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RFC 4855 - Media Type Registration of RTP Payload Formats

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Network Working Group                                          S. Casner
Request for Comments: 4855                                 Packet Design
Obsoletes: 3555                                            February 2007
Category: Standards Track

             Media Type Registration of RTP Payload Formats

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 IETF Trust (2007).


   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.

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................2
   2. Procedure For Registering Media Types for RTP Payload Types .....2
      2.1. Example Media Type Registration ............................4
      2.2. Restrictions on Sharing a Subtype Name .....................5
   3. Mapping to SDP Parameters .......................................6
   4. Changes from RFC 3555 ...........................................7
   5. Security Considerations .........................................8
   6. IANA Considerations .............................................9
   7. References .....................................................10
      7.1. Normative References ......................................10
      7.2. Informative References ....................................10

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
          parameters.  As allowed in Section 4.3 of [1], new parameters
          MAY be added to RTP media types that have been previously

          defined, but the new parameters MUST NOT change existing
          functionality and it MUST be possible for existing
          implementations to ignore the additional parameters without
          impairing operation.

     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 or not the type has already been registered for
   transfer with a non-RTP protocol (e.g., MIME mail or http), 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.

         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/example
   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: example

     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 4566).

          maxptime: maximum amount of media that can be encapsulated in
          each packet, expressed as time in milliseconds (see RFC 4566).

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

     Security considerations: See Section n of RFC nnnn

     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 <fred@example.com>

     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 the 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

   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, 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.

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 that 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

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

      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, respectively.

      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.

   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

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 specifies
   the conditions under which new optional parameters may be added to a
   previously defined RTP media type and 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 4856 [8], 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.  Also, registrations
   conforming to this procedure do not themselves impose security risks.
   However, use of the media type being registered could very well
   impose security risks:

      o  Any media type that contains "active content" imposes the risk
         of malicious side-effects unless execution of that content is
         adequately constrained.

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

      o  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

   Therefore, each media type registration is required to state any
   security considerations that apply to the use of that type.  The
   remainder of this section is copied from RFC 4288 [1], which
   specifies media type registration procedures in general.

   An analysis of security issues MUST be done for all types registered
   in the standards tree.  A similar analysis for media types registered
   in the vendor or personal trees is encouraged but not required.
   However, regardless of what security analysis has or has not been
   done, all descriptions of security issues MUST be as accurate as
   possible regardless of registration tree.  In particular, a statement
   that there are "no security issues associated with this type" MUST
   NOT be confused with "the security issues associated with this type
   have not been assessed".

   There is absolutely no requirement that media types registered in any
   tree be secure or completely free from risks.  Nevertheless, all
   known security risks MUST be identified in the registration of a
   media type, again regardless of registration tree.

   The security considerations section of all registrations is subject
   to continuing evaluation and modification, and in particular MAY be
   extended by use of the "comments on media types" mechanism described
   in RFC 4288, Section 6.

   Some of the issues that should be looked at in a security analysis of
   a media type are:

      o  Complex media types may include provisions for directives that
         institute actions on a recipient's files or other resources.
         In many cases, provision is made for originators to specify
         arbitrary actions in an unrestricted fashion that may then have
         devastating effects.  See the registration of the
         application/postscript media type in RFC 2046 [7] for an
         example of such directives and how they should be described in
         a media type registration.

      o  All registrations MUST state whether or not they employ such
         "active content", and if they do, they MUST state what steps
         have been taken to protect users of the media type from harm.

      o  Complex media types may include provisions for directives that
         institute actions that, while not directly harmful to the
         recipient, may result in disclosure of information that either
         facilitates a subsequent attack or else violates a recipient's
         privacy in some way.  Again, the registration of the
         application/postscript media type illustrates how such
         directives can be handled.

      o  A media type that employs compression may provide an
         opportunity for sending a small amount of data that, when
         received and evaluated, expands enormously to consume all of
         the recipient's resources.  All media types SHOULD state
         whether or not they employ compression, and if they do they
         should discuss what steps need to be taken to avoid such

      o  A media type might be targeted for applications that require
         some sort of security assurance but not provide the necessary
         security mechanisms themselves.  For example, a media type
         could be defined for storage of confidential medical
         information that in turn requires an external confidentiality
         service or is designed for use only within a secure

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.

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., Jacobson, V., and C. Perkins, "SDP: Session
       Description Protocol", RFC 4566, July 2006.

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

   [7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
       Extensions (MIME) Part Two: Media Types", RFC 2046, November

7.2.  Informative References

   [8] Casner, S., "Media Type Registration of Payload Formats in the
       RTP Profile for Audio and Video Conferences", RFC 4856, February

Author's Address

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

   Phone: +1 650 739-1843
   EMail: casner@acm.org

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