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Versions: 00 01 02 03 04                                                
Internet Engineering Task Force                    Avaro-France Telecom
Internet Draft                                               Basso-AT&T
                                                   Casner-Packet Design
                                                          Civanlar-AT&T
                                                        Gentric-Philips
                                                         Herpel-Thomson
                                                      Lifshitz-Optibase
                                                            Lim-mp4cast
                                                            Perkins-ISI
                                                   van der Meer-Philips
                                                             April 2001
                                                      Expires Oct. 2001
Document: draft-gentric-avt-mpeg4-multisl-03.txt


                 RTP Payload Format for MPEG-4 Streams


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.



Abstract

   This document describes a payload format for transporting MPEG-4
   encoded 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 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 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.


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1. Introduction

   MPEG-4 is a recent standard from ISO/IEC for the coding of natural
   and synthetic audio-visual data in the form of audiovisual objects
   that are arranged into an audiovisual scene by means of a scene
   description [1][2][3][4]. This draft specifies an RTP [5] payload
   format for transporting MPEG-4 encoded data streams.

   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].

   The benefits of using RTP for MPEG-4 data stream transport include:

   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.

   1.1 Overview of MPEG-4 End-System Architecture

   Fig. 1 below shows the general layered architecture of MPEG-4
   terminals. The Compression Layer processes individual audio-visual
   media streams. The MPEG-4 compression schemes are defined in the
   ISO/IEC specifications 14496-2 [2] and 14496-3 [3]. The compression
   schemes in MPEG-4 achieve efficient encoding over a bandwidth
   ranging from several kbps to many Mbps. The audio-visual content
   compressed by this layer is organized into Elementary Streams (ESs).
   The MPEG-4 standard specifies MPEG-4 compliant streams. Within the
   constraint of this compliance the compression layer is unaware of a
   specific delivery technology, but it can be made to react to the
   characteristics of a particular delivery layer such as the path-MTU
   or loss characteristics. Also, some compressors can be designed to
   be delivery specific for implementation efficiency. In such cases
   the compressor may work in a non-optimal fashion with delivery
   technologies that are different than the one it is specifically
   designed to operate with.

   The hierarchical relations, location and properties of ESs in a
   presentation are described by a dynamic set of Object Descriptors
   (ODs). Each OD groups one or more ES Descriptors referring to a
   single content item (audio-visual object). Hence, multiple
   alternative or hierarchical representations of each content item are
   possible.

   ODs are themselves conveyed through one or more ESs. A complete set
   of ODs can be seen as an MPEG-4 resource or session description at a


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   stream level. The resource description may itself be hierarchical,
   i.e. an ES conveying an OD may describe other ESs conveying other
   ODs.

   The session description is accompanied by a dynamic scene
   description, Binary Format for Scene (BIFS), again conveyed through
   one or more ESs. At this level, content is identified in terms of
   audio-visual objects. The spatio-temporal location of each object is
   defined by BIFS. The audio-visual content of those objects that are
   synthetic and static are described by BIFS also. Natural and
   animated synthetic objects may refer to an OD that points to one or
   more ESs that carries the coded representation of the object or its
   animation data.

   By conveying the session (or resource) description as well as the
   scene (or content composition) description through their own ESs, it
   is made possible to change portions of the content composition and
   the number and properties of media streams that carry the audio-
   visual content separately and dynamically at well known instants in
   time.

   One or more initial Scene Description streams and the corresponding
   OD stream have to be pointed to by an initial object descriptor
   (IOD). The IOD needs to be made available to the receivers through
   some out-of-band means that are out of scope of this payload
   specification. However in the context of transport on IP networks it
   is defined in a separate document [9].

   A homogeneous encapsulation of ESs carrying media or control (ODs,
   BIFS) data is defined by the Sync Layer (SL) that primarily provides
   the synchronization between streams. The Compression Layer organizes
   the ESs in Access Units (AU), the smallest elements that can be
   attributed individual timestamps. Integer or fractional AUs are then
   encapsulated in SL packets.  All consecutive data from one stream is
   called an SL-packetized stream at this layer. The interface between
   the compression layer and the SL is called the Elementary Stream
   Interface (ESI). The ESI is informative.

   The Delivery Layer in MPEG-4 consists of the Delivery Multimedia
   Integration Framework defined in ISO/IEC 14496-6 [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 interface between the SL and DMIF is called the DMIF
   Application Interface (DAI). It offers content location independent
   procedures for establishing MPEG-4 sessions and access to transport
   channels. The specification of this payload format is considered as
   a part of the MPEG-4 Delivery Layer.

   media aware        +-----------------------------------------+
   delivery unaware   |           COMPRESSION LAYER             |
   14496-2 Visual     |streams from as low as Kbps to multi-Mbps|
   14496-3 Audio      +-----------------------------------------+


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                                                      Elementary
                                                      Stream
   ===================================================Interface

   (ESI)
                     +-------------------------------------------+
   media and         |              SYNC LAYER                   |
   delivery unaware  | manages elementary streams, their synch-  |
   14496-1 Systems   | ronization and hierarchical relations     |
                     +-------------------------------------------+

                                                       DMIF
                                                       Application
   ====================================================Interface

   (DAI)
                     +-------------------------------------------+
   delivery aware    |               DELIVERY LAYER              |
   media  unaware    |provides transparent access to and delivery|
   14496-6 DMIF      | of content irrespective of delivery       |
                     |                technology                 |
                     +-------------------------------------------+

   Figure 1: General MPEG-4 terminal architecture


1.2 MPEG-4 Elementary Stream Data Packetization

   The ESs from the encoders are fed into the SL with indications of AU
   boundaries, random access points, desired composition time and the
   current time.

   The Sync Layer fragments the ESs into SL packets, each containing a
   header that encodes information conveyed through the ESI. If the AU
   is larger than a SL packet, subsequent packets containing remaining
   parts of the AU are generated with subset headers until the complete
   AU is packetized.

   The syntax of the Sync Layer is configurable and can be adapted to
   the needs of the stream to be transported. This includes the
   possibility to select the presence or absence of individual syntax
   elements as well as configuration of their length in bits. The
   configuration for each individual stream is conveyed in a
   SLConfigDescriptor, which is an integral part of the ES Descriptor
   for this stream. The MPEG-4 SLConfigDescriptor, being configuration
   information, is not carried by the media stream itself but is rather
   transported via an ObjectDescriptor Stream encoded using the MPEG-4
   Object Description framework. This can be done in a separate stream
   using this payload format (see section 4.2 for details). The
   SLConfigDescriptor MAY be transported by other means (for example as
   a a=fmtp parameter, see section 5).



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2. Analysis of the carriage of MPEG-4 over IP

   When transporting MPEG-4 audio and video, applications may or may
   not require the use of MPEG-4 systems. To achieve the highest level
   of interoperability between all MPEG-4 applications, it is desirable
   that (a) in both cases the same MPEG-4 transport format can be used
   and that (b) receivers that have no MPEG-4 system knowledge can
   easily skip the MPEG-4 system specific information, if any.

   RTP is perfectly suitable to transport MPEG-4 audio and MPEG-4
   video, but when using MPEG-4 systems a problem arises from the fact
   that both RTP and MPEG-4 systems contain a synchronization layer.
   In particular, the RTP header duplicates some of the information
   provided in SL packet headers such as the composition timestamps
   (CTSs) and the marker bit that signals the end of access units.

   To avoid unnecessary overhead and potential interoperability risks
   when transporting MPEG-4 systems, it is desirable to remove the
   redundancy between the SL packet header and the RTP packet header.
   To be independent on the use of MPEG-4 systems, synchronization can
   rely on the parameters provided in the RTP header.

   In case SL headers are used, the redundant fields are removed from
   the SL header, producing "reduced SL headers".
   The remaining information from the SL header, if any, is contained
   inside the RTP packet payload, together with the SL packet payload.
   The combination of RTP packet headers and reduced SL packet headers
   can be used to logically map the RTP packets to complete SL packets.

   Some of the information contained in the reduced SL headers is also
   useful for transport over RTP when MPEG-4 systems is not used.

   For that reason the information in the "reduced" SL headers is split
   into "general useful information" and "MPEG-4 systems only
   information".

   The "general useful information" hereinafter called Mapped SL Packet
   Header (MSLH) is carried by a number of fields configurable using
   parameters defined in section 5.1; all receivers can parse these
   fields.

   The "MPEG-4 systems only information", if any, is contained in a
   reduced SL header, hereinafter called Remaining SL Packet Header
   (RSLH), also signaled by parameters (see section 5.1)and preceded by
   a length field, so as to enable easy skipping of this information by
   non-MPEG-4 system devices.

   This is depicted in figure 2.


                            <----------SL Packet-------->

                            +---------------------------+

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                            |   SL Packet   | SL Packet |
                            |    Header     | Payload   |
                            +---------------------------+
                                  |                |
                                  |                |
         +-------------+----------+---+            |
         |             |              |            |
         V             V              V            V
   +-----------+ +-----------+ +-------------+ +-----------+
   |RTP Packet | | Mapped SL | | Remaining SL| | SL Packet |
   |  Header   | |  Header   | |    Header   | | Payload   |
   +-----------+ +-----------+ +-------------+ +-----------+

                 <----RTP Packet Payload------------------->


   Figure 2: Mapping of SL Packet into RTP packet

   This RTP payload format has been designed so that it can be
   configured (using parameters described in section 5.1) to be
   identical to RFC 3016 for the recommended MPEG-4 video
   configurations. Hence receivers that comply with this payload
   specification can decode such RTP payload.

3. Payload Format

   The RTP Payload corresponds to an integer number of SL packets.

   SL packets inside RTP packets MUST be in the SL stream order i.e:
   i)   decodingTimeStamp order, if present
   ii)  packetSequenceNumber order, if present
   iii) Implicit decoding order in all other cases.

   The SL Packet Headers are transformed into RSLH with some fields
   extracted to be mapped in the RTP header and others extracted to be
   mapped in the corresponding MSLH. The SL Packet Payload is
   unchanged.

   This payload format has two modes. The "SingleSL" mode is a mode
   where a single SL packet is transported per RTP packet. The
   "MultipleSL" mode is a mode where more than one SL packet are
   transported per RTP packet. The default mode is the Single-SL mode.
   The mode can be set to Multiple-SL by adding a non-zero SLPPSize or
   SLPPSizeLength parameter (see section 5.1).

   RTP Packets SHOULD be sent in the SL stream order (as defined
   above).

   The size (or number) of the SL packet(s) SHOULD be adjusted such
   that the resulting RTP packet is not larger than the path-MTU. To
   handle larger packets, this payload format relies on lower layers
   for fragmentation, which may not be desirable.


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3.1 RTP Header Fields Usage

   Payload Type (PT): The assignment of an RTP payload type for this
   new packet format is outside the scope of this document, and will
   not be specified here. It is expected that the RTP profile for a
   particular class of applications will assign a payload type for this
   encoding, or if that is not done then a payload type in the dynamic
   range shall be chosen.

   Marker (M) bit: The M bit is set to 1 when all SL packets in the RTP
   packet are Access Units ends i.e. the M bit maps to the SL
   accessUnitEndFlag.

   M is set to 1 when the RTP packet contains either:
   . a single SL packet containing a full Access Unit
   . a single SL packet transporting the last fragment of an Access
   Unit
   . multiple SL packets each containing a full Access Unit
   . multiple SL packets each containing the last fragment of an Access
   Unit
   . multiple SL packets each containing either a full Access Unit or
   the last fragment of an Access Unit

   The last 2 cases occur when using specific interleaving schemes. In
   some interleaving schemes it may not be practical to reshuffle the
   SL packets so as to group Access Unit ends in the same RTP packet.
   In that case, Access Unit boundaries SHOULD be transported using one
   or both of the SL flags accessUnitStartFlag and accessUnitEndFlag.

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

   Sequence Number: The RTP sequence number should be generated by the
   sender with a constant random offset and does not have to be
   correlated to any (optional) MPEG-4 SL sequence numbers.

   Timestamp: Set to the value in the compositionTimeStamp field of the
   first SL packet, if present. If compositionTimeStamp has less than
   32 bits length, the MSBs of timestamp MUST be set to zero.

   Although it is available from the SL configuration data, the
   resolution of the timestamp may need to be conveyed explicitly
   through some out-of-band means to be used by network elements which
   are not MPEG-4 aware.

   If compositionTimeStamp has more than 32 bits length, this payload
   format cannot be used.

   In all cases, the sender SHALL always make sure that RTP time stamps
   are identical only for RTP packets transporting fragments of the
   same Access Unit.

   In case compositionTimeStamp is not present in the current SL
   packet, but has been present in a previous SL packet the reason is

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   that this is the same Access Unit that has been fragmented therefore
   the same timestamp value MUST be taken as RTP timestamp.

   If compositionTimeStamp is never present in SL packets for this
   stream, the RTP packetizer SHOULD convey a reading of a local clock
   at the time the RTP packet is created.

   According to RFC1889 [5, Section 5.1] timestamps are recommended to
   start at a random value for security reasons. However then, a
   receiver is not in the general case able to reconstruct the original
   MPEG-4 Time Stamps (CTS, DTS, OCR) which can be of use for
   applications where streams from multiple sources are to be
   synchronized. Therefore the usage of such a random offset SHOULD be
   avoided.

   Note that since RTP devices may re-stamp the stream, all time stamps
   inside of the RTP payload (CTS and DTS in MSLH, OCR in RSLH) MUST be
   expressed as difference to the RTP time stamp. Since this
   subtraction may lead to negative values, the offset MUST be encoded
   as a two's complement signed integer in network byte order. Note
   these offsets (delta) typically require much fewer bits to be
   encoded than the original length, which is another justification.

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

   RTCP SHOULD be used as defined in RFC 1889 [5].

   RTP timestamps in RTCP SR packets: according to the RTP timing
   model, the RTP timestamp that is carried into an RTCP SR packet is
   the same as the compositionTimeStamp that would be applied to an RTP
   packet for data that was sampled at the instant the SR packet is
   being generated and sent. The RTP timestamp value is calculated from
   the NTP timestamp for the current time, which also goes in the RTCP
   SR packet. To perform that calculation, an implementation needs to
   periodically establish a correspondence between the CTS value of a
   data packet and the NTP time at which that data was sampled.

3.2 RTP payload structure

   The packet payload structure consists of 3 byte-aligned sections.

   The first section is the MSLHSection and contains Mapped SL Packet
   Headers (MSLH). The MSLH structure is described in 3.3. In the
   Single-SL mode this section is empty by default.

   The second section is the RSLHSection and contains Remaining SL
   Headers (RSLH). The RSLH structure is described in 3.5. By default
   this section is empty.

   The last section (SLPPSection) contains the SL packet payloads. This
   section is never empty.



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   The Nth MSLH in the MSLHSection, the Nth RSLH in the RSLHSection and
   the Nth SL packet payload in the SLPPSection correspond to the Nth
   SL packet transported by the RTP packet.


   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |V=2|P|X|  CC   |M|     PT      |       sequence number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           timestamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           synchronization source (SSRC) identifier            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   :            contributing source (CSRC) identifiers             :
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |                                                               |
   |                MSLHSection (byte aligned)                     |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                                                               |
   |                RSLHSection (byte aligned)                     |
   |                                                               |
   |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   |                SLPPSection (byte aligned)                     |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               :...OPTIONAL RTP padding        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Figure 3: An RTP packet for MPEG-4

3.3 MSLHSection structure

   If the MSLHSection consumes a non-integer number of bytes, up to 7
   zero-valued padding bits MUST be inserted at the end in order to
   achieve byte-alignment.

   In the Single-SL mode the MSLHSection consists of a single MSLH.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | MSLH (x bits )  : padding bits|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4: MSLHSection structure in Single-SL mode



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   In the Multiple-SL mode this section consist of a 2 bytes field
   giving the size in bits (in network byte order) of the following
   block of bit-wise concatenated MSLHs.

   This size field is absent in the Single-SL mode not because it is
   not needed (which would be a minor gain) but for compatibility with
   RFC 3016.

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | MSLH section size in bits     | MSLH        |         etc     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                 |
   | as many bit-wise concatenated MSLHs                           |
   | as SL packets in this RTP packet                              |
   |                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                 : padding bits|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5: MSLHSection structure in Multiple-SL mode

3.4 MSLH structure

   The Mapped SL Packet Header content depends on parameters (as
   described in section 5.1); by default it is empty for the Single-SL
   mode and contains only the SLPPayloadSize (SL Packet Payload Size)
   field in the Multiple-SL mode.

   When all options are used the MSLH structure is given in figure 6.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     SLPPayloadSize                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     SLPSeqNum/SLPSeqNumDelta          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     CTSFlag                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     CTSDelta                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     DTSFlag                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     DTSDelta                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 6: Mapped SL Packet Header (MSLH) structure

   In the general case a receiver can only discover the size of a MSLH
   by parsing it since for example the presence of CTSDelta is signaled
   by the value of CTSFlag.

3.4.1 Fields of MSLH



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   SLPPayloadSize (SL Packet Payload Size): Indicates the size in bytes
   of the associated SL Packet Payload, which can be found in the
   SLPPSection of the RTP packet. The length in bits of this field is
   signaled by the SLPPSizeLength parameter (see section 5.1).

   SLPSeqNum/SLPSeqNumDelta: Encodes the packetSequenceNumber (serial
   number) of the SL Packet.

   SLPSeqNum is found only for the first SL packet. SLPSeqNumDelta is
   optional and -if present- appears for subsequent (non-first) SL
   packets.

   The length in bits of the SLPSeqNum field is defined by the
   SLPSeqNumLength parameter (see section 5.1).

   The length in bits of the SLPSeqNumDelta field is defined by the
   SLPSeqNumDeltaLength parameter (see section 5.1).

   If the parameter SLPSeqNumDeltaLength is defined, non-first SL
   packets have their packetSequenceNumber encoded as a difference
   named SLPSeqNumDelta. This difference is relative to the previous SL
   packet in the RTP packet according to (with i>=0):
   packetSequenceNumber(0) = SLPSeqNum(0)
   packetSequenceNumber(i+1) = packetSequenceNumber(i) +
   SLPSeqNumDelta(i+1) + 1

   If the parameter SLPSeqNumDeltaLength is not defined the default
   value is zero i.e. the SLPSeqNumDelta field is not present for non-
   first SL packets. Furthermore receivers SHALL then apply the above
   formula with SLPSeqNumDelta equal to zero i.e. by default
   packetSequenceNumber is incremented by 1 for each SL packet in one
   RTP packet. This means that for streams that use
   packetSequenceNumber and are not interleaved the transport of
   packetSequenceNumber in the Multiple-SL mode is "almost free".

   CTSFlag (1 bit): Indicates whether the CTSDelta field is present. A
   value of 1 indicates that the field is present, a value of 0 that it
   is not present.

   If CTSDeltaLength is not zero this field is present in all MSLH
   since the receiver needs it to reconstruct the
   compositionTimeStampFlag of SL Headers.

   CTSDelta: Specifies the value of the CTS as a 2-complement offset
   (delta) from the timestamp in the RTP header of this RTP packet.
   The length in bits of each CTSDelta field is specified by the
   CTSDeltaLength parameter (see section 5.1).

   This field is present if CTSFlag is 1 except for the first MSLH
   since the composition time stamp of the first SL packet is mapped to
   the RTP time stamp, regardless of whether CTSFlag is 1. In all cases
   the sender MUST remove the compositionTimeStamp from the RSLH.


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   DTSFlag (1 bit): Indicates whether the DTSDelta field is present. A
   value of 1 indicates that DTSDelta is present, a value of 0 that it
   is not present.

   If DTSDeltaLength is not zero this field is present in all MSLH
   since the receiver needs it to reconstruct the decodingTimeStampFlag
   of SL Headers.

   DTSDelta: Specifies the value of the decodingTimeStamp as a 2
   complement offset (delta) from the timestamp in the RTP header of
   this packet. The length in bits of each DTSDelta field is specified
   by the DTSDeltaLength parameter (see section 5.1).

   This field appears when DTSFlag is 1. The sender MUST always remove
   the decodingTimeStamp from the RSLH.

3.4.2 Relationship between sizes of MSLH fields and parameters

   The relationship between a Mapped SL Packet Header and the related
   parameters is as follows:

   +===========================+=================================+
   | Fields of MSLPH           | Number of bits (parameters)     |
   +===========================+=================================+
   | SLPPayloadSize            |  SLPPSizeLength                 |
   +---------------------------+---------------------------------+
   | SLPSeqNum                 |  SLPSeqNumLength                |
   +---------------------------+---------------------------------+
   | SLPSeqNumDelta            |  SLPSeqNumDeltaLength           |
   +---------------------------+---------------------------------+
   | CTSFlag                   |   1  If ( CTSDeltaLength > 0 )  |
   +---------------------------+---------------------------------+
   | CTSDelta                  |  CTSDeltaLength If(CTSFlag==1)  |
   +---------------------------+---------------------------------+
   | DTSFlag                   |   1  If ( DTSDeltaLength > 0 )  |
   +---------------------------+---------------------------------+
   | DTSDelta                  |  DTSDeltaLength If(DTSFlag==1)  |
   +---------------------------+---------------------------------+

   Table 1: Relationship between MSLH fieldsÆ size and parameters

3.5 RSLHSection structure

   This section consists of a field (RSLHSectionSize) giving the size
   in bits of the following block of bit-wise concatenated RSLHs.

   If the section consumes a non-integer number of bytes, up to 7 zero
   padding bits MUST be inserted at the end in order to achieve byte-
   alignment.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | RSLHSectionSize (RSLHSectionSizeLength bits)| RSLH (variable  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                 |

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   | number of bits)                                               |
   |                                                               |
   |         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         | RSLH (variable number of bits)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | etc                                                           |
   | as many bit-wise concatenated RSLHs                           |
   | as SL Packets in this RTP packet                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | RSLH (variable number of bits)                                |
   |                                                 +-+-+-+-+-+-+-+
   |                                                 : padding bits|
   |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7: RSLHSection structure

   The length in bits of the RSLHSectionSize field is
   RSLHSectionSizeLength and is specified with a default value of zero
   indicating that the whole RSLHSection is absent.

   +=================================+===============================+
   | Fields of RSLHSection           |         Number of bits        |
   +=================================+===============================+
   | RSLHSectionSize                 |       RSLHSectionSizeLength   |
   +---------------------------------+-------------------------------+
   | all bit-wise concatenated RSLHs |       RSLHSectionSize         |
   +---------------------------------+-------------------------------+

   Table 2: Sizes in bits inside RSLHSection

   Parsing of the bit-wise concatenated RSLHs requires MPEG-4 system
   awareness, specifically it requires to understand the MPEG-4
   Synchronization Layer (SL) syntax and the modifications to this
   syntax described in the next section.

   However thanks to the RSLHSectionSize field non-MPEG-4-system
   receivers MAY skip this part by rounding up RSLPHSize/8 to the next
   integer number of bytes.

3.6 RSLH structure

   A Remaining SL Packet Header (RSLH) is what remains of an SL header
   after modifications for mapping into this payload format.

   The following modifications of the SL packet header MUST be applied.
   The other fields of the SL packet header MUST remain unchanged but
   are bit-shifted to fill in the gaps left by the operations specified
   below.

3.6.1 Removal of fields




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   The following SL Packet Header fields -if present- are removed since
   they are mapped either in the RTP header or in the corresponding
   MSLH:
   . compositionTimeStampFlag
   . compositionTimeStamp
   . decodingTimeStampFlag
   . decodingTimeStamp
   . packetSequenceNumber
   . AccessUnitEndFlag (in Single-SL mode only)

   The AccessUnitEndFlag, when present for a given stream, MUST be
   removed from every RSLH when using the Single-SL mode since it has
   the same meaning as the Marker bit (and for compatibility with RFC
   3016). However when using the Multiple-SL mode, AccessUnitEndFlag
   MUST NOT be removed since it is useful to signal individual AU ends.

3.6.2 Mapping of OCR

   Furthermore if the SL Packet header contains an OCR, then this field
   is encoded in the RSLH as a 2-complement difference (delta) exactly
   like a compositionTimeStamp or a decodingTimeStamp in the MSLH. The
   length in bit of this difference is indicated by the OCRDeltaLength
   parameter (see section 5.1).

   With this payload format OCRs MUST have the same clock resolution as
   Time Stamps.

   If compositionTimeStamp is not present for a SL packet that has OCR
   then the OCR SHALL be encoded as a difference to the RTP time stamp.

3.6.3 Degradation Priority

   For streams that use the optional degradationPriority field in the
   SL Packet Headers, only SL packets with the same degradation
   priority SHALL be transported by one RTP packet so that components
   may dispatch the RTP packets according to appropriate QOS or
   protection schemes. Furthermore only the first RSLH of one RTP
   packet SHALL contain the degradationPriority field since it would be
   otherwise redundant.

3.7 SLPPSection structure

   The SLPPSection (SL Packet Payload Section) contains the
   concatenated SL Packet Payloads. By definition SL Packet Payloads
   are byte aligned.

   For efficiency SL packets do not carry their own payload size. This
   is not an issue for RTP packets that contain a single SL Packet.

   However in the Multiple-SL mode the size of each SL packet payload
   MUST be available to the receiver.



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   If the SL packet payload size is constant for a stream, the size
   information SHOULD NOT be transported in the RTP packet. However in
   that case it MUST be signaled using the SLPPSize parameter (see
   section 5.1).

   If the SL packet payload size is variable then the size of each SL
   packet payload MUST be indicated in the corresponding MSLH. In order
   to do so the MSLH MUST contain a SLPPayloadSize field. The number of
   bits on which this SLPPayloadSize field is encoded MUST be indicated
   using the SLPPSizeLength parameter (see section 5.1).

   The absence of either SLPPSize or SLPPSizeLength indicates the
   Single-SL mode i.e. that a single SL packet is transported in each
   RTP packet for that stream.


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | SLPP (variable number of bytes)                           |
   |                                                           |
   |                                                           |
   |                         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         | SLPP (variable number of bytes) |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+                                 |
   |                                                           |
   |                                                           |
   |                                                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | etc                                                       |
   | as many byte-wise concatenated SLPPs                      |
   | as SL Packets in this RTP packet                          |
   |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 8: SLPPSection structure

3.8 Interleaving

   SL Packets MAY be interleaved. Senders MAY perform interleaving.
   Receivers MUST support interleaving.

   When interleaving of SL packets is used it SHALL be implemented
   using the SLPSeqNum field of MSLH.

   The AUSequenceNumber field of the SL header MUST NOT be used for
   interleaving since firstly it may collide with the Scene Description
   Carousel usage described in section 5.1 and secondly it is not
   visible to non-MPEG-4 system receivers.

   The conjunction of RTP sequence number and SLPSeqNum can produce a
   quasi-unique identifier for each SL packet so that a receiver can
   unambiguously reconstruct the original order even in case of out-of-
   order packets, packet loss or duplication.

3.9 Fragmentation Rules

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   This section specifies rules for senders in order to prevent media
   decoding difficulties at the receiver end.

   MPEG-4 Access Units are the default fragments for MPEG-4 bitstreams
   and SHOULD be mapped one-to-one to RTP packets of this format with
   two exceptions:
   - Access Units larger than the MTU,
   - When using interleaving for better packet loss resilience.

   In all cases Access Unit start MUST be aligned with SL packet start.

   This section gives rules to apply when performing Access Unit
   fragmentation.

   Some MPEG-4 codecs define optional syntax for Access Units sub-
   entities (fragments) that are independently decodable for error
   resilience purposes. Examples are Video Packets for video and Error
   Sensitivity Categories (ESC) for audio. This always corresponds to
   specific bitstream syntax, which is signaled in the
   DecoderSpecificInfo inside the DecoderConfig in SLConfig, and/or
   using the corresponding parameters as described in section 5.1.
   Therefore encoders and decoders are both aware whether they are
   operating in such a mode or not (however since this codec
   configuration is an opaque data block this is not explicitly
   signaled by this payload format).

   If not operating in such a mode it is obvious that the decoder has
   to skip packets after a loss until an Access Unit start is received.
   Similarly decoder implementations that do not implement robust
   decoding of Access Units fragments have to discard all packets after
   a packet loss until an Access Unit start is received. In the same
   way decoder implementations that do not implement re-synchronization
   at any Access Units start have to discard all packets after a packet
   loss until a Random Access Point Access Unit is received.

   One problem would arise however for decoder implementations that try
   to restart decoding after a packet loss if independently decodable
   fragments are signaled (in the decoder configuration) but the
   fragments actually received are not independently decodable because
   the RTP sender has made RTP packets on different boundaries than the
   fragments provided by the encoder (so this issue applies to the
   interface between the encoder and the RTP sender and to the RTP
   sender component itself).

   For this reason the following rules must apply to SL streams that
   are specifically made for transport with this payload format:

   SL packets SHOULD be codec-semantic entities in the spirit of ALF
   i.e. either complete Access Units or fragments of Access Units that
   are independently decodable. Specifically when a given codec has an
   independently decodable Access Unit fragments optional syntax this
   option SHOULD be used.

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   Furthermore when streams are generated using independently decodable
   Access Units fragments these Access Units fragments MUST be mapped
   one-to-one into SL packets. Consequently independently decodable
   Access Units fragments MUST NOT be split across several SL packets
   and therefore MUST NOT be split across several RTP packets.

   For example an MPEG-4 audio stream encoded using the ESC syntax MUST
   NOT split one ESC across 2 RTP packets.

   This rule is relaxed when using MPEG-4 Video Packets for two
   reasons: firstly Video Packets can be much larger than typical MTU
   and secondly all Video Packets start with a specific
   resynchronization marker that can be unambiguously detected.
   Therefore for video streams using the Video Packet syntax Video
   Packets MAY be split across several SL packets although it is
   strongly RECOMMENDED to always adapt the Video Packet size to fit
   the MTU. A Video Packet start MUST always be aligned with a SL
   packet start, except when a GOV is present, in which case the GOV
   and the first Video Packet of the following VOP MUST be included in
   the same SL packet.

4. Other issues

4.1 SL packetized stream reconstruction

   The MPEG-4 over IP framework [9] requires that the way a receiver
   can reconstruct a valid SL packetized stream shall be documented,
   this is the purpose of this section.

   Since this format directly transports SL packets this reconstruction
   is trivial with the following rules:

   - SLPacketHeader.packetSequenceNumber is restored from
   MSLH.SLPSeqNum for the first SL packet in the RTP packet (i= 0):
   SLPacketHeader.packetSequenceNumber(0) = MSLH.SLPSeqNum(0)
   and for subsequent packets using (for i>=0) :
   SLPacketHeader.packetSequenceNumber(i+1) =
   SLPacketHeader.packetSequenceNumber(i) + MSLH.SLPSeqNumDelta(i+1) +1

   - All time stamps (CTS, DTS, OCR), when present, are restored from
   the delta values.
   - Time stamps flags (CTSFlag, DTSFlag) in MSLH are used to
   reconstruct respectively the compositionTimeStampFlag and
   decodingTimeStampFlag of SLPacketHeader.

   Specifically the reconstruction depends on the parameters as
   follows:

   If CTSDeltaLength is absent or equals 0:
   The SL stream reconstruction rules are:
   . for the first (or only) SL packet:
    . if SLConfig.useTimeStamps == true, then:

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     . SLPacketHeader.compositionTimeStampFlag = true
     . SLPacketHeader.compositionTimeStamp = RTP TimeStamp
    . if SLConfig.useTimeStamps == false, then:
     . SLPacketHeader.compositionTimeStampFlag is not defined
   . for the following SL packets:
    . SLPacketHeader.compositionTimeStampFlag = false

   If CTSDeltaLength is not zero:
    . SLPacketHeader.compositionTimeStampFlag = MSLH.CTSFlag
    . SLPacketHeader.compositionTimeStamp = RTP TimeStamp +
   MSLH.CTSDelta

   - The other SL packet header fields SHALL remain as found in RSLH.

   It is obvious that in the general case the reconstruction of the
   original SL packetized stream requires SL-awareness. However this
   payload format allows in all cases a receiver that does not know
   about the SL syntax to reconstruct the semantic of SL for the
   following very useful features:
   - Packet order (decoding order)
   - Access Unit boundaries (using the M bit)
   - Access Unit fragments (i.e. SL packet boundaries using
   MSLH.SLPPayloadSize)
   - Composition Time Stamps (using the RTP Time Stamp and
   MSLH.CTSDelta)
   - Decoding Time Stamps (using the RTP Time Stamp and MSLH.DTSDelta)
   - Packet sequence number (using the RTP Time Sequence number and
   MSLH.SLPSeqNum)

4.2 Handling of scene description streams

   MPEG-4 introduces new stream types as described in section 1 namely
   Object Descriptors and BIFS. In the following both OD and BIFS are
   discussed on the same basis i.e. as "scene description".

   Considering Scene description as a "stream-able" type of content is
   a rather new concept and for that reasons some specific comments are
   needed.

   Typically scene descriptions are encoded in such a way that
   information loss would in the general case cripple the presentation
   beyond any hope of repair by the receiver. Still this is well suited
   for a number of multimedia applications were the scene is first made
   available via reliable channels to the client and then played. This
   payload format is not intended for this type of applications for
   which download of MPEG-4 interchange (.mp4) files is typical.
   However it can also be used if the RTP packets are transported using
   TCP or any other reliable protocol.

   On the other hand MPEG-4 has introduced the possibility to
   dynamically change the scene description by sending animation
   information (changes in parameters) and structural change
   information (updates). Since this information has to be sent in a

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   timely fashion MPEG-4 has defined a number of techniques in order to
   encode the scene description in a manner that makes it behave
   similarly to other temporal encoding schemes such as audio and
   video. This payload format is intended for this usage.

   Note that in many cases the application will consist of first the
   reliable transmission of a static initial scene followed by the
   streaming of animations and updates. For this reason the usage of
   this payload format is attractive since it offers a unique solution.

   Senders must be aware that suitable schemes should be used when
   scene description streams transport sensitive configuration
   information. For example in case the RTP packet transporting an OD-
   update command would be lost, the corresponding media stream would
   not be accessible by the receiver.

   Redundancy is a possibility and may either be added by tools
   hierarchically higher than this payload format, e.g. by packet based
   FEC, re-transmission, or similar tools. In such a case, the general
   congestion control principles have to be observed.

   Since BIFS and OD streams may be modified during the session with
   update commands, there is a need to send both update commands and
   full BIFS/OD refresh. For that reason MPEG-4 defines Random Access
   Points (RAP) for scene description streams (OD and BIFS) where by
   definition a decoder can restart decoding i.e. receives a "full
   update" of the scene. This mechanism is called Scene and Object
   Description Carrousel. The AU Sequence Number field of SL Packet
   Header is used to support this behavior at the Synchronization
   Layer. When two access units are sent consecutively with the same AU
   Sequence Number, the second one is assumed to be a semantic
   repetition of the first. If a receiver starts to listen in the
   middle of a session or has detected losses, it can skip all received
   Access Units until such a RAP. The periodicity of transmission of
   these RAPs should be chosen/adjusted depending on the application
   and the network it is deployed on; i.e. exactly like Intra-coded
   frames for video, it is the responsibility of the sender to make
   sure the periodicity of RAPs is suitable.

4.3 Multiplexing

   An advanced 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 stream may not always be practical. Allocating and
   controlling hundreds of destination addresses for each MPEG-4
   session may pose insurmountable session administration problems.
   The input/output processing overhead at the end-points will be
   extremely high also. Additionally, low delay transmission of low
   bitrate data streams, e.g. facial animation parameters, results in
   extremely high header overheads.




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   To solve these problems, MPEG-4 data transport requires a
   multiplexing scheme that allows selective bundling of several ESs.
   This is beyond the scope of the payload format defined here.

   The MPEG-4's Flexmux multiplexing scheme may be used for this
   purpose and a specific RTP payload format is being developed [11].

   Another approach may be to develop a generic RTP multiplexing scheme
   usable for MPEG-4 data. The multiplexing scheme reported in [8] may
   be a candidate for this approach.

   For MPEG-4 applications, the multiplexing technique needs to address
   the following requirements:

   i. The ESs multiplexed in one stream can change frequently during a
   session. Consequently, the coding type, individual packet size and
   temporal relationships between the multiplexed data units must be
   handled dynamically.

   ii. The multiplexing scheme should have a mechanism to determine the
   ES identifier (ES_ID) for each of the multiplexed packets. ES_ID is
   not a part of the SL header.

   iii. In general, an SL packet does not contain information about its
   size. The multiplexing scheme should be able to delineate the
   multiplexed packets whose lengths may vary from a few bytes to close
   to the path-MTU.

4.5 Overlap with RFC 3016

   This payload format has been designed to have an overlap with RFC
   3016 [7]. The conditions for this overlap are:
   Conditions for RFC 3016:
   i. MPEG-4 video elementary streams only
   ii. Maximum one VOP or Video Packet per RTP packet
   Conditions for this payload format:
   i. No structural parameters defined (or all set to zero), i.e.
   Single-SL mode with empty MSLH and empty RSLH.
   ii. Receivers MUST be ready to accept (ignore) video configuration
   headers (e.g. VOSH, VO and VOL) and visual-object-sequence-end-code
   transported in-band.

5. Types and Names

   This section describes the MIME types and names associated with this
   payload format. Section 5.1 is intended for registration with IANA
   in RFC 2048.

   This format may require additional information about the mapping to
   be made available to the receiver. This is done using parameters
   described in the next section. The absence of any of these fields is
   equivalent to a field set to the default value, which is always


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   zero. The absence of any such parameters resolves into a default
   "basic" configuration.

   In the MPEG-4 framework the SL stream configuration information is
   carried using the Object Descriptor. For compatibility with
   receivers that do not implement the full MPEG-4 system specification
   this information MAY also be signaled using parameters described
   here. When such information is present both in an Object Descriptor
   and as a parameter of this payload format it MUST be exactly the
   same.

   For transport of MPEG-4 audio and video without the use of MPEG-4
   systems, as well as to support non-MPEG-4 system receivers, it is
   also possible to transport information on the profile and level of
   the stream and on the decoder configuration. This is also described
   in the next section.

5.1 MIME type registration

   MIME media type name:  "video" or ôaudio" or "application"

   "video" SHOULD be used for MPEG-4 Video streams (ISO/IEC 14496-2) or
   MPEG-4 Systems streams that convey information needed for an
   audio/visual presentation.

   "audio" SHOULD be used for MPEG-4 Audio streams (ISO/IEC 14496-3) or
   MPEG-4 Systems streams that convey information needed for an audio
   only presentation.

   "application" SHOULD be used for MPEG-4 Systems streams
   (ISO/IEC14496-1) that serve other purposes than audio/visual
   presentation, e.g. in some cases when MPEG-J streams are
   transmitted.

   MIME subtype name: mpeg4-sl

   Required parameters: none

   Optional parameters:

   DTSDeltaLength:
   The number of bits on which the DTSDelta field is encoded in MSLH.
   The default value is zero and indicates the absence of DTSFlag and
   DTSDelta in MSLH (the stream does not transport decodingTimeStamps).
   A value larger than zero indicates that there is a DTSFlag in each
   MSLH. Since decodingTimeStamp -if present- must be encoded as a
   difference to the RTP time stamp, the DTSDeltaLength parameter MUST
   be present in order to transport decodingTimeStamps with this
   payload format.

   CTSDeltaLength:
   The number of bits on which the CTSDelta field is encoded in (non-
   first) MSLH. The default value is zero and indicates the absence of

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   the CTSFlag and CTSDelta fields in MSLH. Non-zero values MOST NOT be
   signaled in the Single-SL mode. Since compositionTimeStamps -if
   present- must be encoded as a difference to the RTP time stamp, the
   CTSDeltaLength parameter MUST be present in order to transport
   compositionTimeStamps using this payload format (in the Multiple-SL
   mode). However CTSDeltaLength SHOULD be set to zero (or not
   signaled) for streams that have a constant Access Unit duration
   (which can be explicitly signaled using the DurationFlag and
   AccessUnitDuration field of SLConfigDescriptor).

   OCRDeltaLength:
   The number of bits on which the OCRDelta field is encoded in RSLH.
   The default value is zero and indicates the absence of OCR for this
   stream. Since objectClockReference -if present- must be encoded as a
   difference to the RTP time stamp, the OCRDeltaLength parameter MUST
   be present in order to transport objectClockReferences with this
   payload format.

   SLPPSizeLength:
   The number of bits on which the SLPPayloadSize field of MSLH is
   encoded. The default value is zero and indicates the Single-SL mode
   (unless SLPPSize is present). Simultaneous presence of this
   parameter and SLPPSize is illegal. Either the SLPPSizeLength or
   SLPPSize parameter MUST be present in order to signal the Multiple-
   SL mode of this payload format.

   SLPPSize:
   The constant size in bytes of each SL Packet Payload for this
   stream. The default value is zero and indicates variable SL Packet
   Payload size (or the Single-SL mode if SLPPSizeLength is absent).
   Simultaneous presence of this parameter and SLPPSizeLength is
   illegal. Either the SLPPSizeLength or SLPPSize parameter MUST be
   present in order to signal the Multiple-SL mode of this payload
   format. When SLPPSize is present the SLPPayloadSize of MSLH in the
   RTP packets MUST NOT be present.

   SLPSeqNumLength:
   The number of bits on which the SLPSeqNum is encoded in the first
   MSLH. The default value is zero and indicates the absence of
   SLPSeqNum and SLPSeqNumDelta for all MSLHs. Since
   packetSequenceNumber -if present- must be mapped in MSLH, the
   SLPSeqNumLength parameter MUST be present in order to transport
   packetSequenceNumber with this payload format.

   SLPSeqNumDeltaLength:
   The number of bits on which the SLPSeqNumDelta are encoded in any
   non-first MSLH. The default value is zero and indicates that
   packetSequenceNumber MUST be incremented by one for each SL packet
   in the RTP packet (see section 3.5). Since when interleaving
   packetSequenceNumber does not increment by 1 inside a RTP packet,
   the SLPSeqNumDeltaLength parameter MUST be present when using
   interleaving with this payload format.


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   RSLHSectionSizeLength:
   The number of bits that is used to encode the RSLHSectionSize field.
   The default value is zero and indicates the absence of the whole
   RSLHSection for all RTP packets of this stream. Compatibility with
   RFC 3016 requires that the RSLHSection must be empty, including the
   RSLHSectionSize field. This is the reason why there is such a
   variable length with a default value indicating absence of the
   RSLHSectionSize field.

   SLConfigDescriptor:
   A base-64 encoding of the SLConfigDescriptor. This SHALL be the
   original SLConfigDescriptor and it SHALL be the same as the one
   transported by the OD framework, if any.

   profile-level-id:
   A decimal representation of the MPEG-4 Profile Level indication
   value. For audio this parameter indicates which MPEG-4 Audio tool
   subsets are applied to encode the audio stream and is defined in
   defined in ISO/IEC 14496-1. For video this parameter indicates which
   MPEG-4 Visual tool subsets are applied to encode the video stream
   and is defined in Table G-1 of ISO/IEC 14496-2. This parameter MAY
   be used in the capability exchange or session setup procedure to
   indicate MPEG-4 Profile and Level combination of which the relevant
   MPEG-4 media codec is capable. If this parameter is not specified by
   the procedure, its default value of 1 (Simple Profile/Level 1) is
   used.

   Config:
   A hexadecimal representation of an octet string that expresses the
   media payload configuration. Configuration data is mapped onto the
   octet string in an MSB-first basis. The first bit of the
   configuration data SHALL be located at the MSB of the first octet.
   In the last octet, zero-valued padding bits, if necessary, shall
   follow the configuration data. For audio this is a
   "StreamMuxConfig", as defined in ISO/IEC 14496-3. For video this
   expresses the MPEG-4 Visual configuration information, as defined in
   subclause 6.2.1 Start codes of ISO/IEC14496-2[2][4][9] and the
   configuration information indicated by this parameter SHALL be the
   same as the configuration information in the corresponding MPEG-4
   Visual stream, except for first-half-vbv-occupancy and latter-half-
   vbv-occupancy, if it exists, which may vary in the repeated
   configuration information inside an MPEG-4 Visual stream (See 6.2.1
   Start codes of ISO/IEC14496-2).

   object-type:
   A decimal representation of the MPEG-4 Audio Object Type value
   defined in ISO/IEC 14496-3. This parameter specifies the tool used
   by the encoder. It CAN be used to limit the capability within the
   specified "profile-level-id".

   Bitrate:
   A decimal representation of the audio bitrate in bits per second for
   the audio bit stream.

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   Encoding considerations:
   System bitstreams MUST be generated according to MPEG-4 System
   specifications (ISO/IEC 14496-1). Video bitstreams MUST be generated
   according to MPEG-4 Visual specifications (ISO/IEC 14496-2). Audio
   bitstreams MUST be generated according to MPEG-4 Visual
   specifications (ISO/IEC 14496-3). All SL streams MUST be generated
   according to MPEG-4 Sync Layer specifications (ISO/IEC 14496-1
   section 10), in order to read this format the SLConfigDescriptor is
   REQUIRED. These bitstream are binary data and MUST be encoded for
   non-binary transport (for Email, the Base64 encoding is sufficient).
   This type is also defined for transfer via RTP.  The RTP packets
   MUST be packetized according to the RTP payload format defined in
   RFC <self-reference-to-this>.

   Security considerations:
   As in RFC <self-reference-to-this>.

   Interoperability considerations:
   MPEG-4 provides a large and rich set of tools for the coding of
   visual objects.  For effective implementation of the standard,
   subsets of the MPEG-4 tool sets have been provided for use in
   specific applications. These subsets, called 'Profiles', limit the
   size of the tool set a decoder is required to implement. In order to
   restrict computational complexity, one or more 'Levels' are set for
   each Profile.  A Profile@Level combination allows:
   o a codec builder to implement only the subset of the standard he
   needs, while maintaining interworking with other MPEG-4 devices
   included in the same combination, and
   o checking whether MPEG-4 devices comply with the standard
   ('conformance testing').
   A stream SHALL be compliant with the MPEG-4 Profile@Level specified
   by the parameter "profile-level-id". Interoperability between a
   sender and a receiver may be achieved by specifying the parameter
   "profile-level-id" in MIME content, or by arranging in the
   capability exchange/announcement procedure to set this parameter
   mutually to the same value.

   Published specification:
   The specifications for MPEG-4 streams are presented in ISO/IEC
   14469-1, 14469-2, and 14469-3.  The RTP payload format is described
   in RFC <self-reference-to-this>.

   Applications which use this media type:
   Multimedia streaming and conferencing tools, Internet messaging and
   Email applications. Also supra-relativistic elementary particle
   hyperspace tunneling trans-galactic communication devices :-)

   Additional information: none

   Magic number(s): none

   File extension(s): none

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   Macintosh File Type Code(s): none

   Person & email address to contact for further information:
   Authors of RFC <self-reference-to-this>.

   Intended usage: COMMON

   Author/Change controller:
   Authors of RFC <self-reference-to-this>.

5.2 Concatenation of parameters

   Multiple parameters SHOULD be expressed as a MIME media type string,
   in the form of a semicolon-separated list of parameter=value pairs
   (see examples in Appendix).

5.3 Usage of SDP

5.3.1 The a=fmtp keyword

   It is assumed that one typical way to transport the above-described
   parameters associated with this payload format is via a SDP message
   for example transported to the client in reply to a RTSP DESCRIBE of
   via SAP. In that case the (a=fmtp) keyword MUST be used as described
   in RFC 2327 [10, section 6]. The syntax being then:

   a=fmtp:<format> <parameter name>=<value>

5.3.2 SDP example

   The following is an example of SDP syntax for the description of a
   session containing one MPEG-4 audio stream, one MPEG-4 video and two
   MPEG-4 system stream, transported using this format and the AVP
   profile [12]. Note that the video stream DTSDelta are encoded on 4
   bits in this example. See the Appendix for more examples.

   o= ....
   I= ....
   c=IN IP4 123.234.71.112
   m=video 1034 RTP/AVP 97
   a=fmtp:DTSDeltaLength=4
   a=rtpmap:97 mpeg4-sl
   m=audio 810  RTP/AVP 98
   a=fmtp: profile-level-id=1; config=7866E7E6EF
   a=rtpmpa:98 mpeg4-sl
   m=application 1234  RTP/AVP 99
   a=rtpmap:99 mpeg4-sl
   m=application 1234  RTP/AVP 99
   a=rtpmap:99 mpeg4-sl

6. Security Considerations


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   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. The packet processing complexity of this payload
   type ûi.e. excluding media data processing- 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.

   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. Acknowledgements
   This document evolved across several years thanks to contributions
   from a large number of people since it is based on work within the
   IETF AVT working group and various ISO MPEG working groups,
   especially the 4-on-IP ad-hoc group in the last stages. The authors
   wish to thank Guido Fransceschini, Art Howarth, Dave Mackie, Dave
   Singer, and Stephan Wenger for their valuable comments.

8. References

    [1] ISO/IEC 14496-1:2000 MPEG-4 Systems October 2000

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   [2] ISO/IEC 14496-2:1999/Amd.1:2000(E) 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, Internet Engineering Task Force, March 1997.

   [7] Y. Kikuchi, T. Nomura, S. Fukunaga, Y. Matsui, H. Kimata, RTP
   payload format for MPEG-4 Audio/Visual streams, Internet Engineering
   Task Force, RFC 3016.

   [8] B. Thompson, T. Koren, D. Wing, Tunneling multiplexed Compressed
   RTP ("TCRTP"), work in progress, draft-ietf-avt-tcrtp-02.txt,
   November 2000.

   [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.

   [11] C.Roux & al, RTP Payload Format for MPEG-4 FlexMultiplexed
   Streams, work in progress, draft-curet-avt-rtp-mpeg4-flexmux-00.txt,
   February 2001.

   [12] H. Schulzrinne, RTP Profile for Audio and Video Conferences
   with Minimal Control, RFC1890, Internet Engineering Task Force,
   January 1996.


9. Authors' Addresses

   Olivier Avaro
   France Telecom
   35 A Schutzenhuttenweg
   60598 Frankfurt am Main
   Deutschland
   e-mail: olivier.avaro@francetelecom.fr

   Andrea Basso
   AT&T Labs Research
   200 Laurel Avenue
   Middletown, NJ 07748
   USA

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   e-mail: basso@research.att.com

   Stephen L. Casner
   Packet Design, Inc.
   66 Willow Place
   Menlo Park, CA 94025
   USA
   e-mail: casner@acm.org

   M. Reha Civanlar
   AT&T Labs - Research
   100 Schultz Drive
   Red Bank, NJ 07701
   USA
   e-mail: civanlar@research.att.com

   Philippe Gentric
   Philips Digital Networks
   22 Avenue Descartes
   94453 Limeil-Brevannes CEDEX
   France
   e-mail: philippe.gentric@philips.com

   Carsten Herpel
   THOMSON multimedia
   Karl-Wiechert-Allee 74
   30625 Hannover
   Germany
   e-mail: herpelc@thmulti.com

   Zvi Lifshitz
   Optibase Ltd.
   7 Shenkar St.
   Herzliya 46120
   Israel
   e-mail: zvil@optibase.com

   Young-kwon Lim
   mp4cast (MPEG-4 Internet Broadcasting Solution Consortium)
   1001-1 Daechi-Dong Gangnam-Gu
   Seoul, 305-333,
   Korea
   e-mail : young@techway.co.kr

   Colin Perkins
   USC Information Sciences Institute
   4350 N. Fairfax Drive #620
   Arlington, VA 22203
   USA
   e-mail : csp@isi.edu

   Jan van der Meer
   Philips Digital Networks

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   Cederlaan 4
   5600 JB Eindhoven
   Netherlands
   e-mail : jan.vandermeer@philips.com




APPENDIX: Examples of usage of this payload format

   This payload format has been designed to transport with flexibility
   a very versatile packetization scheme (the MPEG-4 Synchronization
   Layer); its complexity is therefore larger than the average for RTP
   payload formats. For this reason this section describes a number of
   key examples of how this payload format can be used.

   A C++-like syntax called SDL (Syntactic Description Language)
   defined in [1, section 14] is used to economically describe MPEG-4
   system data structures.

Appendix.1 MPEG-4 Video

   Let us consider the case of a 30 frames per second MPEG-4 video
   stream which bit rate is high enough that Access Units have to be
   split in several SL packets (typically above 300 kb/s).

   Let us assume also that the video codec generates in that case Video
   Packets suitable to fit in one SL packet i.e that the video codec is
   MTU aware and the MTU is 1500 bytes. We assume furthermore that this
   stream contains B frames and that decodingTimeStamps are present.

SLConfigDescriptor

   In this example the SLConfigDescriptor is:

   class SLConfigDescriptor extends BaseDescriptor : bit(8)
   tag=SLConfigDescrTag {
    bit(8) predefined;
    if (predefined==0) {
     bit(1) useAccessUnitStartFlag; = 1
     bit(1) useAccessUnitEndFlag; = 0
     bit(1) useRandomAccessPointFlag; = 1
     bit(1) hasRandomAccessUnitsOnlyFlag; = 0
     bit(1) usePaddingFlag; = 0
     bit(1) useTimeStampsFlag; = 1
     bit(1) useIdleFlag; = 0
     bit(1) durationFlag; = 0
     bit(32) timeStampResolution; = 30
     bit(32) OCRResolution; = 0
     bit(8) timeStampLength; = 32
     bit(8) OCRLength; = 0
     bit(8) AU_Length; = 0
     bit(8) instantBitrateLength; = 0

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     bit(4) degradationPriorityLength; = 0
     bit(5) AU_seqNumLength; = 0
     bit(5) packetSeqNumLength; = 0
     bit(2) reserved=0b11;
    }
    if (durationFlag) {
     bit(32) timeScale; // NOT USED
     bit(16) accessUnitDuration;  // NOT USED
     bit(16) compositionUnitDuration;  // NOT USED
    }
    if (!useTimeStampsFlag) {
     bit(timeStampLength) startDecodingTimeStamp; = 0
     bit(timeStampLength) startCompositionTimeStamp; = 0
    }
   }

   The useRandomAccessPointFlag is set so that the
   randomAccessPointFlag can indicate that the corresponding SL packet
   contains a GOV and the first Video Packet of an Intra coded frame.

SL Packet Header structure

   With this configuration we have the following SL packet header
   structure:

   aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) {
    if (SL.useAccessUnitStartFlag) {
     bit(1) accessUnitStartFlag; // 1 bit
    }
    if (accessUnitStartFlag) {
     if (SL.useRandomAccessPointFlag) {
      bit(1) randomAccessPointFlag; // 1 bit
     }
     if (SL.useTimeStampsFlag) {
      bit(1) decodingTimeStampFlag; // 1 bit
      bit(1) compositionTimeStampFlag; // 1 bit
     }
     if (decodingTimeStampFlag) {
      bit(SL.timeStampLength) decodingTimeStamp;
     }
     if (compositionTimeStampFlag) {
      bit(SL.timeStampLength) compositionTimeStamp;
     }
    }
   }

Parameters

   decodingTimeStamps are encoded on 32 bits, which is much more than
   needed for delta. Therefore the sender will use DTSDeltaLength to
   signal that only 6 bits are used for the coding of relative DTS in
   the RTP packet.


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   The RSLHSectionSize cannot exceed 2 bits, which is encoded on 2 bits
   and signaled by RSLHSectionSizeLength. The resulting concatenated
   fmtp line is:

   a=fmtp:<format> DTSDeltaLength=6;RSLHSectionSizeLength=2

RTP packet structure

   Two cases can occur; for packets that transport first fragments of
   Access Units we have:

   +=========================================+=============+
   | Field                                   |  size       |
   +=========================================+=============+
   | RTP header                              |    -        |
   +-----------------------------------------+-------------+
   | CTSFlag = 1                             |  1 bit      |
   +-----------------------------------------+-------------+
   | DTSFlag = 1                             |  1 bit      |
   +-----------------------------------------+-------------+
   | DTSDelta                                |  6 bits     |
   +-----------------------------------------+-------------+
   | bits to byte alignment                  |  0 bits     |
   +-----------------------------------------+-------------+
   | RSLHSectionSize = 2                     |  2 bits     |
   +-----------------------------------------+-------------+
   | accessUnitStartFlag = 1                 |  1 bit      |
   +-----------------------------------------+-------------+
   | randomAccessPointFlag                   |  1 bit      |
   +-----------------------------------------+-------------+
   | bits to byte alignment                  |  4 bits     |
   +-----------------------------------------+-------------+
   | SL packet payload                       |  N bytes    |
   +-----------------------------------------+-------------+


   For packets that transport non-first fragments of Access Units we
   have:

   +=========================================+=============+
   | Field                                   |  size       |
   +=========================================+=============+
   | RTP header                              |    -        |
   +-----------------------------------------+-------------+
   | CTSFlag = 0                             |  1 bit      |
   +-----------------------------------------+-------------+
   | DTSFlag = 0                             |  1 bit      |
   +-----------------------------------------+-------------+
   | bits to byte alignment                  |  6 bits     |
   +-----------------------------------------+-------------+
   | RSLHSectionSize = 2                     |  2 bits     |
   +-----------------------------------------+-------------+
   | accessUnitStartFlag = 0                 |  1 bit      |

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   +-----------------------------------------+-------------+
   | randomAccessPointFlag                   |  1 bit      |
   +-----------------------------------------+-------------+
   | zero bits to byte alignment             |  4 bits     |
   +-----------------------------------------+-------------+
   | SL packet payload                       |  N bytes    |
   +-----------------------------------------+-------------+


   Note the compositionTimeStamp is never present since it would be
   redundant with the RTP time stamp. However the value of CTSFlag is 1
   to indicate to the receiver that the value of
   compositionTimeStampFlag for the corresponding reconstructed SL
   packed.

Overhead estimation

   In this example we have a RTP overhead of 40 + 2 bytes for 1400
   bytes of payload i.e. 3 % overhead.

Appendix.2 RFC 3016 compatible MPEG-4 Video

   This is an example of a video stream where the SL is configured to
   produce RTP packets compatible with RFC 3016.

SLConfigDescriptor

   In this example the SLConfigDescriptor is:

   class SLConfigDescriptor extends BaseDescriptor : bit(8)
   tag=SLConfigDescrTag {
    bit(8) predefined;
    if (predefined==0) {
     bit(1) useAccessUnitStartFlag; = 0
     bit(1) useAccessUnitEndFlag; = 1
     bit(1) useRandomAccessPointFlag; = 0
     bit(1) hasRandomAccessUnitsOnlyFlag; = 0
     bit(1) usePaddingFlag; = 0
     bit(1) useTimeStampsFlag; = 0
     bit(1) useIdleFlag; = 0
     bit(1) durationFlag; = 0
     bit(32) timeStampResolution; = 0
     bit(32) OCRResolution; = 0
     bit(8) timeStampLength; = 0
     bit(8) OCRLength; = 0
     bit(8) AU_Length; = 0
     bit(8) instantBitrateLength; = 0
     bit(4) degradationPriorityLength; = 0
     bit(5) AU_seqNumLength; = 0
     bit(5) packetSeqNumLength; = 0
     bit(2) reserved=0b11;
    }
    if (durationFlag) {

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     bit(32) timeScale; // NOT USED
     bit(16) accessUnitDuration;  // NOT USED
     bit(16) compositionUnitDuration;  // NOT USED
    }
    if (!useTimeStampsFlag) {
     bit(timeStampLength) startDecodingTimeStamp; = 0
     bit(timeStampLength) startCompositionTimeStamp; = 0
    }
   }

SL Packet Header structure

   With this configuration we have the following SL packet header
   structure:

   aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) {
    if (SL.useAccessUnitEndFlag) {
     bit(1) accessUnitEndFlag; // 1 bit
    }
   }

Parameters

   This configuration is the default one; no parameters are required.

RTP packet structure

   Note that accessUnitEndFlag is mapped to the RTP header M bit.

   +=========================================+=============+
   | Field                                   |  size       |
   +=========================================+=============+
   | RTP header                              |    -        |
   +-----------------------------------------+-------------+
   | SL packet payload                       | 1400 bytes  |
   +-----------------------------------------+-------------+

Overhead

   In this example we have a RTP overhead of 40 bytes for 1400 bytes of
   payload i.e. 3 % overhead.

Appendix.3 Low delay MPEG-4 Audio

   This example is for a low delay audio service. For this reason a
   single SL packet is transported in each RTP packet.

SLConfigDescriptor

   Since CTS=DTS and AccessUnit duration is constant signaling of MPEG-
   4 time stamps is not needed (the durationFlag is set)



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   We also assume here an audio Object Type for which all Access Units
   are Random Access Points, which is signaled using the
   hasRandomAccessUnitsOnlyFlag in the SLConfigDescriptor.

   We assume furtheremore a mode where the Access Unit size is constant
   and 5 bytes (which is signaled with AU_Length).

   In this example the SLConfigDescriptor is:

   class SLConfigDescriptor extends BaseDescriptor : bit(8)
   tag=SLConfigDescrTag {
    bit(8) predefined;
    if (predefined==0) {
     bit(1) useAccessUnitStartFlag; = 0
     bit(1) useAccessUnitEndFlag; = 0
     bit(1) useRandomAccessPointFlag; = 0
     bit(1) hasRandomAccessUnitsOnlyFlag; = 1
     bit(1) usePaddingFlag; = 0
     bit(1) useTimeStampsFlag; = 0
     bit(1) useIdleFlag; = 0
     bit(1) durationFlag; = 1 // signals constant duration
     bit(32) timeStampResolution; = 0
     bit(32) OCRResolution; = 0
     bit(8) timeStampLength; = 0
     bit(8) OCRLength; = 0
     bit(8) AU_Length; = 5
     bit(8) instantBitrateLength; = 0
     bit(4) degradationPriorityLength; = 0
     bit(5) AU_seqNumLength; = 0
     bit(5) packetSeqNumLength; = 0
     bit(2) reserved=0b11;
    }
    if (durationFlag) {
     bit(32) timeScale; = 1000 // for milliseconds
     bit(16) accessUnitDuration; = 10 // ms
     bit(16) compositionUnitDuration; = 10 // ms
    }
    if (!useTimeStampsFlag) {
     bit(timeStampLength) startDecodingTimeStamp; = 0
     bit(timeStampLength) startCompositionTimeStamp; = 0
    }
   }

SL packet header

   With this configuration the SL packet header is empty.

Parameters

   No parameters are required.

RTP packet structure


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   Note that the RTP header M bit should be always set to 1.

   +=========================================+=============+
   | Field                                   |  size       |
   +=========================================+=============+
   | RTP header                              |    -        |
   +-----------------------------------------+-------------+
   | SL packet payload                       |  5 bytes    |
   +-----------------------------------------+-------------+


Overhead estimation

   The overhead is extremely large i.e. more than 800 %, since 40 bytes
   of headers are required to transport 5 bytes of data. Note however
   that RTP header compression would work well since time stamps
   increments are constant.




Appendix.4 Media delivery MPEG-4 Audio

   This example is for a media delivery service where delay is not an
   issue but efficiency is. In this case several SL Packets are
   transported in each RTP packet.

SLConfigDescriptor

   Is the same as in Appendix.3

SL packet header

   With this configuration the SL packet header is empty.

Parameters

   The absence of RSLHSectionSizeLength indicates that the RSLHSection
   is empty.

   The size of SL Packets (which are all complete Access Units in this
   case) is constant and is indicated  with:

   a=fmtp:<format> SLPPSize=5

   This also indicates to the receiver that the Multiple-SL mode will
   be used, i.e. that a 2 bytes field will give the size of the
   MSLHSection. In this case however this field always contains zero
   since the MSLHSection is empty.


RTP packet structure


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   Note that the RTP header M bit should be always set to 1.

   +=========================================+=============+
   | Field                                   |  size       |
   +=========================================+=============+
   | RTP header                              |    -        |
   +-----------------------------------------+-------------+
   | MSLHSection size in bits = 0            |  2 bytes    |
   +-----------------------------------------+-------------+
   | SL packet payload                       |  5 bytes    |
   +-----------------------------------------+-------------+
   | SL packet payload                       |  5 bytes    |
   +-----------------------------------------+-------------+
   | etc, until MTU is reached                             |
   +-----------------------------------------+-------------+
   | SL packet payload                       |  5 bytes    |
   +-----------------------------------------+-------------+

Overhead estimation

   The overhead is 3% i.e. minimal.

Appendix.5 A more complex case: AAC with interleaving

   Let us consider AAC around 130 kb/s where each Access Unit is split
   in 4 SL packets corresponding to Error Sensitivity Categories (ESC)
   of maximum 90 bytes for which interleaving is very useful in terms
   of error resilience. We will therefore use an interleaving scheme
   where 15 SL Packets from 15 consecutive Access Units will be
   interleaved per RTP packet to match a MTU of 1500 bytes.

   The interleaving sequence is 4 RTP packets and 350 ms long, which is
   too long for conferencing but perfectly OK for Internet radio.

   Since the sequence contains 60 SL packets, the sequence number can
   be encoded on 6 bits. But 2 bits are actually enough if the sender
   always resets the SL packet sequence number to zero at the start of
   each sequence, since only the first MSLH in each of the 4 RTP
   packets in the sequence carries an absolute sequence number value
   (0,1,2,3).

   2 bits are also enough for SLPSeqNumDelta, which is constant and
   equal to 3 (since +1 is automatically added)

   Note that the 4th RTP packet in each sequence has its M bit set to 1
   since it contains 15 SL packets transporting the end of 15 different
   Access Units.

   With this scheme a sender (for example upon reception of RTCP
   reports indicating high loss rates) can ûfor example- choose to
   duplicate for each interleaving sequence the first RTP packet that
   contains the most useful data in terms of ESC or apply other error
   protection techniques, with due care to congestion issues.

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   In this example we will also show several other SL features (OCR, AU
   boundary flags, as detailed below).

   One feature demonstrated by this example is the degradation
   priority. We assume degradation priority can take 4 different
   values, one for each SL packet of an Access Unit and is encoded on 2
   bits. This interleaving scheme makes sure that only SL packets of
   identical degradation priorities are grouped in the same RTP packet
   (3.6.3) and that only the first RSLH of each RTP packet transports
   the degradation priority.

   We also assume that for each last SL packet of each RTP packet the
   server inserts an OCR.


SLConfigDescriptor

   In this example the SLConfigDescriptor is:

   class SLConfigDescriptor extends BaseDescriptor : bit(8)
   tag=SLConfigDescrTag {
    bit(8) predefined;
    if (predefined==0) {
     bit(1) useAccessUnitStartFlag; = 1
     bit(1) useAccessUnitEndFlag; = 1
     bit(1) useRandomAccessPointFlag; = 0
     bit(1) hasRandomAccessUnitsOnlyFlag; = 1
     bit(1) usePaddingFlag; = 0
     bit(1) useTimeStampsFlag; = 0
     bit(1) useIdleFlag; = 0
     bit(1) durationFlag; = 1
     bit(32) timeStampResolution; = 0
     bit(32) OCRResolution; = 30
     bit(8) timeStampLength; = 0
     bit(8) OCRLength; = 32
     bit(8) AU_Length; = 0
     bit(8) instantBitrateLength; = 0
     bit(4) degradationPriorityLength; = 2
     bit(5) AU_seqNumLength; = 0
     bit(5) packetSeqNumLength; = 6
     bit(2) reserved=0b11;
    }
    if (durationFlag) {
     bit(32) timeScale; = 1000// milliseconds
     bit(16) accessUnitDuration; = 23.22 // ms
     bit(16) compositionUnitDuration; = 23.22 // ms
    }
    if (!useTimeStampsFlag) {
     bit(timeStampLength) startDecodingTimeStamp; = 0
     bit(timeStampLength) startCompositionTimeStamp; = 0
    }
   }

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SL Packet Header structure

   With this configuration we have the following SL packet header
   structure:

   aligned(8) class SL_PacketHeader (SLConfigDescriptor SL) {
    bit(1) accessUnitStartFlag;
    bit(1) accessUnitEndFlag;
    bit(1) OCRflag;
    bit(SL.packetSeqNumLength) packetSequenceNumber;
    bit(1) DegPrioflag;
    if (DegPrioflag) {
     bit(SL.degradationPriorityLength) degradationPriority;}
    if (OCRflag) {
     bit(SL.OCRLength) objectClockReference;}
    }
   }

Parameters

   The RSLHSectionSize cannot exceed 2 bits, which is encoded on 2 bits
   and signaled by RSLHSectionSizeLength.

   The resulting concatenated fmtp line is:

   a=fmtp:<format>
   SLPPSizeLength=6;RSLHSectionSizeLength=2;SLPSeqNumLength=2;SLPSeqNum
   DeltaLength=2;OCRDeltaLength=16

RTP packet structure

   +=========================================+=============+
   | Field                                   |  size       |
   +=========================================+=============+
   | RTP header                              |    -        |
   +-----------------------------------------+-------------+
                         MSLHSection
   +=========================================+=============+
   | MSLHSection size in bits = 135          |  2 bytes    |
   +-----------------------------------------+-------------+
   | SLPPayloadSize                          |  7 bits     |
   +-----------------------------------------+-------------+
   | SLPSeqNum = 0 or 1 or 2 or 3            |  2 bits     |
   +-----------------------------------------+-------------+
   | SLPPayloadSize                          |  7 bits     |
   +-----------------------------------------+-------------+
   | SLPSeqDeltaNum = 3                      |  2 bits     |
   +-----------------------------------------+-------------+
   |            etc + 12 times 9 bits                      |
   +-----------------------------------------+-------------+
   | SLPPayloadSize                          |  7 bits     |
   +-----------------------------------------+-------------+

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   | SLPSeqDeltaNum = 3                      |  2 bits     |
   +-----------------------------------------+-------------+
   | bits to byte alignment                  |  7 bits     |
   +-----------------------------------------+-------------+
                         RSLHSection
   +=========================================+=============+
   | RSLHSectionSize                         |  6 bits     |
   +-----------------------------------------+-------------+
   | accessUnitStartFlag                     |  1 bit      |
   +-----------------------------------------+-------------+
   | accessUnitEndFlag                       |  1 bit      |
   +-----------------------------------------+-------------+
   | OCRFlag = 0                             |  1 bit      |
   +-----------------------------------------+-------------+
   | DegPrioflag = 1                         |  1 bit      |
   +-----------------------------------------+-------------+
   | degradationPriority                     |  2 bits     |
   +-----------------------------------------+-------------+
   | accessUnitStartFlag                     |  1 bit      |
   +-----------------------------------------+-------------+
   | accessUnitEndFlag                       |  1 bit      |
   +-----------------------------------------+-------------+
   | OCRFlag = 0                             |  1 bit      |
   +-----------------------------------------+-------------+
   | DegPrioflag = 0                         |  1 bit      |
   +-----------------------------------------+-------------+
   |              etc + 12 times 4 bits                    |
   +-----------------------------------------+-------------+
   | accessUnitStartFlag                     |  1 bit      |
   +-----------------------------------------+-------------+
   | accessUnitEndFlag                       |  1 bit      |
   +-----------------------------------------+-------------+
   | OCRFlag = 1                             |  1 bit      |
   +-----------------------------------------+-------------+
   | OCRDelta                                |  16 bits    |
   +-----------------------------------------+-------------+
   | DegPrioflag = 0                         |  1 bit      |
   +-----------------------------------------+-------------+
   | bits to byte alignment                  |  4 bits     |
   +-----------------------------------------+-------------+
                         SLPPSection
   +=========================================+=============+
   | SL packet payload                       |max 90 bytes |
   +-----------------------------------------+-------------+
   |             etc + 13  SL packets                      |
   +-----------------------------------------+-------------+
   | SL packet payload                       |max 90 bytes |
   +-----------------------------------------+-------------+


Overhead estimation



Gentric et al.            Expires July 2001                        39

                RTP Payload Format for MPEG-4 Streams      April 2001


   The MSLHSection is 19 bytes, the RSLHSection is 10 bytes; in this
   example we have therefore a RTP overhead of 40 + 23 bytes for 1350
   bytes (max) of payload i.e. around 5 % overhead.



















































Gentric et al.            Expires July 2001                        40