ntp: Adjustment of time_maxerror with 500ppm instead of 15ppm

[Keno Goertz <contact@kenogo.org>]

Hello,

I've been looking into the kernel's NTP code and found what I understand 
to be a deviation from NTP as standardized by RFC 5905.  The 
documentation of this part of the kernel is pretty sparse, so there may 
be some motivation behind this that I don't know of.  Perhaps someone 
with more knowledge can explain this.

The doc string of `struct ntp_data` states that `time_maxerror` holds 
the "NTP sync distance (NTP dispersion + delay / 2)".

ntpd indeed sets this value to what RFC 5905 calls the "root 
synchronization distance" LAMBDA.

In RFC 5905, this LAMBDA increases over time because the root dispersion 
increases at a rate of PHI, which is set to 15ppm.  Running

$ ntpq -c "rv 0 rootdisp"

a couple of times confirms that the root dispersion reported by ntpd 
increases with this rate.  Consequently, so does the root 
synchronization distance LAMBDA.

However, the function `ntp.c:second_overflow()` instead increases the 
value of `time_maxerror` with the rate MAXFREQ, which is set to 500ppm.

This leads to standard library functions like ntp_gettime() reporting 
much bigger values of `maxerror` than ntpd is working with.  This can be 
confirmed by running

$ adjtimex -p

a couple of times.

MAXFREQ *can* be found in the reference implementation of RFC 5905 and 
is also set to 500ppm there, but it is used in a different context: 
MAXFREQ is an upper bound for the local clock's frequency offset, while 
PHI is an upper bound for the frequency drift of a clock synchronized 
with NTP.

At least this is my understanding.  Can someone explain this?

Best regards
Keno