Internet Engineering Task Force      Audio/Video Transport Working Group
Internet-Draft                                                 S. Casner
draft-ietf-avt-rfc3555bis-02.txt                           Packet Design
Obsoletes: RFC 3555 (if approved)                      February 26, 2006
                                                Expires: August 26, 2006

             Media Type Registration of RTP Payload Formats

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Copyright Notice

Copyright (C) The Internet Society (2006).


     This document specifies the procedure to register RTP payload
     formats as audio, video or other media subtype names.  This is
     useful in a text-based format description or control protocol to
     identify the type of an RTP transmission.

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Table of Contents

   1. Introduction ..................................................  3
      1.2. Terminology ..............................................  3
   2. Procedure For Registering Media Types for RTP Payload Types ...  3
      2.1. Example Media Type Registration ........................... 5
      2.2. Restrictions on Sharing a Subtype Name ...................  6
   3. Mapping to SDP Parameters .....................................  7
   4. Changes from RFC 3555 .........................................  8
   5. Security Considerations .......................................  8
   6. IANA Considerations ...........................................  9
   7. References ....................................................  9
   8. Author's Address ..............................................  9
   9. Intellectual Property Statement ............................... 10
  10. Disclaimer of Validity ........................................ 10
  11. Copyright Statement ........................................... 10

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1.  Introduction

RFC 4288 [1] defines media type specification and registration
procedures that use the Internet Assigned Numbers Authority (IANA) as a
central registry.  That document covers general requirements independent
of particular application environments and transport modes.  This
document defines the specific requirements for registration of media
types for use with the Real-time Transport Protocol (RTP), RFC 3550 [2],
to identify RTP payload formats.

1.1.  Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in RFC 2119 [3] and indicate
requirement levels for implementations compliant with this

2.  Procedure For Registering Media Types for RTP Payload Types

Registering an RTP payload type as a media type follows the same
procedures as described in RFC 4288 [1] and uses the registration
template shown in Section 10 of that RFC.  To specify how the particular
payload format is transported over RTP, some additional information is
required in the following sections of that template:

     Required parameters
          If the payload format does not have a fixed RTP timestamp
          clock rate, then a "rate" parameter is required to specify the
          RTP timestamp clock rate.  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.  The default channel order is as specified
          in RFC 3551 [4].  Any payload format, but most likely audio
          formats, may also include the optional parameters "ptime", to
          specify the recommended length of time in milliseconds
          represented by the media in a packet, and/or "maxptime" to
          specify the maximum amount of media that can be encapsulated
          in each packet, expressed as time in milliseconds.  The
          "ptime" and "maxptime" parameters are defined in the Session
          Description Protocol (SDP) [5].

          A particular payload format may have additional optional

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     Encoding considerations
          Most RTP payload formats include binary or framed data as
          described in Section 4.8 of [1].  The appropriate encoding
          considerations MUST be noted.

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

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

     Restrictions on usage:
          The fact that the media type is defined for transfer via RTP
          MUST be noted, in particular if the transfer depends on RTP
          framing and hence the media type is only defined for transfer
          via RTP.

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
different cases can occur:

     a) Not yet registered as a media type

        A new registration should be constructed using the media type
        registration template.  The registration may specify transfer
        via other means in addition to RTP if that is feasible and
        desired.  The appropriate encoding considerations must be
        specified, and the restrictions on usage must specify whether
        the type is only defined for transfer via RTP or via other modes
        as well.

        Optional parameters may be defined as needed, and it must be
        clearly stated to which mode(s) of transfer the parameters

     b) Media type exists for a non-RTP protocol

        The restrictions on usage of the existing type should be
        changed, if present, or added, if not, to indicate that the type
        can also be transferred via RTP.

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        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 media type for RTP to be used for a non-RTP

        The restrictions on usage 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.

2.1.  Example Media Type Registration

The following sample registration of a fake media type audio/foo
provides examples for some of the required text.  References to RFC nnnn
would be replaced by references to the RFC that contains the payload
format specification and the media type registration.

     Type name: audio

     Subtype name: foo

     Required parameters:
          rate: RTP timestamp clock rate, which is equal to the sampling
          rate.  The typical rate is 8000; other rates may be specified.

     Optional parameters:
          channels: number of interleaved audio streams, either 1 for
          mono or 2 for stereo, and defaults to 1 if omitted.
          Interleaving takes place between on a frame-by-frame basis,
          with the left channel followed by the right channel.

          ptime: recommended length of time in milliseconds represented
          by the media in a packet (see RFC YYYY).

          maxptime: maximum amount of media that can be encapsulated in
          each packet, expressed as time in milliseconds (see RFC YYYY).

     Encoding considerations:
          This media type is framed binary data (see RFC 4288, Section 

     Security considerations: See Section n of RFC nnnn

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     Interoperability considerations:
          Some receivers may only be capable of receiving single-channel

     Published specification: RFC nnnn

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

     Additional information: none

     Person & email address to contact for further information:
          Fred Audio <>

     Intended usage: COMMON

     Restrictions on usage:
          This media type depends on RTP framing, and hence is only
          defined for transfer via RTP (RFC 3550).  Transfer within
          other framing protocols is not defined at this time.

          Fred Audio

     Change controller:
          IETF Audio/Video Transport working group delegated from the

2.2.  Restrictions on Sharing a Subtype Name

The same media subtype name MUST NOT be shared for RTP and non-RTP
(file-based) transfer methods unless the data format is the same for
both methods.  The data format is considered to be same if the file
format is equivalent to a concatenated sequence of payloads from RTP
packets not including the RTP header or any RTP payload-format header.

The file format MAY include a magic number or other header at the start
of the file that is not included when the data is transferred via RTP.

A second requirement for sharing a media subtype name is that the sets
of required parameters must be the same for both methods.

For cases where the data format or required parameters cannot be the
same for RTP and non-RTP transfer methods, then the data formats MUST be
registered as separate types.  It is RECOMMENDED that the subtype names
be related, such as by using a common root plus a suffix.  For those
cases where a suffix is applied in the subtype name for the RTP transfer
method, the suffix "+rtp" is suggested.

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3.  Mapping to SDP Parameters

The representation of a media type is specified in the syntax of the
Content-Type header field in RFC 2045 [6] 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", "ptime", and "maxptime" have general
definitions (given above) that may apply across types and subtypes.  The
format for a parameter is specified in RFC 2045 as

     attribute "=" value

where attribute is the parameter name and the permissible values are
specified for each parameter.  RFC 2045 specifies that a value MUST be
present and that the value MUST be a quoted string if it contains any of
the special characters listed in that RFC.

The information carried in the media type string has a specific mapping
to fields in the Session Description Protocol (SDP) [5], which is
commonly used to describe RTP sessions.  The mapping is as follows:

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

    o  The media 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,

    o  The general (and optional) parameters "ptime" and "maxptime" go
       in the SDP "a=ptime" and "a=maxptime" attributes, respectively.

    o  Any payload-format-specific parameters go in the SDP "a=fmtp"
       attribute.  The set of allowed parameters is defined by the RFC
       that specifies the payload format and MUST NOT be extended by the
       media type registration without a corresponding revision of the
       payload format specification.  The format and syntax of these
       parameters may also be defined by the payload format
       specification, but it is suggested that the parameters be copied
       directly from the media type string as a semicolon separated list
       of parameter=value pairs.  For payload formats that specify some
       other syntax for the fmtp parameters, the registration of that
       payload format as a media type must specify what the parameters
       are in MIME format and how to map them to the "a=fmtp" attribute.

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An example mapping is as follows:

        audio/L16; rate=48000; channels=2; ptime=5; emphasis=50-15

        m=audio 49170 RTP/AVP 97
        a=rtpmap:97 L16/48000/2
        a=fmtp:97 emphasis=50-15

Note that the payload format (encoding) names defined in the RTP Profile
[4] are commonly shown in upper case.  Media subtype names are commonly
shown in lower case.  These names are case-insensitive in both places.
Similarly, parameter names are case-insensitive both in media type
strings and in the default mapping to the SDP a=fmtp attribute.

4.  Changes from RFC 3555

This document updates RFC 3555 to conform to the revised media type
registration procedures in RFC 4288 [1].  Whereas RFC 3555 required the
encoding considerations to specify transfer via RTP, that is now
specified under restrictions on usage.  This document also adds a new
Section 2.2 to clarify the requirements for sharing a media type among
RTP and non-RTP transfer methods.

RFC 3555 included media type registrations for the RTP payload formats
defined in the RTP Profile for Audio and Video Conferences, RFC 3551
[4].  Those media type registrations have been removed from this
document.  Some of them have been assembled into a separate companion
RFC XXXX [7], leaving out those that have been, or are intended to be,
registered in revisions of their own payload format specification RFCs.

Philipp Hoschka is a co-author of RFC 3555; his contributions to the
foundation of this document are appreciated.

5.  Security Considerations

The media type registration procedure specified in this memo does not
impose any security considerations on its own.  Registrations conforming
to this procedure also do not themselves impose security risks, but each
is required to state any security considerations specific to use of the
media type being registered.

Several audio and video encodings are perfect for hiding data using

The RTP specification, RFC 3550, provides security considerations for
the transport of audio and video data over RTP, including the use of
encryption where confidentiality is required.

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6.  IANA Considerations

The purpose of this document is to specify the requirements and
procedures for registering RTP payload formats in the IANA media type
registry.  No registrations are defined here, so no IANA actions are
required for this document.

7.  References

7.1.  Normative References

[1] Freed, N. and J. Klensin, "Media Type Specifications and
    Registration Procedures", BCP 13, RFC 4288, December, 2005.

[2] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP:
    A Transport Protocol for Real-Time Applications", RFC 3550, July

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

[4] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
    Conferences with Minimal Control", RFC 3551, July 2003.

[5] Handley, M., V. Jacobson and C. Perkins, "SDP: Session Description
    Protocol", draft-ietf-mmusic-sdp-new-26.txt, January 2006.
    (Approved for publication as Proposed Standard to obsolete
    RFC 2327, so this reference should be to the RFC when published
    and that number inserted where RFC YYYY appears in this document)

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

7.2.  Informative References

[7] Casner, S., "Media Type Registration of Payload Formats in the
    RTP Profile for Audio and Video Conferences",
    draft-ietf-avt-rfc3555bis-part2-00.txt, February, 2006.
    (Companion to this document, referenced herein as RFC XXXX).

8.  Author's Address

   Stephen L. Casner
   Packet Design
   3400 Hillview Avenue, Building 3
   Palo Alto, CA 94304
   United States

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   Phone: +1 650 739-1843

9.  Intellectual Property Statement

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or
might not be available; nor does it represent that it has made any
independent effort to identify any such rights.  Information on the
procedures with respect to rights in RFC documents can be found in BCP
78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an attempt
made to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can be
obtained from the IETF on-line IPR repository at

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary rights
that may cover technology that may be required to implement this
standard.  Please address the information to the IETF at ietf-

10.  Disclaimer of Validity

This document and the information contained herein are provided on an

11.  Copyright Statement

Copyright (C) The Internet Society (2006).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors retain
all their rights.

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