Media Types with Multiple Suffixes
draft-ietf-mediaman-suffixes-03

MEDIAMAN                                                       M. Sporny
Internet-Draft                                                    A. Guy
Intended status: Standards Track                          Digital Bazaar
Expires: 5 July 2023                                      1 January 2023

                   Media Types with Multiple Suffixes
                    draft-ietf-mediaman-suffixes-03

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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   This Internet-Draft will expire on 5 July 2023.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   2
   2.  Media Types with Multiple Suffixes  . . . . . . . . . . . . .   2
     2.1.  Processing Multiple Suffixes  . . . . . . . . . . . . . .   3
     2.2.  Fragment Identifiers  . . . . . . . . . . . . . . . . . .   4
     2.3.  Security Considerations . . . . . . . . . . . . . . . . .   5
       2.3.1.  Media Type Fibbing  . . . . . . . . . . . . . . . . .   5
   3.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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 final "+"
   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.

2.  Media Types with Multiple Suffixes

   The following paragraphs are additions to RFC 6838.

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   Media types MAY be registered with more than one suffix appended to
   the base subtype name.  The suffixes MUST be interpreted as ordered.
   Valid media type names containing a structured suffix are built from
   right to left (not left to right).  Characters on the left-most side
   of the left-most "+" in a subtype name specify the base subtype name.
   Characters to the right of each "+" in a subtype name denote
   additional structured syntax suffixes.

   Media types with more than one suffix MUST be registered according to
   the procedure defined in [RFC6838].  A new base subtype name MUST
   only be registered with suffix combinations that are already
   registered in their own right in the Structured Syntax Suffixes
   registry (https://www.iana.org/assignments/media-type-structured-
   suffix/media-type-structured-suffix.xhtml).

   For example, a media type that uses two suffixes, such as
   "application/foo+xml+gzip" is only permitted insofar as "+gzip" and
   "+xml" are already registered structured syntax suffixes.

2.1.  Processing Multiple Suffixes

   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 right to left 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 MAY be utilized in a processing
   pipeline where each segment of the pipeline handles a particular
   structured syntax suffix by applying decoding rules associated with
   the structured syntax suffix in the Structured Syntax Suffixes
   Registry (https://www.iana.org/assignments/media-type-structured-
   suffix/media-type-structured-suffix.xhtml).  The segment of the
   pipleine could then remove the structured syntax suffix from the
   media type and then pass the output of the decoding operation as well
   as the modified media type further down the pipeline.

   For example, for the media type "application/did+ld+json",
   applications can choose to process the underlying representation
   according to any of the following processing models: 1) application/
   did+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)), or 3) +json (as specified in the

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   Structured Syntax Suffixes Registry
   (https://www.iana.org/assignments/media-type-structured-suffix/media-
   type-structured-suffix.xhtml)).  As a further example, for the media
   type "image/svg+xml+gzip", applications can choose to process the
   underlying representation according to any of the following
   processing models: 1) image/svg+xml+gzip (as specified in the Media
   Type Registry (https://www.iana.org/assignments/media-types/media-
   types.xhtml)), 2) +gzip (as specified in the Structured Syntax
   Suffixes Registry (https://www.iana.org/assignments/media-type-
   structured-suffix/media-type-structured-suffix.xhtml)), and then +xml
   (as specified in the Structured Syntax Suffixes Registry
   (https://www.iana.org/assignments/media-type-structured-suffix/media-
   type-structured-suffix.xhtml)).

   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 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.

   Given this generic structured syntax processing approach, it is
   possible to perform structured syntax suffix processing on structured
   syntax suffixes that result in an invalid media type that cannot be
   processed further.  For example, when processing image/svg+xml+gzip,
   a processor could choose to process using the +gzip, and then the
   +xml structured syntax suffixes rules which would result in a
   meaningless application/svg media type.  Application developers are
   advised to ensure that the last structured syntax suffix, or valid
   media type, processed is the last one that is expected to be
   meaningfully processed by their application.  Thus, an application
   that processes the +gzip and then the +xml structured syntax suffixes
   from an image/svg+xml+gzip media type expects that the +xml data is
   the last meaningful piece of information that it hopes to extract
   from the processing pipeline.  That is, the application processor is
   expected to make a choice between processing as +xml or as image/
   svg+xml, and by making a choice, other choices might be removed from
   further processing pipeline stages.

2.2.  Fragment Identifiers

   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:

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   1.  For cases defined for the structured syntax suffix, where the
       fragment identifier does resolve per the structured syntax suffix
       rules, then 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 as specified by the specification associated
       with the full media type.

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

   Other advisory information, such as fragment processing not being
   defined in any existing specification, MAY be provided in the
   "Fragment Identifier Considerations" column in the IANA Structured
   Syntax Suffixes registry as long as the text is terse in nature.

2.3.  Security Considerations

2.3.1.  Media Type Fibbing

   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 would then invoke Microsoft Excel directly,
   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.

   While the liklihood of these sorts of attacks are low, they are not
   zero and enterprising attackers might take advantage of applications
   that carelessly register themselves in a structured suffix processing
   toolchain.  These sorts of toolchains need to ensure that the
   incoming media type is not blindly trusted and that proper magic
   header or file structure checking is performed before allowing the
   encoded data to drive operations that might negatively impact the
   application environment or operating system.

3.  Normative References

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   [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>.

Appendix A.  Acknowledgements

   The editors would like to thank the following individuals for
   feedback on the specification (in alphabetical order): Martin J.
   Duerst, Ivan Herman, Graham Klyne, Murray S.  Kucherawy, Mark
   Nottingham, 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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