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Minutes interim-2020-core-03: Wed 16:00

Meeting Minutes Constrained RESTful Environments (core) WG
Date and time 2020-04-29 14:00
Title Minutes interim-2020-core-03: Wed 16:00
State Active
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Last updated 2020-04-29

CoRE Virtual interim - 29-04-2020 - 16:00 UTC

* Marco Tiloca, RISE
* Jaime Jiménez, Ericsson

Materials: Webex:

Minute takers: Christian Amsüss, Jaime and Marco (helping)
Jabber Scribes: Jaime

Marco Tiloca, RISE
Jaime Jiménez, Ericsson
Klaus Hartke, Ericsson
Francesca Palombini, Ericsson
Jim Schaad, August Cellars
Christian Amsüss
Rikard Höglund, RISE
Barry Leiba, FutureWei
Peter Yee, AKAYLA
Ari Keränen, Ericsson
Michael Koster, SmartThings
Thomas Fossati, ARM
Carsten Bormann, TZI
Niklas Widell, Ericsson
Michael Richardson, Sandelman Software Works
Henk Birkholz

Marco doing introductions.

*** draft-bormann-core-senml-versions-01 ***

CB presenting (1603 CEST)

SenML had a long evolution up to RFC8428, called version 10 then without much
thought for extension. Base field can be used to set higher version number, but
does not give meaning to that, deferring to new RFCs.

New fields are an extension points; versions declare from the sender to the
receiver to indicate need to implement something more. Expectation that higher
versions imply lower versions is there somehow, but not realistic (due to

Version numbers work (only) for single-source emissions that develop linearly;
for protocols they create problems. Other mechanisms are used there, and
versions rarely change (cf. HTTP 1.0 from ~2000, extensions done using new

Features as extension points are present in the form of "Must
understand"-fields (indicated by trailing underscores). This could get big over
time (~10 byte per feature).

Idea from Ari: Interpret version number as a bitfield. Current version is 10,
so four LSBs are "gone". Still gives 49 reasonably usable bits. One could be
used for "secondary units" (open point from IESG approval). 53 comes from JSON.
Wouldn't suffice for HTTP evolution, but would last for decates at expected

All in the draft, which updates 8428 to set that versioning mechanism.

Comments? Anything that needs fixing?

Thomas: What's the relation between -versions and -more-units? Does -more-units
need -versions? CB: more units was approved with an informative ref to this
document, not necessarily as *the* way to solve the issue, but -more-units has
a big hole while something like -versions is not there. Thomas: exactly so, if
someone else comes with another way to indicate this, how would it work at then
end? Is the informative reference sufficient. CB: Completely different? Then,
somebody should decide. Thomas: Who is authoritative? CB: Change control is
with the IETF. I'd expect CoRE to make these decisions. Is this completely
hypothetical, or is something on the horizon we don't know? Thomas: It is
hypothetical. Seems like a risk we have. Apart from that, am OK with that.
Small doubts about small code point space and pressure on registry. [...]

JJ: Should the reference to -versions in -more-units be manadatory to implement
(normative) right away? Thomas: Yes. CB: With 7252 we thought to do the right
thing and put in RPK as mandatory-to-implement scheme. Created normative
reference to a not-done-yet document. Some squabble in security area created
one year delay. My perspective is that incurring a normative reference where
it's not necessary is a mistake we should not make. We are not doing that any
more, it's malpractice. For me, that's way more important than the other
considerations. It's kind of weird to have an informative way to the only way
to do it, but we can finish it now and have other SDOs use it in their
documents. -more-units is leaving RFC editor stage and to be published end of
May, any throwbacks would be unfortunate.

Ari: on only-way-is-WIP: There's already a use case where they're only using
units of SenML and not the serialization, so they don't have that problem at
all. Later on they may use SenML serialization. So for that case it's less
weird. On Thomas' point on coexistence: Used to have mandatory-to-implement as
main extension point, and version that was never planned to be used. It's not
that tricky here, those ways can coexist, eg. a set of features could be
grouped inside a single version bit. Different indication ways can coexist.
Thomas: I understand Carsten's point of using non-normative references to avoid
blocking -more-units on -version, still with that we would have a scar in
-more-units that could leave space in the future for potential interop problems.

*** draft-ietf-core-senml-data-ct-01 ***

CB presenting (1624 CEST)

A more conventional extension to SenML, but ran into detail with probably
workable solution.

Problem recap: data fields can be binary or other types. What does binary mean?
content types ("media types") are common to indicate that, in the
Content-Format header in CoAP (for coding), shown in second example on p2
(e.g., "ct":"60" vs "ct":"text/csv@gzip")

How is this handled as a new thing that's not understood by all
implementations? "ct" as field is ignorable, "ct_" is must-understand per SenML
mechanism. What if there are base fields for any? Ugly interactions have been

SenML defines "must understand", but does not define interaction of
not-must-understand items with them ("foo" and "foo_" are unrelated from its
PoV). The resolution process for base values is defined, and only per-field.

Proposed solution #1: per-record overrides any base. (But not backed by SenML)

Proposed solution #2: [...] must-understand overrides optional-to-understand
but "bct_" renders "bct" and "ct" unusable.

Current document proposes #2. We think we are done with this.

JJ: Can you have simultaneously bct_ and bct?
CB: Yes, and #2 means that bct is ignored then.
AK: Clarification: *from that point on* where you use that it's overridden.
Generally, we have not found a case where this would be an actual problem in a
single pack. Would become a problem when merging packs from multiple sources.
KH: Since Carsten has been asking for additional solutions: My favourite
solution is always to not solve the problem. The original problem was that,
without `ct`, we know that there is a bytestring but don't know the semantics
(the content format) of that bytestring. With `ct`, we know the semantics if we
understand `ct`. But, with `ct`, if we don't understand `ct`, then we're not
worse off than we were at the beginning. We still know that there is a
bytestring. Is it actually important that we understand `ct` to make use of a
record? CB: Bit of ambiguity that SenML has. It does not say what "must
understand" means, does it mean "must understand field in principle" or "the
specific value"? That's not clearly defined. KH: I simply assumed what we have
for critical CoAP options: you cannot understand the CoAP message if you do not
understand all the critical options. CB: That's an important distinction – "can
not understand that record" (but may understand that pack). A must-understand
field may influence the rest of the pack. KH: Maybe next step is to clarify
what an underscore means? CB: Not sure if this is the document to do that. KH:
Have draft that clarifies RFC that clarifies RFC... ;-)

CA: Can bct_ be unset ("bct_": null), when unsetting, wouldn't that make the
base thing go away again? Can the composition rule be defined per field? CB:
The resolution process is defined independently of the specific field. We can't
have application-specific resolution.

JLS: Arguments about criticality bother me, X.509 have been discussing that for
ages. CB: Yes.

CB: That's something to take home, clarifying the semantics of "must

AK: Good material for SenML-bis. Original proposal was to have "ct" only, but
"maybe we need it" came up and opened the discussion. For ct, I'd argue that
underscore version is not critical to have (sometimes maybe useful). That part
of the draft could even live without.

CB: To be taken to the list, option of just "ct" might be simpler.

*** Media types and data definition languages ***

CB starting discussion (1642 CEST)

Not about specific document, but touches a number of documents.

(Unrelated WebEx bashing, and philosophical comments on the benefits of

One component of REST: Media types as pre-cooked definitions of data to be
found in a response, and data defined in a more dynamic way.

Back to 1977 (RFC733). If something is text based and has no pre-existing
structure (JSON) and no other data definition, we[1] use ABNF. [1] with some
voices against it but a rough consensus.

Now we have CDDL (RFC8610), and hopefully will arrive at agreement that we use
this for structured data. Machine readable definition enable CI, code
generation, validation of examples etc. Net benefit: fewer doubtful cases in
interops, more clarity. Downside: learning problem for contributors / threshold
effect. So only have a small number to have to learn few. Also, ease to
communicate complexity incites complexity. "ASN.1 effect" that led to GSM
map[?] etc. Format can't help here, discipline can. (Two-line CDDLs can be

Result of data definition: shape (up to ranges and validity, and even,
identification of extension points); augmenting instance with semantics
(implicitly by the name to be referenced in documentation, or explicitly with
RDF rooting).

So far, probably widely agreed on. Now for speculation.

Example: RFC6690. Some structural aspects in structure using ABNF for structure
mapping (crossing levels: mapping web links to ASCII and the higher level).
RFC6690 inherited that. Example implementation of links-json shows
implementation impact. (RFC8... fixes that).

CDDL can describe "plain old CBOR" [sic!], but CoRAL is also there. Is "It's
CoRAL" (which is true) sufficient for a media type? There is a higher level of
structure too, solving a specific application problem, and some minimum
expected components etc are necessary. Not try to apply CDDL to CoRAL (that
would be mixed-level error again).

Look at core-coap-problem. As plain old CBOR this can be described by CDDL.
What to use when using CoRAL? We want validation, minimum properties, probably
not need augmentation (but maybe that's lack of imagination). Do we need
"CoDDL" for class information?

Can we embed definitions of this kind in CoRAL? link to description? Express
expectations on what to be sent? Everywhere where media type goes, there could
go a CDDL or "CoDDL"?

MK: How is this different from a system of shape constraints? Or *is* that a
system for shape constraints, like SHACL CB: We could use SHACL. Suggested
literature to take home. MK: I have also encounter would have found useful a
lightweight mechanism to carry the shape informaiton over the wire. CA [prep]:
CoRAL Fetch to validate? The same format could be used to validate and describe
to the server what the client wants. CB: One aspect of augmentation is adding
flags to say I want this, I don't want that. MK: Ties in to discussion of
selectors, to semantically identify nodes inside a document.

HB: To augment something like from CBOR to CDDL, sounds awesome but [...] how
to sign? As a steering information for the consumer, having a wrong array could
be problematic. CB: There are some security considerations, good point.

MK: Might want to hurry because they're talking about JSON path again.
CB: That's one point I didn't make: It spans tons of working groups. YANG (with
large solutions for large problems), L Carrien[?] who does little JSON schema
because he doesn't like big schema. JSON WG which is having messages back an
forth on how JSON should look like CBOR group working on CDDL. Michael has this
nice matrix of ecosystems, containing a line on what they use for data
definition MK: Will publish that link.

CB: JSON-path to me is a very simple subset, but maybe I'm lacking imagination
on how this is likely to become complex (like XPath got). Interesting about
JSON path is "are you doing this for a single instance" (eg. from inside the
instance, like JSON pointers do) vs. designed to handle a class of data items,
so that you can say "out of this array, give me those that have field type set
to green" (like XPath can do), ending up in a Turing complete language. Is
there a useful subset that's a 80% solution? Who does JSON-path now? MK: Has
come up in WoT group in discovery / directory. In Thing Directory, they use
JSON-Path syntax as query language. Follow-up is standardization of JSON-Path
and -schema. Point is, this is a general issue.

KH: Nobody responded to "Do we need CoDDL". I think yes. Sounds a very good
idea to have a machine readable format to describe the shape of coral
documents, both for documenting and to have simple instance validation. Maybe
fetching is not so distant from this instance validation (e.g., give me all
documents that validate against this shape). Maybe we do not "need" it as we
can use english, but it is good to have something machine readable in our

CB: JSON-Path is a simple matching language. CoDDL would be a powerful matching
language. As usual, "how much complexity do we actually need", "… can we expect
people to imlement"? YANG have used complex XPath stuff, which I can only
imagine being usable in rack hardware. KH: FETCH part should be implentable in
constrained devices. SHACL and ShEx ( looked at, latter looks
usable, shouild try to implement. MK: Dan B. told me he prefers ShEx too. CB:
Some of this is getting too researchy for the WG, maybe we can partition this
into "now" and "after research". Looking into ShEx would help in CoRE.

CA: media types? we already have issues in CoRAL expressing dictionaries as
media types. Restricting them further would make 64k run out fast. KH: Media
types use to say what bytes mean. Now you can the CoAP Accept option, for
instance, to choose among different dictionaries. Then you can say that this
document conforms for some shape. It feels more like a resource type, in the
light of the shape of the document exprssed in the document. But maybe this is
less about declaration of shapes but more about just having the shape (and then
it's working). MK: You're distinguishing between shape and vocabulary. So the
shape includes also the application vocabulary. KH: Yes, e.g. in coap-problems
must have an error code, and may have a text, and may have zero or more other

CB: Seems that problem-details will be guinnea pig here. Sorry, Thomas.
TF: I'm fine with that.
KH: We also have some more CoRAL based applications now, like Marco's group
manager draft, pubsub proposal etc. Most of them define the shape already, but
in English. Could try to come up with something for all of them. MK: Currently
have an issue (kind of related) where people have trouble understanding how
they're supposed to construct URIs or requests. With talk about constraint
languages, this might apply here too. Better way of describing things than
quasi-URI template. But more about interactions and not data descriptions.

CB: Next steps? Extracting requirements / objectives out of CoRAL drafts we
have, in an informal way? JJ: Could it be possible to have some example of a
draft using this? So the other drafts can start working on it as well. For me,
it's easier with examples. CB: Love jumping into examples, but need design
first. What are the properties of the thing we're trying to build? KH: Would
start by creating instance examples for the different drafts, and when we see
the instances we can try to see what we expect of classes.

CB: ..
MK: Is this "Validators vs constructors"?
CB: Can build both for both, but how do you express things? Historically, it's
schematron vs. relax-ng. Inclined to take generative approach.

(Meeting ended 1730 CEST).