Genart last call review of draft-ietf-core-links-json-07

[Elwyn Davies <[e-mail]>]

Reviewer: Elwyn Davies
Review result: Not Ready

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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 : "mumble£ ] 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!