Network Working Group                                         R. Gellens
Request for Comments: 4281                                      Qualcomm
Category: Standards Track                                      D. Singer
                                                                   Apple
                                                               P. Frojdh
                                                                Ericsson
                                                           November 2005

The Codecs Parameter for "Bucket" Media Types

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

Abstract

   Several MIME type/subtype combinations exist that can contain
   different media formats.  A receiving agent thus needs to examine the
   details of such media content to determine if the specific elements
   can be rendered given an available set of codecs.  Especially when
   the end system has limited resources, or the connection to the end
   system has limited bandwidth, it would be helpful to know from the
   Content-Type alone if the content can be rendered.

   This document adds a new parameter, "codecs", to various type/subtype
   combinations to allow for unambiguous specification of the codecs
   indicated by the media formats contained within.

   By labeling content with the specific codecs indicated to render the
   contained media, receiving systems can determine if the codecs are
   supported by the end system, and if not, can take appropriate action
   (such as rejecting the content, sending notification of the
   situation, transcoding the content to a supported type, fetching and
   installing the required codecs, further inspection to determine if it
   will be sufficient to support a subset of the indicated codecs, etc.)

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................4
   3. The Codecs Parameter ............................................4
      3.1. Generic Syntax .............................................5
      3.2. ISO File Format Name Space .................................7
      3.3. ISO Syntax .................................................8
   4. Use in Additional Media Types ...................................8
   5. Examples ........................................................9
   6. Additional Media Feature Details ................................9
   7. IANA Considerations .............................................9
   8. Security Considerations .........................................9
   9. Acknowledgements ...............................................10
   10. Normative References ..........................................10
   11. Informative References ........................................10

1. Introduction

   One of the original motivations for MIME is the ability to identify
   the specific media type of a message part.  However, due to various
   factors, it is not always possible from looking at the MIME type and
   subtype to know which specific media formats are contained in the
   body part, or which codecs are indicated in order to render the
   content.

   There are several media type/subtypes (either currently registered or
   deployed with registration pending) that contain codecs chosen from a
   set.  It is currently necessary to examine each media element in
   order to determine the codecs required to render the content.  For
   example, video/3gpp may contain any of the video formats H.263
   Profile 0, H.263 Profile 3, H.264, MPEG-4 Simple Profile, and/or any
   of the audio formats Adaptive Multi Rate (AMR), Adaptive Multi Rate -
   WideBand (AMR-WB), Extended AMR-WB, Advanced Audio Coding (AAC), or
   Enhanced aacPlus, as specified in [3GPP-Formats].

   In some cases, the specific codecs can be determined by examining the
   header information of the media content.  While this isn't as bad as
   examining the entire content, it still requires specialized knowledge
   of each format and is resource consumptive.

   This ambiguity can be a problem for various clients and servers.  It
   presents a significant burden to Multimedia Messaging (MMS) servers,
   which must examine the media sent in each message in order to
   determine which codecs are required to render the content.  Only then
   can such a server determine if the content requires transcoding or
   specialized handling prior to being transmitted to the handset.

   Additionally, it presents a challenge to smart clients on devices
   with constrained memory, processing power, or transmission bandwidth
   (such as cellular telephones and PDAs).  Such clients often need to
   determine in advance if they are currently capable of rendering the
   content contained in an MMS or email message.

   Current ambiguity:

   o   audio/3gpp can contain AMR, AAC, AMR-WB, Extended AMR-WB, or
       Enhanced aacPlus contents as specified in [3GPP-Formats].
   o   audio/3gpp2 can contain AMR, AAC, 13K (as per [13k]), Enhanced
       Variable Rate Codec (EVRC), Selectable Mode Vocoder (SMV),
       or VMR-WB, as specified in [3GPP2-Formats].
   o   video/3gpp can contain H.263 Profile 0, H.263 Profile 3, H.264,
       MPEG-4 Simple Profile, and/or AMR, AMR-WB, Extended AMR-WB, AAC,
       or Enhanced aacPlus, as specified in [3GPP-Formats].
   o   video/3gpp2 can contain H.263 Profile 0, H.263 Profile 3, H.264,
       MPEG-4 Simple Profile, and/or AMR, AAC, 13K (as per [13k]),
       EVRC, SMV, or VMR-WB, as specified in [3GPP2-Formats].

   Note that there are additional media types that are ambiguous, but
   are outside the scope of this document, including:

   o   video/mpeg4-generic, which can contain anything allowed by the
       MPEG-4 specification, or any codec registered with the MP4
       registration authority [MP4-Reg];
   o   video/quicktime, which can contain anything for which there is a
       QuickTime codec component; since QuickTime is extensible, this
       is not limited to the codecs that are or have been shipped by
       Apple Computer.

   With each "bucket" type, a receiving agent only knows that it has a
   container format.  It doesn't even know whether content labeled
   video/3gpp or video/3gpp2 contains video; it might be audio only,
   audio and video, or video only.

   A solution that permits a receiving agent to determine the specific
   codecs required to render media content would help provide efficient
   and scalable servers, especially for Multimedia Messaging (MMS), and
   aid the growth of multimedia services in wireless networks.

2. Conventions Used in This Document

   The key words "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
   and "MAY" in this document are to be interpreted as described in "Key
   words for use in RFCs to Indicate Requirement Levels" [KEYWORDS].

   The syntax in this document uses the BNF rules specified in
   [MIME-Format] and [MIME-Coding].

3. The Codecs Parameter

   This document adds a parameter to allow unambiguous specification of
   all codecs indicated to render the content in the MIME part.  This
   parameter is optional in all current types to which it is added.
   Future types that contain ambiguity are strongly encouraged to
   include this parameter.

   Media types:
       audio/3gpp,
       audio/3gpp2,
       video/3gpp,
       video/3gpp2,

   Parameter name:
       Codecs

   Parameter value:  A single value, or a comma-separated list of values
        identifying the codec(s) indicated to render the content in the
        body part.

        Each value consists of one or more dot-separated elements.  The
        name space for the first element is determined by the MIME type.
        The name space for each subsequent element is determined by the
        preceding element.

        Note that, per [MIME-Format], some characters (including the
        comma used to separate multiple values) require that the entire
        parameter value be enclosed in quotes.

        An element MAY include an octet that must be encoded in order to
        comply with [MIME-Format].  In this case, [MIME-Coding] is used:
        an asterisk ("*") is placed at the end of the parameter name
        (becoming "codecs*" instead of "codecs"), the parameter value
        usually starts with two single quote ("'") characters
        (indicating that neither character set nor language is
        specified), and each octet that requires encoding is represented
        as a percent sign ("%") followed by two hexadecimal digits.
        Note that, when the [MIME-Coding] form is used, the percent

        sign, asterisk, and single quote characters have special meaning
        and so must themselves be encoded.

        Examples of Generic Syntax:
            codecs=a.bb.ccc.d
            codecs="a.bb.ccc.d, e.fff"
            codecs*=''fo%2e
            codecs*="''%25%20xz, gork"

   When the Codecs parameter is used, it MUST contain all codecs
   indicated by the content present in the body part.  The Codecs
   parameter MUST NOT include any codecs that are not indicated by any
   media elements in the body part.

   In some cases, not all indicated codecs are absolutely required in
   order to render the content.  Therefore, when a receiver does not
   support all listed codecs, special handling MAY be required.  For
   example, the media element(s) MAY need to be examined in order to
   determine if an unsupported codec is actually required (e.g., there
   may be alternative tracks (such as English and Spanish audio), there
   may be timed text that can be dropped, etc.)

   NOTE:  Although the parameter value MUST be complete and accurate in
   'breadth' (that is, it MUST report all four-character codes used in
   all tracks for ISO-family files, for example) systems MUST NOT rely
   on it being complete in 'depth'.  If the hierarchical rules for a
   given code (e.g., 'qvxy') were written after a server was
   implemented, for example, that server will not know what elements to
   place after 'qvxy'.

   If a receiver encounters a body part whose Codecs parameter contains
   codecs that are not indicated by any media elements, then the
   receiver SHOULD process the body part by discarding the information
   in the Codecs parameter.

   If a receiver encounters a body part whose Codecs parameter does not
   contain all codecs indicated by the media elements, then the receiver
   MAY process the body part by discarding the information in the Codecs
   parameter.

3.1. Generic Syntax

   The Codecs parameter takes either of two forms.  The first form is
   used when the value does not contain any octets that require
   encoding.  The second form uses [MIME-Coding] to allow arbitrary
   octets to be encoded.  With either form, quotes allow for commas and
   other characters in <tspecials> (quotes MAY be used even when not
   required).

   This BNF uses the rules specified in [MIME-Format] and [MIME-Coding].

   Implementations MUST NOT add CFWS between the tokens except after
   ",".

   codecs      := cod-simple / cod-fancy

   cod-simple  := "codecs" "=" unencodedv

   unencodedv  := id-simple / simp-list

   simp-list   := DQUOTE id-simple *( "," id-simple ) DQUOTE

   id-simple   := element
               ; "." reserved as hierarchy delimiter

   element     := 1*octet-sim

   octet-sim   := <any TOKEN character>
               ; <TOKEN> defined in [MIME-Format]
               ;
               ; Within a Codecs parameter value, "." is reserved
               ; as a hierarchy delimiter

   cod-fancy   := "codecs*" ":=" encodedv

   encodedv    := fancy-sing / fancy-list

   fancy-sing  := [charset] "'" [language] "'" id-encoded
               ; Parsers MAY ignore <language>
               ; Parsers MAY support only US-ASCII and UTF-8

   fancy-list  := DQUOTE [charset] "'" [language] "'" id-list DQUOTE
               ; Parsers MAY ignore <language>
               ; Parsers MAY support only US-ASCII and UTF-8

   id-list     := id-encoded *( "," id-encoded )

   id-encoded  := encoded-elm *( "." encoded-elm )
               ; "." reserved as hierarchy delimiter

   encoded-elm := 1*octet-fancy

   octet-fancy := ext-octet / attribute-char
               ; <ext-octet> and <attribute-char> defined in
               ; [MIME-Coding]

   DQUOTE      := %x22 ; " (double quote)

   Initial name space:  This document only defines values for files in
   the ISO Base Media File Format family.  Other file formats may also
   define codec naming.

3.2. ISO File Format Name Space

   For the ISO Base Media File Format, the first element of a Codecs
   parameter value is a sample description entry four-character code as
   registered by the MP4 Registration Authority [MP4-Reg].  Values are
   case sensitive.

   Note that there are potentially multiple tracks in a file, each
   potentially carrying multiple sample entries (some but not all uses
   of the ISO File Format restrict the number of sample entries in a
   track to one).

   When the first element of a value is 'mp4a' (indicating some kind of
   MPEG-4 audio) or 'mp4v' (indicating some kind of MPEG-4 part-2
   video), the second element is the hexadecimal representation of the
   MP4 Registration Authority ObjectTypeIndication (OTI), as specified
   in [MP4-Reg] and [MP41] (including amendments).  Note that [MP4-Reg]
   uses a leading "0x" with these values, which is omitted here and
   hence implied.

   One of the OTI values for 'mp4a' is 40 (identifying MPEG-4 audio).
   For this value, the third element identifies the audio
   ObjectTypeIndication (OTI) as defined in [MP4A] (including
   amendments), expressed as a decimal number.

   For example, AAC low complexity has the value 2, so a complete
   string for AAC-LC would be "mp4a.40.2".

   One of the OTI values for 'mp4v' is 20 (identifying MPEG-4 part-2
   video).  For this value, the third element identifies the video
   ProfileLevelIndication as defined in [MP4V] (including amendments),
   expressed as a decimal number.

   For example, MPEG-4 Visual Simple Profile Level 0 has the value 9,
   so a complete string for MPEG-4 Visual Simple Profile Level 0 would
   be "mp4v.20.9".

3.3. ISO Syntax

   id-simple   :=/ id-iso

   id-encoded  :=/ id-iso

   id-iso      := iso-gen / iso-mpega / iso-mpegv

   iso-gen     := cpid *( element / encoded-elm )
               ; <element> used with <codecs-simple>
               ; <encoded-elm> used with <codecs-fancy>
               ;
               ; Note that the BNF permits "." within <element>
               ; and <encoded-elm> but "." is reserved as the
               ; hierarchy delimiter

   iso-mpega   := mp4a "." oti [ "." aud-oti ]
   iso-mpegv   := mp4v "." oti [ "." vid-pli ]

   cpid        := 4(octet-simple / octet-fancy)
               ; <octet-simple> used with <codecs-simple>
               ; <octet-fancy> used with <codecs-fancy>

   mp4a        := %x6d.70.34.61 ; 'mp4a'

   oti         := 2(DIGIT / "A" / "B" / "C" / "D" / "E" / "F")
               ; leading "0x" omitted

   aud-oti     := 1*DIGIT

   mp4v        := %x6d.70.34.76 ; 'mp4v'
   vid-pli     := 1*DIGIT

4. Use in Additional Media Types

   This parameter MAY be specified for use with additional MIME media
   types.

   For ISO file formats where the name space as defined here is
   sufficient, all that needs to be done is to update the media type
   registration to specify the Codecs parameter with a reference to this
   document.  For existing media types, it is generally advisable for
   the parameter to be optional; for new media types, the parameter MAY
   be optional or required, as appropriate.

   For ISO file formats where the name space as defined here needs to be
   expanded, a new document MAY update this one by specifying the
   additional detail.

For non-ISO formats, a new document MAY update this one by specifying
the name space for the media type(s).

5. Examples

   Content-Type: video/3gpp2; codecs="sevc, s263"
       (EVRC audio plus H.263 video)
   Content-Type: audio/3gpp; codecs=samr
       (AMR audio)
   Content-Type: video/3gpp; codecs="s263, samr"
       (H.263 video plus AMR audio)
   Content-Type: audio/3gpp2; codecs=mp4a.E1
       (13k audio)
   Content-Type: video/3gpp2; codecs="mp4v.20.9, mp4a.E1"
       (MPEG-4 Visual Simple Profile Level 0 plus 13K voice)

        Note:  OTI value 20 ("0x20" in [MP4-Reg]) says "Includes
        associated Amendment(s) and Corrigendum(a).  The actual object
        types are defined in [MP4V] and are conveyed in the
        DecoderSpecificInfo as specified in [MP4V], Annex K."
        (references adjusted).

6. Additional Media Feature Details

   It is sometimes helpful to provide additional details for a media
   element (e.g., the number of X and Y pixels, the color depth, etc.).
   These details are sometimes called "media features" or "media
   characteristics".

   When such additional features are included, the [Content-Features]
   header provides a handy way to do so.

7. IANA Considerations

The IANA has added "codecs" as an optional parameter to the media
types listed in Section 3, with a reference to this document.

8. Security Considerations

   The Codecs parameter itself does not alter the security
   considerations of any of the media types with which it is used.  Each
   audio and video media type has its own set of security considerations
   that continue to apply, regardless of the use of the Codecs
   parameter.

   An incorrect Codecs parameter might cause media content to be
   received by a device that is not capable of rendering it, or might
   cause media content to not be sent to a device that is capable of

   receiving it.  An incorrect Codecs parameter is therefore capable of
   some types of denial-of-service attacks.  However, this is most
   likely to arise by accident, as an attacker capable of altering media
   data in transit could cause more harm by altering the media format
   itself, or even the content type header, rather than just the Codecs
   parameter of the content type header.

9. Acknowledgements

   Harinath Garudadri provided a great deal of help, which is very much
   appreciated.  Mary Barnes and Bruce Lilly provided detailed and
   helpful comments.  Reviews and comments by Sam Hartman, Russ Housley,
   and Bert Wijnen were much appreciated.  Chris Newman carefully
   reviewed and improved the BNF.

10. Normative References

   [Content-Features] Klyne, G., "Indicating Media Features for MIME
                      Content", RFC 2912, September 2000.

   [KEYWORDS]         Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", BCP 14, RFC 2119,
                      March 1997.

   [MIME-Coding]      Freed, N. and K. Moore, "MIME Parameter Value and
                      Encoded Word Extensions: Character Sets,
                      Languages, and Continuations", RFC 2231, November
                      1997.

   [MIME-Format]      Freed, N. and N. Borenstein, "Multipurpose
                      Internet Mail Extensions (MIME) Part One: Format
                      of Internet Message Bodies", RFC 2045, November
                      1996.

   [Media-Features]   Holtman, K., Mutz, A., and T. Hardie, "Media
                      Feature Tag Registration Procedure", BCP 31, RFC
                      2506, March 1999.

   [MP4-Reg]          MP4REG, The MPEG-4 Registration Authority, URL:
                      <http://www.mp4ra.org>.

11. Informative References

   [13k]              Gellens, R. and H. Garudadri, "The QCP File Format
                      and Media Types for Speech Data", RFC 3625,
                      September 2003.

   [3GPP-Formats]     TS 26.244, Third Generation Partnership Project
                      (3GPP), "Transparent End-to-End Packet Switched
                      Streaming Service; 3GPP file format (3GP)", URL:
                      <http://www.3gpp.org/ftp/Specs/html-info/
                      26244.htm>.

   [3GPP2-Formats]    Third Generation Partnership Project 2, "3GPP2
                      File Formats for Multimedia Service", URL:
                      <http://www.3gpp2.org/Public_html/specs/
                      C.S0050-0_v1.0_121503.pdf>.

   [MP41]             ISO/IEC 14496-1:2004, "Information technology--
                      Coding of audio-visual objects--Part 1:  Systems".

   [MP4A]             ISO/IEC 14496-3:2001, "Information technology--
                      Coding of audio-visual objects--Part 3:  Audio".

   [MP4V]             ISO/IEC 14496-2:2004, "Information technology--
                      Coding of audio-visual objects--Part 2:  Visual".

Authors' Addresses

   Randall Gellens
   QUALCOMM Incorporated
   5775 Morehouse Drive
   San Diego, CA  92121
   USA

   EMail: randy@qualcomm.com

   David Singer
   Apple Computer, Inc.
   One Infinite Loop, MS:302-3MT
   Cupertino  CA 95014
   USA

   EMail: singer@apple.com
   Phone: +1 408 974 3162

   Per Frojdh
   Ericsson Research
   Multimedia Technologies
   SE-164 80 Stockholm, SWEDEN

   EMail: Per.Frojdh@ericsson.com
   Phone: +46 8 7190000

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