This is a short overview for the reference clocks currently supported
by xntp V3. (Ultimate wisdom can be obtained from xntpd/refclock_*.c
this file was derived from that information - unfortunately some comments
in the files tend to get stale - so use with caution)

Refclock address	Type
127.127.0.x		no clock (fails to configure)
127.127.1.x		local clock - use local clock as reference
127.127.2.x		no clock (fails to configure)
127.127.3.x		PSTI 1010/1020 WWV Clock
127.127.4.x		SPECTRACOM WWVB receiver 8170 and Netclock/2
127.127.5.x		Kinimetric Truetime 468-DC GOES receiver
127.127.6.x		IRIG audio decode (Sun & modified BSD audio driver)
127.127.7.x		CHU Timecode (via normal receiver & Bell 103 modem)
127.127.8.x		PARSE (generic driver for a bunch of DCF/GPS clocks
			       can be extended for other clocks too)
	8.0-3		Meinberg PZF535/TCXO
	8.4-7		Meinberg PZF535/OCXO
	8.8-11		Meinberg DCF U/A 31
	8.12-15		ELV DCF7000
	8.16-19		Walter Schmid DCF receiver (Kit)
	8.20-23		Conrad DCF77 receiver module + level converter (Kit)
	8.24-27		TimeBrick (limited availability ask
				   time@informatik.uni-erlangen.de)
	8.28-31		Meinberg GPS166
	8.32-35		Trimble SV6 GPS receiver
127.127.9.x		MX4200 GPS receiver
127.127.10.x		Austron 2201A GPS Timing Receiver
127.127.11.x		Kinemetrics Truetime OM-DC OMEGA Receiver
127.127.12.x		KSI/Odetecs TPRO-S IRIG-B / TPRO-SAT GPS
127.127.13.x		Leitch: CSD 5300 Master Clock System Driver
127.127.14.x		MSFEES
127/127.15.x		TrueTime GPS/TM-TMD
127.127.16.x		Bancomm GPS/IRIG Ticktock
127.127.17.x		Datum Programmable Time System