<rfc number="5215" category="std">

<front>

<title abbrev="Vorbis RTP Payload Format">RTP Payload Format for Vorbis Encoded Audio</title>

<author initials="L" surname="Barbato" fullname="Luca Barbato">

<organization abbrev="Xiph">Xiph.Org Foundation</organization>

<address>

<email>lu_zero@gentoo.org</email>

<uri>http://xiph.org/</uri>

</address>

</author>

<date month="August" year="2008" />

<area>General</area>

<workgroup>AVT Working Group</workgroup>

<keyword>I-D</keyword>

<keyword>Internet-Draft</keyword>

<keyword>Vorbis</keyword>

<keyword>RTP</keyword>

<keyword>example</keyword>

<abstract>

<t>

This document describes an RTP payload format for transporting Vorbis encoded

audio. It details the RTP encapsulation mechanism for raw Vorbis data and 

the delivery mechanisms for the decoder probability model (referred to

as a codebook), as well as other setup information.

</t>

<t>

Also included within this memo are media type registrations and the details

necessary for the use of Vorbis with the Session Description Protocol (SDP).

</t>

</abstract>

</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 bit rates. 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-4 AAC.

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).

</t>

<t>

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="Conformance and Document Conventions">

<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 BCP 14, <xref target="RFC2119"/> and indicate requirement levels for compliant implementations.  Requirements apply to all implementations unless otherwise stated.</t>

<t>An implementation is a software module that supports one of the media types defined in this document.  Software modules may support multiple media types, but conformance is considered individually for each type.</t>

<t>Implementations that fail to satisfy one or more "MUST" requirements are considered non-compliant.  Implementations that satisfy all "MUST" requirements, but fail to satisfy one or more "SHOULD" requirements, are said to be "conditionally compliant".  All other implementations are "unconditionally compliant".</t>

</section>

</section>

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

<t>

For RTP-based transport of Vorbis-encoded audio, the standard RTP header is

followed by a 4-octet payload header, and then the payload data. The payload

headers are used to associate the Vorbis data with its associated decoding

codebooks as well as indicate if the following packet contains fragmented

Vorbis data and/or the number of whole Vorbis data frames. The payload data

contains the raw Vorbis bitstream information. There are 3 types of Vorbis

data; an RTP payload MUST contain just one of them at a time.

</t>

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

<t>

The format of the RTP header is specified in <xref target="RFC3550"></xref>

and shown in <xref target="RTP Header Figure"/>.  This payload format

uses the fields of the header in a manner consistent with that specification.

</t>

<t>

<figure anchor="RTP Header Figure" title="RTP Header">

<artwork>

    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>

</figure>

</t>

<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>

The Extension bit is used in accordance with <xref target="RFC3550"></xref>.

</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 is 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 that

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 the 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 payload. The clock frequency MUST be set to the sample

rate of the encoded audio data and is conveyed out-of-band (e.g., as an SDP parameter).

</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>

The 4 octets following the RTP Header section are the Payload Header. This

header is split into a number of bit fields detailing the format of the

following payload data packets.

</t>

<figure anchor="Payload Header Figure" title="Payload Header">

<artwork>

    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

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

   |                     Ident                     | F |VDT|# pkts.|

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

]]></artwork>

</figure>

<t>

Ident: 24 bits</t>

<t>

This 24-bit field is used to associate the Vorbis data to a decoding

Configuration. It is stored as a network byte order integer.

</t>

<t>

Fragment type (F): 2 bits</t>

<t>

This field is set according to the following list:

</t>

<vspace blankLines="1" />

<list style="empty">

<t>      0 = Not Fragmented</t>

<t>      1 = Start Fragment</t>

<t>      2 = Continuation Fragment</t>

<t>      3 = End Fragment</t>

</list>

<t>

Vorbis Data Type (VDT): 2 bits</t>

<t>

This field specifies the kind of Vorbis data stored in this RTP packet. There

are currently three different types of Vorbis payloads. Each packet MUST contain only a single type of Vorbis packet (e.g., you must not aggregate configuration and comment packets in the same RTP payload).

</t>

<vspace blankLines="1" />

<list style="empty">

<t>      0 = Raw Vorbis payload</t>

<t>      1 = Vorbis Packed Configuration payload</t>

<t>      2 = Legacy Vorbis Comment payload</t>

<t>      3 = Reserved</t>

</list>

<t> The packets with a VDT of value 3 MUST be ignored.</t>

<t>

The last 4 bits represent the number of complete packets in this payload. This

provides for a maximum number of 15 Vorbis packets in the payload. If the

payload 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 currently unbounded in length; application profiles will

likely define a practical limit. Typical Vorbis packet sizes range 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 setup header packets, which are ~4-12 kilobytes. Within an

RTP context, to avoid fragmentation, the Vorbis data packet size SHOULD be kept

sufficiently small so that after adding the RTP and payload headers, the

complete RTP packet is smaller than the path MTU.

</t>

<figure anchor="Payload Data Figure" title="Payload Data Header">

<artwork>

    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

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

   |            length             |       vorbis packet data     ..

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

]]></artwork>

</figure>

<t>

Each Vorbis payload packet starts with a two octet length header, which is used

to represent the size in bytes of the following data payload, and is followed by the

raw Vorbis data padded to the nearest byte boundary, as explained by the <xref target="VORBIS-SPEC-REF">Vorbis I Specification</xref>. The length value is stored

as a network byte order integer.

</t>

<t>

For payloads that 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 include the length field.

</t>

<t>

The payload packing of the Vorbis data packets MUST follow the guidelines

set out in <xref target="RFC3551"></xref>, where the oldest Vorbis packet occurs

immediately after the RTP packet header. Subsequent Vorbis packets, if any, MUST

follow in temporal order.

</t>

<t>

Audio channel mapping is in accordance with the

<xref target="VORBIS-SPEC-REF">Vorbis I Specification</xref>.

</t>

</section>

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

<t>

Here is an example RTP payload containing two Vorbis packets.

</t>

<figure anchor="Example Raw Vorbis Packet" title="Example Raw Vorbis Packet">

<artwork>

    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             |

   |                              ...                              |

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

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

   |                     Ident                     | 0 | 0 | 2 pks |

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

   |            length             |          vorbis data         ..

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

   ..                        vorbis data                           |

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

   |            length             |   next vorbis packet data    ..

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

   ..                        vorbis data                          ..

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

   ..               vorbis data                    |

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

]]></artwork>

</figure>

<t>

The payload data section of the RTP packet begins with the 24-bit Ident field

followed by the one octet bit field header, which has the number of Vorbis

frames set to 2.  Each of the Vorbis data frames is prefixed by the two octets

length field. The Packet Type and Fragment Type are set to 0. The Configuration

that will be used to decode the packets is the one indexed by the ident value.

</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, Vector Quantization and Huffman models into a data block

that must be transmitted to the decoder with the compressed data.

A decoder also requires information detailing the number of audio 

channels, bitrates, and similar information to configure itself for a 

particular compressed data stream. These two blocks of information are 

often referred to collectively as the "codebooks" for a Vorbis stream,

and are included as special "header" packets at the start 

of the compressed data. In addition,

the <xref target="VORBIS-SPEC-REF">Vorbis I specification</xref>

requires the presence of a comment header packet that gives simple

metadata about the stream, but this information is not required for 

decoding the frame sequence.

</t>

<t>

Thus, these two codebook header packets must be received by the decoder before

any audio data can be interpreted. These requirements pose problems in RTP,

which is often used over unreliable transports.

</t>

<t>

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.

In order to set up an initial state for the client application, the

configuration MUST be conveyed via the signalling channel used to set up

the session. One example of such signalling is

<xref target="RFC4566">SDP</xref> with the

<xref target="RFC3264">Offer/Answer Model</xref>.

Changes to the configuration MAY be communicated via a re-invite,

conveying a new SDP, or sent in-band in the RTP channel.

Implementations MUST support an in-band delivery of updated codebooks,

and SHOULD support out-of-band codebook update using a new SDP file.

The changes may be due to different codebooks as well as 

different bitrates of the RTP stream.

</t>

<t>For non-chained streams, the recommended Configuration delivery

method is inside the <xref target="Packed Configuration">Packed

Configuration</xref> in the SDP as explained the 

target="Mapping Media Type Parameters into SDP"> Mapping Media Type

Parameters into SDP</xref>.

</t>

<t>

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. If the client detects a change in the Ident value and

does not have this information, it MUST NOT decode the raw associated Vorbis

data until it fetches the correct Configuration.

</t>

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

<t>

The <xref target="Packed Configuration">Packed Configuration</xref> Payload is

sent in-band with the packet type bits set to match the Vorbis Data Type.

Clients MUST be capable of dealing with fragmentation and periodic

<xref target="RFC4588">re-transmission of</xref> the configuration headers.

The RTP timestamp value MUST reflect the transmission time of the first data packet for which this configuration applies.

</t>

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

<t>

A Vorbis Packed Configuration is indicated with the Vorbis Data Type field set

to 1. Of the three headers defined in the

<xref target="VORBIS-SPEC-REF">Vorbis I specification</xref>, the

Identification and the Setup MUST be packed as they are, while the Comment

header MAY be replaced with a dummy one.</t>

<t>

The packed configuration stores Xiph codec

configurations in a generic way: the first field stores the number of the following packets

minus one (count field), the next ones represent the size of the headers

(length fields), and the headers immediately follow the list of length fields.

The size of the last header is implicit.</t>

<t>

The count and the length fields are encoded using the following logic: the data

is in network byte order; every byte has the most significant bit used

as a flag, and the following 7 bits are used to store the value.

The first 7 most significant bits are stored in the first byte.

If there are remaining bits, the flag bit is set to 1 and the subsequent

7 bits are stored in the following byte.

If there are remaining bits, set the flag to 1 and the same procedure is

repeated.

The ending byte has the flag bit set to 0. To decode, simply iterate

over the bytes until the flag bit is set to 0. For every byte, the data

is added to the accumulated value multiplied by 128.</t>

<t>

The headers are packed in the same order as they are present in Ogg <xref target="VORBIS-SPEC-REF" />:

Identification, Comment, Setup.</t>

<t>

The 2 byte length tag defines the length of the packed headers as the sum of

the Configuration, Comment, and Setup lengths.</t>

<figure anchor="Packed Configuration Figure" title="Packed Configuration Figure">

<artwork>

    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             |

   |                              ...                              |

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

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

   |                      Ident                    | 0 | 1 |      1|

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

   |           length              | n. of headers |    length1    |

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

   |    length2    |                  Identification              ..

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

   ..                        Identification                       ..

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

   ..                        Identification                       ..

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

   ..                        Identification                       ..

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

   ..               Identification                 |    Comment   ..

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

   ..                            Comment                          ..

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

   ..                            Comment                          ..

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

   ..                            Comment                          ..

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

   ..           Comment            |             Setup            ..

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

   ..                            Setup                            ..

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

   ..                            Setup                            ..

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

]]></artwork>

</figure>

<t>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 because the

packet bears the full Packed configuration. The number of the packet is set to 1.</t>

</section>

</section>

<section anchor="Out of Band Transmission" title="Out of Band Transmission">

<t>

The following packet definition MUST be used when Configuration is inside

in the SDP.

</t>

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

<t>

As mentioned above, the RECOMMENDED delivery vector for Vorbis 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 (network-byte ordered) count field, which details

the number of packed headers that are contained in the bundle. The

following shows the Packed header

payload for each chained Vorbis stream.

</t>

<figure anchor="Packed Headers Overview Figure" title="Packed Headers Overview">

<artwork>

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

   |                     Number of packed headers                  |

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

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

   |                          Packed header                        |

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

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

   |                          Packed header                        |

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

]]></artwork>

</figure>

<figure anchor="Packed Headers Detail Figure" title="Packed Headers Detail">

<artwork>

    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

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

   |                   Ident                       |    length    ..

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

   ..              | n. of headers |    length1    |    length2   ..

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

   ..              |             Identification Header            ..

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

   .................................................................

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

   ..              |         Comment Header                       ..

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

   .................................................................

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

   ..                        Comment Header                        |

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

   |                          Setup Header                        ..

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

   .................................................................

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

   ..                         Setup Header                         |

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

]]></artwork>

</figure>

<t>

The key difference between the in-band format and this one is that there is no

need for the payload header octet. In this figure, the comment has a size bigger

than 127 bytes.

</t>

</section>

</section>

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

<t>

Unlike the loss of raw Vorbis payload data, loss of a configuration header

leads to a situation where it will not be possible to successfully decode the

stream. Implementations MAY try to recover from an error by requesting again the

missing Configuration or, if the delivery method is in-band, by buffering the

payloads waiting for the Configuration needed to decode them.

The baseline reaction SHOULD either be reset or end the RTP session.

</t>

</section>

</section>

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

<t>

Vorbis Data Type flag set to 2 indicates that 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 rather to offer basic

track/song information. Clients MAY ignore it completely. The details on the

format of the comments can be found in the <xref target="VORBIS-SPEC-REF">Vorbis I Specification</xref>.

</t>

<figure anchor="Comment Packet Figure" title="Comment Packet">

<artwork>

    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             |

   |                              ...                              |

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

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

   |                      Ident                    | 0 | 2 |      1|

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

   |            length             |            Comment           ..

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

   ..                           Comment                           ..

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

   ..                           Comment                            |

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

]]></artwork>

</figure>

<t>

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

</t>

</section>

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

<t>

Each RTP payload contains either one Vorbis packet fragment or an integer

number of complete Vorbis packets (up to a maximum of 15 packets, since the

number of packets is defined by a 4-bit value).

</t>

<t>

Any Vorbis data packet that is less than path MTU SHOULD be bundled in the RTP

payload with as many Vorbis packets as will fit, up to a maximum of 15, except

when such bundling would exceed an application's desired transmission latency.

Path MTU is detailed in <xref target="RFC1191"></xref> and <xref target="RFC1981"></xref>.

</t>

<t>

A fragmented packet has a zero in the last four bits of the payload header.

The first fragment will set the Fragment type to 1. Each fragment after the

first will set the Fragment type to 2 in the payload header. The consecutive

fragments MUST be sent without any other payload being sent between the first

and the last fragment. The RTP payload containing the last fragment of the

Vorbis packet will have the Fragment type set to 3. 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 payloads; this will affect the

RTCP jitter measurement. The length field shows the fragment length.

</t>

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

<t>

Here is an example of a fragmented Vorbis packet split over three RTP payloads.

Each of them contains the standard RTP headers as well as the 4-octet Vorbis

headers.

</t>

<figure anchor="Example Fragmented Packet (Packet 1)" title="Example Fragmented Packet (Packet 1)">

<artwork>

   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                |

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

   |                            12345                              |

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

   |           synchronization source (SSRC) identifier            |

   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

   |            contributing source (CSRC) identifiers             |

   |                              ...                              |

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

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

   |                       Ident                   | 1 | 0 |      0|

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

   |             length            |            vorbis data       ..

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

   ..                        vorbis data                           |

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

]]></artwork>

</figure>

<t>

In this payload, the initial sequence number is 1000 and the timestamp is 12345.  The Fragment type is set to 1, the number of packets field is set to 0, and as

the payload is raw Vorbis data, the VDT field is set to 0.

</t>

<figure anchor="Example Fragmented Packet (Packet 2)" title="Example Fragmented Packet (Packet 2)">

<artwork>

   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                |

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

   |                             12345                             |

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

   |           synchronization source (SSRC) identifier            |

   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

   |            contributing source (CSRC) identifiers             |

   |                              ...                              |

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

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

   |                       Ident                   | 2 | 0 |      0|

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

   |             length            |          vorbis data         ..

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

   ..                        vorbis data                           |

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

]]></artwork>

</figure>

<t>

The Fragment type field is set to 2, and the number of packets field is set to 0.

For large Vorbis fragments, there can be several of these types of payloads.

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 payload.

</t>

<figure anchor="Example Fragmented Packet (Packet 3)" title="Example Fragmented Packet (Packet 3)">

<artwork>

   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                |

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

   |                             12345                             |

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

   |           synchronization source (SSRC) identifier            |

   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+

   |            contributing source (CSRC) identifiers             |

   |                              ...                              |

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

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

   |                      Ident                    | 3 | 0 |      0|

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

   |             length            |          vorbis data         ..

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

   ..                        vorbis data                           |

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

]]></artwork>

</figure>

<t>

This is the last Vorbis fragment payload.  The Fragment type is set to 3 and the

packet count remains set to 0. As in the previous payloads, the timestamp remains

set to the first payload timestamp 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

Fragment Type. In case of loss of fragments, the client MUST discard all the

remaining Vorbis fragments and decode the incomplete packet. If we use the

fragmented Vorbis packet example above and the first RTP payload is lost, the

client MUST detect that the next RTP payload has the packet count field set

to 0 and the Fragment type 2 and MUST drop it.

The next RTP payload, which is the final fragmented packet, MUST be dropped

in the same manner.

If the missing RTP payload is the last, the two fragments received will be

kept and the incomplete Vorbis packet decoded.

</t>

<t>

Loss of any of the Configuration fragment will result in the loss of the full

Configuration packet with the result detailed in the <xref target="Loss of Configuration Headers">Loss of Configuration Headers</xref> section.

</t>

</section>

</section>

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

<list style="hanging">

<t hangText="Type name:"> audio </t>

<t hangText="Subtype name:"> vorbis </t>

<t hangText="Required parameters:">

<list style="hanging">

<t hangText="rate:"> indicates the RTP timestamp clock rate as described in <xref target="RFC3551">RTP Profile for Audio and Video Conferences with Minimal Control</xref>.

</t>

<t hangText="channels:"> indicates the number of audio channels as described in <xref target="RFC3551">RTP Profile for Audio and Video Conferences with Minimal Control</xref>.

</t>

<t hangText="configuration:"> the <xref target="RFC4648">base64</xref> representation of the <xref target="Packed Headers">Packed Headers</xref>.

</t>

</list>

</t>

<t hangText="Encoding considerations:">

<vspace blankLines="1" />

This media type is framed and contains binary data.

</t>

<t hangText="Security considerations:">

<vspace blankLines="1" />

See Section 10 of RFC 5215.</t>

<t hangText="Interoperability considerations:">

<vspace blankLines="1" />

None</t>

<t hangText="Published specification:">

<vspace blankLines="1" />

RFC 5215

<vspace blankLines="1" />

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

</t>

<t hangText="Applications which use this media type:">

<vspace blankLines="1"/>

Audio streaming and conferencing tools </t>

<t hangText="Additional information:">

<vspace blankLines="1" />

None </t>

<t hangText="Person & email address to contact for further information:">

<vspace blankLines="1" />

Luca Barbato: <lu_zero@gentoo.org>

<vspace blankLines="0" />

IETF Audio/Video Transport Working Group

</t>

<t hangText="Intended usage:">

<vspace blankLines="1" />

COMMON</t>

<t hangText="Restriction on usage:">

<vspace blankLines="1" />

This media type depends on RTP framing, hence is only defined for transfer via <xref target="RFC3550">RTP</xref>.</t>

<t hangText="Author:">

<vspace blankLines="1"/>Luca Barbato</t>

<t hangText="Change controller:">

<vspace blankLines="1"/>IETF AVT Working Group delegated from the IESG</t>

</list>

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

<t>

The following IANA considerations refers to the split configuration <xref target="Packed Headers">Packed Headers</xref> used within RFC 5215.

</t>