Re: [netmod] [core] CBOR YANG encoding of union & bits [draft-ietf-core-yang-cbor-12]

[Andy Bierman <[e-mail]>]

On Fri, May 8, 2020 at 10:48 AM Jim Schaad <[e-mail]> wrote:

> Does yang consider that there is a difference between a bit being present
> and zero and a bit being absent?
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  leaf bits-leaf {
      type bits {
         bit zero { position 0; }
         bit one { position 1; }
         bit two { position 2; }
         bit 4million { position 4000000; }
     }
  }

I think I see your point.
The value "" is valid for this leaf indicating no bits are set.
In order to send this value:
   { 0, 1, 0x0 }

Any YANG bit that is present is represented by a bit with the value 1 in
the corresponding position and octet.


Andy


*From:* core <[e-mail]> *On Behalf Of *Andy Bierman
> *Sent:* Friday, May 8, 2020 8:58 AM
> *To:* Carsten Bormann <[e-mail]>
> *Cc:* [e-mail]; [e-mail]
> *Subject:* Re: [core] [netmod] CBOR YANG encoding of union & bits
> [draft-ietf-core-yang-cbor-12]
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> On Fri, May 8, 2020 at 8:51 AM Andy Bierman <[e-mail]> wrote:
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> On Thu, May 7, 2020 at 11:22 PM Carsten Bormann <[e-mail]> wrote:
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> On 2020-05-08, at 05:27, Andy Bierman <[e-mail]> wrote:
> >
> > Why is the bit position allowed to be a uint32 in YANG? Who knows, but
> it has to be supported.
>
> If we think that is the way to go, I like Kio’s proposal over in the CBOR
> list:
> <https://mailarchive.ietf.org/arch/msg/cbor/3sZ4YfWLzmVVnNnQbttkovgjTP8>
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> Yes -- this encoding should work well
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> We probably should arm that with some text that says (in nicer words) that
> this is an emergency representation and implementations should not invoke
> it for sane YANG models (with an operable definition of sane).
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> Not sure what this means.
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> Wouldn't this map approach be used all the time for a bits type?
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> The "normal" case would be a small number of octets representing all the
> bits present
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>  (e..g 0x7 for 3 bits : bit 0 to bit 2)
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> This would simply be 1 map entry with {offset=0,length=1,value=0x7}
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> The actual bitmap is conceptually constructed by starting with an array of
> zero bytes.
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> Overlapping offsets are allowed. Duplicate offsets are allowed.
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> There is no requirement to list offsets in ascending order.
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> There is no canonical representation.
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> Each map entry is added to the array using a "bit or" operator.
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> After all map entries are processed the full bits value is known.
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> This allows an implementation to encode bits serially (often done this way
> with YANG bit names)
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> So the same bitset 0x7 could be sent different ways including:
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>   { 0, 1, 0x1 }
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>   { 0, 1, 0x2 }
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>   { 0, 1, 0x4 }
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> If the position is really bit 4 million (bit 1 in byte 500,000)
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>   { 0, 500000, 0x1 }
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> oops: correction:  (bit 0 in byte 500000)
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>   { 500000, 1, 0x1 }
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> Grüße, Carsten
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> Andy
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