vorbis/doc/draft-kerr-avt-vorbis-rtp-04.xml

<?xml version='1.0'?>
<!DOCTYPE rfc SYSTEM 'rfc2629.dtd'>
<?rfc toc="yes" ?>

<rfc ipr="full3667" docName="RTP Payload Format for Vorbis Encoded Audio">

<front>
<title>draft-kerr-avt-vorbis-rtp-04</title>

<author initials="P" surname="Kerr" fullname="Phil Kerr">
<organization>Xiph.Org</organization>
<address>
<email>phil@plus24.com</email>
<uri>http://www.xiph.org/</uri>
</address>
</author>

<date day="31" month="December" year="2004" />

<area>General</area>
<workgroup>AVT Working Group</workgroup>
<keyword>I-D</keyword>

<keyword>Internet-Draft</keyword>
<keyword>Vorbis</keyword>
<keyword>RTP</keyword>

<abstract>
<t>This document describes a RTP payload format for transporting 
Vorbis encoded audio.  It details the RTP encapsulation mechanism 
for raw Vorbis data and details the delivery mechanisms for the 
decoder probability model, referred to as a codebook, metadata 
and other setup information.</t>
</abstract>

<note title="Editors Note">
<t>
All references to RFC XXXX are to be replaced by references to the RFC number of this memo, when published.
</t>
</note>

</front>

<middle>

<section anchor="Introduction" title="Introduction">
<t>
Vorbis is a general purpose perceptual audio codec intended to allow 
maximum encoder flexibility, thus allowing it to scale competitively 
over an exceptionally wide range of bitrates.   At the high 
quality/bitrate end of the scale (CD or DAT rate stereo, 
16/24 bits), it is in the same league as MPEG-2 and MPC. Similarly, 
the 1.0 encoder can encode high-quality CD and DAT rate stereo at 
below 48k bits/sec without resampling to a lower rate.   Vorbis is 
also intended for lower and higher sample rates (from 8kHz 
telephony to 192kHz digital masters) and a range of channel 
representations (monaural, polyphonic, stereo, quadraphonic, 5.1, 
ambisonic, or up to 255 discrete channels).

Vorbis encoded audio is generally encapsulated within an Ogg format 
bitstream <xref target="rfc3533"></xref>, which provides framing and synchronization.  For the 
purposes of RTP transport, this layer is unnecessary, and so raw 
Vorbis packets are used in the payload.
</t>

<section anchor="Terminology" title="Terminology">

<t>
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 <xref target="rfc2119"></xref>.
</t>

</section>
</section>

<section anchor="Payload Format" title="Payload Format">
<t>
For RTP based transportation of Vorbis encoded audio the standard 
RTP header is followed by a 5 octet payload header, then the payload 
data.  The payload headers are used to associate the Vorbis data with 
its associated decoding codebooks as well as indicating if the following packet
contains fragmented Vorbis data and/or the the number of whole Vorbis
data frames.  The payload data contains the raw Vorbis bitstream 
information.
</t>

<section anchor="RTP Header" title="RTP Header">

<artwork><![CDATA[
    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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>

<t>
The RTP header begins with an octet of fields (V, P, X, and CC) to 
support specialized RTP uses (see <xref target="rfc3550"></xref> and <xref target="rfc3551"></xref> for details). For Vorbis RTP, the following values are used.
</t>

<t>
Version (V): 2 bits</t><t>
This field identifies the version of RTP. The version
used by this specification is two (2).
</t>

<t>
Padding (P): 1 bit</t><t>
Padding MAY be used with this payload format according to 
section 5.1 of <xref target="rfc3550"></xref>.  
</t>

<t>
Extension (X): 1 bit</t><t>
Always set to 0, as audio silence suppression is not used by 
the Vorbis codec. 
</t>

<t>
CSRC count (CC): 4 bits</t><t>
The CSRC count is used in accordance with <xref target="rfc3550"></xref>.
</t>

<t>
Marker (M): 1 bit</t><t>
Set to zero.  Audio silence suppression not used.  This conforms
to section 4.1 of <xref target="vorbis-spec-ref"></xref>.
</t>

<t>
Payload Type (PT): 7 bits</t><t>
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 
Vorbis.
</t>

<t>
Sequence number: 16 bits</t><t>
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
<xref target="rfc3550"></xref>.
</t>

<t>
Timestamp: 32 bits</t><t>
A timestamp representing the sampling time of the first sample of
the first Vorbis packet in the RTP packet.  The clock frequency 
MUST be set to the sample rate of the encoded audio data and is 
conveyed out-of-band as a SDP attribute.
</t>

<t>
SSRC/CSRC identifiers: </t><t>
These two fields, 32 bits each with one SSRC field and a maximum 
of 16 CSRC fields, are as defined in <xref target="rfc3550"></xref>.  
</t>

</section>

<section anchor="Payload Header" title="Payload Header">

<t>
After the RTP Header section the following five octets are the Payload Header.  
This header is split into a number of bitfields detailing the format
of the following Payload Data packets.
</t>

<artwork><![CDATA[
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Codebook Ident                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |C|F| T |# pkts.|
   +-+-+-+-+-+-+-+-+
]]></artwork>

<t>
Codebook Ident: 32 bits</t><t>
This 32 bit field is used to associate the Vorbis data to a decoding Codebook.
It is created by making a CRC32 checksum of the codebook required to decode the 
particular Vorbis audio stream.
</t>

<t>
Continuation (C): 1 bit</t><t>
Set to one if this is a continuation of a fragmented packet.
</t>

<t>
Fragmented (F): 1 bit</t><t>
Set to one if the payload contains complete packets or if it
contains the last fragment of a fragmented packet. 
</t>

<t>
Payload Type (T): 2 bits</t><t>
This field sets the packet payload type.  There are currently four type of packet payloads. 
</t>

<vspace blankLines="1" />
<list style="empty">
<t>      0 = Raw Vorbis payload</t>
<t>      1 = Configuration payload</t>
<t>      2 = Codebook payload</t>
<t>      3 = Metadata payload</t>
</list>

<t>
The last 4 bits are the number of complete packets in this payload.  
This provides for a maximum number of 15 Vorbis packets in the 
payload.  If the packet contains fragmented data the number of packets MUST be set to 0.
</t>

</section>

<section anchor="Payload Data" title="Payload Data">

<t>
Raw Vorbis packets are unbounded in length currently, although at some future
point there will likely be a practical limit placed on them.  
Typical Vorbis packet sizes are from very small (2-3 bytes) to 
quite large (8-12 kilobytes).  The reference implementation <xref target="libvorbis"></xref>
typically produces packets less than ~800 bytes, except for the
codebook header packets which are ~4-12 kilobytes.
Within an RTP context the maximum Vorbis packet size, including the RTP and payload 
headers, SHOULD be kept below the path MTU to avoid packet fragmentation.  
</t>

<t>
Each Vorbis payload packet starts with a one octet length header, 
which is used to represent the size of the following data payload, followed 
by the raw Vorbis data.
</t>

<t>
For payloads which consist of multiple Vorbis packets the payload data 
consists of the packet length followed by the packet data for each of 
the Vorbis packets in the payload.
</t>

<t>
The Vorbis packet length header is the length of the Vorbis data 
block only and does not count the length octet.
</t>

<t>
The payload packing of the Vorbis data packets SHOULD follow the
guidelines set-out in <xref target="rfc3551"></xref> where the oldest packet
occurs immediately after the RTP packet header.
</t>

<t>
Channel mapping of the audio is in accordance with BS. 775-1
ITU-R.
</t>

</section>

<section anchor="Example RTP Packet" title="Example RTP Packet">

<t>
Here is an example RTP packet containing two Vorbis packets.
</t>
<t>
RTP Packet Header:
</t>

<artwork><![CDATA[
    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 (in sample rate units)              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          synchronisation source (SSRC) identifier             |
   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
   |            contributing source (CSRC) identifiers             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>

<t>
Payload Data:
</t>

<artwork><![CDATA[
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Codebook Ident                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|1| 0 | 2 pks |      len      |         vorbis data ...       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ..                     ...vorbis data...                       ..
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ..    data      |      len      |   next vorbis packet data...  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>

</section>

</section>

<section anchor="Frame Packetizing" title="Frame Packetizing">

<t>
Each RTP packet contains either one complete Vorbis packet, one 
Vorbis packet fragment, or an integer number of complete Vorbis 
packets (up to a max of 15 packets, since the number of packets 
is defined by a 4 bit value).
</t>

<t>
Any Vorbis data packet that is 256 octets or less SHOULD be bundled in the
RTP packet with as many Vorbis packets as will fit, up to a maximum
of 15.
</t>

<t>
If a Vorbis packet is larger than 256 octets it MUST be
fragmented.  A fragmented packet has a zero in the last four bits 
of the payload header.  Each fragment after the first will also set 
the Continued (C) bit to one in the payload header.  The RTP packet 
containing the last fragment of the Vorbis packet will have the 
Fragmented (F) bit set to one.  To maintain the correct sequence
for fragmented packet reception the timestamp field of fragmented 
packets MUST be the same as the first packet sent, with the sequence 
number incremented as normal for the subsequent RTP packets.  Path 
MTU is detailed in <xref target="rfc1063"></xref>  and <xref target="rfc1981"></xref>.
</t>

<section anchor="Example Fragmented Vorbis Packet" title="Example Fragmented Vorbis Packet">

<t>
Here is an example fragmented Vorbis packet split over three RTP
packets.  
</t>

<artwork><![CDATA[
   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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Codebook Ident                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|0| 0 |      0|      len      |         vorbis data ..        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ..vorbis data..                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>

<t>
In this packet the initial sequence number is 1000 and the 
timestamp is xxxxx.  The number of packets field is set to 0.
</t>

<artwork><![CDATA[
   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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Codebook Ident                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0| 0 |      0|      len      |         vorbis data ...       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ..vorbis data..                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>

<t>
The C bit is set to 1 and the number of packets field is set to 0.
For large Vorbis fragments there can be several of these type of
payload packets.  The maximum 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.
</t>

<artwork><![CDATA[
   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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Codebook Ident                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|1| 0 |      0|      len      |         vorbis data ..        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ..vorbis data..                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>

<t>
This is the last Vorbis fragment packet.  The C and F bits are 
set 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.
</t>
</section>

<section anchor="Packet Loss" title="Packet Loss">

<t>
As there is no error correction within the Vorbis stream, packet 
loss will result in a loss of signal.  Packet loss is more of an 
issue for fragmented Vorbis packets as the client will have to 
cope with the handling of the C and F flags.  If we use the 
fragmented Vorbis packet example above and the first packet is
lost the client SHOULD detect that the next packet has the packet
count field set to 0 and the C bit is set and MUST drop it.  The
next packet, which is the final fragmented packet, SHOULD be dropped
in the same manner, or buffered.  Feedback reports on lost and 
dropped packets MUST be sent back via RTCP.
</t>

</section>

</section>

<section anchor="Configuration Headers" title="Configuration Headers">

<t>
Unlike other mainstream audio codecs Vorbis has no statically 
configured probability model, instead it packs all entropy decoding
configuration, VQ and Huffman models into a self-contained codebook.
This codebook block also requires additional identification 
information detailing the number of audio channels, bitrates and
other information used to initialise the Vorbis stream.
</t>

<t>
To decode a Vorbis stream three configuration header blocks are
needed.  The first header indicates the sample and bitrates, the 
number of channels and the version of the Vorbis encoder used.  
The second header contains the decoders probability model, or 
codebook and the third header details stream metadata.
</t>

<t>
As the RTP stream may change certain configuration data mid-session
there are two different methods for delivering this configuration
data to a client, in-band and SDP which is 
detailed below.  SDP delivery is used to set-up an initial
state for the client application and in-band is used to change state
during the session.  The changes may be due to different metadata
or codebooks as well as different bitrates of the stream.
</t>

<t>
Out of the two delivery vectors the use of an SDP attribute to indicate an URI 
where the configuration and codebook data can be obtained is preferred  
as they can be fetched reliably using TCP.  The in-band codebook delivery SHOULD 
only be used in situations where the link between the client is unidirectional or if 
the SDP-based information is not available. 
</t>

<t>
Synchronizing the configuration and codebook headers to the RTP stream is 
critical.  The 32 bit Codebook Ident field is used to indicate when a change in the stream has
taken place.  The client application MUST have in advance the correct configuration and codebook 
headers and if the client detects a change in the Ident value and does not have this information 
it MUST NOT decode the raw Vorbis data.
</t>

<section anchor="In-band Header Transmission" title="In-band Header Transmission">

<t>
The three header data blocks are sent in-band with the packet type bits set to 
match the payload type.  Normally the codebook and configuration 
headers are sent once per session if the stream is an encoding of live audio, as typically 
the encoder state will not change, but the encoder state can change at the boundary 
of chained Vorbis audio files.  Metadata can be sent at the start as well as any time during 
the life of the session.  Clients MUST be capable of dealing with periodic re-transmission of the 
configuration headers.
</t>

<t>
A Vorbis configuration header is indicated with the payload type field set to 1.
The Vorbis version MUST be set to zero to comply with
this document.  The fields Sample Rate, Bitrate Maximum/Nominal/
Minimum and Num Audio Channels are set in accordance with <xref target="vorbis-spec-ref"></xref> with 
the bsz fields above referring to the blocksize parameters.  The 
framing bit is not used for RTP transportation and so applications 
constructing Vorbis files MUST take care to set this if required.
</t>

<artwork><![CDATA[
    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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Codebook Ident                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|1| 2 |      1| bsz 0 | bsz 1 |       Num Audio Channels      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Vorbis Version                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Audio Sample Rate                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Bitrate Maximum                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Bitrate Nominal                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Bitrate Minimum                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>

<t>
If the payload type field is set to 2, this indicates the packet contains codebook data.
</t>

<t>
The configuration information detailed below MUST be completely 
intact, as a client can not decode a stream with an incomplete 
or corrupted codebook set.
</t>

<t>
A 16 bit codebook length field precedes the codebook datablock.  The length field 
allows for codebooks to be up to 64K in size.  Packet fragmentation, 
as per the Vorbis data, MUST be performed if the codebooks size exceeds
path MTU.  The Codebook Ident field MUST be set to match the associated codebook 
needed to decode the Vorbis stream.
</t>

<t>
The Codebook Ident is the CRC32 checksum of the codebook and 
is used to detect a corrupted codebook as well as 
associating it with its Vorbis data stream.  This Ident value 
MUST NOT be set to the value of the current stream if this header is
being sent before the boundary of the chained file has been reached.
If a checksum failure is detected then this is considered to 
be a failure and MUST be reported to the client application.
</t>

<artwork><![CDATA[
    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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Codebook Ident                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|1| 2 |      1|           Codebook Length                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    length     |           Codebook                           ..
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ..                          Codebook                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>


<t>
With the payload type flag set to 3, this indicates that the packet contain 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 track/song information.  This message MUST be sent at 
the start of the stream, together with the setup and codebook 
headers, even if it contains no information.  During a session the
metadata associated with the stream may change from that specified 
at the start, e.g. a live concert broadcast changing acts/scenes, so
clients MUST have the ability to receive header blocks.  Details
on the format of the comments can be found in the Vorbis 
documentation <xref target="v-comment"></xref>.
</t>


  1) [vendor_length] = read an unsigned integer of 32 bits
  2) [vendor_string] = read a UTF-8 vector as [vendor_length] octets
  3) [user_comment_list_length] = read an unsigned integer of 32 bits
  4) iterate [user_comment_list_length] times {

       5) [length] = read an unsigned integer of 32 bits
       6) this iteration's user comment = read a UTF-8 vector as [length] octets

     }

  7) [framing_bit] = read a single bit as boolean
  8) if ( [framing_bit]  unset or end of packet ) then ERROR
  9) done.


<t>
The format for the data takes the form of a 32 bit codec vendors
name length field followed by the name encoded in UTF-8.  The next 32 
bit field denotes the number of user comments. Each of the user comments 
is prefixed by a 32 bit length field followed by the comment text.   
</t>

<artwork><![CDATA[
    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             |
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Codebook Ident                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|1| 3 |      1|          Vendor string length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    length     |          Vendor string                       ..
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    User comments list length                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ..               User comment length / User comment             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>

</section>

<section anchor="Session Description for Vorbis RTP Streams" title="Session Description for Vorbis RTP Streams"> 
<t>
Session description information concerning the Vorbis stream 
SHOULD be provided if possible and MUST be in accordance with <xref target="rfc2327"></xref>.  
</t>

<t>
If the stream comprises chained Vorbis files the configuration and codebook headers for each
file SHOULD be packaged together and passed to the client using the headers attribute.
</t>

<t>
Below is an outline of the mandatory SDP attributes.
</t>

<vspace blankLines="1" />
<list style="empty">
<t>c=IN IP4/6 </t>
<t>m=audio  RTP/AVP 98</t>
<t>a=rtpmap:98 VORBIS/44100/2</t>
<t>a=fmtp:98 header=&lt;URI of configuration header&gt; </t>
</list>

<t>
The Vorbis configuration specified in the header attribute MUST contain
all of the configuration data and codebooks needed for the life of the session.  
</t>

<t>
The port value is specified by the server application bound to 
the address specified in the c attribute.  The bitrate value 
and channels specified in the m attribute MUST match the Vorbis 
sample rate value.
</t>

</section>

<section anchor="Codebook Caching" title="Codebook Caching"> 

<t>
Codebook caching allows clients that have previously connected to a 
stream to re-use the associated codebooks and configuration data.  
When a client receives a codebook it may store it locally and can 
compare the CRC32 key with that of the new stream and begin decoding 
before it has received any of the headers.
</t>   

</section>

</section>

<section anchor="IANA Considerations" title="IANA Considerations"> 

<t>
MIME media type name: audio
</t>
<t>
MIME subtype: vorbis
</t>

<t>
Required Parameters:</t><t>
header indicates the URI of the decoding configuration headers.
</t>

<t>
Optional Parameters: </t><t>
None.
</t>

<t>
Encoding considerations:</t><t>
This type is only defined for transfer via RTP as specified 
in RFC XXXX.
</t>

<t>
Security Considerations:</t><t>
See Section 6 of RFC 3047.
</t>

<t>
Interoperability considerations: none
</t>

<t>
Published specification:</t>
<t>See the Vorbis documentation <xref target="vorbis-spec-ref"></xref> for details.</t>

<t>
Applications which use this media type:</t><t>
Audio streaming and conferencing tools
</t>

<t>
Additional information: none
</t>

<t>
Person &amp; email address to contact for further information:</t><t>
Phil Kerr: &lt;phil@plus24.com&gt;
</t>

<t>
Intended usage: COMMON
</t>

<t>
Author/Change controller:</t><t>
Author: Phil Kerr
Change controller: IETF AVT Working Group
</t>

</section>

<section anchor="Congestion Control" title="Congestion Control"> 

<t>
Vorbis clients SHOULD send regular receiver reports detailing 
congestion.  A mechanism for dynamically downgrading the stream, 
known as bitrate peeling, will allow for a graceful backing off
of the stream bitrate.  This feature is not available at present
so an alternative would be to redirect the client to a lower 
bitrate stream if one is available.  
</t>

</section> 

<section anchor="Security Considerations" title="Security Considerations"> 
<t>
RTP packets using this payload format are subject to the security 
considerations discussed in the RTP specification <xref target="rfc3550"></xref>.  This implies 
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.
</t>

</section> 

<section anchor="Acknowledgments" title="Acknowledgments"> 

<t>
This document is a continuation of draft-moffitt-vorbis-rtp-00.txt.
The MIME type section is a continuation of draft-short-avt-rtp-
vorbis-mime-00.txt
</t>

<t>
Thanks to the AVT, Ogg Vorbis Communities / Xiph.org including 
Steve Casner, Aaron Colwell, Ross Finlayson, Ramon Garcia, Pascal Hennequin, Ralph Giles, 
Tor-Einar Jarnbjo, Colin Law, John Lazzaro, Jack Moffitt, Christopher Montgomery,
Colin Perkins, Barry Short, Mike Smith, Magnus Westerlund.
</t>

</section> 

</middle>

<back>

<references title="Normative References">

<reference anchor="rfc3533">
<front>
<title>The Ogg Encapsulation Format Version 0</title>
<author initials="S." surname="Pfeiffer" fullname="Silvia Pfeiffer"></author>
</front>
<seriesInfo name="RFC" value="3533" />
</reference>

<reference anchor="rfc2119">
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels </title>
<author initials="S." surname="Bradner" fullname="Scott Bradner"></author>
</front>
<seriesInfo name="RFC" value="2119" />
</reference>   

<reference anchor="rfc3550">
<front>
<title>RTP: A Transport Protocol for real-time applications</title>
<author initials="H." surname="Schulzrinne" fullname=""></author>
<author initials="S." surname="Casner" fullname=""></author>
<author initials="R." surname="Frederick" fullname=""></author>
<author initials="V." surname="Jacobson" fullname=""></author>
</front>
<seriesInfo name="RFC" value="3550" />
</reference> 

<reference anchor="rfc3551">
<front>
<title>RTP Profile for Audio and Video Conferences with Minimal Control.</title>
<author initials="H." surname="Schulzrinne" fullname=""></author>
<author initials="S." surname="Casner" fullname=""></author>
</front>
<date month="July" year="2003" />
<seriesInfo name="RFC" value="3551" />
</reference> 
  
<reference anchor="rfc2327">
<front>
<title>SDP: Session Description Protocol</title>
<author initials="M." surname="Handley" fullname="Mark Handley"></author>
<author initials="V." surname="Jacobson" fullname="Van Jacobson"></author>
</front>
<seriesInfo name="RFC" value="2327" />
</reference>   

<reference anchor="rfc1063">
<front>
<title>Path MTU Discovery</title>
<author initials="J." surname="Mogul et al." fullname="J. Mogul et al."></author>
</front>
<seriesInfo name="RFC" value="1063" />
</reference>   

<reference anchor="rfc1981">
<front>
<title>Path MTU Discovery for IP version 6</title>
<author initials="J." surname="McCann et al." fullname="J. McCann et al."></author>
</front>
<seriesInfo name="RFC" value="1981" />
</reference>   
</references>

<references title="Informative References">
<reference anchor="libvorbis">
<front>
<title>libvorbis: Available from the Xiph website, http://www.xiph.org</title>
</front>
</reference>   

<reference anchor="vorbis-spec-ref">
<front>
<title>Ogg Vorbis I spec:  Codec setup and packet decode.  http://www.xiph.org/ogg/vorbis/doc/vorbis-spec-ref.html</title>
</front>
</reference>   
  
<reference anchor="v-comment">
<front>
<title>Ogg Vorbis I spec:  Comment field and header specification.  http://www.xiph.org/ogg/vorbis/doc/v-comment.html</title>
</front>
</reference>   
  
</references>
</back>
</rfc>