draft-barbato-avt-rtp-theora-01.txt

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   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: <[e-mail]>
      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