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Versions: 00 01 02 03 04 05 RFC 3555

Internet Engineering Task Force      Audio/Video Transport Working Group
INTERNET-DRAFT                                                 S. Casner
draft-ietf-avt-rtp-mime-00.txt                             Cisco Systems
                                                              P. Hoschka
                                                           W3C/INRIA/MIT
                                                           June 25, 1999
                                                       Expires: 12/25/99


             MIME Type Registration of RTP Payload Formats

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

                                Abstract

     This document defines the procedure to register RTP Payload Formats
     as audio or video MIME subtype names.  This is useful in a text-
     based format or control protocol to identify the type of an RTP
     transmission.  This document also registers all the RTP payload
     formats defined in the RTP Profile for Audio and Video Conferences
     as MIME subtypes.  Some of these may also be used for transfer
     modes other than RTP.











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1.  Introduction

The MIME registration procedure described in RFC 2048 [1] was originally
designed for transport of multimedia information via asynchronous Inter-
net mail, but the MIME namespace now provides identification for other
transport modes as well.  This document defines the procedure to regis-
ter MIME subtype names for use with the Real-time Transport Protocol
(RTP), RFC 1889 (under revision as draft-ietf-avt-rtp-new [2]), to iden-
tify RTP payload formats.

This document also registers all the RTP payload formats defined in the
RTP Profile for Audio and Video Conferences, RFC 1890 (under revision as
draft-ietf-avt-profile-new [3]), as MIME subtypes under the "audio" and
"video" MIME types.

[As this document is completed for publication as an RFC, it may be
appropriate to add specifications in some of the registrations for how
the media encodings associated with those RTP payload formats should be
transferred under more traditional MIME transport modes as well.  That
has not been done yet.]

1.1.  Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4] and indicate
requirement levels for implementations compliant with this specifica-
tion.

2.  Procedure For Registering MIME Types for RTP Payload Types

Registering an RTP payload type as MIME type follows the same procedures
as described in RFC 2048 and uses the registration template shown in
Section 2.8 of RFC 2048.  Some additional parameters are required to
specify how a particular payload format is transported over RTP:

     Published specification
          A description of encoding and a specification of the payload
          format must be provided, usually by reference to an RTP pay-
          load format specification RFC.  That RFC may be separate, or
          the MIME subtype registration may be incorporated into the
          payload format specification RFC.  The payload format specifi-
          cation MUST include the RTP timestamp clock rate (or multiple
          rates for audio encodings with multiple sampling rates).

          A reference to a further description of the data compression
          format itself should be provided, if available.




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     Required parameters
          If the payload format does not have a fixed RTP timestamp
          clock rate, then a "rate" parameter is required.  A particular
          payload format may have additional required parameters.

     Optional parameters
          Most audio payload formats can have an optional "channels"
          parameter to specify the number of audio channels included in
          the transmission.  Any payload format, but most likely audio
          formats, may also include an optional "ptime" parameter to
          specify the length of time in milliseconds represented by the
          media in a packet.

          A particular payload format may have additional optional
          parameters.

     Encoding considerations
          The fact that the type can be transferred via RTP MUST be
          noted.

Depending on whether the type has already been registered for transfer
with a non-RTP protocol (e.g. MIME mail or http) or not, several dif-
ferent cases can occur:

     a) Not yet registered as a MIME type

        A new registration should be constructed using the MIME regis-
        tration template.  The registration may specify transfer via
        other means in addition to RTP if that is feasible and desired.
        The encoding considerations must specify how the type is
        transferred via RTP.

        Optional parameters may be defined as needed, and it must be
        clearly stated whether to which mode(s) of transfer the parame-
        ters apply.

     b) MIME type exists for a non-RTP protocol

        The encoding considerations of the existing type should be
        changed to indicate that the type can also be transferred via
        RTP.

        RTP-specific parameters may be added, and it must be clearly
        stated that these are only to be used when the media type is
        transmitted via RTP transport.

     c) Update an existing MIME type for RTP to be used for a non-RTP
        protocol



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        The encoding considerations of the existing type should be
        changed to indicate that the type can also be transferred via a
        non-RTP protocol (e.g. SMTP, HTTP).

        Non-RTP-specific parameters can be added, and it must be clearly
        stated that these are only to be used when the media type is
        transmitted via a non-RTP transport.

3.  Mapping to SDP Parameters

The representation of a MIME media type is specified in the syntax of
the Content-Type header field in RFC 2045 [5] as follows:

     type "/" subtype  *(";" parameter)

Parameters may be required for a particular type or subtype or they may
be optional.  For media types which represent RTP payload formats, the
parameters "rate", "channels" and "ptime" have general definitions
(given above) that may apply across types and subtypes.

The information carried in the media type string has a specific mapping
to fields in the Session Description Protocol (SDP), RFC 2327 [6], which
is commonly used to describe RTP sessions.  The mapping is as follows:

    o  The MIME type (audio, video) goes in SDP "m=" as the media name.

    o  The MIME subtype (payload format) goes in SDP "a=rtpmap" as the
       encoding name.

    o  The general (possibly optional) parameters "rate" and "channels"
       also go in "a=rtpmap" as clock rate and encoding parameters,
       respectively.

    o  The general (and optional) parameter "ptime" goes in SDP
       "a=ptime" attribute.

    o  Any payload-format-specific parameters go in SDP "a=fmtp" line.
       The format and syntax of these parameters may be defined by the
       payload format specification, but it is suggested that the param-
       eters be copied directly from the MIME media type string as a
       semicolon separated list of parameter=value pairs.

An example mapping is as follows:








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        audio/L16; rate=48000; channels=2; ptime=5

        m=audio 49170 RTP/AVP 97
        a=rtpmap:97 L16/48000/2
        a=ptime:5

Note that the payload format (encoding) names defined in the RTP Profile
are commonly shown in upper case.  MIME subtypes are commonly shown in
lower case.  These names are case-insensitive in both places.

4.  Registrations for "Audio/Video Profile"

In the following sections, all RTP payload formats described in draft-
ietf-profile-new-04.txt are registered as MIME subtypes.

The following fields of the registration template in RFC 2048 have gen-
eric values for all of these registrations unless explicitly overridden
in a specific registration.  To reduce the length of this document these
fields will not be replicated unless subtype-specific information must
be given:

     Encoding considerations:
          This type is only defined for transfer via RTP [RFC XXXX,
          draft-ietf-avt-rtp-new].

     Security considerations: none

     Interoperability considerations: none

     Applications which use this media type:
          Audio and video streaming and conferencing tools.

     Additional information: none

     Person & email address to contact for further information:
          See Section describing the respective type in draft-ietf-
          profile-new-04.txt. If no e-mail address is given, contact the
          author of draft-ietf-profile-new-04.txt

     Intended usage: COMMON

     Author/Change controller:
          See Section describing the respective type in draft-ietf-
          profile-new-04.txt






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4.1.  Audio Type Registrations

The following sections register all of the RTP audio payload types
defined in draft-ietf-profile-new-04.txt as MIME types.

For most audio payload formats, the RTP clock rate is equal to the sam-
pling rate.  Some payload formats operate only at one fixed sampling
rate, while others are adjustable.

4.1.1.  Registration of MIME media type audio/1016

     MIME media type name: audio

     MIME subtype name: 1016

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY

4.1.2.  Registration of MIME media type audio/CN

     MIME media type name: audio

     MIME subtype name: CN

     Required parameters: None

     Optional parameters: rate

     Published specification: RFCYYYY

4.1.3.  Registration of MIME media type audio/DVI4

     MIME media type name: audio

     MIME subtype name: DVI4

     Required parameters: rate
          The typical rate is 8000, but other rates may be specified.

     Optional parameters: ptime

     Published specification: RFCYYYY






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4.1.4.  Registration of MIME media type audio/G722

     MIME media type name: audio

     MIME subtype name: G722

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY

4.1.5.  Registration of MIME media type audio/G723

     MIME media type name: audio

     MIME subtype name: G723

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY

4.1.6.  Registration of MIME media type audio/G726-32

     MIME media type name: audio

     MIME subtype name: G726-32

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY

4.1.7.  Registration of MIME media type audio/G728

     MIME media type name: audio

     MIME subtype name: G728

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY




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4.1.8.  Registration of MIME media type audio/G729

     MIME media type name: audio

     MIME subtype name: G729

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY

4.1.9.  Registration of MIME media type audio/GSM

     MIME media type name: audio

     MIME subtype name: GSM

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY

4.1.10.  Registration of MIME media type audio/GSM-HR

     MIME media type name: audio

     MIME subtype name: GSM-HR

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY

4.1.11.  Registration of MIME media type audio/GSM-EFR

     MIME media type name: audio

     MIME subtype name: GSM-EFR

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY




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4.1.12.  Registration of MIME media type audio/L8

     MIME media type name: audio

     MIME subtype name: L8

     Required parameters: rate

     Optional parameters: channels, ptime

     Published specification: RFCYYYY

4.1.13.  Registration of MIME media type audio/L16

MIME subtype audio/L16 has already been registered via RFC 2586 for
transports other than RTP.  That registration is incorporated here and
augmented with additional information for RTP transport.

     MIME media type name: audio

     MIME subtype name: L16

     Required parameters
          rate: number of samples per second -- For non-RTP transport,
          the permissible values for rate are 8000, 11025, 16000, 22050,
          24000, 32000, 44100, and 48000 samples per second.  For RTP
          transport, other values are permissible but the aforementioned
          values are RECOMMENDED.  For RTP, the rate parameter indicates
          the RTP timestamp clock rate, which is equal to the sample
          rate.

     Optional parameters
          channels: how many audio streams are interleaved -- defaults
          to 1; stereo would be 2, etc.  Interleaving takes place
          between individual two-byte samples.  For RTP, ptime: RECOM-
          MENDED duration of each packet in milliseconds.

     Encoding considerations
          Audio data is binary data, and must be encoded for non-binary
          transport; the Base64 encoding is suitable for Email.  Note
          that audio data does not compress easily using lossless
          compression.

          This type is also defined for transfer via RTP [RFC XXXX,
          draft-ietf-avt-rtp-new].

     Security considerations
          Audio data is believed to offer no security risks.



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     Interoperability considerations
          This type is compatible with the encoding used in the WAV
          (Microsoft Windows RIFF) and Apple AIFF union types, and with
          the public domain "sox" and "rateconv" programs.

     Published specification
          RFC 2586 for non-RTP transports, RFCYYYY for RTP

     Applications which use this media
          The public domain "sox" and "rateconv" programs accept this
          type.

          1. Magic number(s) : None
          2. File extension(s) : WAV L16
          3. Macintosh file type code : AIFF


     Person to contact for further information

          1. Name : James Salsman
          2. E-mail : jps-L16@bovik.org


     Intended usage
          Common

          It is expected that many audio and speech applications will
          use this type.  Already the most popular platforms provide
          this type with the rate=11025 parameter referred to as "radio
          quality speech."

     Author/Change controller
          James Salsman for non-RTP transports.  Authors of RFC YYYY for
          RTP.

4.1.14.  Registration of MIME media type audio/LPC

     MIME media type name: audio

     MIME subtype name: LPC

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY





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4.1.15.  Registration of MIME media type audio/MPA

     MIME media type name: audio

     MIME subtype name: MPA (MPEG audio)

     Required parameters: None

     Optional parameters:
          layer: which layer of MPEG audio encoding; permissible values
          are 1, 2, 3.

          samplerate: the rate at which audio is sampled.  MPEG-1 audio
          supports sampling rates of 32, 44.1, and 48 kHz; MPEG-2 sup-
          ports sampling rates of 16, 22.05 and 24 kHz.  This parameter
          is separate from the RTP timestamp clockrate which is always
          90000 Hz for MPA.

          mode: permissible values are "stereo", "joint_stereo",
          "single_channel", "dual_channel".  The "channels" parameter
          does not apply to MPA.  It is undefined to put a number of
          channels in the SDP rtpmap attribute for MPA.

          bitrate: the data rate for the audio bit stream.

          ptime: RECOMMENDED duration of each packet in milliseconds.

          Parameters which are omitted are left to the encoder to choose
          based on the session bandwidth, configuration information, or
          other constraints.  The selected layer as well as the sampling
          rate and mode are indicated in the payload so receivers can
          process the data without these parameters being specified
          externally.

     Published specification: RFCYYYY

4.1.16.  Registration of MIME media type audio/PCMA

     MIME media type name: audio

     MIME subtype name: PCMA

     Required parameters: rate
          The typical rate is 8000, but other rates may be specified.

     Optional parameters: channels, ptime

     Published specification: RFCYYYY



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4.1.17.  Registration of MIME media type audio/PCMU

     MIME media type name: audio

     MIME subtype name: PCMU

     Required parameters: rate
          The typical rate is 8000, but other rates may be specified.

     Optional parameters: channels, ptime

     Published specification: RFCYYYY

4.1.18.  Registration of MIME media type audio/QCELP

     MIME media type name: audio

     MIME subtype name: QCELP

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY

4.1.19.  Registration of MIME media type audio/VDVI

     MIME media type name: audio

     MIME subtype name: VDVI

     Required parameters: None

     Optional parameters: ptime

     Published specification: RFCYYYY

4.2.  Video Type Registrations

For all of the video payload formats registered here, the RTP timestamp
clock rate is always 90000 Hz, so the "rate" parameter is not applica-
ble.  Likewise, the "channel" parameter is not used with video, and
while "ptime" could be used with video, it typically is not.

4.2.1.  Registration of MIME media type video/BT656

     MIME media type name: video




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     MIME subtype name: BT656

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2431

4.2.2.  Registration of MIME media type video/CelB

     MIME media type name: video

     MIME subtype name: CelB

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2029

4.2.3.  Registration of MIME media type video/JPEG

     MIME media type name: video

     MIME subtype name: JPEG

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2435

4.2.4.  Registration of MIME media type video/H261

     MIME media type name: video

     MIME subtype name: H261

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2032

4.2.5.  Registration of MIME media type video/H263

     MIME media type name: video




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     MIME subtype name: H263

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2190

4.2.6.  Registration of MIME media type video/H263-1998

     MIME media type name: video

     MIME subtype name: H263-1998

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2429

4.2.7.  Registration of MIME media type video/MPV

     MIME media type name: video

     MIME subtype name: MPV
          MPEG-1 or -2 Elementary Streams

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2250

4.2.8.  Registration of MIME media type video/MP2T

     MIME media type name: video

     MIME subtype name: MP2T
          MPEG-2 Transport Streams

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2250






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4.2.9.  Registration of MIME media type video/MP1S

     MIME media type name: video

     MIME subtype name: MP1S
          MPEG-1 Systems Streams

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2250

4.2.10.  Registration of MIME media type video/MP2P

     MIME media type name: video

     MIME subtype name: MP2P
          MPEG-2 Program Streams

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2250

4.2.11.  Registration of MIME media type video/BMPEG

     MIME media type name: video

     MIME subtype name: BMPEG

     Required parameters: None

     Optional parameters: None

     Published specification: RFC 2343

4.2.12.  Registration of MIME media type video/nv

     MIME media type name: video

     MIME subtype name: nv

     Required parameters: None

     Optional parameters: None




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     Published specification: RFC YYYY

5.  Security Considerations

The registration procedure specified in this memo does not impose any
security considerations on its own.  This memo also contains several
MIME type registrations, each of which may state any security considera-
tions specific to that registration.

Several audio and video encodings are perfect for hiding data using
steganography.

6.  References

[1] N. Freed, J. Klensin and J. Postel, "Multipurpose Internet Mail
    Extensions (MIME) Part Four: Registration Procedures", RFC 2048,
    November 1996.

[2] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP:
    A Transport Protocol for real-time applications," RFC 1889,
    January 1996, updated by draft-ietf-avt-rtp-new (Work in
    Progress).

[3] H. Schulzrinne, S. Casner, "RTP profile for audio and video
    conferences with minimal control", RFC 1890, January 1996, updated
    by draft-ietf-avt-profile-new (Work in Progress).

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

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

[6] M. Handley and V. Jacobson, "SDP: Session Description Protocol",
    RFC 2327, April 1998.















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7.  Authors' Addresses

   Stephen L. Casner
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134-1706
   Phone: +1 408 525 4782
   Email: casner@cisco.com

   Philipp Hoschka   MIT-LCS                  INRIA
   W3C               545, Technology Square   Route des Lucioles 2004
   ph@w3.org         Cambridge, MA 02139      06904, Sophia-Antipolis Cedex
                     USA                      BP 93, France
                     Tel:(+1) 617.258.0604    Tel:(+33) 4 92 38 79 84
                     Fax:(+1) 617.258.5999    Fax:(+33) 4 92 38 77 65
   http://www.w3.org/people/hoschka



































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