| Internet-Draft | Constrained Resource Identifiers | July 2021 |
| Bormann & Birkholz | Expires 13 January 2022 | [Page] |
The Constrained Resource Identifier (CRI) is a complement to the Uniform Resource Identifier (URI) that serializes the URI components in Concise Binary Object Representation (CBOR) instead of a sequence of characters. This simplifies parsing, comparison and reference resolution in environments with severe limitations on processing power, code size, and memory size.¶
This note is to be removed before publishing as an RFC.¶
The issues list for this Internet-Draft can be found at <https://github.com/core-wg/coral/labels/href>.¶
A reference implementation and a set of test vectors can be found at <https://github.com/core-wg/coral/tree/master/binary/python>.¶
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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].¶
A URI reference is a sequence of characters chosen from the repertoire of US-ASCII characters. The individual components of a URI reference are delimited by a number of reserved characters, which necessitates the use of a character escape mechanism called "percent-encoding" when these reserved characters are used in a non-delimiting function. The resolution of URI references involves parsing a character sequence into its components, combining those components with the components of a base URI, merging path components, removing dot-segments, and recomposing the result back into a character sequence.¶
Overall, the proper handling of URI references is quite intricate. This can be a problem especially in constrained environments [RFC7228], where nodes often have severe code size and memory size limitations. As a result, many implementations in such environments support only an ad-hoc, informally-specified, bug-ridden, non-interoperable subset of half of RFC 3986.¶
This document defines the Constrained Resource Identifier (CRI) by constraining URIs to a simplified subset and serializing their components in Concise Binary Object Representation (CBOR) [RFC8949] instead of a sequence of characters. This allows typical operations on URI references such as parsing, comparison and reference resolution (including all corner cases) to be implemented in a comparatively small amount of code.¶
As a result of simplification, however, CRIs are not capable of expressing all URIs permitted by the generic syntax of RFC 3986 (hence the "constrained" in "Constrained Resource Identifier"). The supported subset includes all URIs of the Constrained Application Protocol (CoAP) [RFC7252], most URIs of the Hypertext Transfer Protocol (HTTP) [RFC7230], Uniform Resource Names (URNs) [RFC8141], and other similar URIs. The exact constraints are defined in Section 2.¶
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".¶
Terms defined in this document appear in cursive where they are introduced (rendered in plain text as the new term surrounded by underscores).¶
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:¶
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:¶
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.)¶
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.¶
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 serialization of CRIs in Concise Binary Object Representation (CBOR) [RFC8949]. To reduce representation size, CRIs are not serialized directly. Instead, CRIs are indirectly referenced through CRI references. These take advantage of hierarchical locality and provide a very compact encoding. The CBOR serialization of CRI references is specified in Section 5.1.¶
The only operation defined on a CRI reference is reference resolution: the act of transforming a CRI reference into a CRI. An application MUST implement this operation by applying the algorithm specified in Section 5.3 (or any algorithm that is functionally equivalent to it).¶
The reverse operation of transforming a CRI into a CRI reference is unspecified; implementations are free to use any algorithm as long as reference resolution of the resulting CRI reference yields the original CRI. Notably, a CRI reference is not required to satisfy all of the constraints of a CRI; the only requirement on a CRI reference is that reference resolution MUST yield the original CRI.¶
When testing for equivalence or difference, applications SHOULD NOT directly compare CRI references; the references should be resolved to their respective CRI before comparison.¶
A CRI reference is encoded as a CBOR array [RFC8949], with the structure as described in the Concise Data Definition Language (CDDL) [RFC8610] as follows:¶
; not expressed in this CDDL spec: trailing nulls to be left off
CRI-Reference = [
((scheme / null, authority / null / true)
// discard), ; relative reference
path / null,
query / null,
fragment / null
]
scheme = scheme-name / scheme-id
scheme-name = text .regexp "[a-z][a-z0-9+.-]*"
scheme-id = (COAP / COAPS / HTTP / HTTPS / other-scheme)
.within nint
COAP = -1 COAPS = -2 HTTP = -3 HTTPS = -4
other-scheme = nint .feature "scheme-id-extension"
authority = host / [host, port]
host = host-name / host-ip
host-name = text
host-ip = bytes .size 4 / bytes .size 16
port = 0..65535
discard = true / 0..127
path = [*text]
query = [*text]
fragment = text
¶
This CDDL specification is simplified for exposition and needs to be augmented by the
following rule for interchange: Trailing null values are removed, and
two leading null values (scheme and authority both not given) are
represented by using the discard alternative instead.
A complete CDDL specification is given in Appendix A.¶
The rules scheme, authority, path, query, fragment
correspond to the (sub-)components of a CRI, as described in
Section 2, with the addition of the discard section.
The discard section can be used when neither a scheme nor an
authority is present.
It then expresses path
prefixes such as "/",
"./", "../", "../../", etc.
The exact semantics of the section values are defined by
Section 5.3.¶
Most URI references that Section 4.2 of [RFC3986] calls "relative
references" (i.e., references that need to undergo a resolution
process to obtain a URI) correspond to the CRI form that starts with
discard. The exception are relative references with an authority
(called a "network-path reference" in Section 4.2 of [RFC3986]),
which in CRI references never carry a discard section (the value of
discard defaults to true).¶
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 /¶
A CRI reference is considered well-formed if it matches the CDDL structure.¶
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.¶
From an abstract point of view, a CRI Reference is a data structure with six sections:¶
scheme, authority, discard, path, query, fragment¶
Each of these sections can be unset ("null"),
except for discard,
which is always an unsigned number or true. If scheme and/or
authority are non-null, discard must be true.¶
When ingesting a CRI Reference that is in the transfer form, those sections are filled in from the transfer form (unset sections are filled with null), and the following steps are performed:¶
[0].¶
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 "").¶
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:¶
true in the CRI reference, replace the
path in the buffer with the empty array, unset query and
fragment, and set a true authority to null. If the value of
discard is an unsigned number, remove as many elements
from the end of the path array; if it is non-zero, unset query and
fragment. Set discard to true in the buffer.¶
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:¶
A CRI is converted to a URI as specified in Section 6.1.¶
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.¶
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].¶
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.¶
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].¶
If the CRI reference contains a scheme section, the scheme
component of the URI reference consists of the value of that
section.
Otherwise, the scheme component is unset.¶
If the CRI reference contains a host-name or host-ip item, the
authority component of the URI reference consists of a host
subcomponent, optionally followed by a colon (":") character and a
port subcomponent. Otherwise, the authority component is unset.¶
The host subcomponent consists of the value of the host-name or
host-ip item.¶
Any character in the value of a host-name item that is not in
the set of unreserved characters (Section 2.3 of [RFC3986]) or
"sub-delims" (Section 2.2 of [RFC3986]) MUST be
percent-encoded.¶
The value of a host-ip item MUST be
represented as a string that matches the "IPv4address" or
"IP-literal" rule (Section 3.2.2 of [RFC3986]).¶
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.¶
If the CRI reference contains a discard item of value true, the
path component is prefixed by a slash ("/") character. If it
contains a discard item of value 0 and the path item is
present, the conversion fails. Otherwise, the path component is
prefixed by as many "../" components as the discard value minus
one indicates.¶
If the discard item is not present and the CRI reference contains an
authority that is true, the path component of the URI reference is
prefixed by the zero-length string. Otherwise, the path component
is prefixed by a slash ("/") character.¶
If the CRI reference contains one or more path items, the prefix
is followed by the value of each item, separated by a slash ("/")
character.¶
Any character in the value of a path item that is not
in the set of unreserved characters or "sub-delims" or a colon
(":") or commercial at ("@") character MUST be
percent-encoded.¶
If the authority component is present (not null or true) and the
path component does not match the "path-abempty" rule (Section 3.3 of [RFC3986]), the conversion fails.¶
If the authority component is not present, but the scheme component
is, and the path component does not match the "path-absolute",
"path-rootless" (authority == true) or "path-empty" rule (Section 3.3 of [RFC3986]), the conversion fails.¶
If neither the authority component nor the scheme component are present, and the path component does not match the "path-absolute", "path-noscheme" or "path-empty" rule (Section 3.3 of [RFC3986]), the conversion fails.¶
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.¶
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.¶
With the exception of the authority=true fix, CRIs are implemented in https://gitlab.com/chrysn/micrurus.¶
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.¶
This document has no IANA actions.¶
The full CDDL specification is somewhat redundant internally in order to express trailing null suppression.¶
; expressing null suppression
CRI-Reference = [
?( ((scheme, (authority / null / true)
// (null, authority))
// discard), ; relative reference
?( (path / null, query / null, fragment) //
(path / null, query) //
path)
)
]
scheme = scheme-name / scheme-id
scheme-name = text .regexp "[a-z][a-z0-9+.-]*"
scheme-id = (COAP / COAPS / HTTP / HTTPS / other-scheme)
.within nint
COAP = -1 COAPS = -2 HTTP = -3 HTTPS = -4
other-scheme = nint .feature "scheme-id-extension"
authority = host / [host, port]
host = host-name / host-ip
host-name = text
host-ip = bytes .size 4 / bytes .size 16
port = 0..65535
discard = true / 0..127
path = [*text]
query = [*text]
fragment = text
¶
This section is to be removed before publishing as an RFC.¶
Changes from -04 to -05¶
Changes from -03 to -04:¶
Changes from -02 to -03:¶
path.type option (#33).¶
append-relation path.type option (#41).¶
Changes from -01 to -02:¶
Changes from -00 to -01:¶
CRIs were developed by Klaus Hartke for use in the Constrained RESTful Application Language (CoRAL). The current author team is completing this work with a view to achieve good integration with the potential use cases, both inside and outside of CoRAL.¶
Thanks to Christian Amsüss, Ari Keränen, Jim Schaad and Dave Thaler for helpful comments and discussions that have shaped the document.¶