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

Barry Leiba <barryleiba@computer.org> Wed, 15 May 2019 13:22 UTC

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From: Barry Leiba <barryleiba@computer.org>
Date: Wed, 15 May 2019 09:22:21 -0400
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To: Alexey Melnikov <aamelnikov@fastmail.fm>
Cc: The IESG <iesg@ietf.org>, draft-ietf-6lo-deadline-time@ietf.org, 6lo-chairs@ietf.org, Samita Chakrabarti <samitac.ietf@gmail.com>, shwethab@cisco.com, 6lo@ietf.org
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Subject: Re: [6lo] Alexey Melnikov's Discuss on draft-ietf-6lo-deadline-time-04: (with DISCUSS and COMMENT)
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> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
>
> I have a few small points (one is confusing enough to warrant a quick
> discussion), but they affect clarity of the specification:
>
> In Section 5:
>
>    o  OTL (3 bits) : Length of OTD field as an unsigned 3-bit integer,
>       encoding the length of the field in hex digits.  If OTL == 0, the
>       OTD field is not present.  The value of OTL MUST NOT exceed the
>       value of DTL plus one.
>
>       *  For example, DTL = 0b0000 means the deadline time in the 6LoRHE
>          is 1 hex digit (4 bits) long.
>
> Ok, so 0b0000 ==> (0 + 1) * 4, means 4 bits.
>
>           OTL = 0b111 means the
>          origination time is 7 hex digits (28 bits) long.
>
> Is my math wrong or is your example wrong?
>
> 0b111 == 7. So (7 + 1) * 4 would be 32 bits.

This fits right into my comment about the +1 inviting confusion.  What
you missed is that the +1 only applies to the DTL, not to the OTL:

> Ok, so 0b0000 ==> (0 + 1) * 4, means 4 bits.

Correct.

> 0b111 == 7. So (7 + 1) * 4 would be 32 bits.

No, for OTL it's 7*4 = 28.

I do think this is unnecessarily confusing, and don't see why there's
a need for the +1 on the DTL.  (Yes, I read Carsten's comment in
response, but I don't find it compelling.)

> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
>
> In Section 4:
>
>    There are multiple ways that a packet can be delayed, including
>    queuing delay, MAC layer contention delay, serialization delay, and
>    propagation delays.  Sometimes there are processing delays as well.
>    For the purpose of determining whether or not the deadline has
>    already passed, these various delays are not distinguished.
>
> Not distinguished from what? Do you mean "not counted"?

It means that they are all delays, and the cause of the delay doesn't
matter -- there's no distinction among the different delays.

Barry