>From Peter Martinez: Thanks to all for helpful coments. I think I can wrap this one up now. I asked my Belgian friend to continue running my new SNTP client program with the logging on, and send me the log. One interesting tell-tale event consisted of a run of three spurious NTP response packets which arrived within a few ms of each other, after evidently leaving the server within ms of each other, but which had 'orig' timestamps from adjacent (5-minute interval) requests. This surely places the source of the spurious repeats very close indeed to the originator, perhaps in his PC, or his router, or at the ISP end of the copper wire run. I have neither the resources nor the inclination to chase that bug further.
A couple of academic final comments from me.. Although clearly a repeated NTP response is meaningless (there's no such thing as 'old' time), you cannot deduce "the network should never repeat an NTP packet". NTP is transported in UDP packets but UDP isn't aware of that. There is no law which says UDP (or the levels below it) must not repeat packets if they thought it was a good idea, and I can see that other uses for UDP (eg DNS) would not malfunction if repeats occured. Classic communication theory knows all about this - it's safe for me to use UDP to say "my name is Peter" twice, but unsafe to for me to repeat "I owe you $100", even if I thought you were a bit deaf! RFC2030 isn't clear about this. It even says the inclusion of the 'time-of-origen' of the request is optional. The requestor can implement the (t1-t1-t2+t4)/2 algorithm without sending t1, but he can't detect duplicates without sending something unique like t1 and matching it in the response. This lack of emphasis in RFC2030 may have been the reason why the NTP client package I originally used, was not written well-enough to protect itself from the situation I encountered in Belgium. I learnt something this week. regards Peter M _______________________________________________ questions mailing list [email protected] https://lists.ntp.isc.org/mailman/listinfo/questions
