[ntp:questions] NTP tunning for OWD measurements
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Sat Oct 27 02:04:27 UTC 2012
John Hasler wrote:
> Read this: <http://www.internet2.edu/performance/owamp/>.
> I think he wants help configuring the four clocks
> mentioned in the "requirements" optimally for OWAMP.
> I don't see why OWAMP is special, though.
OWAMP doesn't appear to mean without a e.g. PPS source,
or perhaps local LAN to a Stratum 1 with a PPS source,
certainly not ntpd synced only via routed connections,
if they want high accuracy and precision,
as micro seconds appear to be a issue?
Effect Of ATOM PPS Driver Edge Change on OWAMP Data
The graph above shows one-way transit times as measured by owamp.
The two hosts are separated by a single Gigabit Ethernet switch,
and at about 11:30 AM, NTP's ATOM refclock was reconfigured
with “fudge flag1 1”, which supposedly uses the falling edge.
This graph suggests, however, that the change has caused the
PPS-configured host (22.214.171.124) to alter its time to 104 μS later,
which suggests that the trailing/rising edge is used when
“fudge flag2 1” is configured.
The fact that this would align the time with the beginning
of the Truetime CR start bit further supports that the ATOM
PPS driver is using the trailing/rising edge of the PPS pulse.
HOTS: An OWAMP-Compliant Hardware Packet Timestamp Recorder
"accepts an external 10-MHz signal as well as 1PPS signal as input"
two mini-BNC jacks are used to obtain clock information
from an external source such as a GPS or CDMA receiver.
HOTS can accept two kinds of signal as input: a 1PPS
signal and a 10-MHz signal.
The precision of the generated timestamp depends on the
accuracy of the provided signals.
In our measurements, we used two kinds of GPS receiver
to generate these signals: the HP 58503A and the TymServe
Both of them can generate extremely precise clock information.
Highly precise OWD measurement is a challenge because of
the difficulty of eliminating errors in process to obtain
an accurate timestamp and other overheads.
In this paper, we introduced HOTS - a hardware packet
timestamper that we developed to measure OWD.
HOTS can generate extremely precise clock information
for OWAMP test packets provided an accurate time source
such as a GPS or CDMA receiver.
RFC4656 A One-way Active Measurement Protocol (OWAMP)
High-precision measurement of these one-way IP performance
metrics became possible with wider availability of good
time sources (such as GPS and CDMA).
With the increasingly wide availability of affordable
global positioning systems (GPS) and CDMA-based time
sources, hosts increasingly have available to them very
accurate time sources, either directly or through their
proximity to Network Time Protocol (NTP) primary
(stratum 1) time servers.
4.1.2. OWAMP-Test Packet Format and Content
The first bit, S, SHOULD be set if the party generating
the timestamp has a clock that is synchronized to UTC
using an external source (e.g., the bit should be set
if GPS hardware is used and it indicates that it has
acquired current position and time or if NTP is used
and it indicates that it has synchronized to an external
source, which includes stratum 0 source, etc.).
If there is no notion of external synchronization for
the time source, the bit SHOULD NOT be set.
Some other threads I read on the subject had people
complaining about issues with too much jitter from their
GPS affecting their attempts at OWAMP measurements in the
10s of microseconds.
To answer the OP's original question, here is a reference:
#server 127.127.5.0 prefer # 5 is trimble TRUETIME/ 0 is ttyd0
#fudge 127.127.5.0 refid CDMA time1 0.013
#fudge 127.127.5.0 refid CDMA
server 127.127.29.0 prefer # 5 is trimble TRUETIME/ 0 is ttyd0
fudge 127.127.29.0 refid CDMA
server 127.127.22.0 minpoll 2 maxpoll 6 # 22 is PPS "ATOM" device
fudge 127.127.22.0 flag2 1 flag3 1
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