CoRE Working Group C. Bormann, Ed.
Internet-Draft Universität Bremen TZI
Intended status: Standards Track H. Birkholz
Expires: 20 July 2022 Fraunhofer SIT
16 January 2022
Constrained Resource Identifiers
draft-ietf-core-href-09
Abstract
The Constrained Resource Identifier (CRI) is a complement to the
Uniform Resource Identifier (URI) that serializes the URI components
in Concise Binary Object Representation (CBOR) instead of a sequence
of characters. This simplifies parsing, comparison and reference
resolution in environments with severe limitations on processing
power, code size, and memory size.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-core-href/.
Discussion of this document takes place on the Constrained RESTful
Environments Working Group mailing list (mailto:core@ietf.org), which
is archived at https://mailarchive.ietf.org/arch/browse/core/.
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/href.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 20 July 2022.
Copyright Notice
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document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 4
2. Constraints . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Constraints by example . . . . . . . . . . . . . . . . . 6
2.2. Constraints not expressed by the data model . . . . . . . 7
3. Creation and Normalization . . . . . . . . . . . . . . . . . 8
4. Comparison . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. CRI References . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. CBOR Serialization . . . . . . . . . . . . . . . . . . . 10
5.2. Ingesting and encoding a CRI Reference . . . . . . . . . 12
5.3. Reference Resolution . . . . . . . . . . . . . . . . . . 13
6. Relationship between CRIs, URIs and IRIs . . . . . . . . . . 14
6.1. Converting CRIs to URIs . . . . . . . . . . . . . . . . . 14
7. Extended CRI: Accommodating Percent Encoding . . . . . . . . 16
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 18
9. Security Considerations . . . . . . . . . . . . . . . . . . . 18
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. The Small Print . . . . . . . . . . . . . . . . . . 20
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 20
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 23
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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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.
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1.1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
In this specification, the term "byte" is used in its now customary
sense as a synonym for "octet".
Terms defined in this document appear in _cursive_ where they are
introduced (rendered in plain text as the new term surrounded by
underscores).
2. Constraints
A Constrained Resource Identifier consists of the same five
components as a URI: scheme, authority, path, query, and fragment.
The components are subject to the following constraints:
C1. The scheme name can be any Unicode string (see Definition D80
in [Unicode]) that matches the syntax of a URI scheme (see
Section 3.1 of [RFC3986], which constrains schemes to ASCII)
and is lowercase (see Definition D139 in [Unicode]). The
scheme is always present.
C2. An authority is always a host identified by an IP address or
registered name, along with optional port information. User
information is not supported (it is often considered to be a
deprecated part of the URI syntax, but then see also
https://www.rfc-editor.org/errata/eid5964 (https://www.rfc-
editor.org/errata/eid5964)).
Alternatively, the authority can be absent; the two cases for
this defined in Section 3.3 of [RFC3986] are modeled by two
different values used in place of an absent authority:
* the path can begin with a root ("/", as when the authority
is present), or
* the path can be rootless.
(Note that in Figure 1, no-authority is marked as a feature, as
not all CRI implementations will support authority-less URIs.)
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C3. An IP address can be either an IPv4 address or an IPv6 address,
optionally with a zone identifier [RFC6874]. Future versions
of IP are not supported (it is likely that a binary mapping
would be strongly desirable, and that cannot be designed ahead
of time, to these versions need to be added as a future
extension if needed).
C4. A registered name is a sequence of one or more _labels_, which,
when joined with dots (".") in between them, result in a
Unicode string that is lowercase and in Unicode Normalization
Form C (NFC) (see Definition D120 in [Unicode]). (The syntax
may be further restricted by the scheme.)
C5. A port is always an integer in the range from 0 to 65535.
Ports outside this range, empty ports (port subcomponents with
no digits, see Section 3.2.3 of [RFC3986]), or ports with
redundant leading zeros, are not supported.
C6. The port is omitted if and only if the port would be the same
as the scheme's default port (provided the scheme is defining
such a default port) or the scheme is not using ports.
C7. 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 not for CRIs in general as they only
perform normalization on the Syntax-Based Normalization level
(Section 6.2.2 of [RFC3986], not on the scheme-specific Scheme-
Based Normalization level (Section 6.2.3 of [RFC3986]).
(A CRI implementation may want to offer scheme-cognizant
interfaces, performing this scheme-specific normalization for
schemes it knows. The interface could assert which schemes the
implementation knows and provide pre-normalized CRIs. This can
also relieve the application from removing a lone zero-length
path segment before putting path segments into CoAP Options,
i.e., from performing the check and jump in item 8 of
Section 6.4 of [RFC7252]. See also SP1 in Appendix A.)
C8. 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
can not be empty. (See also SP1 in Appendix A.)
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C9. A query always consists of one or more query parameters. A
query parameter can be any Unicode string that is in NFC. It
is often in the form of a "key=value" pair. When converting a
CRI to a URI, query parameters are separated by an ampersand
("&") character. (This matches the structure and encoding of
the target URI in CoAP requests.) Queries are optional; there
is a difference between an absent query and a single query
parameter that is the empty string.
C10. A fragment identifier can be any Unicode string that is in NFC.
Fragment identifiers are optional; there is a difference
between an absent fragment identifier and a fragment identifier
that is the empty string.
C11. The syntax of registered names, path segments, query
parameters, and fragment identifiers may be further restricted
and sub-structured by the scheme. There is no support,
however, for escaping sub-delimiters that are not intended to
be used in a delimiting function.
C12. When converting a CRI to a URI, any character that is outside
the allowed character range or is a delimiter in the URI syntax
is percent-encoded. For CRIs, percent-encoding always uses the
UTF-8 encoding form (see Definition D92 in [Unicode]) to
convert the character to a sequence of bytes (that is then
converted to a sequence of %HH triplets).
2.1. Constraints by example
While most URIs in everyday use can be converted to CRIs and back to
URIs matching the input after syntax-based normalization of the URI,
these URIs illustrate the constraints by example:
* https://host%ffname, https://example.com/x?data=%ff
All URI components must, after percent decoding, be valid UTF-8
encoded text. Bytes that are not valid UTF-8 show up, for
example, in BitTorrent web seeds.
* https://example.com/component%3bone;component%3btwo,
http://example.com/component%3dequals
While delimiters can be used in an escaped and unescaped form in
URIs with generally distinct meanings, CRIs only support one
escapable delimiter character per component, which is the
delimiter by which the component is split up in the CRI.
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Note that the separators . (for authority parts), / (for paths), &
(for query parameters) are special in that they are syntactic
delimiters of their respective components in CRIs. Thus, the
following examples _are_ convertible to CRIs:
https://interior%2edot/
https://example.com/path%2fcomponent/second-component
https://example.com/x?ampersand=%26&questionmark=?
* https://alice@example.com/
The user information can not be expressed in CRIs.
2.2. Constraints not expressed by the data model
There are syntactically valid CRIs and CRI references that can not be
converted into a URI or URI reference, respectively.
For CRI references, this is acceptable -- they can be resolved still
and result in a valid CRI that can be converted back. (An example of
this is [0, ["p"]] which appends a slash and the path segment "p" to
its base).
(Full) CRIs that do not correspond to a valid URI are not valid on
their own, and can not be used. Normatively they are characterized
by the Section 6.1 process producing a valid and syntax-normalized
URI. For easier understanding, they are listed here:
* CRIs (and CRI references) containing a path component "." or "..".
These would be removed by the remove_dot_segments algorithm of
[RFC3986], and thus never produce a normalized URI after
resolution.
(In CRI references, the discard value is used to afford segment
removal, and with "." being an unreserved character, expressing
them as "%2e" and "%2e%2e" is not even viable, let alone
practical).
* CRIs without authority whose path starts with two or more empty
segments.
When converted to URIs, these would violate the requirement that
in absence of an authority, a URI's path can not begin with two
slash characters, and they would be indistinguishable from a URI
with a shorter path and a present but empty authority component.
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3. Creation and Normalization
In general, resource identifiers are created on the initial creation
of a resource with a certain resource identifier, or the initial
exposition of a resource under a particular resource identifier.
A Constrained Resource Identifier SHOULD be created by the naming
authority that governs the namespace of the resource identifier (see
also [RFC8820]). For example, for the resources of an HTTP origin
server, that server is responsible for creating the CRIs for those
resources.
The naming authority MUST ensure that any CRI created satisfies the
constraints defined in Section 2. The creation of a CRI fails if the
CRI cannot be validated to satisfy all of the constraints.
If a naming authority creates a CRI from user input, it MAY apply the
following (and only the following) normalizations to get the CRI more
likely to validate:
* map the scheme name to lowercase (C1);
* map the registered name to NFC (C4) and split it on embedded dots;
* elide the port if it is the default port for the scheme (C6);
* map path segments, query parameters and the fragment identifier to
NFC form (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.
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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 5.3 (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.
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5.1. CBOR Serialization
A CRI reference is encoded as a CBOR array [RFC8949], with the
structure as described in the Concise Data Definition Language (CDDL)
[RFC8610] as follows:
; not expressed in this CDDL spec: trailing nulls to be left off
CRI-Reference = [
((scheme / null, authority / no-authority)
// discard), ; relative reference
path / null,
query / null,
fragment / null
]
scheme = scheme-name / scheme-id
scheme-name = text .regexp "[a-z][a-z0-9+.-]*"
scheme-id = (COAP / COAPS / HTTP / HTTPS / URN / DID /
other-scheme)
.within nint
COAP = -1 COAPS = -2 HTTP = -3 HTTPS = -4 URN = -5 DID = -6
other-scheme = nint .feature "scheme-id-extension"
no-authority = NOAUTH-NOSLASH / NOAUTH-LEADINGSLASH
NOAUTH-LEADINGSLASH = null .feature "no-authority"
NOAUTH-NOSLASH = true .feature "no-authority"
authority = [host, ?port]
host = (host-ip // host-name)
host-name = (*text) ; lowercase, NFC labels
host-ip = (bytes .size 4 //
(bytes .size 16, ?zone-id))
zone-id = text
port = 0..65535
discard = DISCARD-ALL / 0..127
DISCARD-ALL = true
path = [*text]
query = [*text]
fragment = text
Figure 1: CDDL for CRI CBOR serialization
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This CDDL specification is simplified for exposition and needs to be
augmented by the following rule for interchange: Trailing null values
MUST be removed, and two leading null values (scheme and authority
both not given) are represented by using the discard alternative
instead.
The rules scheme, authority, path, query, fragment correspond to the
(sub-)components of a CRI, as described in Section 2, with the
addition of the discard section. The discard section can be used
when neither a scheme nor an authority is present. It then expresses
path prefixes such as "/", "./", "../", "../../", etc. The exact
semantics of the section values are defined by Section 5.3.
Most URI references that Section 4.2 of [RFC3986] calls "relative
references" (i.e., references that need to undergo a resolution
process to obtain a URI) correspond to the CRI form that starts with
discard. The exception are relative references with an authority
(called a "network-path reference" in Section 4.2 of [RFC3986]),
which in CRI references never carry a discard section (the value of
discard defaults to true).
| The structure of a CRI is visualized using the somewhat limited
| means of a railroad diagram below.
|
| cri-reference:
| ╭──────────────────────────────────────>───────────────────────────────────────╮
| │ │
| │ ╭─────────────────────>─────────────────────╮ │
| │ │ │ │
| │ │ ╭──────────────>──────────────╮ │ │
| │ │ │ │ │ │
| │ │ │ ╭──────>───────╮ │ │ │
| │ │ │ │ │ │ │ │
| │├──╯──╮── scheme ── authority ──╭──╯── path ──╯── query ──╯── fragment ──╰──╰──╰──╰──┤│
| │ │
| ╰──────── discard ────────╯
|
| This visualization does not go into the details of the
| elements.
Examples:
[-1, / scheme -- equivalent to "coap" /
[h'C6336401', / host /
61616], / port /
[".well-known", / path /
"core"]
]
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[true, / discard /
[".well-known", / path /
"core"],
["rt=temperature-c"]] / query /
[-6, / scheme -- equivalent to "did" /
true, / authority = NOAUTH-NOSLASH /
["web:alice:bob"] / path /
]
A CRI reference is considered _well-formed_ if it matches the
structure as expressed in Figure 1 in CDDL, with the additional
requirement that trailing null values are removed from the array.
A CRI reference is considered _absolute_ if it is well-formed and the
sequence of sections starts with a non-null scheme.
A CRI reference is considered _relative_ if it is well-formed and the
sequence of sections is empty or starts with a section other than
those that would constitute a scheme.
5.2. Ingesting and encoding a CRI Reference
From an abstract point of view, a CRI Reference is a data structure
with six sections:
scheme, authority, discard, path, query, fragment
Each of these sections can be unset ("null"), except for discard,
which is always an unsigned number or true. If scheme and/or
authority are non-null, discard must be true.
When ingesting a CRI Reference that is in the transfer form, those
sections are filled in from the transfer form (unset sections are
filled with null), and the following steps are performed:
* If the array is entirely empty, replace it with [0].
* If discard is present in the transfer form (i.e., the outer array
starts with true or an unsigned number), set scheme and authority
to null.
* If scheme and/or authority are present in the transfer form (i.e.,
the outer array starts with null, a text string, or a negative
integer), set discard to true.
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Upon encoding the abstract form into the transfer form, the inverse
processing is performed: If scheme and/or authority are not null, the
discard value is not transferred (it must be true in this case). If
they are both null, they are both left out and only discard is
transferred. Trailing null values are removed from the array. As a
special case, an empty array is sent in place for a remaining [0]
(URI "").
5.3. Reference Resolution
The term "relative" implies that a "base CRI" exists against which
the relative reference is applied. Aside from fragment-only
references, relative references are only usable when a base CRI is
known.
The following steps define the process of resolving any well-formed
CRI reference against a base CRI so that the result is a CRI in the
form of an absolute CRI reference:
1. Establish the base CRI of the CRI reference and express it in the
form of an abstract absolute CRI reference.
2. Initialize a buffer with the sections from the base CRI.
3. If the value of discard is true in the CRI reference, replace the
path in the buffer with the empty array, unset query and
fragment, and set a true authority to null. If the value of
discard is an unsigned number, remove as many elements from the
end of the path array; if it is non-zero, unset query and
fragment. Set discard to true in the buffer.
4. If the path section is set in the CRI reference, append all
elements from the path array to the array in the path section in
the buffer; unset query and fragment.
5. Apart from the path and discard, copy all non-null sections from
the CRI reference to the buffer in sequence; unset fragment if
query is non-null and thus copied.
6. Return the sections in the buffer as the resolved CRI.
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6. Relationship between CRIs, URIs and IRIs
CRIs are meant to replace both Uniform Resource Identifiers (URIs)
[RFC3986] and Internationalized Resource Identifiers (IRIs) [RFC3987]
in constrained environments [RFC7228]. Applications in these
environments may never need to use URIs and IRIs directly, especially
when the resource identifier is used simply for identification
purposes or when the CRI can be directly converted into a CoAP
request.
However, it may be necessary in other environments to determine the
associated URI or IRI of a CRI, and vice versa. Applications can
perform these conversions as follows:
CRI to URI
A CRI is converted to a URI as specified in Section 6.1.
URI to CRI
The method of converting a URI to a CRI is unspecified;
implementations are free to use any algorithm as long as
converting the resulting CRI back to a URI yields an equivalent
URI.
CRI to IRI
A CRI can be converted to an IRI by first converting it to a URI
as specified in Section 6.1, and then converting the URI to an IRI
as described in Section 3.2 of [RFC3987].
IRI to CRI
An IRI can be converted to a CRI by first converting it to a URI
as described in Section 3.1 of [RFC3987], and then converting the
URI to a CRI as described above.
Everything in this section also applies to CRI references, URI
references and IRI references.
6.1. Converting CRIs to URIs
Applications MUST convert a CRI reference to a URI reference by
determining the components of the URI reference according to the
following steps and then recomposing the components to a URI
reference string as specified in Section 5.3 of [RFC3986].
scheme
If the CRI reference contains a scheme section, the scheme
component of the URI reference consists of the value of that
section. Otherwise, the scheme component is unset.
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authority
If the CRI reference contains a host-name or host-ip item, the
authority component of the URI reference consists of a host
subcomponent, optionally followed by a colon (":") character and a
port subcomponent. Otherwise, the authority component is unset.
The host subcomponent consists of the value of the host-name or
host-ip item.
The host-name is turned into a single string by joining the
elements separated by dots ("."). Any character in the value of a
host-name item that is not in the set of unreserved characters
(Section 2.3 of [RFC3986]) or "sub-delims" (Section 2.2 of
[RFC3986]) MUST be percent-encoded.
The value of a host-ip item MUST be represented as a string that
matches the "IPv4address" or "IP-literal" rule (Section 3.2.2 of
[RFC3986]). Any zone-id is appended to the string, separated by
"%25" as defined in Section 2 of [RFC6874], or as specified in a
successor zone-id specification document; this also leads to a
modified "IP-literal" rule as specified in these documents.
If the CRI reference contains a port item, the port subcomponent
consists of the value of that item in decimal notation.
Otherwise, the colon (":") character and the port subcomponent are
both omitted.
path
If the CRI reference contains a discard item of value true, the
path component is prefixed by a slash ("/") character. If it
contains a discard item of value 0 and the path item is present,
the conversion fails. If it contains a positive discard item, 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.
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If the authority component is present (not null or true) and the
path component does not match the "path-abempty" rule (Section 3.3
of [RFC3986]), the conversion fails.
If the authority component is not present, but the scheme
component is, and the path component does not match the "path-
absolute", "path-rootless" (authority == true) or "path-empty"
rule (Section 3.3 of [RFC3986]), the conversion fails.
If neither the authority component nor the scheme component are
present, and the path component does not match the "path-
absolute", "path-noscheme" or "path-empty" rule (Section 3.3 of
[RFC3986]), the conversion fails.
query
If the CRI reference contains one or more query items, the query
component of the URI reference consists of the value of each item,
separated by an ampersand ("&") character. Otherwise, the query
component is unset.
Any character in the value of a query item that is not in the set
of unreserved characters or "sub-delims" or a colon (":"),
commercial at ("@"), slash ("/") or question mark ("?") character
MUST be percent-encoded. Additionally, any ampersand character
("&") in the item value MUST be percent-encoded.
fragment
If the CRI reference contains a fragment item, the fragment
component of the URI reference consists of the value of that item.
Otherwise, the fragment component is unset.
Any character in the value of a fragment item that is not in the
set of unreserved characters or "sub-delims" or a colon (":"),
commercial at ("@"), slash ("/") or question mark ("?") character
MUST be percent-encoded.
7. Extended CRI: Accommodating Percent Encoding
CRIs have been designed to relieve implementations operating on CRIs
from string scanning, which both helps constrained implementations
and implementations that need to achieve high throughput.
Basic CRI does not support URI components that _require_ percent-
encoding (Section 2.1 of [RFC3986]) to represent them in the URI
syntax, except where that percent-encoding is used to escape the main
delimiter in use.
E.g., the URI
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https://alice/3%2f4-inch
is represented by the basic CRI
[-4, ["alice"], ["3/4-inch"]]
However, percent-encoding that is used at the application level is
not supported by basic CRIs:
did:web:alice:7%3A1-balun
This section presents a method to represent percent-encoded segments
of hostnames, paths, and queries.
The four CDDL rules
host-name = (*text)
path = [*text]
query = [*text]
fragment = text
are replaced with
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 fragment component, 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"]]]
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8. Implementation Status
With the exception of the authority=true fix, host-names split into
labels, and Section 7, CRIs are implemented in
https://gitlab.com/chrysn/micrurus.
9. Security Considerations
Parsers of CRI references must operate on input that is assumed to be
untrusted. This means that parsers MUST fail gracefully in the face
of malicious inputs. Additionally, parsers MUST be prepared to deal
with resource exhaustion (e.g., resulting from the allocation of big
data items) or exhaustion of the call stack (stack overflow). See
Section 10 of [RFC8949] for additional security considerations
relating to CBOR.
The security considerations discussed in Section 7 of [RFC3986] and
Section 8 of [RFC3987] for URIs and IRIs also apply to CRIs.
10. IANA Considerations
This document has no IANA actions.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <https://www.rfc-editor.org/info/rfc3987>.
[RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing
IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
February 2013, <https://www.rfc-editor.org/info/rfc6874>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
13.0.0", ISBN 978-1-936213-26-9, March 2020,
<https://www.unicode.org/versions/Unicode13.0.0/>.
11.2. Informative References
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
<https://www.rfc-editor.org/info/rfc8141>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
[RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 8820, DOI 10.17487/RFC8820, June 2020,
<https://www.rfc-editor.org/info/rfc8820>.
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[W3C.REC-html52-20171214]
Faulkner, S., Eicholz, A., Leithead, T., Danilo, A., and
S. Moon, "HTML 5.2", World Wide Web Consortium
Recommendation REC-html52-20171214, 14 December 2017,
<https://www.w3.org/TR/2017/REC-html52-20171214>.
Appendix A. The Small Print
This appendix lists a few corner cases of URI semantics that
implementers of CRIs need to be aware of, but that are not
representative of the normal operation of CRIs.
SP1. Initial (Lone/Leading) Empty Path Segments:
* _Lone empty path segments:_ As per [RFC3986], s://x is distinct
from s://x/ -- i.e., a URI with an empty path is different from
one with a lone empty path segment. However, in HTTP, CoAP, they
are implicitly aliased (for CoAP, in item 8 of Section 6.4 of
[RFC7252]). As per item 7 of Section 6.5 of [RFC7252],
recomposition of a URI without Uri-Path Options from the other
URI-related CoAP Options produces s://x/, not s://x -- CoAP
prefers the lone empty path segment form.
// TBD: add similar text for HTTP, if that can be
made.Section 6.2.3 of [RFC3986] even states:
| In general, a URI that uses the generic syntax for authority with
| an empty path should be normalized to a path of "/".
* _Leading empty path segments without authority_: Somewhat related,
note also that URIs and URI references that do not carry an
authority cannot represent initial empty path segments (i.e., that
are followed by further path segments): s://x//foo works, but in a
s://foo URI or an (absolute-path) URI reference of the form //foo
the double slash would be mis-parsed as leading in to an
authority.
// (TBD: Add more small print/move that over from above.)
Appendix B. Change Log
This section is to be removed before publishing as an RFC.
Changes from -08 to -09
* Identify more esoteric features with a CDDL ".feature".
* Clarify that well-formedness requires removing trailing nulls.
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* Fragments can contain PET.
* Percent-encoded text in PET is treated as byte strings.
* URIs with an authority but a completely empty path (e.g.,
http://example.com): CRIs with an authority component no longer
always produce at least a slash in the path component.
For generic schemes, the conversion of scheme://example.com to a
CRI is now possible because CRI produces a URI with an authority
not followed by a slash following the updated rules of
Section 6.1. Schemes like http and coap do not distinguish
between the empty path and the path containing a single slash when
an authority is set (as recommended in [RFC3986]). For these
schemes, that equivalence allows implementations to convert the
just-a-slash URI to a CRI with a zero length path array (which,
however, when converted back, does not produce a slash after the
authority).
(Add an appendix "the small print" for more detailed discussion of
pesky corner cases like this.)
Changes from -07 to -08
* Fix the encoding of NOAUTH-NOSLASH / NOAUTH-LEADINGSLASH
* Add URN and DID schemes, add example.
* Add PET
* Remove hopeless attempt to encode "remote trailing nulls" rule in
CDDL (which is not a transformation language).
Changes from -06 to -07
* More explicitly discuss constraints (Section 2), add examples
(Section 2.1).
* Make CDDL more explicit about special simple values.
* Lots of gratuitous changes from XML2RFC redefinition of <tt>
semantics.
Changes from -05 to -06
* rework authority:
- split reg-names at dots;
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- add optional zone identifiers [RFC6874] to IP addresses
Changes from -04 to -05
* Simplify CBOR structure.
* Add implementation status section.
Changes from -03 to -04:
* Minor editorial improvements.
* Renamed path.type/path-type to discard.
* Renamed option to section, substructured into items.
* Simplified the table "resolution-variables".
* Use the CBOR structure inspired by Jim Schaad's proposals.
Changes from -02 to -03:
* Expanded the set of supported schemes (#3).
* Specified creation, normalization and comparison (#9).
* Clarified the default value of the path.type option (#33).
* Removed the append-relation path.type option (#41).
* Renumbered the remaining path.types.
* Renumbered the option numbers.
* 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.
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Acknowledgements
CRIs were developed by Klaus Hartke for use in the Constrained
RESTful Application Language (CoRAL). The current author team is
completing this work with a view to achieve good integration with the
potential use cases, both inside and outside of CoRAL.
Thanks to Christian Amsüss, Thomas Fossati, Ari Keränen, Jim Schaad,
Dave Thaler and Marco Tiloca for helpful comments and discussions
that have shaped the document.
Contributors
Klaus Hartke
Ericsson
Torshamnsgatan 23
SE-16483 Stockholm
Sweden
Email: klaus.hartke@ericsson.com
Authors' Addresses
Carsten Bormann (editor)
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Henk Birkholz
Fraunhofer SIT
Rheinstrasse 75
64295 Darmstadt
Germany
Email: henk.birkholz@sit.fraunhofer.de
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