Re: [core] Benjamin Kaduk's Discuss on draft-ietf-core-senml-data-ct-05: (with DISCUSS and COMMENT)

Benjamin Kaduk <kaduk@mit.edu> Tue, 12 October 2021 17:03 UTC

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Date: Tue, 12 Oct 2021 10:02:20 -0700
From: Benjamin Kaduk <kaduk@mit.edu>
To: Carsten Bormann <cabo@tzi.org>
Cc: The IESG <iesg@ietf.org>, draft-ietf-core-senml-data-ct@ietf.org, core-chairs@ietf.org, core@ietf.org
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Subject: Re: [core] Benjamin Kaduk's Discuss on draft-ietf-core-senml-data-ct-05: (with DISCUSS and COMMENT)
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Hi Carsten,

On Tue, Oct 12, 2021 at 03:37:16PM +0200, Carsten Bormann wrote:
> Hi Ben,
> 
> thank you for bringing up a number of important issues.
> 
> > ----------------------------------------------------------------------
> > DISCUSS:
> > ----------------------------------------------------------------------
> > 
> > I have a couple points for discussion, essentially relating to how much
> > we're diverging from HTTP and to what extent the specifics of the
> > divergence should be specifically mentioned in the document.
> > 
> > (1) I'd like to dig a little more into the analogy with HTTP and
> > whether we are artificially limiting ourselves: currently we only allow
> > 0 or 1 content-codings to be specified, but per
> > https://www.ietf.org/archive/id/draft-ietf-httpbis-semantics-19.html#name-content-encoding
> > the HTTP ecosystem permits multiple codings to be applied in turn to the
> > same representation.  While the sensor data values are likely to be
> > relatively small and applying multiple content-codings is not likely to
> > be useful in such a scenario, this seems like something where we should
> > only consciously diverge from HTTP, rather than inadvertently doing so.
> 
> I agree that this should be handled (if only for     application/json@deflate@aes128gcm and similar cases).
> We discussed this in the previous interim and it seems we like the syntax I proposed in the previous sentence.

That seems reasonable.  I didn't check just now, but I think I looked last
time about whether "@" was allowed in the earlier parts of the
media-type+parameters and there was no problem.

> Combining the numerous changes needed with addressing Christer’s comments on the Abstract:
> 
> https://github.com/core-wg/senml-data-ct/pull/8

Generally seems reasonable, though I've lost track of the surrounding
context for whether "content coding" and "content format" (vs
"Content-Coding" and "Content-Format") make sense in the context of the
list and surrounding discussion.

> > (2) Let's also discuss whether we want to reuse ABNF rule names from
> > HTTP while having the rule content diverge, without specific enumeration
> > of the divergence.  So far I found instances where this document does
> > not allow HTAB or obs-text in places that draft-ietf-httpbis-semantics
> > does, which may well be the right way to spell the rule, but seems to
> > merit a little discussion.
> 
> Very good point.  I’m not sure the ABNF behind “ABNF rule names from HTTP” is stable enough that we need to stick with it in all cases.
> I was certainly surprised by the recent change to
> 
>    parameters = *( OWS ";" OWS [ parameter ] )
> 
> (which makes sequences of ";" legal; see also section B.2 in -semantics.) and wonder whether we should follow this change (and why!?).

I confess I looked at both 7230+7231 and the new -semantics while reviewing
this doc, and didn't make a careful distinction between the two sources.

> Httpbis-semantics uses modified ABNF anyway; so there is no way to have exactly the same ABNF.  But it is worthwhile pointing out that legacy 8-bit text and HTAB have no place in the strings used in this protocol and therefore are not allowed.
> 
> I added a comment:
>  
> -; Cleaned up from RFC 7231:
> +; Cleaned up from RFC 7231, only leaving SP as blank space, and
> +; removing legacy 8-bit characters:

Thanks!

> The other difference from RFC 2616 and its descendants is the absence of “quoted-pair” from the “quoted-string” production, which indeed can be discussed.
> 
> (May need to discuss this in the interim tomorrow for final resolution.)

Ok.

> > ----------------------------------------------------------------------
> > COMMENT:
> > ----------------------------------------------------------------------
> > 
> > Do we want to comment anywhere about the situation where an
> > implementation receives a message using an IANA-registered numeric
> > content-format that is "too new" for that implementation to know about?
> 
> (Should have a discussion about error handling in the interim tomorrow.
> But generally, the important thing is that the implementation *know* that it needs to be updated to understand.)

True.

> > It also feels a little weird that we have to end up using the
> > text-string encoding of a decimal number for the Content-Format, even
> > for the CBOR representation of SenML structures, but I guess that's what
> > RFC 8428 intended and not worth trying to change.
> 
> We started out with a numeric encoding, but then noticed that SenML is usually very specific about only allowing a single data type per field.
> 
> > Abstract
> > 
> > I'm somewhat sympathetic to the gen-art reviewer's contention that the
> > new field is not indicating the "Content-Format" of the binary data
> > values (since Content-Format is a defined term in CoAP and SenML is
> > claimed to not be limited to CoAP usage).  Perhaps we could switch
> > around the order of description, i.e., "for indicating the Internet
> > media type (including parameters) of these binary data values (i.e., the
> > CoAP Content-Format that would apply when CoAp is used), as well as any
> > content-coding that is applied"?
> 
> This is addressed in https://github.com/core-wg/senml-data-ct/pull/8, but not exactly following your blueprint.
> The term “content format” is still used both for a “Content-Format number” and a “Content-Format-String”.
> 
> > 
> > Section 3
> > 
> >   *  a CoAP Content-Format identifier in decimal form with no leading
> >      zeros (except for the value "0" itself).  This value represents an
> >      unsigned integer in the range of 0-65535, similar to the CoRE Link
> >      Format [RFC6690] "ct" attribute).
> > 
> > Should we also reference
> > https://datatracker.ietf.org/doc/html/rfc7252#section-7.2.1 which is
> > where the "ct" link attribute is actually defined?  I spent a bit of
> > time looking for it in 6690 itself only to discover that it was removed
> > in draft-ietf-core-link-format-07 with remark "Moved [...] to the base
> > CoAP specification".
> 
> Good point.  YYY
> 
> > 
> >   The CoAP Content-Format number provides a simple and efficient way to
> >   indicate the type of the data.  [...]
> > 
> > If we're limited to string representation, is it really "efficient" in a
> > CBOR context?
> 
> That’s not what the sentence is about :-), but “11050” is still more efficient than "application/json@deflate”.
> 
> > Section 6
> > 
> >   ; Cleaned up from RFC 7231:
> > 
> > (per the DISCUSS,) I'm a bit anti-enthused about saying "cleaned up" without saying what
> > changed (i.e., whether it's just refactoring, or actual changes to the
> > rule like requiring specifically *SP instead of OWS that allows HTAB as
> > well).
> 
> See above.
> 
> > Section 7
> > 
> > These security considerations are well-thought-out and nicely written.
> > Thank you!
> > 
> > I think there are some (rare) situations where individual media-type
> > (specifications) have their own security considerations, but I'm not
> > really convinced that we need to mention that/incorporate them by
> > reference, here.
> > 
> > NITS
> > 
> > Section 1
> > 
> >   The receiver is expected to know how to interpret the data in the
> >   "vd" field based on the context, e.g., name of the data source and
> >   out-of-band knowledge of the application.  However, this context may
> >   not always be easily available to entities processing the SenML pack.
> > 
> > I'd consider adding ", especially if the pack is propagated to multiple
> > entities".
> 
> That is a bit implied in the context of SenML, but it doesn’t hurt to say it again.
> 
>  field based on the context, e.g., name of the data source and out-of-band
>  knowledge of the application. However, this context may not always be
> -easily available to entities processing the SenML pack. To facilitate
> +easily available to entities processing the SenML pack, especially if
> +the pack is propagated over time and via multiple entities. To facilitate
>  automatic interpretation it is useful to be able to indicate an Internet

Thanks!

> > Section 2
> > 
> >   Content-Coding:  A name registered in the HTTP Content Coding
> >      registry [IANA.http-parameters] as specified by Section 8.5 of
> >      [RFC7230], indicating an encoding transformation with semantics
> >      further specified in Section 3.1.2.1 of [RFC7231].  [...]
> > 
> > (I expect that the RFC Editor will be able to replace the references to
> > point to draft-ietf-httpb-semantics if it has been published before this
> > document.)
> 
> (With the actual changes in this document, I’m not sure that is a mechanical operation.  But let’s wait for httpbis-semantics to emerge from EDIT.)
> 
> > Section 4
> > 
> >   up to the end of the pack otherwise.  Resolution (Section 4.6 of
> >   [RFC8428]) of this base field is performed by adding its value with
> >   the label "ct" to all Records in this range that carry a "vd" field
> >   but do not already contain a Content-Format ("ct") field.
> > 
> > The conjugation "resolution" does not actually appear in RFC 8428
> > itself, just discussion of "resolved records" and "to resolve the
> > records".  It might be helpful to tweak things so that we don't rely on
> > the reader knowing the irregular conjugation (but I don't have any good
> > ideas off the top of my head)...
> 
> -pack otherwise.  Resolution ({{Section 4.6 of -senml}}) of this base
> +pack otherwise.  The process of resolving ({{Section 4.6 of -senml}}) this base
> 
> The changes that are not already accepted in PR 8 are in
> https://github.com/core-wg/senml-data-ct/pull/9

Thanks again for all this,

Ben