Media Types with Multiple Suffixes
draft-ietf-mediaman-suffixes-07

MEDIAMAN                                                       M. Sporny
Internet-Draft                                                    A. Guy
Updates: 6838 (if approved)                               Digital Bazaar
Intended status: Standards Track                            2 March 2024
Expires: 3 September 2024

                   Media Types with Multiple Suffixes
                    draft-ietf-mediaman-suffixes-07

Abstract

   This document updates RFC 6838 "Media Type Specifications and
   Registration Procedures" to describe how to interpret subtypes with
   multiple suffixes.

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 3 September 2024.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   2
   2.  Media Types with Multiple Suffixes  . . . . . . . . . . . . .   3
     2.1.  Processing Multiple Suffixes  . . . . . . . . . . . . . .   3
     2.2.  Fragment Identifiers  . . . . . . . . . . . . . . . . . .   5
     2.3.  Structured Syntax Name Suffixes . . . . . . . . . . . . .   5
     2.4.  Structured Syntax Suffix Registration Template  . . . . .   6
     2.5.  Security Considerations . . . . . . . . . . . . . . . . .   7
       2.5.1.  Document Validity for Suffixes  . . . . . . . . . . .   7
       2.5.2.  Fragment Semantics for Suffixes . . . . . . . . . . .   7
       2.5.3.  Security Characteristics for Suffixes . . . . . . . .   8
       2.5.4.  Partial Processing of Suffixes  . . . . . . . . . . .   8
   3.  Normative References  . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  IANA Considerations  . . . . . . . . . . . . . . . .   9
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   As written, RFC 6838 [RFC6838] permits the registration of media type
   subtype names which contain any number of occurrences of the "+"
   character.  RFC 6838 defines the characters following the first "+"
   character to be a structured syntax suffix, but does not define
   anything further about how to interpret subtype names containing more
   than one "+" character.

   This document updates RFC 6838 to clarify how to interpret subtype
   names containing more than one "+" character as subtypes with
   multiple suffixes.

   As registration of media types which use a structured suffix has
   become widely supported, this enables further specialization of media
   types that build on already registered and well-defined media types
   which themselves use a structured suffix.

1.1.  Conventions Used in This Document

   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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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2.  Media Types with Multiple Suffixes

   This section is an addition to RFC 6838.

   Media types MAY be registered with more than one structured suffix
   appended to the base subtype name.  Characters on the left-most side
   of the left-most "+" in a subtype name specify the base subtype name.
   The entire structured suffix is all of the characters to the right of
   the left-most "+" sign in the media type, including the left-most "+"
   sign itself.  The entire structured suffix MAY be composed of one or
   more broader structured suffixes.  As an example, given the
   "application/foo+bar+baz" media type: "application" is the top-level
   type, "foo" is the base subtype name, "+bar+baz" is the entire
   structured suffix, and "+baz" is the broader structured suffix
   contained in the entire structured suffix.

2.1.  Processing Multiple Suffixes

   This section is an addition to RFC 6838.

   Registered media types have clear processing rules.  In cases where
   specific handling of the exact media type is not required, receivers
   of the media type MAY do generic processing on the underlying
   representation according to their ability to process any subset of
   the suffix(es) from left to right inclusive.  In other words, an
   application can choose to ignore the base subtype name from a media
   type with multiple suffixes, and process according to the remaining
   media type suffix(es).

   This sort of generic processing of a portion of the media type MAY be
   utilized by a processor that is capable of applying decoding rules
   associated with the portion of the structured syntax suffix in the
   Structured Syntax Suffixes Registry
   (https://www.iana.org/assignments/media-type-structured-suffix/media-
   type-structured-suffix.xhtml).

   When the entire structured suffix is composed of multiple structured
   suffixes, those structured suffixes MUST be considered in left-to-
   right order.  This is done by considering the entire structured
   suffix first, and then dropping the left-most suffix for each
   iteration of the structured suffix variations that are considered.
   Note that "considering" a structured suffix variation does not mean
   that an implementation has to process the data associated with each
   variation, but it does mean that the variations that are considered
   do have a valid order in which they are considered.

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   For example, for the media type "application/vc+ld+json",
   applications can choose to process the underlying representation
   according to any one of the following processing models, in the
   following order:

   1.  application/vc+ld+json (as specified in the Media Type Registry
       (https://www.iana.org/assignments/media-types/media-types.xhtml))

   2.  +ld+json (as specified in the Structured Syntax Suffixes Registry
       (https://www.iana.org/assignments/media-type-structured-suffix/
       media-type-structured-suffix.xhtml))

   3.  +json (as specified in the Structured Syntax Suffixes Registry
       (https://www.iana.org/assignments/media-type-structured-suffix/
       media-type-structured-suffix.xhtml))

   This means that a processor considers processing the entire media
   type first, and then considers variations of the structured suffixes
   in most specific to least specific (left-to-right) order; for
   example, "+ld+json" (more specific) and then "+json" (less specific).
   The processor is never expected to process "+ld" alone when presented
   with a "application/vc+ld+json" media type for two reasons 1) doing
   so would be considered interpreting multiple structured suffixes out
   of order, and 2) "+ld" is not a registered structured suffix type.

   If an application choses to utilize a portion of the media type that
   is a structured syntax suffix, the suffix MUST exist as an entry in
   the Structured Syntax Suffixes Registry
   (https://www.iana.org/assignments/media-type-structured-suffix/media-
   type-structured-suffix.xhtml) and the specification referred to in
   the "Encoding Considerations" entry of the registry MUST be used for
   both encoding and decoding the byte stream associated with the media
   type.

   In order to gain the most benefit from the media that is presented,
   and to reduce the security attack surface for processed content,
   implementations that are able to process multiple suffixes SHOULD
   process the most specific media type or structured syntax suffix that
   they are able to process.

   For processors that choose to process data associated with multiple
   structured suffixes, if processing data according to any registered
   structured suffix type fails, further processing on less specific
   structured suffixes SHOULD NOT be performed.

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2.2.  Fragment Identifiers

   This section is an addition to RFC 6838.

   The syntax and semantics for fragment identifiers are specified in
   the "Fragment Identifier Considerations" column in the IANA
   Structured Syntax Suffixes registry.  In general, when processing
   fragment identifiers associated with a structured syntax suffix, the
   following rules SHOULD be followed:

   1.  For cases defined for the structured syntax suffix, where the
       fragment identifier does resolve per the structured syntax suffix
       rules, then proceed as specified by the specification associated
       with the "Fragment Identifier Considerations" column in the IANA
       Structured Syntax Suffixes registry.

   2.  For cases defined for the structured syntax suffix, where the
       fragment identifier does not resolve per the structured syntax
       suffix rules, then proceed as specified by the specification
       associated with the full media type.

   3.  For cases not defined for the structured syntax suffix, then
       proceed as specified by the specification associated with the
       full media type.

2.3.  Structured Syntax Name Suffixes

   The following paragraphs are additional guidance to Section 4.2.8
   "Structured Syntax Name Suffixes", in RFC 6838.

   Media types that make use of a named structured syntax, or similar
   separator such as a dash "-", MUST ensure that the registration is
   semantically aligned, from a data model perspective, with existing
   base subtype names in the media type registry.  For example, for the
   media types "application/foo+bar" and "application/foo+baz", the
   expectation is that the semantics suggested by the base subtype name
   "application/foo" are the same between both media types.  The
   Designated Expert MUST reject a registration if they believe the
   semantics for a media type registration does not align with existing
   base subtype names in the media type registry.

   Registrants MUST prove to the Designated Expert, such as through an
   email to a public mailing list or issue tracker comment, that they
   have consent from the existing Change Controller for the associated
   base subtype name to register the new media type.

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2.4.  Structured Syntax Suffix Registration Template

   This section replaces Section 6.2 "Structured Syntax Suffix
   Registration Template" in RFC 6838.

   Media types containing more than one structured suffix MUST be
   registered according to the procedure defined in [RFC6838] and this
   document.

   For structured suffixes containing more than one broader structured
   suffix, the structured suffix to the right of the one being
   registered MUST also be registered.  For example, for the structured
   suffix "+foo+bar+baz", the registration order is "+baz", then
   "+bar+baz", and finally "+foo+bar+baz".  Structured suffixes
   containing more than one broader structured suffix MAY register all
   of the suffixes at the same time.

   This template describes the fields that must be supplied in a
   structured syntax suffix registration request:

   Name
      Full name of the well-defined structured syntax.

   +suffix
      Suffix used to indicate conformance to the syntax.

   References
      Include full citations for all specifications necessary to
      understand the structured syntax.

   Encoding considerations
      A full citation to a section in a specification that provides
      general guidance regarding encoding considerations for any type
      employing this syntax.  The same requirements for media type
      encoding considerations given in Section 4.8 apply here.

   Interoperability considerations
      A full citation to a section in a specification that documents any
      issues regarding the interoperable use of types employing this
      structured syntax should be given here.  Examples would include
      the existence of incompatible versions of the syntax, issues
      combining certain charsets with the syntax, or incompatibilities
      with other types or protocols.

   Fragment identifier considerations
      A full citation to a section in a specification that documents the
      generic processing rules of fragment identifiers for any type
      employing this syntax should be described here.

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   Security considerations
      A full citation to a section in a specification that provides
      security considerations shared by media types employing this
      structured syntax must be specified here.  The same requirements
      for media type security considerations given in Section 4.6 apply
      here, with the exception that the option of not assessing the
      security considerations is not available for suffix registrations.

   Contact
      Person or organization (including contact information) to contact
      for further information.

   Author/Change controller
      Person or organization (including contact information) authorized
      to change this suffix registration.

2.5.  Security Considerations

   This section is an addition to Section 7 "Security Considerations" in
   RFC 6838.

2.5.1.  Document Validity for Suffixes

   If a toolchain chooses to process a provided media type by using the
   selected structured suffix processing rules, it cannot presume that a
   document that is valid per the decoding rules associated with the
   structured suffix will be valid for a recognized subset of the
   structured suffix.  For example, presuming a media type of
   "application/foo+bar+baz", a toolchain cannot presume that a valid
   "+baz" document will also be a valid "+bar+baz" document nor a valid
   "application/foo+bar+baz" document.  On the other hand, presuming a
   media type of "application/foo+bar+baz", a toolchain _can_ presume
   that a valid "application/foo+bar+baz" document will also be a valid
   "+bar+baz" document and a valid "+baz" document.

2.5.2.  Fragment Semantics for Suffixes

   If a toolchain chooses to process a provided media type by using the
   selected structured suffix processing rules, it cannot presume that
   fragment identifier semantics will be the same across a recognized
   subset of the structured suffix.  For example, presuming a media type
   of "application/foo+bar+baz", a toolchain cannot presume that the
   fragment semantics for a "+baz" document will be the same as for a
   "+bar+baz" document or the same as for an "application/foo+bar+baz"
   document.

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2.5.3.  Security Characteristics for Suffixes

   Toolchains cannot assume that the security characteristics of
   processing based on structured suffixes will be the same for the
   entire media type nor for any combination of recognized structured
   suffixes.  For example, presuming a media type of "application/
   foo+bar+baz", a toolchain cannot presume that the security
   considerations for a "+baz" document will be the same as for a
   "+bar+baz" document nor the same for an "application/foo+bar+baz"
   document.

2.5.4.  Partial Processing of Suffixes

   It is possible for an attacker to utilize multiple structured
   suffixes in a way that tricks unsuspecting toolchains into skipping
   important security checks and allowing viruses to propagate.  For
   example, an attacker might utilize an "application/vnd.ms-
   excel.addin.macroEnabled.12+zip" structured suffix to trigger an
   unzip process that might then directly invoke Microsoft Excel,
   bypassing anti-virus tooling that would otherwise block a macro-
   enabled MS Excel file containing a virus of some kind from being
   scanned or opened.

   Enterprising attackers might take advantage of toolchains that
   partially process media types in this manner.  Toolchains that
   process media types based purely on a structured suffix need to
   ensure that further processing does not blindly trust the decoded
   data, and that proper magic header or file structure checking is
   performed, before allowing the decoded data to drive operations that
   might negatively impact the application environment or operating
   system.

3.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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Appendix A.  IANA Considerations

   [RFC6838] established the Registration Procedure for the Structured
   Syntax Suffixes Registry as "Expert Review".  However, since the
   inception of the registry, the Designated Experts have been operating
   as if the Registration Procedure is "Specification Required" given
   that a specification is required in the registration template for the
   "References" entry, which defines how the structured suffix is to be
   used.  Every entry in the Structured Syntax Suffixes Registry
   contains at least one reference to a specification.  Furthermore,
   this document updates the Structured Syntax Suffixes Registry
   Registration Template to include links to specifications for most
   fields.  Therefore, there is a clear requirement for at least one
   specification when performing a Structured Syntax Suffix
   registration.

   This section updates the Registration Procedure for the Structured
   Syntax Suffixes Registry to "Specification Required" and instructs
   IANA to update the existing registry to reflect this change.

Appendix B.  Acknowledgements

   The editors would like to thank the following individuals for
   feedback on the specification (in alphabetical order): Harald
   Alvestrand, Amanda Baber, Martin J.  Dürst, Ivan Herman, Graham
   Klyne, Murray S.  Kucherawy, Darrel Miller, Mark Nottingham, Roberto
   Polli, Orie Steele, and Ted Thibodeau Jr.

Authors' Addresses

   Manu Sporny
   Digital Bazaar
   203 Roanoke Street W.
   Blacksburg, VA 24060
   United States of America
   Email: msporny@digitalbazaar.com
   URI:   http://manu.sporny.org/

   Amy Guy
   Digital Bazaar
   203 Roanoke Street W.
   Blacksburg, VA 24060
   United States of America
   Email: rhiaro@digitalbazaar.com
   URI:   https://rhiaro.co.uk/

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