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Versions: 00 01 02 03 04                                                
Internet Engineering Task Force        Audio-Video Transport WG & Others
INTERNET-DRAFT                                          D. Singer, Y Lim
draft-singer-mpeg4-ip-01                         Apple Computer, mp4cast
                                                         October 23 2000
                                                 Expires: April 23, 2001
                                             MPEG Document number: N3718

     A Framework for the delivery of MPEG-4 over IP-based Protocols

Status of This Memo

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

   This document is an Internet-Draft.  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."



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Distribution of this document is unlimited.


Abstract

   This document forms an umbrella specification for the carriage and
   operation of MPEG-4 multimedia sessions over IP-based protocols,
   including RTP, RTSP, and HTTP, among others. It addresses IP
   Multicast as well.

   It also serves to document the standard MIME types associated with
   MPEG-4 files.


1 Introduction

   MPEG-4 is a standard designed for the representation and delivery of



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   multimedia information over a variety of transport protocols.  It
   includes interactive scene management, visual and audio
   representations as well as systems functionality like multiplexing,
   synchronization, and an object descriptor framework.

   This document provides a number of specifications for the detailed
   mapping of MPEG-4 into several IP-based protocols, as well as
   references to other specifications.

   Open issues: it might be desirable to signal to the terminal the
   amount of buffering assumed by the encoding/transmission process (in
   addition to any network jitter).

   Editor's note:  the sections that apply to FlexMux have not yet been
   harmonized with the proposed FlexMux format.  Some of the information
   related to FlexMux (e.g. MIME names, FlexMux structures) should
   probably be in that draft and removed from here.

Glossary of terms and acronyms

   AAC - MPEG-4 advanced audio codec
   AU - access unit in an ES (the smallest media data unit to which
   timing can be attributed).
   BIFS - binary format for scenes;  the MPEG-4 scene composition system
   CELP - MPEG-4 speech codec
   CTS - composition time stamp
   DTS - decoding time stamp
   ES - elementary stream
   ESID - elementary stream ID
   FCR - flexmux clock reference
   FlexMux - a multiplex of several PDUs into a single unit;  not used
   for multiplexing in RTP
   IOD - initial object descriptor;  the 'hook' to the MPEG-4 streams
   needed to start a session
   OCR - object clock reference;  an external clock reference for an
   MEG-4 stream
   OD - object descriptor;  declares and defines an MPEG-4 stream
   SL - synchronization layer
   SL Packet - synchronization layer protocol data unit, in MPEG-4
   systems

2 Use of RTP

   There are a number of Internet Drafts describing RTP packetization
   schemes for MPEG-4 data [5] [6] [7] [8] [9].  This draft does not
   specify any new one.  Media-aware packetization (e.g. video frames
   split at recoverable sub-frame boundaries) is a principle in RTP, and
   thus it is likely that several RTP schemes will be needed, to suit



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   both the different kinds of media - audio, video, etc. - and
   different encodings (e.g. AAC and CELP audio codecs) [11].

   This specification requires that, no matter what packetization scheme
   is used, there are a number of common characteristics that all MUST
   have: however, such characteristics depend on the fact that the RTP
   Session contains a single elementary stream or a flexmux stream.

   In case an RTP Session contains a single elementary stream the
   following characteristics apply:

   2.1]  The RTP timestamp corresponds to the presentation time (e.g.
   CTS) of the earliest AU within the packet.

   2.2]  RTP packets have sequence numbers in transmission order. The
   payloads logically or physically have SL Sequence numbers, which are
   in decoding order, for each elementary stream.

   2.3]  The MPEG-4 timescale (clock ticks per second), which is
   timeStampResolution in the case of MPEG-4 Systems, MUST be used as
   the RTP timescale, e.g. as declared in SDP for an RTP stream.

   2.4]  To achieve a base level of interoperability, and to ensure that
   any MPEG-4 stream may be carried, all senders and receivers MUST
   implement a default RTP payload mapping scheme. It is highly
   desirable that this default scheme is common for both pure Audio and
   Visual streams as well as for SL Packetized streams. This default
   scheme is not yet identified.

   2.5]  Streams SHOULD be synchronized using RTP techniques (notable
   RTCP sender reports).  When the MPEG-4 OCR is used, it is logically
   mapped to the NTP time axis used in RTCP.

   2.6]  The RTP packetization schemes may be used for MPEG-4 elementary
   streams 'standing alone' (e.g. without MPEG-4 systems, including
   BIFS);  or they may be used within an overall presentation using the
   object descriptor framework.  In the latter case, an
   SLConfigDescriptor is sent describing the stream.  Logically, each
   RTP stream is passed through a mapping function which is specific to
   the payload format used;  this mapping function yields an SL
   packetized stream.  The SLConfigDescriptor describes this logical
   stream, not the actual bits in the RTP payload.  For example, the RTP
   sequence number may be used to make the SLPacketHeader sequence
   number;  other SL fields may be set in this way, dynamically, or from
   static values in the payload specification. For example, as all RTP
   packets carry a composition time-stamp, the flag in the SL header
   indicating its presence can normally be statically defined as 'true'.
   Each payload format for MPEG-4 content MUST specify the mapping



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   function for the formation of the SLConfigDescriptor and the
   SLPacketHeader.

   In the case of the draft by Kikuchi-san et al., the mapping will be
   defined in a new section.

   In case an RTP Session contains a flexmultiplexed stream the
   following characteristics apply:

   2.6]  There is a single payload format for the carriage of Flexmux
   Streams over RTP [5].  Senders and receivers MAY implement this
   scheme.

   2.7]  The RTP timestamp corresponds to the FCR if present at the
   Flexmux level.

   2.8]  The MPEG-4 Flexmux timescale (FCR resolution in  ticks per
   second) SHOULD be used as the RTP timescale (as can be declared in
   SDP).

   2.9] the MPEG-4 FCR is logically mapped to the NTP time axis used in
   RTCP.

   Other payload formats MAY be used.  They are signalled as dynamic
   payload IDs, defined by a suitable name (e.g. a payload name in an
   SDP RTPMAP attribute).  In particular, the development of specialized
   RTP payloads for video (e.g. respecting video packets) and audio
   (e.g. providing interleave) is expected.  It is possible that these
   schemes can be compatible with the default scheme required here.

   There may be a choice of RTP payload formats for a given stream (e.g.
   as an elementary stream, an SL-packetized stream, using FlexMux, and
   so on).  It is recommended that
      * terminals implementing a given sub-system (e.g. video) accept at
         least an ES and the default SL packings [8] of that stream, if
         they exist;  for example, this means accepting the draft by
         Kikuchi et al. and also the SL draft by Civanlar et al. for
         MPEG-4 video;
      * terminals implementing a given payload format accept any stream
         over that format for which they have a decoder, even if that
         packing is not normally the 'best' packing.

   Future versions of this specification will identify the single
   standard RTP packing format for each MPEG-4 stream type.  However, at
   the time of writing the RTP payload format specifications are still
   being defined, and the set is incomplete.  These recommendations will
   form the basis for improved interoperability.




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   For those streams requiring a certain Quality of Service (specifiable
   appropriately) , the recommendation is to further investigate
   possible solutions such as the leverage of existing work in the IETF
   in this area (including, but not limited to FEC, re-transmission, or
   repetition). However, techniques in data-dependent error correction,
   or combined source/channel coding solutions make other schemes
   attractive [7]. Also, it is recommended that requirement such as
   efficient grouping mechanisms (i.e. the ability to send in a single
   RTP packet multiple consecutive Aus, each with its own SL
   information) and low overhead are also taken into account.



3 SDP Information

   This specification considers only MPEG-4 Systems related issues. The
   usage of elementary streams in other contexts is not addressed here:
   codepoints for this case are specified in [6], and in other places.

   This specification currently assumes that any session described by
   SDP (e.g. in SAP, as a file download, as a DESCRIBE over RTSP) has at
   most one MPEG-4 session.  It is desirable that this restriction be
   lifted.

   3.1] Senders SHOULD alert receivers that an MPEG-4 session is
   included, by means of an SDP attribute that is general (i.e. before
   any "media" lines).  This takes the form of an attribute line:

   a=mpeg4-iod [<location>]

   location:  In an RTSP session, this is an optional attribute. If not
   supplied, the IOD is retrieved over the RTSP session by using
   DESCRIBE with an accept of type application/mpeg4-iod. Where the SDP
   information is supplied by some other means (e.g. as a file, in SAP),
   the location is obligatory. The location should be a URL enclosed in
   double-quotes, which will supply the IOD (e.g. small ones may be
   encoded using "data:", otherwise "http:" or other suitable file-
   access URL). The InitialObjectDescriptor is defined in sub-clause
   8.6.3.1 of ISO/IEC 14496-1.


   3.3] New encoding names for the a = rtpmap attribute It is
   recommended that, no matter what payload format is used, each media
   stream be placed in a media section that is appropriate.  For
   example, a payload format which can carry both video and audio
   streams may be used in sections of SDP starting both with "m=video"
   and "m=audio".  The MIME name for the payload format is thus
   registered under all applicable branches.



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   a = rtpmap:<payload> <name>/<time scale>/<parameters>

   payload is the dynamic payload number
   The <name> is defined and documented in the IETF specification for
   the payload format;  for example, mpeg4-SL might indicate the
   encoding type of the media, one MPEG-4 SL packetized stream, or
   mpeg4-flexmux might indicate the encoding type of the media, one
   MPEG-4 FlexMux stream.

   time scale is the time scale of the RTP time stamps
   parameters if used, is defined in the RTP payload format

   3.3] The mapping of RTP streams to elementary streams needs to cover
   the Flexmux case as well as the single stream.  Within the SDP
   information, a stream-specific attribute SHOULD be present for each
   MPEG-4 stream.  It takes one of two forms, depending on whether a
   single elementary stream, or a flexmux, is carried.

   3.4] In case of a single elementary stream, the following attribute
   is defined:

   a=mpeg4-esid a

   a is the ESID.

   3.5] In case of a flexmux stream, the following attribute is defined:

   a=mpeg4-esids m1:a, m2:b ...

   where m1, m2 are flexmux channels and a, b are ESids

   3.5] In case of a flexmux stream, the following attribute is defined:

   a = mpeg4-flexmuxinfo: <location>
   a = mpeg4-muxcodetable: <location>

   The first form is used to define both the ES mapping and the
   muxcodetable, the second the muxcodetable only.  The mapping of ESs
   to streams and the formatting of the muxcodetable needs to be
   harmonized with the draft on FlexMux.

   <location> is a URL enclosed in double quotes, that will supply the
   required flexmux list of descriptors.  If they are small, a DATA: URL
   will probably suffice to carry them in-line. If not, the URL should
   use a file-retrieval scheme (e.g. HTTP, FTP). The data at the
   indicated URL consists of some number of concatenated descriptors,
   complete, in binary format (but note that DATA URLs allow for base64
   encoding of binary data, which would be needed here). These



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   descriptors have an intrinsic length, so simple concatenation
   suffices. The MPEG-4 descriptors related to FlexMux description can
   be MPEG-4 FlexMuxChannel, MPEG-4 MuxCode, MPEG-4 MultiplexBuffer.
   The MPEG-4 Muxcodetable is defined in MPEG-4 systems,

   The list of MPEG-4 descriptors cannot be empty. Private descriptors
   can complete it.  The MIME name used for this data is defined below.

   3.6] Other SDP attributes should, if used, carry values consistent
   with those carried in MPEG-4 systems (for example, bit rate).



4 MIME Types

   4.1] The historical approach for MPEG data is to declare it under
   "video", and this approach is followed for MPEG-4.  For presentations
   with audio information and no visual aspect, the "audio" top-level
   mime type may be used;  otherwise, "video" is used.

   4.2] Amendment 1 of the MPEG-4 standard (also known as version 2)
   includes a standard file type for encapsulating MPEG-4 data.  This
   file type can be used in a number of ways: perhaps the most important
   are its use as an interchange format for MPEG-4 data, its use as a
   content-download format, and as the format read by streaming media
   servers.

   These first two uses will be greatly facilitated if there is a
   standard MIME type for serving these files (e.g. over HTTP).

   The MPEG-4 standard is broad, and therefore the type of data that may
   be in such a file can vary. In brief, simple compressed video and
   audio (using a number of different compression algorithms) can be
   included; interactive scene information; meta-data about the
   presentation; references to MPEG-4 media streams outside the file and
   so on.

   The MIME types to be assigned to MP4 files SHOULD be "audio/mp4", and
   "video/mp4" , based on the criteria in 4.1. In either case, these
   indicate files conforming to the "MP4" specification (ISO/IEC 14496-
   1:2000, systems file format).

   4.3] When an MP4 file is served (e.g. over HTTP) or otherwise must be
   identified by a MIME type, the type "video/mp4" SHOULD be used.  The
   types "audio/mp4" MAY be used when the MPEG-4 presentation contained
   within the MP4 file has no visual presentation and refers to a pure
   audio presentation.




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   4.4] When a visual MPEG-4 ES is served (e.g. over HTTP or otherwise)
   and must be identified by a MIME type, the type "video/MPEG4-visual"
   SHALL be used. This MIME type may require optional parameters to
   carry all necessary information to configure a receiver: therefore no
   further meta-information (such as that defined by the MP4 file format
   or by the MPEG-4 Object Descriptor framework) has to be provided in
   the data, and the data itself merely represents the media content..
   The format of the bit-stream, including timing etc., is defined in
   ISO/IEC 14496-2.

   4.5]  In some cases, the initial object descriptor needs to be
   identified with a MIME type. In this case, the type
   "application/mpeg4-iod" SHALL be used.

   4.6] When a flexmux stream is served (e.g. over HTTP) or otherwise
   must be identified by a MIME type, the type "application/mpeg4-
   flexmux" SHALL be used.  These files consist of concatenated flexmux
   PDUs in transmission order.

   4.7] In some cases, the information needed by a flexmux decoder needs
   to be identified with a MIME type. In this case, the type
   "application/mpeg4-flexmuxinfo" SHOULD be used.

   4.8] The payload names used in an RTPMAP attribute within SDP, to
   specify the mapping of payload number to its definition, also come
   from the MIME namespace.  Each of the RTP payload mappings defined
   above has a distinct name.  It is recommended that visual streams be
   identified under "video", and audio streams be identified under
   "audio", and otherwise "application" be used.



   Given the broad and general nature of MPEG-4, and the interactive
   environment, it is hard to say that there are no security
   considerations.  However, none are known to the author at this time,
   and the standard was developed with the intent that there be none.

MIME media type name:              video, and audio
MIME subtype name:                 mp4

MIME media type name:              application
MIME subtype name:                 mpeg4-iod, mpeg4-flexmux, mpeg4-
                                   flexmuxinfo
Required parameters:               none
Optional parameters:               none
Encoding considerations:           base64 generally preferred; files are
                                   binary and should be transmitted
                                   without CR/LF conversion, 7-bit



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                                   stripping etc.
Security considerations:           None known at the time of writing
Interoperability considerations:   A number of interoperating
                                   implementations exist within the
                                   MPEG-4 community;  and that community
                                   has reference software for reading
                                   and writing the file format.
Published specification:           Pending (ISO/IEC 14496-1:2000, MPEG-4
                                   Systems).
Applications:                      Multimedia
Additional information:

Magic number(s):                   none
File extension(s):                 mp4 and mpg4 are both declared at
                                   <http://pitch.nist.gov/nics/>
Macintosh File Type Code(s):       mpg4  is registered with Apple

Person to contact for info:        David Singer, singer@apple.com

Intended usage:                    Common

Author/Change controller:          David Singer, MPEG-4 file format
                                   chair

5  RTSP usage

   This specification considers only MPEG-4 Systems related issues. The
   usage of elementary audio or visual streams in other context does not
   require any specific statement about RTSP.

   RTSP may be used as a session control protocol for sessions which
   carry MPEG-4 information.  When RTSP is used as a session-control
   protocol:

   5.1]  RTP SHOULD be used as the transport protocol.

   5.2] The initial DESCRIBE format SHOULD be SDP.  If the SDP
   information reveals that an IOD is needed, and the terminal does not
   already have it, then a second DESCRIBE accepting an IOD SHOULD be
   performed (see above).

   5.3] Note that if all MPEG-4 streams are closed (TEARDOWN) then the
   RTSP session ID will be lost.  The next (re-)opened stream will
   supply a new session ID.  Care should be taken that the target of the
   URL has not changed in the interval;  new DESCRIBEs may be needed.

6 Multicast




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   This specification considers only MPEG-4 Systems related issues.

   When using IP Multicast, the SDP information describing the MPEG-4
   Session SHOULD be made available to the terminal.

   In addition, elementary stream descriptors may use URLs to directly
   address ESs.  The goal of such URL would be to convey information to
   enable the terminal to directly connect to the RTP channel carrying
   the ES. No matter what URL scheme is used ( "rtp:" ....) information
   shall be conveyed for the information which would otherwise be needed
   from SDP, including but not limited to
      * IP Multicast address
      * Port number
      * Any such attributes above as may be needed.

   For these reasons, it is recommended that any multicast session be
   described by SDP.  The default protocol stack SHALL be used, or more
   parameters are required to identify the protocol stack.

Acknowledgments

   This draft has benefited greatly by contributions from many people,
   including Mike Coleman, Jean-Claude Duford, Peter Westerink, Carsten
   Herpel, Olivier Avaro, Paul Christ, Zvi Lifshitz, and many others.
   Their insight, foresight, and contribution is gratefully
   acknowledged.  Little has been invented here by the author;  this is
   mostly a collation of greatness that has gone before.
























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References

   [1] H. Schulzrinne, et. al., "RTP : A Transport Protocol for Real-
   Time Applications", IETF RFC 1889, January 1996.

   [2] H. Schulzrinne, et. al., "RTP Profile for Audio and Video
   Conference with Minimal Control", IETF RFC 1890, January 1996.

   [3] H. Schulzrinne, et. al., "Real Time Streaming Protocol", IETF
   Draft, draft-ietf-mmusic-rtsp-09.txt, February 2 1998, Expires:
   August 2 1998.

   [4] M. Handley, "SDP: Session Description Protocol", IETF Draft,
   draft-ietf-mmusic-sdp-05.txt, November 21 1997, Expires: November 21
   1998.

   [5] C.Roux  et al., "RTP Payload Format for Flexmultiplexed MPEG-4
   Streams", IETF Draft, draft-rgcc-avt-mpeg4flexmux-00, March, 09 2000
   expires Sept 9 2000

   [6] Yoshihiro Kikuchi et al., "RTP payload format for MPEG-4
   Audio/Visual streams", IETF Draft, draft-ietf-avt-rtp-mpeg4-es-
   05.txt, October 11, 2000

   [7] C.Guillemot et al., "RTP Payload Format for MPEG-4 with Flexible
   Error Resiliency", IETF Draft, draft-ietf-avt-mpeg4streams-00, March
   1 2000, expires Sept 1 2000

   [8] R Civanlar et al., " RTP Payload Format for MPEG-4 Streams", IETF
   Draft, draft-ietf-avt-rtp-mpeg4-03.txt, July 13, 2000, expires Jan
   13, 2001

   [9] C.Guillemot et al., "RTP payload format for MPEG-4 Visual
   Advanced Profiles", IETF Draft, draft-gc-avt-mpeg4visual-00.txt,
   March 1 2000, expires Sept 1 2000

   [10] R. Finlayson, "A More Loss-Tolerant RTP Payload Format for MP3
   Audio", IETF Draft, draft-ietf-avt-rtp-mp3-03.txt, Aug 3 2000,
   expires Feb 3 2001

   [11] Kretschmer et al., "RTP Payload Format for MPEG-2 AAC Streams",
   IETF Draft, draft-ietf-avt-rtp-mpeg2aac-00.txt, June 25, 1999,
   expired December 25, 1999


Authors' Contact Information
   David Singer
   Email: singer@apple.com



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   Tel: +1 408 974 3162

   Apple Computer, Inc.
   One Infinite Loop, MS:302-3MT
   Cupertino  CA 95014
   USA


   Young-Kwon LIM
   E-mail : young@techway.co.kr
   TEL : +82-42-863-7800

   mp4cast
   (MPEG-4 Internet Broadcasting Solution Consortium)
   1001-1 Daechi-Dong Gangnam-Gu
   Seoul, 305-333, Korea



































D. Singer & Y Lim                                              [Page 12]