Re: [core] draft-bierman-core-yid-00.txt questions

[Michel Veillette <[e-mail]>]

Hi David, Hi Andy

[MV] Inline

From: core [mailto:[e-mail]] On Behalf Of Andy Bierman
Sent: Wednesday, August 24, 2016 5:23 PM
To: David Reid <[e-mail]>
Cc: Core <[e-mail]>
Subject: Re: [core] draft-bierman-core-yid-00.txt questions



On Wed, Aug 24, 2016 at 1:35 PM, David Reid <[e-mail]<mailto:[e-mail]>> wrote:
I read draft-bierman-core-yid-00.txt and have a few questions.

Will all IETF standard yang modules be included in the YANG identifier
registry or just ones that might be used on constrained devices?


I think all modules.
We want to support unconstrained devices on constrained networks.

[MV] The request we got so far is all YANG modules published in RFCs.


In order to assign numbers, is it correct that I need the first revision of
the module or else the current revision and the registry assignments for the
previous revision?


I think in all numbering schemes (as Michel pointed out) you need
the current registration entry and the new module revision.



What is the benefit of assigning values using hashing over sequentially
assigning values?

There is overhead ir reading and processing a giant list of mappings.
Automatic assignment is risky because both sides need to agree on
the object to number mapping.  Detecting these issues is very difficult.

The YANG Hash algorithm is designed to use the path string,
which is permanent and cannot change no matter how the
YANG is refactored.  The murmur hash is a stable algorithm.
There is not a lot that can go wrong for the 2 peers to disagree
on the hashes (except for collisions)


When there is a hash collision, would it be possible to rehash the value
to get a unique number so that we never have to manually assign numbers and
thus never need to register the numbers in the registry?


yes, this has been suggested.
The problem with auto-rehashing before was inter-module clashes.
Now those are not possible so automatic rehashing is feasible.

[MV] I disagree, see my previous email which explain why all YIDs/SIDs need to be registered even if generated using a hash.


Why does module-id need to be added to the YANG module names registry since
it is also in the YANG identifier registry?


I guess it is not needed since the module name is in both registries already



Would it make sense to put the module-id inside the yang module with a yid
extension so that I would not have to go lookup that information from a
registry?


I suppose -- but how to prevent duplicates and cut-and-paste errors?

[MV] Just adding the module ID in the yang file is not sufficient to use a .yang file, all YIDs/SIDs need to be added.
[MV] Having YIDs/SIDs in yang files will make their maintenance more complex.
[MV] BTW, a pyang plugin already exist to automatically generate and update a .sid file from a .yang file
[MV] See https://github.com/core-wg/yang-cbor/blob/master/sid.py


Would it be possible to assign the module-id based on information in the
module, for example a hash of the namespace and maybe the revision date.
That way, a module-id would not have to be assigned and maintained in a
registry.


I think private module-id would be better, using a range reserved for temporary
assignments.

How do you resolve hash collisions for module-id?


If I want to handle private modules, do I have to create my own registry?
It seems important that private modules be supported. Could we re-use the
SMI enterprise number assignments for creating module-ids for private modules?

The ietf-yid module has a schema for the entire registry.
How to use the registry is a protocol issue that needs to be decided.
If the server advertises  its own registry vs. a public registry,
then it is using a private registry.  If all devices in the system agree
on which registry to use, then everything works.


Who will assign the local-id numbers? Is that done by the working group that
defines the YANG module?

I would expect IETF modules to use hashes by default but for
some modules that seem useful to constrained devices, then
manual numbering could be done instead by the WG.

[MV] To obtain a smaller encoding, I personally believe that the sequential numbering will be used.
[MV] With sequential numbering, all current YANG modules defined in RFCs can be assigned within the first 6500 SIDs which will be encoded as 3 bytes for the first reference and typically as 1 bytes for the following ones using delta encoding.
[MV] With hashes, only one YANG module can be assigned within that range.


-David Reid

Andy


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