[ntpwg] ntpwg Updated NTPv4 Protocol Specification/timestamplocation

David L. Mills mills at udel.edu
Sun Feb 3 20:04:44 UTC 2008


Kurt,

Exactly. A proposal for a follow-up (two-step) function is in the 
architecture briefing on the NTP project page. The intent is that use of 
the original and two-step mode be automatically detected on-fly.

Dave

Kurt Roeckx wrote:

> On Sun, Feb 03, 2008 at 08:32:55AM -0800, TS Glassey wrote:
>
>> I think that the problem is that unlike 1588, the NTP process depends on
>> some stack receiving and framing the data prior to its actually being
>> reliable data. Since there are no precision timer/receipt processes
>> available for logging in TCP, UDP, or the NTP process to address this it
>> becomes a real issue in that the data could sit in the stack for any 
>> amount
>> of time...
>
>
> You can ask the kernel when it received a packet. This is currently
> mostly done in the kernel itself without any help from hardware. Ntpd
> is using this for a long time now. Complete software based PTP
> does the same thing and has exactly the same problem as ntpd.
>
> What could be changed to get better results is:
> - Have more hardware support so that you can capture the time
> in the PHY for both send and receive of a packet;
> - Have the kernel export that to user space;
> - Change ntpd so that is can send a "Follow Up" message that
> contains the real time of when the previous packet was send
> as captured by the PHY.
>
> But those are all just current implementation details. This is not
> a limitation of the NTP protocol.
>
>
> Kurt
>



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