Internet DRAFT - draft-ietf-httpapi-yaml-mediatypes
draft-ietf-httpapi-yaml-mediatypes
HTTPAPI R. Polli
Internet-Draft Digital Transformation Department, Italian Government
Intended status: Informational E. Wilde
Expires: 27 May 2023 Axway
E. Aro
Mozilla
23 November 2022
YAML Media Type
draft-ietf-httpapi-yaml-mediatypes-04
Abstract
This document registers the application/yaml media type and the +yaml
structured syntax suffix on the IANA Media Types registry.
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-httpapi-yaml-mediatypes/.
Discussion of this document takes place on the HTTPAPI Working Group
mailing list (mailto:httpapi@ietf.org), which is archived at
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about/.
Source for this draft and an issue tracker can be found at
https://github.com/ietf-wg-httpapi/mediatypes/labels/yaml.
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This Internet-Draft will expire on 27 May 2023.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
1.2. Fragment identification . . . . . . . . . . . . . . . . . 3
1.2.1. Fragment identification via alias nodes . . . . . . . 4
2. Media Type and Structured Syntax Suffix registrations . . . . 5
2.1. Media Type application/yaml . . . . . . . . . . . . . . . 5
2.2. The +yaml Structured Syntax Suffix . . . . . . . . . . . 6
3. Interoperability Considerations . . . . . . . . . . . . . . . 7
3.1. YAML is an Evolving Language . . . . . . . . . . . . . . 7
3.2. YAML streams . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Filename extension . . . . . . . . . . . . . . . . . . . 7
3.4. YAML and JSON . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Fragment identifiers . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
4.1. Arbitrary Code Execution . . . . . . . . . . . . . . . . 10
4.2. Resource Exhaustion . . . . . . . . . . . . . . . . . . . 10
4.3. YAML streams . . . . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1. Normative References . . . . . . . . . . . . . . . . . . 11
6.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Examples related to fragment identifier
interoperability . . . . . . . . . . . . . . . . . . . . 13
A.1. Unreferenceable nodes . . . . . . . . . . . . . . . . . . 13
A.2. Referencing a missing node . . . . . . . . . . . . . . . 14
A.3. Representation graph with anchors and cyclic
references . . . . . . . . . . . . . . . . . . . . . . . 14
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 15
FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
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Since draft-ietf-httpapi-yaml-mediatypes-02 . . . . . . . . . . 16
Since draft-ietf-httpapi-yaml-mediatypes-01 . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
YAML [YAML] is a data serialization format that is capable of
conveying one or multiple documents in a single presentation stream
(e.g. a file or a network resource). It is widely used on the
Internet, including in the API sector (e.g. see [OAS]), but the
relevant media type and structured syntax suffix previously had not
been registered by IANA.
To increase interoperability when exchanging YAML streams, and
leverage content negotiation mechanisms when exchanging YAML
resources, this specification registers the application/yaml media
type and the +yaml structured syntax suffix.
Moreover, it provides security considerations and interoperability
considerations related to [YAML], including its relation with [JSON].
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. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.
This document uses the Augmented BNF defined in [RFC5234] and updated
by [RFC7405].
The terms "content", "content negotiation", "resource", and "user
agent" in this document are to be interpreted as in [SEMANTICS].
The terms "fragment" and "fragment identifier" in this document are
to be interpreted as in [URI].
The terms "presentation", "stream", "YAML document", "representation
graph", "tag", "node", "alias node", "anchor" and "anchor name" in
this document are to be interpreted as in [YAML].
1.2. Fragment identification
A fragment identifies a node in a stream.
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A fragment identifier starting with "*" is to be interpreted as a
YAML alias node Section 1.2.1.
For single-document YAML streams, a fragment identifier that is empty
or that starts with "/" is to be interpreted as a JSON Pointer
[JSON-POINTER] and is evaluated on the YAML representation graph,
walking through alias nodes; in particular, the empty fragment
identifier references the root node. This syntax can only reference
the YAML nodes that are on a path that is made up of nodes
interoperable with the JSON data model (see Section 3.4).
A fragment identifier is not guaranteed to reference an existing
node. Therefore, applications SHOULD define how an unresolved alias
node ought to be handled.
1.2.1. Fragment identification via alias nodes
This section describes how to use alias nodes (see Section 3.2.2.2
and 7.1 of [YAML]) as fragment identifiers to designate nodes.
A YAML alias node can be represented in a URI fragment identifier by
encoding it into bytes using UTF-8 [UTF-8], while percent-encoding
those characters not allowed by the fragment rule in Section 3.5 of
[URI].
If multiple nodes would match a fragment identifier, the first such
match is selected.
Users concerned with interoperability of fragment identifiers:
* SHOULD limit alias nodes to a set of characters that do not
require encoding to be expressed as URI fragment identifiers: this
is generally possible since anchor names are a serialization
detail;
* SHOULD NOT use alias nodes that match multiple nodes.
In the example resource below, the URL file.yaml#*foo references the
first alias node *foo pointing to the node with value scalar and not
the one in the second document; whereas the URL file.yaml#*document_2
references the root node of the second document { one: [a,
sequence]}.
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%YAML 1.2
---
one: &foo scalar
two: &bar
- some
- sequence
- items
...
%YAML 1.2
---
&document_2
one: &foo [a, sequence]
Figure 1: A YAML stream containing two YAML documents.
2. Media Type and Structured Syntax Suffix registrations
This section describes the information required to register the above
media type according to [MEDIATYPE]
2.1. Media Type application/yaml
The media type for YAML text is application/yaml; the following
information serves as the registration form for this media type.
Type name: application
Subtype name: yaml
Required parameters: N/A
Optional parameters: N/A; unrecognized parameters should be ignored
Encoding considerations: binary
Security considerations: see Section 4 of this document
Interoperability considerations: see Section 3 of this document
Published specification: [YAML]
Applications that use this media type: Applications that need a
human-friendly, cross language, Unicode based data serialization
language designed around the common native data types of dynamic
programming languages.
Fragment identifier considerations: See Section 1.2
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Additional information:
* Deprecated alias names for this type: application/x-yaml, text/
yaml, text/x-yaml
* Magic number(s) N/A
* File extension(s): "yaml" (preferred), "yml". See Section 3.3.
* Macintosh file type code(s): N/A
* Windows Clipboard Name: YAML
Person and email address to contact for further information: See Aut
hors' Addresses section.
Intended usage: COMMON
Restrictions on usage: None.
Author: See Authors' Addresses section.
Change controller: IESG
2.2. The +yaml Structured Syntax Suffix
The suffix +yaml MAY be used with any media type whose representation
follows that established for application/yaml. The media type
structured syntax suffix registration form follows. See [MEDIATYPE]
for definitions of each of the registration form headings.
Name: YAML Ain't Markup Language (YAML)
+suffix: +yaml
References: [YAML]
Encoding considerations: see Section 2.1
Fragment identifier considerations: Differently from application/
yaml, there is no fragment identification syntax defined for
+yaml.
A specific xxx/yyy+yaml media type needs to define the syntax and
semantics for fragment identifiers because the ones in Section 2.1
do not apply unless explicitly expressed.
Interoperability considerations: See Section 2.1
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Security considerations: See Section 2.1
Contact: httpapi@ietf.org or art@ietf.org
Author: See Authors' Addresses section
Change controller: IESG
3. Interoperability Considerations
3.1. YAML is an Evolving Language
YAML is an evolving language and, over time, some features have been
added and others removed.
While this document is based on a given YAML version [YAML], the
media type registration does not imply a specific version. This
allows content negotiation of version-independent YAML resources.
Implementers concerned about features related to a specific YAML
version can specify it in YAML documents using the %YAML directive
(see Section 6.8.1 of [YAML]).
3.2. YAML streams
A YAML stream can contain zero or more YAML documents.
When receiving a multi-document stream, an application that only
expects one-document streams, ought to signal an error instead of
ignoring the extra documents.
Current implementations consider different documents in a stream
independent, similarly to JSON Text Sequences (see [RFC7464]);
elements such as anchors are not guaranteed to be referenceable
across different documents.
3.3. Filename extension
The "yaml" filename extension is the preferred one; it is the most
popular and widely used on the web. The "yml" filename extension is
still used. The simultaneous usage of two filename extensions in the
same context might cause interoperability issues (e.g. when both a
"config.yaml" and a "config.yml" are present).
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3.4. YAML and JSON
When using flow collection styles (see Section 7.4 of [YAML]) a YAML
document could look like JSON [JSON], thus similar interoperability
considerations apply.
When using YAML as a more efficient format to serialize information
intended to be consumed as JSON, information not reflected in the
representation graph and classified as presentation or serialization
detail (see Section 3.2 of [YAML]) can be discarded. This includes
comments (see Section 3.2.3.3 of [YAML]), directives, and alias nodes
(see Section 7.1 of [YAML]) that do not have a JSON counterpart.
# This comment will be lost
# when serializing in JSON.
Title:
type: string
maxLength: &text_limit 64
Name:
type: string
maxLength: *text_limit # Replaced by the value 64.
Figure 2: JSON replaces alias nodes with static values.
Implementers need to ensure that relevant information will not be
lost during the processing. For example, they might consider
acceptable that alias nodes are replaced by static values.
In some cases an implementer may want to define a list of allowed
YAML features, taking into account that the following ones might have
interoperability issues with JSON:
* multi-document YAML streams;
* non UTF-8 encoding, since YAML supports UTF-16 and UTF-32 in
addition to UTF-8;
* mapping keys that are not strings;
* circular references represented using anchor (see Section 4.2 and
Figure 4);
* .inf and .nan float values, since JSON does not support them;
* non-JSON types, including the ones associated with tags like
!!timestamp that were included in the default schema of older YAML
versions;
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* tags in general, and specifically the ones that do not map to JSON
types like custom and local tags such as !!python/object and
!mytag (see Section 2.4 of [YAML]);
%YAML 1.2
---
non-json-keys:
0: a number
[0, 1]: a sequence
? {k: v}
: a map
---
non-json-keys:
!date 2020-01-01: a timestamp
non-json-value: !date 2020-01-01
...
Figure 3: Example of mapping keys and values not supported in
JSON in a multi- document YAML stream
3.5. Fragment identifiers
To allow fragment identifiers to traverse alias nodes, the YAML
representation graph needs to be generated before the fragment
identifier evaluation. It is important that this evaluation will not
cause the issues mentioned in Section 3.4 and in Security
considerations (Section 4) such as infinite loops and unexpected code
execution.
Implementers need to consider that the YAML version and supported
features (e.g. merge keys) can impact on the generation of the
representation graph (see Figure 9).
In Section 2.1, this document extends the use of specifications based
on the JSON data model with support for YAML fragment identifiers.
This is to improve the interoperability of already consolidated
practices, such as the one of writing OpenAPI documents [OAS] in
YAML.
Appendix A provides a non-exhaustive list of examples that could help
understand interoperability issues related to fragment identifiers.
4. Security Considerations
Security requirements for both media type and media type suffix
registrations are discussed in Section 4.6 of [MEDIATYPE].
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4.1. Arbitrary Code Execution
Care should be used when using YAML tags, because their resolution
might trigger unexpected code execution.
Code execution in deserializers should be disabled by default, and
only be enabled explicitly. In those cases, the implementation
should ensure - for example, via specific functions - that the code
execution results in strictly bounded time/memory limits.
Many implementations provide safe deserializers addressing these
issues.
4.2. Resource Exhaustion
YAML documents are rooted, connected, directed graphs and can contain
reference cycles, so they can't be treated as simple trees (see
Section 3.2.1 of [YAML]). An implementation that attempts to do that
can infinite-loop traversing the YAML representation graph at some
point, for example:
* when trying to serialize it JSON;
* or when searching/identifying nodes using specifications based on
the JSON data model (e.g. [JSON-POINTER]).
x: &x
y: *x
Figure 4: A cyclic document
Even if a representaion graph is not cyclic, treating it as a simple
tree could lead to improper behaviors (such as the "billion laughs"
problem).
x1: &a1 ["a", "a"]
x2: &a2 [*a1, *a1]
x3: &a3 [*a2, *a2]
Figure 5: A billion laughs document
This can be addressed using processors limiting the anchor recursion
depth and validating the input before processing it; even in these
cases it is important to carefully test the implementation you are
going to use. The same considerations apply when serializing a YAML
representation graph in a format that does not support reference
cycles (see Section 3.4).
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4.3. YAML streams
Incremental parsing and processing of a YAML stream can produce
partial results and later indicate failure to parse the remainder of
the stream; to prevent partial processing, implementers might prefer
validating all the documents in a stream beforehand.
Repeated parsing and re-encoding of a YAML stream can result in the
addition or removal of document delimiters (e.g. --- or ...) as well
as the modification of anchor names and other serialization details:
this can break signature validation.
5. IANA Considerations
This specification defines the following new Internet media type
[MEDIATYPE].
IANA has updated the "Media Types" registry at
https://www.iana.org/assignments/media-types
(https://www.iana.org/assignments/media-types) with the registration
information provided below.
+==================+==============================+
| Media Type | Section |
+==================+==============================+
| application/yaml | Section 2.1 of this document |
+------------------+------------------------------+
Table 1
IANA has updated the "Structured Syntax Suffixes" registry at
https://www.iana.org/assignments/media-type-structured-suffix
(https://www.iana.org/assignments/media-type-structured-suffix) with
the registration information provided below.
+========+==============================+
| Suffix | Section |
+========+==============================+
| +yaml | Section 2.2 of this document |
+--------+------------------------------+
Table 2
6. References
6.1. Normative References
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[JSON] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/rfc/rfc8259>.
[JSON-POINTER]
Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
"JavaScript Object Notation (JSON) Pointer", RFC 6901,
DOI 10.17487/RFC6901, April 2013,
<https://www.rfc-editor.org/rfc/rfc6901>.
[MEDIATYPE]
Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/rfc/rfc6838>.
[OAS] Darrel Miller, Jeremy Whitlock, Marsh Gardiner, Mike
Ralphson, Ron Ratovsky, and Uri Sarid, "OpenAPI
Specification 3.0.0", 26 July 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[RFC7405] Kyzivat, P., "Case-Sensitive String Support in ABNF",
RFC 7405, DOI 10.17487/RFC7405, December 2014,
<https://www.rfc-editor.org/rfc/rfc7405>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[SEMANTICS]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-19>.
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[URI] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/rfc/rfc3629>.
[YAML] Oren Ben-Kiki, Clark Evans, Ingy dot Net, Tina Müller,
Pantelis Antoniou, Eemeli Aro, and Thomas Smith, "YAML
Ain't Markup Language Version 1.2", 1 October 2021,
<https://yaml.org/spec/1.2.2/>.
6.2. Informative References
[I-D.ietf-jsonpath-base]
Gössner, S., Normington, G., and C. Bormann, "JSONPath:
Query expressions for JSON", Work in Progress, Internet-
Draft, draft-ietf-jsonpath-base-08, 21 November 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-
jsonpath-base-08>.
[RFC7464] Williams, N., "JavaScript Object Notation (JSON) Text
Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015,
<https://www.rfc-editor.org/rfc/rfc7464>.
Appendix A. Examples related to fragment identifier interoperability
A.1. Unreferenceable nodes
In this example, a couple of YAML nodes that cannot be referenced
based on the JSON data model since their mapping keys are not
strings.
%YAML 1.2
---
a-map-cannot:
? {be: expressed}
: with a JSON Pointer
0: no numeric mapping keys in JSON
Figure 6: Example of YAML nodes that are not referenceable based
on JSON data model.
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A.2. Referencing a missing node
In this example the fragment #/0 does not reference an existing node
0: "JSON Pointer `#/0` references a string mapping key."
Figure 7: Example of a JSON Pointer that does not reference an
existing node.
A.3. Representation graph with anchors and cyclic references
In this YAML document, the #/foo/bar/baz fragment identifier
traverses the representation graph and references the string you.
Moreover, the presence of a cyclic reference implies that there are
infinite fragment identifiers #/foo/bat/../bat/bar referencing the
&anchor node.
anchor: &anchor
baz: you
foo: &foo
bar: *anchor
bat: *foo
Figure 8: Example of a cyclic references and alias nodes.
Many YAML implementations will resolve the merge key "<<:"
(https://yaml.org/type/merge.html) defined in YAML 1.1 in the
representation graph. This means that the fragment #/book/author/
given_name references the string Federico and that the fragment
#/book/<< will not reference any existing node.
%YAML 1.1
---
# Many implementations use merge keys.
the-viceroys: &the-viceroys
title: The Viceroys
author:
given_name: Federico
family_name: De Roberto
book:
<<: *the-viceroys
title: The Illusion
Figure 9: Example of YAML merge keys.
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Appendix B. Acknowledgements
Thanks to Erik Wilde and David Biesack for being the initial
contributors of this specification, and to Darrel Miller and Rich
Salz for their support during the adoption phase.
In addition to the people above, this document owes a lot to the
extensive discussion inside and outside the HTTPAPI workgroup. The
following contributors have helped improve this specification by
opening pull requests, reporting bugs, asking smart questions,
drafting or reviewing text, and evaluating open issues:
Tina (tinita) Mueller, Ben Hutton, Manu Sporny and Jason Desrosiers.
FAQ
This section is to be removed before publishing as an RFC.
Q: Why this document? After all these years, we still lack a proper
media-type for YAML. This has some security implications too (eg.
wrt on identifying parsers or treat downloads)
Q: Why using alias nodes as fragment identifiers? Alias nodes are a
native YAML feature that allows addressing any node in a YAML
document. Since YAML is not limited to string keywords, not all
nodes are addressable using JSON Pointers. Alias nodes are thus
the natural choice for fragment identifiers Section 1.2.
Q: Why not use plain names for alias nodes? Why not define plain
names? Using plain name fragments could have limited the ability of
future xxx+yaml media types to define their plain name fragments.
Moreover, alias nodes starts with * so we found no reason to strip
it when using them in fragments.
Preserving * had another positive result: it allows distinguishing
a fragment identifier expressed as an alias node from one
expressed in other formats. In this document we included JSON
Pointer [JSON-POINTER] which is expected to start with /.
Moreover, since JSON Path [I-D.ietf-jsonpath-base] expressions
start with $, this mechanism can be extended to JSON Path too.
Q: Why not just use JSON Pointer as the primary fragment
identifier? Fragment identifiers in YAML always reference YAML
representation graph nodes. JSON Pointer can only rely on string
keywords so it is not able to reference a generic node in the
representation graph.
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Since JSON Pointer is a specification unrelated to YAML, we
decided to isolate the impacts of changes in JSON Pointer on YAML
fragments: only fragments starting with "/" are "delegated" to an
external spec, and if [JSON-POINTER] changes, it will only affect
fragments starting with "/".
The current behaviour for empty fragments is the same for both
JSON Pointer and alias nodes. Incidentally, it's the only
sensible behaviour independently of [JSON-POINTER].
Change Log
This section is to be removed before publishing as an RFC.
Since draft-ietf-httpapi-yaml-mediatypes-02
* clarification on fragment identifiers #50.
Since draft-ietf-httpapi-yaml-mediatypes-01
* application/yaml fragment identifiers compatible with JSON Pointer
#41 (#47).
Authors' Addresses
Roberto Polli
Digital Transformation Department, Italian Government
Italy
Email: robipolli@gmail.com
Erik Wilde
Axway
Switzerland
Email: erik.wilde@dret.net
Eemeli Aro
Mozilla
Finland
Email: eemeli@gmail.com
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