



AVT Working Group                                             L. Barbato
Internet-Draft                                                  Xiph.Org
Expires: December 18, 2006                                 June 16, 2006


                    draft-barbato-avt-rtp-theora-01
              RTP Payload Format for Theora Encoded Video

Status of this Memo

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   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on December 18, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a RTP payload format for transporting Theora
   encoded video.  It details the RTP encapsulation mechanism for raw
   Theora data and configuration headers necessary to configure the
   decoder.

   Also included within the document are the necessary details for the
   use of Theora with MIME and Session Description Protocol (SDP).

Editors Note



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   All references to RFC XXXX are to be replaced by references to the
   RFC number of this memo, when published.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Payload Format . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  RTP Header . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Payload Header . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Payload Data . . . . . . . . . . . . . . . . . . . . . . .  6
     2.4.  Example RTP Packet . . . . . . . . . . . . . . . . . . . .  7
   3.  Configuration Headers  . . . . . . . . . . . . . . . . . . . .  8
     3.1.  In-band Header Transmission  . . . . . . . . . . . . . . .  9
       3.1.1.  Packed Configuration . . . . . . . . . . . . . . . . .  9
     3.2.  Out of Band Transmission . . . . . . . . . . . . . . . . . 10
       3.2.1.  Packed Headers . . . . . . . . . . . . . . . . . . . . 11
     3.3.  Loss of Configuration Headers  . . . . . . . . . . . . . . 13
   4.  Comment Headers  . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Frame Packetizing  . . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Example Fragmented Theora Packet . . . . . . . . . . . . . 15
     5.2.  Packet Loss  . . . . . . . . . . . . . . . . . . . . . . . 17
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  Mapping MIME Parameters into SDP . . . . . . . . . . . . . 19
       6.1.1.  SDP Example  . . . . . . . . . . . . . . . . . . . . . 20
     6.2.  Usage with the SDP Offer/Answer Model  . . . . . . . . . . 20
   7.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     7.1.  Stream Video . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 22
     10.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
   Intellectual Property and Copyright Statements . . . . . . . . . . 25















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1.  Introduction

   Theora is a general purpose, lossy video codec.  It is based on the
   VP3 video codec produced by On2 Technologies and has been donated to
   the Xiph.org Foundation.

   Theora I is a block-based lossy transform codec that utilizes an 8 x
   8 Type-II Discrete Cosine Transform and block-based motion
   compensation.  This places it in the same class of codecs as MPEG-1,
   MPEG-2, MPEG-4, and H.263.  The details of how individual blocks are
   organized and how DCT coefficients are stored in the bitstream differ
   substantially from these codecs, however.  Theora supports only intra
   frames (I frames in MPEG) and inter frames (P frames in MPEG).

   Theora provides none of its own framing, synchronization, or
   protection against transmission errors.  Instead, the codec expects
   to receive a discrete sequence of data packets.  Theora is a free-
   form variable bit rate (VBR) codec, and these packets have no minimum
   size, maximum size, or fixed/expected size.  Theora packets are thus
   intended to be used with a transport mechanism that provides free-
   form framing, synchronization, positioning, and error correction in
   accordance with these design assumptions, such as Ogg [1] or RTP/AVP
   [3].

   Theora I currently supports progressive video data of arbitrary
   dimensions at a constant frame rate in one of several Y'CbCr color
   spaces.  Three different chroma subsampling formats are supported:
   4:2:0, 4:2:2, and 4:4:4.  The Theora I format does not support
   interlaced material, variable frame rates, bit-depths larger than 8
   bits per component, nor alternate color spaces such as RGB or
   arbitrary multi-channel spaces.  Black and white content can be
   efficiently encoded, however, because the uniform chroma planes
   compress well.  Arbitrary frame size will be encoded rounding to the
   upper multiple of 16 both dimension for performance reason.  The
   original width and height will be encoded in the header and the
   decoder will use this information to clip the decoded frame to the
   right dimensions.

   Theora is similar to the Vorbis audio [10] in that it the decoder
   reads the probability model for the entropy coder and all
   quantization parameters from special "header" packets at the start of
   the compressed data.  It is therefore impossible to decode any video
   data without having previously fetched the codec info and codec setup
   headers, although Theora can initiate decode at an arbitrary intra-
   frame packet so long as the codec has been initialized with the
   associated headers.





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1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [2].


2.  Payload Format

   For RTP based transportation of Theora encoded video the standard RTP
   header is followed by a 4 octets payload header, then the payload
   data.  The payload headers are used to associate the Theora data with
   its associated decoding codebooks as well as indicating if the
   following packet contains fragmented Theora data and/or the number of
   whole Theora data frames.  The payload data contains the raw Theora
   bitstream information.

   For RTP based transport of Theora encoded video the standard RTP
   header is followed by a 4 octets payload header, then the payload
   data.

2.1.  RTP Header

   The format of the RTP header is specified in [3] and shown in Figure
   1.  This payload format uses the fields of the header in a manner
   consistent with that specification.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |       sequence number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           timestamp                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: RTP Header

   The RTP header begins with an octet of fields (V, P, X, and CC) to
   support specialized RTP uses (see [3] and [4] for details).  For
   Theora RTP, the following values are used.

   Version (V): 2 bits




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   This field identifies the version of RTP.  The version used by this
   specification is two (2).

   Padding (P): 1 bit

   Padding MAY be used with this payload format according to section 5.1
   of [3].

   Extension (X): 1 bit

   The Extension bit is used in accordance with [3].

   CSRC count (CC): 4 bits

   The CSRC count is used in accordance with [3].

   Marker (M): 1 bit

   The Marker bit is used in accordance with [3].

   Payload Type (PT): 7 bits

   An RTP profile for a class of applications is expected to assign a
   payload type for this format, or a dynamically allocated payload type
   SHOULD be chosen which designates the payload as Theora.

   Sequence number: 16 bits

   The sequence number increments by one for each RTP data packet sent,
   and may be used by the receiver to detect packet loss and to restore
   packet sequence.  This field is detailed further in [3].

   Timestamp: 32 bits

   A timestamp representing the presentation time of the first sample of
   the first Theora packet in the RTP packet.  The clock frequency MUST
   be set to 90kHz.

   SSRC/CSRC identifiers:

   These two fields, 32 bits each with one SSRC field and a maximum of
   16 CSRC fields, are as defined in [3].

2.2.  Payload Header

   The 4 octets following the RTP Header section are the Payload Header.
   This header is split into a number of bitfields detailing the format
   of the following Payload Data packets.



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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Configuration Ident             | F |TDT|# pkts.|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   Figure 2: Payload Header

   Configuration Ident: 24 bits

   This 24 bit field is used to associate the Theora data to a decoding
   Packed Configuration.

   Fragment type (F): 2 bit

   This field is set according to the following list

      0 = Not Fragmented
      1 = Start Fragment
      2 = Continuation Fragment
      3 = End Fragment

   This field must be zero if the number of packets field is non-zero.

   Theora Data Type (TDT): 2 bits

   This field sets the packet payload type for the Theora data.  There
   are currently three Theora payload types.

      0 = Raw Theora payload
      1 = Theora Packed Configuration payload
      2 = Legacy Theora Comment payload
      3 = Reserved

   The packets with a TDT of value 3 MUST be ignored

   The last 4 bits represent the number of complete packets in this
   payload.  This provides for a maximum number of 15 Theora packets in
   the payload.  If the packet contains fragmented data the number of
   packets MUST be set to 0.

2.3.  Payload Data

   Each Theora payload section starts with a two octets length header
   that is used to represent the size of the following data payload,
   followed by the raw Theora packet data.



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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Payload Length         |          Theora Data         ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3: Payload Data

   The Theora codec uses relatively unstructured raw packets containing
   binary integer fields of arbitrary width that often do not fall on an
   octet boundary.  When a Theora encoder produces packets, unused space
   in the last byte of a packet is always zeroed during the encoding
   process.  Thus, should this unused space be read, it will return
   binary zeros.

   For payloads which consist of multiple Theora packets the payload
   data consists of the payload length field followed by the first
   Theora packet's data, then the payload length followed by the second
   Theora packet, and so on for each of the Theora packets in the
   payload.

2.4.  Example RTP Packet

   Here is an example RTP packet containing two Theora packets.

   RTP Packet Header:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | 2 |0|0|  0    |0|      PT     |       sequence number         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                           timestamp                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          synchronisation source (SSRC) identifier             |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4: Example RTP Packet

   Payload Data:








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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Configuration Ident             | 0 | 0 | 2 pks |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Payload Length         |                              ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Theora data                          ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..           data               |        Payload Length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Theora data                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5: Example Theora Payload Packet

   The payload portion of the packet begins with the 24 bit
   Configuration ident field followed by 8 bits describing the payload.
   The Fragment type field is set to 0, indicating that this packet
   contains whole Theora frame data.  The Data type field is set to 0
   since it is theora raw data.  The number of whole Theora data packets
   is set to 2.

   Each of the payload blocks starts with the two octets length field
   followed by the variable length Theora packet data.


3.  Configuration Headers

   To decode a Theora stream three configuration header packets are
   needed.  The first, called the Identification Header, indicates the
   frame dimensions, quality, blocks used and the version of the Theora
   encoder used.  The second, called the Comment Header, contains stream
   metadata and the third, called the Setup Header, contains details of
   the dequantization and Huffman tables.

   Since this information must be transmitted reliably, and as the RTP
   stream may change certain configuration data mid-session, there are
   different methods for delivering this configuration data to a client,
   both in-band and out-of-band which are detailed below.  SDP delivery
   is used to set up an initial state for the client application.  The
   changes may be due to different dequantization and Huffman tables as
   well as different bitrates of the stream.

   The delivery vectors in use are specified by an SDP attribute to
   indicate the method and the optional URI where the Theora Packed
   Configuration (Section 3.1.1) Packets could be fetched.  Different
   delivery methods MAY be advertised for the same session.  The in-band



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   codebook delivery SHOULD be considered as baseline, out-of-band
   delivery methods that don't use RTP will not be described in this
   document.  For non chained streams, the Configuration delivery method
   RECOMMENDED is inline the Packed Configuration (Section 3.1.1) in the
   SDP as explained in the IANA considerations (Section 6.1)

   The 24 bit Ident field is used to map which Configuration will be
   used to decode a packet.  When the Ident field changes, it indicates
   that a change in the stream has taken place.  The client application
   MUST have in advance the correct configuration and if the client
   detects a change in the Ident value and does not have this
   information it MUST NOT decode the raw data associated until it
   fetches the correct Configuration.

3.1.  In-band Header Transmission

   The Packed Configuration (Section 3.1.1) Payload is sent in-band with
   the packet type bits set to match the payload type.  Clients MUST be
   capable of dealing with periodic re-transmission of the configuration
   headers.

3.1.1.  Packed Configuration

   A Theora Packed Configuration is indicated with the payload type
   field set to 1.  Of the three headers, defined in the Theora I
   specification [16], the identification and the setup will be packed
   together, the comment header is completely suppressed.  It is up to
   the client to provide a minimal size comment header to the decoder if
   required by the implementation.






















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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |             xxxx              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Configuration Ident             | 0 | 1 |      1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             length            |          Identification      ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Identification                       ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Identification                       ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Identification                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..              |                      Setup                   ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                            Setup                            ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                            Setup                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 6: Packed Configuration Figure

   The Ident field is set with the value that will be used by the Raw
   Payload Packets to address this Configuration.  The Fragment type is
   set to 0 since the packet bears the full Packed configuration, the
   number of packet is set to 1.  In practice, Packed Headers usually
   need to be fragmented to fit the path MTU.

3.2.  Out of Band Transmission

   This section, as stated above, does not cover all the possible out-
   of-band delivery methods since they rely on different protocols and
   be linked to specific applications.  The following packet definition
   SHOULD be used in out-of-band delivery and MUST be used when
   Configuration is inlined in the SDP.






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3.2.1.  Packed Headers

   As mentioned above, the recommended delivery vector for Theora
   configuration data is via a retrieval method that can be performed
   using a reliable transport protocol.  As the RTP headers are not
   required for this method of delivery the structure of the
   configuration data is slightly different.  The packed header starts
   with a 32 bit count field which details the number of packed headers
   that are contained in the bundle.  Next is the Packed header payload
   for each setup id.

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Number of packed headers                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Packed header                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Packed header                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 7: Packed Headers Overview

   Since the Configuration Ident and the Identification Header are fixed
   length there is only a 16bit Length tag to define the length of the
   packed headers.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Configuration Ident                |              ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..   Length     |              Identification Header           ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                    Identification Header                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Setup Header                        ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                         Setup Header                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 8: Packed Headers Detail

   The key difference from the in-band format is that there is no need
   for the payload header octet.






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3.2.1.1.  Packed Headers IANA Considerations

   The following IANA considerations MUST only be applied to the packed
   headers.

   MIME media type name: audio

   MIME subtype: theora-config

   Required Parameters:

      None

   Optional Parameters:

      None

   Encoding considerations:

      This media type contains binary data.

   Security Considerations:

      See Section 6 of RFC XXXX.

   Interoperability considerations:

      None

   Published specification:

      RFC XXXX [RFC Editor: please replace by the RFC number of this
      memo, when published]

   Applications which use this media type:

      Theora encoded video, configuration data.

   Additional information:

      None

   Person & email address to contact for further information:

      Luca Barbato: <lu_zero@gentoo.org>
      IETF Audio/Video Transport Working Group





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   Intended usage: COMMON

   Restriction on usage:

      This media type does not depend on the transport.

   Author:

      Luca Barbato

   Change controller:

      IETF AVT Working Group

3.3.  Loss of Configuration Headers

   Unlike the loss of raw Theora payload data, the loss of a
   configuration header can lead to a situation where it will not be
   possible to successfully decode the stream.

   A loss of a Configuration Packet results in the halting of stream
   decoding and SHOULD be reported to the client as well as a loss
   report sent via RTCP.


4.  Comment Headers

   When the payload type is set to 2, the packet contains the comment
   metadata, such as artist name, track title and so on.  These metadata
   messages are not intended to be fully descriptive but to offer basic
   title information.  Clients MAY ignore them completely.  The details
   on the format of the comments can be found in the Theora
   documentation [16].


















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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |             xxxx              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Configuration Ident              | 0 | 2 |      1|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            length             |            Comment           ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                           Comment                           ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                           Comment                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 9: Comment Packet

   The 2 byte length field is necessary since this Theora packet could
   be fragmented.


5.  Frame Packetizing

   Each RTP packet contains either one complete Theora packet, one
   Theora packet fragment, or an integer number of complete Theora
   packets (up to a maximum of 15 packets, since the number of packets
   is defined by a 4 bit value).

   Any Theora data packet that is less than path MTU SHOULD be bundled
   in the RTP packet with as many Theora packets as will fit, up to a
   maximum of 15.  Path MTU is detailed in [7] and [8].

   A fragmented packet has a zero in the last four bits of the payload
   header.  The RTP packet containing the first fragment will set the
   Fragment type to 1.  Each RTP packet after the first will set the
   Fragment type to 2 in the payload header.  The RTP packet containing
   the last fragment of the Theora packet will have the Fragment type
   set to 3.  If the fragmented Theora packet spans only two RTP
   packets, the first will set the Fragment type field to 1 and the
   second will set it to 2.  To maintain the correct sequence for
   fragmented packet reception the timestamp field of fragmented packets



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   MUST be the same as the first packet sent, with the sequence number
   incremented as normal for the subsequent RTP packets.

5.1.  Example Fragmented Theora Packet

   Here is an example fragmented Theora packet split over three RTP
   packets.  Each packet contains the standard RTP headers as well as
   the 4 octets Theora headers.

      Packet 1:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |           1000                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Configuration Ident              | 1 | 0 |      0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Payload Length         |           Theora data        ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Theora data                          ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 10: Example Fragmented Packet (Packet 1)

   In this packet the initial sequence number is 1000 and the timestamp
   is xxxxx.  The Fragment type field is set to one, indicating it is
   the start packet of a serie of fragments.  The number of packets
   field is set to 0, and as the payload is raw Theora data the Theora
   payload type field is set to 0.













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      Packet 2:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |           1001                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Configuration Ident              | 2 | 0 |      0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Payload Length         |              ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                        Theora data                          ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 11: Example Fragmented Packet (Packet 2)

   The Fragment type field is set to 2 and the number of packets field
   is set to 0.  For large Theora fragments there can be several of
   these type of payload packets.  The maximum RTP packet size SHOULD be
   no greater than the path MTU, including all RTP and payload headers.
   The sequence number has been incremented by one but the timestamp
   field remains the same as the initial packet.





















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      Packet 3:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|X|  CC   |M|     PT      |           1002                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             xxxxx                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           synchronization source (SSRC) identifier            |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |            contributing source (CSRC) identifiers             |
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Configuration Ident              | 3 | 0 |      0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Payload Length         |                              ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ..                         Theora data                         ..
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 12: Example Fragmented Packet (Packet 3)

   This is the last Theora fragment packet.  The Fragment type filed is
   set to 3 and the packet count remains set to 0.  As in the previous
   packets the timestamp remains set to the first packet in the sequence
   and the sequence number has been incremented.

5.2.  Packet Loss

   As there is no error correction within the Theora stream, packet loss
   will result in a loss of signal.  Packet loss is more of an issue for
   fragmented Theora packets as the client will have to cope with the
   handling of the Fragment type field.  If we use the fragmented Theora
   packet example above and the first packet is lost the client MUST
   detect that the next packet has the packet count field set to 0 and
   the Fragment type is set to 2 and MUST drop it.  The next packet,
   which is the final fragmented packet, MUST be dropped in the same
   manner.  Feedback reports on lost and dropped packets MUST be sent
   back via RTCP.[note: reordering]

   If a particular multicast session has a large number of participants
   care must be taken to prevent an RTCP feedback implosion, [9], in the
   event of packet loss from a large number of participants.

   Loss of any of the Configuration fragment will result in the loss of
   the full Configuration packet as detailed in the Loss of



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   Configuration Headers (Section 3.3) section.


6.  IANA Considerations

   MIME media type name: video

   MIME subtype: theora

   Required Parameters:

      sampling: Determines the chroma subsampling format.

      width: Determines the number of pixels per line.  This is an
         integer between 1 and 1048561 and MUST be in multiples of 16.

      height: Determines the number of lines per frame encoded.  This is
         an integer between 1 and 1048561 and MUST be in multiples of
         16.

      delivery-method: indicates the delivery methods in use, the
         possible values are: inline, in_band, out_band/specific_name
         Where "specific_name" is the name of the out of band delivery
         method.

      configuration: the base16 [11] (hexadecimal) representation of the
         Packed Headers (Section 3.2.1).

   Optional Parameters:

      configuration-uri: the URI of the configuration headers in case of
         out of band transmission.  In the form of
         "protocol://path/to/resource/".  Depending on the specific
         method the single ident packets could be retrived by their
         number or aggregated in a single stream, aggregates MAY be
         compressed using gzip [12] or bzip2 [14] and an sha1 [13]
         checksum MAY be provided in the form of
         "protocol://path/to/resource/aggregated.bz2!sha1hash"

   Encoding considerations:

      This media type is framed and contains binary data.

   Security Considerations:

      See Section 6 of RFC XXXX.





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   Interoperability considerations:

      None

   Published specification:

      RFC XXXX [RFC Editor: please replace by the RFC number of this
      memo, when published]

      Ogg Theora I specification: Codec setup and packet decode.
      Available from the Xiph website, http://www.xiph.org

   Applications which use this media type:

      Audio streaming and conferencing tools

   Additional information:

      None

   Person & email address to contact for further information:

      Luca Barbato: <lu_zero@gentoo.org>
      IETF Audio/Video Transport Working Group

   Intended usage:

      COMMON

   Restriction on usage:

      This media type depends on RTP framing, and hence is only defined
      for transfer via RTP [3]

   Author:

      Luca Barbato

   Change controller:

      IETF AVT Working Group


6.1.  Mapping MIME Parameters into SDP

   The information carried in the MIME media type specification has a
   specific mapping to fields in the Session Description Protocol (SDP)
   [6], which is commonly used to describe RTP sessions.  When SDP is



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   used to specify sessions the mapping are as follows:

   o  The MIME type ("video") goes in SDP "m=" as the media name.

   o  The MIME subtype ("theora") goes in SDP "a=rtpmap" as the encoding
      name.

   o  The clock rate in the "a=rtpmap" line MUST be 90000

   o  The mandated parameters "delivery-method" and "configuration" MUST
      be included in the SDP "a=fmpt" attribute.

   o  The optional parameter "configuration-uri", when present, MUST be
      included in the SDP "a=fmpt" attribute and MUST follow the
      delivery-method that applies.

   If the stream uses multiple decoder setup configurations and all of
   them are known in advance, the Configuration Packet for each file
   SHOULD be packaged together and passed to the client using the
   configuration attribute.

   The URI specified in the configuration-uri attribute MUST point to a
   location where all of the Configuration Packets needed for the life
   of the session reside.

6.1.1.  SDP Example

   The following example shows a basic SDP for a single stream.  The
   first configuration packet is inlined in the sdp, other
   configurations could be fetched at any time from the first provided
   uri using or all the known configuration could be downloaded using
   the second uri.  The inline base16 [11] configuration string is
   omitted because of the lenght.
      c=IN IP4 192.0.0.1
      m=video RTP/AVP 98
      a=rtpmap:98 theora/90000
      a=fmtp:98 sampling=YCbCr-4:2:2; width=1280; height=720; delivery-
      method=inline; configuration=base16string1; delivery-
      method=out_band/rtsp; delivery-method=out_band/rtsp;
      configuration-uri=rtsp://path/to/resource/; delivery-
      method=out_band/http; configuration-uri=http://another/path/to/
      resource/aggregate.bz2!sha1hash;

6.2.  Usage with the SDP Offer/Answer Model

   The offer, as described in An Offer/Answer Model Session Description
   Protocol [5], may contain a large number of delivery methods per
   single fmtp attribute, the answerer MUST remove every delivery-method



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   and configuration-uri not supported.  All the parameters MUST not be
   altered on answer otherwise.


7.  Examples

   The following examples are common usage patterns that MAY be applied
   in such situations, the main scope of this section is to explain
   better usage of the transmission vectors.

7.1.  Stream Video

   This is one of the most common situation: one single server streaming
   content in multicast, the clients may start a session at random time.
   The content itself could be a mix of live stream, as the wj's voice
   or studio scenes, and stored streams, as the music she plays.

   In this situation we don't know in advance how many codebooks we will
   use.  The clients can join anytime and users expect to start the
   fruition of the content in a short time.

   On join the client will receive the current Configuration necessary
   to decode the current streams inlined in the SDP so that the decoding
   will start immediately after.

   When the streamed content changes the new Configuration is sent in-
   band before the actual stream, and the Configuration that has to be
   sent inline in the SDP updated.  Since the inline method is
   unreliable, an out of band fallback is provided.

   The client could choose to fetch the Configuration from the alternate
   source as soon it discovers a Configuration packet got lost inline or
   use selective retransmission [17], if the server supports the
   feature.

   A serverside optimization would be to keep an hash list of the
   Configurations per session to avoid packing all of them and send the
   same Configuration with different Ident tags

   A clientside optimization would be to keep a tag list of the
   Configurations per session and don't process configuration packets
   already known.


8.  Security Considerations

   RTP packets using this payload format are subject to the security
   considerations discussed in the RTP specification [3].  This implies



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   that the confidentiality of the media stream is achieved by using
   encryption.  Because the data compression used with this payload
   format is applied end-to-end, encryption may be performed on the
   compressed data.  Where the size of a data block is set care MUST be
   taken to prevent buffer overflows in the client applications.


9.  Acknowledgments

   This document is a continuation of draft-kerr-avt-theora-rtp-00.txt

   Thanks to the AVT, Ogg Theora Communities / Xiph.org, Fluendo, Ralph
   Giles, Mike Smith, Phil Kerr, Timothy Terriberry, Stefan Ehmann,
   Alessandro Salvatori, Politecnico di Torino (LS)^3/IMG Group in
   particular Federico Ridolfo, Francesco Varano, Giampaolo Mancini,
   Juan Carlos De Martin.


10.  References

10.1.  Normative References

   [1]   Pfeiffer, S., "The Ogg Encapsulation Format Version 0",
         RFC 3533.

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

   [3]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for real-time applications",
         RFC 3550.

   [4]   Schulzrinne, H. and S. Casner, "RTP Profile for video and Video
         Conferences with Minimal Control.", RFC 3551.

   [5]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264.

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

   [7]   Mogul et al., J., "Path MTU Discovery", RFC 1063.

   [8]   McCann et al., J., "Path MTU Discovery for IP version 6",
         RFC 1981.

   [9]   Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
         "Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)",



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         Internet Draft (draft-ietf-avt-rtcp-feedback-11: Work in
         progress).

   [10]  Barbato, L., "RTP Payload Format for Vorbis Encoded Audio -
         draft-ietf-avt-vorbis-rtp-00", Internet Draft (Work in
         progress).

   [11]  Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
         RFC 3548.

   [12]  Deutsch, P., "GZIP file format specification version 4.3",
         RFC 1952.

   [13]  National Institute of Standards and Technology, "Secure Hash
         Standard", May 1993.

   [14]  Seward, J., "libbz2 and bzip2".

10.2.  Informative References

   [15]  "libTheora: Available from the Xiph website,
         http://www.xiph.org".

   [16]  "Theora I specification:  Codec setup and packet decode.
         http://www.xiph.org/theora/doc/Theora_I_spec.pdf".

   [17]  Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol
         Extended Reports (RTCP XR)", RFC 3611, November 2003.

   [18]  "ITU-T Recommendation V.42, 1994, Rev. 1. Error-correcting
         Procedures for DCEs Using Asynchronous-to-Synchronous
         Conversion. International Telecommunications Union. Available
         from the ITU website, http://www.itu.int".

   [19]  "ISO 3309, October 1984, 3rd Edition. Information Processing
         Systems--Data Communication High-Level Data Link Control
         Procedure--Frame Structure. International Organization for
         Standardization.".













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Author's Address

   Luca Barbato
   Xiph.Org

   Email: lu_zero@gentoo.org
   URI:   http://www.xiph.org/












































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