]> Digital Transformation Department, Italian Government

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Axway

Switzerland erik.wilde@dret.net

Finland eemeli@gmail.com

Applications and Real-Time HTTPAPI Internet-Draft 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/. The source code and issues list for this draft can be found at https://github.com/ietf-wg-httpapi/mediatypes.

Introduction YAML is a data serialization format that is widely used on the Internet, including in the API sector (e.g. see ) 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 , including its relation with .

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 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 and updated by . The terms "content", "content negotiation", "resource", and "user agent" in this document are to be interpreted as in .

Media Type registrations This section describes the information required to register the above media types according to

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 of this document

Interoperability considerations:

see of this document

Published specification:

this document

Applications that use this media type:

HTTP

Fragment identifier considerations:

None

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, yml
• Macintosh file type code(s): n/a

Person and email address to contact for further information:

See Authors' Addresses section.

Intended usage:

COMMON

Restrictions on usage:

None.

Author:

See Authors' Addresses section.

Change controller:

n/a

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 for definitions of each of the registration form headings.

Name:

YAML Ain't Markup LanguageML (YAML)

+suffix:

+yaml

References:

Encoding considerations:

see

Fragment identifier considerations:

The syntax and semantics of fragment identifiers specified for +yaml SHOULD be as specified for 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.
3. For cases not defined in +yaml, then process as specified in xxx/yyy+yaml.

Interoperability considerations:

See

Security considerations:

See

Contact:

See Authors' Addresses section.

Author:

See Authors' Addresses section

Change controller:

n/a

Interoperability Considerations

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 , 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 and JSON When using flow collection styles (see Section 7.4 of ) a YAML document could look like 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 ) and alias nodes (see Section 7.1 of ), that do not have a JSON counterpart.

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 and );
• .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 );

Example of mapping keys not supported in JSON

Security Considerations Security requirements for both media type and media type suffix registrations are discussed in Section 4.6 of .

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.

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 ). An implementation that attempts to do that can infinite-loop at some point (e.g. when trying to serialize a YAML document in JSON).

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).

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 ).

IANA Considerations This specification defines the following new Internet media types . IANA has updated the "Media Types" registry at https://www.iana.org/assignments/media-types with the registration information provided below.

Media Type	Section
application/yaml	of this document

IANA has updated the "Structured Syntax Suffixes" registry at https://www.iana.org/assignments/media-type-structured-suffix with the registration information provided below.

Suffix	Section
+yaml	of this document

Normative References JavaScript Object Notation (JSON) is a lightweight, text-based, language-independent data interchange format. It was derived from the ECMAScript Programming Language Standard. JSON defines a small set of formatting rules for the portable representation of structured data. This document removes inconsistencies with other specifications of JSON, repairs specification errors, and offers experience-based interoperability guidance. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] This document extends the base definition of ABNF (Augmented Backus-Naur Form) to include a way to specify US-ASCII string literals that are matched in a case-sensitive manner. Adobe Fastly greenbytes GmbH The Hypertext Transfer Protocol (HTTP) is a stateless application- level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFC 2818, RFC 7231, RFC 7232, RFC 7233, RFC 7235, RFC 7538, RFC 7615, RFC 7694, and portions of RFC 7230. This document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice.

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.