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Thing-to-Thing Research Group                                  K. Hartke
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Experimental                             March 13, 2017
Expires: September 14, 2017


          The Constrained RESTful Application Language (CoRAL)
                      draft-hartke-t2trg-coral-02

Abstract

   The Constrained RESTful Application Language (CoRAL) is a compact,
   binary representation format for building RESTful, hypermedia-driven
   applications that run in constrained environments.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 14, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Model . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Format  . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Options . . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Accept  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Deletable . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Format  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.4.  Href.*  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.5.  Method  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.6.  Relation  . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.7.  Title . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.8.  Updatable . . . . . . . . . . . . . . . . . . . . . . . .  10
   5.  Reference Resolution  . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Establish a Base URI  . . . . . . . . . . . . . . . . . .  11
     5.2.  Transform References  . . . . . . . . . . . . . . . . . .  11
     5.3.  Remove Dot Segments . . . . . . . . . . . . . . . . . . .  12
   6.  Profiles  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Media Type  . . . . . . . . . . . . . . . . . . . . . . .  14
     8.2.  Content Format  . . . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  18
     A.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  18
     A.2.  CoRE Lighting . . . . . . . . . . . . . . . . . . . . . .  23
     A.3.  CoRE Link Format  . . . . . . . . . . . . . . . . . . . .  24
     A.4.  CoRE Interfaces . . . . . . . . . . . . . . . . . . . . .  25
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  26
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  26

1.  Introduction

   Constrained RESTful Environments (CoRE) realize the Web architecture
   [W3C.REC-webarch-20041215] in a suitable form for constrained nodes
   and networks [RFC7228].

   In the Web, hypertext documents contain links and forms that allow a
   user to navigate between resources and submit information to a server
   for processing.  By annotating these elements with machine-readable
   link relation types [RFC5988] and form relation types, it is possible
   to extend this interaction model to machine-to-machine communication.





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   This document describes the Constrained RESTful Application Language
   (CoRAL), a compact serialization format for Web links and forms that
   is based on Concise Binary Object Representation (CBOR) [RFC7049] and
   that aligns closely with the Constrained Application Protocol (CoAP)
   [RFC7252].

1.1.  Terminology

   Readers are expected to be familiar with the terms and concepts
   described in [RFC5988] and [I-D.hartke-core-apps].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

2.  Model

   CoRAL is designed for building hypermedia-driven Web applications in
   which software agents navigate between resources by following links
   and interact with resources by submitting forms.

   Each agent maintains a "browsing context", an environment in which
   resource representations are processed.  (In the traditional Web, the
   browsing context corresponds to a tab or window in a Web browser.)  A
   browsing context has a session history, which lists the resources
   that the browsing context has visited, is visiting, or will visit.
   At any time, one resource in a browsing context is designated the
   "current" resource.  Following a link or submitting a form causes the
   browsing context to navigate to a new resource.

   A link indicates a relationship between two resources, the link
   context and the link target, and affords the navigation between
   these.  The semantics of the relationship are identified by a link
   relation type, which in CoRAL can be IANA-registered or application-
   specific.  To minimize round-trips, a link in CoRAL can optionally
   embed a (complete or partial) representation of the link target.
   Furthermore, a link target can be an anonymous resource in CoRAL; in
   this case, the link turns into a "literal" which consists only of a
   link relation type and a representation.

   A form similarly indicates a relationship between two resources, the
   form context and the form target, and affords the interaction with
   the context through the submission of the form to the target.  In
   many cases, the target of a form is the same resource as the context,
   but this is not required.  The semantics of a form are identified by
   a form relation type, which again in CoRAL can be IANA-registered or
   application-specific.



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   The submission of a form typically requires the agent to construct a
   payload that is included with the request.  For this purpose, a form
   indicates the acceptable content formats for the payload and can
   optionally embed a detailed description of the expected data, for
   example, as a list of form fields.  (The syntax for such a
   description is outside this document's scope.)

   The CoRAL interaction model is as follows:

   1.  The first step for an agent is to decide what to do next, i.e.,
       which type of link to follow or form to submit, based on the link
       relation types and form relation types it understands.

   2.  The agent finds the link(s) or form(s) with the given relation
       type in the current resource.  This may yield one or more
       candidates from which the agent must select the most appropriate
       one.  The set of candidates may be empty if the transition is not
       allowed, for example, when the agent is unauthorized.  The format
       of links and forms in CoRAL is specified in Section 3.

   3.  The agent selects one of the candidates based on the metadata
       associated with the link or form.  Metadata can include the
       content format of the target resource representation, the URI
       scheme, the request method and other attributes that describe the
       target.  Metadata is encoded in CoRAL as CoAP-style options,
       which are specified in Section 4.

   4.  The agent resolves the URI reference in the link or form to its
       absolute form in order to obtain the "request URI".  CoRAL
       encodes URI references like CoAP as a sequence of options, which
       significantly simplifies the implementation of URI processors
       compared to full RFC 3986 support.  The process of reference
       resolution is specified in Section 5.

   5.  The agent constructs a new request with the request URI.  If the
       agent follows a link, the request method is GET.  If the agent
       submits a form, the request method is indicated by an option.
       The agent SHOULD set request parameters according to the link/
       form attributes (e.g., set the CoAP Accept option when the
       content format of the target resource is indicated).  In the case
       of a form, the agent also needs to construct a request payload
       that matches the specifications of the form.

   6.  Finally, the agent sends the request and retrieves the response.
       The agent processes the enclosed representation, updates the
       browsing context to the new resource, and again can decide what
       to do next.




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   An agent can additionally navigate a browsing context by traversing
   the browsing context's session history.  The session history consists
   of a flat list of session history entries, which consist of a URI and
   may have other information associated with them.  Session history
   entries are added to the session history as the agent navigates from
   resource to resource.  An agent can traverse the session history by
   updating the browsing context to the resource for that entry.

3.  Format

   CoRAL can be used as a standalone representation format or embedded
   in representations in other formats.  As a standalone format, CoRAL
   representations have the media type 'application/coral'.  When CoRAL
   is embedded, it is typically embedded in CBOR-based representation
   formats, but other representation formats can embed CoRAL as well.
   The CoRAL format is in all cases the same.

   The top-level structure of CoRAL is called a CoRAL document.  A CoRAL
   document consists of a sequence of links, forms, literals and bases,
   which are collectively called elements.  All elements consist of a
   number indicating the element type, a "href type" that indicates how
   CoRAL-encoded URI references are to be interpreted in reference
   resolution, a sequence of zero or more options and, optionally, a
   body.

   Link, form and literal elements come in two flavors: a "fat" format
   that includes all the items listed above, and a "tiny" format.  The
   tiny formats provide a concise way to express elements that match
   certain patterns, which are specified below.  Base elements are
   always in the "fat" format.  They encode a base URI for reference
   resolution and apply to all subsequent elements until the next base
   element is encountered.

   In the Web, link relation types are identified by strings, such as
   'stylesheet', 'terms-of-service' or 'item'.  In order to minimize the
   overhead of using these relation types in constrained environments,
   [I-D.hartke-core-apps] extends the IANA Link Relation Types registry
   with a numeric identifier for each type.  CoRAL uses these numeric
   identifiers instead of the textual names.  The same optimization is
   applied to form relation types, CoAP content formats and CoAP request
   methods.

   Applications MAY use negative numbers to indicate application-
   specific link relation types and form relation types, which do not
   need to be IANA-registered.  The mapping from numbers to textual
   names needs to be provided by a CoRAL profile, which is indicated by
   the optional 'profile' parameter of the 'application/coral' media
   type (see Section 6).  The 'application/coral' media type without a



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   'profile' parameter does not define any application-specific values.
   A representation format embedding CoRAL documents MAY define
   application-specific values for the CoRAL documents as well.

   CoRAL defines a number of options that can be included in elements.
   Options are used to encode the relation type, the target resource URI
   and target attributes.  Options are serialized in CBOR as a sequence
   of unwrapped pairs where each pair consists of a CoRAL option number
   and an option value.  The pairs in a CoRAL element MUST be sorted
   such that the option numbers appear in ascending order.

   Using the notation of [I-D.greevenbosch-appsawg-cbor-cddl], the CoRAL
   data format can be expressed as follows:

      document      = [*element]
      element       = tiny-link / tiny-literal / tiny-form
                    / fat-link  / fat-literal  / fat-form
                    / base

      tiny-link     = [1, href-type, relation]
      tiny-literal  = [2, href-type, relation, format, body]
      tiny-form     = [3, href-type, relation, accept]
      base          = [4, href-type, options]
      fat-link      = [5, href-type, options, ?body]
      fat-literal   = [6, href-type, options, body]
      fat-form      = [7, href-type, options, ?body]

      href-type     = &(append-relation: 0,
                        absolute-path: 1,
                        append-path: 2,
                        relative-path: 3)

      relation      = int
      format        = uint
      accept        = uint
      options       = [*(option-number, option-value)]
      option-number = uint
      option-value  = uint / int / text / bytes
      body          = bytes

   The tiny formats expand as follows:

              [1, H, R]         ->   [5, H, [1, R]]
              [2, H, R, F, B]   ->   [6, H, [1, R, 4, F], B]
              [3, H, R, A]      ->   [7, H, [1, R, 3, A]]






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4.  Options

   Table 1 summarizes the CoRAL options defined in this document.

       +-----+---+----------------+--------+--------+-------------+
       | No. | R | Name           | Format | Length | Default     |
       +-----+---+----------------+--------+--------+-------------+
       |   1 | x | Relation       | int    |        | (none)      |
       |   2 |   | Method         | uint   |        | 2 (POST)    |
       |   3 | x | Accept         | uint   |        | (none)      |
       |   4 | x | Format         | uint   |        | (none)      |
       |   5 |   | Href.Scheme    | text   | 1-255  | (none)      |
       |   6 |   | Href.Host.Name | text   | 1-255  | (none)      |
       |   7 |   | Href.Host.IPv4 | bytes  | 4      | (none)      |
       |   8 |   | Href.Host.IPv6 | bytes  | 16     | (none)      |
       |   9 |   | Href.Port      | uint   |        | (see below) |
       |  10 | x | Href.Path      | text   | 0-255  | (none)      |
       |  11 | x | Href.Query     | text   | 0-255  | (none)      |
       |  12 |   | Href.Fragment  | text   | 0-255  | (none)      |
       |  13 |   | Title          | text   | 0-255  | (none)      |
       |  14 |   | Updatable      | bool   |        | false       |
       |  15 |   | Deletable      | bool   |        | false       |
       +-----+---+----------------+--------+--------+-------------+

                             Table 1: Options

   The option properties are defined as follows:

   Number:  An option is identified by an option number.

   Repeatable (R):  An option that is repeatable MAY be included one or
      more times in an element.  An option that is not repeatable MUST
      NOT be included more than once.  If an agent encounters an option
      with more occurrences than the option is defined for, each extra
      occurrence MUST be ignored.

   Format:  Option values are defined to have a certain format, which is
      the CBOR encoding of the specified type.

   Length:  Option values with types "text" and "bytes" are defined to
      have a specific length, often in the form of an upper and lower
      bound.  The length of an option value MUST NOT be outside the
      defined range.  If an agent encounters an option with a length
      outside the defined range, that option MUST be ignored.

   Default Value:  Options can be defined to have a default value.  If
      the value of an option is intended to be this default value, the




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      option SHOULD NOT be included in the element.  If the option is
      not present, the default value MUST be assumed.

   The semantics of the individual options are specified in the
   following sections.

4.1.  Accept

   The Accept Option indicates the acceptable content formats for the
   representation included in a form submission.  The option value of an
   Accept Option is one of the content format IDs registered in the CoAP
   Content-Formats registry.  If a form does not include an Accept
   Option, the service accepts any content format.

4.2.  Deletable

   The Deletable Option, when present in a link, defines a form that can
   be used to delete the link target.  The form relation type of that
   form is 'delete', the context and target of the form are identical to
   the target of the link, the submission method is DELETE and no
   representation must be submitted.

4.3.  Format

   The Format Option, when present in a link or a form, provides a hint
   indicating what the content format of the payload of the CoAP
   response should be when following the link or submitting the form.
   Note that this is only a hint; it does not override the Content-
   Format Option included in the CoAP response.  If the Format Option
   occurs more than once, an agent SHOULD set the Accept Option in the
   CoAP request message to request a particular content format.

   The Format Option is REQUIRED if a link embeds a representation in
   the link body.  The Format Option is also REQUIRED in a literal.  In
   both cases, the first occurrence of the option indicates the content
   format of the embedded representation; any additional occurrences
   indicate available alternative content formats.

   The option value of a Format Option is one of the content format IDs
   registered in the CoAP Content-Formats registry.

4.4.  Href.*

   The Href.Scheme, Href.Host.Name, Href.Host.IPv4, Href.IPv6,
   Href.Port, Href.Path, Href.Query and Href.Fragment Options are used
   to specify the target resource URI of a link or form.  They hold the
   following values:




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   o  the Href.Scheme Option specifies the URI scheme name,

   o  the Href.Host.Name Option specifies the host as a registered name,

   o  the Href.Host.IPv4 Option specifies the host as a 32-bit IPv4
      address,

   o  the Href.Host.IPv6 Option specifies the host as a 128-bit IPv6
      address,

   o  the Href.Port Option specifies the port number,

   o  each Href.Path Option specifies one segment of the path,

   o  each Href.Query Option specifies one argument of the query, and

   o  the Href.Fragment Option specifies the fragment identifier.

   The Href.Host.Name, Href.Host.IPv4 and Href.Host.IPv6 options are
   mutually exclusive.

   The default value of the Href.Port Option is the default port for the
   URI scheme.

   Table 2 lists the permitted Href.* options by Href Type.  A 'yes'
   indicates that an option of this type MAY be present; a 'no'
   indicates that an option of this type MUST NOT be present.  The
   resolution of a sequence of Href.* options against a base URI is
   specified in Section 5.

   +----------------+------------+------------+-----------+------------+
   |                | absolute-  | relative-  |  append-  |  append-   |
   |                |    path    |    path    |    path   |  relation  |
   +----------------+------------+------------+-----------+------------+
   | Href.Scheme    |    yes     |     no     |     no    |     no     |
   | Href.Host.Name |    yes     |     no     |     no    |     no     |
   | Href.Host.IPv4 |    yes     |     no     |     no    |     no     |
   | Href.Host.IPv6 |    yes     |     no     |     no    |     no     |
   | Href.Port      |    yes     |     no     |     no    |     no     |
   | Href.Path      |    yes     |    yes     |    yes    |     no     |
   | Href.Query     |    yes     |    yes     |    yes    |     no     |
   | Href.Fragment  |    yes     |    yes     |    yes    |     no     |
   +----------------+------------+------------+-----------+------------+

              Table 2: Permitted Href.* Options by Href Type






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4.5.  Method

   The Method Option, when present in a form, indicates the CoAP method
   to use for form submission.  The option value is one of the CoAP
   method codes registered in the CoAP Method Codes registry.  The
   option value defaults to the POST method when the Method Option is
   not present in a form.

4.6.  Relation

   The Relation Option indicates the link relation type of a link or
   literal and the form relation type of a form.  At least one Relation
   Option is REQUIRED in each link, literal and form element.

   In a link or literal, the option value of a Relation Option is either
   one of the link relation type IDs registered in the Link Relation
   Types registry (>= 0) or one of the application-specific link
   relation type IDs defined by the CoRAL profile (< 0).

   In a form, the option value of Relation Option is either one of the
   form relation type IDs registered in the Form Relation Types registry
   (>= 0) or one of the application-specific form relation type IDs
   defined by the CoRAL profile (< 0).

4.7.  Title

   The Title Option, when present, is used to label the target of a link
   such that it can be used as a human-readable identifier (e.g., a menu
   entry).

4.8.  Updatable

   The Updatable Option, when present in a link, defines a form that can
   be used to update the link target.  The form relation type of that
   form is 'update', the context and target of the form are identical to
   the target of the link, the submission method is PUT and the content
   format of the submitted representation must be one of the formats
   indicated by the Format Option in the link.

5.  Reference Resolution

   This section defines the process of resolving a URI reference within
   a link or form to an absolute URI suitable for inclusion in a CoAP
   request.







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5.1.  Establish a Base URI

   URI references can be relative and thus are only usable when a base
   URI is known.  This means that a base URI must be established before
   the use of all URI references that might be relative.

   The base URI of a reference in a link or form is established as
   specified in Section 5.1 of [RFC3986].  CoRAL supports a "Base URI
   Embedded in Content" in the form of base elements.  A base element
   applies to all subsequent elements in a document until the next base
   element is encountered.  The URI reference in a base element itself
   is resolved relative to the base URI of next lower precedence (which
   is usually the "URI used to retrieve the entity" but can be the "Base
   URI of the encapsulating entity").

5.2.  Transform References

   The following pseudocode describes an algorithm for transforming a
   URI reference R into its target URI T using the base URI B, the href
   type H, and the link or form relation type S.  The URI reference and
   base URI are assumed to be pre-parsed into a sequence of Href.*
   options; the result is returned as a sequence of Href.* options as
   well.

      if (R starts with Href.Scheme) then
         T = R
      elif (R starts with Href.Host.*) then
         T = [ (k, v) | (k, v) <- B, k == Href.Scheme ] ++
             [ (k, v) | (k, v) <- R, k >  Href.Scheme ]
      elif (R starts with Href.Port) then
         T = [ (k, v) | (k, v) <- B, k <  Href.Port ] ++
             [ (k, v) | (k, v) <- R, k >= Href.Port ]
      elif (H is append-relation) then
         T = [ (k, v) | (k, v) <- B, k <= Href.Path ] ++
             [ (Href.Path, (hex S)) ]
      elif (H is append-path) then
         T = [ (k, v) | (k, v) <- B, k <= Href.Path ] ++
             [ (k, v) | (k, v) <- R, k >= Href.Path ]
      elif (H is relative-path) then
         T = [ (k, v) | (k, v) <- B, k <  Href.Path ] ++
       (init [ (k, v) | (k, v) <- B, k == Href.Path ]) ++
             [ (k, v) | (k, v) <- R, k >= Href.Path ]
      elif (H is absolute-path) then
         T = [ (k, v) | (k, v) <- B, k <  Href.Path ] ++
             [ (k, v) | (k, v) <- R, k >= Href.Path ]
      endif





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   The "init" function returns all the elements of the input list except
   the last one.  For example, (init [1, 2, 3]) returns [1, 2] and (init
   []) returns [].

   The "hex" function returns a hexadecimal representation of the input
   number.  For example, (hex -421) returns "-1A5" and (hex 0) returns
   "0".

5.3.  Remove Dot Segments

   After transforming a the URI reference into its target URI, the
   special path segments "." and ".." need to be removed.  Although
   there are many ways to accomplish this removal process, we describe a
   simple method using two string buffers.

   1.  The input buffer is initialized with the sequence of path
       segments and the output buffer is initialized to the empty
       sequence.

   2.  While the input buffer is not empty, loop as follows:

       *  If the input buffer begins with a "." path segment, then
          remove this segment from the input buffer; otherwise,

       *  if the input buffer begins with a ".." path segment, then
          remove this segment from the input buffer and and remove the
          last segment (if any) from the output buffer; otherwise,

       *  move the first path segment in the input buffer to the end of
          the output buffer.

   3.  Finally, the sequence of path segments in the target URI is
       replaced by the sequence of path segments in the output buffer.

6.  Profiles

   Profiles provide an easy way to extend CoRAL with application-
   specific link relation types and form relation types.

   Application-specific link relation types and form relation types are
   encoded in CoRAL as negative numbers.  The meaning of these numbers
   is defined by the profile, which maps them to extension link and form
   relation types.  (An extension relation type is a URI [RFC3986] that
   uniquely identifies the relation type; see Section 4.2 of [RFC5988].)
   For example, a CoRAL profile for online shops could define the
   following set of application-specific link relation types:





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                +-----+----------------------------------+
                | ID  | Link Relation Type               |
                +-----+----------------------------------+
                | -80 | http://example.com/rels/order    |
                | -81 | http://example.com/rels/basket   |
                | -82 | http://example.com/rels/customer |
                +-----+----------------------------------+

                 Table 3: Example profile for online shops

   A good source for existing extension link relation types are RDF
   predicates [W3C.REC-rdf11-concepts-20140225].  An RDF statement says
   that some relationship, indicated by a predicate, holds between two
   resources.  Link relation types and RDF predicates can therefore be
   used interchangeably in many cases.  For example, a CoRAL profile
   could define the following mapping from link relation type numbers to
   the FOAF vocabulary [FOAF]:

               +------+------------------------------------+
               | ID   | RDF Predicate                      |
               +------+------------------------------------+
               | -100 | http://xmlns.com/foaf/0.1/name     |
               | -101 | http://xmlns.com/foaf/0.1/age      |
               | -102 | http://xmlns.com/foaf/0.1/homepage |
               +------+------------------------------------+

             Table 4: Example profile for the FOAF vocabulary

   CoRAL profiles are identified by a URI [RFC3986] and are indicated by
   the 'profile' parameter of the 'application/coral' media type,
   following the recommendations of [RFC6906], Section 3.1.  For
   example, the FOAF profile above could use the media type
   'application/coral; profile="http://xmlns.com/foaf/0.1/"'.

   The profile URI serves only as an identifier, similar to an XML
   namespace URI [W3C.REC-xml-names-20091208].  That is, the profile URI
   does not necessarily have to identify a dereferencable resource (or
   even use a dereferencable URI scheme).  It is possible, though, to
   use a dereferencable URI and serve a representation that provides
   information about the profile in a human- or machine-readable way.
   (The format of such a representation is outside the scope of this
   document.)

   For simplicity, a CoRAL document can use only at most one profile at
   the same time.  The 'profile' parameter of the 'application/coral'
   media type contains a single profile URI, not a whitespace-separated
   list of profile URIs as recommended by [RFC6906].




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   Profile URIs do not have to be registered.  The use of DNS names in
   them allows decentralized creation of new profile identifiers without
   the risk of collisions.  However, in CoAP the media type of a
   representation is indicated by a small numeric identifier, called the
   content format, not by a string of characters.  A content format has
   no internal structure and indicates a media type including specific
   values for its parameters.  For example, content format 0 indicates
   the media type 'text/plain; charset=utf-8'.  A media type 'text/
   plain; charset=iso-8859-1' would require a separate content format.
   Therefore, for use with CoAP, each CoRAL profiles needs to register a
   new content format in the "CoAP Content-Formats" registry, specifying
   the 'application/coral' media type and the profile URI as the value
   of the 'profile' parameter.

7.  Security Considerations

   TODO.

8.  IANA Considerations

8.1.  Media Type

   This document registers the media type 'application/coral' in the
   Media Types Registry.

   Type name:
      application

   Subtype name:
      coral

   Required parameters:
      N/A

   Optional parameters:

      profile
         See Section 6 of [RFCXXXX].

   Encoding considerations:
      CoRAL is a binary encoding.

   Security considerations:
      See Section 7 of [RFCXXXX].

   Interoperability considerations:
      There are no known interoperability issues.




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   Published specification:
      [RFCXXXX]

   Applications that use this media type:
      Hypermedia-driven Web applications that run on constrained nodes
      and networks.

   Fragment identifier considerations:
      N/A

   Additional information:

      Deprecated alias names for this type:  N/A

      Magic number(s):  N/A

      File extension(s):  .coral

      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:
      See "Author's Address" section of [RFCXXXX].

   Intended usage:
      COMMON

   Restrictions on usage:
      N/A

   Author:
      See "Author's Address" section of [RFCXXXX].

   Change controller:
      IESG

8.2.  Content Format

   This document registers a content format for the 'application/coral'
   media type in the CoAP Content-Formats Registry.

   Media type:
      application/coral

   Content coding:
      identity

   ID:
      TBD



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   Reference:
      [RFCXXXX]

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

   [RFC6906]  Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
              DOI 10.17487/RFC6906, March 2013,
              <http://www.rfc-editor.org/info/rfc6906>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <http://www.rfc-editor.org/info/rfc7049>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.

9.2.  Informative References

   [FOAF]     Brickley, D. and L. Miller, "FOAF Vocabulary Specification
              0.99", January 2014,
              <http://xmlns.com/foaf/spec/20140114.html>.

   [I-D.greevenbosch-appsawg-cbor-cddl]
              Vigano, C. and H. Birkholz, "CBOR data definition language
              (CDDL): a notational convention to express CBOR data
              structures", draft-greevenbosch-appsawg-cbor-cddl-09 (work
              in progress), September 2016.






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   [I-D.hartke-core-apps]
              Hartke, K., "CoRE Application Descriptions", draft-hartke-
              core-apps-07 (work in progress), February 2017.

   [I-D.hartke-core-lighting]
              Hartke, K., "CoRE Lighting", draft-hartke-core-lighting-00
              (work in progress), September 2015.

   [I-D.ietf-core-interfaces]
              Shelby, Z., Vial, M., Koster, M., and C. Groves, "Reusable
              Interface Definitions for Constrained RESTful
              Environments", draft-ietf-core-interfaces-08 (work in
              progress), February 2017.

   [I-D.ietf-core-links-json]
              Li, K., Rahman, A., and C. Bormann, "Representing CoRE
              Formats in JSON and CBOR", draft-ietf-core-links-json-06
              (work in progress), July 2016.

   [I-D.kelly-json-hal]
              Kelly, M., "JSON Hypertext Application Language", draft-
              kelly-json-hal-08 (work in progress), May 2016.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <http://www.rfc-editor.org/info/rfc6690>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <http://www.rfc-editor.org/info/rfc7228>.

   [W3C.REC-rdf11-concepts-20140225]
              Cyganiak, R., Wood, D., and M. Lanthaler, "RDF 1.1
              Concepts and Abstract Syntax", World Wide Web Consortium
              Recommendation REC-rdf11-concepts-20140225, February 2014,
              <http://www.w3.org/TR/2014/REC-rdf11-concepts-20140225>.

   [W3C.REC-turtle-20140225]
              Prud&#039;hommeaux, E. and G. Carothers, "RDF 1.1 Turtle",
              World Wide Web Consortium Recommendation REC-turtle-
              20140225, February 2014,
              <http://www.w3.org/TR/2014/REC-turtle-20140225>.








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   [W3C.REC-webarch-20041215]
              Jacobs, I. and N. Walsh, "Architecture of the World Wide
              Web, Volume One", World Wide Web Consortium
              Recommendation REC-webarch-20041215, December 2004,
              <http://www.w3.org/TR/2004/REC-webarch-20041215>.

   [W3C.REC-xml-names-20091208]
              Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
              Thompson, "Namespaces in XML 1.0 (Third Edition)", World
              Wide Web Consortium Recommendation REC-xml-names-20091208,
              December 2009,
              <http://www.w3.org/TR/2009/REC-xml-names-20091208>.

Appendix A.  Examples

A.1.  Overview

A.1.1.  Links

   At its core, CoRAL is just yet another serialization format for Web
   links.  For example, the following Web link (in RFC 5988 syntax):

      <coap://example.com/info/tos>;rel=terms-of-service;type=text/plain

   can be serialized in CoRAL as the following CBOR data item (in CBOR
   extended diagnostic format; text enclosed in forward slashes are
   comments):

      [ [ 5,                     / fat-link                            /
          1,                     / absolute-path                       /
          [  1, 70,              /  relation       = terms-of-service  /
             4, 0,               /  format         = text\plain        /
             5, "coap",          /  href.scheme    = "coap"            /
             6, "example.com",   /  href.host.name = "example.com"     /
            10, "info",          /  href.path      = "info"            /
            10, "tos" ] ] ]      /  href.path      = "tos"             /

   Multiple links are serialized as items of the top-level array:













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      [ [ 5,                     / fat-link                            /
          3,                     / relative-path                       /
          [  1, 26,              /  relation  = first                  /
             4, 0,               /  format    = text\plain             /
            10, "page1" ] ],     /  href.path = "page1"                /
        [ 5,                     / fat-link                            /
          3,                     / relative-path                       /
          [  1, 55,              /  relation  = previous               /
             4, 0,               /  format    = text\plain             /
            10, "page6" ] ],     /  href.path = "page6"                /
        [ 5,                     / fat-link                            /
          3,                     / relative-path                       /
          [  1, 41,              /  relation  = next                   /
             4, 0,               /  format    = text\plain             /
            10, "page8" ] ],     /  href.path = "page8"                /
        [ 5,                     / fat-link                            /
          3,                     / relative-path                       /
          [  1, 34,              /  relation  = last                   /
             4, 0,               /  format    = text\plain             /
            10, "page42" ] ] ]   /  href.path = "page42"               /

   The Format Option, when present, is a hint indicating what the CoAP
   content format of the result of dereferencing the link should be.  If
   more than one format is available, the Format Option can be repeated:

      [ [ 5,                     / fat-link                            /
          3,                     / relative-path                       /
          [  1, 33,              /  relation  = item                   /
             4, 47,              /  format    = application\exi        /
             4, 50,              /  format    = application\json       /
             4, 60,              /  format    = application\cbor       /
            10, "item1" ] ] ]    /  href.path = "item1"                /

A.1.2.  Embedding

   If a representation links to many resources, it may be inefficient to
   retrieve a representation of each link target individually.  For this
   reason, CoRAL supports the embedding of a representation of the link
   target in the link itself:












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      [ [ 5,                     / fat-link                            /
          3,                     / relative-path                       /
          [  1, 33,              /  relation  = item                   /
             4, 50,              /  format    = application\json       /
            10, "item1" ],       /  href.path = "item1"                /
          h'7b20227461736b223a20
            2252657475726e207468
            6520626f6f6b7320746f
            20746865206c69627261
            7279222c202261737369
            676e6565223a2022416c
            69636522207d' ] ]

   where the byte string in this example encodes the following JSON
   object:

      {
        "task":     "Return the books to the library",
        "assignee": "Alice"
      }

   By embedding representations, it is possible to use CoRAL as a (very
   basic) substitute for RDF [W3C.REC-rdf11-concepts-20140225].  For
   example, the RDF graph (in Turtle [W3C.REC-turtle-20140225] syntax):

      @prefix foaf: <http://xmlns.com/foaf/0.1/> .

      <> foaf:name     "John Doe" ;
         foaf:age      32 ;
         foaf:homepage <coap://www.doe.example/> .

   can be serialized in CoRAL as follows:



















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      [ [ 2,                     / tiny-literal                        /
          2,                     / append-path                         /
          -100,                  /  relation       = name              /
          0,                     /  format         = text\plain        /
          h'4a6f686e20446f65' ], / "John Doe"                          /
        [ 2,                     / tiny-literal                        /
          2,                     / append-path                         /
          -101,                  /  relation       = age               /
          7,                     /  format         = uint8             /
          h'20' ],               / 32                                  /
        [ 5,                     / fat-link                            /
          1,                     / absolute-path                       /
          [ 1, -102,             /  relation       = homepage          /
            5, "coap",           /  href.scheme    = "coap"            /
            6, "www.doe.example" /  href.host.name = "www.doe.example" /
      ] ] ]

A.1.3.  Forms

   In addition to Web links, CoRAL supports forms.  An agent can use a
   form to perform an operation on the form context, such as updating a
   resource or creating a new item in a collection.

   Similar to link relation types, the semantics of a form are indicated
   by the form relation type.  The Href.* Options encode the URI of the
   form target to which the agent should submit the form.  A form
   additionally encodes the submission method (POST, PUT, PATCH, DELETE)
   and the description of a representation that the service expects as
   part of form submission:

      [ [ 7,                     / fat-form                            /
          3,                     / relative-path                       /
          [  1, 1,               /  relation  = create-item            /
             2, 2,               /  method    = POST                   /
             3, 60,              /  accept    = application\cbor       /
            10, "items" ] ] ]    /  href.path = "items"                /

   The Accept Option specifies the content format of the expected
   representation.  A content format can use the body of a form to
   describe the expected representation in more detail, for example, by
   specifying a set of form fields that the agent needs to fill out:










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      [ [ 7,                     / fat-form                            /
          3,                     / relative-path                       /
          [  1, 1,               /  relation  = create-item            /
             2, 2,               /  method    = POST                   /
             3, 65535,           /  accept    = example\form           /
            10, "items" ],       /  href.path = "items"                /
          h'6e616d652c206167652c / "name, age, homepage"               /
            20686f6d6570616765' ] ]

A.1.4.  Editing

   The target resource of a link may be editable.  In this case, the
   representation of such a resource typically contains one or more
   forms that allow an agent to edit the resource.  However, it may be
   inefficient to include these forms every time a representation of the
   link target is retrieved and more efficient to include them in
   representations that link to that resource.  CoRAL supports this with
   two options.

   Setting the Updatable Option in a link to true defines a form that
   can be used to update the target resource.  The context and target of
   that form are both the target of the link, the submission method is
   PUT and the content format of the submitted representation must be
   one of the formats indicated by the Format Option in the link.  For
   example, given the following CoRAL representation, an agent can
   change the recipient by making a PUT request to <./to> with the new
   value in 'text/plain' format:
























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      [ [ 5,                     / fat-link                            /
          3,                     / relative-path                       /
          [  1, -120,            /  relation  = sender                 /
             4, 0,               /  format    = text\plain             /
            10, "from",          /  href.path = "from"                 /
            14, true ],          /  updatable = true                   /
          h'4a756c696574' ],     / "Juliet"                            /
        [ 5,                     / fat-link                            /
          3,                     / relative-path                       /
          [  1, -121,            /  relation  = recipient              /
             4, 0,               /  format    = text\plain             /
            10, "to",            /  href.path = "to"                   /
            14, true ],          /  updatable = true                   /
          h'526f6d656f' ],       / "Romeo"                             /
        [ 5,                     / fat-link                            /
          3,                     / relative-path                       /
          [  1, -122,            /  relation  = message                /
             4, 0,               /  format    = text\plain             /
            10, "message",       /  href.path = "message"              /
            14, true ],          /  updatable = true                   /
          h'4172742074686f75206e / "Art thou not Romeo,                /
            6f7420526f6d656f2c20 /  and a Montague?"                   /
            616e642061204d6f6e74
            616775653f' ] ]

   Setting the Deletable Flag in a link to true likewise defines a form
   that can be used to delete the target resource.

A.2.  CoRE Lighting

   CoRE Lighting [I-D.hartke-core-lighting] defines a benchmark scenario
   for the exploration of hypermedia-oriented design in constrained,
   RESTful environments.  The bulletin board example in Section 5.2.1 of
   [I-D.hartke-core-lighting] can be encoded in CoRAL as follows:

     [[7, 3, [1, 1, 3, 65200, 10, "bulletins"]],
      [5, 1, [1, 33, 4, 65200, 6, "light-bulb.example"], <<1>>],
      [5, 1, [1, 33, 4, 65200, 6, "remote-control.example"], <<2>>]]

   where <<1>> is a byte string that encodes the following CoRAL
   structure:

     [[5, 3, [1, -100, 4, 65202, 10, "config"]],
      [2, 3, -101, 0, "Light 2"],
      [2, 3, -102, 0, "Illuminates the couch."],
      [2, 3, -103, 0, "Living Room"]]





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   and <<2>> is a byte string that encodes the following CoRAL
   structure:

     [[5, 3, [1, 1, 4, 65203, 10, "state"]],
      [2, 3, -101, 0, "LRC 1"],
      [2, 3, -102, 0, "Controls Light 2."],
      [2, 3, -103, 0, "Living Room"]]

   Table 5 shows a comparison of sizes of the example encoded in CoRAL
   and JSON.

                          +--------+-----------+
                          | Format | Size      |
                          +--------+-----------+
                          | JSON   | 515 bytes |
                          | CoRAL  | 245 bytes |
                          +--------+-----------+

                         Table 5: Size Comparison

A.3.  CoRE Link Format

   The example in this section is based on an example on page 14 of
   [RFC6690]:

      </sensors>;ct=40;title="Sensor Index",
      </sensors/temp>;rt="temperature-c";if="sensor",
      </sensors/light>;rt="light-lux";if="sensor",
      <http://www.example.com/sensors/t123>;anchor="/sensors/temp"
      ;rel="describedby",
      </t>;anchor="/sensors/temp";rel="alternate"

   The example can be encoded in CoRAL as follows:

     [[4, 1, [10, "sensors"]],
      [5, 2, [4, 40, 13, "Sensor Index"]],
      [5, 2, [10, "temp", 16, "temperature-c", 17, "sensor"]],
      [5, 2, [10, "light", 16, "light-lux", 17, "sensor"]],
      [5, 1, [1, 18, 5, "http", 6, "www.example.com",
              10, "sensors", 10, "t123", 18, "/sensors/temp"]],
      [5, 1, [1, 2, 10, "t", 18, "/sensors/temp"]]]

   Table 6 shows a comparison of sizes of the example encoded in CoRAL
   and a number of Link Format variants [I-D.ietf-core-links-json].







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                    +--------------------+-----------+
                    | Format             | Size      |
                    +--------------------+-----------+
                    | Link Format        | 251 bytes |
                    | Link Format (JSON) | 320 bytes |
                    | Link Format (CBOR) | 203 bytes |
                    | CoRAL              | 181 bytes |
                    +--------------------+-----------+

                         Table 6: Size Comparison

A.4.  CoRE Interfaces

   The example in this section is based on an example in figure 1 of
   [I-D.ietf-core-interfaces]:

      </s/>;rt="simple.sen";if="core.b",
      </s/lt>;rt="simple.sen.lt";if="core.s",
      </s/tmp>;rt="simple.sen.tmp";if="core.s";obs,
      </s/hum>;rt="simple.sen.hum";if="core.s",
      </a/>;rt="simple.act";if="core.b",
      </a/1/led>;rt="simple.act.led";if="core.a",
      </a/2/led>;rt="simple.act.led";if="core.a",
      </d/>;rt="simple.dev";if="core.ll",
      </l/>;if="core.lb"

   The example can be encoded in CoRAL as follows:

     [[5, 1, [10, "d", 10, "", 16, "simple.dev", 17, "core.ll"]],
      [5, 1, [10, "l", 10, "", 17, "core.lb"]],
      [4, 1, [17, "core.b"]],
      [5, 2, [10, "s", 10, "", 16, "simple.sen"]],
      [5, 2, [10, "a", 10, "", 16, "simple.act"]],
      [4, 1, [10, "s", 17, "core.s"]],
      [5, 2, [10, "lt", 16, "simple.sen.lt"]],
      [5, 2, [10, "tmp", 16, "simple.sen.tmp", 19, true]],
      [5, 2, [10, "hum", 16, "simple.sen.hum"]],
      [4, 1, [10, "a", 16, "simple.act.led", 17, "core.a"]],
      [5, 2, [10, "1", 10, "led"]],
      [5, 2, [10, "2", 10, "led"]]]

   Table 7 shows a comparison of sizes of the example encoded in CoRAL
   and a number of Link Format variants.








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                    +--------------------+-----------+
                    | Format             | Size      |
                    +--------------------+-----------+
                    | Link Format        | 332 bytes |
                    | Link Format (JSON) | 456 bytes |
                    | Link Format (CBOR) | 264 bytes |
                    | CoRAL              | 248 bytes |
                    +--------------------+-----------+

                         Table 7: Size Comparison

Acknowledgements

   This specification is heavily inspired by the JSON Hypertext
   Application Language (HAL) [I-D.kelly-json-hal]; the author of and
   contributors to that specification are acknowledged for their great
   work.

   Yassin Nasir Hassan suggested placing the hypermedia controls for
   modifying a link target in the link context rather than in the
   representation of the link target.

   Thanks to Carsten Bormann, Jaime Jimenez and Matthias Kovatsch for
   helpful comments and discussions that have shaped the document.

Author's Address

   Klaus Hartke
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63905
   Email: hartke@tzi.org
















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