Re: [ntpwg] I-D Action: draft-ietf-ntp-checksum-trailer-02.txt

[Greg Dowd <[e-mail]>]

I won't argue about the current state of interleave mode although I think the lack of development effort applied to that mode of operation is more an indication of a lack of interest in this capability in the current ntp market as well as recognition that there are emerging alternatives.   The capability of transferring stratum 1 synchronization in telecom networks using this method was out well before interleave was introduced so there is no question that any high resolution timestamped frame is a good source of frequency and time but the ecosystem may not be able to use it.  Really the main issue with the interleave mode is the need to recognize that it is in use, similar to the issue discussed with the leap "smear" mode.

As for alternatives, this draft is exactly that type of alternative as I'm sure you're already aware.  You can just update the timestamp field and use the checksum trailer to fixup the frame.  Again, a technique used for years, just a matter of whether the market requires that function in this type of application.  

Are you thinking that a discrete field is easier to implement?  Or that the clients would use intelligence to analyze the stability/accuracy/authenticity of the correction creators and include/disclude their correction?  If so, then a chain of extensions may be a better methodology.  One current issue with the PTP correction field is the "bucket" nature that it aggregates all intermediate corrections.

The nice thing is that the way the protocol is evolving, all possibilities are available while maintaining backward compatibility.  There are actually some nice things about using NTP such as the timestamp demarq can be seen as inherently more precise than the PTP definition since it uses the y.1731 beginning of xmit/end of receipt which corrects for network bandwidth steps.



-----Original Message-----
From: Martin Burnicki [mailto:[e-mail]] 
Sent: Monday, September 07, 2015 1:24 AM
To: Greg Dowd; Miroslav Lichvar; Tal Mizrahi
Cc: [e-mail]
Subject: Re: [ntpwg] I-D Action: draft-ietf-ntp-checksum-trailer-02.txt

EXTERNAL EMAIL


Greg Dowd wrote:
> Probably moving further offtopic but why would you need 2?  And why 
> the counters?  The correction field in PTP was originally designed 
> because the hardware couldn't update the timestamps on the fly.  It 
> would post-process and then send a follow-up with the precise
> timestamp.   NTP supports that  with the interleave mode.

IMO the interleave mode is just a hack.

"Interleaved mode can be used only in NTP symmetric and broadcast modes", quoted from https://www.eecis.udel.edu/~mills/ntp/html/xleave.html

I really like Miroslav's idea. If support for latency compensation is introduced then it would be good to not just introduce a first level compensation now and another level later, but just provide a way which allows for the highest degree of accuracy.

Martin
--
Martin Burnicki

Senior Software Engineer

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