Last Call Review of draft-ietf-core-links-json-07
review-ietf-core-links-json-07-genart-lc-davies-2017-04-25-00

Request Review of draft-ietf-core-links-json
Requested rev. no specific revision (document currently at 10)
Type Last Call Review
Team General Area Review Team (Gen-ART) (genart)
Deadline 2017-04-21
Requested 2017-04-07
Authors Kepeng Li, Akbar Rahman, Carsten Bormann
Draft last updated 2017-04-25
Completed reviews Artart Last Call review of -07 by Mark Nottingham (diff)
Secdir Last Call review of -07 by Paul Wouters (diff)
Genart Last Call review of -07 by Elwyn Davies (diff)
Assignment Reviewer Elwyn Davies
State Completed
Review review-ietf-core-links-json-07-genart-lc-davies-2017-04-25
Reviewed rev. 07 (document currently at 10)
Review result Not Ready
Review completed: 2017-04-25

Review
review-ietf-core-links-json-07-genart-lc-davies-2017-04-25

I am the assigned Gen-ART reviewer for this draft. The General Area
Review Team (Gen-ART) reviews all IETF documents being processed
by the IESG for the IETF Chair.  Please treat these comments just
like any other last call comments.

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Document: draft-ietf-core-links-json-07
Reviewer: Elwyn Davies
Review Date: 2017-04-25
IETF LC End Date: 2017-04-21
IESG Telechat date: 2017-04-27

Summary:Not ready for publication.  There a number of issues that need to be addressed  as discussed below.  In particular whether the formats could be returned as the web link specification instead of RFC 6990 format in response to a GET /.well-known/core request.

Having thought about the quote stripping/addition issue cited in Adam Roach's DISCUSS, I would take a slightly different view... see below.

Major issues:

Intentions:
Is it one of the intentions of this draft that a server should be able to return web link descriptions using JSON or CBOR, specifically in response to GET /.well-known/core?  Content formats for the new formats are registered (s3.2) - could a user ask for the alternative formats by specifying at ct filter with the GET request?  It strikes me that if one has a constrained server of sufficiently limited capabilities that it wants to use CBOR then having to encode the RFC 6690 format responses for the web links requests is wasting resources.  Some more thought needs to be given to this as an update to RFC 6690 - If I read correctly, RFC 6690 implicitly requires that a response to GET /.well-known/core MUST be encoded as described in RFC6690.

Minor issues:
Title:  The document appears only to address the CoRE Web Links format rather than any other.  Should the title reflect this more precisely, e.g.,
      Representing CoRE Web Links Format in JSON and CBOR

s1.1: Concerning:
   o  The simplest thing that could possibly work

      *  Do not cater for RFC 5988 complications caused by HTTP header
         character set issues [RFC2047]

Having ferreted around in RFC 5988 and RFC 2047, I can't see what is being referred to here.  However, I observe later that the "title*" attribute (with language specifier) does not appear to be supported (It is missing from Table 1) - is this what is relevant here? If so it needs clarification. 
[Aside: I notice that the relevant ABNF in s5 of RFC 5998 is missing external references to various productions (e.g., ext-value, quoted-string) that are defined in other documents - in the given examples RFC 2987, RFC 2616.]

s2.2/s5: This statement:
   The resulting structure can be represented in CDDL
   [I-D.greevenbosch-appsawg-cbor-cddl] as:

requires that the CDDL draft is a normative reference rather than informative.
[Aside:  Having skimmed the CDDL draft, I am of the opinion that a good deal more work will be needed to get this ready for publication, possibly to the extent that the CDDL quoted here becomes invalid.  Given the simplicity of the specification could it be done without the use of CDDL?] 

s2.2: There needs to be some discussion of handling of double quoted and non-double quoted strings during conversion:
I think it works to require that...
From RFC 6690 to JSON:
- If the parameter value is a double quoted string then it should have the double quotes stripped, any necessary JSON character encodings performed and the double quotes repapplied.
- if the parameter value is anything else, then the necessary JSON character encodings are done and the result enclosed in double quotes.
[what about % encodings on the RFC 6690 side?]

From RFC 6690 to CBOR:
- If the parameter value is a double quoted string, the double quotes are stripped and the result used as the CBOR string type value.
- Otherwise, the parameter value is used as the CBOR string value. 
[what about % encodings on the RFC 6690 side?]

From JSON to RFC 6690: 
- Remove the double quotes from the JSON string value and do any necessary decoding and encoding.  Reapply double quotes.  Note that this may result in values that were originally not enclosed in double quotes in the RFC 6690 repreentation becoming enclosed in double quotes. However, [AFAICS] this does not alter the semantics of any of the predefined parameters.  For example the ABNF productions mean that ct=40 and ct="40" are equivalent (the second case is needed so that one can also have ct="40 41 42").  What IS needed is a statement that this must also apply to any application specific parameters.  For example the case in examples 4 and 5 of ..;foo="bar";foo=3;... transforming to "foo":["bar","3"] and then back to ...;foo="bar";foo="3";.. MUST require that the two RFC 6690 representations are equivalent.

From CBOR to RFC 6690: 
[Essentially the same process - decode/encode and apply double quotes. The discussion of equivalent semantics is equally applicable.]

The conversion from CBOR to JSON or in reverse is similar. 

s2.3: It is not stated whether a CBOR decoder should accept literal use of the encodable parameters - i.e., if the encoded CBOR contains [ "href": "/mumble" ] rather than [1 : "mumbleĀ£ ] in CDDL format.  Similarly, should the use of the encoded values be mandatory on the CBOR encoder?

s2.3, Table 1:  Is the omission of title* from the list of parameter names deliberate?  If so the omission justifies a note and rationale.  Clearly the format of the value for a title* parameter is different from all the others, which may have something to do with this.

s2.3/s5: eEfereences in Table 1 make RFC 7252 and RFC 7641 normative.

Nits/editorial comments: 
s1: s/e.g. /e.g., / (two places)

s1.1: The term "round-tripping" and the associated text are opaque jargon that would normally  be applied to message transmission round a loop rather than format conversion.  A more explicit formulation would help naive readers.  Suggest (if I understand what was intended):
OLD:
   o  Canonical mapping

      *  lossless round-tripping with [RFC6690] and between JSON and
         CBOR

      *  but not trying for bit-preserving (DER-style) round-tripping
NEW:
   o  Canonical mapping

      *  supporting inter-conversion in both directions between any pair 
         of [RFC6690] format and the CBOR and JSON formats defined here 
         with unaltered and unambiguous semantics

      *  but not attempting to ensure that a sequence of conversions from 
         one of the formats through one or both of the others and back to 
         the original would result in an identical representation (c.f., 
         as might be achieved by different BER transcoders rather than by all 
         DER transcoders with ASN.1 [X.690]).
ENDS
This needs an informative reference to X.690 ... but I am not sure that the DER comparison is essential.

s2.2:  Suggest:
OLD:
   We straightforwardly map:

   o  the outer collection to an array of links;

   o  each link to a JSON object or CBOR map, mapping attribute names to
      attribute values.

NEW:
   We straightforwardly map:

   o  the outer collection to an array of parameterized web links;

   o  each parameterized web link to a JSON object or CBOR map, mapping attribute names to
      attribute values.
ENDS

s2.2:
OLD:
The resulting structure can be represented in CDDL
   [I-D.greevenbosch-appsawg-cbor-cddl] as:
NEW:
The resulting structure can be represented in CBOR Data Definition Language (CDDL)
   [I-D.greevenbosch-appsawg-cbor-cddl] as shown in Figure 1.
 
s2.4: Note that the use of ct=40 in RFC 6690 is an anchronism.  The ct parameter appeared in earlier versions of the draft that led to RFC 6690 but was moved out to be used more generally in CoAP and is actually defined in RFC 7252 as mentioned in Table 1 here.  Thus use of ct=40 in the example copied from RFC 6690 really needs an erratum for 6690 but that is for another day!