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v: 3

docname: draft-ietf-core-href-latest cat: std submissiontype: IETF consensus: true lang: en title: Constrained Resource Identifiers updates: 7595 wg: CoRE Working Group

author:

  • name: Carsten Bormann org: Universität Bremen TZI street: Postfach 330440 city: Bremen code: D-28359 country: Germany phone: +49-421-218-63921 email: [email protected] role: editor
  • name: Henk Birkholz org: Fraunhofer SIT email: [email protected] street: Rheinstrasse 75 code: '64295' city: Darmstadt country: Germany

contributor:

  • name: Klaus Hartke org: Ericsson street: Torshamnsgatan 23 city: Stockholm code: '16483' country: Sweden email: [email protected]
  • name: Christian Amsüss street: Hollandstr. 12/4 city: Vienna code: 1020 country: Austria email: [email protected]

venue: group: Constrained RESTful Environments mail: [email protected] github: core-wg/href

informative: RFC7228: term STD97: http

RFC9110

RFC7252: coap RFC8141: urn RFC8288: web-linking BCP190: lawn

BCP205:

RFC8820

W3C.REC-html52-20171214: I-D.ietf-cbor-edn-literals: edn I-D.carpenter-6man-rfc6874bis: zonebis normative: STD66: uri

RFC 3986

RFC3987: iri RFC6874: zone BCP35: schemes

RFC7595

IANA.uri-schemes: BCP26: -: ianacons

=: RFC8126

IANA.core-parameters: RFC8610: cddl Unicode: target: https://www.unicode.org/versions/Unicode13.0.0/ title: The Unicode Standard, Version 13.0.0 author: - org: The Unicode Consortium date: 2020-03 seriesinfo: ISBN: 978-1-936213-26-9 STD94: cbor

RFC8949

RFC9165: cddlcontrol

--- 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 in 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 RFC updates RFC 7595 to add a note on how the URI Schemes registry RFC 7595 describes cooperates with the CRI Scheme Numbers registry created by the present RFC.

1

--- middle

{:cabo: source=" -- cabo"}

Introduction

The Uniform Resource Identifier (URI) and its most common usage, the URI reference, are the Internet standard for linking to resources in hypertext formats such as HTML or the HTTP "Link" header field.

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, 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 {{STD66}}.

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) 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 {{STD66}} (hence the "constrained" in "Constrained Resource Identifier"). The supported subset includes all URIs of the Constrained Application Protocol (CoAP), most URIs of the Hypertext Transfer Protocol (HTTP), Uniform Resource Names (URNs), and other similar URIs. The exact constraints are defined in {{constraints}}.

This RFC creates a "CRI Scheme Numbers" registry and updates RFC 7595 to add a note on how this new registry cooperates with the URI Schemes registry that RFC 7595 describes.

Notational Conventions

{::boilerplate bcp14-tagged-bcp14}

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

Terms defined in this document appear in cursive where they are introduced (in the plaintext form of this document, they are rendered as the new term surrounded by underscores).

The general structure of data items is shown in the Concise Data Definition Language (CDDL) including its control extensions. Specific examples are notated in CBOR Extended Diagnostic Notation (EDN), as originally introduced in {{Section 8 of RFC8949@-cbor}} and extended in {{Appendix G of -cddl}}. ({{-edn}} more rigorously defines and further extends EDN.)

Constraints {#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:

{: type="C%d."}

  1. {:#c-scheme} 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@-uri}}, which constrains scheme names to ASCII) and is lowercase (see Definition D139 in {{Unicode}}). The scheme is always present.

  2. {:#c-authority} 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@-uri}} 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 {{cddl}}, no-authority is marked as a feature, as not all CRI implementations will support authority-less URIs.)

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

  4. {:#c-ip-address} An IP address can be either an IPv4 address or an IPv6 address, optionally with a zone identifier {{-zone}}. 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).

  5. {:#c-reg-name} 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@-uri}}, a registered name can be empty, for which case a scheme can define a default for the host.)

  6. {:#c-port-range} 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@-uri}}), or ports with redundant leading zeros, are not supported.

  7. {:#c-port-omitted} 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.

  8. {:#c-path} 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@-uri}}), not on the scheme-specific Scheme-Based Normalization level ({{Section 6.2.3 of RFC3986@-uri}}).

    (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 -coap}}. See also {{<sp-initial-empty}} in {{the-small-print}}.)

  9. {:#c-path-segment} 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 {{<sp-initial-empty}} in {{the-small-print}}.)

  10. {:#c-query} 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.

  11. {:#c-fragment} 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.

  12. {:#c-escaping} 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.

  13. {:#c-mapping} 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 {{<sp-constraints}} in {{the-small-print}}.

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

(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 {{cri-to-uri}} process not 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 {{STD66}}, 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.

  • {:#naked-rootless} 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:.

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 {{BCP190}}). 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 {{constraints}}. The creation of a CRI fails if the CRI cannot be validated to satisfy all of the constraints.

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

  • map the scheme name to lowercase ({{<c-scheme}});
  • map the registered name to NFC ({{<c-reg-name}}) and split it on embedded dots;
  • elide the port if it is the default port for the scheme ({{<c-port-omitted}});
  • map path segments, query parameters and the fragment identifier to NFC form ({{<c-path-segment}}, {{<c-query}}, {{<c-fragment}}).

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.)

Comparison

One of the most common operations on CRIs is comparison: determining whether two CRIs are equivalent, without dereferencing the CRIs (i.e., 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 {{constraints}} 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.

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.

This section defines the representation of CRIs in Concise Binary Object Representation (CBOR). When reduced representation size is desired, CRIs are often 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 {{cbor-representation}}.

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 {{reference-resolution}} (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.

CBOR Representation {#cbor-representation}

2

A CRI or CRI reference is encoded as a CBOR array (Major type 4 in {{Section 3.1 of RFC8949@-cbor}}), with the structure described in CDDL as follows:

{::include cddl/cri.cddl}

{: #cddl title="CDDL for CRI CBOR representation"}

The rules scheme, authority, path, query, fragment correspond to the (sub‑)components of a CRI, as described in {{constraints}}, with the addition of the discard section.

{:#prose} 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 {{ingest}} 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@-cbor}}); this requirement is relaxed for specifications that embed CRIs into an encompassing CBOR representation that does provide for indefinite length encoding.

scheme-name and scheme-id {#scheme-id}

In the scheme section, a CRI scheme can be given by its scheme-name (a text string giving the scheme name as in URIs' scheme section, mapped to lower case), or as a negative integer scheme-id derived from the scheme number. Scheme numbers are unsigned integers that are mapped to and from URI scheme names by the "CRI Scheme Numbers" registry ({{cri-reg}}). The relationship of a scheme number to its scheme-id is as follows:

$$scheme\text{-}id = -1 - scheme\text{-}number \\\ scheme\text{-}number = -1 - scheme\text{-}id$$

For example, the scheme-name coap has the (unsigned integer) scheme-number 0 which is represented in a (negative integer) scheme-id -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 {{reference-resolution}}.

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

Visualization

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

cri-reference = ((scheme authority) / discard) local-part

local-part = [path [query [fragment]]]

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

Examples

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

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

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

Specific Terminology

A CRI reference is considered well-formed if it matches the structure as expressed in {{cddl}} 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.

Ingesting and encoding a CRI Reference {#ingest}

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 reference "").

Error handling and extensibility {#unprocessable}

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 a 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 {{cbor-representation}}.)

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 cannot 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.

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 {{pet}}).

Reference Resolution {#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.

Relationship between CRIs, URIs, and IRIs

CRIs are meant to replace both Uniform Resource Identifiers (URIs) and Internationalized Resource Identifiers (IRIs) in constrained environments. 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:

{:vspace} CRI to URI : A CRI is converted to a URI as specified in {{cri-to-uri}}.

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 -coap}} and item 7 of {{Section 6.5 of -coap}} do not apply to CRI processing. See {{<sp-initial-empty}} in {{the-small-print}} for more details.

CRI to IRI : A CRI can be converted to an IRI by first converting it to a URI as specified in {{cri-to-uri}}, 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.

Converting CRIs to URIs {#cri-to-uri}

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@-uri}}.

{:vspace} 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-id as per {{scheme-id}}. 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@-uri}}) or "sub-delims" ({{Section 2.2 of RFC3986@-uri}}) 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@-uri}}) or "sub-delims" ({{Section 2.2 of RFC3986@-uri}}) 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.)

{: #host-ip-to-uri} 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@-uri}}). Any zone-id is appended to the string; the details for how this is done are currently in flux in the URI specification: {{Section 2 of -zone}} uses percent-encoding and a separator of "%25", while proposals for a future superseding zone-id specification document (such as {{-zonebis}}) are being prepared; 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 : 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@-uri}}). {:#colon}

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@-uri}}), 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@-uri}}), 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@-uri}}), 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.

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.

Extending CRIs {#extending}

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@-uri}}) 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.

One such extended form is described in the following {{pet}}. 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 {{cddl}}. 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 {{unprocessable}}.

Extended CRI: Accommodating Percent Encoding (PET) {#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.

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.)

{:aside}

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.

CoAP Integration

This section discusses ways in which CRIs can be used in the context of the CoAP protocol {{-coap}}.

Converting Between CoAP CRIs and Sets of CoAP Options

This section provides an analogue to {{Sections 6.4 and 6.5 of -coap}}: Computing a set of CoAP options from a request CRI ({{decompose-coap}}) and computing a request CRI from a set of COAP options ({{compose-coap}}).

This section makes use of the mapping between CRI scheme numbers and URI scheme names shown in {{scheme-map}}:

| CRI scheme number | URI scheme name | |-------------------+-----------------| | 0 | coap | | 1 | coaps | | 6 | coap+tcp | | 7 | coaps+tcp | | 8 | coap+ws | | 9 | coaps+ws | {: #scheme-map title="Mapping CRI scheme numbers and URI scheme names"}

Decomposing a Request CRI into a set of CoAP Options {#decompose-coap}

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 ({{pet}}) 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 {{scheme-id}} and {{scheme-map}}; if a scheme-id that corresponds to a scheme number not in this list is being used, or if a scheme-name 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.

    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 {{host-ip-to-uri}}.

  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.

Composing a Request CRI from a Set of CoAP Options {#compose-coap}

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 {{scheme-id}} and table {{scheme-map}}. 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.

  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.

CoAP Options for Forward-Proxies {#coap-options}

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 -coap}} 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 numbering provided by the present specification.

Proxy-CRI

| No. | C | U | N | R | Name | Format | Length | Default | | TBD235 | x | x | - | | Proxy-Cri | opaque | 1-1023 | (none) | {: #tab-proxy-cri title="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 -coap}}. 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).

Proxy-Scheme-Number

| No. | C | U | N | R | Name | Format | Length | Default | | TBD239 | x | x | - | | Proxy-Scheme-Number | uint | 0-3 | (none) | {: #tab-proxy-scheme-number title="Proxy-Scheme-Number CoAP Option"}

The Proxy-Scheme-Number Option carries a CRI Scheme Number represented as a CoAP unsigned integer. It is used analogously to Proxy-Scheme as defined in {{Section 5.10.2 of -coap}}.

As per {{Section 3.2 of -coap}}, CoAP Options are only defined as one of empty, (text) string, opaque (byte string), or uint (unsigned integer). The Option therefore carries an unsigned integer that represents the CRI scheme-number (which relates to a CRI scheme-id as defined in {{scheme-id}}). For instance, the scheme name "coap" has the scheme-number 0 and is represented as an unsigned integer by a zero-length CoAP Option value.

3

Implementation Status {#impl}

{::boilerplate rfc7942info}

With the exception of the authority=true fix, host-names split into labels, and {{pet}}, 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

Security Considerations {#security}

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@-cbor}} for additional security considerations relating to CBOR.

The security considerations discussed in {{Section 7 of RFC3986@-uri}} and {{Section 8 of RFC3987}} for URIs and IRIs also apply to CRIs.

IANA Considerations

CRI Scheme Numbers Registry {#cri-reg}

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 RFC8126@-ianacons}}). 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.

Instructions for the Designated Expert {#de-instructions}

The expert is instructed to be frugal in the allocation of CRI scheme number values whose scheme-id values ({{scheme-id}}) have 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.

When the expert notices that a registration has been made in the Uniform Resource Identifier (URI) Schemes registry (see also {{upd}}), the expert is requested to initiate a parallel registration in the CRI Scheme Numbers registry. CRI scheme number 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 registrations in the CRI Scheme Numbers registry are oblivious to the details of any URI Schemes registry registration, so if a registration is later made in the URI Schemes registry that uses such a previously unregistered text string as a name, the CRI Scheme Numbers registration simply stays in place, even if the URI Schemes registration happens to be for something different from what the expert had in mind at the time for the CRI Scheme Numbers registration. Also note that the initial registrations in {{tab-numbers}} in {{sec-numbers}} 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 [email protected] mailing list for a time-limited discussion.

Structure of Entries

Each entry in the registry must include:

{:vspace} CRI scheme number: : An unsigned 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

Initial Registrations

The initial registrations for the CRI Scheme Numbers registry are provided in {{tab-numbers}} in {{sec-numbers}}.

Update to "Uniform Resource Identifier (URI) Schemes" Registry {#upd}

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

{:quote}

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.

CBOR Diagnostic Notation Application-extension Identifiers Registry {#cri-iana}

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 {{tab-iana}} and defined in {{edn-cri}}.

Application-extension Identifier Description Change Controller Reference
cri Constrained Resource Identifier IETF RFC-XXXX, {{edn-cri}}
{: #tab-iana title="CBOR Extended Diagnostic Notation (EDN) Application-extension Identifier for CRI"}

2

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 {{tab-iana-options}} and defined in {{coap-options}}.

| No. | Name | Reference | | TBD235 | Proxy-Cri | RFC-XXXX | | TBD239 | Proxy-Scheme-Number | RFC-XXXX | {: #tab-iana-options title="New CoAP Option Numbers"}

2

--- back

Mapping Scheme Numbers to Scheme Names {#sec-numbers}

2

{{tab-numbers}} defines the initial mapping from CRI scheme numbers to URI scheme names.

{::include code/schemes-numbers.md} {: #tab-numbers title="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:

{::include code/extract-schemes-numbers.rb}

The Small Print

{:sp: type="SP%d." group="SP"}

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.

{:sp}

  1. {:#sp-initial-empty} Initial (Lone/Leading) Empty Path Segments:
  • Lone empty path segments: As per {{-uri}}, 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 -coap}}). As per item 7 of {{Section 6.5 of -coap}}, 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@-uri}} states:

{:quote}

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.

{:sp} 2. {:#sp-constraints} Constraints ({{constraints}}) 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.

  • 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 {{pet}}) 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://[email protected]/

    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.

CBOR Extended Diagnostic Notation (EDN): The "cri" Extension {#edn-cri}

{{-edn}} more rigorously defines and further extends the CBOR Extended Diagnostic Notation (EDN), as originally introduced in {{Section 8 of RFC8949@-cbor}} and extended in {{Appendix G of -cddl}}. 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 ({{cri-iana}}) the application-extension identifier "cri", which can be used to notate an EDN literal for a CRI reference as defined in this document.

The text of the literal is a URI Reference as per {{-uri}} or an IRI Reference as per {{-iri}}.

The value of the literal is a CRI reference that can be converted to the text of the literal using the procedure of {{cri-to-uri}}. 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 {{cri-grammar}} for an ABNF definition for the content of cri literals.

cri: ABNF Definition of URI Representation of a CRI {#cri-grammar}

The syntax of the content of cri literals can be described by the ABNF for URI-reference in {{Section 4.1 of RFC3986@-uri}} with certain re-arrangements taken from {{figure-5 (Figure 5)<I-D.ietf-cbor-edn-literals}} of {{I-D.ietf-cbor-edn-literals}}; these are reproduced in {{abnf-grammar-cri}}. 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

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 / ":" )

; Use IPv6address, h16, ls32, IPv4adress, dec-octet as re-arranged
; for PEG Compatibility in Figure 5 of [I-D.ietf-cbor-edn-literals]:

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

h16           = 1*4HEXDIG
ls32          = ( h16 ":" h16 ) / IPv4address
IPv4address   = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet     = "25" %x30-35         ; 250-255
              / "2" %x30-34 DIGIT    ; 200-249
              / "1" 2DIGIT           ; 100-199
              / %x31-39 DIGIT        ; 10-99
              / DIGIT                ; 0-9
ALPHA         = %x41-5a / %x61-7a
DIGIT         = %x30-39
HEXDIG        = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
; case insensitive matching, i.e., including lower case

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 ":"

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    = "!" / "$" / "&" / "'" / "(" / ")"
                 / "*" / "+" / "," / ";" / "="

{: #abnf-grammar-cri sourcecode-name="cbor-edn-cri.abnf" title="ABNF Definition of URI Representation of a CRI" }

Change Log

{:removeinrfc}

Changes from -15 to -16

  • Add note that CRI Scheme Number registrations are oblivious of the actual URI Scheme registrations (if any).

  • Add information about how this RFC updates RFC7595 to abstract and introduction.

Changes from -14 to -15

  • Make scheme numbers unsigned and map them to negative numbers used as scheme-id values

Changes from -09 to -14

  • Editorial changes; move some examples to {{the-small-print}}, 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 {{naked-rootless}} (rootless CRIs without authority that do not have a path component)

  • Generalize {{extending}}, discuss PET (percent-encoded text) extension in more detail

  • Add registry of URI scheme numbers ({{sec-numbers}}, {{iana-considerations}})

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

  • {{cddl}}: Use separate rule for CRI, allow [] for query in CRI Reference; generalize scheme numbers, add userinfo; add list of additional requirements in prose ({{prose}})

  • Discuss {{<<unprocessable}} ({{unprocessable}})

  • Conversion to URI: Handle : in first pathname component of a CRI-Reference ({{colon}})

  • Add Christian Amsüss as contributor

  • Add CBOR EDN application-extension "cri" (see {{edn-cri}} and {{cri-iana}}).

  • 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 {{cri-to-uri}}. 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 {{STD66}}). 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

  • More explicitly discuss constraints ({{constraints}}), add examples ({{sp-constraints}}).

  • 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 {{-zone}} 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.

  • 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

{: numbered="false"}

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.

Footnotes

  1. (This "cref" paragraph will be removed by the RFC editor:)\ The present revision –16 of this draft continues -15 by picking up more comments; it was made specifically for IETF 120. \ This revision still contains open issues and is intended to serve as a snapshot. ↩

  2. RFC Ed.: throughout this section, please replace RFC-XXXX with the RFC number of this specification and remove this note. ↩ ↩2 ↩3 ↩4

  3. TO DO: Discuss the need for a location-scheme-numeric option? ↩