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Versions: 00 01 02 03 04 05 06 07 08 RFC 3640

Internet Engineering Task Force                         J. van der Meer
Internet Draft                                      Philips Electronics
                                                              D. Mackie
                                                         Apple Computer
                                                         V. Swaminathan
                                                  Sun Microsystems Inc.
                                                              D. Singer
                                                         Apple Computer
                                                             P. Gentric
                                                    Philips Electronics

                                                          December 2002
                                                      Expires June 2003

   Document: draft-ietf-avt-mpeg4-simple-06.txt


   Transport of MPEG-4 Elementary Streams

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of section 10 of RFC 2026.

   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.

   This specification is a product of the Audio/Video Transport working
   group within the Internet Engineering Task Force. Comments are
   solicited and should be addressed to the working group's mailing
   list at avt@ietf.org and/or the authors.

   << Note for the RFC editor: xxxx should be replaced with the RFC
   number that will be assigned. >>

Abstract

   The MPEG Committee (ISO/IEC JTC1/SC29 WG11) is a working group in
   ISO that produced the MPEG-4 standard. MPEG defines tools to
   compress content such as audio-visual information into elementary
   streams. This specification defines a simple, but generic RTP
   payload format for transport of any non-multiplexed MPEG-4
   elementary stream.


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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Carriage of MPEG-4 elementary streams over RTP . . . . . . .   6
   2.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . .   6
   2.2.  MPEG Access Units  . . . . . . . . . . . . . . . . . . . .   6
   2.3.  Concatenation of Access Units  . . . . . . . . . . . . . .   6
   2.4.  Fragmentation of Access Units  . . . . . . . . . . . . . .   7
   2.5.  Interleaving . . . . . . . . . . . . . . . . . . . . . . .   7
   2.6.  Time stamp information . . . . . . . . . . . . . . . . . .   8
   2.7.  State indication of MPEG-4 system streams  . . . . . . . .   8
   2.8.  Random Access Indication . . . . . . . . . . . . . . . . .   8
   2.9.  Carriage of auxiliary information  . . . . . . . . . . . .   9
   2.10. MIME format parameters and configuring conditional field .   9
   2.11. Global structure of payload format . . . . . . . . . . . .   9
   2.12. Modes to transport MPEG-4 streams  . . . . . . . . . . . .  10
   2.13. Alignment with RFC 3016  . . . . . . . . . . . . . . . . .  10
   3.  Payload format . . . . . . . . . . . . . . . . . . . . . . .  11
   3.1.  Usage of RTP header fields and RTCP  . . . . . . . . . . .  11
   3.2.  RTP payload structure  . . . . . . . . . . . . . . . . . .  12
   3.2.1.  The AU Header Section  . . . . . . . . . . . . . . . . .  12
   3.2.1.1.  The AU-header  . . . . . . . . . . . . . . . . . . . .  12
   3.2.2.  The Auxiliary Section  . . . . . . . . . . . . . . . . .  14
   3.2.3.  The Access Unit Data Section . . . . . . . . . . . . . .  15
   3.2.3.1.  Fragmentation  . . . . . . . . . . . . . . . . . . . .  16
   3.2.3.2.  Interleaving . . . . . . . . . . . . . . . . . . . . .  16
   3.2.3.3.  Constraints for interleaving . . . . . . . . . . . . .  17
   3.2.3.4.  Crucial and non-crucial AUs with MPEG-4 System data  .  20
   3.3.  Usage of this specification  . . . . . . . . . . . . . . .  22
   3.3.1.  General  . . . . . . . . . . . . . . . . . . . . . . . .  22
   3.3.2.  The generic mode . . . . . . . . . . . . . . . . . . . .  22
   3.3.3.  Constant bit rate CELP . . . . . . . . . . . . . . . . .  23
   3.3.4.  Variable bit rate CELP . . . . . . . . . . . . . . . . .  23
   3.3.5.  Low bit rate AAC . . . . . . . . . . . . . . . . . . . .  24
   3.3.6.  High bit rate AAC  . . . . . . . . . . . . . . . . . . .  25
   3.3.7.  Additional modes . . . . . . . . . . . . . . . . . . . .  26
   4.  IANA considerations  . . . . . . . . . . . . . . . . . . . .  27
   4.1.  MIME type registration . . . . . . . . . . . . . . . . . .  27
   4.2.  Registration of mode definitions with IANA . . . . . . . .  32
   4.3.  Concatenation of parameters  . . . . . . . . . . . . . . .  32
   4.4.  Usage of SDP . . . . . . . . . . . . . . . . . . . . . . .  33
   4.4.1.  The a=fmtp keyword . . . . . . . . . . . . . . . . . . .  33
   5.  Security considerations  . . . . . . . . . . . . . . . . . .  33
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  34
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . .  34
   7.1 Normative references . . . . . . . . . . . . . . . . . . . .  34
   7.2 Informative references . . . . . . . . . . . . . . . . . . .  34
   8.  Author addresses . . . . . . . . . . . . . . . . . . . . . .  35





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       APPENDIX: Usage of this payload format . . . . . . . . . . .  37
       A. Examples of delay analysis with interleave  . . . . . . .  37
       A.1 Introduction . . . . . . . . . . . . . . . . . . . . . .  37
       A.2 De-interleaving and error concealment  . . . . . . . . .  37
       A.3 Simple Group interleave  . . . . . . . . . . . . . . . .  37
       A.3.1 Introduction . . . . . . . . . . . . . . . . . . . . .  37
       A.3.2 Determining the de-interleave buffer size  . . . . . .  38
       A.3.3 Determining the maximum displacement . . . . . . . . .  38
       A.4 More subtle group interleave . . . . . . . . . . . . . .  38
       A.4.1 Introduction . . . . . . . . . . . . . . . . . . . . .  38
       A.4.2 Determining the de-interleave buffer size  . . . . . .  39
       A.4.3 Determining the maximum displacement . . . . . . . . .  39
       A.5 Continuous interleave  . . . . . . . . . . . . . . . . .  39
       A.5.1 Introduction . . . . . . . . . . . . . . . . . . . . .  39
       A.5.2 Determining the de-interleave buffer size  . . . . . .  40
       A.5.3 Determining the maximum displacement . . . . . . . . .  41





































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1. Introduction

   The MPEG Committee is Working Group 11 (WG11) in ISO/IEC JTC1 SC29
   that specified the MPEG-1, MPEG-2 and, more recently, the MPEG-4
   standards [1]. The MPEG-4 standard specifies compression of
   audio-visual data into for example an audio or video elementary
   stream. In the MPEG-4 standard, these streams take the form of
   audio-visual objects that may be arranged into an audio-visual scene
   by means of a scene description. Each MPEG-4 elementary stream
   consists of a sequence of Access Units; examples of an Access Unit
   (AU) are an audio frame and a video picture.

   This specification defines a general and configurable payload
   structure to transport MPEG-4 elementary streams, in particular
   MPEG-4 audio (including speech) streams, MPEG-4 video streams and
   also MPEG-4 systems streams, such as BIFS (BInary Format for
   Scenes), OCI (Object Content Information), OD (Object Descriptor)
   and IPMP (Intellectual Property Management and Protection) streams.
   The RTP payload defined in this document is simple to implement and
   reasonably efficient. It allows for optional interleaving of Access
   Units (such as audio frames) to increase error resiliency in packet
   loss.

   Some types of MPEG-4 elementary streams include "crucial"
   information whose loss cannot be tolerated, but RTP does not provide
   reliable transmission so receipt of that crucial information is not
   assured.  Section 3.2.3.4 specifies how stream state is conveyed so
   that the receiver can detect the loss of crucial information and
   cease decoding until the next random access point is received.
   Applications transmitting streams that include crucial information,
   such as OD commands, BIFS commands, or programmatic content such as
   MPEG-J (Java) and ECMAScript, should include random access points
   sufficiently often, depending upon the probability of loss, to
   reduce stream corruption to an acceptable level.  An example is the
   carousel mechanism as defined by MPEG in ISO/IEC 14496-1.

   Such applications may also employ additional protocols or services
   to reduce the probability of loss.  At the RTP layer, these measures
   include payload formats and profiles for retransmission or forward
   error correction (such as in RFC 2733 [10]), which must be employed
   with due consideration to congestion control.  Another solution that
   may be appropriate for some applications is to carry RTP over TCP
   (such as in RFC 2326 [8], section 10.12).  At the network layer,
   resource allocation or preferential service may be available to
   reduce the probability of loss. For a general description of methods
   to repair streaming media see RFC 2354 [9].







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   Though the RTP payload format defined in this document is capable
   of transporting any MPEG-4 stream, other, more specific, formats
   may exist, such as RFC 3016 [12] for transport of MPEG-4 video
   (ISO/IEC 14496 [1] part 2).

   Configuration of the payload is provided to accommodate transport
   of any MPEG-4 stream at any possible bit rate. However, for a
   specific MPEG-4 elementary stream typically only very few
   configurations are needed. So as to allow for the design of
   simplified, but dedicated receivers, this specification requires
   that specific modes are defined for transport of MPEG-4 streams.
   This document defines modes for MPEG-4 CELP and AAC streams, as
   well as a generic mode that can be used to transport any MPEG-4
   stream. In the future new RFCs are expected to specify additional
   modes for transport of MPEG-4 streams.

   The RTP payload format defined in this document specifies carriage
   of system-related information that is often equivalent to the
   information that may be contained in the MPEG-4 Sync Layer (SL) as
   defined in MPEG-4 Systems [1]. This document does not prescribe how
   to transcode or map information from the SL to fields defined in
   the RTP payload format. Such processing, if any, is left to the
   discretion of the application. However, to anticipate the need for
   transport of any additional system-related information in future,
   an auxiliary field can be configured that may carry any such data.

   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 [4].

























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2. Carriage of MPEG-4 elementary streams over RTP

2.1 Introduction

   With this payload format a single MPEG-4 elementary stream can be
   transported. Information on the type of MPEG-4 stream carried in
   the payload is conveyed by MIME format parameters, for example in
   an SDP [5] message or by other means (see section 4). These MIME
   format parameters specify the configuration of the payload. To
   allow for simplified and dedicated receivers, a MIME format
   parameter is available to signal a specific mode of using this
   payload. A mode definition MAY include the type of MPEG-4
   elementary stream as well as the applied configuration, so as to
   avoid the need in receivers to parse all MIME format parameters.
   The applied mode MUST be signaled.

2.2 MPEG Access Units

   For carriage of compressed audio-visual data MPEG defines Access
   Units. An MPEG Access Unit (AU) is the smallest data entity to
   which timing information is attributed. In case of audio an Access
   Unit may represent an audio frame and in case of video a picture.
   MPEG Access Units are by definition octet-aligned. If for example
   an audio frame is not octet-aligned, up to 7 zero-padding bits MUST
   be inserted at the end of the frame to achieve the octet-aligned
   Access Units, as required by the MPEG-4 specification. MPEG-4
   decoders MUST be able to decode AUs in which such padding is
   applied.

   Consistent with the MPEG-4 specification, this document requires
   that each MPEG-4 part 2 video Access Unit includes all the coded
   data of a picture, any video stream headers that may precede the
   coded picture data, and any video stream stuffing that may follow
   it, up to, but not including the startcode indicating the start of
   a new video stream or the next Access Unit.

2.3 Concatenation of Access Units

   Frequently it is possible to carry multiple Access Units in one RTP
   packet. This is particularly useful for audio; for example, when
   AAC is used for encoding of a stereo signal at 64 kbits/sec, AAC
   frames contain on average approximately 200 octets. On a LAN with a
   1500 octet MTU this would allow on average 7 complete AAC frames to
   be carried per AAC packet.

   Access Units may have a fixed size in octets, but a variable size
   is also possible. To facilitate parsing in case of multiple
   concatenated AUs in one RTP packet, the size of each AU is made
   known to the receiver. When concatenating in case of a constant AU
   size, this size is communicated "out of band" through a MIME format
   parameter. When concatenating in case of variable size AUs, the RTP
   payload carries "in band" an AU size field for each contained AU.

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   In combination with the RTP payload length the size information
   allows the RTP payload to be split by the receiver back into the
   individual AUs.

   To simplify the implementation of RTP receivers, it is required
   that when multiple AUs are carried in an RTP packet, each AU MUST
   be complete, i.e. the number of AUs in an RTP packet MUST be
   integral. In addition, an AU MUST NOT be repeated in other RTP
   packets; hence repetition of an AU is only possible by using a
   duplicate RTP packet.

2.4 Fragmentation of Access Units

   MPEG allows for very large Access Units. Since most IP networks
   have significantly smaller MTU sizes, this payload format allows
   for the fragmentation of an Access Unit over multiple RTP packets
   so as to avoid IP layer fragmentation. To simplify the
   implementation of RTP receivers, an RTP packet SHALL either carry
   one or more complete Access Units or a single fragment of one
   Access Unit (i.e. packets MUST NOT contain fragments of multiple
   Access Units).

2.5 Interleaving

   When an RTP packet carries a contiguous sequence of Access Units,
   the loss of such a packet can result in a "decoding gap" for the
   user. One method to alleviate this problem is to allow for the
   Access Units to be interleaved in the RTP packets. For a modest
   cost in latency and implementation complexity, significant error
   resiliency to packet loss can be achieved.

   To support optional interleaving of Access Units, this payload
   format allows for index information to be sent for each Access Unit.
   After informing receivers about buffer resources to allocate for
   de-interleaving, the RTP sender is free to choose the interleaving
   pattern without propagating this information a priori to the
   receiver(s). Indeed the sender could dynamically adjust the
   interleaving pattern based on the Access Unit size, error rates,
   etc. The RTP receiver does not need to know the interleaving
   pattern used, it only needs to extract the index information of the
   Access Unit and insert the Access Unit into the appropriate
   sequence in the decoding or rendering queue. An example of
   interleaving is given below.

   Assume that an RTP packet contains 3 AUs, and that the AUs are
   numbered 0, 1, 2, 3, 4, etc. If an interleaving group length of 9 is
   chosen, then RTP packet(i) contains the following AU(n):
   RTP packet(0):  AU(0),  AU(3),  AU(6)
   RTP packet(1):  AU(1),  AU(4),  AU(7)
   RTP packet(2):  AU(2),  AU(5),  AU(8)
   RTP packet(3):  AU(9),  AU(12), AU(15)
   RTP packet(4):  AU(10), AU(13), AU(16)
   Etc.

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2.6 Time stamp information

   The RTP time stamp MUST carry the sampling instant of the first AU
   (fragment) in the RTP packet. When multiple AUs are carried within
   an RTP packet, the time stamps of subsequent AUs can be calculated
   if the frame period of each AU is known. For audio and video this
   is possible if the frame rate is constant. However, in some cases
   it is not possible to make such calculation, for example for
   variable frame rate video and for MPEG-4 BIFS streams carrying
   composition information. To support such cases, this payload format
   can be configured to carry a time stamp in the RTP payload for each
   contained Access Unit. A time stamp MAY be conveyed in the RTP
   payload only for non-first AUs in the RTP packet, and SHALL NOT be
   conveyed for the first AU (fragment), as the time stamp for the
   first AU in the RTP packet is carried by the RTP time stamp.

   MPEG-4 defines two type of time stamps, the composition time stamp
   (CTS) and the decoding time stamp (DTS). The CTS represents the
   sampling instant of an AU, and hence the CTS is equivalent to the
   RTP time stamp. The DTS may be used in MPEG-4 video streams that
   use bi-directional coding, i.e. when pictures are predicted in both
   forward and backward direction by using either a reference picture
   in the past, or a reference picture in the future. The DTS cannot
   be carried in the RTP header. In some cases the DTS can be derived
   from the RTP time stamp using frame rate information; this requires
   deep parsing in the video stream, which may be considered
   objectionable. But if the video frame rate is variable, the required
   information may not even be present in the video stream. For both
   reasons, the capability has been defined to optionally carry the
   DTS in the RTP payload for each contained Access Unit.

   To keep the coding of time stamps efficient, each time stamp
   contained in the RTP payload is coded differentially, the CTS from
   the RTP time stamp, and the DTS from the CTS.

2.7 State indication of MPEG-4 system streams

   ISO/IEC 14496-1 defines states for MPEG-4 system streams. So as to
   convey state information when transporting MPEG-4 system streams,
   this payload format allows for the optional carriage in the RTP
   payload of the stream state for each contained Access Unit. Stream
   states are used to signal "crucial" AUs that carry information whose
   loss cannot be tolerated and are also useful when repeating AUs
   according to the carousel mechanism defined in ISO/IEC 14496-1.

2.8 Random access indication

   Random access to the content of MPEG-4 elementary streams may be
   possible at some but not all Access Units. To signal Access Units
   where random access is possible, a random access point flag can




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   optionally be carried in the RTP payload for each contained Access
   Unit. Carriage of random access points is particularly useful for
   MPEG-4 system streams in combination with the stream state.

2.9 Carriage of auxiliary information.

   This payload format defines a specific field to carry auxiliary
   data. The auxiliary data field is preceded by a field that specifies
   the length of the auxiliary data, so as to facilitate skipping of
   the data without parsing it. The coding of the auxiliary data is not
   defined in this document; instead the format, meaning and signaling
   of auxiliary information is expected to be specified in one or more
   future RFCs. Auxiliary information MUST NOT be transmitted until its
   format, meaning and signaling have been specified and its use has
   been signaled. Receivers that have knowledge of the auxiliary data
   MAY decode the auxiliary data, but receivers without knowledge of
   such data MUST skip the auxiliary data field.

2.10 MIME format parameters and configuring conditional fields

   To support the features described in the previous sections several
   fields are defined for carriage in the RTP payload. However, their
   use strongly depends on the type of MPEG-4 elementary stream that
   is carried. Sometimes a specific field is needed with a certain
   length, while in other cases such field is not needed at all. To be
   efficient in either case, the fields to support these features are
   configurable by means of MIME format parameters. In general, a MIME
   format parameter defines the presence and length of the associated
   field. A length of zero indicates absence of the field. As a
   consequence, parsing of the payload requires knowledge of MIME
   format parameters. The MIME format parameters are conveyed to the
   receiver via SDP [5] messages, as specified in section 4.4.1, or
   through other means.

2.11 Global structure of payload format

   The RTP payload following the RTP header, contains three
   octet-aligned data sections, of which the first two MAY be empty.
   See figure 1.

          +---------+-----------+-----------+---------------+
          | RTP     | AU Header | Auxiliary | Access Unit   |
          | Header  | Section   | Section   | Data Section  |
          +---------+-----------+-----------+---------------+

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

   Figure 1: Data sections within an RTP packet


   The first data section is the AU (Access Unit) Header Section, that
   contains one or more AU-headers; however, each AU-header MAY be
   empty, in which case the entire AU Header Section is empty. The

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   second section is the Auxiliary Section, containing auxiliary data;
   this section MAY also be configured empty. The third section is the
   Access Unit Data Section, containing either a single fragment of
   one Access Unit or one or more complete Access Units. The Access
   Unit Data Section MUST NOT be empty.

2.12 Modes to transport MPEG-4 streams

   While it is possible to build fully configurable receivers capable
   of receiving any MPEG-4 stream, this specification also allows for
   the design of simplified, but dedicated receivers, that are capable
   for example of receiving only one type of MPEG-4 stream. This
   is achieved by requiring that specific modes be defined for using
   this specification. Each mode may define constraints for transport
   of one or more type of MPEG-4 streams, for instance on the payload
   configuration.

   The applied mode MUST be signaled. Signaling the mode is
   particularly important for receivers that are only capable of
   decoding one or more specific modes. Such receivers need to
   determine whether the applied mode is supported, so as to avoid
   problems with processing of payloads that are beyond the
   capabilities of the receiver.

   In this document several modes are defined for transport of MPEG-4
   CELP and AAC streams, as well as a generic mode that can be used
   for any MPEG-4 stream. In the future, new RFCs may specify other
   modes of using this specification. However, each mode MUST be in
   full compliance with this specification (see section 3.3.7).

2.13 Alignment with RFC 3016

   This payload can be configured to be nearly identical to the
   payload format defined in RFC 3016 [12] for the MPEG-4 video
   configurations recommended in RFC 3016. Hence, receivers that
   comply with RFC 3016 can decode such RTP payload, providing that
   additional packets containing video decoder configuration (VO,
   VOL, VOSH) are inserted in the stream, as required by RFC 3016.
   Conversely, receivers that comply with the specification in this
   document should be able to decode payloads, names and parameters
   defined for MPEG-4 video in RFC 3016. In this respect it is
   strongly RECOMMENDED to implement the ability to ignore "in band"
   video decoder configuration packets in the RFC 3016 payload.

   Note the "out of band" availability of the video decoder
   configuration is optional in RFC 3016. To achieve maximum
   interoperability with the RTP payload format defined in this
   document, applications that use RFC 3016 to transport MPEG-4 video
   (part 2) are recommended to make the video decoder configuration
   available as a MIME parameter.




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3. Payload Format

3.1 Usage of RTP Header Fields and RTCP

   Payload Type (PT): The assignment of an RTP payload type for this
   packet format is outside the scope of this document; it is
   specified by the RTP profile under which this payload format is
   used.

   Marker (M) bit: The M bit is set to 1 to indicate that the RTP
   packet payload contains either the final fragment of a fragmented
   Access Unit or one or more complete Access Units.

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

   Sequence Number: The RTP sequence number SHOULD be generated by the
   sender in the usual manner with a constant random offset.

   Timestamp: Indicates the sampling instant of the first AU
   contained in the RTP payload. This sampling instant is equivalent
   to the CTS in the MPEG-4 time domain. When using SDP the clock rate
   of the RTP time stamp MUST be expressed using the "rtpmap"
   attribute. If an MPEG-4 audio stream is transported, the rate SHOULD
   be set to the same value as the sampling rate of the audio stream.
   If an MPEG-4 video stream is transported, it is RECOMMENDED to set
   the rate to 90 kHz.

   In all cases, the sender SHALL make sure that RTP time stamps
   are identical only if the RTP time stamp refers to fragments of the
   same Access Unit.

   According to RFC 1889 [2] (section 5.1), RTP time stamps are
   RECOMMENDED to start at a random value for security reasons. This
   is not an issue for synchronization of multiple RTP streams. When,
   however, streams from multiple sources are to be synchronized (for
   example one stream from local storage, another from an RTP streaming
   server), synchronization may become impossible if the receiver only
   knows the original time stamp relationships. Synchronization in such
   cases, may require to provide the correct relationship between time
   stamps for obtaining synchronization by out of band means. The
   format of such information as well as methods to convey such
   information are beyond the scope of this specification.

   SSRC: set as described in RFC 1889 [2].

   CC and CSRC fields are used as described in RFC 1889 [2].

   RTCP SHOULD be used as defined in RFC 1889 [2]. Note that time
   stamps in RTCP Sender Reports may be used to synchronize multiple
   MPEG-4 elementary streams and also to synchronize MPEG-4 streams
   with non-MPEG-4 streams, in case the delivery of these streams uses
   RTP.


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3.2 RTP Payload Structure

3.2.1 The AU Header Section

   When present, the AU Header Section consists of the
   AU-headers-length field, followed by a number of AU-headers. See
   figure 2.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+-+
   |AU-headers-length|AU-header|AU-header|      |AU-header|padding|
   |                 |   (1)   |   (2)   |      |   (n)   | bits  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+-+

   Figure 2: The AU Header Section

   The AU-headers are configured using MIME format parameters and MAY
   be empty. If the AU-header is configured empty, the
   AU-headers-length field SHALL NOT be present and consequently the
   AU Header Section is empty. If the AU-header is not configured
   empty, then the AU-headers-length is a two octet field that
   specifies the length in bits of the immediately following
   AU-headers, excluding the padding bits.

   Each AU-header is associated with a single Access Unit (fragment)
   contained in the Access Unit Data Section in the same RTP packet.
   For each contained Access Unit (fragment) there is exactly one
   AU-header. Within the AU Header Section, the AU-headers are
   bit-wise concatenated in the order in which the Access Units are
   contained in the Access Unit Data Section. Hence, the n-th
   AU-header refers to the n-th AU (fragment). If the concatenated
   AU-headers consume a non-integer number of octets, up to 7
   zero-padding bits MUST be inserted at the end in order to achieve
   octet-alignment of the AU Header Section.

3.2.1.1 The AU-header

   Each AU-header may contain the fields given in figure 3. The length
   in bits of the above fields with the exception of the CTS-flag, the
   DTS-flag and the RAP-flag fields is defined by MIME format
   parameters; see section 4.1. If a MIME format parameter has the
   default value of zero, then the associated field is not present.
   The number of bits for fields that are present and that represent
   the value of a parameter MUST be chosen large enough to correctly
   encode the largest value of that parameter during the session.

   If present, the fields MUST occur in the mutual order given in
   figure 3. In the general case a receiver can only discover the size
   of an AU-header by parsing it since the presence of the CTS-delta
   and DTS-delta fields is signaled by the value of the CTS-flag and
   DTS-flag, respectively.



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   +---------------------------------------+
   |     AU-size                           |
   +---------------------------------------+
   |     AU-Index / AU-Index-delta         |
   +---------------------------------------+
   |     CTS-flag                          |
   +---------------------------------------+
   |     CTS-delta                         |
   +---------------------------------------+
   |     DTS-flag                          |
   +---------------------------------------+
   |     DTS-delta                         |
   +---------------------------------------+
   |     RAP-flag                          |
   +---------------------------------------+
   |     Stream-state                      |
   +---------------------------------------+

   Figure 3: The fields in the AU-header. If used, the AU-Index field
             only occurs in the first AU-header within an AU Header
             Section; in any other AU-header the AU-Index-delta field
             occurs instead.

   AU-size: Indicates the size in octets of the associated Access Unit
         in the Access Unit Data Section in the same RTP packet. When
         the AU-size is associated with an AU fragment, the AU size
         indicates the size of the entire AU and not the size of the
         fragment. In this case, the size of the fragment is known
         from the size of the AU data section. This can be exploited
         to determine whether a packet contains an entire AU or a
         fragment, which is particularly useful after losing a packet
         carrying the last fragment of an AU.

   AU-Index: Indicates the serial number of the associated Access Unit
         (fragment). For each (in decoding order) consecutive AU or AU
         fragment, the serial number is incremented with 1. When
         present, the AU-Index field occurs in the first AU-header in
         the AU Header Section, but MUST NOT occur in any subsequent
         (non-first) AU-header in that Section. To encode the serial
         number in any such non-first AU-header, the AU-Index-delta
         field is used.

   AU-Index-delta: The AU-Index-delta field is an unsigned integer
         that specifies the serial number of the associated AU as the
         difference with respect to the serial number of the previous
         Access Unit. Hence, for the n-th (n>1) AU the serial number
         is found from:
         AU-Index(n) = AU-Index(n-1) + AU-Index-delta(n) + 1
         If the AU-Index field is present in the first AU-header in




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         the AU Header Section, then the AU-Index-delta field MUST be
         present in any subsequent (non-first) AU-header. When the
         AU-Index-delta is coded with the value 0, it indicates that
         the Access Units are consecutive in decoding order. An
         AU-Index-delta value larger than 0 signals that interleaving
         is applied.

   CTS-flag: Indicates whether the CTS-delta field is present.
         A value of 1 indicates that the field is present, a value
         of 0 that it is not present.
         The CTS-flag field MUST be present in each AU-header if the
         length of the CTS-delta field is signaled to be larger than
         zero. In that case, the CTS-flag field MUST have the value 0
         in the first AU-header and MAY have the value 1 in all
         non-first AU-headers. The CTS-flag field SHOULD be 0 for
         any non-first fragment of an Access Unit.

   CTS-delta: Encodes the CTS by specifying the value of CTS as a 2's
         complement offset (delta) from the time stamp in the RTP
         header of this RTP packet. The CTS MUST use the same clock
         rate as the time stamp in the RTP header.

   DTS-flag: Indicates whether the DTS-delta field is present. A value
         of 1 indicates that DTS-delta is present, a value of 0 that
         it is not present.
         The DTS-flag field MUST be present in each AU-header if the
         length of the DTS-delta field is signaled to be larger than
         zero. The DTS-flag field MUST have the same value for all
         fragments of an Access Unit.

   DTS-delta: Specifies the value of the DTS as a 2's complement
         offset (delta) from the CTS. The DTS MUST use the
         same clock rate as the time stamp in the RTP header. The
         DTS-delta field MUST have the same value for all fragments of
         an Access Unit.

   RAP-flag: Indicates when set to 1 that the associated Access Unit
         provides a random access point to the content of the stream.
         If an Access Unit is fragmented, the RAP flag, if present,
         MUST be set to 0 for each non-first fragment of the AU.

   Stream-state:  Specifies the state of the stream for an AU of an
         MPEG-4 system stream; each state is identified by a value of
         a modulo counter. In ISO/IEC 14496-1, MPEG-4 system streams
         use the AU_SequenceNumber to signal stream states. When the
         stream state changes, the value of stream-state MUST be
         incremented by one.

         Note: no relation is required between stream-states of
         different streams.



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3.2.2 The Auxiliary Section

   The Auxiliary Section consists of the auxiliary-data-size field
   followed by the auxiliary-data field. Receivers MAY (but are not
   required to) parse the auxiliary-data field; to facilitate skipping
   of the auxiliary-data field by receivers, the auxiliary-data-size
   field indicates the length in bits of the auxiliary-data. If the
   concatenation of the auxiliary-data-size and the auxiliary-data
   fields consume a non-integer number of octets, up to 7 zero padding
   bits MUST be inserted immediately after the auxiliary data in order
   to achieve octet-alignment. See figure 4.

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+
   | auxiliary-data-size   | auxiliary-data       |padding bits |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+

   Figure 4: The fields in the Auxiliary Section


   The length in bits of the auxiliary-data-size field is configurable
   by a MIME format parameter; see section 4.1. The default length of
   zero indicates that the entire Auxiliary Section is absent.

   auxiliary-data-size: specifies the length in bits of the immediately
         following auxiliary-data field;

   auxiliary-data: the auxiliary-data field contains data of a format
         not defined by this specification.

3.2.3 The Access Unit Data Section

   The Access Unit Data Section contains an integer number of complete
   Access Units or a single fragment of one AU. The Access Unit Data
   Section is never empty. If data of more than one Access Unit is
   present, then the AUs are concatenated into a contiguous string
   of octets. See figure 5. The AUs inside the Access Unit Data
   Section MUST be in decoding order, though not necessarily contiguous
   in the case of interleaving.

   The size and number of Access Units 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 result in reduced performance.











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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AU(1)                                                          |
   +                                                               |
   |                                                               |
   |               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               |AU(2)                                          |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               | AU(n)                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |AU(n) continued|
   |-+-+-+-+-+-+-+-+

   Figure 5: Access Unit Data Section; each AU is octet-aligned.


   When multiple Access Units are carried, the size of each AU MUST be
   made available to the receiver. If the AU size is variable then the
   size of each AU MUST be indicated in the AU-size field of the
   corresponding AU-header. However, if the AU size is constant for a
   stream, this mechanism SHOULD NOT be used, but instead the fixed
   size SHOULD be signaled by the MIME format parameter
   "ConstantSize", see section 4.1.

   The absence of both AU-size in the AU-header and the ConstantSize
   MIME format parameter indicates carriage of a single AU (fragment),
   i.e. that a single Access Unit (fragment) is transported in each
   RTP packet for that stream.

3.2.3.1 Fragmentation

   A packet SHALL carry either one or more complete Access Units, or
   a single fragment of an Access Unit.  Fragments of the same Access
   Unit have the same time stamp but different RTP sequence numbers.
   The marker bit in the RTP header is 1 on the last fragment of an
   Access Unit, and 0 on all other fragments.

3.2.3.2 Interleaving

   Access Units MAY be interleaved. Senders MAY perform interleaving.
   Receivers MUST support interleaving, except if the receiver only
   supports modes in which no interleaving is allowed. When Access
   Units are interleaved, it SHALL be implemented using the AU-Index
   and the AU-Index-delta fields in the AU-header.

   When a sender interleaves Access Units, then the transmitter needs
   to provide sufficient information to enable a receiver to
   unambiguously reconstruct the original order, even in case of
   out-of-order packets, packet loss or duplication. The information


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   that senders need to provide depends on whether or not the Access
   Units have a constant time duration. Access Units have a constant
   time duration, if:

      TS(i+1) รป TS(i) = constant, for any i, where

            i indicates the index of the AU in original order
            TS(i) denotes the time stamp of AU(i)

   If Access Units have a constant time duration then a receiver can
   unambiguously reconstruct the original order based on the RTP
   time stamp, the AU-Index and the AU-Index-delta. Note that for this
   purpose the AU-Index is redundant, as the RTP time stamp and the
   AU-Index-delta values are sufficient for placing the AUs correctly
   in time. The RTP time stamp usually provides better robustness to
   large bursts of packet losses, and is therefore to be preferred.
   In order to unambiguously determine the index of each AU in the
   most convenient way when the AUs have a constant time duration, the
   value of the time duration SHOULD be signaled by the MIME format
   parameter "constantDuration", see section 4.1.

   If the "constantDuration" parameter is present, then the transmitter
   MUST encode the AU-Index, if present, with the value 0 and the
   receiver MUST use the RTP time stamp to determine the index of the
   first AU in the RTP packet.

   If the "constantDuration" parameter is not present, then Access
   Units are assumed to have a variable duration. In this case, the
   AU-Index is not redundant, and MUST provide the index information
   required for re-ordering, and the receiver MUST use that value to
   determine the index of the first AU in the RTP packet. The number
   of bits of the AU-Index field MUST be chosen so that valid index
   information is provided at the applied interleaving scheme, without
   causing problems due to roll-over of the AU-Index field. For
   variable duration AUs, index information is needed to reconstruct
   the original order and to identify missing AUs, but to place the
   AUs correctly in time, for each AU the time stamp is needed.
   Therefore, if the "constantDuration" parameter is not present, then
   the CTS-delta MUST be coded in the AU header for each non-first AU
   in the RTP packet.

   When interleaving is applied, a de-interleave buffer is needed in
   receivers to put the Access Units in their correct logical
   consecutive decoding order. This requires the computation of the
   time stamp for each Access Unit. In case of a constant time duration
   per Access Unit, the time stamp of the i-th access unit in an RTP
   packet with RTP time stamp T is calculated as follows:

   Timestamp[0] = T
   Timestamp[i, i > 0] = T +(Sum(for k=1 to i of (AU-Index-delta[k]
                         + 1))) * access-unit-duration


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   When AU-Index-delta is always 0, this reduces to T + i * (access-
   unit-duration). This is the non-interleaved case, where the frames
   are consecutive in decoding order. Note that the AU-Index field
   (present for the first Access Unit) is indeed not needed in this
   calculation.


3.2.3.3 Constraints for interleaving

   The size of the packets should be suitably chosen to be appropriate
   to both the path MTU and the capacity of the receiver's
   de-interleave buffer. The maximum packet size for a session SHOULD
   be chosen not to exceed the path MTU.

   To allow receivers to allocate sufficient resources for
   de-interleaving, senders MUST provide the information to receivers
   as specified in this section.

   AUs enter the decoder in decoding order. The de-interleave buffer
   is used to re-order a stream of interleaved AUs back into decoding
   order. When interleaving is applied, the decoding of "early" AUs
   has to be postponed until all AUs that precede in decoding order
   are present. Therefore these "early" AUs are stored in the
   de-interleave buffer. As an example in figure 6 the interleaving
   pattern from section 2.5 is considered.

                             +--+--+--+--+--+--+--+--+--+--+--+-
   Interleaved AUs           | 0| 3| 6| 1| 4| 7| 2| 5| 8| 9|12|..
                             +--+--+--+--+--+--+--+--+--+--+--+-
   Storage of "early" AUs         3  3  3  3  3  3
                                     6  6  6  6  6  6
                                           4  4  4
                                              7  7  7
                                                            12 12

   Figure 6: Storage of "early" AUs in the de-interleave buffer per
             interleaved AU.

   AU(3) is to be delivered to the decoder after AU(0), AU(1)and
   AU(2); of these AUs, AU(2) is most late and hence AU(3) needs to be
   stored until AU(2) is present in the pattern. Similarly, AU(6) is
   to be stored until AU(5) is present, while AU(4) and AU(7) are to
   be stored until AU(2) and AU(5) are present, respectively. Note
   that the fullness of the de-interleave buffer varies in time. In
   figure 6, the de-interleave buffer contains at most 4, but often
   less AUs.

   So as to give a rough indication of the resources needed in the
   receiver for de-interleaving, the maximum displacement in time of
   an AU is defined. For any AU in the pattern it can be verified

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   which AUs are not yet present. The maximum displacement in time of
   an AU is the maximum difference between the time stamp of an AU in
   the pattern and the time stamp of the earliest AU that is not yet
   present. In other words, when considering a sequence of interleaved
   AUs, then:

   Maximum displacement = max{TS(i) - TS(j)}, for any i and any j>i,

            where i and j indicate the index of the AU in the
                  interleaving pattern and TS denotes the time stamp
                  of the AU

   As an example in figure 7 the interleaving pattern from section 2.5
   is considered. For each AU in the pattern the earliest not yet
   present AU  is indicated. A "-" indicates that all previous AUs
   are present. If the AU period is constant, the maximum displacement
   equals 5 AU periods, as found for AU(6) and AU(7).

                                 +--+--+--+--+--+--+--+--+--+--+--+-
   Interleaved AUs               | 0| 3| 6| 1| 4| 7| 2| 5| 8| 9|12|..
                                 +--+--+--+--+--+--+--+--+--+--+--+-

   Earliest not yet present AU     -  1  1  -  2  2  -  -  -  - 10

   Figure 7: The earliest not yet present AU for each AU in the
             interleaving pattern.


   When interleaving, senders MUST signal the maximum displacement
   in time during the session via the MIME format parameter
   "maxDisplacement"; see section 4.1.

   An estimate of the size of the de-interleave buffer is found by
   multiplying the maximum displacement by the maximum bit rate:

   size(de-interleave buffer) = {(maxDisplacement) * Rate(max)} / (RTP
                                clock frequency),

   where Rate(max) is the maximum bit-rate of the transported stream.

   Note that receivers can derive Rate(max) from the MIME format
   parameters StreamType, Profile-level-id, and config.

   However, this calculation estimates the size of the de-interleave
   buffer and the really required size may differ from the calculated
   value. If this calculation under-estimates the size of the
   de-interleave buffer, then senders, when interleaving, MUST signal
   a size of the de-interleave buffer via the MIME format parameter
   "de-interleaveBufferSize"; see section 4.1. If the calculation



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   over-estimates the size of the de-interleave buffer, then senders,
   when interleaving, MAY signal a size of the de-interleave buffer
   via the MIME format parameter "de-interleaveBufferSize".

   The signaled size of the de-interleave buffer MUST be large enough
   to contain all "early" AUs at any point in time during the session,
   that is:

   minimum de-interleave buffer size = max [sum {if TS(i) > TS(j) then
                                       AU-size(i) else 0}] for any j
                                                    and any i<j, where

              i and j indicate the index of an AU in the interleaving
                      pattern,
              TS(i) denotes the time stamp of AU(i), and
              AU-size(i) denotes the size of AU(i) in number of octets.

   If the "de-interleaveBufferSize" parameter is present, then the
   applied buffer for de-interleaving in a receiver MUST have a size
   that is at least equal to the signaled size of the de-interleave
   buffer, else a size that is at least equal to the calculated size
   of the de-interleave buffer.

   No matter what interleaving scheme is used, the scheme must be
   analyzed to calculate the applicable maxDisplacement value, as well
   as the required size of the de-interleave buffer. Senders SHOULD
   signal values that are not larger than the strictly required
   values; if larger values are signalled, the receiver will buffer
   excessively.

   Note that for low bit-rate material, the applied interleaving
   may make packets shorter than the MTU size.

3.2.3.4. Crucial and non-crucial AUs with MPEG-4 System data

   Some Access Units with MPEG-4 system data, called "crucial" AUs,
   carry information whose loss cannot be tolerated, either in the
   presentation or in the decoder. At each crucial AU in an MPEG-4
   system stream, the stream state changes. The stream-state MAY
   remain constant at non-crucial AUs. In ISO/IEC 14496-1, MPEG-4
   system streams use the AU_SequenceNumber to signal stream states.

   Example: Given three AUs, AU1 = "Insertion of node X", AU2 = "Set
   position of node X", AU3 = "Set position of node X". AU1 is crucial,
   since if it is lost, AU2 cannot be executed. However, AU2 is not
   crucial, since AU3 can be executed even if AU2 is lost.

   When a crucial AU is (possibly) lost, the stream is corrupted. For
   example, when an AU is lost and the stream state has changed at the
   next received AU, then it is possible that the lost AU was crucial.
   Once corrupted, the stream remains corrupted until the next random
   access point. Note that loss of non-crucial AUs does not corrupt the


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   stream. When a decoder starts receiving a stream, the decoder MUST
   consider the stream corrupted until an AU is received that provides
   a random access point.

   An AU that provides a random access point, as signaled by the
   RAP-flag, may be crucial or not. Non-crucial RAP AUs provide a
   "repeated" random access point for use by decoders that recently
   joined the stream or that need to re-start decoding after a stream
   corruption. Non-crucial RAP AUs MUST include all updates since the
   last crucial RAP AU.

   Upon receiving AUs, decoders are to react as follows:
   a) if the RAP-flag is set to 1 and the stream-state changes, then
      the AU is a crucial RAP AU, and the AU MUST be decoded.
   b) if the RAP-flag is set to 1 and the stream state does not change,
      then the AU is a non-crucial RAP AU, and the receiver SHOULD
      decode it if the stream is corrupted. Otherwise, the decoder MUST
      ignore the AU.
   c) if the RAP-flag is set to 0, then the AU MUST be decoded, unless
      the stream is corrupted, in which case the AU MUST be ignored.


































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3.3 Usage of this specification

3.3.1 General

   Usage of this specification requires definition of a mode. A mode
   defines how to use this specification, as deemed appropriate.
   Senders MUST signal the applied mode via the MIME format parameter
   "Mode", as specified in section 4.1. This specification defines a
   generic mode that can be used for any MPEG-4 stream, as well as
   specific modes for transport of MPEG-4 CELP and MPEG-4 AAC streams,
   defined in ISO/IEC 14496-3.

   When use of this payload format is signaled using SDP [5], an
   "rtpmap" attribute is part of that signaling.  The same requirements
   apply for the rtpmap attribute in any mode compliant to this
   specification. The general form of an rtpmap attribute is:
   a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding
             parameters>]
   For audio streams, <encoding parameters> specifies the number of
   audio channels: 2 for stereo material (see RFC 2327 [5]) and 1 for
   mono. Provided no additional parameters are needed, this parameter
   may be omitted for mono material, hence its default value is 1.

3.3.2 The generic mode

   The generic mode can be used for any MPEG-4 stream. In this mode
   no mode-specific constraints are applied; hence, in the generic
   mode the full flexibility of this specification can be exploited.
   The generic mode is signaled by mode=generic.

   An example is given below for transport of a BIFS stream. In this
   example carriage of multiple BIFS Access Units is allowed in one
   RTP packet. The AU-header contains the AU-size field, the CTS-flag
   and, if the CTS flag is set to 1, the CTS-delta field. The number
   of bits of the AU-size and the CTS-delta fields is 10 and 16,
   respectively. The AU-header also contains the RAP-flag and the
   Stream-state of 4 bits. This results in an AU-header with a
   total size of two or four octets per BIFS AU. The RTP time stamp
   uses a 1 kHz clock. Note that the media type name is video,
   because the BIFS stream is part of an audio-visual presentation. For
   conventions on media type names see section 4.1.

   In detail:
   m=video 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/1000
   a=fmtp:96 streamtype=3; profile-level-id=257; mode=generic;
   ObjectType=2; config=BIFSConfiguration(); SizeLength=10;
   CTSDeltaLength=16; RandomAccessIndication=1;
   StreamStateIndication=4
   Note: The a=fmtp line has been wrapped to fit the page, it comprises
         a single line in the SDP file.
   BIFSConfiguration() is the hexadecimal string as defined in ISO/IEC
   14496-1; for the description of MIME parameters see section 4.1.

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3.3.3 Constant bit-rate CELP

   This mode is signaled by mode=CELP-cbr. In this mode one or more
   complete CELP frames of fixed size can be transported in one RTP
   packet; there is no support for interleaving. The RTP payload
   consists of one or more concatenated CELP frames, each of the same
   size. CELP frames MUST not be fragmented when using this mode. Both
   the AU Header Section and the Auxiliary Section MUST be empty.

   The MIME format parameter ConstantSize MUST be provided to specify
   the length of each CELP frame.

   For example:

   m=audio 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/44100/2
   a=fmtp:96 streamtype=5; profile-level-id=15; mode=CELP-cbr; config=
   AudioSpecificConfig(); ConstantSize=xxx;

   Note: The a=fmtp line has been wrapped to fit the page, it comprises
         a single line in the SDP file.

   AudioSpecificConfig() is the hexadecimal string as defined in
   ISO/IEC 14496-3. AudioSpecificConfig() specifies that the audio
   stream type is CELP. For the description of MIME parameters see
   section 4.1.

3.3.4 Variable bit-rate CELP

   This mode is signaled by mode=CELP-vbr. With this mode one or more
   complete CELP frames of variable size can be transported in one RTP
   packet with optional interleaving. As CELP frames are very small,
   while the largest possible AU-size in this mode is greater than the
   maximum CELP frame size, there is no support for fragmentation of
   CELP frames. Hence CELP frames MUST not be fragmented when using
   this mode.

   In this mode the RTP payload consists of the AU Header Section,
   followed by one or more concatenated CELP frames. The Auxiliary
   Section MUST be empty. For each CELP frame contained in the payload
   there MUST be a one octet AU-header in the AU Header Section to
   provide:
   (a) the size of each CELP frame in the payload and
   (b) index information for computing the sequence (and hence timing)
       of each CELP frame.

   Transport of CELP frames requires that the AU-size field is coded
   with 6 bits. In this mode therefore 6 bits are allocated to the
   AU-size field, and 2 bits to the AU-Index(-delta) field. Each
   AU-Index field MUST be coded with the value 0. In the AU Header




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   Section, the concatenated AU-headers are preceded by the 16-bit
   AU-headers-length field, as specified in section 3.2.1.

   In addition to the required MIME format parameters, the following
   parameters MUST be present: SizeLength, IndexLength, and
   IndexDeltaLength. CELP frames have fixed time duration per Access
   Unit; when interleaving in this mode, the applicable duration MUST
   be signaled by the MIME format parameter constantDuration. In
   addition, the parameter maxDisplacement MUST be present when
   interleaving.

   For example:

   m=audio 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/8000/1
   a=fmtp:96 streamtype=5; profile-level-id=15; mode=CELP-vbr; config=
   AudioSpecificConfig(); SizeLength=6; IndexLength=2;
   IndexDeltaLength=2; constantDuration=xxx; maxDisplacement=yyy

   Note: The a=fmtp line has been wrapped to fit the page, it comprises
         a single line in the SDP file.

   AudioSpecificConfig() is the hexadecimal string as defined in
   ISO/IEC 14496-3, AudioSpecificConfig() specifies that the audio
   stream type is CELP. For the description of MIME parameters see
   section 4.1.

3.3.5 Low bit-rate AAC

   This mode is signaled by mode=AAC-lbr. This mode supports transport
   of one or more complete AAC frames of variable size. In this mode
   the AAC frames are allowed to be interleaved and hence receivers
   MUST support de-interleaving. The maximum size of an AAC frame in
   this mode is 63 octets. AAC frames MUST not be fragmented when
   using this mode.


   The payload configuration in this mode is the same as in the
   variable bit-rate CELP mode as defined in 3.3.4. The RTP payload
   consists of the AU Header Section, followed by concatenated AAC
   frames. The Auxiliary Section MUST be empty. For each AAC frame
   contained in the payload the one octet AU-header MUST provide:
   (a) the size of each AAC frame in the payload and
   (b) index information for computing the sequence (and hence timing)
       of each AAC frame.
   In the AU-header, the AU-size MUST be coded with 6 bits and the
   AU-Index(-delta) with 2 bits; the AU-Index field MUST have the
   value 0 in each AU-header.





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   In the AU-header Section, the concatenated AU-headers MUST be
   preceded by the 16-bit AU-headers-length field, as specified in
   section 3.2.1.

   In addition to the required MIME format parameters, the following
   parameters MUST be present: SizeLength, IndexLength, and
   IndexDeltaLength. AAC frames have fixed time duration per Access
   Unit; when interleaving in this mode, the applicable duration MUST
   be signaled by the MIME format parameter constantDuration. In
   addition, the parameter maxDisplacement MUST be present when
   interleaving.

   For example:

   m=audio 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/44100/2
   a=fmtp:96 streamtype=5; profile-level-id=15; mode=AAC-lbr; config=
   AudioSpecificConfig(); SizeLength=6; IndexLength=2;
   IndexDeltaLength=2; constantDuration=xxx; maxDisplacement=yyy

   Note: The a=fmtp line has been wrapped to fit the page, it comprises
         a single line in the SDP file.

   AudioSpecificConfig() is the hexadecimal string as defined in ISO/IEC
   14496-3. AudioSpecificConfig() specifies that the audio
   stream type is AAC. For the description of MIME parameters see
   section 4.1.

3.3.6 High bit-rate AAC

   This mode is signaled by mode=AAC-hbr. This mode supports transport
   of variable size AAC frames. In one RTP packet either one or more
   complete AAC frames are carried, or a single fragment of an AAC
   frame. In this mode the AAC frames are allowed to be interleaved
   and hence receivers MUST support de-interleaving. The maximum size
   of an AAC frame in this mode is 8191 octets.

   In this mode the RTP payload consists of the AU Header Section,
   followed by either one AAC frame, several concatenated AAC frames
   or one fragmented AAC frame. The Auxiliary Section MUST be empty.
   For each AAC frame contained in the payload there MUST be an
   AU-header in the AU Header Section to provide:
   (a) the size of each AAC frame in the payload and
   (b) index information for computing the sequence (and hence timing)
       of each AAC frame.

   To code the maximum size of an AAC frame requires 13 bits. Therefore
   in this configuration 13 bits are allocated to the AU-size, and
   3 bits to the AU-Index(-delta) field. Thus each AU-header has a size
   of 2 octets. Each AU-Index field MUST be coded with the value 0. In
   the AU Header Section, the concatenated AU-headers MUST be preceded
   by the 16-bit AU-headers-length field, as specified in
   section 3.2.1.

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   In addition to the required MIME format parameters, the following
   parameters MUST be present: SizeLength, IndexLength, and
   IndexDeltaLength. AAC frames have fixed time duration per Access
   Unit; when interleaving in this mode, the applicable duration MUST
   be signaled by the MIME format parameter constantDuration. In
   addition, the parameter maxDisplacement MUST be present when
   interleaving.

   For example:

   m=audio 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/44100/2
   a=fmtp:96 streamtype=5; profile-level-id=15; mode=AAC-hbr;
   config=AudioSpecificConfig(); SizeLength=13; IndexLength=3;
   IndexDeltaLength=3; constantDuration=xxx; maxDisplacement=yyy

   Note: The a=fmtp line has been wrapped to fit the page, it comprises
         a single line in the SDP file.

   AudioSpecificConfig() is the hexadecimal string as defined in
   ISO/IEC 14496-3. AudioSpecificConfig() specifies that the audio
   stream type is AAC. For the description of MIME parameters see
   section 4.1.

3.3.7 Additional modes

   This specification only defines the modes specified in sections
   3.3.2 up to 3.3.6. Additional modes are expected to be defined in
   future RFCs. Each additional mode MUST be in full compliance with
   this specification.

   Any new mode MUST be defined such that an implementation including
   all the features of this specification can decode the payload format
   corresponding to this new mode. For this reason a mode MUST NOT
   specify new default values for MIME parameters. In particular, MIME
   parameters that configure the RTP payload MUST be present (unless
   they have the default value), even if its presence is redundant in
   case the mode assigns a fixed value to a parameter. A mode may
   define additionally that some MIME parameters are required instead
   of optional, that some MIME parameters have fixed values (or
   ranges), and that there are rules restricting the usage.












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4. IANA considerations

   This section describes the MIME types and names associated with
   this payload format. Section 4.1 registers the MIME types, as per
   RFC 2048 [3].

   This format may require additional information about the mapping to
   be made available to the receiver. This is done using parameters
   also described in the next section.

4.1 MIME type registration

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

   "video" MUST be used for MPEG-4 Visual streams (ISO/IEC 14496-2)
   or MPEG-4 Systems streams (ISO/IEC 14496-1) that convey information
   needed for an audio/visual presentation.

   "audio" MUST 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" MUST be used for MPEG-4 Systems streams (ISO/IEC
   14496-1) that serve purposes other than audio/visual presentation,
   e.g. in some cases when MPEG-J (Java) streams are transmitted.

   Depending on the required payload configuration, MIME format
   parameters need to be available to the receiver. This is done using
   the parameters described in the next section. There are required
   and optional parameters.

   Optional parameters are of two types: general parameters and
   configuration parameters. The configuration parameters are used to
   configure the fields in the AU Header section and in the auxiliary
   section. The absence of any configuration parameter is equivalent to
   the associated field set to its default value, which is always zero.
   The absence of all configuration parameters resolves into a default
   "basic" configuration with an empty AU-header section and an empty
   auxiliary section in each RTP packet.

   MIME subtype name: mpeg4-generic

   Required parameters:

   MIME format parameters are not case dependent; however for clarity
   both upper and lower case are used in the names of the parameters
   described in this specification.

      StreamType:
      The integer value that indicates the type of MPEG-4 stream that
      is carried; its coding corresponds to the values of the
      streamType as defined in Table 9 (streamType Values) in ISO/IEC
      14496-1.

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      Profile-level-id:
      A decimal representation of the MPEG-4 Profile Level indication.
      This parameter MUST be used in the capability exchange or
      session set-up procedure to indicate the MPEG-4 Profile and Level
      combination of which the relevant MPEG-4 media codec is capable
      of.
      For MPEG-4 Audio streams, this parameter is the decimal value
         from Table 5 (audioProfileLevelIndication Values) in ISO/IEC
         14496-1, indicating which MPEG-4 Audio tool subsets are
         required to decode the audio stream.
      For MPEG-4 Visual streams, this parameter is the decimal value
         from Table G-1 (FLC table for profile and level indication) of
         ISO/IEC 14496-2, indicating which MPEG-4 Visual tool subsets
         are required to decode the visual stream.
      For BIFS streams, this parameter is the decimal value that is
         obtained from (SPLI + 256*GPLI), where:
         SPLI is the decimal value from Table 4 in ISO/IEC 14496-1 with
            the applied sceneProfileLevelIndication;
         GPLI is the decimal value from Table 7 in ISO/IEC 14496-1 with
            the applied graphicsProfileLevelIndication.
      For MPEG-J streams, this parameter is the decimal value from
         table 13 (MPEGJProfileLevelIndication) in ISO/IEC 14496-1,
         indicating the profile and level of the MPEG-J stream.
      For OD streams, this parameter is the decimal value from table 3
         (ODProfileLevelIndication) in ISO/IEC 14496-1, indicating the
         profile and level of the OD stream.
      For IPMP streams, this parameter has either the decimal value 0,
         indicating an unspecified profile and level, or a value larger
         than zero, indicating an MPEG-4 IPMP profile and level as
         defined in a future MPEG-4 specification.
      For Clock Reference streams and Object Content Info streams, this
         parameter has the decimal value zero, indicating that profile
         and level information is conveyed through the OD framework.

      Config:
      A hexadecimal representation of an octet string that expresses
      the media payload configuration. Configuration data is mapped
      onto the hexadecimal 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, if necessary to achieve
      octet-alignment, up to 7 zero-valued padding bits shall follow
      the configuration data.
      For MPEG-4 Audio streams, config is the audio object type
         specific decoder configuration data AudioSpecificConfig() as
         defined in ISO/IEC 14496-3. For Structured Audio, the
         AudioSpecificConfig() may be conveyed by other means, not
         defined by this specification. If the AudioSpecificConfig()
         is conveyed by other means for Structured Audio, then the
         config MUST be a quoted empty hexadecimal octet string, as
         follows: config="".
         Note that a future mode of using this RTP payload format for
         Structured Audio may define such other means.

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      For MPEG-4 Visual streams, config is the MPEG-4 Visual
         configuration information as defined in subclause 6.2.1 Start
         codes of ISO/IEC 14496-2. 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/IEC 14496-2).
      For BIFS streams, this is the BIFSConfig() information as defined
         in ISO/IEC 14496-1. For version 1, BIFSConfig is defined in
         section 9.3.5.2, and for version 2 in section 9.3.5.3. The
         MIME format parameter ObjectType signals the version of
         BIFSConfig.
      For IPMP streams, this is either a quoted empty hexadecimal octet
         string, indicating the absence of any decoder configuration
         information (config=""), or the IPMPConfiguration() as
         defined in a future MPEG-4 IPMP specification.
      For Object Content Info (OCI) streams, this is the
         OCIDecoderConfiguration() information of the OCI stream, as
         defined in section 8.4.2.4 in ISO/IEC 14496-1.
      For OD streams, Clock Reference streams and MPEG-J streams, this
         is a quoted empty hexadecimal octet string (config=""), as
         no information on the decoder configuration is required.

      Mode:
      The mode in which this specification is used. The following modes
      can be signaled:
      mode=generic,
      mode=CELP-cbr,
      mode=CELP-vbr,
      mode=AAC-lbr and
      mode=AAC-hbr.
      Other modes are expected to be defined in future RFCs. See also
      section 3.3.7 and 4.2 of RFC xxxx.

   Optional general parameters:

      ObjectType:
      The decimal value from Table 8 in ISO/IEC 14496-1, indicating
      the value of the objectTypeIndication of the transported stream.
      For BIFS streams this parameter MUST be present to signal the
      version of BIFSConfiguration(). Note that ObjectTypeIndication
      may signal a non-MPEG-4 stream and that the RTP payload format
      defined in this document may not be suitable to carry a stream
      that is not defined by MPEG-4. ObjectType SHOULD NOT be set to
      a value that signals a stream that cannot be carried by this
      payload format.

      ConstantSize:
      The constant size in octets of each Access Unit for this stream.
      The ConstantSize and the SizeLength parameters MUST NOT be
      simultaneously present.

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      ConstantDuration:
      The constant duration of each Access Unit for this stream,
      measured with the same units as the RTP time stamp.

      maxDisplacement:
      The decimal representation of the maximum displacement in time
      of an interleaved AU, as defined in section 3.2.3.3, expressed
      in units of the RTP time stamp clock.
      This parameter MUST be present when interleaving is applied.

      de-interleaveBufferSize:
      The decimal representation in number of octets of the size of
      the de-interleave buffer, described in section 3.2.3.3.
      When interleaving, this parameter MUST be present if the
      calculation of the de-interleave buffer size given in 3.2.3.3
      and based on maxDisplacement and rate(max) under-estimates the
      size of the de-interleave buffer. If this calculation does not
      under-estimate the size of the de-interleave buffer, then the
      de-interleaveBufferSize parameter SHOULD NOT be present.

   Optional configuration parameters:

      SizeLength:
      The number of bits on which the AU-size field is encoded in the
      AU-header. The SizeLength and the ConstantSize parameters MUST
      NOT be simultaneously present.

      IndexLength:
      The number of bits on which the AU-Index is encoded in the first
      AU-header. The default value of zero indicates the absence of
      the AU-Index field in each first AU-header.

      IndexDeltaLength:
      The number of bits on which the AU-Index-delta field is encoded
      in any non-first AU-header. The default value of zero indicates
      the absence of the AU-Index-delta field in each non-first
      AU-header.

      CTSDeltaLength:
      The number of bits on which the CTS-delta field is encoded in
      the AU-header.

      DTSDeltaLength:
      The number of bits on which the DTS-delta field is encoded in
      the AU-header.

      RandomAccessIndication:
      A decimal value of zero or one, indicating whether the RAP-flag
      is present in the AU-header. The decimal value of one indicates
      presence of the RAP-flag, the default value zero its absence.




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      StreamStateIndication:
      The number of bits on which the Stream-state field is encoded in
      the AU-header. This parameter MAY be present when transporting
      MPEG-4 system streams, and SHALL NOT be present for MPEG-4 audio
      and MPEG-4 video streams.

      AuxiliaryDataSizeLength:
      The number of bits that is used to encode the auxiliary-data-size
      field.

   Applications MAY use more parameters, in addition to those defined
   above. Each additional parameter MUST be registered with IANA, to
   ensure that there is no clash of names. Each additional parameter
   MUST be accompanied by a specification in the form of an RFC, MPEG
   standard, or other permanent and readily available reference (the
   "Specification Required" policy defined in RFC 2434 [6]). Receivers
   MUST tolerate the presence of such additional parameters, but these
   parameters SHALL NOT impact the decoding of receivers that comply to
   this specification.

   Encoding considerations:
   This MIME subtype is defined for RTP transport only. System
   bitstreams MUST be generated according to MPEG-4 Systems
   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 Audio
   specifications (ISO/IEC 14496-3). The RTP packets MUST be packetized
   according to the RTP payload format defined in RFC xxxx.

   Security considerations:
   As defined in section 5 of RFC xxxx.

   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:
   . a codec builder to implement only the subset of the standard he
     needs, while maintaining interworking with other MPEG-4 devices
     that implement the same combination, and
   . 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 is achieved by specifying the parameter
   "profile-level-id" in MIME content. In the capability exchange /
   announcement procedure this parameter may mutually be set to the
   same value.


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   Published specification:
   The specifications for MPEG-4 streams are presented in ISO/IEC
   14496-1, 14496-2, and 14496-3. The RTP payload format is described
   in RFC xxxx.

   Applications which use this media type:
   Multimedia streaming and conferencing tools.

   Additional information: none

   Magic number(s): none

   File extension(s):
   None. A file format with the extension .mp4 has been defined for
   MPEG-4 content but is not directly correlated with this MIME type
   for which the sole purpose is RTP transport.

   Macintosh File Type Code(s): none

   Person & email address to contact for further information:
   Authors of RFC xxxx, IETF Audio/Video Transport working group.

   Intended usage: COMMON

   Author/Change controller:
   Authors of RFC xxxx, IETF Audio/Video Transport working group.

4.2 Registration of mode definitions with IANA

   This specification can be used in a number of modes. The mode of
   operation is signaled using the "Mode" MIME parameter, with the
   initial set of values specified in section 4.1. New modes may be
   defined at any time, as described in section 3.3.7. These modes
   MUST be registered with IANA, to ensure that there is no clash
   of names.

   A new mode registration MUST be accompanied by a specification in
   the form of an RFC, MPEG standard, or other permanent and readily
   available reference (the "Specification Required" policy defined
   in RFC 2434 [6]).

4.3 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
   (for parameter usage examples see sections 3.3.2 up to 3.3.6).








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4.4 Usage of SDP

4.4.1 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
   [5] for example transported to the client in reply to a RTSP
   DESCRIBE [8] or via SAP [11]. In that case the (a=fmtp) keyword
   MUST be used as described in RFC 2327 [5], section 6, the syntax
   being then:

   a=fmtp:<format> <parameter name>=<value>[; <parameter name>=<value>]

5. Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [2]. 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 (i.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 the receiver or make it
   temporarily unavailable. Senders that transport MPEG-4 content
   SHOULD ensure that such content is MPEG compliant, as defined in the
   compliance part of IEC/ISO 14496 [1]. Receivers that support MPEG-4
   content should prevent malfunctioning of the receiver in case of
   non MPEG compliant content.

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

   In ISO/IEC 14496-1 a security model is defined for MPEG-4 Systems
   streams carrying MPEG-J access units which comprise 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



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   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.


6. Acknowledgements

   This document evolved through several revisions thanks to
   contributions by people from the ISMA forum, from the IETF AVT
   Working Group and from the 4-on-IP ad-hoc group within MPEG. The
   authors wish to thank all involved people, and in particular Andrea
   Basso, Stephen Casner, M. Reha Civanlar, Carsten Herpel, John
   Lazaro, Zvi Lifshitz, Young-kwon Lim, Alex MacAulay, Bill May,
   Colin Perkins, Dorairaj V and Stephan Wenger for their valuable
   comments and support.


7. References

7.1 Normative references

   [1] ISO/IEC International Standard 14496 (MPEG-4); "Information
   technology - Coding of audio-visual objects", January 2000

   [2] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson RTP, "A
   Transport Protocol for Real Time Applications", RFC 1889, Internet
   Engineering Task Force, January 1996.

   [3] N. Freed, J. Klensin, J. Postel, " Multipurpose Internet Mail
   Extensions (MIME) Part Four: Registration Procedures", RFC 2048,
   Internet Engineering Task Force, November 1996.

   [4] S. Bradner, "Key words for use in RFCs to Indicate Requirement
   Levels", RFC 2119, March 1997.

   [5] M. Handley, V. Jacobson, "SDP: Session Description Protocol",
   RFC 2327, Internet Engineering Task Force, April 1998.

   [6] T. Narten, H. Alvestrand, " Guidelines for Writing an IANA
   Considerations Section in RFCs", RFC 2434, October 1998.


7.2 Informative references

   [7] D. Hoffman, G. Fernando, V. Goyal, M. Civanlar, "RTP payload
   format for MPEG1/MPEG2 Video", RFC 2250, January 1998.


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   [8] H. Schulzrinne, A. Rao, R. Lanphier, "RTSP: Real-Time Session
   Protocol", RFC 2326, Internet Engineering Task Force, April 1998.

   [9] C. Perkins, O. Hudson, "Options for Repair of Streaming Media"
   RFC 2354, Internet Engineering Task Force, June 1998.

   [10] H. Schulzrinne, J. Rosenberg, "An RTP Payload Format for
   Generic Forward Error Correction", RFC 2733, Internet Engineering
   Task Force, December 1999.

   [11] M. Handley, C. Perkins, E. Whelan, "SAP: Session Announcement
   Protocol", RFC 2974, Internet Engineering Task Force, October 2000.

   [12] Y. Kikuchi, T. Nomura, S. Fukunaga, Y. Matsui, H. Kimata, "RTP
   payload format for MPEG-4 Audio/Visual streams", RFC 3016, Internet
   Engineering Task Force, November 2000.


8. Author Addresses

   Jan van der Meer
   Philips Digital Networks
   Cederlaan 4
   5600 JB Eindhoven
   Netherlands
   Email : jan.vandermeer@philips.com

   David Mackie
   Apple Computer, Inc.
   One Infinite Loop, MS:302-2LF
   Cupertino  CA 95014
   Email: dmackie@apple.com

   Viswanathan Swaminathan
   Sun Microsystems Inc.
   901 San Antonio Road, M/S UMPK15-214
   Palo Alto, CA 94303
   Email: viswanathan.swaminathan@sun.com

   David Singer
   Apple Computer, Inc.
   One Infinite Loop, MS:302-3MT
   Cupertino  CA 95014
   Email: singer@apple.com

   Philippe Gentric
   Philips Digital Networks, MP4Net
   51 rue Carnot
   92156 Suresnes
   France
   e-mail: philippe.gentric@philips.com



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   Full Copyright Statement

   Copyright (C) The Internet Society (December 2002). All Rights
   Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain
   it or assist in its implementation may be prepared, copied,
   published and distributed, in whole or in part, without restriction
   of any kind, provided that the above copyright notice and this
   paragraph are included on all such copies and derivative works.
   However, this document itself may not be modified in any way, such
   as by removing the copyright notice or references to the Internet
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   The limited permissions granted above are perpetual and will
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   ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
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APPENDIX: Usage of this payload format

Appendix A. Interleave analysis


A.1 Introduction

   In this appendix interleaving issues are discussed. Some general
   notes are provided on de-interleaving and error concealment, while
   a number of interleaving patterns are examined, in particular
   for determining the maximum displacement in time and the size of
   the de-interleave buffer. In these examples, the maximum
   displacement is cited in terms of an access unit count, for ease of
   reading. In actual streams, it is signalled in units of the RTP
   time stamp clock.


A.2 De-interleaving and error concealment

   This appendix does not describe any details on de-interleaving and
   error concealment, as the control of the AU decoding and error
   concealment process has little to do with interleaving. If the
   next AU to be decoded is present and there is sufficient storage
   available for the decoded AU, then decode it now. If not, wait.
   When the decoding deadline is reached (i.e., the time when decoding
   must begin in order to be completed by the time the AU is to be
   presented), or if the decoder is some hardware that presents a
   constant delay between initiation of decoding of an AU and
   presentation of that AU, then decoding must begin at that deadline
   time.

   If the next AU to be decoded is not present when the decoding
   deadline is reached, then that AU is lost so the receiver must take
   whatever error concealment measures is deemed appropriate. The
   play-out delay may need to be adjusted at that point (especially if
   other AUs have also missed their deadline recently).  Or, if it was
   a momentary delay, and maintaining the latency is important, then
   the receiver should minimize the glitch and continue processing
   with the next AU.


A.3 Simple Group interleave

A.3.1 Introduction

   An example of regular interleave is when packets are formed into
   groups. If the 'stride' of the interleave (the distance between
   interleaved AUs) is N, packet 0 could contain AU(0), AU(N), AU(2N),
   and so on; packet 1 could contain AU(1), AU(1+N), AU(1+2N), and so
   on. If there are M access units in a packet, then there are M*N
   access units in the group.



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   An example with N=M=3 follows; note that this is the same example
   as given in section 2.5 and that a fixed time duration per Access
   Unit is assumed:

   Packet   Time stamp   Carried AUs      AU-Index, AU-Index-delta
   P(0)     T[0]         0, 3, 6          0, 2, 2
   P(1)     T[1]         1, 4, 7          0, 2, 2
   P(2)     T[2]         2, 5, 8          0, 2, 2
   P(3)     T[9]         9,12,15          0, 2, 2

   In this example the AU-Index is present in the first AU-header and
   coded with the value 0, as required for fixed duration AUs. The
   position of the first AU of each packet within the group is defined
   by the RTP time stamp, while the AU-Index-delta field indicates the
   position of subsequent AUs relative to the first AU in the packet.
   All AU-Index-delta fields are coded with the value N-1, equal to 2
   in this example. Hence the RTP time stamp and the AU-Index-delta are
   used to reconstruct the original order. See also section 3.2.3.2.

A.3.2 Determining the de-interleave buffer size

   For the regular pattern as in this example, figure 6 in section
   3.2.3.3 shows that the de-interleave buffer stores at most 4 AUs. A
   de-interleaveBufferSize value may be signaled that is at least
   equal to the total number of octets of any 4 "early" AUs that are
   stored at the same time.

A.3.3 Determining the maximum displacement

   For the regular pattern as in this example, figure 7 in section 3.3
   shows that the maximum displacement in time equals 5 AU periods.
   Hence the minimum maxDisplacement value that must be signaled is 5
   AU periods. In case each AU has the same size, this maxDisplacement
   value over-estimates the de-interleave buffer size with one AU.
   However, note that in case of variable AU sizes the total size of
   any 4 "early" AUs that must be stored at the same time may exceed
   maxDisplacement times the maximum bitrate, in which case the
   de-interleaveBufferSize must be signaled.


A.4 More subtle group interleave

A.4.1 Introduction

   Another example of forming packets with group interleave is given
   below. In this example the packets are formed such that the loss of
   two subsequent RTP packets does not cause the loss of two subsequent
   AUs. Note that in this example the RTP time stamps of packet 3 and
   packet 4 are earlier than the RTP time stamps of packets 1 and 2,
   respectively; a fixed time duration per Access Unit is assumed.




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   Packet   Time stamp   Carried AUs      AU-Index, AU-Index-delta
   0        T[0]         0,  5            0, 4
   1        T[2]         2,  7            0, 4
   2        T[4]         4,  9            0, 4
   3        T[1]         1,  6            0, 4
   4        T[3]         3,  8            0, 4
   5        T[10]       10, 15            0, 4
   and so on ..

   In this example the AU-Index is present in the first AU-header and
   coded with the value 0, as required for AUs with a fixed duration.
   To reconstruct the original order, the RTP time stamp and the
   AU-Index-delta (coded with the value 4) are used. See also
   section 3.2.3.2.

A.4.2 Determining the de-interleave buffer size

   From figure 8 it can be to determined that at most 5 "early" AUs
   are to be stored. If the AUs are of constant size, then this value
   equals 5 times the AU size. The minimum size of the de-interleave
   buffer equals the maximum total number of octets of the "early" AUs
   that are to be stored at the same time. This gives the minimum
   value of the de-interleaveBufferSize that may be signaled.

                              +--+--+--+--+--+--+--+--+--+--+
   Interleaved AUs            | 0| 5| 2| 7| 4| 9| 1| 6| 3| 8|
                              +--+--+--+--+--+--+--+--+--+--+
                                -  -  5  -  5  -  2  7  4  9
                                            7     4  9  5
   "Early" AUs                                    5     6
                                                  7     7
                                                  9     9

   Figure 8: Storage of "early" AUs in the de-interleave buffer per
             interleaved AU.


A.4.2 Determining the maximum displacement

   From figure 9 it can be seen that the maximum displacement in time
   equals 8 AU periods. Hence the minimum maxDisplacement value to be
   signaled is 8 AU periods.

                                    +--+--+--+--+--+--+--+--+--+--+
   Interleaved AUs                  | 0| 5| 2| 7| 4| 9| 1| 6| 3| 8|
                                    +--+--+--+--+--+--+--+--+--+--+

   Earliest not yet present AU        -  1  1  1  1  1  -  3  -  -

   Figure 9: The earliest not yet present AU for each AU in the
             interleaving pattern.


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   In case each AU has the same size, the found maxDisplacement value
   over-estimates the de-interleave buffer size with three AUs.
   However, in case of variable AU sizes the total size of any 5
   "early" AUs stored at the same time may exceed maxDisplacement
   times the maximum bitrate, in which case de-interleaveBufferSize
   must be signaled.


A.5 Continuous interleave

A.5.1 Introduction

   In continuous interleave, once the scheme is 'primed', the number
   of AUs in a packet exceeds the 'stride' (the distance between
   them). This shortens the buffering needed, smooths the data-flow,
   and gives slightly larger packets -- and thus lower overhead -- for
   the same interleave. For example, here is a continuous interleave
   also over a stride of 3 AUs, but with 4 AUs per packet, for a run
   of 20 AUs. This shows both how the scheme 'starts up' and how it
   finishes. Once again, the example assumes fixed time duration per
   Access Unit.

   Packet   Time-stamp   Carried AUs         AU-Index, AU-Index-delta
   0        T[0]                      0      0
   1        T[1]                  1   4      0  2
   2        T[2]              2   5   8      0  2  2
   3        T[3]          3   6   9  12      0  2  2  2
   4        T[7]          7  10  13  16      0  2  2  2
   5        T[11]        11  14  17  20      0  2  2  2
   6        T[15]        15  18              0  2
   7        T[19]        19                  0

   In this example the AU-Index is present in the first AU-header and
   coded with the value 0, as required for AUs with a fixed duration.
   To reconstruct the original order, the RTP time stamp and the
   AU-Index-delta (coded with the value 2) are used. See also 3.2.3.2.
   Note that this example has RTP time-stamps in increasing order.

A.5.2 Determining the de-interleave buffer size

   For this example the de-interleave buffer size can be derived from
   figure 10. The maximum number of "early" AUs is three. If the AUs
   are of constant size, then this value equals 3 times the AU size.
   Compared to the example in A.2, for constant size AUs the
   de-interleave buffer size is reduced from 4 to 3 times the AU size,
   while maintaining the same 'stride'.







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                        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
   Interleaved AUs      | 0| 1| 4| 2| 5| 8| 3| 6| 9|12| 7|10|13|16|
                        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
                          -  -  -  4  -  -  4  8  -  -  8 12  -  -
                                            5           9
   "Early" AUs                              8          12


   Figure 10: Storage of "early" AUs in the de-interleave buffer per
              interleaved AU.


A.5.3 Determining the maximum displacement

   For this example the maximum displacement has a value of 5 AU
   periods. See figure 11. Compared to the example in A.2, the maximum
   displacement does not decrease, though in fact less de-interleave
   buffering is required.



                        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
   Interleaved AUs      | 0| 1| 4| 2| 5| 8| 3| 6| 9|12| 7|10|13|16|
                        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
   Earliest not yet
        present AU        -  -  2  -  3  3  -  -  7  7  -  - 11 11


   Figure 11: The earliest not yet present AU for each AU in the
              interleaving pattern.























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