mills@DCN6.ARPA (09/03/85)
Folks,
You might be interested in the following survey of Internet timetellers,
the results of which provide an interesting insight into the things that
can break in such a service. The section that follows was extracted from
a research report that may become an RFC. The original (promise not to
redistribute it) is in the file SYNCH.DOC in the MILLS directory at ISID.
A3. Comparison of UDP and ICMP Time
The third experiment was designed to assess the accuracies produced
by the various host implementations of the UDP Time protocol and ICMP
Timestamp messages. For each of the hosts responding to the UDP Time
protocol in the first experiment a separate test was conducted using
both UDP and ICMP in the same test, so as to minimize the effect of
clock drift. Of the 162 hosts responding to UDP requests, 45 also
responded to ICMP requests with apparently correct time, but the
remainder either responded with unknown or nonstandard ICMP time (29) or
failed to respond to ICMP requests at all (88).
Table A8 shows both the UDP time (seconds) and ICMP time
(milliseconds) returned by each of the 45 hosts responding to both UDP
and ICMP requests. The data are ordered first by indicated UDP offset
and then by indicated ICMP offset. The seven hosts at the top of the
table are continuously synchronized, directly or indirectly to a radio
clock, as described earlier under the first experiment. It is probable,
but not confirmed, that those hosts below showing discrepancies of a
second or less are synchronized on occasion to one of these hosts.
Host UDP time ICMP time
-------------------------------------------------
DCN6.ARPA 0 sec 0 msec
DCN7.ARPA 0 0
DCN1.ARPA 0 -6
DCN5.ARPA 0 -7
UMD1.ARPA 0 8
UMICH1.ARPA 0 -21
FORD1.ARPA 0 31
TESLA.EE.CORNELL.EDU 0 132
SEISMO.CSS.GOV 0 174
UT-SALLY.ARPA -1 -240
CU-ARPA.CS.CORNELL.EDU -1 -514
UCI-ICSE.ARPA -1 -1896
UCI-ICSC.ARPA 1 2000
DCN9.ARPA -7 -6610
TRANTOR.ARPA 10 10232
COLUMBIA.ARPA 11 12402
GVAX.CS.CORNELL.EDU -12 -11988
UCI-CIP5.ARPA -15 -17450
RADC-MULTICS.ARPA -16 -16600
SU-WHITNEY.ARPA 17 17480
UCI-ICSD.ARPA -20 -20045
SU-COYOTE.ARPA 21 21642
MIT-MULTICS.ARPA 27 28265
BBNA.ARPA -34 -34199
UCI-ICSA.ARPA -37 -36804
ROCHESTER.ARPA -42 -41542
SU-AIMVAX.ARPA -50 -49575
UCI-CIP4.ARPA -57 -57060
SU-SAFE.ARPA -59 -59212
SU-PSYCH.ARPA -59 -58421
UDEL-MICRO.ARPA 62 63214
UIUCDCSB.ARPA 63 63865
BELLCORE-CS-GW.ARPA 71 71402
USGS2-MULTICS.ARPA 76 77018
BBNG.ARPA 81 81439
UDEL-DEWEY.ARPA 89 89283
UCI-CIP3.ARPA -102 -102148
UIUC.ARPA 105 105843
UCI-CIP2.ARPA -185 -185250
UCI-CIP.ARPA -576 -576386
OSLO-VAX.ARPA 3738 3739395
DEVVAX.TN.CORNELL.EDU 3657 3657026
PATCH.ARPA -86380 20411
IPTO-FAX.ARPA -86402 -1693
NETWOLF.ARPA 10651435 -62164450
Table A8. Comparison of UDP and ICMP Host Clock Offsets
Allowing for various degrees of truncation and roundoff abuse in
the various implmentations, discrepancies of up to a second could be
expected between UDP and ICMP time. While the results for most hosts
confirm this, some discrepancies are far greater, which may indicate
defective implementations, excessive swapping delays and so forth. For
instance, the ICMP time indicated by UCI-CIP5.ARPA is almost 2.5 seconds
less than the UDP time.
Even though the UDP and ICMP times indicated by OSLO-VAX.ARPA and
DEVVAX.TN.CORNELL.EDU agree within nominals, the fact that they are
within a couple of minutes or so of exactly one hour early (3600
seconds) lends weight to the conclusion they were incorrectly set,
probably by an operator who confused local summer and standard time.
Something is clearly broken in the case of PATCH.ARPA,
IPTO-FAX.ARPA and NETWOLF.ARPA. Investigation of the PATCH.ARPA and
IPTO-FAX.ARPA reveals that these hosts were set by hand accidently one
day late (-86400 seconds), but otherwise close to the correct
time-of-day. Since the ICMP time rolls over at 2400 UT, the ICMP offset
was within nominals. No explanation is available for the obviously
defective UDP and ICMP times indicated by NETWOLF.ARPA, although it was
operating within nominals at least in the first experiment.
Dave
-------