]> Digital Transformation Department, Italian Government

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Switzerland erik.wilde@dret.net

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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, intended to be used to identify document components serialized according to the YAML specification. About This Document Status information for this document may be found at . Discussion of this document takes place on the HTTPAPI Working Group mailing list (), which is archived at . Subscribe at . Working Group information can be found at . Source for this draft and an issue tracker can be found at .

Introduction 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 ), but a corresponding media type and structured syntax suffix had not previously 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 , 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. 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 "presentation", "stream", "YAML document", "representation graph", "tag", "serialization detail", "node", "alias node", "anchor" and "anchor name" in this document are to be interpreted as in . Figures containing YAML code always start with the "%YAML 1.2" directive to improve readability.

Fragment identification A fragment identifies a node in a stream. A fragment identifier starting with "*" is to be interpreted as a YAML alias node (see ). For single-document YAML streams, a fragment identifier that is empty or that starts with "/" is to be interpreted as a 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 ). A fragment identifier is not guaranteed to reference an existing node. Therefore, applications SHOULD define how an unresolved alias node ought to be handled.

Fragment identification via alias nodes 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 bytes using UTF-8 , but percent-encoding of those characters is not allowed by the fragment rule in . If multiple nodes would match a fragment identifier, the first occurrence of 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]}.

A YAML stream containing two YAML documents.

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:

N/A

Optional parameters:

N/A; 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:

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

Additional information:

• Deprecated alias names for this type: application/x-yaml, text/yaml, text/x-yaml. These names are used, but not registered.
• Magic number(s) N/A
• File extension(s): "yaml" (preferred), "yml". See .
• Macintosh file type code(s): N/A
• Windows Clipboard Name: YAML

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:

IESG

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:

httpapi@ietf.org or art@ietf.org

Author:

See Authors' Addresses section

Change controller:

IESG

Interoperability Considerations

YAML is an Evolving Language YAML is an evolving language and, over time, some features have been added and others removed. This media type registration is independent of YAML 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 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 ); elements such as anchors are not guaranteed to be referenceable across different documents.

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).

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 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 ) can be discarded. This includes comments (see Section 3.2.3.3 of ), directives, 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 :

• multi-document YAML streams;
• non UTF-8 encoding. Before encoding YAML streams in UTF-16 or UTF-32, it is important to note that mandates the use of UTF-8 when exchanging JSON texts between systems that are not part of a closed ecosystem, and that Section 5.2 of recommends the use of 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 and values not supported in JSON in a multi-document YAML stream

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 affect 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 understand 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 treats them as simple trees risks going into an infinite loop while traversing the YAML representation graph. This can happen:

• when trying to serialize it as JSON;
• or when searching/identifying nodes using specifications based on the JSON data model (e.g., ).

A cyclic document

Even if a representation graph is not cyclic, treating it as a simple tree could lead to improper behaviors (such as the "billion laughs" or "Exponential Entity Expansion" 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 ).

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, which can break signature validation.

IANA Considerations This specification defines the following new Internet media type . IANA is asked to update the "Media Types" registry at https://www.iana.org/assignments/media-types with the registration information provided in the section below.

Media Type	Registration information section
application/yaml	of this document

IANA is asked to update the "Structured Syntax Suffixes" registry at https://www.iana.org/assignments/media-type-structured-suffix with the registration information provided in the section below.

Suffix	Registration information 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. 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. 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. 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. Informative References Fachhochschule Dortmund Universität Bremen TZI JSONPath defines a string syntax for selecting and extracting JSON (RFC 8259) values from a JSON value. This document describes the JavaScript Object Notation (JSON) text sequence format and associated media type "application/json-seq". A JSON text sequence consists of any number of JSON texts, all encoded in UTF-8, each prefixed by an ASCII Record Separator (0x1E), and each ending with an ASCII Line Feed character (0x0A).

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) Müller, 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 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 .

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 which is expected to start with /. Moreover, since JSON Path 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. 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 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 .

Q: Why describe the YAML/JSON so closely?

In the context of Web APIs, YAML is widely used as a more compact way to serialize content inteded to be consumed according to the JSON data model. Typical examples are OpenAPI specifications and Kubernetes manifest files, that can be serialized in both formats. The YAML media type registration I-D is a spin-off and a building block for the OpenAPI specification media type registration. The YAML/JSON section aims at clarifying what developers should expect when using YAML instead of JSON, and its content arose from common mistakes and FAQs. Please note that we are not imposing any normative restriction on YAML streams; this is because YAML is defined outside this document. Instead, we only provide Interoperability and Security considerations that, by their nature, are not normative.

Q: Do we forbid using non-UTF-8 YAML serialization?

No. Since recommends UTF-8 in interoperability context we suggest that using UTF-8 is an interoperable behavior. This is aligned with Section 5.2 of that explicitly recommends UTF-8.

Q: Why media type registration information is outside the IANA Considerations?

We decided to follow the style adopted in where the IANA Considerations in references the multipart/byteranges media type registration form contained in the specification body .

Change Log

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).