[ntpwg] NTPv4 draft-8 to draft-9 changes / alternative paragraph

Greg Dowd GDowd at symmetricom.com
Thu Feb 28 21:59:04 UTC 2008


I still think the quasi-leading edge (SOF) is a good compromise.  If you
can only timestamp after you have received a complete packet, I wonder
if you have the intended definition of access to the physical layer.  If
you do, common practice is to strike a timestamp on SOF and then make a
decision to save once you've analyzed the frame. If you have physical
layer access only at the end (but it's deterministic), it seems like you
could make an empirical or predefined adjustment to approximate ingress
time.


Greg Dowd
gdowd at symmetricom dot com (antispam format)
Symmetricom, Inc.
www.symmetricom.com
"Everything should be made as simple as possible, but no simpler" Albert
Einstein

-----Original Message-----
From: ntpwg-bounces+gdowd=symmetricom.com at lists.ntp.org
[mailto:ntpwg-bounces+gdowd=symmetricom.com at lists.ntp.org] On Behalf Of
STUART VENTERS
Sent: Thursday, February 28, 2008 1:25 PM
To: jack.burbank at jhuapl.edu
Cc: ntpwg at lists.ntp.org
Subject: [ntpwg] NTPv4 draft-8 to draft-9 changes / alternative
paragraph

Jack,

It looks like the changes from NTPv4 draft-8 to draft-9 are editorial
except for a couple of new paragraphs.

The editorial changes mostly clean up how references are cited.  (Nice
job, it's much better.)

The new paragraphs are as follows:

On page 23 the following was added:
   If NTP has access to the physical layer, then the timestamps are
   associated with the beginning of the symbol after the start of frame.
   Otherwise, implementations should attempt to associate the timestamp
   to the earliest accessible point in the frame.


On page 25, the following was added:
   The Receive Timestamp and the Transmit Timestamp (set by the server)
   are undefined when in a KoD packet and MUST NOT be relied upon to
   have valid values and MUST be discarded.


As an implementer, I'm ok with everything except the new page 23
paragraph.


I'm working on transporting time to the edge of an access network for
one-way packet delay measurements.  The server is back in the cloud with
1G interfaces.  The client is at the end of a T1 link.  The goals are
modest, to make 1mS accurate measurements.  After adding up all the
tolerances, in order to get enough accuracy, I'm going to have to
violate the page 23 paragraph.


Here's an alternative paragraph which documents current practice, works
on existing packet paths, and leaves wiggle room for future spec efforts
with more time for careful study.


For interoperatability with existing equipment (switches, routers, and
the reference NTP implementation), implementations should use the
following guidelines for associating timestamps with physical layer
transitions.  Receive timestamps should be at the first point which
indicates a complete, good frame has gone by.  Transmit timestamps
should be at the first point which indicates that a frame transmission
has started.  Accuracy requirements and media specific details are FFS.


Regards,

Stuart Venters





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