[ntp:hackers] Leap stuff

David L. Mills mills at udel.edu
Mon Jul 9 22:14:43 UTC 2007


Kurt,

There is no change in the interpretation of the leap bits; it's been 
that way for many years. The LEAP_NOWARNING means exactly what is says; 
no leap second is anticipated in future.

If no leapseconds values are available, either from file or server, the 
TAI is ordinaril zero. However, if a leap is signaled from a server and 
passes the majority test, the kernel will leap as directed. In the 
current design the kernel TAI is incremented, so an application program 
will see the incremented value. Since leaps occur so rarely, the 
application should assume no valid TAI information is available if the 
TAI value is less than 10, which was the intial offset in 1972,

Dave

Kurt Roeckx wrote:

> On Mon, Jul 02, 2007 at 05:00:01PM +0000, David L. Mills wrote:
>
>> Terje Mathisen wrote:
>>
>>> Here it would have been good to have effectively four possible values:
>>>
>>> 0) Unknown, i.e. no authoritative source available.
>>
>> TAI = 0 and no leap bits
>>
>>> 1) Known, no leapsecond. (with tracing back to NIST)
>>
>> Leapsec = 0 and current time less than file expiration time.
>>
>>> 2) Known, positive leapsecond. (ditto)
>>
>> Leapsec > 0. This is the number of seconds until the leap.
>>
>>> 3) Known, negative leapsecond. (ditto)
>>
>> No way to tell, unless using DUT1 trend.
>>
>>> The main idea being the difference between the first two, which for
>>> many users would be quite significant.
>>>
>>> Or is this effectively what the new code does?
>>
>
> If I understood Terje, he was talking about the Leap Indicator
> in the protocol, which is now defined as:
> | 0 | no warning |
> | 1 | last minute of the day has 61 seconds |
> | 2 | last minute of the day has 59 seconds |
> | 3 | alarm condition (the clock is not synchronized) |
>
> And that 3 maybe should say: "I have no idea if there is going to be a
> leap second or not".
>
> The way I understand things is that we can't just change this.
>
>
> Kurt
>



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