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Updated by: 5691 PROPOSED STANDARD

Network Working Group                                    J. van der Meer
Request for Comments: 3640                           Philips Electronics
Category: Standards Track                                      D. Mackie
                                                          Apple Computer
                                                          V. Swaminathan
                                                   Sun Microsystems Inc.
                                                               D. Singer
                                                          Apple Computer
                                                              P. Gentric
                                                     Philips Electronics
                                                           November 2003


     RTP Payload Format for Transport of MPEG-4 Elementary Streams

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   The Motion Picture Experts Group (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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Carriage of MPEG-4 Elementary Streams Over RTP . . . . . . . .  4
       2.1.  Signaling by MIME Format Parameters  . . . . . . . . . .  4
       2.2.  MPEG Access Units  . . . . . . . . . . . . . . . . . . .  5
       2.3.  Concatenation of Access Units  . . . . . . . . . . . . .  5
       2.4.  Fragmentation of Access Units  . . . . . . . . . . . . .  6
       2.5.  Interleaving . . . . . . . . . . . . . . . . . . . . . .  6
       2.6.  Time Stamp Information . . . . . . . . . . . . . . . . .  7
       2.7.  State Indication of MPEG-4 System Streams  . . . . . . .  8
       2.8.  Random Access Indication . . . . . . . . . . . . . . . .  8



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       2.9.  Carriage of Auxiliary Information  . . . . . . . . . . .  8
       2.10. MIME Format Parameters and Configuring Conditional Field  8
       2.11. Global Structure of Payload Format . . . . . . . . . . .  9
       2.12. Modes to Transport MPEG-4 Streams  . . . . . . . . . . .  9
       2.13. Alignment with RFC 3016  . . . . . . . . . . . . . . . . 10
   3.  Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 10
       3.1.  Usage of RTP Header Fields and RTCP  . . . . . . . . . . 10
       3.2.  RTP Payload Structure  . . . . . . . . . . . . . . . . . 11
             3.2.1.  The AU Header Section  . . . . . . . . . . . . . 11
                     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. . . . . . . . . . . . . . . 21
             3.3.1.  General. . . . . . . . . . . . . . . . . . . . . 21
             3.3.2.  The Generic Mode . . . . . . . . . . . . . . . . 22
             3.3.3.  Constant Bit Rate CELP . . . . . . . . . . . . . 22
             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 . . . . . . . 33
       4.3.  Concatenation of Parameters. . . . . . . . . . . . . . . 33
       4.4.  Usage of SDP . . . . . . . . . . . . . . . . . . . . . . 34
             4.4.1.  The a=fmtp Keyword . . . . . . . . . . . . . . . 34
   5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 34
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
   APPENDIX: Usage of this Payload Format. . .  . . . . . . . . . . . 36
   Appendix A.  Interleave Analysis . . . . . . . . . . . . . . . . . 36
   A.  Examples of Delay Analysis with Interleave. . .  . . . . . . . 36
       A.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . 36
       A.2.  De-interleaving and Error Concealment  . . . . . . . . . 36
       A.3.  Simple Group Interleave  . . . . . . . . . . . . . . . . 36
             A.3.1.  Introduction . . . . . . . . . . . . . . . . . . 36
             A.3.2.  Determining the De-interleave Buffer Size  . . . 37
             A.3.3.  Determining the Maximum Displacement . . . . . . 37
       A.4.  More Subtle Group Interleave . . . . . . . . . . . . . . 38
             A.4.1.  Introduction . . . . . . . . . . . . . . . . . . 38
             A.4.2.  Determining the De-interleave Buffer Size. . . . 38
             A.4.3.  Determining the Maximum Displacement . . . . . . 39
       A.5.  Continuous Interleave  . . . . . . . . . . . . . . . . . 39
             A.5.1.  Introduction . . . . . . . . . . . . . . . . . . 39



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             A.5.2.  Determining the De-interleave Buffer Size  . . . 40
             A.5.3.  Determining the Maximum Displacement . . . . . . 40
   References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
   Normative References . . . . . . . . . . . . . . . . . . . . . . . 41
   Informative References . . . . . . . . . . . . . . . . . . . . . . 41
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 42
   Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 43

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.  However, 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 has been 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, at
   a suitable periodicity depending upon the probability of loss, in
   order to reduce stream corruption to an acceptable level.  An example
   is the carousel mechanism as defined by MPEG in ISO/IEC 14496-1 [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]), that must be employed



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   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].

   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 the
   transportation 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 be 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 the
   transportation 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
   the transportation of any additional system-related information in
   the 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 BCP 14, RFC 2119 [4].

2.  Carriage of MPEG-4 Elementary Streams over RTP

2.1.  Signaling by MIME Format Parameters

   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, as 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



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   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 for 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 the case of audio, an Access
   Unit may represent an audio frame and in the case of video, a
   picture.  MPEG Access Units are octet-aligned by definition.  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 include 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 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 an average of 7 complete AAC frames to be
   carried per RTP packet.

   Access Units may have a fixed size in octets, but a variable size is
   also possible.  To facilitate parsing in the case of multiple
   concatenated AUs in one RTP packet, the size of each AU is made known
   to the receiver.  When concatenating in the 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.

   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.





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   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 when 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.  Hence,
   when an IP packet is lost after IP-level fragmentation, only an AU
   fragment may get lost instead of the entire AU.  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 AU, 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 of alleviating 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.












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   For example, if we assume that an RTP packet contains 3 AUs, and that
   the AUs are numbered 0, 1, 2, 3, 4, and so forth, and 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.

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 a calculation (for example, for variable
   frame rate video, or 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 types 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.  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 as a difference.  For the CTS,
   the offset from the RTP time stamps is provided, and for the DTS, the
   offset from the CTS.





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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
   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 the skipping of
   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 a field is not needed.  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.




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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
   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 for example,
   capable of receiving only one type of MPEG-4 stream.  This is
   achieved by requiring that specific modes be defined in order to use
   this specification.  Each mode may define constraints for transport
   of one or more types 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 the transportation 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).




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2.13.  Alignment with RFC 3016

   This payload can be configured as 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, provided that additional packets
   containing video decoder configuration (VO, VOL, VOSH) are inserted
   in the stream, as required by RFC 3016 [12].  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 [12].  In this respect, it is strongly RECOMMENDED that the
   implementation provide the ability to ignore "in band" video decoder
   configuration packets that may be found in streams conforming to the
   RFC 3016 video payload.

   Note the "out of band" availability of the video decoder
   configuration is optional in RFC 3016 [12].  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.

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, or signaled dynamically out-of-band (e.g., using SDP).

   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 that the
      rate be set to 90 kHz.



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   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 3550 [2] (section 5.1), it is RECOMMENDED that RTP
   time stamps start at a random value for security reasons.  This is
   not an issue for synchronization of multiple RTP streams.  However,
   when 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.  In such cases the time
   stamp relationship required for obtaining synchronization may be
   provided 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 3550 [2].

   CC and CSRC fields are used as described in RFC 3550 [2].

   RTCP SHOULD be used as defined in RFC 3550 [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.

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.



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   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 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.

      +---------------------------------------+
      |     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.



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   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 by 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 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 indicates
      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.










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   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 indicates
      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: When set to 1, indicates 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 the stream-state MUST be incremented by one.

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

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.







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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .. -+-+-+-+-+-+-+-+-+
      | 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.

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

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





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   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; 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 the 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

   Unless prohibited by the signaled mode, a sender MAY interleave
   Access Units.  Receivers that are capable of receiving modes that
   support interleaving MUST be able to decode interleaved Access Units.

   When a sender interleaves Access Units, it needs to provide
   sufficient information to enable a receiver to unambiguously
   reconstruct the original order, even in the case of out-of-order
   packets, packet loss or duplication.  The information 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) - TS(i) = constant

       for any i, where:
          i indicates the index of the AU in the original order, and
          TS(i) denotes the time stamp of AU(i)

   The MIME parameter "constantDuration" SHOULD be used to signal that
   Access Units have a constant time duration; see section 4.1.

   If the "constantDuration" parameter is present, the receiver can
   reconstruct the original Access Unit timing based solely on the RTP
   timestamp and AU-Index-delta.  Accordingly, when transmitting Access
   Units of constant duration, the AU-Index, if present, MUST be set to



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   the value 0.  Receivers of constant duration Access Units MUST use
   the RTP timestamp to determine the index of the first AU in the RTP
   packet.  The AU-Index-delta header and the signaled
   "constantDuration" are used to reconstruct AU timing.

   If the "constantDuration" parameter is not present, then senders MAY
   signal AUs of constant duration by coding the AU-Index with zero in
   each RTP packet.  In the absence of the constantDuration parameter
   receivers MUST conclude that the AUs have constant duration if the
   AU-index is zero in two consecutive RTP packets.

   When transmitting Access Units of variable duration, then the
   "constantDuration" parameter MUST NOT be present, and the transmitter
   MUST use the AU-Index to encode the index information required for
   re-ordering, and the receiver MUST use that value to determine the
   index of each 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.  In addition, the CTS-delta
   MUST be coded in the AU header for each non-first AU in the RTP
   packet, so that receivers can place the AUs correctly in time.

   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

   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 to
   not exceed the path MTU.






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   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 it 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) arrives from the network last 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(j) in the pattern, each AU(i) with i<j
   that is not yet present can be determined.  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.



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   As an example in Figure 7, the interleaving pattern from section 2.5
   is considered.  For each AU in the pattern, the index is given of the
   earliest of any earlier AUs not yet present.  Hence for each AU(n) in
   the interleaving pattern the smallest index k (with k<n) of not yet
   delivered AUs 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: For each AU in the interleaving pattern, the earliest of
             any earlier AUs not yet present

   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 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
   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".









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   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 signaled, 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
   stream.  When a decoder starts receiving a stream, the decoder MUST



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   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 or may not be crucial.  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.

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 the transportation of MPEG-4 CELP and MPEG-4 AAC
   streams, defined in ISO/IEC 14496-3 [1].

   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.




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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 the transportation of a BIFS-Anim
   stream.  In this example carriage of multiple BIFS-Anim 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
   are 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-Anim AU.  The RTP
   time stamp uses a 1 kHz clock.  Note that the media type name is
   video, because the BIFS-Anim 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=1807; mode=generic;
   objectType=2; config=0842237F24001FB400094002C0; 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.

   The hexadecimal value of the "config" parameter is the
   BIFSConfiguration() as defined in ISO/IEC 14496-1.  The
   BIFSConfiguration() specifies that the BIFS stream is a BIFS-Anim
   stream.  For the description of MIME parameters, see section 4.1.

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; interleaving MUST NOT be used with this mode.  The RTP
   payload consists of one or more concatenated CELP frames, each of
   equal 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.




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   For example:

   m=audio 49230 RTP/AVP 96
   a=rtpmap:96 mpeg4-generic/16000/1
   a=fmtp:96 streamtype=5; profile-level-id=14; mode=CELP-cbr; config=
   440E00; constantSize=27; constantDuration=240

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

   The hexadecimal value of the "config" parameter is the
   AudioSpecificConfig()as defined in ISO/IEC 14496-3.
   AudioSpecificConfig() specifies a mono CELP stream with a sampling
   rate of 16 kHz at a fixed bitrate of 14.4 kb/s and 6 sub-frames per
   CELP frame.  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.  In this mode, the largest
   possible value for AU-size is greater than the maximum CELP frame
   size. Because CELP frames are very small, 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 be coded
   with 6 bits.  Therefore, in this mode 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
   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 always have a fixed duration per
   Access Unit; when interleaving in this mode, this specific duration



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   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/16000/1
   a=fmtp:96 streamtype=5; profile-level-id=14; mode=CELP-vbr; config=
   440F20; sizeLength=6; indexLength=2; indexDeltaLength=2;
   constantDuration=160; maxDisplacement=5

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

   The hexadecimal value of the "config" parameter is the
   AudioSpecificConfig() as defined in ISO/IEC 14496-3.
   AudioSpecificConfig() specifies a mono CELP stream with a sampling
   rate of 16 kHz, at a bitrate that varies between 13.9 and 16.2 kb/s
   and with 4 sub-frames per CELP frame.  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 the
   transportation 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.  Hence, when using this mode, encoders MUST
   ensure that the size of each AAC frame is at most 63 octets.

   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 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 always have a fixed duration per Access
   Unit; when interleaving in this mode, this specific 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/22050/1
   a=fmtp:96 streamtype=5; profile-level-id=14; mode=AAC-lbr; config=
   1388; sizeLength=6; indexLength=2; indexDeltaLength=2;
   constantDuration=1024; maxDisplacement=5

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

   The hexadecimal value of the "config" parameter is the
   AudioSpecificConfig(), as defined in ISO/IEC 14496-3.
   AudioSpecificConfig() specifies a mono AAC stream with a sampling
   rate of 22.05 kHz.  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 the
   transportation 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 is carried.  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



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   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.

   In addition to the required MIME format parameters, the following
   parameters MUST be present: sizeLength, indexLength, and
   indexDeltaLength.  AAC frames always have a fixed duration per Access
   Unit; when interleaving in this mode, this specific 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/48000/6
   a=fmtp:96 streamtype=5; profile-level-id=16; mode=AAC-hbr;
   config=11B0; sizeLength=13; indexLength=3;
   indexDeltaLength=3; constantDuration=1024

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

   The hexadecimal value of the "config" parameter is the
   AudioSpecificConfig(), as defined in ISO/IEC 14496-3.
   AudioSpecificConfig() specifies a 5.1 channel AAC stream with a
   sampling rate of 48 kHz.  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
   through 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
   additionally define that some MIME parameters are required instead of
   optional, that some MIME parameters have fixed values (or ranges),
   and that there are rules restricting its 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
   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 may 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 results in 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; for clarity however,
   both upper and lower case are used in the names of the parameters
   described in this specification.




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      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.

      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.

      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 [1], indicating which MPEG-4 Visual tool
         subsets are required to decode the visual stream.

      For BIFS streams, this parameter is the decimal value 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.







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      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.

      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.  Version 1 of BIFSConfig is defined in
         section 9.3.5.2, and version 2 is defined 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 will be
         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.






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      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 3640.

   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 for carrying a stream that is
      not defined by MPEG-4.  The objectType parameter 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.

      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.






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      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 indicates its
      absence.

      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.





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      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 not a 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
   with 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 3640.

   Security considerations:
   As defined in section 5 of RFC 3640.

   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 3640.

   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 3640, IETF Audio/Video Transport working group.

   Intended usage: COMMON

   Author/Change controller:
   Authors of RFC 3640, 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 not a 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 an SDP message
   [5] for example transported to the client in reply to an 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 then
   being:

   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) so that the receiver/decoder's buffers are
   overloaded, 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 support 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 MPEG-4
   scripts.  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.





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   In ISO/IEC 14496-1, a security model is defined for MPEG-4 Systems
   streams carrying MPEG-J access units that 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
   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, MPEG-4 scripts) commands in the streams.
   However, this can increase the complexity significantly.

   Implementors of MPEG-4 streaming over RTP who also implement MPEG-4
   scripts (subset of ECMAScript) MUST ensure that the action of such
   scripts is limited solely to the domain of the single presentation in
   which they reside (thus disallowing session to session communication,
   access to local resources and storage, etc).  Though loading static
   network-located resources (such as media) into the presentation
   should be permitted, network access by scripts MUST be restricted to
   such a (media) download.

6.  Acknowledgements

   This document evolved into RFC 3640 after several revisions.  Thanks
   to contributions from people in the ISMA forum, the IETF AVT Working
   Group and the 4-on-IP ad-hoc group within MPEG.  The authors wish to
   thank all people involved, particularly 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.




















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APPENDIX: Usage of this Payload Format

Appendix A.  Interleave Analysis

A.  Examples of Delay Analysis with Interleave

A.1.  Introduction

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



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   on.  If there are M access units in a packet, then there are M*N
   access units in the group.

   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 that is at least equal to the total
   number of octets of any 4 "early" AUs that are stored at the same
   time may be signaled.

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.










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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.

   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.





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A.4.3.  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: For each AU in the interleaving pattern, the earliest of
             any earlier AUs not yet present

   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, smoothes 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



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   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 3.  If the AUs are
   of constant size, then the de-interleave buffer size 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'.

                        +--+--+--+--+--+--+--+--+--+--+--+--+--+--+-
   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: For each AU in the interleaving pattern, the earliest of
              any earlier AUs not yet present












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References

Normative References

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

   [2]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
        "RTP:  A Transport Protocol for Real-Time Applications", RFC
        3550, July 2003.

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

   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [5]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.

   [6]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

Informative References

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

   [8]  Schulzrinne, H., Rao, A. and R. Lanphier, "Real-Time Session
        Protocol (RTSP)", RFC 2326, April 1998.

   [9]  Perkins, C. and O. Hodson, "Options for Repair of Streaming
        Media", RFC 2354, June 1998.

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

   [11] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
        Protocol", RFC 2974, October 2000.

   [12] Kikuchi, Y., Nomura, T., Fukunaga, S., Matsui, Y. and H. Kimata,
        "RTP Payload Format for MPEG-4 Audio/Visual Streams", RFC 3016,
        November 2000.







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Authors' Addresses

   Jan van der Meer
   Philips Electronics
   Prof Holstlaan 4
   Building WAH-1
   5600 JZ Eindhoven
   Netherlands

   EMail: jan.vandermeer@philips.com


   David Mackie
   Apple Computer, Inc.
   One Infinite Loop, MS:302-3KS
   Cupertino  CA 95014

   EMail: dmackie@apple.com


   Viswanathan Swaminathan
   Sun Microsystems Inc.
   2600 Casey Avenue
   Mountain View, CA 94043

   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 Electronics
   51 rue Carnot
   92156 Suresnes
   France

   EMail: philippe.gentric@philips.com








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Full Copyright Statement

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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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