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Versions: 00 01 02 RFC 2038

Internet Engineering Task Force             Don Hoffman
INTERNET-DRAFT                              Gerard Fernando
                                            Sun Microsystems, Inc.

                                            Vivek Goyal
                                            Precept Software, Inc.

                                            October, 1996
                                            Expires: February 1, 1997


               RTP Payload Format for MPEG1/MPEG2 Video


                          Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups.  Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
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ftp.isi.edu (US West Coast).

Distribution of this memo is unlimited.

                                Abstract

This draft describes a packetization scheme for MPEG video and audio
streams.  The scheme proposed can be used to transport such a video or
audio flow over the transport protocols supported by RTP.  Two
approaches are described. The first is designed to support maximum
interoperability with MPEG System environments.  The second is designed
to provide maximum compatibility with other RTP-encapsulated media
streams and future conference control work of the IETF.


1. Introduction

ISO/IEC JTC1/SC29 WG11 (also referred to as the MPEG committee) has



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defined the MPEG1 standard (ISO/IEC 11172)[1] and the MPEG2 standard
(ISO/IEC 13818)[2].  This draft describes a packetization scheme to
transport MPEG video and audio streams using the Real-time Transport
Protocol (RTP), version 2 [3, 4].

The MPEG1 specification is defined in three parts: System, Video and
Audio.  It is designed primarily for CD-ROM-based applications, and is
optimized for approximately 1.5 Mbits/sec combined data rates. The video
and audio portions of the specification describe the basic format of the
video or audio stream.  These formats define the Elementary Streams
(ES).  The MPEG1 System specification defines an encapsulation of the ES
that contains Presentation Time Stamps (PTS), Decoding Time Stamps and
System Clock references, and performs multiplexing of MPEG1 compressed
video and audio ES's with user data.

The MPEG2 specification is structured in a similar way. However, it
hasn't been restricted only to CD-ROM applications. The MPEG2 System
specification defines two system stream formats:  the MPEG2 Transport
Stream (MTS) and the MPEG2 Program Stream (MPS).  The MTS is tailored
for communicating or storing one or more programs of MPEG2 compressed
data and also other data in relatively error-prone environments. The MPS
is tailored for relatively error-free environments.

We seek to achieve interoperability among 4 types of end-systems in the
following specification. The 4 types are:

        1. Transmitting Interworking Unit (TIU)

           Receives MPEG information from a native MTS system for
           distribution over packet networks using a native RTP-based
           system layer (such as an IP-based internetwork). Examples:
           real-time encoder, MTS satellite link to Internet, video
           server with MTS-encoded source material.

        2. Receiving Interworking Unit (RIU)

           Receives MPEG information in real time from an RTP-based
           network for forwarding to a native MTS environment.
           Examples: Internet-based video server to MTS-based cable
           distribution plant.

        3. Transmitting Internet End-System (TAES)

           Transmits MPEG information generated or stored within the
           internet end-system itself, or received from internet-based
           computer networks.  Example: video server.





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        4. Receiving Internet End-System (RAES)

           Receives MPEG information over an RTP-based internet for
           consumption at the internet end-system or forwarding to
           traditional computer network.  Example: desktop PC or
           workstation viewing training video.

Each of the 2 types of transmitters must work with each of the 2 types
of receivers.  Because it is probable that the TAES, and certain that
the RAES, will be based on existing and planned internet-connected
computers, it is highly desirable for the interoperable protocol to be
based on RTP.

Because of the range of applications that might employ MPEG streams, we
propose to define two payload formats.

Much interest in the MPEG community is in the use of one of the MPEG
System encodings, and hence, in Section 2 we propose encapsulations of
MPEG1 System streams and MPEG2 Transport and Program Streams with RTP.
This profile supports the full semantics of MPEG System and offers basic
interoperability among all four end-system types.

When operating only among internet-based end-systems (i.e., TAES and
RAES) a payload format that provides greater compatibility with the
Internet architecture is desired, deferring some of the system issues to
other protocols being defined in the Internet community (such as the
MMUSIC WG).  In Section 3 we propose an encapsulation of compressed
video and audio data (referred to in MPEG documentation as "Elementary
Streams" (ES)) complying with either MPEG1 or MPEG2. Here, neither of
the System standards of MPEG1 or MPEG2 are utilized.  The ES's are
directly encapsulated with RTP.

Throughout this specification, we make extensive use of MPEG
terminology.  The reader should consult the primary MPEG references for
definitive descriptions of this terminology.


2. Encapsulation of MPEG System and Transport Streams


Each RTP packet will contain a timestamp derived from the sender's 90KHz
clock reference.  This clock is synchronized to the system stream
Program Clock Reference (PCR) or System Clock Reference (SCR) and
represents the target transmission time of the first byte of the packet
payload.  The RTP timestamp will not be passed to the MPEG decoder.
This use of the timestamp is somewhat different than normally is the
case in RTP, in that it is not considered to be the media display or
presentation timestamp. The primary purposes of the RTP timestamp will



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be to estimate and reduce any network-induced jitter and to synchronize
relative time drift between the transmitter and receiver.


For MPEG2 Transport Streams the RTP payload will contain an integral
number of MPEG transport packets.  To avoid end system inefficiencies,
data from multiple small MTS packets (normally fixed in size at 188
bytes) are aggregated into a single RTP packet.  The number of transport
packets contained is computed by dividing RTP payload length by the
length of an MTS packet (188).

For MPEG2 Program streams and MPEG1 system streams there are no
packetization restrictions; these streams are treated as a packetized
stream of bytes.


2.1 RTP header usage

The RTP header fields are used as follows:

        Payload Type: Distinct payload types should be assigned for
          of MPEG1 System Streams, MPEG2 Program Streams and MPEG2
          Transport Streams.  See [4] for payload type assignments.

        M bit:  Set to 1 whenever the timestamp is discontinuous
          (such as might happen when a sender switches from one data
          source to another). This allows the receiver and any
          intervening RTP mixers or translators that are synchronizing
          to the flow to ignore the difference between this timestamp
          and any previous timestamp in their clock phase detectors.

        timestamp: 32 bit 90K Hz timestamp representing the target
          transmission time for the first byte of the packet.

3. Encapsulation of MPEG Elementary Streams

The following ES types may be encapsulated directly in RTP:
        (a) MPEG1 Video (ISO/IEC 11172-2)
        (b) MPEG2 Video (ISO/IEC 13818-2)
        (c) MPEG1 Audio (ISO/IEC 11172-3)
        (d) MPEG2 Audio (ISO/IEC 13818-3)

A distinct RTP payload type is assigned to MPEG1/MPEG2 Video and
MPEG1/MPEG2 Audio, respectively. Further indication as to whether the
data is MPEG1 or MPEG2 need not be provided in the RTP or MPEG-specific
headers of this encapsulation, as this information is available in the
ES headers.




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Presentation Time Stamps (PTS) of 32 bits with an accuracy of 90 kHz
shall be carried in the fixed RTP header. All packets that make up a
audio or video frame shall have the same time stamp.

3.1 MPEG Video elementary streams

MPEG1 Video can be distinguished from MPEG2 Video at the video sequence
header, i.e. for MPEG2 Video a sequence_header() is followed by
sequence_extension().  The particular profile and level of MPEG2 Video
(MAIN_Profile@MAIN_Level, HIGH_Profile@HIGH_Level, etc) are determined
by the profile_and_level_indicator field of the sequence_extension
header of MPEG2 Video.

The MPEG bit-stream semantics were designed for relatively error-free
environments, and there is significant amount of dependency (both
temporal and spatial) within the stream such that loss of some data make
other uncorrupted data useless.  The format as defined in this
encapsulation uses application layer framing information plus additional
information in the RTP stream-specific header to allow for certain
recovery mechanisms.  Appendix 1 suggests several recovery strategies
based on the properties of this encapsulation.

Since MPEG pictures can be large, they will normally be fragmented into
packets of size less than a typical LAN/WAN MTU.  The following
fragmentation rules apply:

        1. The MPEG Video_Sequence_Header, when present, will always
           be at the beginning of an RTP payload.
        2. An MPEG GOP_header, when present, will always be at the
           beginning of the RTP payload, or will follow a
           Video_Sequence_Header.
        3. An MPEG Picture_Header, when present, will always be at the
           beginning of a RTP payload, or will follow a GOP_header.

Each ES header must be completely contained within the packet.
Consequently, a minimum RTP payload size of 261 bytes must be supported
to contain the largest single header defined in the ES (that is, the
extension_data() header containing the quant_matrix_extension()).
Otherwise, there are no restrictions on where headers may appear within
packet payloads.

In MPEG, each picture is made up of one or more "slices," and a slice is
intended to be the unit of recovery from data loss or corruption. An
MPEG-compliant decoder will normally advance to the beginning of next
slice whenever an error is encountered in the stream.  MPEG slice begin
and end bits are provided in the encapsulation header to facilitate
this.




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The beginning of a slice must either be the first data in a packet
(after any MPEG ES headers) or must follow after some integral number of
slices in a packet.  This requirement insures that the beginning of the
next slice after one with a missing packet can be found without
requiring that the receiver scan the packet contents.  Slices may be
fragmented across packets as long as all the above rules are met.

An implementation based on this encapsulation assumes that the
Video_Sequence_Header is repeated periodically in the MPEG bit-stream.
In practice (though not required by MPEG standard) this is used to allow
channel switching and to receive and start decoding a continuously
relayed MPEG bit-stream at arbitrary points in the media stream.  It is
suggested that when playing back from an MPEG stream from a file format
(where the Video_Sequence_Header may only be represented at the
beginning of the stream) that the first Video_Sequence_Header (preceded
by an end-of-stream indicator) be saved by the packetizer for periodic
injection in to the network stream.


3.2 MPEG Audio elementary streams

MPEG1 Audio can be distinguished from MPEG2 Audio from the MPEG
ancillary_data() header.  For either MPEG1 or MPEG2 Audio, distinct
Presentation Time Stamps may be present for frames which correspond to
either 384 samples for Layer-I, or 1152 samples for Layer-II or Layer-
III.  The actual number of bytes required to represent this number of
samples will vary depending on the encoder parameters.

Multiple audio frames may be encapsulated within one RTP packet.  In
this case, an integral number of audio frames must be contained within
the packet and the fragmentation header defined in Section 3.5 shall be
set to 0.

Also, if relatively short packets are to be used, one frame may be so
large that it may straddle multiple RTP packets.  For example, for
Layer-II MPEG audio sampled at a rate of 44.1 KHz each frame would
represent a time slot of 26.1 msec. At this sampling rate if the
compressed bit-rate is 384 kbits/sec (i.e.  48 kBytes/sec) then the
average audio frame size would be 1.25 KBytes.  If packets were to be
500 Bytes long, then each audio frame would straddle 3 RTP packets.  The
audio fragmentation indicator header (See Section 3.5) shall be present
for an MPEG1/2 Audio payload type to provide for this fragmentation.

3.3 RTP Fixed Header for MPEG ES encapsulation

The RTP header fields are used as follows:

        Payload Type: Distinct payload types should be assigned



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          for video elementary streams and audio elementary streams.
          See [4] for payload type assignments.

        M bit:  For video, set to 1 on packet containing MPEG frame
          end code, 0 otherwise.  For audio, set to 1 on first packet
          of a "talk-spurt," 0 otherwise.

        PT:  MPEG video or audio stream ID.

        timestamp: 32-bit 90K Hz timestamp representing presentation
          time of MPEG picture or audio frame.  Same for all packets
          that make up a picture or audio frame.  May not be
          monotonically increasing in video stream if B pictures
          present in stream.  For packets that contain only a video
          sequence and/or GOP header, the timestamp is that of the
          subsequent picture.

3.4 MPEG Video-specific header

This header shall be attached to each RTP packet after the RTP fixed
header.


 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    MBZ    |         TR        |MBZ|S|B|E|  P  | | BFC | | FFC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                FBV     FFV

        MBZ: Unused. Must be set to zero in current
           specification. This space is reserved for future use.

        TR: Temporal-Reference (10 bits). The temporal reference of
           the current picture within the current GOP. This value
           ranges from 0-1023 and is constant for all RTP packets of a
           given picture.

        MBZ: Unused. Must be set to zero in current
           specification. This space is reserved for future use.

        S: Sequence-header-present (1 bit). Normally 0 and set to 1 at
           the occurrence of each MPEG sequence header.  Used to
           detect presence of sequence header in RTP packet.

        B: Beginning-of-slice (BS) (1 bit). Set when the start of the
           packet payload is a slice start code, or when a slice start
           code is preceded only by one or more of a



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           Video_Sequence_Header, GOP_header and/or Picture_Header.

        E: End-of-slice (ES) (1 bit). Set when the last byte of the
           payload is the end of an MPEG slice.

        P: Picture-Type (3 bits). I (1), P (2), B (3) or D (4). This
           value is constant for each RTP packet of a given picture.
           Value 000B is forbidden and 101B - 111B are reserved to
           support future extensions to the MPEG ES specification.

        FBV: full_pel_backward_vector
        BFC: backward_f_code
        FFV: full_pel_forward_vector
        FFC: forward_f_code
           Obtained from the most recent picture header, and are
           constant for each RTP packet of a given picture. None of
           these values are used for I frames and must be set to zero
           in the RTP header. For P frames only the last two values
           are present and FBV and BFC must be set to zero in the RTP
           header. For B frames all the four values are present.


3.5 MPEG Audio-specific header

This header shall be attached to each RTP packet at the start of the
payload and after any RTP headers for an MPEG1/2 Audio payload type.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             MBZ               |          Frag_offset          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Frag_offset: Byte offset into the audio frame for the data
           in this packet.



Appendix 1. Error Recovery and Resynchronization Strategies.

The following error recovery and resynchronization strategies are
intended to be guidelines only.  A compliant receiver is free to employ
alternative (or no) strategies.

When initially decoding an RTP-encapsulated MPEG Elementary Stream, the
receiver may discard all packets until the Sequence-header-present bit
is set to 1.  At this point, sufficient state information is contained
in the stream to allow processing by an MPEG decoder.



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Loss of packets containing the GOP_header and/or Picture_Header are
detected by an unexpected change in the Temporal-Reference and Picture-
Type values.  Consider the following example GOP sequence:

        In display order: 0B 1B 2I 3B 4B 5P 6B 7B 8P GOP_HDR 0B ...
        In stream order:  2I 0B 1B 5P 3B 4B 8P 6B 7B GOP_HDR 2I ...

Consider also two counters:

        ref_pic_temp (Reference Picture (I,P) Temporal Reference)
        dep_pic_temp (Dependent Picture (B) Temporal Reference)

At each GOP beginning, set these counters to the temporal reference
value of the corresponding picture type. For our example GOP sequence,
ref_pic_temp = 2 and dep_pic_temp = 0. Keep incrementing BOTH counters
by unity with each following picture. Ref_pic_temp should match the
temporal references of the I and P frames, and dep_pic_temp should match
the temporal references of the B frames.

    dep_pic_temp: -  0  1  2  3  4  5  6  7        8  9
In stream order:  2I 0B 1B 5P 3B 4B 8P 6B 7B GOP_H 2I 0B 1B ...
    ref_pic_temp: 2  3  4  5  6  7  8  9  10  ^    11
                  --------------------------  |    ^
                             Match            Drop |
                                                   Mismatch
                                                    in ref_pic_temp

The loss of a GOP header can be detected by matching the appropriate
counter (based on picture type) to the temporal reference value. A
mismatch indicates a lost GOP header. If desired, a GOP header can be
re-constructed using a "null" time_code, repeating the closed_gop flag
from previous GOP headers, and setting the broken_link flag to 1.

The loss of a Picture_Header can also be detected by a mismatch in the
Temporal Reference contained in the RTP packet from the appropriate
dep_pic_temp or ref_pic_temp counters at the receiver.  After scanning
to the next Beginning-of-slice the Picture_Header is reconstructed from
the P, TR, FBV, BFC, FFV and FFC contained in that packet, and from
stream-dependent default values.

Any time an RTP packet is lost (as indicated by a gap in the RTP
sequence number), the receiver may discard all packets until the
Beginning-of-slice bit is set.  At this point, sufficient state
information is contained in the stream to allow processing by an MPEG
decoder starting at the next slice boundary (possibly after
reconstruction of the GOP_header and/or Picture_Header as described
above).




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References:

[1] ISO/IEC International Standard 11172; "Coding of moving pictures
    and associated audio for digital storage media up to about 1,5
    Mbits/s", November 1993.

[2] ISO/IEC International Standard 13818; "Generic coding of moving
    pictures and associated audio information", November 1994.

[3] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson,
    "RTP: A Transport Protocol for Real-Time Applications",
    RFC 1889, January 1996.

[4] H. Schulzrinne, "RTP Profile for Audio and Video Conferences
    with Minimal Control", RFC 1890, January 1996.


Authors' Addresses:

        Gerard Fernando
        Sun Microsystems, Inc.
        Mail-stop UMPK14-305
        2550 Garcia Avenue
        Mountain View, California 94043-1100
        USA
        phone: +1 415-786-6373
        email: gerard.fernando@eng.sun.com

        Vivek Goyal
        Precept Software, Inc.
        1072 Arastradero Rd,
        Palo Alto, CA 94304
        USA
        phone: +1 415-845-5200
        e-mail: goyal@precept.com

        Don Hoffman
        Sun Microsystems, Inc.
        Mail-stop UMPK14-305
        2550 Garcia Avenue
        Mountain View, California 94043-1100
        USA
        phone: +1 503-297-1580
        email: don.hoffman@eng.sun.com







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