Time synchronization and distribution plan

Sat, 23 Jan 88 18:20:24 EST


There is good news for clockwatchers, timewarpers and chron daemons. All radio
clocks known to me are now finally repaired and ticking to standard time.
Also, venerable WWVB ticker dcn1.arpa (aka pogo), which some of you may fondly
remember from years past, has resumed this life. There is even a NTP secondary
time server on the European MILNET which is keeping pretty good time. There
are a half-dozen or so gents who have scrounged up an old PDP11-compatible
system and volunteered additional GOES or WWV servers as well. It may be time
(!) to bring some order to the clockwatching business. A treaty is suggested
in this note.

As in the telephone systems NTP uses a model of stratified clocks and servers.
A primary server is one directly synchronized to a radio clock, so it can keep
accurate time even in the absence of NTP itself. However, from long experience
with such things, the radio clocks sometimes feature scrambled time due radio
propagation conditions or broken logic. Thus, the primary servers ordinarily
run NTP with a couple of their buddies as a sanity check. Should the radio
clock itself become suspect, time synchronization shifts to the NTP peer
group. Secondary servers are synchronized with NTP using special filtering and
deglitching mechanisms ordinarily accurate to a few tens of milliseconds, even
across intransigent gateways.

NTP can be used in either a remote-procedure-call (asymmetric) mode, in which
a client sends a single message to the server and receives the time in reply,
or a distributed (symmetric) mode, in which the protocol runs continuously and
time is continuously compared between the peers and corrected as required. The
asymmetric mode is designed for casual use, such as setting the time and date
when a PC comes up, for example, while the symmetric mode is designed for more
accurate and precise applications, such as transaction timestamping and time
redistribution. This note is concerned only with use of the symmetric mode.

Not everybody can chime with a primary server, since this would eventually
lead to severe congestion and degraded service. Therefore, a system of
hierarchical time servers is suggested. Assume that each of the 400-odd
networks now active has a secondary time server synchronized to one or more
primary servers and providing time service to other hosts in its community. In
order to provide the highest robustness, the secondary server should chime
with more than one primary server, perhaps three, so we are talking about 1200
peer paths. The existing LSI-11 fuzzball gateways can support at least 60 peer
paths each, so some twenty primary servers would be needed. At the moment NTP
chimes one packet each direction per peer per minute, so the aggregate time
traffic works out to about one packet each direction per fuzzball per second.
It is planned to introduce NTP protocol modifications that would reduce this
rate by a factor of ten.

The twenty-odd primary servers should be located at strategic spots designed
to minimize the impact of the NTP traffic itself, yet provide low delay
dispersion for their customers. The existing and planned NSFNET Backbone sites
would seem ideal candidates and, indeed, time-synchronized fuzzballs are
already installed at seven of these sites. Without admitting agenda on how the
time-synchronization capability came to pass or on the likely disposition of
the fuzzballs once the new NSFNET Backbone is deployed, I suggest a nucleus of
timetellers is already in place. Additional timetellers are now ticking on
ARPANET and local nets gatewayed to ARPANET, MILNET and elsewhere.

At the U Delaware campus and at several other campuses known to me, one or
more relatively inexpensive WWV clocks are installed as backups should
connectivity to a primary server be lost for one reason or another. The WWV
clocks are distinctly inferior in accuracy and reliability with respect to the
WWVB and GOES clocks now used at the primary servers; however, as some may
remember, there have been occasions over the last several years when all
primary servers in the experimental system were down and the entire
NTP-speaking Internet was synchronized to a dinky WWV clock on my desk at
home. I suspect several institutions that cherish accurate time will install
GOES, WWVB or even GPS clocks and join the NTP chorus as well, so there may be
in fact no need for an overt program to buy and install additional clocks.

I saved specific recommendations for last. I suggest an appropriate first step
is that those sites with good connectivity to an NSFNET regional system chime
NTP with the NSFNET Backbone fuzzball serving that regional system. Other
sites may wish to choose one or another fuzzball listed below. However, it is
most important to understand that time service is provided by each of these
gizmos on a secondary basis only, is still in an experimental phase and may be
limited or curtailed should it interfere with the primary functions of the

Speaking for myself and I suspect the other operators listed, we would expect
users to set up their own time-redistribution network, perhaps using the 4.3
bsd ntpd daemon specifically designed for this purpose, and to avoid ganging
up on the servers with many hosts from the same net. We would also expect
users planning long-term use of the servers to express their intent to the
operators and comply with requests to reaffiliate with other servers should
that become necessary. Finally, we are looking for volunteers to install
additional primary servers and join the chorus as well.

Name Address Clock Operator and notes
Primary servers
umd1.umd.edu WWVB Mike Petry (petry@trantor.umd.edu)
                                        U Maryland (gatewayed to NSFNET
                                        Backbone, ARPANET PSNs 17 and 20 and
                                        MILNET PSN 57.
wwvb.isi.edu WWVB Steve Casner (casner@isi.edu)
                                        ISI (gatewayed to ARPANET PSN 22)
ncar.nsf.net WWVB Scott Brim (swb@devvax.tn.cornell.edu)
                                        NCAR (NSFNET Backbone gateway)
dcn1.arpa WWVB Dave Mills (mills@udel.edu)
       U Delaware (directly connected to
                                        ARPANET PSN 96)
ford1.arpa GOES Fred Ball (ball@ford-vax.arpa)
                                        Ford Research (gatewayed via 9600-bps
                                        line to ARPANET PSN 111.
Secondary servers (please do NOT chime with these except by permission)
macom1.arpa NTP Woody Woodburn (woody@macom2.arpa)
       Linkabit, Vienna, VA
swamprat.arpa NTP Woody Woodburn (woody@macom2.arpa)
                                        Linkabit, Vienna, VA
patch.arpa NTP Dave Park (dpark@dca-eur.arpa)
                                        USECOM Stuttgart, FRG
gw.umich.edu NTP Hans-Werner Braun (hwb@mcr.umich.edu)
                                        U Michigan (WWV backup)
xyzzy.umich.edu NTP Hans-Werner Braun (hwb@mcr.umich.edu)
                                        U Michigan
libra.rice.edu NTP Paul Milazzo (milazzo@rice.edu)
       Rice U
dcn6.arpa NTP Dave Mills (mills@udel.edu)
                                        Newark, DE (WWV backup)
sdsc.nsf.net NTP Scott Brim (swb@devvax.tn.cornell.edu)
                                        San Diego Supercomputing Center
uiuc.nsf.net NTP Scott Brim (swb@devvax.tn.cornell.edu)
                                        National Center for Supercomputing
psc.nsf.net NTP Scott Brim (swb@devvax.tn.cornell.edu)
                                        Pittsburg Supercomputing Center
cornell.nsf.net NTP Scott Brim (swb@devvax.tn.cornell.edu)
                                        Cornell U (NYSERNET)
jvnc.nsf.net NTP Scott Brim (swb@devvax.tn.cornell.edu)
                                        John von Neumann Center (JVNCNET)
nsfnet-gw.umd.edu NTP Mike Petry (petry@trantor.umd.edu)
                                        U Maryland (SURANET)

Corrections or additions to this list would be appreciated


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