Constrained Resource Identifiers

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Table of Contents

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].¶

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.¶

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]) and is lowercase (see Definition D139 in [Unicode]).¶

C2.

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

C3.

An IP address can be either an IPv4 address or an IPv6 address. IPv6 scoped addressing zone identifiers and future versions of IP are not supported.¶

C4.

A registered name can be any Unicode string that is lowercase and in Unicode Normalization Form C (NFC) (see Definition D120 in [Unicode]). (The syntax may be further restricted by the scheme.)¶

C5.

A port is always an integer in the range from 0 to 65535. Empty ports or ports outside this range are not supported.¶

C6.

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

C7.

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

C8.

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

C9.

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

C10.

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

C11.

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

C12.

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

3. Creation and Normalization

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

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

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

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

• map the scheme name to lowercase (C1);¶
• map the registered name to NFC (C4);¶
• elide the port if it is the default port for the scheme (C6);¶
• elide a single zero-length path segment (C7);¶
• map path segments, query parameters and the fragment identifier to NFC (C8, C9, C10).¶

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

4. Comparison

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

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

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

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

5. CRI References

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

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 5.2 (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.¶

CRI-Reference = [
  (authority // discard),
  *path,
  ? (([], fragment)              ; include array only if
     //([+query], ?fragment))    ; at least one query and/or fragment
]

authority   = (?scheme, ?(host, ?port))
scheme      = (scheme-name
               // COAP // COAPS // HTTP // HTTPS)
scheme-name = (false, text .regexp "[a-z][a-z0-9+.-]*")
COAP = -1 COAPS = -2 HTTP = -3 HTTPS = -4
host        = (host-name // host-ip)
host-name   = (true, text)
host-ip     = bytes .size 4 / bytes .size 16
port        = 0..65535
discard     = 0..127
path        = text
query       = text
fragment    = text

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.¶

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.¶

7. 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.¶

8. IANA Considerations

This document has no IANA actions.¶

Appendix A. Change Log

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

Changes from -03 to -04:¶

• Minor editorial improvements.¶
• Renamed path.type/path-type to discard.¶
• Renamed option to element.¶
• Simplied Table 1.¶
• 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 discard option (#33).¶
• Removed the append-relation discard option (#41).¶
• Renumbered the remaining discards.¶
• 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

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

Authors' Addresses