigation
clock update
exit (n)
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19. system process: select algorithm
For each of m associations construct a correctness interval [q – rootdist, q + rootdist]
Consider the lowpoint, midpoint and highpoint of these intervals. Sort these values in a list from lowest to highest. Set the number of falsetickers f = 0.
Set the number of midpoints d = 0. Set c = 0. Scan from lowest endpoint to highest. Add one to c for every lowpoint, subtract one for every highpoint, add one to d for every midpoint. If c ≥ m - f, stop; set l = current lowpoint
Set c = 0. Scan from highest endpoint to lowest. Add one to c for every highpoint, subtract one for every lowpoint, add one to d for every midpoint. If c ≥ m - f, stop; set u = current highpoint.
If d ≤ f and l < u? no
yes
yes
Add one to f. Is f < m / 2?
Success; the intersection interval is [l, u]. no
Failure; a majority clique could not be found..
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20. system process: cluster algorithm
Let (q, jR, L) represent a candidate with peer offset q, jitter j and a weight factor L equal to stratum as the high order field and root distance as the low order field.
Sort the candidates by increasing L. Let n be the number of candidates and nmin ≤ n the minimum number of survivors.
For each candidate compute the selection jitter jS (RMS peer offset differences between this and all other candidates).
Select jmax as the candidate with maximum LjS.
Select jmin as the candidate with minimum j.
jmax < jmin or n ≤ nmin or jmax is prefer peer?
yes no
Delete the outlyer candidate with jmax; reduce n by one.
Done. The remaining cluster survivors are the pick of the litter.
20-Sep-18

21. system process: mitigation procedures
valid
mitigation
is new peer the same as the last chosen and is its stratum lowest?
type = local clock or modem?
prefer? no
yes
type = PPS?
yes no no
yes
yes no
prefer?
sys = PPS
sys = OLD
sys = NEW no
yes NO
acceptable?
sys = PREF
y = z = 0
YES
> 1 s
rootdist
scan clock select
survivors
x = rootdist
< 1 s
exit (NO)
exit (YES)
done
mitigation procedure Variable Process Description
Q system clock offset jS system select jitter J system combine jitter sys system system peer q peer clock offset j peer filter jitter type peer type prefer peer prefer enable x, y local temp
exit
valid procedure Variable Process Description
type peer type prefer peer prefer enable
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22. clock discipline process
Clock update procedure
Discipline system clock using adaptiver-parameter, phase/frequency-lock loop.
trans procedure
Transition to new state and initialize variables.
adjust freq procedure
Adjust oscillator frequency using PLL/FLL feedback loop.
step freq procedure
Step oscillator frequency when first starting and no previous information.
tc procedure
Adjust time constant as a function of prevailing jitter and oscillator stability.
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23. clock discipline process: variables and procedures
clock discipline process Variable Process Description
clock discipline process Procedure Process Called by
QL clock last offset QR clock current offset D peer root delay E peer root dispersion jC clock clock jitter f clock frequency h clock frequency stability t clock time constant m clock time since last update state clock state jiggle clock jiggle counter Q system clock offset J system combine jitter tS system epoch of last update sys system peer leap system leap indicator stratum system stratum refid system reference ID reftime system reference timestamp tP peer epoch of last update max poll max poll interval min poll min poll interval F constant frequency tolerance
local clock clock clock update trans clock local clock adjust freq clock local clock step freq clock local clock tc clock local clock poll update poll packet, clock, poll step time kernel step time
20-Sep-18

24. clock discipline process: clock update procedure
clock update procedure System Process Copied from
clock_update
system mitigate proc J system mitigate proc sys system mitigate proc D peer DR + d E peer ER + e + Fm + j + |q| leap peer leap stratum peer stratum refid peer refid reftime peer reftime
poll_update no
yes
local_clock
IGNOR
PANIC
ADJ
STEP
panic exit
update sys variables
reset all associations
exit
20-Sep-18

25. clock discipline process: local clock function
loca_ clock
|Q| > PANIC?
yes
exit (panic) no
SPIK no
|Q| > STEP?
yes
SYNC
FREQ
FSET
NSET
SYNC
FSET
SPIK
FREQ
NSET
state = SPIK
QR = Q
m > 900 s?
QR = Q
yes no
m > 900 s?
trans (FREQ)
adjust_freq no
yes
step_freq
exit (IGNOR)
step_freq
step_time (Q)
step_time (Q)
trans (SYNC)
Q = 0
t = min
Q = 0
t = min tc
trans (SYNC)
trans (FREQ)
exit (ADJ) tc
20-Sep-18
exit (STEP)

26. Clock discipline process: state transition function
|Q| < STEP
|Q| > STEP
Comments
NSET
>FREQ; adjust time
>FREQ; step time
no frequency file
FSET
>SYNC; adjust time
>SYNC; step time
frequency file
SPIK
>SYNC adjust freq; adjust time
if (< 900 s) >SPIK else >SYNC; step freq; step time
outlyer detected
FREQ
if (< 900 s) >FREQ else >SYNC; step freq; adjust time
if (< 900 s) >FREQ else >SYNC; step freq; step time
Initial frequency
SYNC
>SYNC; adjust freq; adjust time
If (< 900 s) >SPIK else >SYNC; step freq; step time
normal operation
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27. clock discipline process: procedures Itc
trans (x)
state = x
QB = Q - QR
QL = QR = Q
m = 0
jC > h and |Q| < 4jC no
yes
exit
jiggle -= 2t
jiggle += t
tc procedure Variable Process Description
jiggle < -30
jiggle > 30 no
yes no
yes
jiggle = -30
jiggle = 30
clock offset t clock time constant jC clock jitter h clock stability jiggle clock jiggle counter max poll max poll interval min poll min poll interval
t > min
t < max no
yes no
yes
t--
t++
exit
trans procedure Variable Process Description
Q clock offset QB clock base offset QR clock current offset m clock time since last update state clock current state x, y local temp
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28. clock discipline process: procedures II
adjust freq
adjust freq procedure Variable Process Description
t > 750
yes no
clock clock offset QB clock base offset QR clock last offset m clock time since last update f clock frequency h clock frequency stability t clock time constant padj, fadj local temp*
step freq
f += fadj + padj*
* The fadj and padj comtributions are weighted by functions depending on t.
exit
20-Sep-18

29. clock adjust process clock adjust procedure
Called by kernel timer routines once each second.
Adjusts system clock frequency as computed by PLL/FLL.
system process computes initial system clock offset.
Reduce residual clock offset as exponential decay.
This procedure can also be implemented in the kernel for reduced sawtooth error.
20-Sep-18

30. clock adjust process: clock adjust procedure
clock adjust process Variables Process Description
f clock frequency t clock time constant QR clock current offset E system root dispersion tmp local temp F constant tolerance (15 PPM)
clock adjust
E += F
tmp = QR / 16t
QR -= tmp
adjust time (f + tmp)
clock adjust process Procedures Process Called by
adjust time kernel clock adjust
wait 1 s
20-Sep-18

31. poll process poll procedure transmit procedure poll update procedure
Determine when to transmit a packet according to poll and burst schedules.
transmit procedure
Format and transmit an NTP packet.
poll update procedure
Mitigate the poll interval as a function of the peer interval and designated lower and upper values.
20-Sep-18

32. poll process: variables and procedures
poll process Variable Process Description
r peer precision D peer root delay E peer root dispersion refid peer reference ID reftime peer reference timestamp org peer origin timestamp rec peer receive timestamp xmt peer transmit timestamp hpoll peer host poll interval poll poll poll interval min poll min poll interval max poll max poll interval reach poll reach shift register unreach poll unreach counter burst poll burst counter b1_enable poll enable unreach burst b2_enable poll enable reach burst leap system leap indicator version system version mode system mode stratum system stratum addr system hashed local address t system current epoch timer kernel timer
poll process Procedure Process Called by
poll poll kernel timer poll update poll packet, clock, poll transmit poll receive, poll clock select system clock filter, poll acceptable system select, poll clock filter peer packet, poll crypto xmit Autokey transmit sndpkt kernel transmit
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33. poll process: poll procedure
peer = NULL
yes
mode= BCST
yes no
wait timer
transmit no
burst = 0?
yes
reach << 1
burst-- no
reach = 0?
yes
unreach > 8
poll_update
burst = 0?
unreach = 0 no
yes
yes no no
yes
hpoll++
clock_select
reach & 0x7=0?
yes
clock_filter
(0, 0, ∞, t)
b2_enable?
yes no
transmit
acceptable?
hpoll = poll
yes no no
burst = 8
mode = BCL
yes no
b1_enable?
yes no
unreach=0?
yes
unreach++
burst = 8
20-Sep-18

34. poll process: transmit procedure
Transmit Procedures
transmit
Peer Transmit Packet Process Variable
Fast Transmit Packet Process Variable
copy header
r system r D system D E system E refid system refid reftime system reftime T1 peer org T2 peer rec T3 kernel clock MAC MD5 hash leap system leap version peer version mode peer pmode stratum system stratum poll peer hpoll
r system r D system D E system E refid system refid reftime system reftime T1 packet T2 T2 packet T3 T3 kernel clock MAC MD5 hash leap system leap version packet version mode system mode stratum system stratum poll packet poll
T1, T2
Crypto_xmit
T3 = clock
make MAC
sndpkt
exit
20-Sep-18

35. poll process: procedures
poll_update
burst = 0? no
timer running?
yes
yes no
timer = max(min(ppoll, hpoll), min)
timer = 2 s
exit
poll update procedure Variable Process Description
ppoll peer peer poll interval hpoll peer host poll interval min poll min poll interval max poll max poll interval burst poll burst counter timer kernel timer n
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36. Miscellaneous functions
rootdist
acceptable
exit ((DR + d) / 2 + ER + e + Fm + j)
reach = 0? leap = 11? stratum >15?
any yes
server not synchronized
all no
rootdist > MAXDIST?
yes
root distance exceeded
rootdist procedure Variable Process Description no
refid = addr?
yes
server/client sync loop
DR peer root delay d peer delay ER peer root dispersion e peer dispersion F constant frequency tolerance m peer time since last update j peer jitter no
exit (YES)
exit (NO)
reset
persistent? no
yes
acceptable procedure Variable Process Description
initialize association variables
demobilize association
leap peer leap indicator stratum peer stratum refid peer reference ID addr system hashed local IP addr reach poll reach shift register
exit
20-Sep-18

37. 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
20-Sep-18