                      Notes on Xntpd Configuration

                    David L. Mills (mills@udel.edu)
                         University of Delaware
                            14 January 1993

Introduction

This document is a collection of notes concerning the use of xntpd and
related programs, and on coping with the Network Time Protocol (NTP) in
general. It is a major rewrite and update of an earlier document written
by Dennis Ferguson of the University of Toronto dated 5 November 1989.
It includes many changes and additions resulting from the NTP Version 3
specification and new implementation features. It supersedes the earlier
document, which should no longer be used for new configurations.

Xntpd is a complete implementation of the NTP Version 3 specification as
defined in RFC 1305. It also retains compatibility with both NTP Version
2, as defined in RFC 1119, and NTP Version 1, as defined in RFC 1059,
although this compatibility is sometimes strained and only
semiautomatic. In order to support in principle the ultimate precision
of about 232 picoseconds in the NTP specification, xntpd does no
floating-point arithmetic and instead manipulates the 64-bit NTP
timestamps as unsigned 64-bit integers. Xntpd fully implements NTP
Versions 2 and 3 authentication and a mode-6 control-message facility.
As extensions to the specification, a flexible address-and-mask
restriction facility has been included, along with a private mode-7
control-message facility used to remotely reconfigure the system and
monitor a considerable amount of internal detail.

The code is biased towards the needs of a busy time server with
numerous, possibly hundreds, of clients and other servers. Tables are
hashed to allow efficient handling of many associations, though at the
expense of additional overhead when the number of associations is small.
Many fancy features have been included to permit efficient management
and monitoring of a busy primary server, features which are simply
excess baggage for a server on a high stratum client. The code was
written with near demonic attention to details which can affect
precision and as a consequence should be able to make good use of high
performance, special purpose hardware such as precision oscillators and
radio clocks. The present code supports a number of radio clocks,
including those for the WWV, CHU, WWVB, DCF77, GOES and GPS radio and
satellite services. The server methodically avoids the use of Unix-
specific library routines where possible by implementing local versions,
in order to aid in porting the code to perverse Unix and non-Unix
platforms.

While this implementation slavishly obeys the NTP specification RFC
1305, it has been specifically tuned to achieve the highest accuracy
possible on whatever hardware and operating-system platform is
available. In general, its precision is limited only by that of the
onboard time-of-day clock maintained by the hardware and operating
system, while its stability is limited only by that of the onboard
frequency source, usually an uncompensated crystal oscillator. On modern
RISC-based processors connected directly to radio clocks via serial-
asynchronous interfaces, the accuracy is usually limited by that of the
radio clock and interface to the order of a few milliseconds. The code
includes special features to support a one-pulse-per-second (1-pps)
signal generated by some radio clocks. When used in conjunction with a
suitable hardware level converter, the accuracy can be improved to the
order of 100 microseconds. Further improvement is possible using an
outboard, stabilized frequency source, in which the accuracy and
stability are limited only by the characteristics of that source.

The xntp3 distribution includes, in addition to the daemon itself
(xntpd), several utility programs, including two remote-monitoring
programs (ntpq, xntpdc), a remote clock-setting program similar to the
Unix rdate program (ntpdate), a traceback utility useful to discover
suitable synchronization sources (ntptrace), and various programs used
to configure the local platform and calibrate the intrinsic errors. NTP
has been ported to a large number of platforms, including most RISC and
CISC workstations and mainframes manufactured today. Example
configuration files for many models of these machines are included in
the xntp3 distribution. While in most cases the standard version of the
implementation runs with no hardware or operating-system modifications,
not all features of the distribution are available on all platforms. For
instance, a special feature allowing Sun 4s to achieve accuracies in the
order of 100 microseconds requires some minor changes and additions to
the kernel and input/output support.

There are, however, several drawbacks to all of this. Xntpd is very,
very fat. This is rotten if your intended platform for the daemon is
memory-limited. Xntpd uses SIGIO for all input, a facility which appears
to not enjoy universal support and whose use seems to exercise the parts
of your vendors' kernels which are most likely to have been done poorly.
The code is unforgiving in the face of kernel problems which affect
performance, and generally requires that you repair the problems in
order to achieve acceptable performance. The code has a distinctly
experimental flavour and contains features which could charitably be
termed failed experiments, but which have not been hacked out yet. There
is code which has not been thoroughly tested (e.g. leap-second support)
due to the inconvenience of setting up tests. Much was learned from the
addition of support for a variety of radio clocks, with the result that
this support could use some rewriting.

How NTP Works

The approach used by NTP to achieve reliable time synchronization from a
set of possibly unreliable remote time servers is somewhat different
than other such protocols. In particular, NTP does not attempt to
synchronize clocks to each other. Rather, each server attempts to
synchronize to UTC (i.e., Universal Coordinated Time) using the best
available source and available transmission paths to that source. This
is a fine point which is worth understanding. A group of NTP-
synchronized clocks may be close to each other in time, but this is not
a consequence of the clocks in the group having synchronized to each
other, but rather because each clock has synchronized closely to UTC via
the best source it has access to. As such, trying to synchronize a set
of clocks to a set of servers whose time is not in mutual agreement may
not result in any sort of useful synchronization of the clocks, even if
you don't care about UTC. NTP operates on the premise that there is one
true standard time, and that if several servers which claim
synchronization to standard time disagree about what that time is, then
one or more of them must be broken. There is no attempt to resolve
differences more gracefully since the premise is that substantial
differences cannot exist. In essence, NTP expects that the time being
distributed from the root of the synchronization subnet will be derived
from some external source of UTC (e.g. a radio clock). This makes it
somewhat inconvenient (though not impossible) to synchronize hosts
together without a reliable source of UTC to synchronize them to. If
your network is isolated and you cannot access other people's servers
across the Internet, a radio clock may make a good investment.

Time is distributed through a hierarchy of NTP servers, with each server
adopting a "stratum" which indicates how far away from an external
source of UTC it is operating at. Stratum-1 servers, which are at the
top of the pile (or bottom, depending on your point of view), have
access to some external time source, usually a radio clock synchronized
to time signal broadcasts from radio stations which explicitly provide a
standard time service. A stratum-2 server is one which is currently
obtaining time from a stratum-1 server, a stratum-3 server gets its time
from a stratum-2 server, and so on. To avoid long lived synchronization
loops the number of strata is limited to 15.

Each client in the synchronization subnet (which may also be a server
for other, higher stratum clients) chooses exactly one of the available
servers to synchronize to, usually from among the lowest stratum servers
it has access to. It is thus possible to construct a synchronization
subnet where each server has exactly one source of lower stratum time to
synchronize to. This is, however, not an optimal configuration, for
indeed NTP operates under another premise as well, that each server's
time should be viewed with a certain amount of distrust. NTP really
prefers to have access to several sources of lower stratum time (at
least three) since it can then apply an agreement algorithm to detect
insanity on the part of any one of these. Normally, when all servers are
in agreement, NTP will choose the best of these, where "best" is defined
in terms of lowest stratum, closest (in terms of network delay) and
claimed precision, along with several other considerations. The
implication is that, while one should aim to provide each client with
three or more sources of lower stratum time, several of these will only
be providing backup service and may be of lesser quality in terms of
network delay and stratum (i.e. a same-stratum peer which receives time
from lower stratum sources the local server doesn't access directly can
also provide good backup service).

Finally, there is the issue of association modes. There are a number of
modes in which NTP servers can associate with each other, with the mode
of each server in the pair indicating the behaviour the other server can
expect from it. In particular, when configuring a server to obtain time
from other servers, there is a choice of two modes which may be
alternatively used. Configuring an association in symmetric-active mode
(usually indicated by a "peer" declaration in configuration files)
indicates to the remote server that one wishes to obtain time from the
remote server and that one is also willing to supply time to the remote
server if need be. This mode is appropriate in configurations involving
a number of redundant time servers interconnected via diverse network
paths, which is presently the case for most stratum-1 and stratum-2
servers on the Internet today. Configuring an association in client mode
(usually indicated by a "server" declaration in configuration files)
indicates that one wishes to obtain time from the remote server, but that
one is not willing to provide time to the remote server. This mode is
appropriate for file-server and workstation clients that do not provide
synchronization to other local clients. Client mode is also useful for
boot-date-setting programs and the like, which really have no time to
provide and which don't retain state about associations over the longer
term.

Configuring Your Subnet

At startup time the xntpd daemon running on a host reads the initial
configuration information from a file, usually /etc/ntp.conf, unless a
different name has been specified at compile time. Putting something in
this file which will enable the host to obtain time from somewhere else
is usually the first big hurdle after installation of the software
itself, which is described in other documents included in the xntp3
distribution. At its simplest, what you need to do in the configuration
file is declare the servers that the daemon should poll for time
synchronization. In principle, no such list is needed if some other time
server explicitly mentions the host and is willing to provide
synchronization; however, this is considered dangerous, unless the
access control or authentication features (described later) are in use.

In the case of a workstation operating in an enterprise network for a
public or private organization, there is often an administrative
department that coordinates network services, including NTP. Where
available, the addresses of appropriate servers can be provided by that
department. However, if this infrastructure is not available, it is
necessary to explore some portion of the existing NTP subnet now running
in the Internet. There are at present many thousands of time servers
running NTP in the Internet, a significant number of which are willing
to provide a public time-synchronization service. Some of these are
listed in a file maintained on the Internet host louie.udel.edu
(128.175.1.3) on the path pub/ntp/doc/clock.txt. This file is updated on
a regular basis using information provided voluntarily by various site
administrators. There are other ways to explore the nearby subnet using
the ntptrace and ntpq programs. See the man pages for further
information on these programs.

It is vital to carefully consider the issues of robustness and
reliability when selecting the sources of synchronization. Normally, not
less than three sources should be available, preferably selected to
avoid common points of failure. It is usually better to choose sources
which are likely to be "close" to you in terms of network topology,
though you shouldn't worry overly about this if you are unable to
determine who is close and who isn't. Normally, it is much more serious
when a server becomes faulty and delivers incorrect time than when it
simply stops operating, since an NTP-synchronized host normally can
coast for hours or even days without its clock accumulating serious
error over one second, for instance. Selecting at least three sources
from different operating administrations, where possible, is the minimum
recommended, although a lesser number could provide acceptable service
with a degraded degree of robustness.

Normally, it is not considered good practice for a single workstation to
request synchronization from a primary (stratum-1) time server. At
present, these servers provide synchronization for hundreds of clients
in many cases and could, along with the network access paths, become
seriously overloaded if large numbers of workstation clients requested
synchronization directly. Therefore, workstations located in sparsely
populated administrative domains with no local synchronization
infrastructure should request synchronization from nearby stratum-2
servers instead. In most cases the keepers of those servers listed in
the clock.txt file provide unrestricted access without prior permission;
however, in all cases it is considered polite to notify the
administrator listed in the file upon commencement of regular service.
In all cases the access mode and notification requirements listed in the
file must be respected.

In the case of a gateway or file server providing service to a
significant number of workstations or file servers in an enterprise
network it is even more important to provide multiple, redundant sources
of synchronization and multiple, diversity-routed, network access paths.
The preferred configuration is at least three administratively
coordinated time servers providing service throughout the administrative
domain including campus networks and subnetworks. Each of these should
obtain service from at least two different outside sources of
synchronization, preferably via different gateways and access paths.
These sources should all operate at the same stratum level, which is one
less than the stratum level to be used by the local time servers
themselves. In addition, each of these time servers should peer with all
of the other time servers in the local administrative domain at the
stratum level used by the local time servers, as well as at least one
(different) outside source at this level. This configuration results in
the use of six outside sources at a lower stratum level (toward the
primary source of synchronization, usually a radio clock), plus three
outside sources at the same stratum level, for a total of nine outside
sources of synchronization. While this may seem excessive, the actual
load on network resources is minimal, since the interval between polling
messages exchanged between peers usually ratchets back to no more than
one message every 17 minutes.

The stratum level to be used by the local time servers is an engineering
choice. As a matter of policy, and in order to reduce the load on the
primary servers, it is desirable to use the highest stratum consistent
with reliable, accurate time synchronization throughout the
administrative domain. In the case of enterprise networks serving
hundreds or thousands of client file servers and workstations,
conventional practice is to obtain service from stratum-1 primary
servers such as listed in the clock.txt file. When choosing sources away
from the primary sources, the particular synchronization path in use at
any time can be verified using the ntptrace program included in the
xntp3 distribution. It is important to avoid loops and possible common
points of failure when selecting these sources. Note that, while NTP
detects and rejects loops involving neighboring servers, it does not
detect loops involving intervening servers. In the unlikely case that
all primary sources of synchronization are lost throughout the subnet,
the remaining servers on that subnet can form temporary loops and, if
the loss continues for an interval of many hours, the servers will drop
off the subnet and free-run with respect to their internal (disciplined)
timing sources.

In many cases the purchase of one or more radio clocks is justified, in
which cases good engineering practice is to use the configurations
described above and connect the radio clock to one of the local servers.
This server is then encouraged to participate in a special primary-
server subnetwork in which each radio-equipped server peers with several
other similarly equipped servers. In this way the radio-equipped server
may provide synchronization, as well as receive synchronization, should
the local or remote radio clock(s) fail or become faulty. Xntpd treats
attached radio clock(s) in the same way as other servers and applies the
same criteria and algorithms to the time indications, so can detect when
the radio fails or becomes faulty and switch to alternate sources of
synchronization. It is strongly advised, and in practice for most
primary servers today, to employ the authentication or access-control
features of the xntp3 distribution in order to protect against hostile
penetration and possible destabilization of the time service.

Using this or similar strategies, the remaining hosts in the same
administrative domain can be synchronized to the three (or more)
selected time servers. Assuming these servers are synchronized directly
to stratum-1 sources and operate normally as stratum-2, the next level
away from the primary source of synchronization, for instance various
campus file servers, will operate at stratum 3 and dependent
workstations at stratum 4. Engineered correctly, such a subnet will
survive all but the most exotic failures or even hostile penetrations of
the various, distributed timekeeping resources.

The above arrangement should provide very good, robust time service with
a minimum of traffic to distant servers and with manageable loads on the
local servers. While it is theoretically possible to extend the
synchronization subnet to even higher strata, this is seldom justified
and can make the maintenance of configuration files unmanageable.
Serving time to a higher stratum peer is very inexpensive in terms of
the load on the lower stratum server if the latter is located on the
same concatenated LAN. When justified by the accuracy expectations, NTP
can be operated in broadcast mode, so that clients need only listen for
periodic broadcasts and do not need to send anything.

When planning your network you might, beyond this, keep in mind a few
generic don'ts, in particular:

1.   Don't synchronize a local time server to another peer at the same
     stratum, unless the latter is receiving time from lower stratum
     sources the former doesn't talk to directly. This minimizes the
     occurance of common points of failure, but does not eliminate them
     in cases where the usual chain of associations to the primary
     sources of synchronization are disrupted due to failures.
2.   Don't configure peer associations with higher stratum servers. Let
     the higher strata configure lower stratum servers, but not the
     reverse. This greatly simplifies configuration file maintenance,
     since there is usually much greater configuration churn in the high
     stratum clients such as personal workstations.

3.   Don't synchronize more than one time server in a particular
     administrative domain to the same time server outside that domain.
     Such a practice invites common points of failure, as well as raises
     the possibility of massive abuse, should the configuration file be
     automatically distributed do a large number of clients.

There are many useful exceptions to these rules. When in doubt, however,
follow them.

Dennis Ferguson writes: Note that mention was made of machines with
"good" clocks versus machines with "bad" ones. There are two things that
make a clock good, the precision of the clock (e.g. how many low order
bits in a time value are actually significant) and the frequency of
occurance (or lack thereof) of such things as lost clock interrupts.
Among the most common computers I have observed there to be a fairly
simple algorithm for determining the goodness of its clock. If the
machine is a Vax, it probably has a good clock (the low order bit in the
time is in the microseconds and most of these seem to manage to get
along without losing clock interrupts). If the machine is a Sun 3 it
probably doesn't (the low order clock bit is at the 10 or 20 millisecond
mark and Sun 3s like to lose clock interrupts, particularly if they have
a screen and particularly if they run SunOS 4.0.x). If you have IBM RTs
running AOS 4.3, they have fair clocks (low order clock bit at about a
millisecond and they don't lose clock interrupts, though they do have
trouble with clock rollovers while reading the low order clock bits) but
I recommend them as low stratum NTP servers anyway since they aren't
much use as anything else. Sun 4s running SunOS 4.1.1 make very good
time servers, once some native foolishness mentioned below is
surmounted. [However, it is very important to avoid using the keyboard
firmware, which can cause severe interrupt latencies, in favor of the
software drivers ordinarily used in conjunction with a windowing system.
- DLM] For other machines you are on your own since I don't have enough
data points to venture an opinion. In any event, if at all possible you
should try to use machines with good clocks for the lower strata.

Configuring Your Server or Client

As mentioned previously, the configuration file is usually called
/etc/ntp.conf. This is an ASCII file conforming to the usual comment and
whitespace conventions. A working configuration file might look like (In
this and other examples, do not copy this directly.):

     # peer configuration for 128.100.100.7
     # (expected to operate at stratum 2)

     server 128.4.1.1         # rackety.udel.edu
     server 128.8.10.1        # umd1.umd.edu
     server 192.35.82.50      # lilben.tn.cornell.edu
     driftfile /etc/ntp.drift

This particular host is expected to operate as a client at stratum 2 by
virtue of the "server" keyward and the fact that two of the three
servers declared (the first two, actually) have radio clocks and usually
run at stratum 1. The third server in the list has no radio clock, but
is known to maintain associations with a number of stratum 1 peers and
usually operates at stratum 2. Of particular importance with the last
host is that it maintains associations with peers besides the two
stratum 1 peers mentioned. This can be verified using the ntpq program
included in the xntp3 distribution. When configured using the "server"
keyword, this host can receive synchronization from any of the listed
servers, but can never provide synchronization to them.

Unless restricted using facilities described later, this host can
provide synchronization to dependent clients, which do not have to be
listed in the configuration file. Associations maintained for these
clients are transitory and result in no persistent state in the host.
These clients are normally not visible using the ntpq program included
in the xntp3 distribution; however, xntpd includes a monitoring feature
(described later) which caches a minimal amount of client information
useful for debugging administrative purposes.

A time server expected to both receive synchronization from another
server, as well as to provide synchronization to it, is delared using
the "peer" keyword instead of the "server" keyword. In all other aspects
the server operates the same in either mode and can provide
synchronization to dependent clients or other peers. It is considered
good engineering practice to declare time servers outside the
administrative domain as "peer" and those inside as "server" in order to
provide redundancy in the global Internet, while minimizing the
possibility of instability within the domain itself. A time server in
one domain can in principle heal another domain temporarily isolated
from all other sources of synchronization. However, it is probably
unwise for a casual workstation to bridge fragments of the local domain
which have become temporarily isolated.

Note the inclusion of a "driftfile" declaration. One of the things the
NTP daemon does when it is first started is to compute the error in the
intrinsic frequency of the clock on the computer it is running on. It
usually takes about a day or so after the daemon is started to compute a
good estimate of this (and it needs a good estimate to synchronize
closely to its server). Once the initial value is computed, it will
change only by relatively small amounts during the course of continued
operation. The "driftfile" declaration indicates to the daemon the name
of a file where it may store the current value of the frequency error so
that, if the daemon is stopped and restarted, it can reinitialize itself
to the previous estimate and avoid the day's worth of time it will take
to recompute the frequency estimate. Since this is a desireable feature,
a "driftfile" declaration should always be included in the configuration
file.

An implication in the above is that, should xntpd be stopped for some
reason, the local platform time will diverge from UTC by an amount that
depends on the intrinsic error of the clock oscillator and the time
since last synchronized. In view of the length of time necessary to
refine the frequency estimate, every effort should be made to operate
the daemon on a continuous basis and minimize the intervals when for
some reason it is not running.

Xntpd3 Versus Previous Versions

There are several items of note when dealing with a mixture of xntp3 and
and previous distributions of xntp (NTP Version 2 xntpd) and ntp3.4 (NTP
Version 1 ntpd). The xntp3 implementation of xntpd is an NTP Version 3
implementation. As such, by default when no additional information is
available concerning the preferences of the peer, xntpd claims to be
version 3 in the packets that it sends.

An NTP implementation conforming to a previous version specification
ordinarily discards packets from a later version. However, in most
respects documented in RFC 1305, the previous version is compatible with
the version-3 algorithms and protocol. Ntpd, while implementing most of
the version-2 algorithms, still believes itself to be a version-1
implementation. The sticky part here is that, when either xntpd version
2 or ntpd version 1 receives a packet claiming to be from a version-3
server, it discards it without further processing. Hence there is a
danger that in some situations synchronization with previous versions
will fail.

Xntpd is aware of this problem. In particular, when xntpd is polled
first by a host claiming to be a previous version 1 or version 2
implementation, xntpd claims to be a version 1 or 2 implementation,
respectively, in packets returned to the poller. This allows xntpd to
serve previous version clients transparently. The trouble occurs when an
previous version is to be included in an xntpd configuration file. With
no further indication, xntpd will send packets claiming to be version 3
when it polls. To get around this, xntpd allows a qualifier to be added
to configuration entries to indicate which version to use when polling.
Hence the entry

     # specify NTP version 1

     peer 130.43.2.2 version 1     # apple.com (running ntpd version 1)
     peer 130.43.2.2 version 2     # apple.com (running xntpd version 2)

will cause version 1 packets to be sent to the host address 130.43.2.2.
If you are testing xntpd against previous version servers you will need
to be careful about this. Note that, as indicated in the RFC 1305
specification, there is no longer support for the original NTP
specification, popularly called NTP Version 0.

There are a few other items to watch when converting an ntpd
configuration file for use with xntpd. The first is to reconsider the
precision entry from the configuration file, if there is one. There was
a time when the precision claimed by a server was mostly commentary,
with no particularly useful purpose. This is no longer the case,
however, and so changing the precision a server claims should only be
done with some consideration as to how this alters the performance of
the server. The default precision claimed by xntpd will be right for
most situations. A section later on will deal with when and how it is
appropriate to change a server's precision without doing things you
don't intend.

Second, note that in the example configuration file above numeric
addresses are used in the peer and server declarations. It is also
possible to use names requiring resolution instead, but only if some
additional configuration is done (xntpd doesn't include the resolver
routines itself, and requires that a second program be used to do name
resolution). If you find numeric addresses offensive, see below.

Finally, "passive" and "client" entries in an ntpd configuration file
have no useful equivalent semantics for xntpd and should be deleted.
Xntpd won't reset the kernel variable tickadj when it starts, so you can
remove anything dealing with this in the configuration file. The
configuration of radio clock peers is done using different language in
xntpd configuration files, so you will need to delete these entries from
your ntpd configuration file and see below for the equivalent language.

Traffic Monitoring

Xntpd handles peers whose stratum is higher than the stratum of the
local server and pollers using client mode by a fast path which
minimizes the work done in responding to their polls, and normally
retains no memory of these pollers. Sometimes, however, it is
interesting to be able to determine who is polling the server, and how
often, as well as who has been sending other types of queries to the
server.

To allow this, xntpd implements a traffic monitoring facility which
records the source address and a minimal amount of other information
from each packet which is received by the server. This can be enabled by
adding the following line to the server's configuration file:

     # enable monitoring feature

     monitor yes

The recorded information can be displayed using the xntpdc query
program, described briefly below.

Address-and-Mask Restrictions

The address-and-mask configuration facility supported by xntpd is quite
flexible and general, but is not an integral part of the NTP Version 3
specification. The major drawback is that, while the internal
implementation is very nice, the user interface sucks. For this reason
it is probably worth doing an example here. Briefly, the facility works
as follows. There is an internal list, each entry of which holds an
address, a mask and a set of flags. On receipt of a packet, the source
address of the packet is compared to each entry in the list, with a
match being posted when the following is true:

     (source_addr & mask) == (address & mask)

A particular source address may match several list entries. In this case
the entry with the most one bits in the mask is chosen. The flags
associated with this entry are used to control the access.

In the current implementation the flags always add restrictions. In
effect, an entry with no flags set leaves matching hosts unrestricted.
An entry can be added to the internal list using a "restrict"
declaration. The flags associated with the entry are specified
textually. For example, the "notrust" flag indicates that hosts matching
this entry, while treated normally in other respects, shouldn't be
trusted to provide synchronization even if otherwise so enabled. The
"nomodify" flag indicates that hosts matching this entry should not be
allowed to do run time configuration. There are many more flags, see the
xntpd.8 man page.

Now the example. Suppose you are running the server on a host whose
address is 128.100.100.7. You would like to ensure that run time
reconfiguration requests can only be made from the local host and that
the server only ever synchronizes to one of a pair of off-campus servers
or, failing that, a time source on net 128.100. The following entries in
the configuration file would implement this policy:

     # by default, don't trust and don't allow modifications

     restrict default notrust nomodify

     # these guys are trusted for time, but no modifications allowed

     restrict 128.100.0.0 mask 255.255.0.0 nomodify
     restrict 128.8.10.1 nomodify
     restrict 192.35.82.50 nomodify

     # the local addresses are unrestricted

     restrict 128.100.100.7
     restrict 127.0.0.1

The first entry is the default entry, which all hosts match and hence
which provides the default set of flags. The next three entries indicate
that matching hosts will only have the nomodify flag set and hence will
be trusted for time. If the mask isn't specified in the restrict
keyward, it defaults to 255.255.255.255. Note that the address
128.100.100.7 matches three entries in the table, the default entry
(mask 0.0.0.0), the entry for net 128.100 (mask 255.255.0.0) and the
entry for the host itself (mask 255.255.255.255). As expected, the flags
for the host are derived from the last entry since the mask has the most
bits set.

The only other thing worth mentioning is that the restrict declarations
apply to packets from all hosts, including those that are configured
elsewhere in the configuration file and even including your clock
pseudopeer(s), in any. Hence, if you specify a default set of
restrictions which you don't wish to be applied to your configured
peers, you must remove those restrictions for the configured peers with
additional restrict declarations mentioning each peer separately.

Authentication

Xntpd supports the optional authentication procedure specified in the
NTP Version 2 and 3 specifications. Briefly, when an association runs in
authenticated mode, each packet transmitted has appended to it a 32-bit
key ID and a 64-bit crypto checksum of the contents of the packet
computed using either the Data Encryption Standard (DES) or Message
Digest (MD5) algorithms. Note that while either of these algorithms
provide sufficient protection from message-modification attacks,
distribution of the former algorithm implementation is restricted to the
U.S. and Canada, while the latter presently is free from such
restrictions. With either algorithm the receiving peer recomputes the
checksum and compares it with the one included in the packet. For this
to work, the peers must share at least one encryption key and,
furthermore, must associate the shared key with the same key ID.

This facility requires some minor modifications to the basic packet
processing procedures, as required by the specification. These
modifications are enabled by the "authenticate" configuration
declaration. In particular, in authenticated mode, peers which send
unauthenticated packets, peers which send authenticated packets which
the local server is unable to decrypt and peers which send authenticated
packets encrypted using a key we don't trust are all marked
untrustworthy and unsuitable for synchronization. Note that, while the
server may know many keys (identified by many key IDs), it is possible
to declare only a subset of these as trusted. This allows the server to
share keys with a client which requires authenticated time and which
trusts the server but which is not trusted by the server. Also, some
additional configuration language is required to specify the key ID to
be used to authenticate each configured peer association. Hence, for a
server running in authenticated mode, the configuration file might look
similar to the following:

     # peer configuration for 128.100.100.7
     # (expected to operate at stratum 2)
     # fully authenticated this time

     peer 128.100.49.105 key 22    # suzuki.ccie.utoronto.ca
     peer 128.8.10.1 key 4    # umd1.umd.edu
     peer 192.35.82.50 key 6  # lilben.tn.cornell.edu
     authenticate yes         # enable authentication
     keys /usr/local/bin/ntp.keys  # path for key file
     trustedkey 1 2 14 15     # define trusted keys
     requestkey 15            # key (7) for accessing server variables
     controlkey 15            # key (6) for accessing server variables

     #authdelay 0.000047      # authentication delay (Sun4c/50 IPX DES)
     authdelay 0.000094       # authentication delay (Sun4c/50 IPX MD5)

There are a couple of previously unmentioned things in here. The
"authenticate yes" line enables authentication processing, while the
"keys /usr/local/bin/ntp.keys" specifies the path to the keys file (see
below and the xntpd.8 man page for detaiils of the file format). The
"trustedkey" declaration identifies those keys that are known to be
uncompromised; the remainder presumably represent the expired or
possibly compromised keys. Both sets of keys must be declared by key
identifier in the ntp.keys file described below. This provides a way to
retire old keys while minimrequestkey 15izing the frequency of delicate
key-distribution procedures. The "requestkey 15" line establishes the
key to be used for mode-6 control messages as specified in RFC 1305 and
used by the ntpq utility program, while the "controlkey 15" establishes
the key to be used for mode-7 private control messages used by the
xntpdc utility program these keys are used to prevent unauthorized
modification of daemon variables.

The "authdelay" declaration is an estimate of the amount of processing
time taken between the freezing of a transmit timestamp and the actual
transmission of the packet when authentication is enabled (i.e. more or
less the time it takes for the DES or MD5 routine to encrypt a single
block), and is used as a correction for the transmit timestamp. This can
be computed for your CPU by the authspeed program included in the
authstuff directory in the xntp3 distribution. The usage is illustrated
to the following:

     # for DES keys

     authspeed -n 30000 auth.samplekeys

     # for MD5 keys

     authspeed -nd 30000 auth.samplekeys

Additional utility programs included in the authstuff directory can be
used to generate random keys, certify implementation correctness and
display sample keys. As a general rule, keys should be chosen randomly,
except possibly the request and control keys, which must be entered by
the user as a password.

The ntp.keys file contains the list of keys and associated key IDs the
server knows about (for obvious reasons this file is better left
unreadable by anyone except the server). The contents of this file might
look like:

     # ntp keys file (ntp.keys)

     1    N    29233E0461ECD6AE    # des key in NTP format
     2    M    RIrop8KPPvQvYotM    # md5 key as an ASCII random string
     14   M    sundial             # md5 key as an ASCII string
     15   A    sundial             # des key as an ASCII string

     # the following 3 keys are identical

     10   A    SeCReT
     10   N    d3e54352e5548080
     10   S    a7cb86a4cba80101

In the keys file the first token on each line indicates the key ID, the
second token the format of the key and the third the key itself. There
are four key formats. An "A" indicates a DES key written as a 1-to-8
character string in 7-bit ASCII representation, with each character
standing for a key octet (like a Unix password). An "S" indicates a DES
key written as a hex number in the DES standard format, with the low
order bit (LSB) of each octet being the (odd) parity bit. An "N"
indicates a DES key again written as a hex number, but in NTP standard
format with the high order bit of each octet being the (odd) parity bit
(confusing enough?). An "M" indicates an MD5 key written as a 1-to-31
character ASCII string in the "A" format. Note that, because of the
simple tokenizing routine, the characters ' ', '#', '\t', '\n' and '\0'
can't be used in either a DES or MD5 ASCII key. Everything else is fair
game, though. Key 0 (zero) is used for special purposes and should not
appear in this file.

The big trouble with the authentication facility is the keys file. It is
a maintenance headache and a security problem. This should be fixed some
day. Presumably, this whole bag of worms goes away if/when a generic
security regime for the Internet is established.

Query Programs

Three utility query programs are included with the xntp3 distribution,
ntpq, ntptrace and xntpdc. Ntpq is a rather handy program which sends
queries and receives responses using NTP standard mode-6 control
messages. Since it uses the standard control protocol specified in RFC
1305, it may be used with NTP Version 2 and Version 3 implementations
for both Unix and Fuzzball, but not Version 1 implementations. It is
most useful to query remote NTP implementations to assess timekeeping
accuracy and expose bugs in configuration or operation.

Ntptrace can be used to display the current synchronization path from a
selected host through possibly intervening servers to the primary source
of synchronization, usually a radio clock. It works with both version 2
and version 3 servers, but not version 1.

Xnptdc is a horrid program which uses NTP private mode-7 control
messages to query local or remote servers. The format and and contents
of these messages are specific to xntpd. The program does allow
inspection of a wide variety of internal counters and other state data,
and hence does make a pretty good debugging tool, even if it is
frustrating to use. The other thing of note about xntpdc is that it
provides a user interface to the run time reconfiguration facility.

See the respective man pages for details on the use of these programs.
The primary reason for mentioning them here is to point out an
inconsistancy which can be awfully annoying if it catches you, and which
is worth keeping firmly in mind. Both xntpdc and xntpd demand that
anything which has dimensions of time be specified in units of seconds,
both in the configuration file and when doing run time reconfiguration.
Both programs also print the values in seconds. Ntpq on the other hand,
obeys the standard by printing all time values in milliseconds. This
makes the process of looking at values with ntpq and then changing them
in the configuration file or with xntpdc very prone to errors (by three
orders of magnitude). I wish this problem didn't exist, but xntpd and
its love of seconds predate the mode-6 protocol and the latter's
(Fuzzball-inspired) millisecond orientation, making the inconsistancy
irresolvable without considerable work.

Run Time Reconfiguration

Xntpd was written specifically to allow its configuration to be fully
modifiable at run time. Indeed, the only way to configure the server is
at run time. The configuration file is read only after the rest of the
server has been initialized into a running, but default unconfigured,
state. This facility was included not so much for the benefit of Unix,
where it is handy but not strictly essential, but rather for dedicated
platforms where the feature is more important for maintenance.
Nevertheless, run time configuration works very nicely for Unix servers
as well.

Nearly all of the things it is possible to configure in the
configuration file may be altered via NTP mode-7 messages using the
xntpdc program. Mode-6 messages may also provide some limited
configuration functionality (though the only thing you can currently do
with mode-6 messages is set the leap-second warning bits) and the ntpq
program provides generic support for the latter. The leap bits that can be
set in the leap_warning variable (up to one month ahead) and in the
leap_indication variable have a slighly different encoding than the
usual interpretation:

	Value		Action
	 00		The daemon passes the leap bits of its 
			synchronisation source (usual mode of operation)
	01/10		A leap second is added/deleted
	 11		Leap information from the sychronisation source
			is ignored (thus LEAP_NOWARNING is passed on)

Mode-6 and mode-7 messages which would modify the configuration of the
server are required to be authenticated using standard NTP
authentication. To enable the facilities one must, in addition to
specifying the location of a keys file, indicate in the configuration
file the key IDs to be used for authenticating reconfiguration commands.
Hence the following fragment might be added to a configuration file to
enable the mode-6 (ntpq) and mode-7 (xntpdc) facilities in the daemon:

     # specify mode-6 and mode-7 trusted keys

     requestkey 65535    # for mode-7 requests
     controlkey 65534    # for mode-6 requests

If the "requestkey" and/or the "controlkey" configuration declarations
are omitted from the configuration file, the corresponding run time
reconfiguration facility is disabled.

The query programs require the user to specify a key ID and a key to use
for authenticating requests to be sent. The key ID provided should be
the same as the one mentioned in the configuration file, while the key
should match that corresponding to the key ID in the keys file. As the
query programs prompt for the key as a password, it is useful to make
the request and control authentication keys typable (in ASCII format)
from the keyboard.

Name Resolution

Xntpd includes the cability to specify host names requiring resolution
in "peer" and "server" declarations in the configuration file. There are
several reasons why this was not permitted in the past. Chief among
these is the fact that name service is unreliable and the interface to
the Unix resolver routines is synchronous. The hangups and delays
resulting from name-resolver clanking can be unacceptable once the NTP
server is running (and remember it is up and running before the
configuration file is read). However, it is advantageous to resolve time
server names, since their addresses are occasionally changed.

Instead of running the resolver itself the daemon can defer this task to
a separate program, xntpres. When the daemon comes across a "peer" or
"server" entry with a non-numeric host address it records the relevant
information in a temporary file and continues on. When the end of the
configuration file has been reached and one or more entries requiring
name resolution have been found, the server runs an instance of xntpres
with the temporary file as an argument. The server then continues on
normally but with the offending peers/servers omitted from its
configuration.

When xntpres successfully resolves a name from this file, it configures
the associated entry into the server using the same mode-7 run time
reconfiguration facility that xntpdc uses. If temporary resolver
failures occur, xntpres will periodically retry the offending requests
until a definite response is received. The program will continue to run
until all entries have been resolved.
There are several configuration requirements if xntpres is to be used.
The path to the xntpres program must be made known to the daemon via a
"resolver" configuration entry, and mode-7 run time reconfiguration must
be enabled. The following fragment might be used to accomplish this:

     # specify host name resolver data

     resolver /local/etc/xntpres
     keys /etc/ntp.keys
     requestkey 65535

Note that xntpres sends packets to the server with a source address of
127.0.0.1. You should obviously avoid "restrict" modification requests
from this address or xntpres will fail.

Dealing with Frequency Tolerance Violations (Tickadj and Friends)

The NTP Version 3 specification RFC 1305 calls for a maximum oscillator
frequency tolerance of +-100 parts-per-million (ppm), which is
representative of those components suitable for use in relatively
inexpensive workstation platforms. For those platforms meeting this
tolerance, NTP will automatically compensate for the frequency errors of
the individual oscillator and no further adjustments are required,
either to the configuration file or to various kernel variables.

However, in the case of certain notorious platforms, in particular Sun
4s, the 100-ppm tolerance is routinely violated. In such cases it may be
necessary to adjust the values of certain kernel variables; in
particular, "tick" and "tickadj". The variable tick is the increment in
microseconds added to the system time on each interval-timer interrupt,
while the variable tickadj is used by the time adjustment code as a slew
rate. When the time is being adjusted via a call to the system routine
adjtime(), the kernel increases or reduces tick by tickadj microseconds
until the specified adjustment has been completed. Unfortunately, in
most Unix implementations the tick increment must be either zero or
plus/minus exactly tickadj microseconds, meaning that adjustments are
truncated to be an integral multiple of tickadj (this latter behaviour
is a misfeature, and is the only reason the xntpd code needs to concern
itself with the internal implementation of adjtime() at all). In
addition, the stock Unix implementation considers it an error to request
another adjustment before a prior one has completed.

Thus, to make very sure it avoids problems related to the roundoff, the
xntpd daemon reads the values of tick and tickadj from /dev/kmem when it
starts. It then ensures that all adjustments given to adjtime() are an
even multiple of tickadj microseconds and computes the largest
adjustment that can be completed in the adjustment interval (using both
the value of tickadj and the value of tick) so it can avoid exceeding
this limit.

Unfortunately, the value of tickadj set by default is almost always too
large for xntpd. NTP operates by continuously making small adjustments
to the clock, usually at one-second intervals. If tickadj is set too
large, the adjustments will disappear in the roundoff; while, if tickadj
is too small, NTP will have difficulty if it needs to make an occasional
large adjustment. While the daemon itself will read the kernel's values
of tick and tickadj, it will not change the values, even if they are
unsuitable. You must do this yourself before the daemon is started,
either with adb or, in the running kernel only, with the tickadj program
included in the util directory of the xntp3 distribution. Note that the
latter program will also computes an optimal value of tickadj for NTP
use based on the kernel's value of tick.

The tickadj program can reset several other kernel variables if asked.
It can also change the value of tick if asked, this being necessary on a
few machines with very broken clocks, like Sun 4s. With these machines
it should also set the value of the kernel dosynctodr variable to zero.
This variable controls whether to synchronize the system clock to the
time-of-day clock, something you really don't want to be happen when
xntpd is trying to keep it under control.

In order to maintain reasonable correctness bounds, as well as
reasonably good accuracy with acceptable polling intervals, xntpd will
complain if the frequency error is greater than 100 ppm. For machines
with a value of tick in the 10-ms range, a change of one in the value of
tick will change the frequency by about 100 ppm. In order to determine
the value of tick for a particular CPU, disconnect the machine from all
sources of time (dosynctodr = 0) and record its actual time compared to
an outside source (eyeball-and-wristwatch will do) over a day or more.
Multiply the time change over the day by 0.116 and add or subtract the
result to tick, depending on whether the CPU is fast or slow. An example
call to tickadj useful on Sun 4s is:

     tickadj -t 9999 -a 5 -s

which sets tick 100 ppm fast, tickadj to 5 microseconds and turns off
the clock/calendar chip fiddle. This line can be added to the rc.local
configuration file to automatically set the kernel variables at boot
time.

All this stuff about diddling kernel variables so the NTP daemon will
work is really silly. If vendors would ship machines with clocks that
kept reasonable time and would make their adjtime() system call apply
the slew it is given exactly, independent of the value of tickadj, all
this could go away.

Tuning Your Subnet

There are several parameters available for tuning the NTP subnet for
maximum accuracy and minimum jitter. Two important parameters are the
the "precision" and "prefer" configuration declarations. The precision
declaration specifies the number of significant bits of the system clock
representation relative to one second. For instance, the default value
of -6 corresponds to 1/64 second or about 16 milliseconds.

The NTP protocol makes use of the precision parameter in several places.
It is included in packets sent to peers and is used by them to calculate
the maximum absolute error and maximum statistical error. When faced
with selecting one of several servers of the same stratum and about the
same network path delay for synchronization purposes, clients will
usually prefer to synchronize to those servers claiming the smallest
(most negative) precision, since this maximizes the accuracy and
minimizes the jitter apparent to application programs running on the
client platform. Therefore, when the maximum attainable accuracy is
required, it is important that every platform configure an accurate
value for the precision variable. This can be done using the optional
"precision" declaration in the configuration file:

     # precision declaration

     precision -18       # for microsecond clocks (Sun 4s, DEC 5000/240)

When more than one eligible server exists, the NTP clock-selection and
combining algorithms act to winnow out all except the "best" set of
servers using several criteria based on differences between the readings
of different servers and between successive readings of the same server.
The result is usually a set of surviving servers that are apparently
statistically equivalent in accuracy, jitter and stability. The
population of survivors remaining in this set depends on the individual
server characteristics measured during the selection process and may
vary from time to time as the result of normal statistical variations.
In LANs with high speed RISC-based time servers, the population can
become somewhat unstable, with individual servers popping in and out of
the surviving population, generally resulting in a regime called
clockhopping.

When only the smallest residual jitter can be tolerated, it may be
convenient to elect one of the servers at each stratum level as the
preferred one using the keyword "prefer" on the configuration
declaration for the selected server:

     # prefered server declaration

     peer 128.4.1.1 prefer    # preferred server

The preferred server will always be included in the surviving
population, regardless of its characteristics and as long as it survives
preliminary sanity checks and validation procedures.

The most useful application of the prefer keyword is in high speed LANs
equipped with precision radio clocks, such as a GPS receiver. In order
to insure robustness, the hosts need to include outside peers as well as
the GPS-equipped server; however, as long as that server is running, the
synchronization preference should be that server. The keyword should
normally be used in all cases in order to prefer an attached radio
clock. It is probably inadvisable to use this keyword for peers outside
the LAN, since it interferes with the carefully crafted judgement of the
selection and combining algorithms.

Provisions for Leap Seconds and Accuracy Metrics

Xntpd understands leap seconds and will attempt to take appropriate
action when one occurs. In principle, every host running xntpd will
insert a leap second in the local timescale in precise synchronization
with UTC. This requires that the leap-warning bits be manually activated
some time prior to the occurance of a leap second at the primary
(stratum 1) servers. Subsequently, these bits are propagated throughout
the subnet depending on these servers by the NTP protocol itself and
automatically implemented by xntpd and the time-conversion routines of
each host. The implementation is independent of the idiosyncracies of
the particular radio clock, which vary widely among the various devices,
as long as the idiosyncratic behavior does not last for more than about
20 minutes following the leap. Provisions are included to modify the
behavior in cases where this cannot be guaranteed.

While provisions for leap seconds have been carefully crafted so that
correct timekeeping immediately before, during and after the occurance
of a leap second is scrupulously correct, stock Unix systems are mostly
inept in responding to the available information. This caveat goes also
for the maximum-error and statistical-error bounds carefully calculated
for all clients and servers, which could be very useful for application
programs needing to calibrate the delays and offsets to achieve a near-
simulataneous commit procedure, for example. While this information is
maintained in the xntpd data structures, there is at present no way for
application programs to access it. This may be a topic for further
development.

Clock Support Overview

Xntpd was designed to support radio (and other external) clocks and does
some parts of this function with utmost care. Clocks are treated by the
protocol as ordinary NTP peers, even to the point of referring to them
with an (invalid) IP host address. Clock addresses are of the form
127.127.t.u, where t specifies the particular type of clock (i.e. refers
to a particular clock driver) and u is a unit number whose
interpretation is clock-driver dependent. This is analogous to the use
of major and minor device numbers by Unix and permits multiple
instantiations of clocks of the same type on the same server, should
such magnificant redundancy be required.

Because clocks look much like peers, both configuration file syntax and
run time reconfiguration commands can be be used to control clocks in
the same way as ordinary peers. Clocks are configured via "server"
declarations in the configuration file, can be started and stopped using
xntpdc and are subject to address-and-mask restrictions much like a
normal peer, should this stretch of imagination ever be useful. As a
concession to the need to sometimes transmit additional information to
clock drivers, an additional configuration file is available: the
"fudge" statement. This enables one to specify the values two time
quantities, two integral values and two flags, the use of which is
dependent on the particular clock driver. For example, to configure a
PST radio clock which can be accessed through the serial device
/dev/pst1, with propagation delays to WWV and WWVH of 7.5 and 26.5
milliseconds, respectively, on a machine with an imprecise system clock
and with the driver set to disbelieve the radio clock once it has gone
30 minutes without an update, one might use the following configuration
file entries:

     # radio clock fudge fiddles

     server 127.127.3.1
     fudge 127.127.3.1 time1 0.0075 time2 0.0265
     fudge 127.127.3.1 value2 30 flag1 1

Additional information on the interpretation of these data with respect
to various radio clock drivers is given in the xntpd.8 man page.

Towards the Ultimate Tick

This section consideres issues in providing precision time
synchronization in NTP subnets which need the highest quality time
available in the present technology. These issues are important in
subnets supporting real-time services such as distributed multimedia
conferencing and wide-are experiment control and monitoring.

In the Internet of today synchronization paths often span continents and
oceans with moderate to high variations in delay due to traffic spasms.
NTP is specifically designed to minimize timekeeping jitter due to delay
variations using intricately crafted filtering and selection algorithms;
however, in cases where these variations are as much as a second or
more, the residual jitter following these algorithms may still be
excessive. Sometimes, as in the case of some isolated NTP subnets where
a local source of precision time is available, such as a 1-pps signal
produced by a calibrated cesium clock, it is possible to remove the
jitter and retime the local clock oscillator of the NTP server. This has
turned out to be a useful feature to improve the synchronization quality
of time distributed in remote places where radio clocks are not
available. In these cases special features of the xntp3 distribution are
used together with the 1-pps signal to provide a jitter-free timing
signal, while NTP itself is used to provide the coarse timing and
resolve the seconds numbering.

Most available radio clocks can provide time to an accuracy in the order
of milliseconds, depending on propagation conditions, local noise levels
and so forth. However, as a practical matter, all clocks can
occasionally display errors significantly exceeding nominal
specifications. Usually, the algorithms used by NTP for ordinary network
peers, as well as radio clock "peers" will detect and discard these
errors as discrepancies between the disciplined local clock oscillator
and the decoded time message produced by the radio clock. Some radio
clocks can produce a special 1-pps signal which can be interfaced to the
server platform in a number of ways and used to substantially improve
the (disciplined) clock oscillator jitter and wander characteristics by
at least an order of magnitude. Using these features it is possible to
achieve accuracies in the order of 100 microseconds with a fast RISC-
based platform.

There are three ways to implement 1-pps support, depending on the radio
clock model, platform model and serial line interface. Each of these
requires circuitry to convert the TTL signal produced by most clocks to
the the EIA levels used by most serial interfaces. An example of a
device designed to do this is presented in the gadget subdirectory
included in the xntp3 distribtuion. Besides being useful for this
purpose, this device includes an inexpensive modem designed for use with
the Canadian CHU time/frequency radio station.

In order to select the appropriate implementation, it is important to
understand the underlying 1-pps mechanism used by xntpd. The 1-pps
suport depends on a continuous source of 1-pps pulses used to calculate
an offset within +-500 milliseconds relative to the local clock. The
serial timecode produced by the radio or the time determined by NTP in
absence of the radio is used to adjust the local clock within +-128
milliseconds of the actual time. As long as the local clock is within
this interval the 1-pps support is used to discipline the local clock
and the timecode used only to verify that the local clock is in fact
within the interval. Outside this interval the 1-pps support is disabled
and the timecode used directly to control the local clock.

The first method of implementation uses a dedicated serial port and
either the bsd line discipline or System V streams module, which can be
found in the kernel directory of the xntp3 distribution. This method can
be used with any radio clock or in the absence of any clock. The line
discipline and streams modules take receive timestamps in the kernel,
specifically the interrupt routine of the serial port hardware driver.
Using this method the port is dedicated to serve the 1-pps signal and
cannot be used for other purposes. Instructions for implementing the
feature, which requires rebuilding the kernel, are included in the
modules themselves. Note that xndpd must be compiled with the -DPPSDEV
compiler switch in this case. There is an inherent error in this method
due to the latency of the interrupt system and remaining serial-line
protocol modules in the order of a millisecond with Sun 4s. While the
jitter in this latency is unavoidable, the systematic component can be
calibrated out using a special configuration declaration:

     # pps delay and baud rate

     pps delay .0017 baud 19200    # pps delay (ms) and baud rate

Note that the delay defaults to zero and the baud to 38400.

The second method uses mechanisms embedded in the radio clock driver,
which call the 1-pps support directly and do not require a dedicated
serial port. Currently, only the DCF77 (German radio time service)
driver uses this method. Instructions for implementing this are given in
README files in the xntp3 distribution.

The third method and the most accurate and intrusive of all uses the
carrier-detect modem-control lead monitored by the serial port driver.
This method can be used with any radio clock and 1-pps interface
mentioned above. It requires in addition to a special streams module,
replacement of the kernel high resolution time-of-day clock routine.
This method is applicable only to Sun 4 platforms running SunOS 4.1.1
and then only with either of the two onboard serial ports. It does not
work with other platforms, operating systems or external (SBus) serial
multiplexors.

Swatting Bugs

Let's say you have compiled and installed the code and put up an
apparently relevant configuration file. In many Unix systems the xntpd
daemon and utility programs (ntpq, ntptrace and xntpdc) are usually
installed in the /usr/local directory along with the key file
(ntp.keys), while the configuration file (ntp.conf) and drift file
(ntp.drift) are installed in the /etc directory. The daemon can is
usually started from the rc.local shell script at system boot time, but
could be started (and stopped) at other times for debugging, etc. How do
you verify that the daemon can form associations with remote peers and
verify correct synchronization? For this you need the ntpq utility
described in the ntpq.8 man page.

After starting the daemon, run the ntpq program using the -n switch,
which will avoid possible distractions due to name resolutions. Use the
peer command to display a billboard showing the status of configured
peers and possibly other clients poking the daemon. After operating for
a few minutes, the display should be something like:

  remote           refid      st when poll reach   delay  offset    disp
========================================================================
+128.4.2.6    132.249.16.1     2  131  256  373     9.89   16.28   23.25
*128.4.1.20   .WWVB.           1  137  256  377   280.62   21.74   20.23
-128.8.2.88   128.8.10.1       2   49  128  376   294.14    5.94   17.47
+128.4.2.17   .WWVB.           1  173  256  377   279.95   20.56   16.40

The hosts shown in the "remote" column should agree with the entries in
the configuration file, plus any peers not mentioned in the file at the
same or lower than your stratum that happen to be configured to peer
with you. The "refid" entry shows the current source of synchronization
for that peer, while the "st" reveals its stratum and the "poll" entry
the polling interval, in seconds. The "when" entry shows the time since
the peer was last heard, in seconds, while the "reach" entry shows the
status of the reachability register (see specification), which is in
octal format. The remaining entries show the latest delay, offset and
dispersion computed for the peer, in milliseconds.

*** This section incomplete. Soon.

status=0664 leap_none, sync_ntp, 6 events, event_peer/strat_chg
system="UNIX", leap=00, stratum=2, rootdelay=280.62,
rootdispersion=45.26, peer=11673, refid=128.4.1.20,
reftime=af00bb42.56111000  Fri, Jan 15 1993  4:25:38.336, poll=8,
clock=af00bbcd.8a5de000  Fri, Jan 15 1993  4:27:57.540, phase=21.147,
freq=13319.46, compliance=2

status=7414 reach, auth, sel_sync, 1 event, event_reach
srcadr=128.4.2.6, srcport=123, dstadr=128.4.2.7, dstport=123, keyid=1,
stratum=2, precision=-10, rootdelay=362.00, rootdispersion=21.99,
refid=132.249.16.1,
reftime=af00bb44.849b0000  Fri, Jan 15 1993  4:25:40.517,
delay=    9.89, offset=   16.28, dispersion=23.25, reach=373, valid=8,
hmode=2, pmode=1, hpoll=8, ppoll=10, leap=00, flash=0x0,
org=af00bb48.31a90000  Fri, Jan 15 1993  4:25:44.193,
rec=af00bb48.305e3000  Fri, Jan 15 1993  4:25:44.188,
xmt=af00bb1e.16689000  Fri, Jan 15 1993  4:25:02.087,
filtdelay=  16.40   9.89 140.08   9.63   9.72   9.22  10.79 122.99,
filtoffset= 13.24  16.28 -49.19  16.04  16.83  16.49  16.95 -39.43,
filterror=  16.27  20.17  27.98  31.89  35.80  39.70  43.61  47.52

ind assID status  conf reach auth condition  last_event cnt
===========================================================
  1 11670  7414    no   yes   ok    synchr.   reachable  1
  2 11673  7614    no   yes   ok   sys.peer   reachable  1
  3 11833  7314    no   yes   ok    outlyer   reachable  1
  4 11868  7414    no   yes   ok    synchr.   reachable  1
Parting Shots

There are several undocumented programs which are useful if you are
trying to set up a clock. They can be found in the clockstuff directory
of the xntp3 distribution. The most useful of these is the propdelay
program, which can compute high frequency radio propagation delays
between any two points whose latitude and longitude are known. The
program understands something about the phenomena which allow high
frequency radio propagation to occur, and will generally provide a
better estimate than a calculation based on the great circle distance.
The other two programs in the directory are clktest, which allows one to
exercise the generic clock line discipline, and chutest, which runs the
basic reduction algorithms used by the daemon on data received from a
serial port.
