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rfc:rfc4855

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).

Abstract

 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

Casner Standards Track [Page 1] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [3] and
 indicate requirement levels for implementations compliant with this
 specification.

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

Casner Standards Track [Page 2] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

        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
        rates).
        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
       apply.

Casner Standards Track [Page 3] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

    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
       protocol
       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).

Casner Standards Track [Page 4] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

   Encoding considerations:
        This media type is framed binary data (see RFC 4288, Section
        4.8).
   Security considerations: See Section n of RFC nnnn
   Interoperability considerations:
        Some receivers may only be capable of receiving single-channel
        audio.
   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.
   Author:
        Fred Audio
   Change controller:
        IETF Audio/Video Transport working group delegated from the
        IESG.

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
 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.

Casner Standards Track [Page 5] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

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

Casner Standards Track [Page 6] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

    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
       a=ptime:5
 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 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.

Casner Standards Track [Page 7] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

 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
       required.
 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.

Casner Standards Track [Page 8] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

 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
       attacks.
    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
       environment.

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.

Casner Standards Track [Page 9] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

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
     2003.
 [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
     1996.

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
     2007.

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

Casner Standards Track [Page 10] RFC 4855 Media Type Reg. of RTP Payload Formats February 2007

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 contained in BCP 78, and except as set forth therein, the authors
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Acknowledgement

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Casner Standards Track [Page 11]

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