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 C.Roux & al. Page 1 23/02/01 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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 C.Roux & al. expires 14/08/01 2 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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 C.Roux & al. expires 14/08/01 3 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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 C.Roux & al. expires 14/08/01 4 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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 C.Roux & al. expires 14/08/01 5 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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 C.Roux & al. expires 14/08/01 6 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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 C.Roux & al. expires 14/08/01 7 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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 C.Roux & al. expires 14/08/01 8 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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). C.Roux & al. expires 14/08/01 9 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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 C.Roux & al. expires 14/08/01 10 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 ,.., - these 16-bit fields specify the identifiers of ISO/IEC 14496-1 SL-packetized streams defined in [1]. ,..,,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]. C.Roux & al. expires 14/08/01 11 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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. C.Roux & al. expires 14/08/01 12 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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 C.Roux & al. expires 14/08/01 13 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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. C.Roux & al. expires 14/08/01 14 RTP payload format for MPEG-4 FlexMultiplexed streams 23/02/01 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: /