<s>
The	O
Network	B-Protocol
Time	I-Protocol
Protocol	I-Protocol
(	O
NTP	O
)	O
is	O
a	O
networking	O
protocol	O
for	O
clock	O
synchronization	O
between	O
computer	O
systems	O
over	O
packet-switched	B-Protocol
,	O
variable-latency	O
data	O
networks	O
.	O
</s>
<s>
NTP	O
is	O
intended	O
to	O
synchronize	O
all	O
participating	O
computers	O
to	O
within	O
a	O
few	O
milliseconds	O
of	O
Coordinated	O
Universal	O
Time	B-Protocol
(	O
UTC	O
)	O
.	O
</s>
<s>
It	O
uses	O
the	O
intersection	B-Operating_System
algorithm	I-Operating_System
,	O
a	O
modified	O
version	O
of	O
Marzullo	B-Operating_System
's	I-Operating_System
algorithm	I-Operating_System
,	O
to	O
select	O
accurate	O
time	B-Protocol
servers	I-Protocol
and	O
is	O
designed	O
to	O
mitigate	O
the	O
effects	O
of	O
variable	O
network	B-Protocol
latency	I-Protocol
.	O
</s>
<s>
NTP	O
can	O
usually	O
maintain	O
time	B-Protocol
to	O
within	O
tens	O
of	O
milliseconds	O
over	O
the	O
public	O
Internet	O
,	O
and	O
can	O
achieve	O
better	O
than	O
one	O
millisecond	O
accuracy	O
in	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
under	O
ideal	O
conditions	O
.	O
</s>
<s>
Asymmetric	O
routes	B-Protocol
and	O
network	B-Protocol
congestion	I-Protocol
can	O
cause	O
errors	O
of	O
100ms	O
or	O
more	O
.	O
</s>
<s>
The	O
protocol	O
is	O
usually	O
described	O
in	O
terms	O
of	O
a	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
model	I-Operating_System
,	O
but	O
can	O
as	O
easily	O
be	O
used	O
in	O
peer-to-peer	O
relationships	O
where	O
both	O
peers	O
consider	O
the	O
other	O
to	O
be	O
a	O
potential	O
time	B-Protocol
source	O
.	O
</s>
<s>
Implementations	O
send	O
and	O
receive	O
timestamps	O
using	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
on	O
port	B-Protocol
number	I-Protocol
123	O
.	O
</s>
<s>
They	O
can	O
also	O
use	O
broadcasting	B-Operating_System
or	O
multicasting	O
,	O
where	O
clients	O
passively	O
listen	O
to	O
time	B-Protocol
updates	O
after	O
an	O
initial	O
round-trip	O
calibrating	O
exchange	O
.	O
</s>
<s>
NTP	O
supplies	O
a	O
warning	O
of	O
any	O
impending	O
leap	O
second	O
adjustment	O
,	O
but	O
no	O
information	O
about	O
local	O
time	B-Device
zones	I-Device
or	O
daylight	O
saving	O
time	B-Protocol
is	O
transmitted	O
.	O
</s>
<s>
The	O
current	O
protocol	O
is	O
version	O
4	O
(	O
NTPv4	O
)	O
,	O
which	O
is	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
version	O
3	O
.	O
</s>
<s>
In	O
1979	O
,	O
network	B-Protocol
time	I-Protocol
synchronization	O
technology	O
was	O
used	O
in	O
what	O
was	O
possibly	O
the	O
first	O
public	O
demonstration	O
of	O
Internet	O
services	O
running	O
over	O
a	O
trans-Atlantic	O
satellite	O
network	O
,	O
at	O
the	O
National	O
Computer	O
Conference	O
in	O
New	O
York	O
.	O
</s>
<s>
The	O
technology	O
was	O
first	O
deployed	O
in	O
a	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
as	O
part	O
of	O
the	O
Hello	O
routing	B-Protocol
protocol	O
and	O
implemented	O
in	O
the	O
Fuzzball	B-Protocol
router	I-Protocol
,	O
an	O
experimental	O
operating	O
system	O
used	O
in	O
network	O
prototyping	O
,	O
where	O
it	O
ran	O
for	O
many	O
years	O
.	O
</s>
<s>
They	O
include	O
the	O
Daytime	B-Protocol
and	O
Time	B-Protocol
protocols	I-Protocol
for	O
recording	O
the	O
time	B-Protocol
of	O
events	O
,	O
as	O
well	O
as	O
the	O
ICMP	O
Timestamp	O
messages	O
and	O
IP	O
Timestamp	O
option	O
(	O
)	O
.	O
</s>
<s>
More	O
complete	O
synchronization	O
systems	O
,	O
although	O
lacking	O
NTP	O
's	O
data	O
analysis	O
and	O
clock	O
disciplining	O
algorithms	O
,	O
include	O
the	O
Unix	B-Operating_System
daemon	I-Operating_System
timed	B-Protocol
,	O
which	O
uses	O
an	O
election	O
algorithm	O
to	O
appoint	O
a	O
server	O
for	O
all	O
the	O
clients	O
;	O
and	O
the	O
Digital	O
Time	B-Protocol
Synchronization	O
Service	O
(	O
DTSS	O
)	O
,	O
which	O
uses	O
a	O
hierarchy	O
of	O
servers	O
similar	O
to	O
the	O
NTP	O
stratum	O
model	O
.	O
</s>
<s>
In	O
1985	O
,	O
NTP	O
version	O
0	O
(	O
NTPv0	O
)	O
was	O
implemented	O
in	O
both	O
Fuzzball	O
and	O
Unix	B-Application
,	O
and	O
the	O
NTP	O
packet	O
header	O
and	O
round-trip	O
delay	O
and	O
offset	O
calculations	O
,	O
which	O
have	O
persisted	O
into	O
NTPv4	O
,	O
were	O
documented	O
in	O
.	O
</s>
<s>
Despite	O
the	O
relatively	O
slow	O
computers	O
and	O
networks	O
available	O
at	O
the	O
time	B-Protocol
,	O
accuracy	O
of	O
better	O
than	O
100	O
milliseconds	O
was	O
usually	O
obtained	O
on	O
Atlantic	O
spanning	O
links	O
,	O
with	O
accuracy	O
of	O
tens	O
of	O
milliseconds	O
on	O
Ethernet	O
networks	O
.	O
</s>
<s>
It	O
drew	O
on	O
the	O
experimental	O
results	O
and	O
clock	O
filter	O
algorithm	O
documented	O
in	O
and	O
was	O
the	O
first	O
version	O
to	O
describe	O
the	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
and	O
peer-to-peer	O
modes	O
.	O
</s>
<s>
In	O
1989	O
,	O
was	O
published	O
defining	O
NTPv2	O
by	O
means	O
of	O
a	O
state	B-Architecture
machine	I-Architecture
,	O
with	O
pseudocode	B-Language
to	O
describe	O
its	O
operation	O
.	O
</s>
<s>
However	O
the	O
design	O
of	O
NTPv2	O
was	O
criticized	O
for	O
lacking	O
formal	O
correctness	O
by	O
the	O
DTSS	O
community	O
,	O
and	O
the	O
clock	O
selection	O
procedure	O
was	O
modified	O
to	O
incorporate	O
Marzullo	B-Operating_System
's	I-Operating_System
algorithm	I-Operating_System
for	O
NTPv3	O
onwards	O
.	O
</s>
<s>
Broadcast	B-Operating_System
mode	O
was	O
introduced	O
.	O
</s>
<s>
Following	O
the	O
retirement	O
of	O
Mills	O
from	O
the	O
University	O
of	O
Delaware	O
,	O
the	O
reference	O
implementation	O
is	O
currently	O
maintained	O
as	O
an	O
open	B-License
source	I-License
project	O
led	O
by	O
Harlan	O
Stenn	O
.	O
</s>
<s>
On	O
the	O
IANA	O
side	O
,	O
a	O
ntp	O
(	O
network	B-Protocol
time	I-Protocol
protocols	I-Protocol
)	O
work	O
group	O
is	O
in	O
charge	O
of	O
reviewing	O
proposed	O
drafts	O
.	O
</s>
<s>
An	O
unrelated	O
draft	O
termed	O
"	O
NTPv5	O
"	O
by	O
M	O
.	O
Lichvar	O
of	O
chrony	B-Application
was	O
initiated	O
in	O
2020	O
and	O
includes	O
security	O
,	O
accuracy	O
,	O
and	O
scaling	O
changes	O
.	O
</s>
<s>
As	O
NTP	O
replaces	O
the	O
use	O
of	O
the	O
old	O
Time	B-Protocol
Protocol	I-Protocol
,	O
some	O
use-cases	O
nevertheless	O
finds	O
the	O
full	O
protocol	O
too	O
complicated	O
.	O
</s>
<s>
In	O
1992	O
,	O
Simple	O
Network	B-Protocol
Time	I-Protocol
Protocol	I-Protocol
(	O
SNTP	O
)	O
was	O
defined	O
to	O
fill	O
this	O
niche	O
.	O
</s>
<s>
The	O
SNTPv3	O
standard	O
describes	O
a	O
way	O
to	O
use	O
NTPv3	O
,	O
such	O
that	O
no	O
storage	O
of	O
state	B-Application
over	O
extended	O
periods	O
of	O
time	B-Protocol
is	O
needed	O
.	O
</s>
<s>
The	O
topology	O
becomes	O
essentially	O
the	O
same	O
as	O
with	O
the	O
Time	B-Protocol
Protocol	I-Protocol
,	O
as	O
only	O
one	O
server	O
is	O
used	O
.	O
</s>
<s>
However	O
,	O
the	O
simple	O
algorithms	O
provide	O
times	O
of	O
reduced	O
accuracy	O
and	O
thus	O
it	O
is	O
inadvisable	O
to	O
sync	O
time	B-Protocol
from	O
an	O
SNTP	O
source	O
.	O
</s>
<s>
NTP	O
uses	O
a	O
hierarchical	O
,	O
semi-layered	O
system	O
of	O
time	B-Protocol
sources	O
.	O
</s>
<s>
Stratum	O
is	O
not	O
always	O
an	O
indication	O
of	O
quality	O
or	O
reliability	O
;	O
it	O
is	O
common	O
to	O
find	O
stratum	O
3	O
time	B-Protocol
sources	O
that	O
are	O
higher	O
quality	O
than	O
other	O
stratum	O
2	O
time	B-Protocol
sources	O
.	O
</s>
<s>
These	O
are	O
high-precision	O
timekeeping	O
devices	O
such	O
as	O
atomic	O
clocks	O
,	O
GNSS	O
(	O
including	O
GPS	O
)	O
or	O
other	O
radio	O
clocks	O
,	O
or	O
a	O
PTP-synchonized	O
clock	O
.	O
</s>
<s>
They	O
generate	O
a	O
very	O
accurate	O
pulse	O
per	O
second	O
signal	O
that	O
triggers	O
an	O
interrupt	B-Application
and	O
timestamp	O
on	O
a	O
connected	O
computer	O
.	O
</s>
<s>
NTP	B-Protocol
servers	I-Protocol
cannot	O
advertise	O
themselves	O
as	O
stratum	O
0	O
.	O
</s>
<s>
These	O
are	O
computers	O
whose	O
system	B-Operating_System
time	I-Operating_System
is	O
synchronized	O
to	O
within	O
a	O
few	O
microseconds	O
of	O
their	O
attached	O
stratum	O
0	O
devices	O
.	O
</s>
<s>
Stratum	O
1	O
servers	O
may	O
peer	O
with	O
other	O
stratum	O
1	O
servers	O
for	O
sanity	B-Error_Name
check	I-Error_Name
and	O
backup	O
.	O
</s>
<s>
They	O
are	O
also	O
referred	O
to	O
as	O
primary	O
time	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
Stratum	O
2	O
computers	O
may	O
also	O
peer	O
with	O
other	O
stratum	O
2	O
computers	O
to	O
provide	O
more	O
stable	O
and	O
robust	O
time	B-Protocol
for	O
all	O
devices	O
in	O
the	O
peer	O
group	O
.	O
</s>
<s>
The	O
NTP	O
algorithms	O
on	O
each	O
computer	O
interact	O
to	O
construct	O
a	O
Bellman-Ford	B-Algorithm
shortest-path	O
spanning	O
tree	O
,	O
to	O
minimize	O
the	O
accumulated	O
round-trip	O
delay	O
to	O
the	O
stratum	O
1	O
servers	O
for	O
all	O
the	O
clients	O
.	O
</s>
<s>
For	O
servers	O
on	O
stratum	O
2	O
and	O
below	O
,	O
the	O
refid	O
is	O
an	O
encoded	O
form	O
of	O
the	O
upstream	O
time	B-Protocol
server	I-Protocol
's	O
IP	O
address	O
.	O
</s>
<s>
Some	O
examples	O
are	O
INIT	O
(	O
initialization	O
)	O
,	O
STEP	O
(	O
step	O
time	B-Protocol
change	O
)	O
,	O
and	O
RATE	O
(	O
client	O
requesting	O
too	O
fast	O
)	O
.	O
</s>
<s>
The	O
64-bit	O
binary	O
fixed-point	O
timestamps	O
used	O
by	O
NTP	O
consist	O
of	O
a	O
32-bit	O
part	O
for	O
seconds	O
and	O
a	O
32-bit	O
part	O
for	O
fractional	O
second	O
,	O
giving	O
a	O
time	B-Protocol
scale	O
that	O
rolls	B-Error_Name
over	I-Error_Name
every	O
232	O
seconds	O
(	O
136	O
years	O
)	O
and	O
a	O
theoretical	O
resolution	O
of	O
2−32	O
seconds	O
(	O
233	O
picoseconds	O
)	O
.	O
</s>
<s>
According	O
to	O
Mills	O
,	O
"	O
The	O
64-bit	O
value	O
for	O
the	O
fraction	O
is	O
enough	O
to	O
resolve	O
the	O
amount	O
of	O
time	B-Protocol
it	O
takes	O
a	O
photon	O
to	O
pass	O
an	O
electron	O
at	O
the	O
speed	O
of	O
light	O
.	O
</s>
<s>
The	O
64-bit	O
second	O
value	O
is	O
enough	O
to	O
provide	O
unambiguous	O
time	B-Protocol
representation	O
until	O
the	O
universe	O
goes	O
dim.	O
"	O
</s>
<s>
A	O
typical	O
NTP	O
client	O
regularly	O
polls	B-General_Concept
one	O
or	O
more	O
NTP	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
The	O
client	O
must	O
compute	O
its	O
time	B-Protocol
offset	O
and	O
round-trip	O
delay	O
.	O
</s>
<s>
Time	B-Protocol
offset	O
θ	O
is	O
positive	O
or	O
negative	O
(	O
client	O
time	B-Protocol
>	O
server	O
time	B-Protocol
)	O
difference	O
in	O
absolute	O
time	B-Protocol
between	O
the	O
two	O
clocks	O
.	O
</s>
<s>
Solving	O
for	O
θ	O
yields	O
the	O
definition	O
of	O
the	O
time	B-Protocol
offset	O
.	O
</s>
<s>
Outliers	O
are	O
discarded	O
and	O
an	O
estimate	O
of	O
time	B-Protocol
offset	O
is	O
derived	O
from	O
the	O
best	O
three	O
remaining	O
candidates	O
.	O
</s>
<s>
Accurate	O
synchronization	O
is	O
achieved	O
when	O
both	O
the	O
incoming	O
and	O
outgoing	O
routes	B-Protocol
between	O
the	O
client	O
and	O
the	O
server	O
have	O
symmetrical	O
nominal	O
delay	O
.	O
</s>
<s>
If	O
the	O
routes	B-Protocol
do	O
not	O
have	O
a	O
common	O
nominal	O
delay	O
,	O
a	O
systematic	O
bias	O
exists	O
of	O
half	O
the	O
difference	O
between	O
the	O
forward	O
and	O
backward	O
travel	O
times	O
.	O
</s>
<s>
A	O
number	O
of	O
approaches	O
have	O
been	O
proposed	O
to	O
measure	O
asymmetry	O
,	O
but	O
among	O
practical	O
implementations	O
only	O
chrony	B-Application
seems	O
to	O
have	O
one	O
included	O
.	O
</s>
<s>
Backwards	B-General_Concept
compatibility	I-General_Concept
has	O
been	O
maintained	O
as	O
new	O
features	O
have	O
been	O
added	O
.	O
</s>
<s>
It	O
runs	O
as	O
a	O
daemon	B-Operating_System
called	O
ntpd	B-Protocol
under	O
Unix	B-Application
or	O
as	O
a	O
service	O
under	O
Windows	O
.	O
</s>
<s>
All	O
Microsoft	B-Application
Windows	I-Application
versions	O
since	O
Windows	B-Application
2000	I-Application
include	O
the	O
Windows	O
Time	B-Protocol
service	O
(	O
W32Time	O
)	O
,	O
which	O
has	O
the	O
ability	O
to	O
synchronize	O
the	O
computer	O
clock	O
to	O
an	O
NTP	B-Protocol
server	I-Protocol
.	O
</s>
<s>
W32Time	O
was	O
originally	O
implemented	O
for	O
the	O
purpose	O
of	O
the	O
Kerberos	B-Protocol
version	O
5	O
authentication	O
protocol	O
,	O
which	O
required	O
time	B-Protocol
to	O
be	O
within	O
5	O
minutes	O
of	O
the	O
correct	O
value	O
to	O
prevent	O
replay	O
attacks	O
.	O
</s>
<s>
The	O
version	O
in	O
Windows	B-Application
2000	I-Application
and	O
Windows	O
XP	O
only	O
implements	O
SNTP	O
,	O
and	O
violates	O
several	O
aspects	O
of	O
the	O
NTP	O
version	O
3	O
standard	O
.	O
</s>
<s>
Beginning	O
with	O
Windows	B-Application
Server	I-Application
2003	I-Application
and	O
Windows	B-Application
Vista	I-Application
,	O
W32Time	O
became	O
compatible	O
with	O
a	O
significant	O
subset	O
of	O
NTPv3	O
.	O
</s>
<s>
Microsoft	O
states	O
that	O
W32Time	O
cannot	O
reliably	O
maintain	O
time	B-Protocol
synchronization	O
with	O
one	O
second	O
accuracy	O
.	O
</s>
<s>
Beginning	O
with	O
Windows	B-Operating_System
10	I-Operating_System
version	O
1607	O
and	O
Windows	B-Device
Server	I-Device
2016	I-Device
,	O
W32Time	O
can	O
be	O
configured	O
to	O
reach	O
time	B-Protocol
accuracy	O
of	O
1	O
s	O
,	O
50	O
ms	O
or	O
1ms	O
under	O
certain	O
specified	O
operating	O
conditions	O
.	O
</s>
<s>
In	O
2004	O
,	O
Henning	O
Brauer	O
of	O
OpenBSD	B-Operating_System
presented	O
OpenNTPD	B-Application
,	O
an	O
NTPv3/SNTPv4	O
implementation	O
with	O
a	O
focus	O
on	O
security	O
and	O
encompassing	O
a	O
privilege	O
separated	O
design	O
.	O
</s>
<s>
Whilst	O
it	O
is	O
aimed	O
more	O
closely	O
at	O
the	O
simpler	O
generic	O
needs	O
of	O
OpenBSD	B-Operating_System
users	O
,	O
it	O
also	O
includes	O
some	O
protocol	O
security	O
improvements	O
while	O
still	O
being	O
compatible	O
with	O
existing	O
NTP	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
NTPsec	O
is	O
a	O
fork	B-Application
of	O
the	O
reference	O
implementation	O
that	O
has	O
been	O
systematically	O
security-hardened	O
.	O
</s>
<s>
The	O
fork	B-Application
point	O
was	O
in	O
June	O
2015	O
and	O
was	O
in	O
response	O
to	O
a	O
series	O
of	O
compromises	O
in	O
2014	O
.	O
</s>
<s>
Between	O
removal	O
of	O
unsafe	O
features	O
,	O
removal	O
of	O
support	O
for	O
obsolete	O
hardware	O
,	O
and	O
removal	O
of	O
support	O
for	O
obsolete	O
Unix	B-Application
variants	O
,	O
NTPsec	O
has	O
been	O
able	O
to	O
pare	O
away	O
75%	O
of	O
the	O
original	O
codebase	O
,	O
making	O
the	O
remainder	O
easier	O
to	O
audit	O
.	O
</s>
<s>
chrony	B-Application
is	O
an	O
independent	O
NTP	O
implementation	O
mainly	O
sponsored	O
by	O
Red	O
Hat	O
,	O
who	O
uses	O
it	O
as	O
the	O
default	O
time	B-Protocol
program	O
in	O
their	O
distributions	O
.	O
</s>
<s>
Being	O
written	O
from	O
scratch	O
,	O
chrony	B-Application
has	O
a	O
simpler	O
codebase	O
allowing	O
for	O
better	O
security	O
and	O
lower	O
resource	O
consumption	O
.	O
</s>
<s>
It	O
does	O
not	O
however	O
compromise	O
on	O
accuracy	O
,	O
instead	O
syncing	O
faster	O
and	O
better	O
than	O
the	O
reference	O
ntpd	B-Protocol
in	O
many	O
circumstances	O
.	O
</s>
<s>
Chrony	B-Application
has	O
been	O
evaluated	O
as	O
"	O
trustworthy	O
"	O
,	O
with	O
only	O
a	O
few	O
incidents	O
.	O
</s>
<s>
It	O
is	O
able	O
to	O
achieve	O
improved	O
precision	O
on	O
LAN	B-General_Concept
connections	O
,	O
using	O
hardware	O
timestamping	O
on	O
the	O
network	O
adapter	O
.	O
</s>
<s>
Support	O
for	O
Network	B-Protocol
Time	I-Protocol
Security	I-Protocol
(	O
NTS	O
)	O
was	O
added	O
on	O
version	O
4.0	O
.	O
is	O
available	O
under	O
GNU	O
General	O
Public	O
License	O
version	O
2	O
,	O
was	O
created	O
by	O
Richard	O
Curnow	O
in	O
1997	O
and	O
is	O
currently	O
maintained	O
by	O
Miroslav	O
Lichvar	O
.	O
</s>
<s>
was	O
started	O
by	O
Poul-Henning	O
Kamp	O
of	O
FreeBSD	B-Operating_System
in	O
2014	O
and	O
abandoned	O
in	O
2015	O
.	O
</s>
<s>
The	O
implementation	O
was	O
sponsored	O
by	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
.	O
</s>
<s>
is	O
the	O
SNTP	O
client	O
built	O
into	O
systemd	B-Application
.	O
</s>
<s>
On	O
the	O
day	O
of	O
a	O
leap	O
second	O
event	O
,	O
ntpd	B-Protocol
receives	O
notification	O
from	O
either	O
a	O
configuration	B-Protocol
file	I-Protocol
,	O
an	O
attached	O
reference	O
clock	O
,	O
or	O
a	O
remote	O
server	O
.	O
</s>
<s>
Although	O
the	O
NTP	O
clock	O
is	O
actually	O
halted	O
during	O
the	O
event	O
,	O
because	O
of	O
the	O
requirement	O
that	O
time	B-Protocol
must	O
appear	O
to	O
be	O
strictly	O
increasing	O
,	O
any	O
processes	B-Operating_System
that	O
query	O
the	O
system	B-Operating_System
time	I-Operating_System
cause	O
it	O
to	O
increase	O
by	O
a	O
tiny	O
amount	O
,	O
preserving	O
the	O
order	O
of	O
events	O
.	O
</s>
<s>
An	O
alternative	O
implementation	O
,	O
called	O
leap	O
smearing	O
,	O
consists	O
in	O
introducing	O
the	O
leap	O
second	O
incrementally	O
during	O
a	O
period	O
of	O
24	O
hours	O
,	O
from	O
noon	O
to	O
noon	O
in	O
UTC	O
time	B-Protocol
.	O
</s>
<s>
This	O
implementation	O
is	O
used	O
by	O
Google	O
(	O
both	O
internally	O
and	O
on	O
their	O
public	O
NTP	B-Protocol
servers	I-Protocol
)	O
,	O
Amazon	O
AWS	O
,	O
and	O
Facebook	O
.	O
</s>
<s>
Chrony	B-Application
supports	O
leap	O
smear	O
in	O
and	O
configurations	O
,	O
but	O
such	O
use	O
is	O
not	O
to	O
be	O
mixed	O
with	O
a	O
public	O
NTP	B-Protocol
pool	I-Protocol
as	O
leap	O
smear	O
is	O
non-standard	O
and	O
will	O
throw	O
off	O
client	O
calculation	O
in	O
a	O
mix	O
.	O
</s>
<s>
A	O
stack	B-General_Concept
buffer	I-General_Concept
overflow	I-General_Concept
exploit	O
was	O
discovered	O
and	O
patched	O
in	O
2014	O
.	O
</s>
<s>
Apple	O
was	O
concerned	O
enough	O
about	O
this	O
vulnerability	O
that	O
it	O
used	O
its	O
auto-update	O
capability	O
for	O
the	O
first	O
time	B-Protocol
.	O
</s>
<s>
Some	O
implementation	O
errors	O
are	O
basic	O
,	O
such	O
as	O
a	O
missing	O
return	O
statement	O
in	O
a	O
routine	O
,	O
that	O
can	O
lead	O
to	O
unlimited	O
access	O
to	O
systems	O
that	O
are	O
running	O
some	O
versions	O
of	O
NTP	O
in	O
the	O
root	O
daemon	B-Operating_System
.	O
</s>
<s>
Systems	O
that	O
do	O
not	O
use	O
the	O
root	O
daemon	B-Operating_System
,	O
such	O
as	O
those	O
derived	O
from	O
Berkeley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
(	O
BSD	B-Operating_System
)	O
,	O
are	O
not	O
subject	O
to	O
this	O
flaw	O
.	O
</s>
<s>
A	O
2017	O
security	O
audit	O
of	O
three	O
NTP	O
implementations	O
,	O
conducted	O
on	O
behalf	O
of	O
the	B-Operating_System
Linux	I-Operating_System
Foundation	I-Operating_System
's	O
Core	O
Infrastructure	O
Initiative	O
,	O
suggested	O
that	O
both	O
NTP	O
and	O
NTPsec	O
were	O
more	O
problematic	O
than	O
Chrony	B-Application
from	O
a	O
security	O
standpoint	O
.	O
</s>
<s>
NTP	B-Protocol
servers	I-Protocol
can	O
be	O
susceptible	O
to	O
man-in-the-middle	O
attacks	O
unless	O
packets	O
are	O
cryptographically	O
signed	O
for	O
authentication	O
.	O
</s>
<s>
Some	O
of	O
the	O
services	O
affected	O
by	O
fake	O
NTP	O
messages	O
identified	O
are	O
TLS	B-Protocol
,	O
DNSSEC	B-Protocol
,	O
various	O
caching	O
schemes	O
(	O
such	O
as	O
DNS	O
cache	O
)	O
,	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
,	O
Bitcoin	O
and	O
a	O
number	O
of	O
persistent	O
login	O
schemes	O
.	O
</s>
<s>
A	O
small	O
query	O
is	O
sent	O
to	O
an	O
NTP	B-Protocol
server	I-Protocol
with	O
the	O
return	O
IP	O
address	O
spoofed	O
to	O
be	O
the	O
target	O
address	O
.	O
</s>
<s>
To	O
avoid	O
participating	O
in	O
an	O
attack	O
,	O
NTP	B-Protocol
server	I-Protocol
software	O
can	O
be	O
upgraded	O
or	O
servers	O
can	O
be	O
configured	O
to	O
ignore	O
external	O
queries	O
.	O
</s>
<s>
NTPv3	O
supports	O
a	O
symmetric	B-Algorithm
key	I-Algorithm
mode	O
,	O
which	O
is	O
not	O
useful	O
against	O
MITM	O
.	O
</s>
<s>
The	O
public	B-Application
key	I-Application
system	O
known	O
as	O
"	O
autokey	O
"	O
in	O
NTPv4	O
adapted	O
from	O
IPSec	B-Protocol
offers	O
useful	O
authentication	O
,	O
but	O
is	O
not	O
practical	O
for	O
a	O
busy	O
server	O
.	O
</s>
<s>
Autokey	O
was	O
also	O
later	O
found	O
to	O
suffer	O
from	O
several	O
design	O
flaws	O
,	O
with	O
no	O
correction	O
published	O
,	O
save	O
for	O
a	O
change	O
in	O
the	O
message	B-Algorithm
authentication	I-Algorithm
code	I-Algorithm
in	O
.	O
</s>
<s>
Network	B-Protocol
Time	I-Protocol
Security	I-Protocol
(	O
NTS	O
)	O
is	O
a	O
secure	O
version	O
of	O
NTPv4	O
with	O
TLS	B-Protocol
and	O
AEAD	B-Algorithm
.	O
</s>
<s>
The	O
main	O
improvement	O
over	O
previous	O
attempts	O
is	O
that	O
a	O
separate	O
"	O
key	O
establishment	O
"	O
server	O
handles	O
the	O
heavy	O
asymmetric	B-Application
cryptography	I-Application
,	O
which	O
needs	O
to	O
be	O
done	O
only	O
once	O
.	O
</s>
<s>
If	O
the	O
server	O
goes	O
down	O
,	O
previous	O
users	O
would	O
still	O
be	O
able	O
to	O
fetch	O
time	B-Protocol
without	O
fear	O
of	O
MITM	O
.	O
</s>
<s>
NTS	O
is	O
currently	O
supported	O
by	O
several	O
time	B-Protocol
servers	I-Protocol
,	O
including	O
CloudFlare	B-Application
.	O
</s>
<s>
It	O
is	O
supported	O
by	O
NTPSec	O
and	O
chrony	B-Application
.	O
</s>
<s>
Microsoft	O
also	O
has	O
an	O
approach	O
to	O
authenticate	O
NTPv3/SNTPv4	O
packets	O
using	O
a	O
Windows	B-General_Concept
domain	I-General_Concept
identity	O
,	O
known	O
as	O
MS-SNTP	O
.	O
</s>
<s>
This	O
system	O
is	O
implemented	O
in	O
the	O
reference	O
ntpd	B-Protocol
and	O
chrony	B-Application
,	O
using	O
samba	B-Language
for	O
the	O
domain	O
connection	O
.	O
</s>
