[ntp:questions] reachability register

brian utterback brian.utterback at oracle.com
Thu Apr 9 13:49:17 UTC 2015

On 4/8/2015 9:23 AM, MAYER Hans wrote:
> Dear all,
> I have a question about the reachability register. For my opinion this is a left shift 8-bit register.
> I looked in one of our internal ntp-server with "ntpq" and found a value of 376 to a peer configured internal server. After waiting the time difference between "poll" and "when" I can find the value of 377. 
> How is it possible ? For my understanding this could only happen after 8 times the poll interval. The next value should be 375 and after that 373. And so on shifting the 0-bit to the left. 

You are mostly correct. You are missing only one small detail. The
register gets shifted when NTP sends a packet and ORs in a one when it
receives a packet. Thus, if the low order bit were zero because a packet
got dropped, then your reasoning would be correct. However, if the low
order bit were zero because you happened to look between the time a
request was sent and a reply was received then what you observed is correct.
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