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Applications and Real-Time HTTPAPI Internet-Draft This document registers the following media types used in APIs on the IANA Media Types registry: application/yaml, application/schema+json, application/schema-instance+json, application/openapi+json, and application/openapi+yaml. 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 OpenAPI Specification version 3 and above is a consolidated standard for describing HTTP APIs using the JSON and YAML data format. To increase interoperability when processing API specifications and leverage content negotiation mechanisms when exchanging OpenAPI Specification resources this specification register the following media-types: application/yaml, application/schema+json, application/schema-instance+json, application/openapi+json and application/openapi+yaml. Moreover it defines and registers the +yaml structured syntax suffix.

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: Same as 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: Same as for application/json 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: Interoperability considerations: See Security considerations: See Contact: See Authors' Addresses section. Author: See Authors' Addresses section Change controller: n/a

The OpenAPI Media Types The OpenAPI Specification Media Types convey OpenAPI document (OAS) files as defined in for version 3.0.0 and above. Those files can be serialized in or . Since there are multiple OpenAPI Specification versions, those media-types support the version parameter. The following examples conveys the desire of a client to receive an OpenAPI Specification resource preferably in the following order:

1. openapi 3.1 in YAML
2. openapi 3.0 in YAML
3. any openapi version in json

Media Type application/openapi+json The following information serves as the registration form for the application/openapi+json media type. Type name: application Subtype name: openapi+json Required parameters: None Optional parameters: version; unrecognized parameters should be ignored Encoding considerations: Same as Security considerations: see of this document Interoperability considerations: None Published specification: (this document) Applications that use this media type: HTTP Fragment identifier considerations: Same as for application/json Additional information: Deprecated alias names for this type: n/a Magic number(s): n/a File extension(s): json 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

Media Type application/openapi+yaml The following information serves as the registration form for the application/openapi+yaml media type. Type name: application Subtype name: openapi+yaml Required parameters: None Optional parameters: version; unrecognized parameters should be ignored Encoding considerations: Same as Security considerations: see of this document Interoperability considerations: see Published specification: (this document) Applications that use this media type: HTTP Fragment identifier considerations: Same as for application/json Additional information: Deprecated alias names for this type: n/a 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

JSON Schema Media Types JSON Schema is a declarative domain-specific language for validating and annotating JSON documents (see ). This document registers the media types associated with JSON Schema. There are many dialects of JSON Schema in wide use today. The JSON Schema maintainers have released several dialects including draft-04, draft-07, and draft 2020-12. There are also several third-party JSON Schema dialects in wide use including the ones defined for use in OpenAPI and MongoDB. This specification defines little more than how to identify the dialect while leaving most of the semantics of the schema up to the dialect to define. Clients MUST use the following order of precedence for determining the dialect of a schema.

• The $schema keyword ()
• The "schema" media type parameter ()
• The context of the enclosing document. This applies only when a schema is embedded within a document. The enclosing document could be another schema in the case of a bundled schema or it could be another type of document that includes schemas such as an OpenAPI document.
• If none of the above result in identifying the dialect, client behavior is undefined.

The "$schema" Keyword The $schema keyword is used as a JSON Schema dialect identifier. The value of this keyword MUST be a URI . This URI SHOULD identify a meta-schema that can be used to validate that the schema is syntactically correct according to the dialect the URI identifies. The dialect SHOULD define where the $schema keyword is allowed and/or recognized in a schema, but it is RECOMMENDED that dialects do not allow the schema to change within the same Schema Resource.

Identifying a Schema via a Media Type Parameter Media types MAY allow for a schema media type parameter, to support content negotiation based on schema identifier (see ). The schema media type parameter MUST be a URI-reference . The schema parameter identifies a schema that provides semantic information about the resource the media type represents. When using the application/schema+json media type, the schema parameter identifies the dialect of the schema the media type represents. The schema URI is opaque and SHOULD NOT automatically be dereferenced. Since schema doesn't necessarily point to a network location, the "describedby" relation is used for linking to a downloadable schema. The following is an example of content negotiation where a user agent can accept two different versions of a "pet" resource. Each resource version is identified by a unique JSON Schema. Request: Response: In the following example, the user agent is able to accept two possible dialects of JSON Schema and the server replies with the latest one. Request: Response:

Linking to a Schema It is RECOMMENDED that instances described by a schema provide a link to a downloadable JSON Schema using the link relation describedby, as defined by Linked Data Protocol 1.0, section 8.1 . In HTTP, such links can be attached to any response using the Link header . ; rel="describedby" ]]>

Fragment Identifiers Two fragment identifier structures are supported: JSON Pointers and plain-names. The use of JSON Pointers as URI fragment identifiers is described in . Fragment identifiers that are empty or start with a /, MUST be interpreted as JSON Pointer fragment identifiers. Plain-name fragment identifiers reference locally named locations in the document. The dialect determines how plain-name identifiers map to locations within the document. All fragment identifiers that do not match the JSON Pointer syntax MUST be interpreted as plain name fragment identifiers.

Media Type application/schema+json The application/schema+json media type represents JSON Schema. This schema can be an official dialect or a third-party dialect. The following information serves as the registration form for the application/schema+json media type. Type name: application Subtype name: schema+json Required parameters: N/A Optional parameters:

• schema: A URI identifying the JSON Schema dialect the schema was written for. If this value conflicts with the value of the $schema keyword in the schema, the $schema keyword takes precedence.

Encoding considerations: Same as Security considerations: See the "Security Considerations" section of Interoperability considerations: See the "General Considerations" section of Published specification: (this document) Applications that use this media type: JSON Schema is used in a variety of applications including API servers and clients that validate JSON requests and responses, IDEs that valid configuration files, databases that store JSON, and more. Fragment identifier considerations: See Additional information:

• Deprecated alias names for this type: N/A
• Magic number(s): N/A
• File extension(s): json, schema.json
• 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: N/A. Author: See Authors' Addresses section. Change controller: N/A

Media Type application/schema-instance+json The application/schema-instance+json media type is an extension of the media type that just adds the schema media type parameter and fragment identification. The following information serves as the registration form for the application/schema-instance+json media type. Type name: application Subtype name: schema-instance+json Required parameters: N/A Optional parameters:

• schema: A URI identifying a JSON Schema that provides semantic information about this JSON representation.

Encoding considerations: Same as Security considerations: Same as Interoperability considerations: Same as Published specification: (this document) Applications that use this media type: JSON Schema is used in a variety of applications including API servers and clients that validate JSON requests and responses, IDEs that valid configuration files, databases that store JSON, and more. Fragment identifier considerations: See Additional information:

• Deprecated alias names for this type: N/A
• Magic number(s): N/A
• File extension(s): json
• 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: N/A Author: See Authors' Addresses section. Change controller: N/A

Interoperability Considerations

YAML Media Types

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 to tags like !!timestamp that were deployed in older YAML versions;
• tags in general, and specifically 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 .

YAML Media Types

Arbitrary Code Execution Care should be used when using YAML tags, because their implementation 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 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 do 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 ThisRFC
application/openapi+yaml	of ThisRFC
application/openapi+json	of ThisRFC
application/schema+json	of ThisRFC
application/schema-instance+json	of ThisRFC

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 ThisRFC

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. A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] This specification defines a model for the relationships between resources on the Web ("links") and the type of those relationships ("link relation types"). It also defines the serialisation of such links in HTTP headers with the Link header field. JSON Pointer defines a string syntax for identifying a specific value within a JavaScript Object Notation (JSON) document.

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: Eemeli Aro, Tina (tinita) Mueller, Ben Hutton 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.