thoughts on fake IP addresses and clock specification

[Eric S. Raymond]

Hal Murray <hmurray at megapathdsl.net>:
> 
> esr at thyrsus.com said:
> > I was under the impression that ntpd is internally set up so that clock
> > samples come in as though they were packets from peers, with the fake IPv4
> > addresses for their type and unit ID in the refid field. If so, then the
> > fake addresses really are part of the wire protocol. Am I in error about
> > this? 
> 
> I don't think so, but I can't point a finger at the right place in the code.
> 
> I think they bypass that layer and join in at the peer level.
> 
> There are peer control blocks that have an IP Address in them.  That's what 
> the main control logic works with.
> 
> Internally, each refclock has a FIFO holding the last ~64 samples.  For 
> things like NMEA, each time the driver gets a valid sample it adds it to the 
> FIFO.  For the Atom/PPS driver there is a hook that gets called/polled each 
> second.  If it finds good data, it adds a sample to the FIFO.  The FIFO is 
> actually a ring buffer.  On overflow, old samples are dropped.
> 
> At the polling interval, the driver is "polled".  (Note the possible 
> confusion on "poll".)  That is parallel with sending a packet.  The driver 
> can call back and say "process everything in the FIFO", or do something or 
> set a flag and call back later.
> 
> 
> The process everything step sorts the contents of the FIFO, then discards 
> outliers, roughly 1/3 of the samples, and then figures out the average and 
> injects that into the peer buffer.
> 
> The peer buffer holds the last 8 samples from the wire.  The normal logic 
> uses the one with the lowest round trip time.  That's a hack to minimize 
> errors from queuing delays out on the big bad internet.  The refclock data 
> always has a round trip time of 0.  I assume the code is setup to take the 
> most recent of equal samples.

Interesting read.  Keeping a copy because one of my near-term to-do
items is to write a fairly detailed narrative description of how the
code works.  This looks like it should be a section in same.
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>