[ntp:questions] NTP client basic

mills at udel.edu mills at udel.edu
Tue Mar 6 15:33:21 UTC 2007


In the far distant past, the most likely source of retransmissions was 
link-level retransmissions. In any case, the NTP on-wire protocol 
discards duplicates and bogus packets anyway. It does no good whatsoever 
to retransmit unanswered packets. If necessary, transmit a new packet 
with current timestamps.


Hal Murray wrote:

>>When the repeats still appeared in my own SNTP client implementation (now 
>>trapped, but logged), it seemed to me that I might have stumbled on a 
>>possible source of trouble. RFC2030 didn't seem to say that you MUST take 
>>steps to discard repeats which COULD arise in the underlying UDP protocol, 
>>it seemed to be saying that you could keep an eye on the xmit timestamp in 
>>the request - but it's not essential.  RFC768 (which describes UDP) doesn't 
>>shed any light on whether a corrupted UDP packet could be repeated.
> It's possible but very very unlikely for the network to duplicate
> a packet.
> Is your code retransmitting if you don't get an answer?  If so, a
> likely explanation is that you aren't waiting long enough.  In
> any case, your code should filter out duplicates and/or process
> them correctly.

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