]> Digital Transformation Department, Italian Government

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Switzerland erik.wilde@dret.net

Mozilla

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 previously had not been 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 . The terms "fragment" and "fragment identifier" in this document are to be interpreted as in . The terms "node", "alias node", "anchor" and "named anchor" in this document are to be intepreded as in .

Fragment identification This section describes how to use alias nodes (see Section 3.2.2.2 and 7.1 of ) as fragment identifiers to designate nodes. A YAML alias node can be represented in a URI fragment identifier by encoding it into octects using UTF-8 , while percent-encoding those characters not allowed by the fragment rule in . If multiple nodes would match a fragment identifier, the first such match is selected. A fragment identifier is not guaranteed to reference an existing node. Therefore, applications SHOULD define how an unresolved alias node ought to be handled. 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 named anchors 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 alias node *foo pointing to the node with value scalar; whereas the URL file.yaml#*bar references the alias node *bar pointing to the node with value [ some, sequence, items ].

Media Type and Structured Syntax Suffix registrations This section describes the information required to register the above media type according to

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:

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:

Applications that use this media type:

HTTP

Fragment identifier considerations:

An empty fragment identifier references the root node. A fragment identifier starting with "*" is to be interpreted as a YAML alias node . A fragment identifier starting with "/" is to be interpreted as a JSON Pointer and is evaluated on the YAML representation graph, walking through alias nodes; this syntax can only reference YAML nodes that are on a path that is made up of nodes interoperable with the JSON data model (see ).

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 Language (YAML)

+suffix:

+yaml

References:

Encoding considerations:

see

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 do not apply unless explicitly expressed.

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, over time, some features have been added and others removed. While this document is based on a given YAML version , 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 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

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 and in Security considerations 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 ). In , 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 in YAML. provides a non exhaustive list of examples that could help understanding interoperability issues related to fragment identifiers.

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 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. ).

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 type . 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

References Normative References JSON Pointer defines a string syntax for identifying a specific value within a JavaScript Object Notation (JSON) document. 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 RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. 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] ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. 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. Informative References Fachhochschule Dortmund Universität Bremen TZI JSONPath defines a string syntax for selecting and extracting values within a JavaScript Object Notation (JSON, RFC 8259) value.

Examples related to fragment identifier interoperability

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.

Example of YAML nodes that are not referenceable based on JSON data model.

Referencing a missing node In this example the fragment #/0 does not reference an existing node

Example of a JSON Pointer that does not reference an existing node.

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.

Example of a cyclic references and alias nodes.

Many YAML implementations will resolve the merge key "<<:" 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.

Example of YAML merge keys.

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)

Q: Why using alias nodes as fragment identifiers?

Alias nodes starts with *. This allow to distinguish a fragment identifier expressed as an alias node from one expressed in JSON Pointer which is expected to start with /. Moreover, since json-path expressions start with $, this mechanism is even extensible that specification.

Change Log

Since draft-ietf-httpapi-yaml-mediatypes-01

• application/yaml fragment identifiers compatible with JSON Pointer #41 (#47).