[ntpwg] REQUEST FOR TEXT - Protocol Doc Changes

[Mark Elliot]

In the comments below, IEEE 1588 timestamps the first data bit of
Ethernet packet.  The start of frame delimiter mentioned below is the
Ethernet start of packet.  So you are time stamping on the leading edge
of the very first data bit of Ethernet packet.  If you wish to have
consistency, and I do, all time packets that are hardware time stamped
should follow the same rule.  

This way the hardware, that does the time stamping, need not be as aware
of the details of the upper layer protocols.  There are problems with
the timestamp being located in the data part of the packet.  For example
when using protocols like IPv6, with extension headers, the location of
the timestamp in the UDP payload would literally be dependent on IPv6
header options used.  The options would vary over time without the
timing protocol's knowledge.  Here again is one of many good arguments
for the Ethernet layer timestamp.

-----Original Message-----
From: ntpwg-bounces+melliot=symmetricom.com at lists.ntp.org
[mailto:ntpwg-bounces+melliot=symmetricom.com at lists.ntp.org] On Behalf
Of rich.osman at nsn.com
Sent: Monday, November 12, 2007 6:21 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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