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

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   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: <[e-mail]>
      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