Re: [6lo] Barry Leiba's Discuss on draft-ietf-6lo-deadline-time-04: (with DISCUSS and COMMENT)

Carsten Bormann <cabo@tzi.org> Sun, 12 May 2019 18:49 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Sun, 12 May 2019 20:49:08 +0200
Cc: The IESG <iesg@ietf.org>, draft-ietf-6lo-deadline-time@ietf.org, 6lo-chairs@ietf.org, Samita Chakrabarti <samitac.ietf@gmail.com>, Shwetha Bhandari <shwethab@cisco.com>, 6lo@ietf.org
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To: Barry Leiba <barryleiba@computer.org>
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Subject: Re: [6lo] Barry Leiba's Discuss on draft-ietf-6lo-deadline-time-04: (with DISCUSS and COMMENT)
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On May 12, 2019, at 15:54, Barry Leiba via Datatracker <noreply@ietf.org>; wrote:
> 
> Why is DTL the length *minus 1*?  Doesn’t that invite mistakes?  Is there a
> reason not to make it the length, and to say that 0 is not a valid value?

Fundamentally, a small integer encoded into a bitfield is best encoded as a value starting from 0, instead of creating holes at the bottom *and* the top.  Also, in this case, the 4-bit value encoding DTL can be between 0 and 15, so the length in bits could be 0 to 60, which is not enough for 64 bits.
So adding one sounds fine, even if you believe holes are good.  

But then, I don’t understand why both DT and OTD are said to be “8..64” bits — that needs 15 possible values, not 16; OTL has only 3 bits (which cannot express 15 possible values, which are probably not needed anyway), and the example for OTL has a DT of 4 bits.

Grüße, Carsten