HTTPAPI R. Polli
Internet-Draft Digital Transformation Department, Italian Government
Intended status: Informational E. Wilde
Expires: 3 October 2022 Axway
E. Aro
1 April 2022
YAML Media Types
draft-ietf-httpapi-yaml-mediatypes-00
Abstract
This document registers the application/yaml media type and the +yaml
structured syntax suffix on the IANA Media Types registry.
Note to Readers
_RFC EDITOR: please remove this section before publication_
Discussion of this draft takes place on the HTTP APIs working group
mailing list (httpapi@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/httpapi/
(https://mailarchive.ietf.org/arch/browse/httpapi/).
The source code and issues list for this draft can be found at
https://github.com/ietf-wg-httpapi/mediatypes (https://github.com/
ietf-wg-httpapi/mediatypes).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 3 October 2022.
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Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3
2. Media Type registrations . . . . . . . . . . . . . . . . . . 3
2.1. Media Type application/yaml . . . . . . . . . . . . . . . 3
2.2. The +yaml Structured Syntax Suffix . . . . . . . . . . . 4
3. Interoperability Considerations . . . . . . . . . . . . . . . 5
3.1. YAML is an Evolving Language . . . . . . . . . . . . . . 5
3.2. YAML and JSON . . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4.1. Arbitrary Code Execution . . . . . . . . . . . . . . . . 6
4.2. Resource Exhaustion . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Normative References . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
YAML [YAML] is a data serialization format that is widely used on the
Internet, including in the API sector (e.g. see [oas]) but the
relevant media type and structured syntax suffix are not registered
by IANA.
To increase interoperability when exchanging YAML data 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].
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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. 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].
2. Media Type registrations
This section describes the information required to register the above
media types according to [MEDIATYPE]
2.1. Media Type application/yaml
The following information serves as the registration form for the
application/yaml media type.
Type name: application
Subtype name: yaml
Required parameters: None
Optional parameters: None; 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: this document
Applications that use this media type: HTTP
Fragment identifier considerations: None
Additional information:
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* Deprecated alias names for this type: application/x-yaml, text/
yaml, text/x-yaml
* Magic number(s) n/a
* File extension(s): yaml, yml
* Macintosh file type code(s): n/a
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: n/a
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 LanguageML (YAML)
+suffix: +yaml
References: [YAML]
Encoding considerations: see Section 2.1
Fragment identifier considerations: The syntax and semantics of
fragment identifiers specified for +yaml SHOULD be as specified
for Section 2.1 The syntax and semantics for fragment identifiers
for a specific xxx/yyy+yaml SHOULD be processed as follows:
1. For cases defined in +yaml, where the fragment identifier
resolves per the +yaml rules, then process as specified in
+yaml.
2. For cases defined in +yaml, where the fragment identifier does
not resolve per the +yaml rules, then process as specified in
xxx/yyy+yaml.
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3. For cases not defined in +yaml, then process as specified in
xxx/yyy+yaml.
Interoperability considerations: See Section 2.1
Security considerations: See Section 2.1
Contact: See Authors' Addresses section.
Author: See Authors' Addresses section
Change controller: n/a
3. Interoperability Considerations
3.1. YAML is an Evolving Language
YAML is an evolving language and, in time, some features have been
added, and others removed.
While this document is based on a given YAML version [YAML], media
types 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 the documents using the %YAML directive
(see Section 6.8.1 of [YAML]).
3.2. 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
intented to be consumed as JSON, information can be discarded: this
includes comments (see Section 3.2.3.3 of [YAML]) 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.
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Figure 1: 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:
* 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 3);
* .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;
* 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]);
non-json-keys:
2020-01-01: a timestamp
[0, 1]: a sequence
? {k: v}
: a map
non-json-value: 2020-01-01
Figure 2: Example of mapping keys not supported in JSON
4. Security Considerations
Security requirements for both media type and media type suffix
registrations are discussed in Section 4.6 of [MEDIATYPE].
4.1. Arbitrary Code Execution
Care should be used when using YAML tags, because their resolution
might trigger unexpected code execution.
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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 at some point (e.g. when trying to serialize a YAML
document in JSON).
x: &x
y: *x
Figure 3: A cyclic document
Even if a document 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 4: 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 3.2).
5. IANA Considerations
This specification defines the following new Internet media types
[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.
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+==================+==============================+
| 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. Normative References
[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>.
[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>.
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[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>.
[YAML] Oren Ben-Kiki, Clark Evans, and Ingy dot Net, "YAML Ain't
Markup Language Version 1.2", 1 October 2021,
<https://yaml.org/spec/1.2/spec.html>.
Appendix A. 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
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)
Change Log
RFC EDITOR PLEASE DELETE THIS SECTION.
Authors' Addresses
Roberto Polli
Digital Transformation Department, Italian Government
Italy
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Email: robipolli@gmail.com
Erik Wilde
Axway
Switzerland
Email: erik.wilde@dret.net
Eemeli Aro
Finland
Email: eemeli@gmail.com
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