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YAML Media Type

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This is an older version of an Internet-Draft that was ultimately published as RFC 9512.
Authors Roberto Polli , Erik Wilde , Eemeli Aro
Last updated 2023-05-25 (Latest revision 2023-05-08)
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Stream WG state Submitted to IESG for Publication
Document shepherd Darrel Miller
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HTTPAPI                                                         R. Polli
Internet-Draft     Digital Transformation Department, Italian Government
Intended status: Informational                                  E. Wilde
Expires: 9 November 2023                                           Axway
                                                                  E. Aro
                                                              8 May 2023

                            YAML Media Type


   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

   This note is to be removed before publishing as an RFC.

   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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 9 November 2023.

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   Copyright (c) 2023 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
     1.2.  Fragment identification . . . . . . . . . . . . . . . . .   4
       1.2.1.  Fragment identification via alias nodes . . . . . . .   4
   2.  Media Type and Structured Syntax Suffix registrations . . . .   5
     2.1.  Media Type application/yaml . . . . . . . . . . . . . . .   5
     2.2.  The +yaml Structured Syntax Suffix  . . . . . . . . . . .   6
   3.  Interoperability Considerations . . . . . . . . . . . . . . .   7
     3.1.  YAML is an Evolving Language  . . . . . . . . . . . . . .   7
     3.2.  YAML streams  . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Filename extension  . . . . . . . . . . . . . . . . . . .   8
     3.4.  YAML and JSON . . . . . . . . . . . . . . . . . . . . . .   8
     3.5.  Fragment identifiers  . . . . . . . . . . . . . . . . . .   9
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     4.1.  Arbitrary Code Execution  . . . . . . . . . . . . . . . .  10
     4.2.  Resource Exhaustion . . . . . . . . . . . . . . . . . . .  10
     4.3.  YAML streams  . . . . . . . . . . . . . . . . . . . . . .  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Examples related to fragment identifier
           interoperability  . . . . . . . . . . . . . . . . . . . .  13
     A.1.  Unreferenceable nodes . . . . . . . . . . . . . . . . . .  13
     A.2.  Referencing a missing node  . . . . . . . . . . . . . . .  14

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     A.3.  Representation graph with anchors and cyclic
           references  . . . . . . . . . . . . . . . . . . . . . . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  15
   FAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Change Log  . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     Since draft-ietf-httpapi-yaml-mediatypes-02 . . . . . . . . . .  17
     Since draft-ietf-httpapi-yaml-mediatypes-01 . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   YAML [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 [OAS]), 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 [MEDIATYPE].

   Moreover, it provides security considerations and interoperability
   considerations related to [YAML], including its relation with [JSON].

1.1.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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

   The terms "content", "content negotiation", "resource", and "user
   agent" in this document are to be interpreted as in [HTTP].

   The terms "fragment" and "fragment identifier" in this document are
   to be interpreted as in [URI].

   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

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   Figures containing YAML code always start with the "%YAML 1.2"
   directive to improve readability.

1.2.  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 Section 1.2.1).

   For single-document YAML streams, a fragment identifier that is empty
   or that starts with "/" is to be interpreted as a JSON Pointer
   [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 Section 3.4).

   A fragment identifier is not guaranteed to reference an existing
   node.  Therefore, applications SHOULD define how an unresolved alias
   node ought to be handled.

1.2.1.  Fragment identification via alias nodes

   This section describes how to use alias nodes (see Section
   and 7.1 of [YAML]) 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 [UTF-8], but percent-encoding of
   those characters is not allowed by the fragment rule in Section 3.5
   of [URI].

   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

   *  SHOULD NOT use alias nodes that match multiple nodes.

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   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,

    %YAML 1.2
    one: &foo scalar
    two: &bar
      - some
      - sequence
      - items
    %YAML 1.2
    one: &foo [a, sequence]

           Figure 1: A YAML stream containing two YAML documents.

2.  Media Type and Structured Syntax Suffix registrations

   This section describes the information required to register the above
   media type according to [MEDIATYPE]

2.1.  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 Section 4 of this document

   Interoperability considerations:  see Section 3 of this document

   Published specification:  [YAML]

   Applications that use this media type:  Applications that need a

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      human-friendly, cross language, Unicode based data serialization
      language designed around the common native data types of dynamic
      programming languages.

   Fragment identifier considerations:  See Section 1.2

   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 Section 3.3.

   *  Macintosh file type code(s): N/A

   *  Windows Clipboard Name: YAML

   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:  IESG

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 Language (YAML)

   +suffix:  +yaml

   References:  [YAML]

   Encoding considerations:  Same as "application/yaml"

   Fragment identifier considerations:  Differently from application/
      yaml, there is no fragment identification syntax defined for

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      A specific xxx/yyy+yaml media type needs to define the syntax and
      semantics for fragment identifiers because the ones defined for
      "application/yaml" do not apply unless explicitly expressed.

   Interoperability considerations:  Same as "application/yaml"

   Security considerations:  Same as "application/yaml"

   Contact: or

   Author:  See Authors' Addresses section

   Change controller:  IESG

3.  Interoperability Considerations

3.1.  YAML is an Evolving Language

   YAML is an evolving language and, over time, some features have been
   added and others removed.

   This [YAML] media type registration is independent of YAML version.
   This allows content negotiation of version-independent YAML

   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]).

3.2.  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 [RFC7464]);
   elements such as anchors are not guaranteed to be referenceable
   across different documents.

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3.3.  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).

3.4.  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
   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 [YAML]) can be discarded.  This includes
   comments (see Section of [YAML]), directives, and alias nodes
   (see Section 7.1 of [YAML]) that do not have a JSON counterpart.

    %YAML 1.2
    # This comment will be lost
    # when serializing in JSON.
      type: string
      maxLength: &text_limit 64

      type: string{{Section 8.1 of JSON}}
      maxLength: *text_limit  # Replaced by the value 64.

          Figure 2: 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]:

   *  multi-document YAML streams;

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   *  non UTF-8 encoding.  Before encoding YAML streams in UTF-16 or
      UTF-32, it is important to note that Section 8.1 of [JSON]
      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 [YAML] recommends the use of UTF-8;

   *  mapping keys that are not strings;

   *  circular references represented using anchor (see Section 4.2 and
      Figure 4);

   *  .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

   *  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]);

    %YAML 1.2
      0: a number
      [0, 1]: a sequence
      ? {k: v}
      : a map
      !date 2020-01-01: a timestamp
    non-json-value: !date 2020-01-01

       Figure 3: Example of mapping keys and values not supported in
                   JSON in a multi- document YAML stream

3.5.  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 Section 3.4 and in Security
   considerations (Section 4) such as infinite loops and unexpected code

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   Implementers need to consider that the YAML version and supported
   features (e.g., merge keys) can affect the generation of the
   representation graph (see Figure 9).

   In Section 2.1, 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 [OAS] in

   Appendix A provides a non-exhaustive list of examples that could help
   understand interoperability issues related to fragment identifiers.

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.

   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

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 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., [JSON-POINTER]).

    %YAML 1.2
    x: &x
      y: *x

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                        Figure 4: 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).

    %YAML 1.2
    x1: &a1 ["a", "a"]
    x2: &a2 [*a1, *a1]
    x3: &a3 [*a2, *a2]

                    Figure 5: 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 3.4).

4.3.  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 and processing all the documents in a stream at the same

   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.

5.  IANA Considerations

   This specification defines the following new Internet media type

   IANA is asked to update the "Media Types" registry at
   ( with the registration
   information provided in the section below.

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          | Media Type       | Registration information section |
          | application/yaml | Section 2.1 of this document     |

                                  Table 1

   IANA is asked to update the "Structured Syntax Suffixes" registry at
   ( with
   the registration information provided in the section below.

               | Suffix | Registration information section |
               | +yaml  | Section 2.2 of this document     |

                                  Table 2

6.  References

6.1.  Normative References

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

   [JSON]     Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

              Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,

              Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

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   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [UTF-8]    Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <>.

   [YAML]     Oren Ben-Kiki, Clark Evans, Ingy dot Net, Tina Müller,
              Pantelis Antoniou, Eemeli Aro, and Thomas Smith, "YAML
              Ain't Markup Language Version 1.2", 1 October 2021,

6.2.  Informative References

              Gössner, S., Normington, G., and C. Bormann, "JSONPath:
              Query expressions for JSON", Work in Progress, Internet-
              Draft, draft-ietf-jsonpath-base-13, 15 April 2023,

   [RFC7464]  Williams, N., "JavaScript Object Notation (JSON) Text
              Sequences", RFC 7464, DOI 10.17487/RFC7464, February 2015,

Appendix A.  Examples related to fragment identifier interoperability

A.1.  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

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    %YAML 1.2
      ? {be: expressed}
      : with a JSON Pointer

    0: no numeric mapping keys in JSON

      Figure 6: Example of YAML nodes that are not referenceable based
                            on JSON data model.

A.2.  Referencing a missing node

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

    %YAML 1.2
    0: "JSON Pointer `#/0` references a string mapping key."

       Figure 7: Example of a JSON Pointer that does not reference an
                               existing node.

A.3.  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.

    %YAML 1.2
    anchor: &anchor
      baz: you
    foo: &foo
      bar: *anchor
      bat: *foo

         Figure 8: 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.

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    %YAML 1.1
    # Many implementations use merge keys.
    the-viceroys: &the-viceroys
      title: The Viceroys
        given_name: Federico
        family_name: De Roberto
      <<: *the-viceroys
      title: The Illusion

                   Figure 9: Example of YAML merge keys.


   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.


   This section is to be removed before publishing as an RFC.

   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 Section 1.2.

   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.

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      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 [JSON-POINTER] which is expected to start with /.
      Moreover, since JSON Path [I-D.ietf-jsonpath-base] 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 [JSON-POINTER] 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 [JSON-POINTER].

   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
      [JSON] recommends UTF-8 in interoperability context we suggest
      that using UTF-8 is an interoperable behavior.  This is aligned
      with Section 5.2 of [YAML] that explicitly recommends UTF-8.

   Q: Why media type registration information is outside the IANA
   Considerations?  We decided to follow the style adopted in [HTTP]

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      where the IANA Considerations in Section 18.8 of [HTTP] references
      the multipart/byteranges media type registration form contained in
      the specification body Section 14.6 of [HTTP].

Change Log

   This section is to be removed before publishing as an RFC.

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).

Authors' Addresses

   Roberto Polli
   Digital Transformation Department, Italian Government

   Erik Wilde

   Eemeli Aro

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