Network Working Group                                             A. Guy
Internet-Draft                                            Digital Bazaar
Intended status: Standards Track                        January 11, 2021
Expires: July 15, 2021

                   Media Types with Multiple Suffixes


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

Status of This Memo

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   This Internet-Draft will expire on July 15, 2021.

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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
   3.  Normative References  . . . . . . . . . . . . . . . . . . . .   3
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

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",
   document are to be interpreted as described in [RFC2119] when they
   appear in ALL CAPS.  They may also appear in lower or mixed case as
   plain English words, without any normative meaning.

2.  Media Types with Multiple Suffixes

   The following paragraphs are additions to RFC 6838.

   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 specifier suffixes.

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   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.

   For example, a media type that uses the two suffixes "+svg+xml" is
   only permitted insofar as "svg+xml" is already registered.  In this
   case, the suffix "+svg" does not need to be registered individually,
   but "+xml" and "svg+xml" MUST be registered.

2.1.  Processing Multiple Suffixes

   Registered subtypes 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 and left-most
   "+" from a media type with multiple suffixes, and process according
   to the remaining media type suffix(es).  An application can ignore as
   many of the left-most suffixes as necessary to achieve a media type
   that can be processed.

   For example, for the media type "application/did+ld+json", the
   following are all valid subtypes with their own individually
   specified processing rules:


   Thus applications can choose to process the underlying representation
   according any of the following valid media types:


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,

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

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, Manu Sporny,
   Ted Thibodeau Jr.

Author's Address

   Amy Guy
   Digital Bazaar
   203 Roanoke Street W.
   Blacksburg, VA  24060


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