[ntpwg] REQUEST FOR TEXT - Protocol Doc Changes

[Odonoghue, Karen F CIV NSWCDD, W13]

Just out of curiosity, if there isn't a strong rationale to differ from 1588 why would we do so. Part of the point of this would be to potentially leverage hardware in the future. If 1588 and 802.1AS define the same timestamp point, why would we differ?
 
Karen

________________________________

From: ntpwg-bounces+karen.odonoghue=navy.mil at lists.ntp.org on behalf of rich.osman at nsn.com
Sent: Mon 11/12/2007 8:20 PM
To: ntpwg at lists.ntp.org
Subject: Re: [ntpwg] REQUEST FOR TEXT - Protocol Doc Changes





> -----Original Message-----
> From: ntpwg-bounces+rich.osman=nsn.com at lists.ntp.org
> [mailto:ntpwg-bounces+rich.osman=nsn.com at lists.ntp.org] On
> Behalf Of ext Jim Martin
> Sent: Thursday, November 08, 2007 3:47 PM
> To: ntpwg at lists.ntp.org
> Subject: [ntpwg] REQUEST FOR TEXT - Protocol Doc Changes
>
> Gentlepeople,
>       Based upon the notes (and my recollection) from the meeting in 
> Chicago, there were four  main points to be addressed in the
> next (and 
> hopefully final) rev of the protocol spec.  They were:
>

 <stuff deleted>

> > -Comment from Stewart Bryant:  need to specify precisely the start
bit
> > * Yakov and Stewart Bryant agreed to contribute text to resolve this

> > issue
>
>       This is where I'm looking for input from the community. Yakov, 
> Stewart, Rich Osman, or anyone who has a good grasp on the issue and 
> can generate a few lines to precisely characterize the
> details.  This 
> is currently the blocker on getting this out.

NTP has a lot in common with IEEE-1588 and there can be value in
consistency if it isn't foolish.

IEEE P1588 D2.2 paragraph 7.3.4.1 "Event message timestamp point" says:
"Unless otherwise specified in a transport specific Annex to this
standard, the message timestamp point for an event message shall be the
beginning of the first symbol following the start of frame delimiter."

I'd propose that the leading edge of the first bit of the LI Leap
Indicator (first transport independent point) of the NTP packet header
as shown in Figure 8 be the reference point.  This isn't completely
consistent with 1588, but I believe it's consistent with the intent and
with the rest of NTP. Regardless of where it ends up, it should be in
the first 12 words of the message so that there is no need to compensate
for varying message lengths.  A brief look at some sample code seems to
indicate that this is about the location that assumed but I've done no
exhaustive or scientific search.

Given the structure of the document I believe that the end of section 4
"Definitions" is probably the best place to include the text: "The
timestamp reference point is the leading edge of the first bit of the LI
Leap Indicator bit pair of the NTP packet header shown in Figure 8."

--

Rich Osman
Nokia Siemens Networks
mailto:rich.osman at nsn.com
6000 Connection Drive; Irving, TX  75039






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