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 -------