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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-02                         Apple Computer, mp4cast
                                                              Nov 16 2000
                                                     Expires: May 16 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.''

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

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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.



    +----------------+        +---------------+       +---------+
    |   RTP Packet   |        |   Normative   |       |         |
    |                | -----> |    mapping    | ----->|         |
    |(visual, audio) |        |   function    |       |         |
    +----------------+        +---------------+       |         |
                                                      |         |
    +----------------+        +---------------+       |         |
    |   RTP Packet   |        |   Normative   |       | MPEG-4  |
    |                | -----> |    mapping    | ----->|         |
    |(generic format)|        |   function    |       |   SL    |
    +----------------+        +---------------+       |         |
             .                         .              | packets |
             .                         .              |         |
             .                         .              |         |
    +----------------+        +---------------+       |         |
    |   RTP Packet   |        |   Normative   |       |         |
    |                | -----> |    mapping    | ----->|         |
    |(FlexMux format)|        |   function    |       |         |
    +----------------+        +---------------+       +---------+



    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



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    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.

    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.




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    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.

    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




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    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
    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



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    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.

    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.




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    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.




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
                                    stripping etc.
Security considerations:           See below
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



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    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 Security Considerations

    RTP packets using the payload formats referred to in this
    specification are subject to the security considerations discussed in
    the RTP specification [5]. This implies that confidentiality of the
    media streams is achieved by encryption. Because the data compression
    used with this payload format is applied end-to-end, encryption may
    be performed on the compressed data so there is no conflict between
    the two operations. The packet processing complexity of this payload
    type does not exhibit any significant non-uniformity in the receiver
    side to cause a denial-of-service threat.

    However, it is possible to inject non-compliant MPEG streams (Audio,
    Video, and Systems) to overload the receiver/decoder's buffers which
    might compromise the functionality of the receiver or even crash it.
    This is especially true for end-to-end systems like MPEG where the
    buffer models are precisely defined.

    MPEG-4 Systems supports stream types including commands that are
    executed on the terminal like OD commands, BIFS commands, etc. and
    programmatic content like MPEG-J (Java(TM) Byte Code) and ECMASCRIPT.
    It is possible to use one or more of the above in a manner non-
    compliant to MPEG to crash or temporarily make the receiver
    unavailable.

    Authentication mechanisms can be used to validate of the sender and
    the data to prevent security problems due to non-compliant malignant
    MPEG-4 streams.




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    A security model is defined in MPEG-4 Systems streams carrying MPEG-J
    access units which comprises Java(TM) classes and objects. MPEG-J
    defines a set of Java APIs and a secure execution model. MPEG-J
    content can call this set of APIs and Java(TM) methods from a set of
    Java packages supported in the receiver within the defined security
    model. According to this security model, downloaded byte code is
    forbidden to load libraries, define native methods, start programs,
    read or write files, or read system properties.

    Receivers can implement intelligent filters to validate the buffer
    requirements or parametric (OD, BIFS, etc.) or programmatic (MPEG-J,
    ECMAScript) commands in the streams. However, this can increase the
    complexity significantly.



7 Multicast

    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, Viswanathan Swaminathan,
    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 13]