Network Working Group R. Gellens

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.)

Gellens, et al. Standards Track [Page 1]

RFC 4281 The Codecs Parameter November 2005

Table of Contents

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.

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RFC 4281 The Codecs Parameter November 2005

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.

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RFC 4281 The Codecs Parameter November 2005

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

Gellens, et al. Standards Track [Page 4]

RFC 4281 The Codecs Parameter November 2005

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).

Gellens, et al. Standards Track [Page 5]

RFC 4281 The Codecs Parameter November 2005

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)

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RFC 4281 The Codecs Parameter November 2005

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".

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RFC 4281 The Codecs Parameter November 2005

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.

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RFC 4281 The Codecs Parameter November 2005

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

Gellens, et al. Standards Track [Page 9]

RFC 4281 The Codecs Parameter November 2005

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.

Gellens, et al. Standards Track [Page 10]

RFC 4281 The Codecs Parameter November 2005

[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: [email protected]

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

EMail: [email protected]
Phone: +1 408 974 3162

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

EMail: [email protected]
Phone: +46 8 7190000

Gellens, et al. Standards Track [Page 11]

RFC 4281 The Codecs Parameter November 2005

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Gellens, et al. Standards Track [Page 12]