[core] Eric Rescorla's No Objection on draft-ietf-core-senml-14: (with COMMENT)

Ari Keränen

2018-05-07 18:03:09 UTC

Thank you for the review Eric!

We have now submitted a new revision of the SenML draft that addresses all the IESG review comments:
https://tools.ietf.org/html/draft-ietf-core-senml-15

For answers to your review comments, please see below.

Thanks,
Ari

Post by Eric Rescorla
Eric Rescorla has entered the following ballot position for
draft-ietf-core-senml-14: No Objection
When responding, please keep the subject line intact and reply to all
email addresses included in the To and CC lines. (Feel free to cut this
introductory paragraph, however.)
Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
for more information about IESG DISCUSS and COMMENT positions.
https://datatracker.ietf.org/doc/draft-ietf-core-senml/
----------------------------------------------------------------------
----------------------------------------------------------------------
https://mozphab-ietf.devsvcdev.mozaws.net/D4594
COMMENTS

Abstract
This specification defines media types for representing simple sensor
measurements and device parameters in the Sensor Measurement Lists
(SenML). Representations are defined in JavaScript Object Notation

You're kind of burying the lede here. This document defines SenML, it
doesn't just define media type.

True, and also commented by others. Changed now to:

This specification defines a format for representing simple sensor
measurements and device parameters in the Sensor Measurement Lists
(SenML).

Post by Eric Rescorla

measurement every second, batch up 60 of them, and then send the
batch to a server. It would include the relative time each
measurement was made compared to the time the batch was sent in each
SenML Record. The server might have accurate NTP time and use the
time it received the data, and the relative offset, to replace the
times in the SenML with absolute times before saving the SenML Pack

You use "Pack" here before you define it in S 3.

Changed to:

The server might have accurate NTP time and use the
time it received the data, and the relative offset, to replace the
times in the SenML with absolute times before saving the SenML
information in a document database.

Post by Eric Rescorla

kinds of fields: base and regular. The base fields can be included
in any SenML Record and they apply to the entries in the Record.
Each base field also applies to all Records after it up to, but not
including, the next Record that has that same base field. All base
fields are optional. Regular fields can be included in any SenML
Record and apply only to that Record.

It looks like it's permissible to intermix Base and Regular fields in
the same record. Assuming that's so, it would be helpful to say so.

Yes, made this now more explicit:

kinds of fields: base and regular. Both the base fields and the
regular fields can be included in any SenML Record. The base fields
apply to the entries in the Record and also to all Records after it
up to, but not including, the next Record that has that same base
field. All base fields are optional. Regular fields can be included
in any SenML Record and apply only to that Record.

Post by Eric Rescorla

("vs" for "String Value") and binary data ("vd" for "Data Value").
Exactly one value field MUST appear unless there is Sum field in
which case it is allowed to have no Value field.
Sum: Integrated sum of the values over time. Optional. This field
is in the unit specified in the Unit value multiplied by seconds.

This text is hard to read, but the dimensional analysis seems
potentially wrong, for at least some measurements. I assume you're
thinking of something like watts and watt-hours. but if the value is
for instance bits, then "bit-seconds" is not a meaningful unit, and
yet you might want to sum these.

True. The name "sum" is actually not fully correct, but there are historical reasons why it's called so. Updated the text to reflect this:

Sum: Integrated sum of the values over time. Optional. This field
is in the unit specified in the Unit value multiplied by seconds.
For historical reason it is named sum instead of integral.

Also clarified more the use of the sum in Section 10.

Post by Eric Rescorla

The SenML format can be extended with further custom fields. Both
new base and regular fields are allowed. See Section 12.2 for
details. Implementations MUST ignore fields they don't recognize
unless that field has a label name that ends with the '_' character
in which case an error MUST be generated.

How does this map to CBOR? I see you explain this later, but here
would be helpful

We discussed this and concluded that the SenML is format agnostic in this matter so there should be no surprise here.

Post by Eric Rescorla

concatenated name MUST consist only of characters out of the set "A"
to "Z", "a" to "z", "0" to "9", "-", ":", ".", "/", and "_";
furthermore, it MUST start with a character out of the set "A" to
"Z", "a" to "z", or "0" to "9". This restricted character set was
chosen so that concatenated names can be used directly within various
URI schemes (including segments of an HTTP path with no special

Assuming I am understanding this correctly, then "/" is a problem
inside a path component.

Yes, clarified this with:

URI schemes (including segments of an HTTP path with no special
encoding; note that a name that contains "/" characters maps into
multiple URI path segments)

Post by Eric Rescorla

specified above puts strict limits on the URI schemes and URN
namespaces that can be used. As a result, implementers need to take
care in choosing the naming scheme for concatenated names, because
such names both need to be unique and need to conform to the
restricted character set. One approach is to include a bit string
that has guaranteed uniqueness (such as a 1-wire address). Some of

citation for 1-wire please.

Added informative reference:

[AN1796] Linke, B., "Overview of 1-Wire Technology and Its Use",
June 2008,
<http://pdfserv.maximintegrated.com/en/an/AN1796.pdf>.

Post by Eric Rescorla

| Encoding | Size | Compressed Size |
+----------+------+-----------------+
| JSON | 573 | 206 |
| XML | 649 | 235 |
| CBOR | 254 | 196 |
| EXI | 161 | 184 |

Compression not working out so well for EXI

Indeed.

Post by Eric Rescorla

To select a single SenML Record, the "rec" scheme followed by a
single number is used. For the purpose of numbering records, the
first record is at position 1. A range of records can be selected by
giving the first and the last record number separated by a '-'
character. Instead of the second number, the '*' character can be

This is an odd notation as * usually means "all". Why not just omit
the terminal #?

Yes, we re-used the fragment format from RFC7111 that was using the terminal "*":
https://tools.ietf.org/html/rfc7111#section-2.1

Seems to make sense to be consistent here, but no strong opinions.

Post by Eric Rescorla

New entries can be added to the registration by Expert Review as
defined in [RFC8126]. Experts should exercise their own good
judgment but need to consider that shorter labels should have more
strict review. New entries should not be made that counteract the
advice at the end of Section 4.4.

Note that you say earlier that you don't expect to define new CBOR
integers. That should probably be repeated here.

We had a second look at this and the recommendation two paragraphs earlier seemed to be quite sufficient to cover this. This paragraph was now slightly modified to reference new structure of Section 4 though.

Post by Eric Rescorla

Sensor data can range from information with almost no security
considerations, such as the current temperature in a given city, to
highly sensitive medical or location data. This specification
provides no security protection for the data but is meant to be used
inside another container or transport protocol such as S/MIME
[RFC5751] or HTTP with TLS [RFC5246] that can provide integrity,

This is the wrong citation for HTTP over TLS. That's RFC 2818.

Fixed.

Post by Eric Rescorla

We would like to thank Alexander Pelov, Alexey Melnikov, Andrew
McClure, Andrew McGregor, Bjoern Hoehrmann, Christian Amsuess,
Christian Groves, Daniel Peintner, Jan-Piet Mens, Jim Schaad, Joe
Hildebrand, John Klensin, Karl Palsson, Lennart Duhrsen, Lisa
Dusseault, Lyndsay Campbell, Martin Thomson, Michael Koster, Peter
Saint-Andre, Roni Even, and Stephen Farrell, for their review

Nit: no comma after Farrell

Fixed.