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Versions: 00 01 02 03                                                   
   Internet Engineering Task Force          Audio Visual Transport WG
   Internet-Draft                           J.Van der Meer/
                                            C.Roux,E.Gouleau,
                                            D.Curet,S.Relier/
                                            P.Clement/
                                            G.Cherry
                                            Philips/FT R&D /Thomcast/
                                            nCube
   February, 2001
   Expires: August, 2001

            RTP Payload Format for MPEG-4 FlexMultiplexed Streams
                draft-curet-avt-rtp-mpeg4-flexmux-00.txt


1. Status of this Memo

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

   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
   The list of Internet-Draft Shadow Directories can be accessed
   at http://www.ietf.org/shadow.html.

2. Abstract

   This document describes a payload format for transporting
   MPEG-4 encoded and multiplexed data using RTP. MPEG-4 is a
   recent standard from ISO/IEC for the coding of natural and
   synthetic audio-visual data.
   Several services provided by RTP are beneficial for MPEG-4
   encoded and multiplexed data transport over the Internet.
   Additionally, the use of RTP makes it possible to synchronize
   MPEG-4 data with other real-time data types.
   This specification is a product of the Audio/Video Transport

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   working group within the Internet Engineering Task Force and
   ISO/IEC MPEG-4 ad hoc group on MPEG-4 over Internet. Comments
   are solicited and should be addressed to the working group's
   mailing list at rem-conf@es.net and/or the authors.

RTP PAYLOAD FORMAT FOR MPEG-4 FLEXMULTIPLEXED STREAMS 1
1. STATUS OF THIS MEMO  1
2. ABSTRACT   1
3. INTRODUCTION TO THE MPEG-4 STANDARD 3
4. OVERVIEW OF MPEG-4 END-SYSTEM ARCHITECTURE    4
5. BENEFITS OF USING RTP FOR TRANSPORT:5
6. CONVENTIONS USED IN THIS DOCUMENT   5
6.1 GENERAL:  5
6.2 MPEG-4 GLOSSARY:    5
7. THE RTP PACKET  6
7.1 THE RTP PACKET HEADER    6
7.2 RTP HEADER FIELDS USAGE STREAMS:   6
7.3 THE TWO RTP PACKET PAYLOADS   7
7.4  MPEG-4 FLEXMUX SIGNALING DESCRIPTORS   8
8. FLEX MULTIPLEXING    12
8.1 SOME OF THE ADVANTAGES : 12
8.2 DISADVANTAGES: 13
8.3. TRANSPORT OF MPEG-4 FLEXMUX STREAMS    13
9. SDP SYNTAX 15
9.1 ATTRIBUTES15
9.2 MIME TYPES16

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10. RTSP USAGE:    16
11. SECURITY CONSIDERATIONS  17
12. REFERENCES17
13. AUTHORS' ADDRESSES  18

3. Introduction to the MPEG-4 standard

   The MPEG-4 standard (ISO/IEC 14496) can be represented in a
   layered architecture, where three layers can be identified as
   follows:
                    +---------------------------------------+
      media aware   |           COMPRESSION LAYER:          |
    and             |     Elementary Streams (ES) encoding  |
    delivery unaware|       MPEG-4 part 2 Visual            |
    layer           |              MPEG-4 part 3  Audio     |
                    |       MPEG-4 part 1 Bifs,OD,IPMP,OCI  |
                    +---------------------------------------+
   ================================================ ESI Interface
                    +---------------------------------------+
       media and    |         SYNC LAYER (SL)               |
    delivery unaware|      Elementary streams management    |
         layer      |         and synchronisation           |
                    +---------------------------------------+
    ================================================DAI Interface
                    +---------------------------------------+
    delivery aware  |          DELIVERY LAYER (DMIF)        |
    media  unaware  |provides Flexmultiplexing of SL streams|
         layer      |        and transparent access         |
                    |      to the delivery technology       |
                    +---------------------------------------+
   Although the Delivery Layer mostly focuses on the control plane
   it also encompasses  multiplexing tools and defines a bitstream
   syntax, called the Flexmux, to multiplex MPEG-4 SL streams. The
   reconstruction of the correct timing of MPEG-4 bitstreams is
   supported both by the MPEG-4 SL stream syntax and by the MPEG-4
   FlexMux stream syntax. The reconstruction of the correct timing

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   of an MPEG-4 Flexmux stream is possible under various QoS
   constraints related to the reduction of network jitter. MPEG
   makes the assumption that all Flexmux packets transmitted over
   the network should have a nearly constant transmission delay.
   The reconstruction of the correct timing of the MPEG-4 Flexmux
   streams is based on the fact that this assumption can be
   verified, in accordance with the required accuracy of the
   reconstructed timing of the MPEG-4 FlexMux stream. This
   document will specify a RTP payload format to enable the
   carriage of Flexmux streams.

4. Overview of MPEG-4 End-System Architecture

   The Compression Layer processes the traditional individual
   audio/visual elementary streams (ES) and some associated
   'systems' elementary streams (ES) such as Bifs, OD, IPMP and
   OCI elementary streams.
   The MPEG-4 audio/visual ES syntaxes are defined in[2] and[3].
   The ôsystemsö ES syntaxes are described in [1]: the Bifs ES
   syntax allows a dynamic scene description. The OD ES syntax
   allows the description of the hierarchical relations, location
   and properties of different ESs through a dynamic set of Object
   Descriptors. The ôsystemö ES may require to be carried with a
   better protection than the traditional audio/visual ESs.
   The compressed content produced by this layer are organised
   into Elementary Streams (ESs). The compression layer is unaware
   of a specific delivery technology, but it can react to the
   characteristics of a particular delivery layer such as the
   path-MTU or packet loss or bit error characteristics.
   The MPEG-4 SL stream syntax is defined in [1]. It provides a
   unique and homogeneous encapsulation of any of the MPEG-4 ESs.
   That layer primarily provides the synchronisation between ESs.
   ESs are organised in Access Units (AU). An AU is the smallest
   element to which timestamps can be assigned. Integer or
   fractional AUs are then encapsulated in SL packets.
   The MPEG-4 Delivery Layer consists of the Delivery Multimedia
   Integration Framework defined in [4]. This layer is media
   unaware but delivery technology aware. It provides transparent
   access to and delivery of content irrespective of the
   technologies used. The DAI interface between the SL layer and
   the DMIF layer is called the DMIF Application Interface. This
   interface supports content location independent protocols
   firstly for establishing the MPEG-4 session and secondly for
   accessing to transport channels. DMIF monitors transport
   channels on the QoS requirements assigned to the SL streams,
   and supports the multiplexing of the SL streams, by the means

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   of the MPEG-4 FlexMux tools. There are two possible FlexMux
   tools. FlexMux streams delivery is defined in [4], FlexMux
   tools are defined within [1].
   MPEG makes the assumption that the carriage of MPEG-4 Flexmux
   streams over the network should affect packets with an ôidealö
   constant transmission delay; the reconstruction of the correct
   timing of a MPEG-4 Flexmux streams is based on that assumption.
   This draft specifies an RTP [5] payload format for transporting
   multiplexed MPEG-4 encoded data streams. It can be presented as
   an instance of the MPEG-4 Delivery layer.

5. Benefits of using RTP for transport:

     i. Ability to synchronize MPEG-4 streams with other RTP
        payloads

     ii. Monitoring MPEG-4 delivery performance through RTCP

     iii. Combining MPEG-4 and other real-time data streams
          received from multiple end-systems into a set of
          consolidated streams through RTP mixers

     iv. Converting data types, etc. through the use of RTP
         translators

6. Conventions used in this document

6.1 general:

   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 [6].

6.2 MPEG-4 glossary:

   AU :Access Unit                 Bifs: Binary format for scene
   DMIF: Delivery Multimedia Integration Framework,
   DAI: DMIF Application Interface,  ES: Elementary stream,
   ESI: Elementary stream Interface, FlexMux: Flexible Multiplex.
   IPMP: Intellectual Property Management and Protection,
   OCI: Object Content Information,  OD: Object descriptor,
   SL: Synchronization layer

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7. The RTP packet

7.1 The RTP packet header

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |V=2|P|X|   CC  |M|      PT     |     sequence number           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          timestamp                            |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         synchronization source (SSRC) identifier              |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 :        contributing source (CSRC) identifiers                 |
 |=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                                                               |
 |                                                               |
 |                    RTP Packet Payload                         |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 1 - An RTP packet for MPEG-4 FlexMux stream

7.2 RTP header fields usage streams:

   Payload Type (PT): The assignment of a particular  RTP payload
   type to this new packet format, is outside the scope of this
   document, and is not specified here. If the dynamic payload
   type assignment is used, it can be specified by some out of
   band means (e.g. SDP, according to the syntax proposed in the
   paragraph 9) that the MPEG-4 FlexMux payload format
   is used for the corresponding RTP packets. The specification
   of the usage of MPEG-4 FlexMux payload format can also include,
   if needed, the specification of the usage of the MPEG-4 FlexMux
   signaling format.

   Marker (M) bit:  set to zero.

   Extension (X) bit: Defined by the RTP profile used.

   Sequence Number:Increment by one for each RTP data packet sent.
   It starts with a random initial value for security reasons.

   Timestamp: it represents the target transmission time for the

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   first byte of the RTP packet. Unless specified by an out of
   band means (e.g. SDP), the resolution of the timestamp is set
   to its default (90KHz).

   SSRC, CC and CSRC fields are used as described in RFC 1889 [5].

7.3 The two RTP packet payloads

    The MPEG-4 FlexMux payload format encompasses two payload
   types:
   the first one for MPEG-4 FlexMux packets and the second one for
   MPEG-4 FlexMux signaling.

   When the Payload type specifies the usage of the MPEG-4
   FlexMux payload format, the RTP packet payload is built from
   an integer number of complete, defined in [1].

   When the Payload type specifies the usage of the MPEG-4
   FlexMux signaling payload format, the RTP packet payload is
   built from an integer number of descriptors, defined in the
   following 7.4. paragraph.

   7.3.1 payload for the MPEG-4 FlexMux payload format

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         FlexMux Packet Header |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                     |
 |            FlexMux Packet Payload (byte aligned)              |
 |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    FlexMux Packet Header      |     FlexMux                   |
 |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
 |                        Packet                                 |
 |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             Payload           :...optional RTP padding        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Figure 3 - An RTP Payload carrying MPEG-4 FlexMux packets

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   7.3.2 payload for the MPEG-4 FlexMux signaling payload format

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        MPEG-4 FlexMux signaling descriptor                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
 |               | MPEG-4 FlexMux signaling descriptor           |
 |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | signaling descriptor          |...optional RTP padding        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 4 - An RTP Payload carrying MPEG-4 FlexMux signaling
   descriptors

7.4  MPEG-4 FlexMux signaling descriptors

   7.4.1 signaling descriptor usage:

   They are used to describe in-band a FlexMux stream
   characteristics. Their use is mostly static . Only one of these
   descriptors (the FlexMux codetable entry descriptor) may have a
   dynamic use, when the FlexMux stream characteristic is to be
   dynamic.

   They are built from a signaling descriptor header followed by a
   signaling descriptor content.
   The signaling descriptor header structure is identical for all
   the signaling descriptors.
   Their overall length is an integer number of bytes.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           HEADER              |          CONTENT              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 5 - A signaling descriptor scheme

   7.4.1 the signaling descriptor header

   The signaling descriptor header is built from a tag field on
   one byte, followed by a length field on one byte

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 0                   1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      TAG      |    LENGTH     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 6 - A signaling descriptor header

   TAG gives the type of the descriptor:

   0x60 for FlexMux Channel Table descriptor
   0x61 for FlexMux buffersize descriptor
   0x62 for FlexMux timing descriptor
   0x63 for FlexMux codetable entry descriptor
   0x64 for FlexMux declaration descriptor

   values from 0x00 to 0x5F, & value  0xFF  are forbidden
   values from 0x65 to 0xBF are  ISO reserved
   values from 0xC0 to 0xDF are IETF reserved
   values from 0xE0 to 0xFE are Ad Hoc values

   LENGTH - is the byte length of the descriptor's content that
   follows.

   7.4.2 FlexMux declaration descriptor

   This descriptor is used to allow identification of some
   different FlexMux streams within an MPEG-4 scene. This can
   be the case when scalable coding is used.
   When no FlexMux descriptor is used, the default TYPE is the
   first FlexMux tool, and the default MODE is static.
 0                   1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      MUXID    |   TYPE|  MODE |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 7 - declaration descriptor content

   MUXID - is the identifier of the FlexMux stream
   TYPE - is the type of the Multiplexing tool used to generate
   the FlexMux stream. AS they are two FlexMux tools defined,
   indicated type values shall be either 0 (for the first FlexMux
   tool), 1 (for the second FlexMux tool).

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   Values from 2 to 7 are IETF reserved, values 8 to 15 are Ad Hoc
   values.
   MODE û is the mode of management used by the Multiplexing tool,
   to generate the FlexMux stream. Indicated mode values shall be
   either 0 (static), 1 (dynamic).  Values from 2 to 7 are IETF
   reserved, values 8 to 15 are Ad Hoc values.

   7.4.3 FlexMux timing descriptor

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           FCR-ES-ID           |        FCRRESOLUTION          |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         FCRRESOLUTION (end)   |  FCRLENGTH    | FMXRATELENGTH |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
   Figure 8 - timing descriptor content

   FCR-ES-ID û (16 bits)is the ES_ID associated to this clock
   reference stream.
   FCRRESOLUTION û(32 bits) is the resolution of the object time
   base in cycles per second.
   FCRLENGTH -(8 bits) is the length of the  fmxClockReference
   field in FlexMux packets with index = 238. A length of zero
   shall indicate that no FlexMux packets with index = 238 are
   present in this FlexMux stream. FCRLENGTH shall take values
   between zero and 64.
   FMXRATELENGTH û (8 bits)is the length of the fmxRate field
   in FlexMux packets with index = 238. FMXRATELENGTH shall take
   values between 1 and 32.

   7.4.4 FlexMux Channel Table descriptor

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             ES1               |        M1     | ES2(start)    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |  ES2 (end)    |       M2      |            ......             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |    ........   |                  ESn          |       Mn      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 9 - FLexMuxChannel Table descriptor content

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   <ES1>,..,<ESn> - these 16-bit fields specify the identifiers of
   ISO/IEC 14496-1 SL-packetized streams defined in [1].

   <M1>,..,<M2>,à  This 8-bit fields specify the number of the
   FlexMux channels used for these SL-packetized streams.

   7.4.5 FlexMux codetable entry descriptor

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |MUXCODE|VERSION| SUBSTRUCTCNT  | SLOTCNT |RCNT |      M1       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |     NB1       |     M2        |    NB2        |.............. |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | ..............| SlotCNT |RCNT |     Mi        |    NBi        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |      Mi+1     |      NBi+1    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 10 û FLexMuxcode table descriptor content

   MUXCODE - the number through which this FlexMux codetable entry
   is referenced

   VERSION - the version of the FlexMux codetable entry.
   Only the latest received version of a FlexMux codetable entry
   is valid.

   SUBSTRUCTCNT is the number of substructures of this FlexMux
   codetable entry.

   SLOTCNT -  the number of slots with data from different FlexMux
   channels that are described by this substructure

   RCNT  -  indicates how often this substructure is to be repeated.
   A zero indicates that this substructure is to be repeated
   infinitely. Zero is only permitted in the last substructure
   of a FlexMux codetable entry.

   M1,..,Mi - the FlexMux channels to which the data in this slot
   belongs.

   NB1,..,NBi -  the number of data bytes in this slot associated
   to the FlexMux channel M1, Mi. This number of bytes corresponds
   to one SL packet. SL packets are defined in [1].

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   7.4.5 FlexMux buffersize descriptor

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |        DEFAULTSIZE                            |      M1       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             SIZE1                             |      M2       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             SIZE2                             |      M3       |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |             SIZE3                             |.............. |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | ............................................................. |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Figure 11 - FlexMux buffersize descriptor content

   DEFAULTSIZE - the default size of multiplex buffers for each
   individual channel in a FlexMux stream. FlexMux channels that
   use a different buffer size may signal this using the following
   Mi,SIZEi assignments

   M1,M2,M3 - the FlexMux channels

   SIZE1, SIZE2, SIZE3 - the exact sizes of FlexMux buffers, for
   FlexMux channels M1,M2 and M3 of this FlexMux stream. Sizes are
   expressed in bytes.

8. Flex Multiplexing

8.1 Some of the advantages :

   1. Since a typical MPEG-4 session may involve a large number of
   objects, that may be as many as a few hundred, transporting
   each ES as an individual RTP session may not always be
   practical. The use of one session per elementary stream cannot
   be much cost effective, both on the server side and on the
   client side in terms of performance, when the number of
   elementary treams will increase within a scene.

   2. The use of one single session for a multiplexed bitstream
   enables to send a bunch of ESs that are tightly synchronized
   together. Some of these ESs can themselves be Bifs and OD
   ESs when a scene description is used with Audio-Visual ES,
   and some other ESs can be OCI ES, and even IPMP ES when such
   systems are involved.

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   3. The FlexMultiplexing management supports embedding multiple
   SL packets into one FlexMux packet, by the use of FlexMux
   codetable entries.

   4. If the multiplexing policy used is smoothing at most the
   multiplexed SL streams, mutual synchronization between these
   SL streams can be easily preserved when packet losses occur.

   5. The use of the FlexMux technology enables possible
   interconnection between Internet network and digital television
   network, as MPEG normatively defines the use the MPEG-4 FlexMux
   syntax to carry MPEG-4 over MPEG-2 transport channels[8].

   6. The reconstruction of the correct timing of the FlexMux
   stream is possible, if some QoS requirements are supported.

   7. The overall MPEG-4 receiver buffer size is reduced, as
   MPEG-4 compliant Flexmultiplexed streams, by the use of the
   MPEG-4 timestamps, respect the MPEG-4 system decoder model.

   8. The overall bandwidth management is easier. The FlexMux
   syntax allows having piecewise constant bitrate FlexMux
   bitstream, with an inband signaling mechanism.

   9. Protection can be enhanced by means of repetitions of
   vital SL packets.

   10. Content providers are able to bundle together a single
   stream with assurance that associated streams will be kept
   together and synchronized.

8.2 Disadvantages:

   The major disadvantage with the packetization of the MPEG-4
   Flexmultiplexed streams is the added packet header overhead.
   MPEG-4 does not support a reduction mechanism of the carried
   MPEG-4 Flexmultiplexed streams packet headers. This issue needs
   certainly be resolved using a mechanism similar to what was
   proposed with [7]. During their transport, FlexMux packets
   need not be compliant to the MPEG-4 standard, it is only when
   they are delivered to the Flexdemultiplexer that the MPEG-4
   compliance point is defined.

8.3. Transport of MPEG-4 FlexMux streams

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   An MPEG-4 FlexMux stream is mapped directly onto the RTP
   payload without any addition of extra header fields or removal
   of any FlexMux packet header syntactic elements.
   Each RTP packet will contain a timestamp derived from the
   sender's clock reference. This clock is synchronized to the
   FlexMux Clock Reference (FCR) and represents the target
   transmission time of the first byte of the RTP packet payload.
   On the receiving side, the RTP packet timestamp will not be
   passed to the MPEG-4 Flexdemultiplexor.
   This use of the timestamp is slightly different from the normal
   use in RTP, in that it is not considered to be the media
   display  time-stamp. The first purpose of this RTP timestamp
   will then be to reduce (after estimation) the network  jitter,
   and  the relative time drift between the transmitter and the
   receiver.

   There are packetization restrictions due to the fact that no
   synchronization pattern is part of the FlexMux packet header:
   An RTP packet payload should start with the start of a FlexMux
   packet. An RTP packet will contain an integer number of FlexMux
   packets.

   The FlexMux characteristics (declaration descriptor, timing
   descriptor, Channel Table descriptor, codetable entry
   descriptor, buffersize descriptor) may be provided by out of
   band means (e.g. SDP), or by the inband signaling mechanism
   supported by the use of the FLexMuxChannel signalling
   descriptors.

   The FlexMux declaration descriptor, FlexMux timing descriptor,
   FlexMux Channel Table descriptor, FlexMux buffersize
   descriptor, may only be used to define statically the FlexMux
   stream characteristics.

   The FlexMux codetable entry descriptor may change dynamically.

   The IP packet marking facility may be needed. If this is the
   case, as it is  based on the 'degradationPriority' field
   present in each FlexMux packet, all the FlexMux packets
   grouped in the same RTP packet should have the
   'degradationPriority' field filled with the same value.

   The size of the FlexMux packets should be adjusted such that
   the resulting RTP packet (embedding one or several FlexMux
   packets) is not larger than the path-MTU.

   Protection mechanisms for FlexMux streams within RTP packets
   are outside of the scope of this specification.

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9. SDP syntax

9.1 attributes

   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.

   <name> when equal to mpeg4-flexmux indicates the encoding type
          of the media, one MPEG-4 FlexMux stream. When equal to
          mpeg4-flexsig indicates the encoding type of the media,
          one MPEG-4 FlexMux signaling stream.

   <time scale> is the time scale of the RTP time stamps.

   <parameters> if used, can be a way  to  specialise  <name>.

   In order to be able to define either in the clear, or in binary
   format, or to point out to some already defined characteristics
   (MPEG-4 FlexMux descriptors) of a FlexMux stream, the following
   attribute is defined
   a = mpeg4-flexmuxinfo: <location of descriptor list>

   <location of descriptor list> is a URL enclosed in double
          quotes, that will supply the required flexmux list of
          MPEG-4 FlexMux 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). The data at the indicated URL consists of
          some number of concatenated complete FlexMux
          descriptors. These descriptors have an intrinsic length,
          so simple concatenation suffices.
   Examples:
   a= mpeg4-flexmuxinfo:"http://example.com/FlexMuxdescriptor.dat"
   a= mpeg4-flexmuxinfo:"http://example.com/FlexMuxdescriptor.xml"
   a= mpeg4-flexmuxinfo:
   "data:application/mpeg4- flexmuxinfo,7ab3742134bab347"
   a= mpeg4-flexmuxinfo:"data:text/xml;<FlexMuxdescriptor.../>"

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   The MPEG-4 FlexMux descriptors related to FlexMux description
   are defined within the paragraph 7.4. The list of MPEG-4
   descriptors cannot be empty. Ad Hoc descriptors can complete
   it.  The MIME name used for this data is defined below.

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

9.2 MIME Types

   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.
   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 packets in transmission order.
   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.
   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: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
   onversion, 7-bit stripping etc.

10. RTSP usage:

   When RTSP is used as a session-control protocol:
   RTP SHOULD be used as the transport protocol.
   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

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   IOD SHOULD be performed.
   Note that if an MPEG-4 FlexMux stream is closed (TEARDOWN) then
   the RTSP session ID will be lost.  The next (re-)opened FlexMux
   stream will supply a new session ID. New DESCRIBEs may be
   needed.

11. Security Considerations

   RTP packets using the payload format defined 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.
   This payload type does not exhibit any significant
   non-uniformity in the receiver side computational complexity
   for packet processing  to cause a potential denial-of-service
   threat.

12. References

   [1] ISO/IEC 14496-1:2000 MPEG-4 Systems October 2000
   [2] ISO/IEC 14496-2:1999/Amd.1:2000 MPEG-4 Visual January 2000
   [3] ISO/IEC 14496-3:1999/FDAM 1:20000 MPEG-4 Audio January 2000
   [4] ISO/IEC 14496-6 FDIS Delivery Multimedia Integration
       Framework,November 1998
   [5] Schulzrinne, Casner, Frederick, Jacobson RTP: A
     Transport Protocol for Real Time Applications  RFC 1889,
     Internet Engineering Task Force, January 1996.
   [6] S. Bradner, Key words for use in RFCs to Indicate
     Requirement Levels, RFC 2119, March 1997.
   [7] Y. Kikuchi, T. Nomura, S. Fukunaga, Y. Matsui,
     H. Kimata, RTP payload format for MPEG-4 Audio/Visual
     streams, draft-ietf-avt-rtp-mpeg4-es-05.txt, September 2000.
   [8] B. Thompson, T. Koren, D. Wing, Tunneling multiplexed
     Compressed RTP ('TCRTP'), work in progress,
     draft-ietf-avt-tcrtp-01.txt, July 2000.

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   [9] D. Singer, Y Lim, A Framework for the delivery of MPEG-4
       over IP-based Protocols, work in progress,
       draft-singer-mpeg4-ip-01.txt,October 2000.
   [10] Handley, Jacobson, SDP: Session Description Protocol,
       RFC 2327, Internet Engineering Task Force, April 1998.

13. Authors' Addresses

   Jan Van der Meer
   Philips Digital Networks
   Cederlaan 4
   5600 JB Eindhoven
   Netherlands
   e-mail : jan.vandermeer@philips.com

   C.Roux, D.Curet,
   E.Gouleau, S.Relier
   France Telecom
   rue du Clos Courtel
   35512 Cesson-Sevigne
   France
   e-mails: catherine.roux, dominique.curet
   emmanuel.gouleau, stephanie.relier@rd.francetelecom.fr

   Pierre Clement
   Thomcast
   40 r Bray
   35510 Cesson-Sevigne
   France
   pierre.clement@thomcast.thomson-csf.com

   Guy Cherry
   nCUBE
   110 Marsh Drive, Suite 200
   Foster City, California 94404-1184
   USA
   guyc@ncube.com

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