Re: [dtn] Genart last call review of draft-ietf-dtn-bpbis-17

Stewart Bryant <stewart.bryant@gmail.com> Fri, 08 November 2019 12:36 UTC

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From: Stewart Bryant <stewart.bryant@gmail.com>
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Date: Fri, 8 Nov 2019 12:35:52 +0000
Cc: "gen-art@ietf.org" <gen-art@ietf.org>, "last-call@ietf.org" <last-call@ietf.org>, "draft-ietf-dtn-bpbis.all@ietf.org" <draft-ietf-dtn-bpbis.all@ietf.org>, "dtn@ietf.org" <dtn@ietf.org>
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To: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: Re: [dtn] Genart last call review of draft-ietf-dtn-bpbis-17
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>> 
>> SB> I think you are saying that you use Unix time, but Unix time
>> includes leap seconds by double increment, so I don’t think
>> you are using that because that would give you the measurement
>> error you are concerned about. I think that what you are using is
>> a monotonically increasing time based on the Unix epoch. I think
>> that is what PTP (IEEE1588) is using and PTP might be a better
>> reference. PTP is likely to become more available in spacecraft
>> anyway, since it is finding deployment in precision measurement
>> applications. Thus I am not sure I understand why UET is more
>> accessible on spacecraft than TAI. Presumably the spacecraft are using
>> free-running clocks and so will drift, although I understand that
>> work is in progress to provide time sync to spacecraft for
>> navigation purposes.
>> 
>> The argument in this section seems long and will become dated.
>> Surely all you need to say is that you need a monotonically
>> increasing time system such as TAI or UNIX time(), and out of
>> software convenience you choose the latter. However I don’t
>> think that is what you are actually doing. What I think you are doing
>> is using TAI with a free running clock that you accept will drift.
>> 
> 
> So, when I did the AD review I did raise similar concerns, and the note was
> really intended to answer these questions of why the chosen clock rather than
> TAI or other clock definitions that would avoid some other issues. I got enough
> good answers to progress, but apparently the motivation part isn't clear enough
> to make you comfortable over the choices. 

Hi Magnus

I will address the other points in a later email, but on this I am concerned that WG have a misunderstanding of the timebase they are using. UNIX/POSIX time does have leap seconds it just handles them silently so you will get double increments. However since my follow-up note yesterday, I was wondering how the remote remote system knew to add a leap second in order to keep in track with the local system? Isn’t there a risk that the two systems will drift apart as leap seconds are added locally? Also there is the rollover problem to worry about.

I think the section needs to go to a specialist group for review such as TicToc/NTP to verify that it works exactly as the WG expect.

Best regards

Stewart