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Constrained Resource Identifiers
draft-ietf-core-href-14

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Carsten Bormann , Henk Birkholz
Last updated 2024-01-09 (Latest revision 2023-07-10)
Replaces draft-hartke-t2trg-ciri
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draft-ietf-core-href-14
CoRE Working Group                                       C. Bormann, Ed.
Internet-Draft                                    Universität Bremen TZI
Intended status: Standards Track                             H. Birkholz
Expires: 13 July 2024                                     Fraunhofer SIT
                                                         10 January 2024

                    Constrained Resource Identifiers
                        draft-ietf-core-href-14

Abstract

   The Constrained Resource Identifier (CRI) is a complement to the
   Uniform Resource Identifier (URI) that represents 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.

   // (This "cref" paragraph will be removed by the RFC editor:) The
   // present revision –14 of this draft picks up comments from the
   // shepherd review and adds sections on CoAP integration and on cri
   // application-oriented literals for the Extended Diagnostic
   // Notation.  This revision still contains open issues and is
   // intended to serve as a snapshot while the processing of the
   // shepherd review is being completed.

About This Document

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

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-core-href/.

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

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   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   4
   2.  Constraints . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Constraints not expressed by the data model . . . . . . .   7
   3.  Creation and Normalization  . . . . . . . . . . . . . . . . .   8
   4.  Comparison  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  CRI References  . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  CBOR Representation . . . . . . . . . . . . . . . . . . .  10
       5.1.1.  The discard Section . . . . . . . . . . . . . . . . .  12
       5.1.2.  Visualization . . . . . . . . . . . . . . . . . . . .  13
       5.1.3.  Examples  . . . . . . . . . . . . . . . . . . . . . .  13
       5.1.4.  Specific Terminology  . . . . . . . . . . . . . . . .  14
     5.2.  Ingesting and encoding a CRI Reference  . . . . . . . . .  14
       5.2.1.  Error handling and extensibility  . . . . . . . . . .  15
     5.3.  Reference Resolution  . . . . . . . . . . . . . . . . . .  16
   6.  Relationship between CRIs, URIs, and IRIs . . . . . . . . . .  17
     6.1.  Converting CRIs to URIs . . . . . . . . . . . . . . . . .  17
   7.  Extending CRIs  . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Extended CRI: Accommodating Percent Encoding (PET)  . . .  21
   8.  CoAP Integration  . . . . . . . . . . . . . . . . . . . . . .  22

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     8.1.  Converting Between CoAP CRIs and Sets of CoAP Options . .  22
       8.1.1.  Decomposing a Request CRI into a set of CoAP
               Options . . . . . . . . . . . . . . . . . . . . . . .  23
       8.1.2.  Composing a Request CRI from a Set of CoAP Options  .  24
     8.2.  CoAP Options for Forward-Proxies  . . . . . . . . . . . .  25
       8.2.1.  Proxy-CRI . . . . . . . . . . . . . . . . . . . . . .  25
       8.2.2.  Proxy-Scheme-Number . . . . . . . . . . . . . . . . .  26
   9.  Implementation Status . . . . . . . . . . . . . . . . . . . .  26
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     11.1.  CRI Scheme Numbers Registry  . . . . . . . . . . . . . .  27
       11.1.1.  Instructions for the Designated Expert . . . . . . .  27
       11.1.2.  Structure of Entries . . . . . . . . . . . . . . . .  28
       11.1.3.  Initial Registrations  . . . . . . . . . . . . . . .  28
     11.2.  Update to "Uniform Resource Identifier (URI) Schemes"
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  29
     11.3.  CBOR Diagnostic Notation Application-extension Identifiers
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  29
     11.4.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  29
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     12.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  Mapping Scheme Numbers to Scheme Names . . . . . . .  33
   Appendix B.  The Small Print  . . . . . . . . . . . . . . . . . .  49
   Appendix C.  CBOR Extended Diagnostic Notation (EDN): The "cri"
           Extension . . . . . . . . . . . . . . . . . . . . . . . .  50
     C.1.  cri: ABNF Definition of URI Representation of a CRI . . .  51
   Appendix D.  Change Log . . . . . . . . . . . . . . . . . . . . .  53
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  56
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  56
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  56

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 [RFC3986].

   This document defines the _Constrained Resource Identifier (CRI)_ by
   constraining URIs to a simplified subset and representing 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 [RFC3986]
   (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) [RFC9110], 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, and
         optionally preceded by user information.

         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 be root-based (zero or more path components
            that are each started in the URI with "/", as when the
            authority is present), or

         *  the path can be rootless, which requires at least one path
            component.

         (Note that, in Figure 1, no-authority is marked as a feature,
         as not all CRI implementations will support authority-less
         URIs.)

   C3.   A userinfo is a text string built out of unreserved characters
         (Section 2.3 of [RFC3986]) or "sub-delims" (Section 2.2 of
         [RFC3986]); any other character needs to be percent-encoded
         (Section 7.1).  Note that this excludes the ":" character,
         which is commonly deprecated as a way to delimit a cleartext
         password in a userinfo.

   C4.   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 (it is likely that a binary mapping
         would be strongly desirable, and that cannot be designed ahead
         of time, so these versions need to be added as a future
         extension if needed).

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   C5.   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.  As per Section 3.2.2
         of [RFC3986], a registered name can be empty, for which case a
         scheme can define a default for the host.)

   C6.   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.

   C7.   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.

   C8.   A path consists of zero or more path segments.  Note that a
         path of just a single zero-length path segment is allowed —
         this is considered equivalent to a path of zero path segments
         by HTTP and CoAP, but this equivalence does not hold for CRIs
         in general as they only perform normalization on the Syntax-
         Based Normalization level (Section 6.2.2 of [RFC3986]), not on
         the scheme-specific Scheme-Based Normalization level
         (Section 6.2.3 of [RFC3986]).

         (A CRI implementation may want to offer scheme-cognizant
         interfaces, performing this scheme-specific normalization for
         schemes it knows.  The interface could assert which schemes the
         implementation knows and provide pre-normalized CRIs.  This can
         also relieve the application from removing a lone zero-length
         path segment before putting path segments into CoAP Options,
         i.e., from performing the check and jump in item 8 of
         Section 6.4 of [RFC7252].  See also SP1 in Appendix B.)

   C9.   A path segment can be any Unicode string that is in NFC, with
         the exception of the special "." and ".." complete path
         segments.  Note that this includes the zero-length string.

         If no authority is present in a CRI, the leading path segment
         cannot be empty.  (See also SP1 in Appendix B.)

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   C10.  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.

   C11.  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.

   C12.  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.

   C13.  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).

   Examples for URIs at or beyond the boundaries of these constraints
   are in SP2 in Appendix B.

2.1.  Constraints not expressed by the data model

   There are syntactically valid CRIs and CRI references that cannot 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.  Examples of
   this are:

   *  [0, ["p"]]: appends a slash and the path segment "p" to its base
      (and unsets the query and the fragment)

   *  [0, null, []]: leaves the path alone but unsets the query and the
      fragment

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   (Full) CRIs that do not correspond to a valid URI are not valid on
   their own, and cannot 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 cannot begin with two
      slash characters, and they would be indistinguishable from a URI
      with a shorter path and a present but empty authority component.

   *  CRIs without authority that are rootless and do not have a path
      component (e.g., ["a", true]), which would be indistinguishable
      from its root-based equivalent (["a"]) as both would have the URI
      a:.

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:

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   *  map the scheme name to lowercase (C1);

   *  map the registered name to NFC (C5) and split it on embedded dots;

   *  elide the port if it is the default port for the scheme (C7);

   *  map path segments, query parameters and the fragment identifier to
      NFC form (C9, C10, C11).

   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 representation of CRIs in Concise Binary
   Object Representation (CBOR) [RFC8949].  When reduced representation
   size is desired, CRIs are not represented directly.  Instead, CRIs
   are indirectly referenced through _CRI references_. These take
   advantage of hierarchical locality and provide a very compact
   encoding.  The CBOR representation 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
   not specified in detail in this document; 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 Representation

   // RFC Ed.: throughout this section, please replace RFC-XXXX with the
   // RFC number of this specification and remove this note.

   A CRI or CRI reference is encoded as a CBOR array [RFC8949], with the
   structure as described in the Concise Data Definition Language (CDDL)
   [RFC8610] including its control extensions [RFC9165] as follows:

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

   RFC-XXXX-Definitions = [CRI, CRI-Reference]

   CRI = [
     scheme,
     authority / no-authority,
     path / null,
     query / null,
     fragment / null
   ]

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

   scheme      = scheme-name / scheme-id
   scheme-name = text .regexp "[a-z][a-z0-9+.-]*"
   scheme-id   = nint

   no-authority = NOAUTH-ROOTBASED / NOAUTH-ROOTLESS
   NOAUTH-ROOTBASED = null .feature "no-authority"
   NOAUTH-ROOTLESS = true .feature "no-authority"

   authority   = [?userinfo, host, ?port]
   userinfo    = (false, text .feature "userinfo")
   host        = (host-ip // host-name)
   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

                 Figure 1: CDDL for CRI CBOR representation

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   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.

   This CDDL specification is simplified for exposition and needs to be
   augmented by the following rules for interchange of CRIs and CRI
   references:

   *  Trailing null values MUST be removed,

   *  two leading null values (scheme and authority both not given) MUST
      be represented by using the discard alternative instead, and

   *  an empty path in a CRI MUST be represented as the empty array []
      (note that for CRI-Reference there is a difference between empty
      and absent paths, represented by [] and null, respectively),

   *  an entirely empty outer array is not a valid CRI (but a valid CRI
      reference, as per Section 5.2 equivalent to [0], which essentially
      copies the base CRI).

   For interchange as separate encoded data items, CRIs MUST NOT use
   indefinite length encoding (see Section 3.2 of [RFC8949]); this
   requirement is relaxed for specifications that embed CRIs into an
   encompassing CBOR representation that does provide for indefinite
   length encoding.

5.1.1.  The discard Section

   The discard section can be used in a CRI reference when neither a
   scheme nor an authority is present.  It then expresses the operations
   performed on a base CRI by CRI references that are equivalent to URI
   references with relative paths and path prefixes such as "/", "./",
   "../", "../../", etc. "." and ".." are not available in CRIs and are
   therefore expressed using discard after a normalization step, as is
   the presence or absence of a leading "/".

   E.g., a simple URI reference "foo" specifies to remove one leading
   segment from the base URI's path, which is represented in the
   equivalent CRI reference discard section as the value 1; similarly
   "../foo" removes two leading segments, represented as 2; and "/foo"
   removes all segments, represented in the discard section as the value
   true.  The exact semantics of the section values are defined by
   Section 5.3.

   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 reference form that

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   starts with discard.  The exception are relative references with an
   authority (called a "network-path reference" in Section 4.2 of
   [RFC3986]), which discard the entire path of the base CRI.  These CRI
   references never carry a discard section: the value of discard
   defaults to true.

5.1.2.  Visualization

   The structure of a CRI reference is visualized using the somewhat
   limited means of a railroad diagram:

   cri-reference:
       │├──╮── scheme ── authority ──╭── local-part ──┤│
           │                         │
           ╰──────── discard ────────╯

   local-part:
           ╭─────────────────────>─────────────────────╮
           │                                           │
           │          ╭──────────────>──────────────╮  │
           │          │                             │  │
           │          │           ╭──────>───────╮  │  │
           │          │           │              │  │  │
       │├──╯── path ──╯── query ──╯── fragment ──╰──╰──╰──┤│

   This visualization does not go into the details of the elements.

5.1.3.  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-ROOTLESS /
    ["web:alice:bob"] / path /
   ]

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5.1.4.  Specific Terminology

   A CRI reference is considered _well-formed_ if it matches the
   structure as expressed in Figure 1 in CDDL, with the additional
   requirement that trailing null values are removed from the array.

   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 integer 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 integer), 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 "").

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5.2.1.  Error handling and extensibility

   It is recommended that specifications that describe the use of CRIs
   in CBOR-based protocols use the error handling mechanisms outlined in
   this section.  Implementations of this document MUST adhere to these
   rules unless the containing document overrides them.

   When encountering a CRI that is well-formed in terms of CBOR, but
   that

   *  is not well-formed as a CRI,

   *  does not meet the other requirements on CRIs that are not covered
      by the term "well-formed", or

   *  uses features not supported by the implementation,

   the CRI is treated as "unprocessable".

   When encountering an unprocessable CRI, the processor skips the
   entire CRI top-level array, including any CBOR items contained in
   there, and continues processing the CBOR items surrounding the
   unprocessable CRI.  (Note: this skipping can be implemented in
   bounded memory for CRIs that do not use indefinite length encoding,
   as mandated in Section 5.1.)

   The unprocessable CRI is treated as an opaque identifier that is
   distinct from all processable CRIs, and distinct from all
   unprocessable CRIs with different CBOR representations.  It is up to
   implementation whether unprocessable CRIs with identical
   representations are treated as identical to each other or not.
   Unprocessable CRIs can not be dereferenced, and it is an error to
   query any of their components.

   This mechanism ensures that CRI extensions (using originally defined
   features or later extensions) can be used without extending the
   compatibility hazard to the containing document.  For example, if a
   collection of possible interaction targets contains several CRIs,
   some of which use the "no-authority" feature, an application
   consuming that collection that does not support that feature can
   still offer the supported interaction targets.

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   The duty of checking validity is with the recipients that rely on
   this validity.  An intermediary that does not use the detailed
   information in a CRI (or merely performs reference resolution) MAY
   pass on a CRI/CRI reference without having fully checked it, relying
   on the producer having generated a valid CRI/CRI reference.  This is
   true for both basic CRIs (e.g., checking for valid UTF-8) and for
   extensions (e.g., checking both for valid UTF-8 and the minimal use
   of PET elements in extended-cris as per Section 7.1).

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.

   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 (which is
       implicitly the case when scheme and/or authority are present in
       the 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 integer, 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 query in the
       buffer if query is the empty array [] in the CRI reference; unset
       fragment in the buffer if query is non-null in the CRI reference.

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

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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.

      Note that CRIs are defined to enable implementing conversions from
      or to URIs analogously to processing URIs into CoAP Options and
      back, with the exception that item 8 of Section 6.4 of [RFC7252]
      and item 7 of Section 6.5 of [RFC7252] do not apply to CRI
      processing.  See SP1 in Appendix B for more details.

   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].

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   scheme
      If the CRI reference contains a scheme section, the scheme
      component of the URI reference consists of the value of that
      section, if text (scheme-name); or, if a negative integer is given
      (scheme-id), the lower case scheme name corresponding to the
      scheme number as per the CRI Scheme Numbers registry
      (Section 11.1).  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, optionally preceded by a userinfo subcomponent.
      Otherwise, the authority component is unset.

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

      The userinfo subcomponent, if present, is turned into a single
      string by appending a "@".  Otherwise, both the subcomponent and
      the "@" sign are omitted.  Any character in the value of the
      userinfo elements 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 host-name is turned into a single string by joining the
      elements separated by dots (".").  Any character in the elements
      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.  If there are dots (".") in
      such elements, the conversion fails (percent-encoding is not able
      to represent such elements, as normalization would turn the
      percent-encoding back to the unreserved character that a dot is.)

      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
      superseding zone-id specification document
      [I-D.carpenter-6man-rfc6874bis]; this also leads to a modified
      "IP-literal" rule as specified in these documents.

      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

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      If the CRI reference contains a discard item of value true, the
      path component is considered _rooted_.  If it contains a discard
      item of value 0 and the path item is present, the conversion
      fails.  If it contains a positive discard item, the path component
      is considered _unrooted_ and prefixed by as many "../" components
      as the discard value minus one indicates.  If the discard value is
      1 and the first element of the path contains a :, the path
      component is prefixed by "./" (this avoids the first element to
      appear as supplying a URI scheme; compare path-noscheme in
      Section 4.2 of [RFC3986]).

      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 considered unrooted.  Otherwise, the path component is
      considered rooted.

      If the CRI reference contains one or more path items, the path
      component is constructed by concatenating the sequence of
      representations of these items.  These representations generally
      contain a leading slash ("/") character and the value of each
      item, processed as discussed below.  The leading slash character
      is omitted for the first path item only if the path component is
      considered "unrooted".

      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.  Extending CRIs

   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.

   The CRI structure described up to this point is termed the _Basic
   CRI_. It should be sufficient for all applications that use the CoAP
   protocol, as well as most other protocols employing URIs.

   However, Basic CRIs have one limitation: They do 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

   Extended forms of CRIs may be defined to enable these applications.
   They will generally extend the potential values of text components of
   URIs, such as userinfo, hostnames, paths, queries, and fragments.

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   One such extended form is described in the following Section 7.1.
   Consumers of CRIs will generally notice when an extended form is in
   use, by finding structures that do not match the CDDL rules given in
   Figure 1.  Future definitions of extended forms need to strive to be
   distinguishable in their structures from the extended form presented
   here as well as other future forms.

   Extensions to CRIs MUST NOT allow indefinite length items.  This
   provision ensures that recipients of CRIs can deal with unprocessable
   CRIs as described in Section 5.2.1.

7.1.  Extended CRI: Accommodating Percent Encoding (PET)

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

   The four CDDL rules

   userinfo    = (false, text .feature "userinfo")
   host-name   = (*text)
   path        = [*text]
   query       = [+text]
   fragment    = text

   are replaced with

   userinfo    = (false, text-or-pet .feature "userinfo")
   host-name   = (*text-or-pet)
   path        = [*text-or-pet]
   query       = [+text-or-pet]
   fragment    = text-or-pet

   text-or-pet = text /
       text-pet-sequence .feature "extended-cri"

   ; text1 and pet1 alternating, at least one pet1:
   text-pet-sequence = [?text1, ((+(pet1, text1), ?pet1) // pet1)]
   ; pet is percent-encoded bytes
   pet1 = bytes .ne ''
   text1 = text .ne ""

   That is, for each of the host-name, path, and query segments, and for
   the userinfo and fragment components, an alternate representation is
   provided besides a simple text string: a non-empty array of
   alternating non-blank text and byte strings, the text strings of
   which stand for non-percent-encoded text, while the byte strings
   retain the special semantics of percent-encoded text without actually
   being percent-encoded.

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   The above DID URI can now be represented as:

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

   (Note that, in CBOR diagnostic notation, single quotes delimit
   literals for byte strings, double quotes for text strings.)

   To yield a valid extended-cri, the use of byte strings MUST be
   minimal.  Both the following examples are therefore not valid:

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

   An algorithm for constructing a valid text-pet-sequence might
   repeatedly examine the byte sequences in each byte string; if such a
   sequence stands for an unreserved ASCII character, or constitutes a
   valid UTF-8 character ≥ U+0080, move this character over into a text
   string by appending it to the end of the preceding text string,
   prepending it to the start of the following text string, or splitting
   the byte string and inserting a new text string with this character,
   all while preserving the order of the bytes.  (Note that the
   properties of UTF-8 make this a simple linear process.)

      |  Unlike the text elements of a path or a query, which through
      |  CoAP's heritage are designed to be processable element by
      |  element, a text-pet-sequence does not usually produce a
      |  semantically meaningful division into array elements.  This
      |  consequence of the flexibility in delimiters offered in URIs is
      |  demonstrated by this example, which structurally singles out
      |  the one ':' that is _not_ a delimiter at the application level.
      |  Applications designed for using CRIs will generally avoid using
      |  the extended-cri feature.  Applications using existing URI
      |  structures that require text-pet-sequence elements for their
      |  representation typically need to process them byte by byte.

8.  CoAP Integration

   This section discusses ways in which CRIs can be used in the context
   of the CoAP protocol [RFC7252].

8.1.  Converting Between CoAP CRIs and Sets of CoAP Options

   This section provides an analogue to Sections 6.4 and 6.5 of
   [RFC7252]: Computing a set of CoAP options from a request CRI
   Section 8.1.1 and computing a request CRI from a set of COAP options
   Section 8.1.2.

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   This section makes use of the mapping between CRI scheme ids and URI
   scheme names shown in Table 1:

                      +===============+============+
                      | CRI scheme id | URI scheme |
                      +===============+============+
                      | -1            | coap       |
                      +---------------+------------+
                      | -2            | coaps      |
                      +---------------+------------+
                      | -7            | coap+tcp   |
                      +---------------+------------+
                      | -8            | coaps+tcp  |
                      +---------------+------------+
                      | -9            | coap+ws    |
                      +---------------+------------+
                      | -10           | coaps+ws   |
                      +---------------+------------+

                           Table 1: Mapping CRI
                        scheme ids and URI scheme
                                  names

8.1.1.  Decomposing a Request CRI into a set of CoAP Options

   The steps to parse a request's options from a CRI »cri« are as
   follows.  These steps either result in zero or more of the Uri-Host,
   Uri-Port, Uri-Path, and Uri-Query Options being included in the
   request or they fail.

   Where the following speaks of deriving a text-string for a CoAP
   Option value from a data item in the CRI, the presence of any text-
   pet-sequence subitem (Section 7.1) in this item fails this algorithm.

   1.  If »cri« is not an absolute CRI reference, then fail this
       algorithm.

   2.  Translate the scheme id into a URI scheme name as per Table 1; if
       a scheme id not in this list is being used, fail this algorithm.
       Remember the specific variant of CoAP to be used based on this
       URI scheme name.

   3.  If »cri« has a fragment component, then fail this algorithm.

   4.  If the host component of »cri« is a host-name, include a Uri-Host
       Option and let that option's value be the text string value of
       the host-name.

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       If the host component of »cri« is a host-ip, check whether the IP
       address given represents the request's destination IP address
       (and, if present, zone-id).  Only if it does not, include a Uri-
       Host Option, and let that option's value be the text value of the
       URI representation of the IP address, as derived in Section 6.1,
       Paragraph 2.4.5.

   5.  If »cri« has a port component, then let »port« be that
       component's unsigned integer value; otherwise, let »port« be the
       default port number for the scheme.

   6.  If »port« does not equal the request's destination UDP port,
       include a Uri-Port Option and let that option's value be »port«.

   7.  If the value of the path component of »cri« is empty or consists
       of a single empty string, then move to the next step.

       Otherwise, for each element in the »path« component, include a
       Uri-Path Option and let that option's value be the text string
       value of that element.

   8.  If »cri« has a query component, then, for each element in the
       query component, include a Uri-Query Option and let that option's
       value be the be the text string value of that element.

8.1.2.  Composing a Request CRI from a Set of CoAP Options

   The steps to construct a CRI from a request's options are as follows.
   These steps either result in a CRI or they fail.

   1.  Based on the variant of CoAP used in the request, choose a
       scheme-id as per table Table 1.  Use that as the first value in
       the resulting CRI array.

   2.  If the request includes a Uri-Host Option, insert an authority
       with its value determined as follows: If the value of the Uri-
       Host Option is a reg-name, include this as the host-name.  If the
       value is an IP-literal or IPv4address, extract any zone-id, and
       represent the IP address as a byte string of the correct length
       in host-ip, followed by any zone-id extracted if present.  If the
       value is none of the three, fail this algorithm.

       If the request does not include a Uri-Host Option, insert an
       authority with host-ip being the byte string that represents the
       request's destination IP address and, if one is present in the
       request's destination, add a zone-id.

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   3.  If the request includes a Uri-Port Option, let »port« be that
       option's value.  Otherwise, let »port« be the request's
       destination UDP port.  If »port« is not the default port for the
       scheme, then insert the integer value of »port« as the value of
       port in the authority.  Otherwise, elide the port.

   4.  Insert a path component that contains an array built from the
       text string values of the Uri-Path Options in the request, or an
       empty array if no such options are present.

   5.  Insert a query component that contains an array built from the
       text string values of the Uri-Query Options in the request, or an
       empty array if no such options are present.

8.2.  CoAP Options for Forward-Proxies

   Apart from the above procedures to convert CoAP CRIs to and from sets
   of CoAP Options, two additional CoAP Options are defined in
   Section 5.10.2 of [RFC7252] that support requests to forward-proxies:

   *  Proxy-Uri, and

   *  its more lightweight variant, Proxy-Scheme

   This section defines analogues of these that employ CRIs and the URI
   Scheme numering provided by the present specification.

8.2.1.  Proxy-CRI

    +========+===+===+===+===+===========+========+========+=========+
    | No.    | C | U | N | R | Name      | Format | Length | Default |
    +========+===+===+===+===+===========+========+========+=========+
    | TBD235 | x | x | - |   | Proxy-Cri | opaque | 1-1023 | (none)  |
    +--------+---+---+---+---+-----------+--------+--------+---------+

                      Table 2: Proxy-Cri CoAP Option

   The Proxy-CRI Option carries an encoded CBOR data item that
   represents an absolute CRI reference.  It is used analogously to
   Proxy-Uri as defined in Section 5.10.2 of [RFC7252].  The Proxy-Cri
   Option MUST take precedence over any of the Uri-Host, Uri-Port, Uri-
   Path or Uri-Query options, as well as over any Proxy-Uri Option (each
   of which MUST NOT be included in a request containing the Proxy-Cri
   Option).

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8.2.2.  Proxy-Scheme-Number

   +========+=+=+=+=+=====================+========+========+=========+
   | No.    |C|U|N|R| Name                | Format | Length | Default |
   +========+=+=+=+=+=====================+========+========+=========+
   | TBD239 |x|x|-| | Proxy-Scheme-Number | uint   | 0-3    | (none)  |
   +--------+-+-+-+-+---------------------+--------+--------+---------+

                 Table 3: Proxy-Scheme-Number CoAP Option

   The Proxy-Scheme-Number Option carries a URI Scheme Number
   represented as a CoAP unsigned integer.  It is used analogously to
   Proxy-Scheme as defined in Section 5.10.2 of [RFC7252].

   Since CoAP Options are only defined as empty, (text) string, opaque
   (byte string), or unsigned integer, the Option carries an unsigned
   integer that gives the 1's complement of the URI Scheme Number, i.e.:

   option value = -1 - scheme number

   // DISCUSS: Should the scheme registry simply use unsigned numbers so
   // it can be used here right away and the 1's complement form is only
   // used specifically for the nint that scheme-id at the start of CRIs
   // is?

   // TO DO: Discuss the need for a location-scheme-numeric option?

9.  Implementation Status

   (Boilerplate as per Section 2.1 of [RFC7942]:)

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

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   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   With the exception of the authority=true fix, host-names split into
   labels, and Section 7.1, CRIs are implemented in
   https://gitlab.com/chrysn/micrurus.  A golang implementation of
   version -10 of this document is found at: https://github.com/thomas-
   fossati/href

10.  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.

11.  IANA Considerations

11.1.  CRI Scheme Numbers Registry

   This specification defines a new "CRI Scheme Numbers" sub-registry in
   the "CoRE Parameters" registry [IANA.core-parameters], with the
   policy "Expert Review" (Section 4.5 of RFC 8126 [BCP26]).  The
   objective is to have CRI scheme number values registered for all
   registered URI schemes (Uniform Resource Identifier (URI) Schemes
   registry), as well as exceptionally for certain text strings that the
   Designated Expert considers widely used in constrained applications
   in place of URI scheme names.

11.1.1.  Instructions for the Designated Expert

   The expert is instructed to be frugal in the allocation of CRI values
   with short representations (1+0 and 1+1 encoding), keeping them in
   reserve for applications that are likely to enjoy wide use and can
   make good use of their shortness.

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   When the expert notices that a registration has been made in the
   Uniform Resource Identifier (URI) Schemes registry (see also
   Section 11.2), the expert is requested to initiate a parallel
   registration in the CRI Scheme Numbers registry.  CRI values in the
   range between 1000 and 20000 (inclusive) should be assigned unless a
   shorter representation in CRIs appears desirable.

   The expert exceptionally also may make such a registration for text
   strings that have not been registered in the Uniform Resource
   Identifier (URI) Schemes registry if and only if the expert considers
   them to be in wide use in place of URI scheme names in constrained
   applications.  (Note that the initial registrations in Table 6 in
   Appendix A already include such registrations for the text strings
   "mqtt" and "mqtts".)

   A registration in the CRI Scheme Numbers registry does not imply that
   a URI scheme under this name exists or has been registered in the
   Uniform Resource Identifier (URI) Schemes registry -- it essentially
   is only providing an integer identifier for an otherwise
   uninterpreted text string.

   Any questions or issues that might interest a wider audience might be
   raised by the expert on the core-parameters@ietf.org mailing list for
   a time-limited discussion.

11.1.2.  Structure of Entries

   Each entry in the registry must include:

   CRI value:
      A negative integer unique in this registry

   URI scheme name:
      a text string that would be acceptable for registration as a URI
      Scheme Name in the Uniform Resource Identifier (URI) Schemes
      registry

   Reference:
      a reference to a document, if available, or the registrant

11.1.3.  Initial Registrations

   The initial registrations for the CRI Scheme Numbers registry are
   provided in Table 6 in Appendix A.

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11.2.  Update to "Uniform Resource Identifier (URI) Schemes" Registry

   [RFC7595] is updated to add the following note in the "Uniform
   Resource Identifier (URI) Schemes" Registry [IANA.uri-schemes]:

   |  The CRI Scheme Numbers Registry registers numeric identifiers for
   |  what essentially are URI Scheme names.  Registrants for the
   |  Uniform Resource Identifier (URI) Schemes Registry are requested
   |  to make a parallel registration in the CRI Scheme Numbers
   |  registry.  The number for this registration will be assigned by
   |  the Designated Expert for that registry.

11.3.  CBOR Diagnostic Notation Application-extension Identifiers
       Registry

   In the "Application-Extension Identifiers" registry in the "CBOR
   Diagnostic Notation" registry group [IANA.cbor-diagnostic-notation],
   IANA is requested to register the application-extension identifier
   cri as described in Table 4 and defined in Appendix C.

     +=======================+=============+============+===========+
     | Application-extension | Description | Change     | Reference |
     | Identifier            |             | Controller |           |
     +=======================+=============+============+===========+
     | cri                   | Constrained | IETF       | RFC-XXXX, |
     |                       | Resource    |            | Appendix  |
     |                       | Identifier  |            | C         |
     +-----------------------+-------------+------------+-----------+

             Table 4: CBOR Extended Diagnostic Notation (EDN)
                 Application-extension Identifier for CRI

   // RFC Ed.: throughout this section, please replace RFC-XXXX with the
   // RFC number of this specification and remove this note.

11.4.  CoAP Option Numbers Registry

   In the "CoAP Option Numbers" registry in the "CoRE Parameters"
   registry group [IANA.core-parameters], IANA is requested to register
   the CoAP Option Numbers as described in Table 5 and defined in
   Section 8.2.

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               +========+=====================+===========+
               | No.    | Name                | Reference |
               +========+=====================+===========+
               | TBD235 | Proxy-Cri           | RFC-XXXX  |
               +--------+---------------------+-----------+
               | TBD239 | Proxy-Scheme-Number | RFC-XXXX  |
               +--------+---------------------+-----------+

                     Table 5: New CoAP Option Numbers

   // RFC Ed.: throughout this section, please replace RFC-XXXX with the
   // RFC number of this specification and remove this note.

12.  References

12.1.  Normative References

   [BCP26]    Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, June 2017.

              <https://www.rfc-editor.org/info/bcp26>

   [I-D.carpenter-6man-rfc6874bis]
              Carpenter, B. E., Cheshire, S., and R. M. Hinden,
              "Representing IPv6 Zone Identifiers in Address Literals
              and Uniform Resource Identifiers", Work in Progress,
              Internet-Draft, draft-carpenter-6man-rfc6874bis-03, 8
              February 2022, <https://datatracker.ietf.org/doc/html/
              draft-carpenter-6man-rfc6874bis-03>.

   [IANA.core-parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              <https://www.iana.org/assignments/core-parameters>.

   [IANA.uri-schemes]
              IANA, "Uniform Resource Identifier (URI) Schemes",
              <https://www.iana.org/assignments/uri-schemes>.

   [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/rfc/rfc2119>.

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   [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/rfc/rfc3986>.

   [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
              Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
              January 2005, <https://www.rfc-editor.org/rfc/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/rfc/rfc6874>.

   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,
              <https://www.rfc-editor.org/rfc/rfc7595>.

   [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/rfc/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/rfc/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/rfc/rfc8949>.

   [RFC9165]  Bormann, C., "Additional Control Operators for the Concise
              Data Definition Language (CDDL)", RFC 9165,
              DOI 10.17487/RFC9165, December 2021,
              <https://www.rfc-editor.org/rfc/rfc9165>.

   [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/>.

12.2.  Informative References

   [I-D.ietf-cbor-edn-literals]
              Bormann, C., "CBOR Extended Diagnostic Notation (EDN):
              Application-Oriented Literals, ABNF, and Media Type", Work

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              in Progress, Internet-Draft, draft-ietf-cbor-edn-literals-
              07, 21 December 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
              edn-literals-07>.

   [RFC4180]  Shafranovich, Y., "Common Format and MIME Type for Comma-
              Separated Values (CSV) Files", RFC 4180,
              DOI 10.17487/RFC4180, October 2005,
              <https://www.rfc-editor.org/rfc/rfc4180>.

   [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/rfc/rfc7228>.

   [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/rfc/rfc7252>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/rfc/rfc7942>.

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

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

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

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

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

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Appendix A.  Mapping Scheme Numbers to Scheme Names

   // RFC Ed.: throughout this section, please replace RFC-XXXX with the
   // RFC number of this specification and remove this note.

   Table 6 defines the initial mapping from CRI scheme numbers to URI
   scheme names.

     +===========+======================================+============+
     | CRI value | URI scheme                           | Reference  |
     +===========+======================================+============+
     | -1        | coap                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2        | coaps                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3        | http                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4        | https                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5        | urn                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6        | did                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7        | coap+tcp                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8        | coaps+tcp                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9        | coap+ws                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10       | coaps+ws                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1025     | telnet                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1046     | ldap                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1056     | ms-virtualtouchpad                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1091     | fax                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1107     | ves                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1147     | submit                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1192     | gg                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1219     | simplex                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

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     | -1240     | ms-settings-nfctransactions          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1241     | secret-token                         | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1249     | acap                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1276     | openpgp4fpr                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1300     | ms-mixedrealitycapture               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1307     | ymsgr                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1320     | iris.xpcs                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1351     | turns                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1367     | opaquelocktoken                      | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1499     | platform                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1597     | sftp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1613     | vscode                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1649     | mqtt                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1664     | ms-settings                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1690     | doi                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1720     | file                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1729     | dvb                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1760     | magnet                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1768     | calculator                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1836     | ssh                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1928     | openid                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1966     | gopher                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1985     | ms-gamingoverlay                     | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -1997     | z39.50                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

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     | -2032     | ms-secondary-screen-setup            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2038     | fido                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2085     | mumble                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2095     | ms-settings-cloudstorage             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2106     | imap                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2152     | ms-officeapp                         | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2233     | pwid                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2236     | drm                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2264     | tag                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2369     | feed                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2460     | ipps                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2463     | ms-launchremotedesktop               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2484     | xmlrpc.beeps                         | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2492     | jms                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2542     | wpid                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2669     | barion                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2675     | onenote                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2695     | icon                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2769     | message                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2800     | ms-enrollment                        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2804     | bolo                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2817     | diaspora                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2833     | microsoft.windows.camera.picker      | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2864     | notes                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

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     | -2866     | amss                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -2873     | tip                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3018     | fm                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3042     | rtmfp                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3060     | reload                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3111     | pres                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3232     | acd                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3362     | prospero                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3364     | geo                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3414     | snmp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3483     | iris.beep                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3510     | maps                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3575     | content                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3618     | pack                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3619     | keyparc                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3632     | mongodb                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3693     | smb                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3796     | graph                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3818     | filesystem                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3839     | payment                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3840     | ms-settings-bluetooth                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -3951     | palm                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4014     | at                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4027     | hyper                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

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     | -4043     | microsoft.windows.camera             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4067     | mvn                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4098     | mtqp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4130     | jabber                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4275     | mms                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4343     | skype                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4351     | oid                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4420     | dict                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4454     | attachment                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4662     | ocf                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4807     | isostore                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4816     | redis                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4862     | ms-settings-privacy                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -4877     | ms-settings-wifi                     | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5004     | v-event                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5020     | com-eventbrite-attendee              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5039     | ms-remotedesktop                     | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5105     | teliaeid                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5222     | itms                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5234     | fish                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5285     | dtn                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5298     | vscode-insiders                      | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5304     | tftp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5347     | rtsp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 37]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -5358     | adiumxtra                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5464     | smp                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5470     | ms-eyecontrolspeech                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5479     | ms-settings-language                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5491     | mqtts                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5595     | wyciwyg                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5596     | hcp                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5619     | go                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5673     | rediss                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5683     | ms-settings-cellular                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5743     | ldaps                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5803     | dhttp                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5843     | z39.50s                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5886     | bitcoincash                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -5960     | ms-mobileplans                       | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6182     | pttp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6208     | facetime                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6289     | gtalk                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6348     | afp                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6361     | mss                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6426     | ms-settings-notifications            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6448     | psyc                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6488     | tv                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6514     | wifi                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 38]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -6523     | sarif                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6539     | moz                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6659     | ms-lockscreencomponent-config        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6716     | cabal                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6734     | ms-media-stream-id                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6780     | mupdate                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6793     | dis                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6804     | nih                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6809     | ms-help                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6909     | soap.beep                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -6998     | iotdisco                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7027     | acr                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7040     | ms-newsandinterests                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7089     | hxxp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7096     | ms-settings-location                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7125     | soap.beeps                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7301     | ipn                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7309     | nntp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7316     | query                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7334     | smtp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7335     | ms-spd                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7400     | ni                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7403     | ms-excel                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7421     | ms-settings-power                    | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 39]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -7435     | pop                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7447     | session                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7582     | ms-infopath                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7701     | ms-word                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7715     | web+ap                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7791     | steam                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -7995     | cstr                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8008     | web3                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8064     | videotex                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8069     | nfs                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8094     | udp                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8102     | ed2k                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8138     | ms-getoffice                         | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8203     | sgn                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8331     | data                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8364     | swidpath                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8385     | fuchsia-pkg                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8395     | ms-screensketch                      | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8426     | hxxps                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8487     | unreal                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8555     | ens                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8585     | ms-settings-camera                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8619     | stun                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8673     | ms-stickers                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 40]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -8734     | lid                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8775     | spotify                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8860     | starknet                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8890     | ms-settings-emailandaccounts         | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8907     | market                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -8967     | ms-powerpoint                        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9001     | rtsps                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9064     | p1                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9128     | aw                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9132     | mailserver                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9186     | irc6                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9338     | ms-settings-lock                     | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9339     | hcap                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9350     | drop                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9419     | icap                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9437     | xcon-userid                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9457     | leaptofrogans                        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9461     | ipfs                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9479     | bitcoin                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9555     | apt                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9605     | ms-whiteboard-cmd                    | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9669     | ssb                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9725     | aaas                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9734     | ar                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 41]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -9767     | proxy                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9773     | res                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9780     | msrps                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9795     | aim                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9826     | tool                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9842     | finger                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9900     | turn                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9901     | num                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9903     | svn                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9904     | ut2004                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -9932     | ms-visio                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10008    | eid                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10100    | wss                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10103    | gizmoproject                         | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10172    | dlna-playsingle                      | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10224    | swh                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10337    | dat                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10348    | cap                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10355    | z39.50r                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10412    | xcon                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10430    | gitoid                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10524    | hydrazone                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10565    | example                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10699    | crid                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 42]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -10717    | teamspeak                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10743    | elsi                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10769    | dtmi                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10840    | ftp                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10902    | ms-drive-to                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10903    | upt                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -10911    | appdata                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11039    | callto                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11131    | ms-remotedesktop-launch              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11139    | dweb                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11264    | lastfm                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11307    | xmlrpc.beep                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11342    | ms-whiteboard                        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11465    | first-run-pen-experience             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11473    | webcal                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11553    | adt                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11566    | vemmi                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11590    | cvs                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11629    | taler                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11688    | ms-inputapp                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11864    | git                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11893    | irc                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -11936    | ms-settings-workplace                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12171    | blob                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 43]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -12173    | modem                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12188    | msnim                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12268    | iris.lwz                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12302    | ms-sttoverlay                        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12321    | lbry                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12334    | rmi                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12346    | ms-restoretabcompanion               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12482    | ms-useractivityset                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12485    | dab                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12491    | about                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12500    | embedded                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12501    | rtmp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12526    | ircs                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12558    | mid                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12573    | sip                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12593    | ipns                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12666    | dvx                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12706    | android                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12747    | wtai                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12831    | ms-search-repair                     | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12838    | microsoft.windows.camera.multipicker | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12857    | ms-settings-screenrotation           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12879    | rtspu                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12914    | ms-screenclip                        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 44]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -12943    | aaa                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12954    | xmpp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -12988    | soldat                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13041    | lorawan                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13054    | beshare                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13077    | sips                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13081    | iris.xpc                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13113    | simpleledger                         | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13127    | vsls                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13207    | matrix                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13307    | otpauth                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13336    | cid                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13352    | service                              | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13417    | h323                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13438    | ms-settings-connectabledevices       | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13452    | payto                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13463    | ms-settings-displays-topology        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13505    | lvlt                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13596    | ms-walk-to                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13672    | dns                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13730    | quic-transport                       | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13762    | paparazzi                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13766    | ms-people                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13889    | xri                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 45]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -13894    | onenote-cmd                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -13934    | dav                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14003    | content-type                         | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14068    | sms                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14119    | ms-publisher                         | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14197    | xfire                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14250    | secondlife                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14260    | ark                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14301    | iax                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14312    | msrp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14475    | swid                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14590    | tn3270                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14596    | ms-appinstaller                      | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14627    | stuns                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14688    | dpp                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14701    | ms-secondary-screen-controller       | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14764    | browserext                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14820    | chrome                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -14878    | pkcs11                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15066    | dlna-playcontainer                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15155    | spiffe                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15207    | uuid-in-package                      | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15261    | ms-settings-proximity                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15356    | things                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

Bormann & Birkholz        Expires 13 July 2024                 [Page 46]
Internet-Draft      Constrained Resource Identifiers        January 2024

     | -15377    | ms-gamebarservices                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15379    | shc                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15547    | ipp                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15552    | mailto                               | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15558    | ms-browser-extension                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15838    | shttp (OBSOLETE)                     | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15842    | acct                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15849    | w3                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15869    | wais                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15928    | qb                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -15947    | ms-search                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16043    | ms-settings-airplanemode             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16045    | jar                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16069    | tel                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16074    | dntp                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16160    | chrome-extension                     | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16193    | cast                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16326    | view-source                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16356    | im                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16358    | resource                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16378    | ms-calculator                        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16380    | news                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16415    | wcr                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16523    | casts                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

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     | -16689    | ms-access                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16723    | grd                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16750    | rsync                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16773    | lpa                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16850    | afs                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16874    | bb                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16884    | ham                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16926    | info                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -16972    | ms-meetnow                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17117    | ms-project                           | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17172    | ethereum                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17225    | thismessage                          | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17226    | vnc                                  | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17232    | snews                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17245    | sieve                                | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17269    | feedready                            | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17271    | mt                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17288    | ws                                   | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17338    | ms-transit-to                        | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17346    | ventrilo                             | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+
     | -17357    | iris                                 | [RFC-XXXX] |
     +-----------+--------------------------------------+------------+

              Table 6: Mapping Scheme Numbers to Scheme Names

   The assignments from this table can be extracted from the XML form of
   this document (when stored in a file "this.xml") into CSV form
   [RFC4180] using this short Ruby program:

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   require 'rexml/document'; include REXML
   XPath.each(Document.new(File.read("this.xml")),"//tr") {|row|
     puts XPath.each(row,"td").map{|d|d.text()}[0..1].join(",")}

Appendix B.  The Small Print

   This appendix lists a few corner cases of URI semantics that
   implementers of CRIs need to be aware of, but that are not
   representative of the normal operation of CRIs.

   SP1.  Initial (Lone/Leading) Empty Path Segments:

   *  _Lone empty path segments:_ As per [RFC3986], s://x is distinct
      from s://x/ -- i.e., a URI with an empty path ([] in CRI) is
      different from one with a lone empty path segment ([""]).
      However, in HTTP and CoAP, they are implicitly aliased (for CoAP,
      in item 8 of Section 6.4 of [RFC7252]).  As per item 7 of
      Section 6.5 of [RFC7252], recomposition of a URI without Uri-Path
      Options from the other URI-related CoAP Options produces s://x/,
      not s://x -- CoAP prefers the lone empty path segment form.
      Similarly, after discussing HTTP semantics, Section 6.2.3 of
      [RFC3986] states:

   |  In general, a URI that uses the generic syntax for authority with
   |  an empty path should be normalized to a path of "/".

   *  _Leading empty path segments without authority_: Somewhat related,
      note also that URIs and URI references that do not carry an
      authority cannot represent initial empty path segments (i.e., that
      are followed by further path segments): s://x//foo works, but in a
      s://foo URI or an (absolute-path) URI reference of the form //foo
      the double slash would be mis-parsed as leading in to an
      authority.

   SP2.  Constraints (Section 2) of CRIs/basic CRIs

         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, basic CRIs
            (i.e., without percent-encoded text Section 7.1) 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
            basic 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 be expressed in CRIs if the
            "userinfo" feature is present.  The URI https://@example.com
            is represented as [-4, [false, "", "example", "com"]]; the
            false serves as a marker that the next element is the
            userinfo.

            The rules do not cater for unencoded ":" in userinfo, which
            is commonly considered a deprecated inclusion of a literal
            password.

Appendix C.  CBOR Extended Diagnostic Notation (EDN): The "cri"
             Extension

   [I-D.ietf-cbor-edn-literals] more rigorously defines and further
   extends the CBOR Extended Diagnostic Notation (EDN), as originally
   introduced in Section 8 of [RFC8949] and extended in Appendix G of
   [RFC8610].  Among others, it provides an extension point for
   "application-extension identifiers" that can be used to notate CBOR
   data items in application-specific ways.

   The present document defines and registers (Section 11.3) the
   application-extension identifier "cri", which can be used to notate
   an EDN literal for a CRI reference as defined in this document.

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   The text of the literal is a URI Reference as per [RFC3986] or an IRI
   Reference as per [RFC3987].

   The value of the literal is a CRI reference that can be converted to
   the text of the literal using the procedure of Section 6.1.  Note
   that there may be more than one CRI reference that can be converted
   to the URI/IRI reference given; implementations are expected to favor
   the simplest variant available and make non-surprising choices
   otherwise.

   As an example, the CBOR diagnostic notation

   cri'https://example.com/bottarga/shaved'

   is equivalent to

   [-4, ["example", "com"], ["bottarga", "shaved"]]

   See Appendix C.1 for an ABNF definition for the content of cri
   literals.

C.1.  cri: ABNF Definition of URI Representation of a CRI

   The syntax of the content of cri literals can be described by the
   ABNF for URI-reference in Section 4.1 of [RFC3986], as reproduced in
   Figure 2.  If the content is not ASCII only (i.e., for IRIs), first
   apply Section 3.1 of [RFC3987] and apply this grammar to the result.

   app-string-cri = URI-reference
   ; ABNF from RFC 3986:

   URI           = scheme ":" hier-part [ "?" query ] [ "#" fragment ]

   hier-part     = "//" authority path-abempty
                    / path-absolute
                    / path-rootless
                    / path-empty

   URI-reference = URI / relative-ref

   absolute-URI  = scheme ":" hier-part [ "?" query ]

   relative-ref  = relative-part [ "?" query ] [ "#" fragment ]

   relative-part = "//" authority path-abempty
                    / path-absolute
                    / path-noscheme
                    / path-empty

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   scheme        = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

   authority     = [ userinfo "@" ] host [ ":" port ]
   userinfo      = *( unreserved / pct-encoded / sub-delims / ":" )
   host          = IP-literal / IPv4address / reg-name
   port          = *DIGIT

   IP-literal    = "[" ( IPv6address / IPvFuture  ) "]"

   IPvFuture     = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )

   IPv6address   =                            6( h16 ":" ) ls32
                    /                       "::" 5( h16 ":" ) ls32
                    / [               h16 ] "::" 4( h16 ":" ) ls32
                    / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
                    / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
                    / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
                    / [ *4( h16 ":" ) h16 ] "::"              ls32
                    / [ *5( h16 ":" ) h16 ] "::"              h16
                    / [ *6( h16 ":" ) h16 ] "::"

   h16           = 1*4HEXDIG
   ls32          = ( h16 ":" h16 ) / IPv4address
   IPv4address   = dec-octet "." dec-octet "." dec-octet "." dec-octet
   dec-octet     = DIGIT                 ; 0-9
                    / %x31-39 DIGIT         ; 10-99
                    / "1" 2DIGIT            ; 100-199
                    / "2" %x30-34 DIGIT     ; 200-249
                    / "25" %x30-35          ; 250-255

   reg-name      = *( unreserved / pct-encoded / sub-delims )

   path          = path-abempty    ; begins with "/" or is empty
                    / path-absolute   ; begins with "/" but not "//"
                    / path-noscheme   ; begins with a non-colon segment
                    / path-rootless   ; begins with a segment
                    / path-empty      ; zero characters

   path-abempty  = *( "/" segment )
   path-absolute = "/" [ segment-nz *( "/" segment ) ]
   path-noscheme = segment-nz-nc *( "/" segment )
   path-rootless = segment-nz *( "/" segment )
   path-empty    = 0<pchar>

   segment       = *pchar
   segment-nz    = 1*pchar
   segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
                    ; non-zero-length segment without any colon ":"

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   pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"

   query         = *( pchar / "/" / "?" )

   fragment      = *( pchar / "/" / "?" )

   pct-encoded   = "%" HEXDIG HEXDIG

   unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"
   reserved      = gen-delims / sub-delims
   gen-delims    = ":" / "/" / "?" / "#" / "[" / "]" / "@"
   sub-delims    = "!" / "$" / "&" / "'" / "(" / ")"
                    / "*" / "+" / "," / ";" / "="

          Figure 2: ABNF Definition of URI Representation of a CRI

Appendix D.  Change Log

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

   Changes from -09 to -14

   *  Editorial changes; move some examples to Appendix B, break up
      railroad diagram; mention commonalities with (and tiny difference
      from) CoAP Options; mention failure of percent-encoding for dots
      in host-name components

   *  Explicitly mention invalid case in Section 2.1, Paragraph 5, Item
      3 (rootless CRIs without authority that do not have a path
      component)

   *  Generalize Section 7, discuss PET (percent-encoded text) extension
      in more detail

   *  Add registry of URI scheme numbers (Appendix A, Section 11)

   *  Add user information to the authority ("userinfo" feature)

   *  Figure 1: Use separate rule for CRI, allow [] for query in CRI
      Reference; generalize scheme numbers, add userinfo; add list of
      additional requirements in prose (Section 5.1, Paragraph 5)

   *  Discuss Error handling and extensibility (Section 5.2.1)

   *  Conversion to URI: Handle : in first pathname component of a CRI-
      Reference (Section 6.1, Paragraph 2.6.1)

   *  Add Christian Amsüss as contributor

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   *  Add CBOR EDN application-extension "cri" (see Appendix C and
      Section 11.3).

   *  Add Section on CoAP integration (and new CoAP Options Proxy-Cri
      and Proxy-Scheme-Number).

   Changes from -08 to -09

   *  Identify more esoteric features with a CDDL ".feature".

   *  Clarify that well-formedness requires removing trailing nulls.

   *  Fragments can contain PET.

   *  Percent-encoded text in PET is treated as byte strings.

   *  URIs with an authority but a completely empty path (e.g.,
      http://example.com): CRIs with an authority component no longer
      always produce at least a slash in the path component.

      For generic schemes, the conversion of scheme://example.com to a
      CRI is now possible because CRI produces a URI with an authority
      not followed by a slash following the updated rules of
      Section 6.1.  Schemes like http and coap 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 implementations to convert the
      just-a-slash URI to a CRI with a zero length path array (which,
      however, when converted back, does not produce a slash after the
      authority).

      (Add an appendix "the small print" for more detailed discussion of
      pesky corner cases like this.)

   Changes from -07 to -08

   *  Fix the encoding of NOAUTH-NOSLASH / NOAUTH-LEADINGSLASH

   *  Add URN and DID schemes, add example.

   *  Add PET

   *  Remove hopeless attempt to encode "remote trailing nulls" rule in
      CDDL (which is not a transformation language).

   Changes from -06 to -07

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   *  More explicitly discuss constraints (Section 2), add examples
      (Appendix B, Paragraph 6, Item 1).

   *  Make CDDL more explicit about special simple values.

   *  Lots of gratuitous changes from XML2RFC redefinition of <tt>
      semantics.

   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:

   *  Minor editorial improvements.

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

   *  Renamed option to section, substructured into items.

   *  Simplified 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.

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   *  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, Thomas Fossati, Ari Keränen, Jim Schaad,
   Dave Thaler, and Marco Tiloca for helpful comments and discussions
   that have shaped the document.

Contributors

   Klaus Hartke
   Ericsson
   Torshamnsgatan 23
   SE-16483 Stockholm
   Sweden
   Email: klaus.hartke@ericsson.com

   Christian Amsüss
   Hollandstr. 12/4
   1020 Vienna
   Austria
   Email: christian@amsuess.com

Authors' Addresses

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   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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