Incestuous clockwatching
16-Feb-86 22:10:23-UT


I have had several messages of the general form "Why didn't my host show up in
your list of clock respondents?" If said host did not respond to at least two
of the three protocols: ICMP Timestamp, UDP/NTP or UDP/TIME, it was
intentionally not included. In almost all cases those hosts that responded
with UDP/NTP or UDP/TIME responded also to ICMP Timestamp. Accordingly, for
those of you who are interested, I have put up the file TIME.TXT on DCN1
(ANONYMOUS/GUEST) containing all the IMCP Timestamp responses ordered by
(apparent) subnet.

Examination of the data in that file, by the way, suggests some revealing
insights. The intent of the experiment that produced the file was to
determine to what extent local nets tend to cluster closely around the "wrong"
time, presumably due to incestuous clockwatching. In some local nets this
tendency is readily apparent, with almost all hosts eagerly tracking some tens
of seconds or minutes off with maybe a few broken or defiant exceptions. This
has interesting implications for the design of synchronization algorithms such
as suggested in RFC-956, since a large local net could in principle "take
over" the world of clocks with the wrong time. Interestingly enough, however,
the clustering algorithm of RFC-956 was fed the entire data base collected in
last wwek's experiment, but it still came up with the "real" time to within a
few millisceonds.

For the crew working on NTP implementations, the following list of known
reference hosts and clocks is as follows, along with the unsmoothed offset
(milliseconds) measured with respect to DCN6 just to give you some idea of the
accuracies that can be expected. The best accuracies are normally obtained
with ICMP Timestamp (I), with UDP/NTP (N) next and UDP/TIME (U) last. The
hosts marked * are corrected to agree to zero offset relative to radio time,
but only with ICMP Timestamp. The offsets shown are almost completely
dominated by network-delay dispersion, except for UDP/TIME, which has an
inherent precision of one second.

P Host Name Address Offset
N GW.UMICH.EDU [] -152
U GW.UMICH.EDU [] -118
I DCN1.ARPA [] -9
N DCN1.ARPA [] 8
U DCN1.ARPA [] -73
I DCN-WWV.ARPA [] -14 *
I DCN-WWVB.ARPA [] -60 *
I FORD1.ARPA [] 49
N FORD1.ARPA [] 40
U FORD1.ARPA [] -167
I UMD1.UMD.EDU [] -183
N UMD1.UMD.EDU [] -81
U UMD1.UMD.EDU [] 656
I UMD-WWVB.UMD. [] 174 *


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