Re: [netconf] YANG encoding in CBOR

Carsten Bormann <cabo@tzi.org> Sat, 23 March 2019 09:03 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Sat, 23 Mar 2019 10:03:32 +0100
Cc: "core@ietf.org" <core@ietf.org>, "netconf@ietf.org" <netconf@ietf.org>, "Rob Wilton (rwilton)" <rwilton@cisco.com>
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To: Michel Veillette <Michel.Veillette@trilliant.com>
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Subject: Re: [netconf] YANG encoding in CBOR
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Well, if that is a problem, we can go for a longer representation within unions (section 6.12).  Theoretically, we could do that only of there is more than one enum in the union type (so things stay efficient if there is only one), but that might pose difficulties with model evolution.

Going for a string representation repeats the feature of XML YANG (which was ported over to JSON YANG):

typedef foo {
  type union {
    type enumeration {
      enum red { value 1; }
      enum breen { value 2; }
      enum glue { value 3; }
    }
    type enumeration {
      enum tacks { value 1; }
      enum nails { value 2; }
      enum glue { value 3; }
    }
  }
}

If you use “glue”, you don’t know which of the enumerations are being used.

Using SIDs, we can do better.

So what do we have to do to get the SID tool to allocate SIDs for enum values?

We could then define the CBOR tag for enums in unions to take the usual SID difference (delta relative to the environment, I’d think), not an integer value.

Several of us are at the hackathon and could make something happen today and tomorrow.

Grüße, Carsten


> On Mar 22, 2019, at 18:30, Rob Wilton (rwilton) <rwilton@cisco.com>; wrote:
> 
> I guess that the concern is that this introduces more variation in how data is interpreted between the different XML/JSON/CBOR encodings.
> 
> E.g. if someone switched from XML to CBOR, suddenly the configuration or state data may have a different meaning.
> 
> Thanks,
> Rob
> 
> 
>> -----Original Message-----
>> From: Carsten Bormann <cabo@tzi.org>;
>> Sent: 22 March 2019 16:08
>> To: Michel Veillette <Michel.Veillette@trilliant.com>;
>> Cc: Rob Wilton (rwilton) <rwilton@cisco.com>;; core@ietf.org;
>> netconf@ietf.org
>> Subject: Re: [netconf] YANG encoding in CBOR
>> 
>> On Mar 22, 2019, at 16:45, Michel Veillette <Michel.Veillette@trilliant.com>;
>> wrote:
>>> 
>>> The only potential problem I aware is when multiple enumerations are part of
>> the same union.
>>> Value 4 from enumeration A will be encoded the same way as Value 4 from
>> enumeration B.
>> 
>> … and that is not a problem for the XML version, because the string is being used
>> instead of the value.  (But then if two enumerations share a string, you have the
>> equivalent problem in the XML serialization.)
>> 
>> Anyway, I haven’t seen a piece of real-world YANG that actually has this
>> problem, so I would be a bit reluctant to make CBOR-based implementations
>> more complex (and less efficient) so solve this (non-?)problem.
>> 
>> Grüße, Carsten
> 
> 
>