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

Stewart Bryant <stewart.bryant@gmail.com> Fri, 08 November 2019 14:34 UTC

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From: Stewart Bryant <stewart.bryant@gmail.com>
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Date: Fri, 8 Nov 2019 14:33:10 +0000
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Cc: Magnus Westerlund <magnus.westerlund@ericsson.com>, "last-call@ietf.org" <last-call@ietf.org>, "gen-art@ietf.org" <gen-art@ietf.org>, "draft-ietf-dtn-bpbis.all@ietf.org" <draft-ietf-dtn-bpbis.all@ietf.org>, "dtn@ietf.org" <dtn@ietf.org>
To: lloyd.wood@yahoo.co.uk
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Subject: Re: [dtn] [Last-Call] Genart last call review of draft-ietf-dtn-bpbis-17
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> On 8 Nov 2019, at 13:03, lloyd.wood@yahoo.co.uk wrote:
> 
> http://lloydwood.users.sourceforge.net/Personal/L.Wood/publications/wood-ieee-aerospace-2009-bundle-problems.pdf <http://lloydwood.users.sourceforge.net/Personal/L.Wood/publications/wood-ieee-aerospace-2009-bundle-problems.pdf>
The critical text in that paper is:

"The Bundle Protocol uses Coordinated Universal Time (UTC), where leap seconds are added at irregular, unpredictable, intervals to reflect slowing of the Earth’s rotation. For nodes ‘in the field’ for a long time (decades), some way of communicating newly-decided UTC leap seconds will be required to prevent clock drift over long time scales that would eventually lead to bundles expiring before delivery. This is most likely to be a significant issue for real-time traffic with very short bundle lifetimes."
That says that leap-seconds need to be transmitted to the remote site, but the ID does not say anything about that, indeed it silently implies that this is handled.

The draft text says

Like TAI, Unix epoch time
   is simply a count of seconds elapsed since a standard epoch.  Unlike
   TAI, the current value of Unix epoch time is provided by virtually
   all operating systems on which BP is likely to run.

Which is not quite right. The TAI is a continuous count of the number of seconds since the epoch. The UNIX tine is the continuous count + leap seconds since the epoch. Unix knows how many leap seconds have happened by a background process and adds them in, but for that to work it has to have a method of knowing the current leap second state. BTW leap seconds can be removed as well as added.

- Stewart