Constrained Resource Identifiers
draft-ietf-core-href-08

CoRE Working Group                                       C. Bormann, Ed.
Internet-Draft                                    Universität Bremen TZI
Intended status: Standards Track                             H. Birkholz
Expires: 11 May 2022                                      Fraunhofer SIT
                                                         7 November 2021

                    Constrained Resource Identifiers
                        draft-ietf-core-href-08

Abstract

   The Constrained Resource Identifier (CRI) is a complement to the
   Uniform Resource Identifier (URI) that serializes the URI components
   in Concise Binary Object Representation (CBOR) instead of a sequence
   of characters.  This simplifies parsing, comparison and reference
   resolution in environments with severe limitations on processing
   power, code size, and memory size.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Constrained RESTful
   Environments Working Group mailing list (core@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/href.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 May 2022.

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Copyright Notice

   Copyright (c) 2021 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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   extracted from this document must include Simplified BSD License text
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Constraints . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Constraints by example  . . . . . . . . . . . . . . . . .   5
     2.2.  Constraints not expressed by the data model . . . . . . .   6
   3.  Creation and Normalization  . . . . . . . . . . . . . . . . .   7
   4.  Comparison  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  CRI References  . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  CBOR Serialization  . . . . . . . . . . . . . . . . . . .   9
     5.2.  Ingesting and encoding a CRI Reference  . . . . . . . . .  12
     5.3.  Reference Resolution  . . . . . . . . . . . . . . . . . .  12
   6.  Relationship between CRIs, URIs and IRIs  . . . . . . . . . .  13
     6.1.  Converting CRIs to URIs . . . . . . . . . . . . . . . . .  14
   7.  Extended CRI: Accommodating Percent Encoding  . . . . . . . .  16
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .  17
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     11.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  19
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  20
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The Uniform Resource Identifier (URI) [RFC3986] and its most common
   usage, the URI reference, are the Internet standard for linking to
   resources in hypertext formats such as HTML [W3C.REC-html52-20171214]
   or the HTTP "Link" header field [RFC8288].

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   A URI reference is a sequence of characters chosen from the
   repertoire of US-ASCII characters.  The individual components of a
   URI reference are delimited by a number of reserved characters, which
   necessitates the use of a character escape mechanism called "percent-
   encoding" when these reserved characters are used in a non-delimiting
   function.  The resolution of URI references involves parsing a
   character sequence into its components, combining those components
   with the components of a base URI, merging path components, removing
   dot-segments, and recomposing the result back into a character
   sequence.

   Overall, the proper handling of URI references is quite intricate.
   This can be a problem especially in constrained environments
   [RFC7228], where nodes often have severe code size and memory size
   limitations.  As a result, many implementations in such environments
   support only an ad-hoc, informally-specified, bug-ridden, non-
   interoperable subset of half of RFC 3986.

   This document defines the _Constrained Resource Identifier (CRI)_ by
   constraining URIs to a simplified subset and serializing their
   components in Concise Binary Object Representation (CBOR) [RFC8949]
   instead of a sequence of characters.  This allows typical operations
   on URI references such as parsing, comparison and reference
   resolution (including all corner cases) to be implemented in a
   comparatively small amount of code.

   As a result of simplification, however, CRIs are not capable of
   expressing all URIs permitted by the generic syntax of RFC 3986
   (hence the "constrained" in "Constrained Resource Identifier").  The
   supported subset includes all URIs of the Constrained Application
   Protocol (CoAP) [RFC7252], most URIs of the Hypertext Transfer
   Protocol (HTTP) [RFC7230], Uniform Resource Names (URNs) [RFC8141],
   and other similar URIs.  The exact constraints are defined in
   Section 2.

1.1.  Notational Conventions

   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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   In this specification, the term "byte" is used in its now customary
   sense as a synonym for "octet".

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   Terms defined in this document appear in _cursive_ where they are
   introduced (rendered in plain text as the new term surrounded by
   underscores).

2.  Constraints

   A Constrained Resource Identifier consists of the same five
   components as a URI: scheme, authority, path, query, and fragment.
   The components are subject to the following constraints:

   C1.   The scheme name can be any Unicode string (see Definition D80
         in [Unicode]) that matches the syntax of a URI scheme (see
         Section 3.1 of [RFC3986], which constrains schemes to ASCII)
         and is lowercase (see Definition D139 in [Unicode]).  The
         scheme is always present.

   C2.   An authority is always a host identified by an IP address or
         registered name, along with optional port information.  User
         information is not supported.

         Alternatively, the authority can be absent; the two cases for
         this defined in Section 3.3 of [RFC3986] are modeled by two
         different values used in place of an absent authority:

         *  the path can begin with a root ("/", as when the authority
            is present), or

         *  the path can be rootless.

   C3.   An IP address can be either an IPv4 address or an IPv6 address,
         optionally with a zone identifier [RFC6874].  Future versions
         of IP are not supported.

   C4.   A registered name is a sequence of one or more _labels_, which,
         when joined with dots (".") in between them, result in a
         Unicode string that is lowercase and in Unicode Normalization
         Form C (NFC) (see Definition D120 in [Unicode]).  (The syntax
         may be further restricted by the scheme.)

   C5.   A port is always an integer in the range from 0 to 65535.
         Ports outside this range, empty ports (port subcomponents with
         no digits, see Section 3.2.3 of [RFC3986]), or ports with
         redundant leading zeros, are not supported.

   C6.   The port is omitted if and only if the port would be the same
         as the scheme's default port (provided the scheme is defining
         such a default port) or the scheme is not using ports.

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   C7.   A path consists of zero or more path segments.  A path must not
         consist of a single zero-length path segment, which is
         considered equivalent to a path of zero path segments.

   C8.   A path segment can be any Unicode string that is in NFC, with
         the exception of the special "." and ".." complete path
         segments.  It can be the zero-length string.  No special
         constraints are placed on the first path segment.

   C9.   A query always consists of one or more query parameters.  A
         query parameter can be any Unicode string that is in NFC.  It
         is often in the form of a "key=value" pair.  When converting a
         CRI to a URI, query parameters are separated by an ampersand
         ("&") character.  (This matches the structure and encoding of
         the target URI in CoAP requests.)  Queries are optional; there
         is a difference between an absent query and a single query
         parameter that is the empty string.

   C10.  A fragment identifier can be any Unicode string that is in NFC.
         Fragment identifiers are optional; there is a difference
         between an absent fragment identifier and a fragment identifier
         that is the empty string.

   C11.  The syntax of registered names, path segments, query
         parameters, and fragment identifiers may be further restricted
         and sub-structured by the scheme.  There is no support,
         however, for escaping sub-delimiters that are not intended to
         be used in a delimiting function.

   C12.  When converting a CRI to a URI, any character that is outside
         the allowed character range or is a delimiter in the URI syntax
         is percent-encoded.  For CRIs, percent-encoding always uses the
         UTF-8 encoding form (see Definition D92 in [Unicode]) to
         convert the character to a sequence of bytes (that is then
         converted to a sequence of %HH triplets).

2.1.  Constraints by example

   While most URIs in everyday use can be converted to CRIs and back to
   URIs matching the input after syntax-based normalization of the URI,
   these URIs illustrate the constraints by example:

   *  https://host%ffname, https://example.com/x?data=%ff

      All URI components must, after percent decoding, be valid UTF-8
      encoded text.  Bytes that are not valid UTF-8 show up, for
      example, in BitTorrent web seeds.

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   *  https://example.com/component%3bone;component%3btwo,
      http://example.com/component%3dequals

      While delimiters can be used in an escaped and unescaped form in
      URIs with generally distinct meanings, CRIs only support one
      escapable delimiter character per component, which is the
      delimiter by which the component is split up in the CRI.

      Note that the separators . (for authority parts), / (for paths), &
      (for query parameters) are special in that they are syntactic
      delimiters of their respective components in CRIs.  Thus, the
      following examples _are_ convertible to CRIs:

      https://interior%2edot/

      https://example.com/path%2fcomponent/second-component

      https://example.com/x?ampersand=%26&questionmark=?

   *  https://alice@example.com/

      The user information can not be expressed in CRIs.

   *  URIs with an authority but a completely empty path (eg.
      http://example.com)

      CRIs with an authority component always produce at least a slash
      in the path component.

      For generic schemes, the conversion of scheme://example.com to a
      CRI is impossible because no CRI produces a URI with an authority
      not followed by a slash following the rules of Section 6.1.  Most
      schemes do not distinguish between the empty path and the path
      containing a single slash when an authority is set (as recommended
      in [RFC3986]).  For these schemes, that equivalence allows
      converting even the slash-less URI to a CRI (which, when converted
      back, produces a slash after the authority).

2.2.  Constraints not expressed by the data model

   There are syntactically valid CRIs and CRI references that can not be
   converted into a URI or URI reference, respectively.

   For CRI references, this is acceptable -- they can be resolved still
   and result in a valid CRI that can be converted back.  (An example of
   this is [0, ["p"]] which appends a slash and the path segment "p" to
   its base).

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   (Full) CRIs that do not correspond to a valid URI are not valid on
   their own, and can not be used.  Normatively they are characterized
   by the Section 6.1 process producing a valid and syntax-normalized
   URI.  For easier understanding, they are listed here:

   *  CRIs (and CRI references) containing a path component "." or "..".

      These would be removed by the remove_dot_segments algorithm of
      [RFC3986], and thus never produce a normalized URI after
      resolution.

      (In CRI references, the discard value is used to afford segment
      removal, and with "." being an unreserved character, expressing
      them as "%2e" and "%2e%2e" is not even viable, let alone
      practical).

   *  CRIs without authority whose path starts with two or more empty
      segments.

      When converted to URIs, these would violate the requirement that
      in absence of an authority, a URI's path can not begin with two
      slash characters, and they would be indistinguishable from a URI
      with a shorter path and a present but empty authority component.

3.  Creation and Normalization

   In general, resource identifiers are created on the initial creation
   of a resource with a certain resource identifier, or the initial
   exposition of a resource under a particular resource identifier.

   A Constrained Resource Identifier SHOULD be created by the naming
   authority that governs the namespace of the resource identifier (see
   also [RFC8820]).  For example, for the resources of an HTTP origin
   server, that server is responsible for creating the CRIs for those
   resources.

   The naming authority MUST ensure that any CRI created satisfies the
   constraints defined in Section 2.  The creation of a CRI fails if the
   CRI cannot be validated to satisfy all of the constraints.

   If a naming authority creates a CRI from user input, it MAY apply the
   following (and only the following) normalizations to get the CRI more
   likely to validate:

   *  map the scheme name to lowercase (C1);

   *  map the registered name to NFC (C4) and split it on embedded dots;

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   *  elide the port if it is the default port for the scheme (C6);

   *  elide a single zero-length path segment (C7);

   *  map path segments, query parameters and the fragment identifier to
      NFC (C8, C9, C10).

   Once a CRI has been created, it can be used and transferred without
   further normalization.  All operations that operate on a CRI SHOULD
   rely on the assumption that the CRI is appropriately pre-normalized.
   (This does not contradict the requirement that when CRIs are
   transferred, recipients must operate on as-good-as untrusted input
   and fail gracefully in the face of malicious inputs.)

4.  Comparison

   One of the most common operations on CRIs is comparison: determining
   whether two CRIs are equivalent, without dereferencing the CRIs
   (using them to access their respective resource(s)).

   Determination of equivalence or difference of CRIs is based on simple
   component-wise comparison.  If two CRIs are identical component-by-
   component (using code-point-by-code-point comparison for components
   that are Unicode strings) then it is safe to conclude that they are
   equivalent.

   This comparison mechanism is designed to minimize false negatives
   while strictly avoiding false positives.  The constraints defined in
   Section 2 imply the most common forms of syntax- and scheme-based
   normalizations in URIs, but do not comprise protocol-based
   normalizations that require accessing the resources or detailed
   knowledge of the scheme's dereference algorithm.  False negatives can
   be caused, for example, by CRIs that are not appropriately pre-
   normalized and by resource aliases.

   When CRIs are compared to select (or avoid) a network action, such as
   retrieval of a representation, fragment components (if any) should be
   excluded from the comparison.

5.  CRI References

   The most common usage of a Constrained Resource Identifier is to
   embed it in resource representations, e.g., to express a hyperlink
   between the represented resource and the resource identified by the
   CRI.

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   This section defines the serialization of CRIs in Concise Binary
   Object Representation (CBOR) [RFC8949].  To reduce representation
   size, CRIs are not serialized directly.  Instead, CRIs are indirectly
   referenced through _CRI references_. These take advantage of
   hierarchical locality and provide a very compact encoding.  The CBOR
   serialization of CRI references is specified in Section 5.1.

   The only operation defined on a CRI reference is _reference
   resolution_: the act of transforming a CRI reference into a CRI.  An
   application MUST implement this operation by applying the algorithm
   specified in Section 5.3 (or any algorithm that is functionally
   equivalent to it).

   The reverse operation of transforming a CRI into a CRI reference is
   unspecified; implementations are free to use any algorithm as long as
   reference resolution of the resulting CRI reference yields the
   original CRI.  Notably, a CRI reference is not required to satisfy
   all of the constraints of a CRI; the only requirement on a CRI
   reference is that reference resolution MUST yield the original CRI.

   When testing for equivalence or difference, applications SHOULD NOT
   directly compare CRI references; the references should be resolved to
   their respective CRI before comparison.

5.1.  CBOR Serialization

   A CRI reference is encoded as a CBOR array [RFC8949], with the
   structure as described in the Concise Data Definition Language (CDDL)
   [RFC8610] as follows:

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   ; not expressed in this CDDL spec: trailing nulls to be left off

   CRI-Reference = [
     ((scheme / null, authority / no-authority)
      // discard),                 ; relative reference
     path / null,
     query / null,
     fragment / null
   ]

   scheme      = scheme-name / scheme-id
   scheme-name = text .regexp "[a-z][a-z0-9+.-]*"
   scheme-id   = (COAP / COAPS / HTTP / HTTPS / other-scheme)
                 .within nint
   COAP = -1 COAPS = -2 HTTP = -3 HTTPS = -4 URN = -5 DID = -6
   other-scheme = nint .feature "scheme-id-extension"

   no-authority = NOAUTH-NOSLASH / NOAUTH-LEADINGSLASH
   NOAUTH-LEADINGSLASH = null
   NOAUTH-NOSLASH = true

   authority   = [host, ?port]
   host        = (host-name // host-ip)
   host-name   = (*text) ; lowercase, NFC labels
   host-ip     = (bytes .size 4 //
                  (bytes .size 16, ?zone-id))
   zone-id     = text
   port        = 0..65535

   discard     = DISCARD-ALL / 0..127
   DISCARD-ALL = true
   path        = [*text]
   query       = [*text]
   fragment    = text

   This CDDL specification is simplified for exposition and needs to be
   augmented by the following rule for interchange: Trailing null values
   are removed, and two leading null values (scheme and authority both
   not given) are represented by using the discard alternative instead.

   The rules scheme, authority, path, query, fragment correspond to the
   (sub-)components of a CRI, as described in Section 2, with the
   addition of the discard section.  The discard section can be used
   when neither a scheme nor an authority is present.  It then expresses
   path prefixes such as "/", "./", "../", "../../", etc.  The exact
   semantics of the section values are defined by Section 5.3.

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   Most URI references that Section 4.2 of [RFC3986] calls "relative
   references" (i.e., references that need to undergo a resolution
   process to obtain a URI) correspond to the CRI form that starts with
   discard.  The exception are relative references with an authority
   (called a "network-path reference" in Section 4.2 of [RFC3986]),
   which in CRI references never carry a discard section (the value of
   discard defaults to true).

   |  The structure of a CRI is visualized using the somewhat limited
   |  means of a railroad diagram below.
   |  
   |  cri-reference:
   |          ╭──────────────────────────────────────>───────────────────────────────────────╮
   |          │                                                                              │
   |          │                               ╭─────────────────────>─────────────────────╮  │
   |          │                               │                                           │  │
   |          │                               │          ╭──────────────>──────────────╮  │  │
   |          │                               │          │                             │  │  │
   |          │                               │          │           ╭──────>───────╮  │  │  │
   |          │                               │          │           │              │  │  │  │
   |      │├──╯──╮── scheme ── authority ──╭──╯── path ──╯── query ──╯── fragment ──╰──╰──╰──╰──┤│
   |             │                         │
   |             ╰──────── discard ────────╯
   |  
   |  This visualization does not go into the details of the
   |  elements.

   Examples:

   [-1,             / scheme -- equivalent to "coap" /
    [h'C6336401',   / host /
     61616],        / port /
    [".well-known", / path /
     "core"]
   ]

   [true,                  / discard /
    [".well-known",        / path /
     "core"],
    ["rt=temperature-c"]]  / query /

   [-6,             / scheme -- equivalent to "did" /
    true,           / authority = NOAUTH-NOSLASH /
    ["web:alice:bob"] / path /
   ]

   A CRI reference is considered _well-formed_ if it matches the CDDL
   structure.

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   A CRI reference is considered _absolute_ if it is well-formed and the
   sequence of sections starts with a non-null scheme.

   A CRI reference is considered _relative_ if it is well-formed and the
   sequence of sections is empty or starts with a section other than
   those that would constitute a scheme.

5.2.  Ingesting and encoding a CRI Reference

   From an abstract point of view, a CRI Reference is a data structure
   with six sections:

   scheme, authority, discard, path, query, fragment

   Each of these sections can be unset ("null"), except for discard,
   which is always an unsigned number or true.  If scheme and/or
   authority are non-null, discard must be true.

   When ingesting a CRI Reference that is in the transfer form, those
   sections are filled in from the transfer form (unset sections are
   filled with null), and the following steps are performed:

   *  If the array is entirely empty, replace it with [0].

   *  If discard is present in the transfer form (i.e., the outer array
      starts with true or an unsigned number), set scheme and authority
      to null.

   *  If scheme and/or authority are present in the transfer form (i.e.,
      the outer array starts with null, a text string, or a negative
      integer), set discard to true.

   Upon encoding the abstract form into the transfer form, the inverse
   processing is performed: If scheme and/or authority are not null, the
   discard value is not transferred (it must be true in this case).  If
   they are both null, they are both left out and only discard is
   transferred.  Trailing null values are removed from the array.  As a
   special case, an empty array is sent in place for a remaining [0]
   (URI "").

5.3.  Reference Resolution

   The term "relative" implies that a "base CRI" exists against which
   the relative reference is applied.  Aside from fragment-only
   references, relative references are only usable when a base CRI is
   known.

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   The following steps define the process of resolving any well-formed
   CRI reference against a base CRI so that the result is a CRI in the
   form of an absolute CRI reference:

   1.  Establish the base CRI of the CRI reference and express it in the
       form of an abstract absolute CRI reference.

   2.  Initialize a buffer with the sections from the base CRI.

   3.  If the value of discard is true in the CRI reference, replace the
       path in the buffer with the empty array, unset query and
       fragment, and set a true authority to null.  If the value of
       discard is an unsigned number, remove as many elements from the
       end of the path array; if it is non-zero, unset query and
       fragment.  Set discard to true in the buffer.

   4.  If the path section is set in the CRI reference, append all
       elements from the path array to the array in the path section in
       the buffer; unset query and fragment.

   5.  Apart from the path and discard, copy all non-null sections from
       the CRI reference to the buffer in sequence; unset fragment if
       query is non-null and thus copied.

   6.  Return the sections in the buffer as the resolved CRI.

6.  Relationship between CRIs, URIs and IRIs

   CRIs are meant to replace both Uniform Resource Identifiers (URIs)
   [RFC3986] and Internationalized Resource Identifiers (IRIs) [RFC3987]
   in constrained environments [RFC7228].  Applications in these
   environments may never need to use URIs and IRIs directly, especially
   when the resource identifier is used simply for identification
   purposes or when the CRI can be directly converted into a CoAP
   request.

   However, it may be necessary in other environments to determine the
   associated URI or IRI of a CRI, and vice versa.  Applications can
   perform these conversions as follows:

   CRI to URI
      A CRI is converted to a URI as specified in Section 6.1.

   URI to CRI
      The method of converting a URI to a CRI is unspecified;
      implementations are free to use any algorithm as long as
      converting the resulting CRI back to a URI yields an equivalent
      URI.

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   CRI to IRI
      A CRI can be converted to an IRI by first converting it to a URI
      as specified in Section 6.1, and then converting the URI to an IRI
      as described in Section 3.2 of [RFC3987].

   IRI to CRI
      An IRI can be converted to a CRI by first converting it to a URI
      as described in Section 3.1 of [RFC3987], and then converting the
      URI to a CRI as described above.

   Everything in this section also applies to CRI references, URI
   references and IRI references.

6.1.  Converting CRIs to URIs

   Applications MUST convert a CRI reference to a URI reference by
   determining the components of the URI reference according to the
   following steps and then recomposing the components to a URI
   reference string as specified in Section 5.3 of [RFC3986].

   scheme
      If the CRI reference contains a scheme section, the scheme
      component of the URI reference consists of the value of that
      section.  Otherwise, the scheme component is unset.

   authority
      If the CRI reference contains a host-name or host-ip item, the
      authority component of the URI reference consists of a host
      subcomponent, optionally followed by a colon (":") character and a
      port subcomponent.  Otherwise, the authority component is unset.

      The host subcomponent consists of the value of the host-name or
      host-ip item.

      The host-name is turned into a single string by joining the
      elements separated by dots (".").  Any character in the value of a
      host-name item that is not in the set of unreserved characters
      (Section 2.3 of [RFC3986]) or "sub-delims" (Section 2.2 of
      [RFC3986]) MUST be percent-encoded.

      The value of a host-ip item MUST be represented as a string that
      matches the "IPv4address" or "IP-literal" rule (Section 3.2.2 of
      [RFC3986]).  Any zone-id is appended to the string, separated by
      "%25" as defined in Section 2 of [RFC6874], or as specified in a
      successor zone-id specification document; this also leads to a
      modified "IP-literal" rule as specified in these documents.

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      If the CRI reference contains a port item, the port subcomponent
      consists of the value of that item in decimal notation.
      Otherwise, the colon (":") character and the port subcomponent are
      both omitted.

   path
      If the CRI reference contains a discard item of value true, the
      path component is prefixed by a slash ("/") character.  If it
      contains a discard item of value 0 and the path item is present,
      the conversion fails.  Otherwise, the path component is prefixed
      by as many "../" components as the discard value minus one
      indicates.

      If the discard item is not present and the CRI reference contains
      an authority that is true, the path component of the URI reference
      is prefixed by the zero-length string.  Otherwise, the path
      component is prefixed by a slash ("/") character.

      If the CRI reference contains one or more path items, the prefix
      is followed by the value of each item, separated by a slash ("/")
      character.

      Any character in the value of a path item that is not in the set
      of unreserved characters or "sub-delims" or a colon (":") or
      commercial at ("@") character MUST be percent-encoded.

      If the authority component is present (not null or true) and the
      path component does not match the "path-abempty" rule (Section 3.3
      of [RFC3986]), the conversion fails.

      If the authority component is not present, but the scheme
      component is, and the path component does not match the "path-
      absolute", "path-rootless" (authority == true) or "path-empty"
      rule (Section 3.3 of [RFC3986]), the conversion fails.

      If neither the authority component nor the scheme component are
      present, and the path component does not match the "path-
      absolute", "path-noscheme" or "path-empty" rule (Section 3.3 of
      [RFC3986]), the conversion fails.

   query
      If the CRI reference contains one or more query items, the query
      component of the URI reference consists of the value of each item,
      separated by an ampersand ("&") character.  Otherwise, the query
      component is unset.

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      Any character in the value of a query item that is not in the set
      of unreserved characters or "sub-delims" or a colon (":"),
      commercial at ("@"), slash ("/") or question mark ("?") character
      MUST be percent-encoded.  Additionally, any ampersand character
      ("&") in the item value MUST be percent-encoded.

   fragment
      If the CRI reference contains a fragment item, the fragment
      component of the URI reference consists of the value of that item.
      Otherwise, the fragment component is unset.

      Any character in the value of a fragment item that is not in the
      set of unreserved characters or "sub-delims" or a colon (":"),
      commercial at ("@"), slash ("/") or question mark ("?") character
      MUST be percent-encoded.

7.  Extended CRI: Accommodating Percent Encoding

   CRIs have been designed to relieve implementations operating on CRIs
   from string scanning, which both helps constrained implementations
   and implementations that need to achieve high throughput.

   Basic CRI does not support URI components that _require_ percent-
   encoding (Section 2.1 of [RFC3986]) to represent them in the URI
   syntax, except where that percent-encoding is used to escape the main
   delimiter in use.

   E.g., the URI

   https://alice/3%2f4-inch

   is represented by the basic CRI

   [-4, ["alice"], ["3/4-inch"]]

   However, percent-encoding that is used at the application level is
   not supported by basic CRIs:

   did:web:alice:7%3A1-balun

   This section presents a method to represent percent-encoded segments
   of hostnames, paths, and queries.

   The three CDDL rules

   host-name   = (*text)
   path        = [*text]
   query       = [*text]

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   are replaced with

   host-name   = (*text-or-pet)
   path        = [*text-or-pet]
   query       = [*text-or-pet]

   text-or-pet = text /
       ([*(text, pet), ?text]) .feature "extended-cri"

   ; pet is perent-encoded text
   pet = text

   That is, for each of the host-name, path, and query segments, for
   each segment an alternate representation is provided: an array of
   text strings, the even-numbered ones of which are normal text
   strings, while the odd-numbered ones are text strings that retain the
   special semantics of percent-encoded text without actually being
   percent-encoded.

   The above DID URI can now be represented as:

   [-6, true, [["web:alice:7", ":", "1-balun"]]]

8.  Implementation Status

   With the exception of the authority=true fix and host-names split
   into labels, CRIs are implemented in https://gitlab.com/chrysn/
   micrurus.

9.  Security Considerations

   Parsers of CRI references must operate on input that is assumed to be
   untrusted.  This means that parsers MUST fail gracefully in the face
   of malicious inputs.  Additionally, parsers MUST be prepared to deal
   with resource exhaustion (e.g., resulting from the allocation of big
   data items) or exhaustion of the call stack (stack overflow).  See
   Section 10 of [RFC8949] for additional security considerations
   relating to CBOR.

   The security considerations discussed in Section 7 of [RFC3986] and
   Section 8 of [RFC3987] for URIs and IRIs also apply to CRIs.

10.  IANA Considerations

   This document has no IANA actions.

11.  References

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11.1.  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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
              January 2005, <https://www.rfc-editor.org/info/rfc3987>.

   [RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
              IPv6 Zone Identifiers in Address Literals and Uniform
              Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
              February 2013, <https://www.rfc-editor.org/info/rfc6874>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [Unicode]  The Unicode Consortium, "The Unicode Standard, Version
              13.0.0", ISBN 978-1-936213-26-9, March 2020,
              <https://www.unicode.org/versions/Unicode13.0.0/>.

11.2.  Informative References

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

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   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

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

   [RFC8141]  Saint-Andre, P. and J. Klensin, "Uniform Resource Names
              (URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
              <https://www.rfc-editor.org/info/rfc8141>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [RFC8820]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 8820, DOI 10.17487/RFC8820, June 2020,
              <https://www.rfc-editor.org/info/rfc8820>.

   [W3C.REC-html52-20171214]
              Faulkner, S., Eicholz, A., Leithead, T., Danilo, A., and
              S. Moon, "HTML 5.2", World Wide Web Consortium
              Recommendation REC-html52-20171214, 14 December 2017,
              <https://www.w3.org/TR/2017/REC-html52-20171214>.

Appendix A.  Change Log

   This section is to be removed before publishing as an RFC.

   Changes from -05 to -06

   *  rework authority:

      -  split reg-names at dots;

      -  add optional zone identifiers [RFC6874] to IP addresses

   Changes from -04 to -05

   *  Simplify CBOR structure.

   *  Add implementation status section.

   Changes from -03 to -04:

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   *  Minor editorial improvements.

   *  Renamed path.type/path-type to discard.

   *  Renamed option to section, substructured into items.

   *  Simplied the table "resolution-variables".

   *  Use the CBOR structure inspired by Jim Schaad's proposals.

   Changes from -02 to -03:

   *  Expanded the set of supported schemes (#3).

   *  Specified creation, normalization and comparison (#9).

   *  Clarified the default value of the path.type option (#33).

   *  Removed the append-relation path.type option (#41).

   *  Renumbered the remaining path.types.

   *  Renumbered the option numbers.

   *  Restructured the document.

   *  Minor editorial improvements.

   Changes from -01 to -02:

   *  Changed the syntax of schemes to exclude upper case characters
      (#13).

   *  Minor editorial improvements (#34 #37).

   Changes from -00 to -01:

   *  None.

Acknowledgements

   CRIs were developed by Klaus Hartke for use in the Constrained
   RESTful Application Language (CoRAL).  The current author team is
   completing this work with a view to achieve good integration with the
   potential use cases, both inside and outside of CoRAL.

   Thanks to Christian Amsüss, Ari Keränen, Jim Schaad and Dave Thaler
   for helpful comments and discussions that have shaped the document.

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Contributors

   Klaus Hartke
   Ericsson
   Torshamnsgatan 23
   SE-16483 Stockholm
   Sweden

   Email: klaus.hartke@ericsson.com

Authors' Addresses

   Carsten Bormann (editor)
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

   Henk Birkholz
   Fraunhofer SIT
   Rheinstrasse 75
   64295 Darmstadt
   Germany

   Email: henk.birkholz@sit.fraunhofer.de

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