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From: Mills@UDEL.EDU.UUCP
Newsgroups: mod.protocols.tcp-ip
Subject: Re:  Time RFC 868
Message-ID: <8704051517.a011510@Huey.UDEL.EDU>
Date: Sun, 5-Apr-87 15:16:59 EST
Article-I.D.: Huey.8704051517.a011510
Posted: Sun Apr  5 15:16:59 1987
Date-Received: Mon, 6-Apr-87 01:45:24 EST
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The ARPA Internet
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Approved: tcp-ip@sri-nic.arpa

You can't keep "exact time" with RFC-868 time servers, since that protocol
provides resolution only to the second. You should really and truly avoid
TCP-based time, since not only is the accuracy udually degraded by the
clank-and-bump of the connection-open sequence, but the sometimes meager
resources of the time server can be strained. The protocol of choice is
the Network Time Protocol (NTP), documented in RFC-958, for which Unix-
based server and client programs are available (e.g. Mike Petry
petry@trantor.umd.edu, Milo Medin medin@orion.arpa, both for 4.3 systems).

As for the most accurate clocks in town, a fair number of "fuzzball"
gateways and hosts are equipped with WWVB and WWV radio clocks that can
deliver time accurate to a millisecond or two. Typical accuracies using
NTP via ARPANET/MILNET are within 20 to 100 milliseconds, depending on
the path and state of congestion. The fuzzballs are interconnected with
NTP and, in some cases an interior-gateway protocol called hellospeak
(c.f. RFC-891), so a failure in one radio clock does not frustrate the
clockwatchers.

At present the NSFNET Backbone sites are all synchronized to a WWVB
radio clock at Boulder, CO, and capable of very accurate and robust
time service using RFC-868 or NTP protocols. Use of TCP is adamantly
discourged with the former and not available with the latter. UDP is
the prefered envelope in any case. In addition, several WWVB-equipped
servers are scattered about, including macom1.arpa (192.5.8.1), umd1.umd.edu
(128.8.10.6) and ford1.arpa (182.5.0.1 - actually a GOES clock). Finally,
a few hosts scattered over the swamps support less-accurate WWV radio
which are also used as backups for the NTP system, including gw.umich.edu
(35.1.1.1) and udel2.udel.edu (192.5.39.87).

If present plans work out, the best places to watch clocks will be the
NSFNET Backbone fuzzballs or other hosts synchronized directly to them.
An announcement will be posted to this list when the details stabilize.
Meanwhile, feel free watch one or more of the clocks above, with the
first two (macom1.arpa or umd1.umd.edu) as the primary choice, but please
don't use TCP.

Dave