gpsd/www/gpsd-time-service-howto.adoc

= GPSD Time Service HOWTO
:description: How to set up an NTP Stratum 1 server using GPSD.
:keywords: time, GPSD, NTP, time, precision, 1PPS, PPS, stratum, jitter
Gary E. Miller <gem@rellim.com>, Eric S. Raymond <esr@thyrsus.com>
28 June 2020

This document is mastered in asciidoc format.  If you are reading it in HTML,
you can find the original at the GPSD project website.

== Introduction

GPSD, NTP and a GPS receiver supplying 1PPS (one pulse-per-second)
output can be used to set up a high-quality NTP time server. This
HOWTO explains the method and various options you have in setting it
up.

Here is the quick-start sequence. The rest of this document goes
into more detail about the steps.

. Ensure that gpsd and either ntpd or chronyd are installed on your
  system. (Both gpsd and ntpd are pre-installed in many stock Linux
  distributions; chronyd is normally not.) You don't have to choose
  which to use yet if you have easy access to both, but knowing which
  alternatives are readily available to you is a good place to start.

. Verify that your gpsd version is at least 3.20.  Many problems are
  caused by the use of old versions.  When in doubt, reinstall
  gpsd from the upstream source.  Many distributions ship old
  and/or broken versions of gpsd.

. Connect a PPS-capable GPS receiver to one of your serial or USB
  ports.  A random cheap consumer-grade GPS receiver won't do; you
  may have to do some hunting to find a usable one.

. Check that it actually emits PPS by pointing GPSD's gpsmon utility
  at the port.  If it has a good (3D-mode) fix, lines marked "PPS"
  should scroll by in the packet-logging window.  A new device out of
  the box may take up to 30 minutes for the first 3D fix.  If gpsmon
  shows a 3D fix, but does not show PPS lines, try running ppscheck.

. If you persistently fail to get live PPS, (a) you may have a
  skyview problem, (b) you may have a cabling problem, (c) your GPS
  may not support PPS, (d) you may have a gpsd or kernel configuration
  problem, (e) you may have a device problem, (e) there may be a bug
  in the core GPSD code used by gpsmon.  These are listed in roughly
  decreasing probability.  Troubleshoot appropriately.

. Edit your ntpd or chronyd configuration to tell your NTP daemon to
  listen for time hints. (This step is somewhat tricky.)

. Start up gpsd.  If you are using ntpd, you can use ipcrm(1) to check that
  verify that the shared-memory segment that gpsd and ntpd want to
  use to communicate has been attached; or you can impatiently skip
  to the next step and look for the segment only if that fails.

. Use cgps or xgps to verify that your GNSS receiver has a good 3D fix.

. Use ntpshmmon to verify that gpsd is sending time corrections to SHM
  memory.

. Use ntpq or the chronyc sources command to verify that your device
  is feeding time corrections to your NTP daemon.

. (Optional and challenging.) Hand-tune your installation for the
  best possible performance.

This document does not attempt to explain all the intricacies of time
service; it is focused on practical advice for one specific deployment
case.  There is an introduction <<TIME-INTRO>> to basic concepts and
terminology for those new to time service. An overview of the NTP
protocols can be found at <<WIKI-NTP>>, and the official NTP FAQ
<<NTP-FAQ>> is probably as gentle an introduction to the NTP reference
implementation as presently exists.

We encourage others to contribute additions and corrections.

.Units table
|====================================================
| nSec    | nanoSecond  | 1/1,000,000,000 of a second
| uSec    | microSecond | 1/1,000,000 of a second
| mSec    | milliSecond | 1/1,000 of a second
|====================================================

There are a few important terms we need to define up front.  *Latency*
is delay from a time measurement until a report on it arrives where it
is needed. *Jitter* is short-term variation in latency. *Wobble* is a
jitter-like variation that is long compared to typical measurement
periods.  *Accuracy* is the traceable offset from 'true' time as
defined by a national standard institute.

A good analogy to jitter vs wobble is changes in sea level on a beach.
Jitter is caused by wave action, wobble is the daily effect of tides.
For a time server, the most common causes of wobble are varying GPS
satellite geometries and the effect of daily temperature variations on
the oscillators in your equipment.

== NTP with GPSD

See <<TIME-INTRO>> for a technical description of how NTP corrects
your computer's clock against wobble. For purposes of this how-to, the
important concepts to take way are those of time strata, servers, and
reference clocks.

Ordinary NTP client computers are normally configured to get time from
one or more Stratum 2 (or less commonly Stratum 3) NTP
servers. However, with GPSD and a suitable GPS receiver, you can easily
condition your clock to higher accuracy than what you get from typical
Stratum 2; with a little effort, you can do better than you can get
from most public Stratum 1 servers.

You can then make your high-quality time available to other systems on
your network, or even run a public NTP server.  Anyone can do this;
there is no official authority, and any NTP client may choose to use
your host as a server by requesting time from it. The time-service
network is self-regulating, with NTP daemons constantly pruning
statistical outliers so the timebase cannot be accidentally or
deliberately compromised.

In fact many public and widely-trusted Stratum 1 servers use GPS
receivers as their reference clocks, and a significant fraction of
those use GPSD in the way we will describe here.

== GPS time

The way time is shipped from GPS satellites causes problems to
beware of in certain edge cases.

Date and time in GPS is represented as number of weeks from the start
of zero second of 6 January 1980, plus number of seconds into the
week. GPS time is *not* leap-second corrected, though satellites also
broadcast a current leap-second correction which is updated on
six-month boundaries according to rotational bulletins issued by the
International Earth Rotation and Reference Systems Service (IERS).

The leap-second correction is only included in the multiplexed satellite
subframe broadcast, once every 12.5 minutes.  While the satellites do
notify GPSes of upcoming leap-seconds, this notification is not
necessarily processed correctly on consumer-grade devices, and may not
be available at all when a GPS receiver has just cold-booted. Thus,
reported UTC time may be slightly inaccurate between a cold boot or leap
second and the following subframe broadcast.

GPS date and time are subject to a rollover problem in the 10-bit week
number counter, which will re-zero every 1024 weeks (roughly every 19.6
years). The first rollover since GPS went live in 1980 was in Aug-1999,
followed by Apr-2019, the next will be in Nov-2038 (the 32-bit and POSIX
issues will probably be more important by then).  The new "CNAV" data
format extends the week number to 13 bits, with the first rollover
occurring in Jan-2137, but this is only used with some newly added GPS
signals, and is unlikely to be usable in most consumer-grade receivers
currently.

For accurate time reporting, therefore, a GPS requires a supplemental
time references sufficient to identify the current rollover period,
e.g. accurate to within 512 weeks. Many GPSes have a wired-in (and
undocumented) assumption about the UTC time of the last rollover and
will thus report incorrect times outside the rollover period they were
designed in.

For accurate time service via GPSD, you require three things:

* A GPS made since the last rollover, so its hidden assumption about
 the epoch will be correct.

* Enough time elapsed since a cold boot or IERS leap-second adjustment
  for the current leap-second to get update.

* A GPS that properly handles leap-second adjustments.  Anything
  based on a u-blox from v6 onward should be good; the status of
  SiRFs is unknown and doubtful.

== 1PPS quality issues

GPSD is useful for precision time service because it can use the 1PPS
pulse delivered by some GPS receivers to discipline (correct) a local
NTP instance.

It's tempting to think one could use a GPS receiver for time service
just by timestamping the arrival of the first character in the report
on each fix and correcting for a relatively small fixed latency
composed of fix-processing and RS232 transmission time.

At one character per ten bits (counting framing and stopbits) a
9600-bps serial link introduces about a mSec of latency *per
character*; furthermore, your kernel will normally delay delivery
of characters to your application until the next timer tick, about
every 4 mSec in modern kernels. Both USB and RS232 will incur that
approximately 5mSec-per-char latency overhead.  You'll have to deal
with this latency even on chips like the Venus 6 that claim the
beginning of their reporting burst is synced to PPS.  (Such claims are
not always reliable, in any case.)

Unfortunately, fix reports are also delayed in the receiver and on
the link by as much as several hundred mSec, and this delay is not
constant. This latency varies (wobbles) throughout the day.  It may be
stable to 10 mSec for hours and then jump by 200mSec.  Under these
circumstances you can't expect accuracy to UTC much better than 1
second from this method.

For example: SiRF receivers, the make currently most popular in
consumer-grade GPS receivers, exhibit a wobble of about 170mSec in the
offset between actual top-of-second and the transmission of the first
sentence in each reporting cycle.

To get accurate time, then, the in-band fix report from the GPS
receiver needs to be supplemented with an out-of-band signal that has
a low and constant or near-constant latency with respect to the time
of the fix.  GPS satellites deliver a top-of-GPS-second
notification that is nominally accurate to 50nSec; in capable GPS
receivers that becomes the 1PPS signal.

1PPS-capable GPS receivers use an RS-232 control line to ship the 1PPS
edge of second to the host system (usually Carrier Detect or Ring
Indicator; GPSD will quietly accept either).  Satellite top-of-second
loses some accuracy on the way down due mainly to variable delays in
the ionosphere; processing overhead in the GPS receiver itself adds a
bit more latency, and your local host detecting that pulse adds still
more latency and jitter.  But it's still often accurate to on the
order of 1 uSec.

Under most Unixes there are two ways to watch 1PPS; Kernel PPS (KPPS)
and plain PPS latching.  KPPS is an implementation of RFC 2783 <<RFC-2783>>.
Plain PPS just references the pulse to the system clock as
measured in user space.  These have different error budgets.

Kernel PPS uses a kernel function to accurately timestamp the status
change on the PPS line.  Plain PPS has the kernel wake up the GPSD PPS
thread and then the PPS thread reads the current system clock.  As
noted in the GPSD code, having the kernel do the time stamp yields
lower latency and less jitter. Both methods have accuracy degraded by
interrupt-processing latency in the kernel serial layer, but plain
PPS incurs additional context-switching overhead that KPPS does not.

With KPPS it is very doable to get the system clock stable to &plusmn;1
uSec.  Otherwise, you are lucky to get &plusmn;5 uSec, and there will be
about 20uSec of jitter. All these figures were observed on
plain-vanilla x86 PCs with clock speeds in the 2GHz range.

All the previous figures assume you're using PPS delivered over RS232.
USB GPS receivers that deliver 1PPS are rare, but do exist. Notably,
there's the Navisys GR-601W/GR-701W/GR-801W <<MACX-1>>. In case these devices go
out of production it's worth noting that they are a trivial
modification of the stock two-chip-on-a-miniboard
commodity-GPS-receiver design of engine plus USB-to-serial adapter;
the GR-[678]01W wires a u-blox 6/7/8 to a Prolific Logic PL23203.  To
get 1PPS out, this design just wires the 1PPS pin from the GPS engine
to the Carrier Detect pin on the USB adapter. (This is known as the
"Macx-1 mod".)

With this design, 1PPS from the engine will turn into a USB event that
becomes visible to the host system (and GPSD) the next time the USB
device is polled. USB 1.1 polls 1024 slots every second.  Each slot is
polled in the same order every second.  When a device is added it is
assigned to one of those 1024 polling slots.  It should then be clear
that the accuracy of a USB 1.1 connected GPS receiver would be about 1
mSec.

As of mid-2016 no USB GPS receiver we know of implements the higher
polling-rate options in USB 2 and 3 or the interrupt capability in USB
3.  When one does, and if it has the Macx-1 mod, higher USB accuracy
will ensue.

.Summary of typical accuracy
|=====================================================
| GPS atomic clock      | &plusmn;50nSec
| KPPS                  | &plusmn;1uSec
| PPS                   | &plusmn;5uSec
| USB 1.1 poll interval | &plusmn;1mSec
| USB 2.0 poll interval | &plusmn;100&mu;Sec (100000 nSec)
| Network NTP time      | ~&plusmn;30mSec footnote:[RFC5905 says "a few tens of milliseconds", but asymmetric routing can produce 100mSec offset]
|=====================================================

Observed variations from the typical figure increase towards the bottom
of the table.  Notably, a heavily loaded host system can reduce PPS
accuracy further, though not KPPS accuracy except in the most extreme
cases.  The USB poll interval tends to be very stable (relative to its
1mSec or 100&mu;Sec base).

Network NTP time accuracy can be degraded below RFC5905's "a few tens
of milliseconds" by a number of factors. Almost all have more to do
with the quality of your Internet connection to your servers than with
the time accuracy of the servers themselves.  Some negatives:

* Having a cable modem.  That is, as opposed to DSL or optical fiber, which
  tend to have less variable latencies.

* Path delay asymmetries due to peering policy.  These can confuse
  NTP's reconciliation algorithms.

With these factors in play, worst-case error can reach up to
&plusmn;100mSec.  Fortunately, errors of over &plusmn;100mSec are
unusual and should occur only if all your network routes to servers
have serious problems.

== Software Prerequisites

If your kernel provides the RFC 2783 KPPS (kernel PPS) API, gpsd will
use that for extra accuracy. Many Linux distributions have a package
called "pps-tools" that will install KPPS support and the timepps.h
header file.  We recommend you do that.  If your kernel is built in
the normal modular way, this package installation will suffice.

=== Building gpsd

A normal gpsd build includes support for interpreting 1PPS pulses that
is mostly autoconfiguring and requires no special setup.  If the
current system, and GNSS receiver, supports pps.

You can build a version stripped to the minimum configuration required
for time service.  This reduces the size of the binary and may be
helpful on embedded systems or for SBCs like the Raspberry Pi, Odroid,
or BeagleBone.  Only do this if you have serious size constraints, much
functionality will be lost.

When gpsd is built with timeservice=yes:

1.  The -n (nowait) option is forced: gpsd opens its command-line devices
    immediately on startup.  Assuming you do not start gpsd with systemd.

2.  Forces the building of ntpshmmon and cgps.  Those programs would
    be built by default anyway, unless gpsdclients=no.

3.  The configure will fail if pps is not available.

4.  Most drivers will not be built.  You must specify the ones you need
    when configuring.

To configure the minimal timeservice build:

-----------------------------------------------------------------------------
$ scons -c
$ scons timeservice=yes nmea0183=yes
-----------------------------------------------------------------------------

You may substitute a different GNSS reciver protocol (e.g. "ublox" or
"sirf").  Besides the daemon, this also builds cgps and ntpshmmon.

If you do not use timeservice=yes, then make sure the build is with
pps=yes and ntpshm=yes (the defaults).  Like this:

-----------------------------------------------------------------------------
$ scons -c
$ scons pps=yes ntpshm=yes
-----------------------------------------------------------------------------

More complete, and distro specific, build instructions can be found in
the files INSTALL.adoc and build.adoc in the source distribution.


=== Kernel support

If you are scratch-building a Linux kernel, the configuration
must include either these two lines, or the same with "y" replaced
by "m" to enable the drivers as modules:

-----------------------------------------------------------------------------
CONFIG_PPS=y
CONFIG_PPS_CLIENT_LDISC=y
-----------------------------------------------------------------------------

Some embedded systems, like the Raspberry Pi, detect PPS on a GPIO
line instead of on a serial port line.  For those systems you will
also need these two lines:

-----------------------------------------------------------------------------
CONFIG_PPS_CLIENT_GPIO=y
CONFIG_GPIO_SYSFS=y
-----------------------------------------------------------------------------

Your Linux distribution may ship a file /boot/config-XXX (where XXX is
the name of a kernel) or one called /proc/config.gz (for the running
kernel).  This will have a list of the configuration options that were
used to build the kernel.  You can check if the above options are
set. Usually they will be set to "m", which is sufficient.

NetBSD has included the RFC2783 Pulse Per Second API for real serial
ports by default since 1998, and it works with ntpd.  NetBSD 7
(forthcoming) includes RFC2783 support for USB-serial devices, and
this works (with ntpd) with the GR-601W/GR-701W/GR-801W.  However,
gpsd's code interacts badly with the NetBSD implementation, and gpsd's
support for RFC2783 PPS does not yet work on NetBSD (for serial or
USB).

Other OSes have different ways to enable KPPS in their kernels.
When we learn what those are, we'll document them or point
at references.

=== Time service daemon

You will need to have either ntpd or chrony installed. If you are
running a Unix variant with a package system, the packages will
probably be named 'ntp' (or 'ntpsec') and either 'chrony' or 'chronyd'.

Between ntpd and chrony, ntpd is the older and more popular choice --
thus, the one with the best-established peer community if you need
help in unusual situations.  On the other hand, chrony has a
reputation for being easier to set up and configure, and is better in
situations where your machine has to be disconnected from the Internet
for long enough periods of time for the clock to drift significantly.

ntpd and chrony have differing philosophies, with ntpd more interested
in deriving consensus time from multiple sources while chrony tries to
identify a single best source and track it closely.

A feature comparison, part of the chrony documentation, is at
<<CHRONY-COMPARE>>. An informative email thread about the differences
is <<CHRONYDEFAULT>>. If you don't already know enough about time
service to have a preference, the functional differences between them
are unlikely to be significant to you; flip a coin.

=== NTPSec ===
If you choose the ntpd option, it's best to go with the NTPsec version
rather than legacy ntpd.  NTPsec shares some maintainers with GPSD,
and has some significant improvements in security and performance.

As of June 2020 2019, NTPsec is available as a package in:

* Alpine
* archlinux
* Debian (and variants like Ubuntu and Raspbian)
* Gentoo
* OpenSUSE

If it is not available as a package, you can build it from source,
<<GITLAB-SOURCE>>, it is not especially difficult.


== Choice of Hardware

To get 1PPS to your NTP daemon, you first need to get it from a
PPS-capable GPS receiver. As of early 2015 this means either the
previously mentioned GR devices or a serial GPS receiver with 1PPS.

You can find 1PPS-capable devices supported by GPSD at <<HARDWARE>>.
Note that the most popular consumer-grade GPS receivers do not usually
deliver 1PPS through USB or even RS232.  The usual run of cheap GPS
mice won't do.  In general, you can't use a USB device for time
service unless you know it has the Macx-1 mod.

In the past, the RS232 variant of the Garmin GPS-18 has been very
commonly used for time service (see <<LVC>> for a typical setup very
well described).  While it is still a respectable choice, newer
devices have better sensitivity and signal discrimination. This makes
them superior for indoor use as time sources.

In general, use a GPS receiver with an RS232 interface for time
service if you can.  The GR-601W was designed (by one of the authors,
as it happens) for deployment with commodity TCP/IP routers that only
have USB ports.  RS232 is more fiddly to set up (with older devices
like the GPS-18 you may even have to make your own cables) but it can
deliver three orders of magnitude better accuracy and repeatability --
enough to meet prevailing standards for a public Stratum 1 server.

Among newer receiver designs the authors found the u-blox line of
receivers used in the GR-[678]01W to be particularly good.  Very
detailed information on its timing performance can be found at
<<UBLOX-TIMING>>. One of us (Raymond) has recent experience with an
eval kit, the EVK 6H-0-001, that would make an excellent Stratum 0
device.

Both the EVK 6H and GR-601W are built around the LEA-6H module, which
is a relatively inexpensive but high-quality navigation GPS
receiver. We make a note of this because u-blox also has a specialized
timing variant, the LEA 6T, which would probably be overkill for an
NTP server. (The 6T does have the virtue that you could probably get a
good fix from one satellite in view once it knows its location, but
the part is expensive and difficult to find.)

Unfortunately as of early 2015 the LEA-6H is still hard to find in a
packaged RS232 version, as opposed to a bare OEM module exporting TTL
levels or an eval kit like the EVK 6H-0-001 costing upwards of
US$300. Search the web; you may find a here-today-gone-tomorrow offer
on alibaba.com or somewhere similar.

The LEA-6T, and some other higher-end GPS receivers (but not the
LEA-6H) have a stationary mode which, after you initialize it with the
device's location, can deliver time service with only one good
satellite lock (as opposed to the three required for a fix in its
normal mode). For most reliable service we recommend using stationary
mode if your device has it. GPSD tools don't yet directly support
this, but that capability may be added in a future release.

The design of your host system can also affect time quality.  The
&plusmn;5uSec error bound quoted above is for a dual-core or better
system with clock in the 2GHz range on which the OS can schedule the
long-running PPS thread in GPSD on an otherwise mostly unused
processor (the Linux scheduler, in particular, will do this). On a
single-core system, contention with other processes can pile
on several additional microseconds of error.

If you are super-serious about your time-nuttery, you may want to look
into the newest generation of dedicated Stratum 1 microservers being
built out of open-source SBCs like the Raspberry Pi and Beaglebone, or
sometimes with fully custom designs. A representative build is well
described at <<RPI>>.

These microserver designs avoid load-induced jitter by being fully
dedicated to NTP service.  They are small, low-powered devices and
often surprisingly inexpensive, as in costing less than US$100.  They
tend to favor the LEA-6H, and many of them use preinstalled GPSD on
board.

== Enabling PPS

You can determine whether your GPS receiver emits 1PPS, and gpsd is
detecting it, by running the gpsmon utility (giving it the GPS
receiver's serial-device path as argument).  Watch for lines of dashes
marked 'PPS' in the packet-logging window; for most GPS receiver types
there will also be a "PPS offset:" field in the data panels above
showing the delta between PPS and your local clock.

If you don't have gpsmon available, or you don't see PPS lines in it,
you can run ppscheck.  As a last resort you can gpsd at -D 5 and watch
for PPS state change messages in the logfile.

If you don't see evidence of incoming PPS, here are some trouble
sources to check:

1. The skyview of your GPS receiver may be poor.  Suspect this if,
   when you watch it with cgps, it wanders in and out of having a
   good 3D fix. Unfortunately, the only fix for this is to re-site
   your GPS where it can see more sky; fortunately, this is not as
   common a problem as it used to be, because modern receivers are
   often capable of getting a solid fix indoors.

2. If you are using an RS232 cable, examine it suspiciously, ideally
   with an RS232 breakout box. Cheap DB9 to DB9 cables such as those
   issued with UPSs often carry TXD/RXD/SG only, omitting handshake
   lines such as DCD, RI, and DSR that are used to carry 1PPS.
   Suspect this especially if the cable jacket looks too skinny to
   hold more than three leads!

3. Verify that your gpsd and kernel were both built with PPS support,
   as previously described in the section on software prerequisites.

4. Verify that the USB or RS232 device driver is accepting the ioctl
   that tells it to wait on a PPS state change from the device.  The
   messages you hope *not* to see look like "KPPS cannot set PPS line
   discipline" and "PPS ioctl(TIOCMIWAIT) failed".  The former
   can probably be corrected by running as root; the latter (which
   should never happen with an RS232 device) probably means your USB
   device driver lacks this wait capability entirely and cannot be
   used for time service.

5. If you have a solid 3D fix, a known-good cable, your software is
   properly configured, the wait ioctl succeeded, but you still get no
   PPS, then you might have a GPS receiver that fails to deliver PPS
   off the chip to the RS232 or USB interface.  You get to become
   intimate with datasheets and pinouts, and might need to acquire a
   different GPS receiver.

== Running GPSD

If you're going to use gpsd for time service, you must run in -n mode
so the clock will be updated even when no clients are active.  This option
is forced if you built GPSD with timeservice=yes as an option.

Note that gpsd assumes that after each fix the GPS receiver will
assert 1PPS first and ship sentences reporting time of fix
second (and the sentence burst will end before the next 1PPS). Every
GPS we know of does things in this order.  (However, on some very old
GPSes that defaulted to 4800 baud, long sentence bursts -- notably
those containing a skyview -- could slop over into the next second.)

If you ever encounter an exception, it should manifest as reported
times that look like they're from the future and require a negative
fudge. If this ever happens, please report the device make and model
to the GPSD maintainers, so we can flag it in our GPS hardware
database.

There is another possible cause of small negative offsets which
shows up on the GR-601W: implementation bugs in your USB driver,
combining with quantization by the USB poll interval.  This
doesn't mean the u-blox 6 inside it is actually emitting PPS
after the GPS timestamp is shipped.

In order to present the smallest possible attack surface to
privilege-escalation attempts, gpsd, if run as root, drops its root
privileges very soon after startup - just after it has opened any
serial device paths passed on the command line.

Thus, KPPS can only be used with devices passed that way, not with
GPSes that are later presented to gpsd by the hotplug system.  Those
hotplug devices may, however, be able to use plain, non-kernel
PPS. gpsd tries to automatically fall back to this when absence of
root permissions makes KPPS unavailable.

In general, if you start gpsd as other than root, the following things
will happen that degrade the accuracy of reported time:

1. Devices passed on the command line will be unable to use KPPS and
will fall back to the same plain PPS that all hotplug devices must
use, increasing the associated error from ~1 uSec to about ~5 uSec.

2. gpsd will be unable to renice itself to a higher priority.  This
action helps protect it against jitter induced by variable system
load. It's particularly important if your NTP server is a general-use
computer that's also handling mail or web service or development.

3. The way you have to configure ntpd and chrony will change away
from what we show you here; ntpd will need to be told different
shared-memory segment numbers, and chronyd will need a different
socket location.

4. gpsd will be unable to change to user nobody.  This means gpsd will
paradoxically run with higher privileges than if it was started as root.
This increases the attack surface and decreases your security.

You may also find gpsd can't open serial devices at all if your
OS distribution has done "secure" things with the permissions.

== Feeding NTPD from GPSD

Most Unix systems get their time service through ntpd, a very old and
stable open-source software suite which is the reference
implementation of NTP.  The project home page is <<NTP.ORG>>. We
recommend using NTPsec, a recent fork that is improved and
security-hardened <<NTPSEC.ORG>>.

When gpsd receives a sentence with a timestamp, it packages the
received timestamp with current local time and sends it to a
shared-memory segment with an ID known to ntpd, the network time
synchronization daemon.  If ntpd has been properly configured to
receive this message, it will be used to correct the system clock.

When in doubt, the preferred method to start your timekeeping is:

-----------------------------------------------------------------------------
$ su -
# killall -9 gpsd ntpd
# gpsd -n /dev/ttyXX
# sleep 2
# ntpd -gN
# sleep 2
# cgps
-----------------------------------------------------------------------------

where /dev/ttyXX is whatever 1PPS-capable device you have.  In a
binary-package-based Linux distribution it is probable that ntpd
will already have been launched at boot time.

It's best to have gpsd start first.  That way when ntpd restarts it has
a good local time handy.  If ntpd starts first, it will set the local
clock using a remote, probably pool, server.  Then ntpd has to spend a
whole day slowly resyncing the clock.

If you're using dhcp3-client to configure your system, make sure
you disable /etc/dhcp3/dhclient-exit-hooks.d/ntp, as dhclient would
restart ntpd with an automatically created ntp.conf otherwise -- and
gpsd would not be able to talk with ntpd anymore.

While gpsd may be runnable as non-root, you will get significantly
better accuracy of time reporting in root mode; the difference, while
almost certainly insignificant for feeding Stratum 1 time to clients
over the Internet, may matter for PTP service over a LAN.  Typically
only root can access kernel PPS, whereas in non-root mode you're limited to
plain PPS (if that feature is available).  As noted in the previous
section on 1PPS quality issues, this difference has performance
implications.

The rest of these setup instructions will assume that you are starting
gpsd as root, with occasional glances at the non-root case.

Now check to see if gpsd has correctly attached the shared-memory
segments in needs to communicate with ntpd.  ntpd's rules for the
creation of these segments are:

Segments 0 and 1::
	 Permissions are 0600 -- other programs can only read and
	 write this segment when running as root.

Segments 2, 3 and above::
	 Permissions are 0666 -- other programs can read
 	 and write as any user. If ntpd has been
 	 configured to use these segments, any
 	 unprivileged user is allowed to provide data
 	 for synchronization.

Because gpsd can be started either as root or non-root, it checks and
attaches the more privileged segment pair it can -- either 0 and 1 or 2
and 3.

For each GPS receiver that gpsd controls, it will use the attached ntpshm
segments in pairs (for coarse clock and pps source, respectively)
starting from the first found segments.

To debug, try looking at the live segments this way:

-----------------------------------------------------------------------------
# ipcs -m
-----------------------------------------------------------------------------

If gpsd was started as root, the results  should look like this:

-----------------------------------------------------------------------------
 ------ Shared Memory Segments --------
  key        shmid      owner      perms      bytes      nattch     status
  0x4e545030 0          root       700        96         2
  0x4e545031 32769      root       700        96         2
  0x4e545032 163842     root       666        96         1
  0x4e545033 196611     root       666        96         1
-----------------------------------------------------------------------------

For a bit more data try this:

-----------------------------------------------------------------------------
cat /proc/sysvipc/shm
-----------------------------------------------------------------------------

If gpsd cannot open the segments, check that you are not running SELinux
or apparmor. Either may require you to configure a security exception.

If you see the shared segments (keys 1314148400 -- 1314148403), and
no gpsd or ntpd is running then try removing them like this:

-----------------------------------------------------------------------------
# ipcrm -M 0x4e545030
# ipcrm -M 0x4e545031
# ipcrm -M 0x4e545032
# ipcrm -M 0x4e545033
-----------------------------------------------------------------------------

Here is a minimal sample ntp.conf configuration to work with GPSD run
as root, telling ntpd how to read the GPS notifications

-----------------------------------------------------------------------------
pool us.pool.ntp.org iburst

driftfile /var/lib/ntp/ntp.drift
logfile /var/log/ntp.log

restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery
restrict 127.0.0.1 mask 255.255.255.0
restrict -6 ::1

# GPS Serial data reference (NTP0)
server 127.127.28.0
fudge 127.127.28.0 time1 0.9999 refid GPS

# GPS PPS reference (NTP1)
server 127.127.28.1 prefer
fudge 127.127.28.1 refid PPS
-----------------------------------------------------------------------------

The number "0.9999" is a placeholder, to be explained shortly.  It
is *not a number to be used in production* - it's too large. If you
can't replace it with a real value, it would be best to leave out the
clause entirely so the entry looks like:

-----------------------------------------------------------------------------
fudge 127.127.28.0 refid GPS
-----------------------------------------------------------------------------

This is equivalent to declaring a time1 of 0.

The pool statement adds a variable number of servers (often 10) as
additional time references needed by ntpd for redundancy and to give you
a reference to see how well your local GPS receiver is performing.  If
you are outside of the USA replace the pool servers with one in your
local area. See <<USE-POOL>> for further information.

The pool statement, and the driftfile and logfile declarations after it,
will not be strictly necessary if the default ntp.conf that your
distribution supplies gives you a working setup. The two pairs of
server and fudge declarations are the key.

ntpd can be used in Denial of Service (DoS) attacks.  To prevent that,
but still allow clients to request the local time, be sure the 'restrict'
statements are in your ntpd config file.  For more information see
<<CVE-2009-3563>>.

Users of ntpd versions older than revision ntp-4.2.5p138 should instead use
this ntp.conf, when gpsd is started as root:

-----------------------------------------------------------------------------
pool us.pool.ntp.org iburst

driftfile /var/lib/ntp/ntp.drift
logfile /var/log/ntp.log

restrict default kod nomodify notrap nopeer noquery
restrict -6 default kod nomodify notrap nopeer noquery
restrict 127.0.0.1 mask 255.255.255.0
restrict -6 ::1

# GPS Serial data reference (NTP0)
server 127.127.28.0 minpoll 4 maxpoll 4
fudge 127.127.28.0 time1 0.9999 refid GPS

# GPS PPS reference (NTP1)
server 127.127.28.1 minpoll 4 maxpoll 4 prefer
fudge 127.127.28.1 refid PPS
-----------------------------------------------------------------------------

Users of ntpd versions prior to ntp-4.2.5 do not have the "pool" option.
Alternative configurations exist, but it is recommended that you upgrade
ntpd, if feasible.

The magic pseudo-IP address 127.127.28.0 identifies unit 0 of the ntpd
shared-memory driver (NTP0); 127.127.28.1 identifies unit 1 (NTP1).
Unit 0 is used for in-band message timestamps and unit 1 for the (more
accurate, when available) time derived from combining in-band message
timestamps with the out-of-band PPS synchronization pulse.  Splitting
these notifications allows ntpd to use its normal heuristics to weight
them.

Different units -- 2 (NTP2) and 3 (NTP3), respectively -- must be used
when gpsd is not started as root.  Some GPS HATs put PPS time on a GPIO
pin and will also use unit 2 (NTP2) for the PPS time correction.

With this configuration, ntpd will read the timestamp posted by gpsd
every 64 seconds (16 if non-root) and send it to unit 0.

The number after the parameter time1 (0.9999 in the example above) is a
"fudge", offset in seconds.  It's an estimate of the latency between
the time source and the 'real' time. You can use it to compensate out
some of the fixed delays in the system. An 0.9999 fudge would be
ridiculously large.

You may be able to find a value for the fudge by looking at the entry
for your GPS receiver type on <<HARDWARE>>.  Later in this document
we'll explain methods for estimating a fudge factor on unknown
hardware.

There is nothing magic about the refid fields; they are just labels
used for generating reports.  You can name them anything you like.

When you start gpsd, it will wait for a few good fixes before attempting
to process PPS.  You should run cgps to verify your GPS receiver has a
3D lock before worrying about timekeeping.

After starting (as root) ntpd, then gpsd, a listing similar to the one
below should appear as the output of the command "ntpq -p" (after
allowing the GPS receiver to acquire a 3D fix).  This may take up to
30 minutes if your GPS receiver has to cold-start or has a poor
skyview.

-----------------------------------------------------------------------------
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
xtime-a.timefreq .ACTS.           1 u   40   64  377   59.228   -8.503   0.516
-bonehed.lcs.mit 18.26.4.106      2 u   44   64  377   84.259    4.194   0.503
+clock.sjc.he.ne .CDMA.           1 u   41   64  377   23.634   -0.518   0.465
+SHM(0)          .GPS.            0 l   50   64  377    0.000    6.631   5.331
-----------------------------------------------------------------------------

The line with refid ".GPS." represents the in-band time reports from
your GPS receiver.  When you are getting PPS then it may look like
this:

-----------------------------------------------------------------------------
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
xtime-a.timefreq .ACTS.           1 u   40   64  377   59.228   -8.503   0.516
-bonehed.lcs.mit 18.26.4.106      2 u   44   64  377   84.259    4.194   0.503
+clock.sjc.he.ne .CDMA.           1 u   41   64  377   23.634   -0.518   0.465
+SHM(0)          .GPS.            0 l   50   64  377    0.000    6.631   5.331
*SHM(1)          .PPS.            0 l   49   64  377    0.000    0.222   0.310
-----------------------------------------------------------------------------

Note the additional ".PPS." line.

If the value under "reach" for the SHM lines remains zero, check that
gpsd is running; cgps reports a 3D fix; and the '-n' option was used.
Some GNSS receivers specialized for time service can report time with signal
lock on only one satellite, but with most devices a 3D fix is
required.

When the SHM(0) line does not appear at all, check your ntp.conf and
the system logs for error messages from ntpd.

Notice the 1st and 3rd servers, stratum 1 servers, disagree by more than
8 mSec.  The 1st and 2nd servers disagree by over 12 mSec.  Our local
PPS reference agrees to the clock.sjc.he.net server within the expected
jitter of the GR-601W in use.

When no other servers or local reference clocks appear in the NTP
configuration, the system clock will lock onto the GPS clock, but this
is a fragile setup -- you can lose your only time reference if the GPS
receiver is temporarily unable to get satellite lock.

You should always have at least two (preferably four) fallback servers
in your ntpd.conf for proper ntpd operation, in case your GPS receiver
fails to report time.  The 'pool' command makes this happen.  And
you'll need to adjust the offsets (fudges) in your ntp.conf so the
SHM(0) time is consistent with your other servers (and other local
reference clocks, if you have any). We'll describe how to diagnose and
tune your server configuration in a later section.

Also note that after cold-starting ntpd it will calibrate for up to 15
minutes before it starts adjusting the clock. Because the frequency
error estimate ("drift") that NTP uses isn't right when you first
start NTP, there will be a phase error that persists while the
frequency is estimated.  So if your clock is a little slow, then it
will keep getting behind, and the positive offset will cause NTP to
adjust the phase forward and also increase the frequency offset error.
After a day or so or maybe less the frequency estimate will be very
close and there won't be a persistent offset.

The GPSD developers would like to receive information about the
offsets (fudges) observed by users for each type of receiver. If your
GPS receiver is not present in <<HARDWARE>>, or doesn't have a
recommended fudge, or you see a fudge value very different from what's
there, send us the output of the "ntpq -p" command and the make and
type of receiver.

== Feeding chrony from GPSD

chrony is an alternative open-source implementation of NTP service,
originally designed for systems with low-bandwidth or intermittent
TCP/IP service.  It interoperates with ntpd using the same NTP
protocols.  Unlike ntpd which is designed to always be connected to
multiple internet time sources, chrony is designed for long periods
of offline use.  Like ntpd, it can either operate purely as a client
or provide time service. The chrony project has a home page at
<<CHRONY>>. Its documentation includes an instructive feature comparison
with ntpd at <<CHRONY-COMPARE>>.

gpsd, when run as root, may feed time information to chronyd using
sockets.  The sockets are named /run/chrony.XXXX.sock.  Where XXXX is
replaced by the basenames of the device names gpsd is using.  If your
receiver outputs serial data on /dev/ttyS0, then the corresponding
socket is /run/chrony.ttyS0.sock.  If your PPS is on /dev/pps0, then the
corresponding socket is /run/chrony.pps0.sock.

Older systems may use the /var/run directory instead of /run.  If gpsd
can not open the sockets there, it falls back to try /tmp.

No gpsd configuration is required to talk to chronyd. chronyd is
configured using the file /etc/chrony.conf or /etc/chrony/chrony.conf.
Check your distributions documentation for the correct location.

To get chronyd to connect to gpsd using the socket method add the
following lines your chrony.conf file.  Except, replace XXXX with
the basename of your device's serial port, often ttyS0, ttyACM0, or
ttyAMA0. Replace YYYY with the basename of your PPS device, usually
pps0.

When running as root:

-----------------------------------------------------------------------------
server 0.us.pool.ntp.org
server 1.us.pool.ntp.org
server 2.us.pool.ntp.org
server 3.us.pool.ntp.org

driftfile /var/lib/chrony/drift

allow

# set larger delay to allow the NMEA source to overlap with
# the other sources and avoid the falseticker status
refclock SOCK /run/chrony.XXXX.sock refid GPS precision 1e-1 offset 0.9999
refclock SOCK /run/chrony.YYYY.sock refid PPS precision 1e-7
-----------------------------------------------------------------------------

Older systems may use the /var/run directory for the socket file instead
of /run.  /run is compliant with FHS 3.0.  If that file can not be opened
then gpsd falls back to trying in /tmp:

-----------------------------------------------------------------------------
refclock SOCK /tmp/chrony.XXXX.sock refid GPS precision 1e-1 offset 0.9999
refclock SOCK /tmp/chrony.YYYY.sock refid PPS precision 1e-7
-----------------------------------------------------------------------------

You can also get gpsd to connect to chronyd using the basic ntpd
compatible SHM method.  To use that instead of sockets, add these lines
to the basic chrony.conf file:

-----------------------------------------------------------------------------
server 0.us.pool.ntp.org
server 1.us.pool.ntp.org
server 2.us.pool.ntp.org
server 3.us.pool.ntp.org

driftfile /var/lib/chrony/drift

allow

# set larger delay to allow the NMEA source to overlap with
# the other sources and avoid the falseticker status
refclock SHM 0 refid GPS precision 1e-1 offset 0.9999 delay 0.2
refclock SHM 1 refid PPS precision 1e-7
-----------------------------------------------------------------------------

You need to add the "precision 1e-7" on the SHM 1 line as chronyd fails
to read the precision from the SHM structure.  Without knowing the high
precision of the PPS on SHM 1 it may not place enough importance on its
data.

If you are outside of the USA replace the pool servers with one in your
local area. See <<USE-POOL>> for further information.

The offset option is functionally like ntpd's time1 option, used to
correct known and constant latency.

The 'allow' option allows anyone on the internet to query your server's
time.

See the chrony man page for more detail on the configuration options
<<CHRONY-MAN>>.

Finally note that chronyd needs to be started before gpsd so the
sockets are ready before gpsd starts up.

If running as root, the preferred starting procedure is:

-----------------------------------------------------------------------------
$ su -
# killall -9 gpsd chronyd
# chronyd -f /etc/chrony/chrony.conf
# sleep 2
# gpsd -n /dev/ttyXX
# sleep 2
# cgps
-----------------------------------------------------------------------------

After you have verified with cgps that your GPS receiver has a good 3D
lock you can check that gpsd is outputting good time by running ntpshmmon.

-----------------------------------------------------------------------------
# ntpshmmon
ntpshmmon version 1
#      Name   Seen@                Clock                Real               L Prec
sample NTP0 1461537438.593729271 1461537438.593633306 1461537438.703999996 0 -1
sample NTP1 1461537439.000421494 1461537439.000007374 1461537439.000000000 0 -20
sample NTP0 1461537439.093844900 1461537438.593633306 1461537438.703999996 0 -1
sample NTP0 1461537439.621309382 1461537439.620958240 1461537439.703999996 0 -1
sample NTP1 1461537440.000615395 1461537439.999994105 1461537440.000000000 0 -20
sample NTP0 1461537440.122079148 1461537439.620958240 1461537439.703999996 0 -1
^C
-----------------------------------------------------------------------------

If you see only "NTP2", instead, you forgot to go root before starting gpsd.

Once ntpshmmon shows good time data you can see how chrony is doing by
running 'chronyc sources'.  Your output will look like this:

-----------------------------------------------------------------------------
# chronyc sources

210 Number of sources = 7
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#- GPS                           0   4   377    12  +3580us[+3580us] +/- 101ms
#* PPS                           0   4   377    10    -86ns[ -157ns] +/- 181ns
^? vimo.dorui.net                3   6   377    23   -123ms[ -125ms] +/- 71ms
^? time.gac.edu                  2   6   377    24   -127ms[ -128ms] +/- 55ms
^? 2001:470:1:24f::2:3           2   6   377    24   -122ms[ -124ms] +/- 44ms
^? 142.54.181.202                2   6   377    22   -126ms[ -128ms] +/- 73ms
-----------------------------------------------------------------------------

The stratum is as in ntpq.  The Poll is how many seconds elapse between samples.
The Reach is as in ntpq. LastRx is the time since the last successful
sample.  Last sample is the offset and jitter of the source.

To keep chronyd from preferring NMEA time over PPS time, you can add an
overlarge fudge to the NMEA time.  Or add the suffix 'noselect' so it
is never used, just monitored.

== Performance Tuning

This section is general and can be used with either ntpd or chronyd.
We'll have more to say about tuning techniques for the specific
implementations in later sections.

The clock crystals used in consumer electronics have two properties we
are interested in: accuracy and stability.  *Accuracy* is how well the
measured frequency matches the number printed on the can.  *Stability*
is how well the frequency stays the same even if it isn't accurate.
(Long term aging is a third property that is interesting, but ntpd and
chrony both a use a drift history that is relatively short; thus,
this is not a significant cause of error.)

Typical specs for oscillator packages are 20, 50, 100 ppm.  That includes
everything; initial accuracy, temperature, supply voltage, aging, etc.

With a bit of software, you can correct for the inaccuracy.  ntpd and
chrony both call it *drift*.  It just takes some extra low order bits
on the arithmetic doing the time calculations.  In the simplest case,
if you thought you had a 100 MHz crystal, you need to change that to
something like 100.000324. The use of a PPS signal from gpsd
contributes directly to this measurement.

Note that a low drift contributes to stability, not necessarily accuracy.

The major source of instability is temperature.  Ballpark is the drift
changes by 1 PPM per degree C.  This means that the drift does not stay
constant, it may vary with a daily and yearly pattern.  This is why the
value of drift the ntpd uses is calculated over a (relatively) short time.

So how do we calculate the drift?  The general idea is simple.
Measure the time offset every N seconds over a longer window of time
T, plot the graph, and fit a straight line.  The slope of that line is
the drift.  The units cancel out.  Parts-per-million is a handy scale.

How do you turn that hand waving description into code?  One easy way
is to set N=2 and pick the right T, then run the answer through a
low pass filter.  In that context, there are two competing sources of
error.  If T is too small, the errors on each individual measurement
of the offset time will dominate.  If T is too big, the actual drift
will change while you are measuring it.  In the middle is a sweet
spot.  (For an example, see <<ADEV-PLOT>>.)

Both ntpd and chrony use this technique; ntpd also uses a more
esoteric form of estimation called a "PLL/FLL hybrid loop". How T and N are
chosen is beyond the scope of this HOWTO and varies by implementation
and tuning parameters.

If you turn on the right logging level ("statistics loopstats peerstats"
for ntpd, "log measurements tracking" for chronyd), that will record
both offset, drift, and the polling interval. The ntpd stats are easy to
feed to gnuplot, see the example script in the GPSD contrib directory.
The most important value is the offset reported in the 3rd field in
loopstats and the last field in tracking.log. With gnuplot you can
compare them (after concatenating the rotated logs):

-----------------------------------------------------------------------------
plot "tracking.log" using 7 with lines, "loopstats" using 3 with lines
-----------------------------------------------------------------------------

While your NTP daemon (ntpd or chrony) is adjusting the polling
interval, it is assuming that the drift is not changing.  It gets
confused if your drift changes abruptly, say because you started some
big chunk of work on a machine that's usually idle and that raises the
temperature.

Your NTP daemon writes out the drift every hour or so.  (Less often if
it hasn't changed much to reduce the workload on flash file systems.)
On startup, it reloads the old value.

If you restart the daemon, it should start with a close old drift
value and quickly converge to the newer slightly different value.  If
you reboot, expect it to converge to a new/different drift value and
that may take a while depending on how different the basic calibration
factors are.

=== ARP is the sound of your server choking

By default, ntpd and chronyd poll remote servers every 64 seconds.  This
is an unfortunate choice.  Linux by default only keeps an ARP table
entry for 60 seconds, anytime thereafter it may be flushed.

If the ARP table has flushed the entry for a remote peer or server then
when the NTP server sends a request to the remote server an entire ARP
cycle will be added to the NTP packet round trip time (RTT).  This will
throw off the time measurements to servers on the local lan.

On a Raspberry Pi ARP has been shown to impact the remote offset by up to
600 uSec in some rare cases.

The solution is the same for both ntpd and chronyd, add the "maxpoll 5"
command to any 'server" or "peer directive.  This will cause the maximum
polling period to be 32 seconds, well under the 60 second ARP timeout.

=== Watch your temperatures

The stability of the system clock is very temperature dependent.  A one
degree change in room temperature can create 0.1 ppm of clock frequency
change.  One simple way to see the effect is to place your running
NTP server inside bubble wrap.  The time will take a quick and noticeable
jump.

If you leave your NTP server in the bubble wrap you will notice some
improved local and remote offsets.

=== Power saving is not your friend

Normally enabling power saving features is a good thing: it saves you power.
But when your CPU changes power saving modes (cstates for Intel CPUs) the
impact on PPS timing is noticeable.  For some reason the NO_HZ kernel
mode has a similar bad effect on timekeeping.

To improve your timekeeping, turn off both features on Intel CPUs by
adding this to your boot command line:

-----------------------------------------------------------------------------
nohz=off intel_idle.max_cstate=0
-----------------------------------------------------------------------------

For ARM, be sure NO_HZ is off:

-----------------------------------------------------------------------------
nohz=off
-----------------------------------------------------------------------------

You will also need to select the 'performance' CPU governor to keep your
CPU set to the maximum speed for continuous usage.  How you see and set
your governor will be distribution specific.  The easiest way it to
recompile your kernel to only provide the performance governor.

== NTP tuning and performance details

This section deals specifically with ntpd.  It discusses algorithms
used by the ntpd suite to measure and correct the system time.  It is not
directly applicable to chronyd, although some design considerations
may be similar.

You can't optimize what you can't visualize.  The easiest way to
visualize ntpd performance is with ntpviz from <<NTPSEC.ORG>>.  Once you
are regularly graphing your server performance it is much easier to see
the results of changes.

=== NTP performance tuning

For good time stability, you should always have at least four other
servers in your ntpd or chrony configuration besides your GPS receiver
-- in case, for example, your GPS receiver is temporarily unable to achieve
satellite lock, or has an attack of temporary insanity. You can find
public NTP servers to add to your configuration at <<USE-POOL>>.

To minimize latency variations, use the national and regional
pool domains for your country and/or nearby ones.  Your ntp.conf
configuration line should probably look like this:

-----------------------------------------------------------------------------
pool us.pool.ntp.org iburst
-----------------------------------------------------------------------------

Where "us" may be replaced by one of the zone/country codes the Pool
project supports (list behind the "Global" link at <<ZONES>>). The
"pool" tag expands to four randomly chosen servers by default.  "iburst"
implements a fast start algorithm that also weeds out bad servers.

Note that a server can be a poor performer (what the NTP documentation
colorfully calls a "falseticker") for any of three reasons. It may be
shipping bad time, or the best routes between you and it has large
latency variations (jitter), or it may have a time-asymmetric route,
to you (that is, B-to-A time is on average very different from A-to-B
time).  Asymmetric routing is the most common cause of serious
problems.

The standard tool for tuning ntpd is "ntpq" ("NTP query program"). To
show a list of all servers declared in ntp.conf and their statistics,
invoke it with the "-p" option. On a sample system configured with 7
servers from the NTP pool project and one PPS GPS receiver attached
via RS232, this is the output:

------------------------------------------------------------------------
$ ntpq -p
 remote          refid         st t when poll reach delay offset jitter
========================================================================
-arthur.testserv 162.23.41.56   2 u 62     64 377  5.835 -1.129   8.921
-ntppublic.uzh.c 130.60.159.7   3 u 62     64 377  6.121 -4.102   6.336
-smtp.irtech.ch  162.23.41.56   2 u 35     64 377 15.521 -1.677   8.678
+time2.ethz.ch   .PPS.          1 u 27     64 377  5.938 -1.713  16.404
-callisto.mysnip 192.53.103.104 2 u 53     64 377 49.357 -0.274   5.125
-shore.naturalne 122.135.113.81 3 u 22     64 377 14.676 -0.528   2.601
-ntp.univ-angers 195.220.94.163 2 u 41     64 377 40.678 -1.847  13.391
+SHM(0)          .GPS.          0 l  4     64 377  0.000 34.682   7.952
*SHM(1)          .PPS.          0 l  3     64 377  0.000 -2.664   0.457
------------------------------------------------------------------------

The interesting columns are "remote", "st", "reach" and "offset".

"remote" is the name of the remote NTP server. The character in its
first column shows its current state: "-" or "x" for out-of-tolerance
servers, "+" for good servers ("truechimers"), and "*" for the one good
server currently used as the primary reference. The calculations used to
determine a server's state are outside the scope of this document;
details are available in NTPv4 RFC 5905.

"st" shows the remote server's stratum.

"reach" is the octal representation of the remote server's reachability.
A bit is set if the corresponding poll of the server was successful,
i.e. the server returned a reply. New poll results are shifted in from
the least significant bit; results older than 8 polls are discarded. In
the absence of network problems, this should show "377".

"offset" shows the mean offset in the times reported between this local
host and the remote server in milliseconds. This is the value that can
be fudged with the "time1" parameter of the GPS server line in ntp.conf.
If the offset is positive, reduce the time1 value and vice versa.

The asterisk in this example indicates that ntpd has correctly
preferred '.PPS.'  over '.GPS.', as it should.  If for some reason it
locks on to GPS time as a preferred source, you can add an overlarge
fudge to the NMEA time to discourage it.  Or add the suffix 'noselect'
so GPS time is never used, just monitored.

A more detailed description of the output is available at
<<NTPQ-OUTPUT>>.

In order to determine the correct GPS offset, do one of the following:

==== Peerstats-based procedure

[start=1]
. Add these lines to ntp.conf:

-----------------------------------------------------------------------------
statsdir /var/log/ntpstats/
statistics peerstats
filegen peerstats file peerstats type day enable
-----------------------------------------------------------------------------

This enables logging of the peer server statistics.

. Make sure the directory exists properly.  For ntpd as root do:

-----------------------------------------------------------------------------
   # mkdir -p /var/log/ntpstats
   # chown ntp:ntp /var/log/ntpstats
-----------------------------------------------------------------------------

. Start ntpd and let it run for at least four hours.
Periodically check progress with "ntpq -p" and wait
until change has settled out.

. Calculate the average GPS offset using this script (a copy is
included as contrib/ntpoffset in the GPSD distribution):

-----------------------------------------------------------------------------
awk '
     /127\.127\.28\.0/ { sum += $5 * 1000; cnt++; }
     END { print sum / cnt; }
' </var/log/ntpstats/peerstats
-----------------------------------------------------------------------------

This prints the average offset.

. Adjust the "time1" value for unit 0 of your ntp.conf (the non-PPS
   channel) by subtracting the average offset from step 4.

. Restart ntpd.

==== Loopstats-based procedure

Recall that magic pseudo-IP address 127.127.28.0 identifies unit 0 of
the ntpd shared-memory driver (NTP0); 127.127.28.1 identifies unit
1 (NTP1).  Unit 0 is used for in-band message timestamps (IMT) and unit
1 for the (more accurate, when available) time derived from combining
IMT with the out-of-band PPS synchronization pulse.  Splitting these
notifications allows ntpd to use its normal heuristics to weight them.

We assume that the 1PPS signal, being just one bit long, and directly
triggering an interrupt, is always on time (sic).  Correcting for latency
in the 1PPS signal is beyond the scope of this document.  The IMT,
however, may be delayed, due to it being emitted, copied to shared
memory, etc.

Based on advice and script fragments on the GPSD list, the following
may help to calculate the 'time1' factor.  You may need to modify
these scripts for your particular setup.

These scripts are for the combination of GPSD and ntpd.  If you use
chronyd, you *will* need to modify these, at the least.

==== ntpviz procedure

If all this calculating and graphing looks painful, then grab a copy
of ntpviz from <<NTPSEC.ORG>>. ntpviz generates lots of pretty graphs
and html pages.  It even calculates the correct IMT offset, and other
performance metrics for you.

===== Format of the loopstats and peerstats files

The following is incorporated from the ntpd website, see <<NTP-MONOPT>>

.loopstats

Record clock discipline loop statistics. Each system clock update
appends one line to the loopstats file set:

Example:    50935 75440.031 0.000006019 13.778 0.000351733 0.013380 6

|===========================================================
|Item 		|Units 	|Description
|50935 		|MJD 	|date
|75440.031 	|s 	|time past midnight (UTC)
|0.000006019 	|s 	|clock offset
|13.778 	|PPM 	|frequency offset
|0.000351733 	|s 	|RMS jitter
|0.013380 	|PPM 	|RMS frequency jitter (aka wander)
|6 		|log2 s |clock discipline loop time constant
|===========================================================


.peerstats

Record peer statistics. Each NTP packet or reference clock update
received appends one line to the peerstats file set:

Example:    48773 10847.650 127.127.4.1 9714 -0.001605376 0.000000000 0.001424877 0.000958674

|================================================
|Item 		|Units 	|Description
|48773 		|MJD 	|date
|10847.650 	|s 	|time past midnight (UTC)
|127.127.4.1 	|IP 	|source address
|9714 		|hex 	|status word
|-0.001605376 	|s 	|clock offset
|0.000000000 	|s 	|roundtrip delay
|0.001424877 	|s 	|dispersion
|0.000958674 	|s 	|RMS jitter
|================================================

===== Measurement of delay

There are three parts to measuring and correcting for the delay in
processing the 1PPS signal.

1. Running ntpd without using the IMT (but using the 1PPS time).
2. Measuring the delay between the two messages.
3. Applying the correction factor.

We assume that you have successfully integrated GPSD with ntpd already.
You should also have a decent set of NTP servers you are syncing to.

[start=1]
. Running ntpd without IMT

Locate the line in your ntp.conf that refers to the SHM0 segment and
append 'noselect' to it.  As an example, the first two lines in the sample
above will become:

--------------------
server 127.127.28.0 minpoll 4 maxpoll 4 noselect
fudge 127.127.28.0 time1 0.420 refid GPS
--------------------

ntpd will now continue to monitor the IMT from GPSD, but not use it
for its clock selection algorithm.  It will still write out statistics to
the peerstats file.  Once ntpd is stable (a few hours or so), we can
process the peerstats file.

. Measuring the delay between the two messages.

From the 'peerstats' file, extract the lines corresponding to
127.127.28.0

-----------
grep 127.127.28.0 peerstats > peerstats.shm
-----------

You can now examine the offset and jitter of the IMT.  <<ANDY-POST>>
suggests the following gnuplot fragment (you will need to set output
options before plotting).

----------------
	set term gif
	set output "fudge.gif"
----------------

If your gnuplot has X11 support, and you do not wish to save the plot,
the above may not be required.  Use:

---------------
	set term x11
---------------

Now plot the GPSD shared memory clock deviations from the system
clock.  (You will get the GPSD shared memory clock fudge value
estimate from this data when NTP has converged to your
satisfaction.)

------------------
        gnuplot> plot "peerstats.shm" using ($2):($5):($8) with yerrorbars
        gnuplot> replot "peerstats.shm" using ($2):($5) with lines
------------------

. Applying the correction factor.

By examining the plot generated above, you should be able to estimate
the offset between the 1PPS time and the GPS time.

If, for example, your estimate of the offset is -0.32s, your time1 fudge
value will be '0.32'.  Note the change of sign.

=== Polling Interval

ntpd seems to better use a PPS refclock when the polling interval is
as small as possible.  The ntpd default minpoll is 6, and can be set to
as low as 4.  NTPsec versions 0.9.5 and above of ntpd allow you to
set minpoll and maxpoll as low as 0.  Changing minpoll from 4 to 3, 2, 1
or maybe even as low as 0, may reduce your PPS jitter by over a factor of 4.

Any change will require several hours for ntpd to converge with the new
settings.  Use ntpviz to find the best poll interval for your system.

The value that yields the lowest jitter may not be the one that yields
the best Local Clock Frequency Offset.

-----------------------------------------------------------------------------
server 127.127.28.1 minpoll 0 maxpoll 0 prefer
-----------------------------------------------------------------------------

== Chrony performance tuning

The easiest way to determine the offset with chronyd is probably to
configure the source whose offset should be measured with the noselect
option and a long poll, let chronyd run for at least 4 hours and
observe the offset reported in the chronyc sourcestats output.  If the
offset is unstable, wait longer.  For example:

SHM 0 configured as:
refclock SHM 0 poll 8 filter 1000 noselect

-----------------------------------------------------------------------------
# chronyc sourcestats
210 Number of sources = 6
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
==============================================================================
SHM0                       21   9   85m      4.278      4.713   +495ms  8896us
SHM1                       20   8   307      0.000      0.002     +0ns   202ns
mort.cihar.com             21   8   72m      0.148      0.798   +668us   490us
vps2.martinpoljak.net       6   4   17m    -53.200    141.596    -24ms    15ms
ntp1.kajot.cz              25  16   91m     -0.774      1.494    -11ms  1859us
ntp1.karneval.cz           17  10   89m      0.127      0.539  -4131us   574us
-----------------------------------------------------------------------------

In this case (Garmin 18x) the offset specified in the config for the
SHM 0 source should be around 0.495.

//FIXME: What more can we say about chronyd tuning?

== Providing local NTP service using PTP

By now if you have a good serial PPS signal your local clock should
have jitter on the order of 1 uSec.  You do not want the hassle of
placing a GPS receiver on each of your local computers.  So you
install chrony or ntp on your other hosts and configure them to use
your NTP PPS server as their local server.

With your best setup on a lightly loaded GigE network you find that your
NTP clients have jitter on the order of 150 uSec, or 150 times worse
than your master.  Bummer, you want to do much better, so you look to
the Precision Time Protocol <<PTP>> for help.  PTP is also known as IEEE
1588.

With PTP you can easily synchronize NTP hosts to 5 uSec with some
generic NIC hardware and newer Linux kernels.  Some of the Ethernet
drivers have been modified to time stamp network packets when sending and
receiving.  This is done with the new SO_TIMESTAMPING socket option.  No
hardware support is required.

A more recent addition is PTP Hardware Clock (PHC) support.  This requires
hardware support in the NIC.

Software timestamping is more mature, available on more NICs, and almost
as accurate as hardware timestamping.  Try it first.  This HOWTO will
build on those results.

One final wrinkle before proceeding with PTP.  Ethernet ports have
something called <<EEE>> (IEEE 802.3az).  Percentage wise EEE can save
50% of the Ethernet energy needs.  Sadly this is 50% of an already small
energy usage.  Only important in large data centers.  EEE can be very
disruptive to timekeeping.  Up to almost 1 Sec of errors in offset,
wander and jitter.  To see if you have EEE enabled, and then turn it
off:

-----------------------------------------------------------------------------
# ethtool --show-eee eth0
EEE Settings for eth0:
	EEE status: enabled - inactive
	Tx LPI: 0 (us)
	Supported EEE link modes:  100baseT/Full
	                           1000baseT/Full
	Advertised EEE link modes:  100baseT/Full
	                            1000baseT/Full
	Link partner advertised EEE link modes:  Not reported
# ethtool --set-eee eth0 eee off
# ethtool --show-eee eth0
EEE Settings for eth1:
	EEE status: disabled
	Tx LPI: disabled
	Supported EEE link modes:  100baseT/Full
	                           1000baseT/Full
	Advertised EEE link modes:  Not reported
	Link partner advertised EEE link modes:  Not reported
-----------------------------------------------------------------------------

=== PTP with software timestamping

To start you need to verify that your running Linux kernel configuration
includes these two lines, or the same with "y" replaced by "m" to enable
the drivers as modules:

-----------------------------------------------------------------------------
CONFIG_NETWORK_PHY_TIMESTAMPING=y
PTP_1588_CLOCK=y
-----------------------------------------------------------------------------

Then you need to verify that your Ethernet driver supports PTP
by running this command as root:

-----------------------------------------------------------------------------
# ethtool -T eth0 | fgrep SOFTWARE
	software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
	software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
	software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
-----------------------------------------------------------------------------

If the result includes those three lines then you have support for
software PTP timestamping.  We will leave hardware timestamping
for later.

Next you will need the <<LINUX-PTP>> package, just follow the simple
instructions on their web page to download, compile and install on your
NTP server and its slaves.  Be sure to also follow their instructions on
how to configure your Linux kernel.

In this setup we will just use the ptp4l program.  This program measures
the delay and offset between a master and slaves and shares that information
with chronyd or ntpd using an SHM.  Since gpsd also uses SHM be very careful
not to have the two SHM servers stepping on the same shmid.

If you are using ntpd, then add the last three lines below to your
master ntp.conf file to configure the SHM.

-----------------------------------------------------------------------------
# GPS Serial data reference (NTP0)
server 127.127.28.0
fudge 127.127.28.0 time1 0.9999 refid GPS

# GPS PPS reference (NTP1)
server 127.127.28.1 prefer
fudge 127.127.28.1 refid PPS

# local PTP reference (NTP2)
server 127.127.28.2
fudge 127.127.28.2 refid PTP
-----------------------------------------------------------------------------

If you are using chronyd, then add the last one line below to your
master chronyd.conf file to configure the SHM.

-----------------------------------------------------------------------------
refclock SHM 0 refid GPS precision 1e-1 offset 0.9999 delay 0.2
refclock SHM 1 refid PPS precision 1e-7
refclock SHM 2 refid PTP precision 1e-7
-----------------------------------------------------------------------------

To configure the master ptp4l, create a new file
/usr/local/etc/ptp4l.conf with these contents:

-----------------------------------------------------------------------------
[global]
# only syslog every 1024 seconds
summary_interval 10

# send to chronyd/ntpd using SHM 2
clock_servo ntpshm
ntpshm_segment 2

# set our priority high since we have PPS
priority1 10
priority2 10

[eth0]
-----------------------------------------------------------------------------

Now as root on the master, start the ptp4l daemon:

-----------------------------------------------------------------------------
# ethtool --set-eee eth0 eee off
# ptp4l -S -f /usr/local/etc/ptp4l.conf &
-----------------------------------------------------------------------------

Configuration of the master server is now complete.  Now to configure
the slaves.  If the slaves also have PPS then configure them as masters.
Otherwise you will stomp on your SHMs.

If you are using ntpd, then add the last three lines below to your
master ntp.conf file to configure your one and only SHM.

-----------------------------------------------------------------------------
# local PTP reference (NTP0)
server 127.127.28.0
fudge 127.127.28.0 refid PTP
-----------------------------------------------------------------------------

If you are using chronyd, then add the one line below to your master
chronyd.conf file to configure your one and only SHM.

-----------------------------------------------------------------------------
refclock SHM 0 refid PTP precision 1e-7
-----------------------------------------------------------------------------

To configure the slave ptp4l, create a new file
/usr/local/etc/ptp4l.conf with these contents:

-----------------------------------------------------------------------------
[global]
# only syslog every 1024 seconds
summary_interval 10

# send to chronyd/ntpd using SHM 0
clock_servo ntpshm
ntpshm_segment 0

[eth0]
-----------------------------------------------------------------------------

Now as root on the slave, as with the master, turn off EEE and start the
ptp4l daemon:

-----------------------------------------------------------------------------
# ethtool --set-eee eth0 eee off
# ptp4l -S -f /usr/local/etc/ptp4l.conf &
-----------------------------------------------------------------------------

Configuration of the slave server is now complete.  Follow the earlier
procedures for checking the jitter on the SHM on the slaves.  Give it
a few hours to settle and your hosts will now be synced to around 5 uSec.

=== PTP with hardware timestamping

Some NICs requires two additional kernel options.  Just in case, verify
that your running Linux kernel configuration includes these lines, or
the same with "y" replaced by "m" to enable the drivers as modules:

-----------------------------------------------------------------------------
CONFIG_DP83640_PHY=y
CONFIG_PTP_1588_CLOCK_PCH=y
-----------------------------------------------------------------------------

Then you need to verify that your Ethernet driver supports PTP
by running ethtool  as root and verify at least the following lines are
present in the output:

-----------------------------------------------------------------------------
# ethtool -T eth0
	hardware-transmit     (SOF_TIMESTAMPING_TX_HARDWARE)
	hardware-receive      (SOF_TIMESTAMPING_RX_HARDWARE)
	all                   (HWTSTAMP_FILTER_ALL)
-----------------------------------------------------------------------------

Your NIC may have more features, and your driver may support them for
better results.

In the software timestamping above the ptp4l program took care of all steps to
determine the slave offset from the master and feeding that to a SHM for
ntpd or chronyd to use.

In hardware timestamping mode ptp4l will continue to perform most of
the work.  An additional program, phc2sys, will take over the duties of
reading the hardware timestamps from the NIC, computing the offset, and
feeding that to the SHM.

phc2sys will use the SHM exactly as ptp4l did previously so no
change is required to your ntpd or chronyd configuration.

To keep things simple, for now, we will not touch the already configured
and working software timestamping master server.  We will proceed to
configure a slave.

To configure the slave ptp4l, edit your /usr/local/etc/ptp4l.conf
to remove the ntpshm options:

-----------------------------------------------------------------------------
[global]
# only syslog every 1024 seconds
summary_interval 10

clock_servo linreg

[eth0]
-----------------------------------------------------------------------------

Now as root on the slave, as with the master, turn off EEE and start the
ptp4l daemon:

-----------------------------------------------------------------------------
# ethtool --set-eee eth0 eee off
# ptp4l -H -f /usr/local/etc/ptp4l.conf &
# sleep 3
# phc2sys -a -r -E ntpshm -m -M 0 &
-----------------------------------------------------------------------------


Configuration of the slave server is now complete.  Follow the earlier
procedures for checking the jitter on the SHM on the slaves.

Sadly, theory and practice diverge here.  I have never succeeded in
making hardware timestamping work.  I have successfully trashed my
host system clock.  Tread carefully.  If you make progress please
pass on some clue.

== Providing public NTP service

<<NTP-FAQ>> has good advice on things to be sure you have done -- and
are ready to do -- before becoming a public server. One detail it
doesn't mention is that you'll need to un-firewall UDP port 123.  The
NTP protocol does not use TCP, so no need to unblock TCP port 123.

If and when you are ready to go public, see <<JOIN-POOL>>.

== Acknowledgments
Beat Bolli <bbolli@ewanet.ch> wrote much of the section on NTP
performance tuning. Hal Murray <hmurray@megapathdsl.net> wrote
much of the section on NTP working and performance details.
Sanjeev Gupta <ghane0@gmail.com> assisted with editing.
Shawn Kohlsmith <skohlsmith@gmail.com> tweaked the Bibliography.
Jaap Winius <jwinius@rjsystems.nl> cleaned up some terminology.

The loopstats-based tuning procedure for ntpd was drafted by Sanjeev
Gupta <ghane0@gmail.com>, based on discussions on the GPSD list
<<GPSD-LIST>> in Oct and Nov 2013.  Code examples are based on work by
Andy Walls <andy@silverblocksystems.net>.  A copy of the original
email can be found at <<ANDY-POST>>. A thorough review was contributed
by Jaap Winius <jwinius@rjsystems.nl>.

== References

[bibliography]

- [[[TIME-INTRO]]] link:time-service-intro.html[Introduction to Time Service]

- [[[WIKI-NTP]]] https://en.wikipedia.org/wiki/Network_Time_Protocol[Network Time Protocol]

- [[[NTP-FAQ]]] http://www.ntp.org/ntpfaq/[NTP FAQ]

- [[[RFC-2783]]] https://tools.ietf.org/html/rfc2783[RFC 2783]

- [[[RFC-5905]]] https://tools.ietf.org/html/rfc5905[RFC 5905]

- [[[MACX-1]]] https://www.etsy.com/listing/280336400/navisys-gr-601w-u-blox-6-macx-1-usb-gnss[Navisys GR-601W u-blox-6 "Macx-1" USB GPS receiver]

- [[[CHRONY-COMPARE]]] https://chrony.tuxfamily.org/manual.html#Comparison-with-ntpd[ntpd (comparison with chrony)]

- [[[CHRONYDEFAULT]]] https://lists.fedoraproject.org/pipermail/devel/2010-May/135679.html

- [[[HARDWARE]]] https://gpsd.gitlab.io/gpsd/hardware.html[Compatible Hardware]

- [[[UBLOX-TIMING]]] https://www.u-blox.com/sites/default/files/products/documents/Timing_AppNote_(GPS.G6-X-11007).pdf[GPS-based timing considerations with u-blox 6 receivers]

- [[[RPI]]] https://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html[The Raspberry Pi as a Stratum-1 NTP Server]

- [[[NTP.ORG]]] http://www.ntp.org/[Home of the Network Time Protocol project]

- [[[NTPSEC.ORG]]] https://www.ntpsec.org/[Wecome to NTPsec]

- [[[USE-POOL]]] https://www.pool.ntp.org/en/use.html[How do I use pool.ntp.org?]

- [[[CVE-2009-3563]]] https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2009-3563

- [[[CHRONY]]] https://chrony.tuxfamily.org/[Chrony Home]

- [[[CHRONY-MAN]]] https://chrony.tuxfamily.org/manual.html

- [[[ADEV-PLOT]]] http://www.leapsecond.com/pages/adev-avg/[Allan deviation and Averaging]

- [[[ZONES]]] https://www.pool.ntp.org/zone

- [[[NTPQ-OUTPUT]]] https://nlug.ml1.co.uk/2012/01/ntpq-p-output/831[ntpq output description]

- [[[JOIN-POOL]]] https://www.pool.ntp.org/en/join.html[How do I join pool.ntp.org?]

- [[[ANDY-POST]]] https://lists.gnu.org/archive/html/gpsd-dev/2013-10/msg00152.html[Clarifications needed for the time-service HOWTO]

- [[[NTP-MONOPT]]] https://www.eecis.udel.edu/~mills/ntp/html/monopt.html[NTP Monitoring]

- [[[GPSD-LIST]]]https://lists.gnu.org/archive/html/gpsd-dev/[gpsd-dev Archives]

- [[[PTP]]] https://www.nist.gov/el/isd/ieee/ieee1588[PTP]

- [[[LINUX-PTP]]] http://linuxptp.sourceforge.net/[Linux PTP]

- [[[EEE]]] https://en.wikipedia.org/wiki/Energy-Efficient_Ethernet[Energy-Efficient Ethernet]

- [[[LVC]]] https://www.rjsystems.nl/en/2100-ntpd-garmin-gps-18-lvc-gpsd.php

- [[[GITLAB-SOURCE]]] https://gitlab.com/NTPsec/ntpsec[NTPSec source on Gitlab]

== Changelog ==

1.1, Nov 2013::
      Initial release.

1.2, Aug 2014::
      Note that NetBSD now has PPS support.

1.3, Aug 2014::
      Add a note about the GR-601W.

1.4, Dec 2014::
      Cleaned up Bibliography

2.0, Feb 2015::
     More about troubleshooting PPS delivery.  Folded in Sanjeev
     Gupta's Calibration Howto describing the loopstats-based
     procedure.  Added preliminary information on PTP.

2.1 Mar 2015::
    More on PTP. Added link to Jaap Winius's page on GPS-18 setup.

2.2 Mar 2015::
    Detailed explanation of NTP has moved to a new page,
    link:time-service-intro.html[Introduction to Time Service].

2.3 Mar 2015::
    Use the NTP accuracy estimate from RFC 5905.

2.4 Mar 2015::
    Removed some typos, corrected formatting, and minor changes.
    A bit more specificity about root vs. non-root.

2.5 Apr 2016::
    Note the existence of the GR-701W.

2.6 May 2016::
    New section on GPS time.  Note the existence of the GR-801W.
    Describe the special timeserver build of GPSD. Recommend NTPsec.
    Add Macx-1 link.
    Add sections on ARP and temperature problems

2.7 June 2016::
    Add section on avoiding power saving.

2.8 July 2016::
    Mention required version of gpsd
    Fix Typos.

2.9 August 2016::
    Fix typos.

2.10 September 2016::
    Mention ntpviz
    Recommend minpoll=maxpoll=0 for PPS refclocks
    Recommend NTPsec.

2.11 June 2019::
    Check all links, and update to https where possible

// end