Re: [chrony-users] GPS stratum

[Miroslav Lichvar <mlichvar@xxxxxxxxxx>]

On Fri, Jun 13, 2014 at 04:01:53PM +0200, Mattias Walström wrote:
> Hi!
> 
> When I do chronyc sources the GPS is presented as a stratum 0 server,
> shouldnt it say 1, or have I misunderstood something?

The reference clock is handled internally as a stratum 0 server, but
chronyd will reply to its clients as a stratum 1 server, you can see
this in the "chronyc tracking" output.

> ~ # chronyc sources
> 210 Number of sources = 1
> MS Name/IP address         Stratum Poll Reach LastRx Last sample
> ===============================================================================
> #* GPS                           0   4   377    21  -6811us[-9719us]
> +/-  253ms

> >> According http://www.faqs.org/rfcs/rfc5905.html GPS shall have stratum 1
> >> if it is connected as an external receiver, this is the case for me, I
> >> use a GPS receiver connected with RS422.

Were you referring to the following text of the RFC?

  For packet stratum 0 (unspecified or invalid), this is a
  four-character ASCII [RFC1345] string, called the "kiss code", used
  for debugging and monitoring purposes.  For stratum 1 (reference
  clock), this is a four-octet, left-justified, zero-padded ASCII
  string assigned to the reference clock. 

This is about value used in NTP packets when synchronized to a
reference clock, chronyd is following that.

Here is a part that might explain it better:

  It is customary to map the stratum value 0 in received packets to
  MAXSTRAT (16) in the peer variable p.stratum and to map p.stratum
  values of MAXSTRAT or greater to 0 in transmitted packets. This
  allows reference clocks, which normally appear at stratum 0, to be
  conveniently mitigated using the same clock selection algorithms
  used for external sources (see Appendix A.5.5.1 for an example).

> >> But is it possible to change stratum of the GPS in chrony, so it is
> >> correct according to the RFC?

Currently, it's not possible to change the stratum of a refclock. An
option could be implemented for that, but I'm not sure how useful it
would be.

-- 
Miroslav Lichvar

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