Internet Draft R. Gellens
Document: draft-gellens-mime-bucket-03.txt Qualcomm
Expires: August 2005 D. Singer
Apple
P. Frojdh
Ericsson
February 2005
The Codecs Parameter for "Bucket" Media Types
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Abstract
Several MIME type/subtype combinations exist which 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
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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.)
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Table of Contents
1. Conventions Used in this Document . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Codecs Parameter . . . . . . . . . . . . . . . . . . . . 5
3.1. Generic Syntax . . . . . . . . . . . . . . . . . . . . 6
3.2. ISO File Format Name Space . . . . . . . . . . . . . . . 7
3.3. ISO Syntax . . . . . . . . . . . . . . . . . . . . . . 7
4. Use in Additional Media Types . . . . . . . . . . . . . . . 8
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 8
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
12. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property Statement . . . . . . . . . . . . . . . 12
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 12
Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . 12
1. 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].
Syntax is given using [ABNF], including the Core Rules.
2. 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) which may 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 AMR, AMR-WB, Extended
AMR-WB, AAC, or Enhanced aacPlus, as specified in [3GPP-Formats].
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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 it 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.
Specifically,
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]), EVRC, SMV,
or VMR-WB, as specified in [3GPP2-Formats] (video/3gpp2 MIME
registration pending).
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]
(video/3gpp2 MIME registration pending).
Note that there are additional media types which 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 audio 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.
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A solution which 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.
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 which contain ambiguity are strongly encouraged to
include this parameter, as mandatory if possible, as optional
otherwise.
Media types:
audio/3gpp,
audio/3gpp2*,
video/3gpp,
video/3gpp2*,
*registration pending
Parameter name:
Codecs
Parameter value:
A single value, or a comma-separated list of values (which must
be enclosed in quotes to comply with [MIME-Format]) which
identifies 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.
An element MAY includes characters that (per [MIME-Format]) must
be enclosed in quotes. In this case the entire value MUST be
enclosed in quotes.
An element MAY include a character that must be encoded in order
to comply with [MIME-format] or with additional restrictions in
this document. In this case, the character is represented as a
percent-sign ("%") followed by two characters which express the
character as two hexadecimal digits. Note that due to the use
of the percent sign in this way, the percent sign itself must be
so encoded. (See the ABNF below.)
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Examples of Generic Syntax:
a.bb.ccc.d
"a.bb, c.dd"
fo%2e
"%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 which 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'.
3.1. Generic Syntax
See [ABNF] Core Rules for ALPHA, DIGIT, HEXDIGIT, DQUOTE, SP
codecs = "Codecs" *SP "=" *SP value
value = id / DQUOTE *SP id *(*SP "," SP id) *SP DQUOTE
id = id-gen
id-gen = element *( "." element )
element = 1*(idchar / idchar-spec)
idchar = "!" / "#" / "$" / "&" /
"'" / "*" / "+" / "-" /
"^" / "_" / "`" / "/" /
"(" / ")" / "<" / ">" /
"@" / ";" / ":" / "\" /
"{" / "}" / "~" /
ALPHA / DIGIT
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; non-ALPHA and non-DIGIT MUST be in quotes
idchar-spec = "%" 2HEXDIG ; one character in hex
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).
One of the OTI values for 'mp4a' is 0x40 (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.0x40.2'.
One of the OTI values for 'mp4v' is 0x20 (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.0x20.9'.
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3.3. ISO Syntax
id =/ id-iso
id-iso = iso-gen / iso-mpega / iso-mpegv
iso-gen = cpid *( "." element )
iso-mpega = mp4a "." oti [ "." aud-oti ]
iso-mpegv = mp4v "." oti [ "." vid-pli ]
cpid = 4(idchar / idchar-spec)
mp4a = %x6d.70.34.61 ;; 'mp4a'
oti = "0x" 2HEXDIG
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).
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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.0xE1
(13k audio)
Content-Type: video/3gpp2; Codecs="mp4v.0x20, mp4a.0xE1"
(Visual ISO/IEC 14496-2 [MP4V] plus 13K voice)
Note: 0x20 OTI value 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.
6. Additional Media Feature Details
For the same reasons that the Codecs parameter is useful, 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" and sometimes "media
characteristics".
When such additional features are included, the [Content-Features]
header provides a handy way to do so.
7. IANA Considerations
The IANA is kindly requested to add "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 for which it is available.
Each audio and video media type has its own set of security
considerations which continue to apply, regardless of the use of the
codecs parameter.
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An incorrect codecs parameter might cause media content to be
received by a device which is not capable of rendering it, or might
cause media content to not be sent to a device which 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.
10. Normative References
[ABNF] Crocker, Overell, "Augmented BNF for Syntax Specifications:
ABNF", RFC 2234, Internet Mail Consortium, Demon Internet Ltd.,
November 1997.
[Content-Features] Kline, G., "Indicating Media Features for MIME
Content", RFC 2912, September 2000.
[MIME-Format] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 2996.
[MIME-Types] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November
1996.
[Media-Features] Holtman, K., A. Mutz and T. Hardie, "Media Feature
Tag Registration Procedure", RFC 2506, BCP 31, March 1999.
[MP4-reg] MP4REG, The MPEG-4 Registration Authority,
<url://www.mp4ra.org>
11. Informative References
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[13k] Gellens, R and H. Garudadri, "The QCP File Format and Media
Types for Speech Data", RFC 3625, September 2003.
[AMR] Sjoberg, J., M. Westerlund, A. Lakaniemi, Q. Xie, "Real-Time
Transport Protocol (RTP) Payload Format and File Storage Format for
the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband
(AMR-WB) Audio Codecs", RFC 3267, June 2002.
[3GPP-Codecs] TS 26.234, Third Generation Partnership Project
(3GPP), "Transparent End-to-End Packet-Switched Streaming Service
(PSS); 3GPP file format (3GP)", URL:
<http://www.3gpp.org/ftp/Specs/html-info/26234.htm>
[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 14496-1:2001, "Information technology--Coding of
audio-visual objects--Part 1: Systems".
[MP4A] ISO/IEC 14496-3:2001, "Information Technology--Coding of
Audio/Visual Object--Part 3: Audio".
[MP4V] ISO/IEC 14496-2:2001, "Information Technology--Coding of
Audio/Visual Object--Part 2: Video".
12. Authors' Addresses
Randall Gellens
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego, CA 92121
USA
randy@qualcomm.com
David Singer
Apple Computer, Inc.
One Infinite Loop, MS:302-3MT
Cupertino CA 95014
USA
singer@apple.com
+1 408 974 3162
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Per Frojdh
Ericsson Research
Multimedia Technologies
SE-164 80 Stockholm, SWEDEN
+46 8 7190000
Per.Frojdh@ericsson.com
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