AVT Working Group L. Barbato
Internet-Draft Xiph.Org
Expires: April 24, 2006 October 21, 2005
draft-kerr-avt-vorbis-rtp-05
RTP Payload Format for Vorbis Encoded Audio
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes an 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 and other setup
information.
Also included within the document are the necessary details for the
use of Vorbis with MIME and Session Description Protocol (SDP).
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Editors Note
All references to RFC XXXX are to be replaced by references to the
RFC number of this memo, when published.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. RTP Header . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Payload Header . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Payload Data . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Example RTP Packet . . . . . . . . . . . . . . . . . . . . 7
3. Configuration Headers . . . . . . . . . . . . . . . . . . . . 8
3.1. In-band Header Transmission . . . . . . . . . . . . . . . 9
3.1.1. Packed Configuration . . . . . . . . . . . . . . . . . 9
3.2. Out of Band Transmission . . . . . . . . . . . . . . . . . 10
3.2.1. Packed Headers . . . . . . . . . . . . . . . . . . . . 10
3.3. Loss of Configuration Headers . . . . . . . . . . . . . . 12
4. Comment Headers . . . . . . . . . . . . . . . . . . . . . . . 12
5. Frame Packetizing . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Example Fragmented Vorbis Packet . . . . . . . . . . . . . 14
5.2. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
6.1. Mapping MIME Parameters into SDP . . . . . . . . . . . . . 18
7. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 19
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Stream Radio . . . . . . . . . . . . . . . . . . . . . . . 19
9. Security Considerations . . . . . . . . . . . . . . . . . . . 20
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
11.1. Normative References . . . . . . . . . . . . . . . . . . . 21
11.2. Informative References . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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1. Introduction
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 version 1.1
reference 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 [1], 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.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].
2. Payload Format
For RTP based transportation of Vorbis encoded audio the standard RTP
header is followed by a 4 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.
2.1. RTP Header
The format of the RTP header is specified in [3] and shown in Figure
Figure 1. This payload format uses the fields of the header in a
manner consistent with that specification.
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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 | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP Header
The RTP header begins with an octet of fields (V, P, X, and CC) to
support specialized RTP uses (see [3] and [4] for details). For
Vorbis RTP, the following values are used.
Version (V): 2 bits
This field identifies the version of RTP. The version used by this
specification is two (2).
Padding (P): 1 bit
Padding MAY be used with this payload format according to section 5.1
of [3].
Extension (X): 1 bit
The Extension bit is used in accordance with [3].
CSRC count (CC): 4 bits
The CSRC count is used in accordance with [3].
Marker (M): 1 bit
Set to zero. Audio silence suppression not used. This conforms to
section 4.1 of [12].
Payload Type (PT): 7 bits
An RTP profile for a class of applications is expected to assign a
payload type for this format, or a dynamically allocated payload type
SHOULD be chosen which designates the payload as Vorbis.
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Sequence number: 16 bits
The sequence number increments by one for each RTP data packet sent,
and may be used by the receiver to detect packet loss and to restore
packet sequence. This field is detailed further in [3].
Timestamp: 32 bits
A timestamp representing the 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.
SSRC/CSRC identifiers:
These two fields, 32 bits each with one SSRC field and a maximum of
16 CSRC fields, are as defined in [3].
2.2. Payload Header
After the RTP Header section the following 4 octets are the Payload
Header. This header is split into a number of bitfields detailing
the format of the following payload data packets.
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.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Payload Header
Ident: 24 bits
This 24 bit field is used to associate the Vorbis data to a decoding
Configuration.
Fragment type (F): 2 bits
This field is set accordingly the following list
0 = Not Fragmented
1 = Start Fragment
2 = Continuation Fragment
3 = End Fragment
Vorbis Data Type (VDT): 2 bits
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This field sets the packet payload type for the Vorbis data. There
are currently three type of Vorbis payloads.
0 = Raw Vorbis payload
1 = Vorbis Packed Configuration payload
2 = Legacy Vorbis Comment payload
3 = Reserved
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.
2.3. Payload Data
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 [11]
typically produces packets less than ~800 bytes, except for the setup
header packets which are ~4-12 kilobytes. Within an RTP context the
maximum packet size, including the RTP and payload headers, SHOULD be
kept below the path MTU to avoid packet fragmentation.
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 ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Payload Data Header
Each Vorbis payload packet starts with a two octet length header,
which is used to represent the size of the following data payload,
followed by the raw Vorbis data padded to the nearest byte boundary.
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.
The Vorbis packet length header is the length of the Vorbis data
block only and does not count the length field.
The payload packing of the Vorbis data packets MUST follow the
guidelines set-out in [4] where the oldest packet occurs immediately
after the RTP packet header.
Channel mapping of the audio is in accordance with the Vorbis I
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Specification [12].
2.4. Example RTP Packet
Here is an example RTP packet containing two Vorbis 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 (in sample rate units) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronisation source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Example Packet (RTP Headers)
Payload Data:
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 | 0 | 0 | 2 pks |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | vorbis data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. vorbis data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | next vorbis packet data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. vorbis data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Example Packet (Payload Data)
The payload data section of the RTP packet starts with the 24 bit
Ident field followed by the one octet bitfield header, which has the
number of Vorbis frames set to 2. Each of the Vorbis data frames is
prefixed by the two octet length field. The Packet Type and Fragment
Type are set to 0. The decode Configuration that will be used to
decode the packets is the one indexed by the ident value.
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3. Configuration Headers
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 data block that must be
transmitted to the decoder along with the compressed data. A decoder
also requires identification information detailing the number of
audio channels, bitrates and other 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 nominally included as special "header"
packets at the start of the compressed data.
Thus these two codebook header packets must be received by the
decoder before any audio data can be interpreted. In addition, the
Vorbis I specification [12] requires the presense of a comment header
packet which gives simple metadata about the stream. This
requirement poses problems in RTP, which is often used over
unreliable transports.
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 is detailed below. SDP delivery
is used to setup an initial state for the client application. The
changes may be due to different codebooks 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 Vorbis Packed
Configuration (Section 3.1.1) Packets could be fetched. Different
delivery methods MAY be advertised for the same session. The in-band
Configuration 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) section.
The 24 bit Ident field is used to map which Configuration will be
used to decodea 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 Vorbis data associated until
it fetches the correct Configuration.
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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 fragmentation and periodic re-transmission of
the configuration headers.
3.1.1. Packed Configuration
A Vorbis Packed Configuration is indicated with the payload type
field set to 1. Of the three headers, defined in the Vorbis I
specification [12], the identification and the setup will be packed
together, the comment header is completely suppressed. Is up to the
client provide a minimal size comment header to the decoder if
required by the implementation.
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 | Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. | Setup ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Setup ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Setup |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Packed Configuration Figure
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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.
3.2. Out of Band Transmission
This section, as stated before, won't cover all the possible out-of-
band delivery methods since they rely to different protocols and be
linked to a specific application. The following packet definition
SHOULD be used in out-of-band delivery and MUST be used when
Configuration is inlined in the SDP.
3.2.1. Packed Headers
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 count field which details the number of packed headers
that are contained in the bundle. Next is the Packed header payload
for each chained Vorbis stream.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 2 byte length tag to define the length of the
packed headers.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ident | ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. length | Identification Header ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Setup Header ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Setup Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Packed Headers Detail
The key difference between the in-band format and this one, is there
is no need for the payload header octet.
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: vorbis-config
Required Parameters:
None.
Optional Parameters:
None.
Encoding considerations:
This type is only defined for transfer via non RTP protocols as
specified in RFC XXXX.
Security Considerations:
See Section 6 of RFC 3047.
Interoperability considerations: none
Published specification:
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See RFC XXXX for details.
Applications which use this media type:
Vorbis encoded audio, configuration data.
Additional information: none
Person & email address to contact for further information:
Luca Barbato: <lu_zero@gentoo.org>
Intended usage: COMMON
Author/Change controller:
Author: Luca Barbato
Change controller: IETF AVT Working Group
3.3. Loss of Configuration Headers
Unlike the loss of raw Vorbis payload data, loss of a configuration
header can lead to a situation where it will not be possible to
successfully decode the stream.
Loss of 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
With the payload type flag set to 2, 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. Clients MAY ignore it
completely. The details on the format of the comments can be found
in the Vorbis documentation [12].
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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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ident | 0 | 2 | 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | Comment ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Comment ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Comment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Comment Packet
The 2 bytes length field is necessary since this packet could be
fragmented.
5. Frame Packetizing
Each RTP packet contains either 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).
Any Vorbis data packet that is less than path MTU SHOULD be bundled
in the RTP packet with as many Vorbis packets as will fit, up to a
maximum of 15. Path MTU is detailed in [6] and [7].
If a Vorbis packet, not only data but also Configuration and Comment,
is larger than 65535 octets it MUST be fragmented. 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
RTP packet 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 packets. The
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length field shows the fragment length.
5.1. Example Fragmented Vorbis Packet
Here is an example fragmented Vorbis packet split over three RTP
packets. Each packet contains the standard RTP headers as well as
the 4 octet Vorbis 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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ident | 1 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | vorbis data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. vorbis 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 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.
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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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ident | 2 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | vorbis data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. vorbis 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 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.
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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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ident | 3 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | vorbis data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. vorbis data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Example Fragmented Packet (Packet 3)
This is the last Vorbis fragment packet. The Fragment type 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 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 fragments and decode the
incomplete packet. If we use the fragmented Vorbis 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
2 and MUST drop it. The next packet, which is the final fragmented
packet, MUST be dropped in the same manner. If the missing packet is
the last, the received two fragments will be kept and the incomplete
vorbis packet decoded. Feedback reports on lost and dropped packets
MUST be sent back via RTCP.
If a particular multicast session has a large number of participants
care must be taken to prevent an RTCP feedback implosion, [10], in
the event of packet loss from a large number of participants.
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Loss of any of the Configuration fragment will result in the loss of
the full Configuration packet with the result detailed in the Loss of
Configuration Headers (Section 3.3) section.
6. IANA Considerations
MIME media type name: audio
MIME subtype: vorbis
Required Parameters:
delivery-method: indicates the delivery methods in use, the possible
values are:inline, in_band, out_band
configuration: the base16 [9] (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 packet could be retrived by their number, or aggregated
in a single stream.
Encoding considerations:
This type is only defined for transfer via RTP as specified in RFC
XXXX.
Security Considerations:
See Section 6 of RFC 3047.
Interoperability considerations: none
Published specification:
See the Vorbis documentation [12] for details.
Applications which use this media type:
Audio streaming and conferencing tools
Additional information: none
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Person & email address to contact for further information:
Luca Barbato: <lu_zero@gentoo.org>
Intended usage: COMMON
Author/Change controller:
Author: Luca Barbato
Change controller: IETF AVT Working Group
6.1. Mapping MIME Parameters into SDP
The information carried in the MIME media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[5], which is commonly used to describe RTP sessions. When SDP is
used to specify sessions the mapping are as follows:
o The MIME type ("audio") goes in SDP "m=" as the media name.
o The MIME subtype ("vorbis") goes in SDP "a=rtpmap" as the encoding
name.
o The parameter "rate" also goes in "a=rtpmap" as clock rate.
o The parameter "channels" also goes in "a=rtpmap" as channel count.
o The mandated parameters "delivery-method" and "configuration" MUST
be included in the SDP "a=fmpt" attribute.
o The optional parameter "configuration-uri", when present, MUST be
included in the SDP "a=fmpt" attribute.
If the stream comprises chained Vorbis files and all of them are
known in advance, the Configuration Packet for each file SHOULD be
passed to the client using the configuration attribute.
The URI specified in the configuration-uri attribute MUST point to a
location where all of the Configuration Packets needed for the life
of the session reside.
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 rtpmap attribute MUST match the Vorbis sample rate
value. An example is found below.
The answer to any offer, [8], MUST NOT change the URI specified in
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the configuration-uri attribute. The Configuration inlined in the
configuration parameter MAY change.
c=IN IP4/6
m=audio RTP/AVP 98
a=rtpmap:98 VORBIS/44100/2
a=delivery:out_band/http
a=fmtp:98 delivery-method:in_band,out_band/http;
configuration=base16string1;
configuration-uri=http://path/to/the/resource
Note that the payload format (encoding) names are commonly shown in
upper case. MIME subtypes are commonly shown in lower case. These
names are case-insensitive in both places. Similarly, parameter
names are case-insensitive both in MIME types and in the default
mapping to the SDP a=fmtp attribute. The exception regarding case
sensitivity is the configuration-uri URI which MUST be regarded as
being case sensitive.
7. Congestion Control
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.
If a particular multicast session has a large number of participants
care must be taken to prevent an RTCP feedback implosion, [10], in
the event of congestion.
8. Examples
The following examples are common usage patterns that MAY be applied
in such situations, the main scope of this section is to explain
better usage of the transmission vectors.
8.1. Stream Radio
That is one of the most common situation: one single server streaming
content in multicast, the clients may start a session at random time.
The content itself could be a mix of live stream as the dj's speech
and stored streams as the music she plays.
In this situation we don't know in advance how many codebooks we will
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use and. The clients can join anytime and users expect to start
listening to the content in a short time
On join the client will receive the current Configuration necessary
to decode the current stream inlined in the SDP. And can start
decoding the current stream.
When the streamed content changes the new Configuration is sent in-
band befoe the actual stream, and the Configuration that has to be
sent inline in the SDP updated.
A serverside optimization would be keep an hash list of the
Configurations per session to avoid packing them and send the same
Configuration with different Ident tags
A clientside optimization would be keep a tag list of the
Configurations per session and don't process configuration packets
already known.
Let's assume that the client playout buffer can store at least 7
packets and that is the maximum latency.
9. Security Considerations
RTP packets using this payload format are subject to the security
considerations discussed in the RTP specification [3]. 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.
10. Acknowledgments
This document is a continuation of draft-moffitt-vorbis-rtp-00.txt
and draft-kerr-avt-vorbis-rtp-04.txt. The MIME type section is a
continuation of draft-short-avt-rtp-vorbis-mime-00.txt.
Thanks to the AVT, Ogg Vorbis Communities / Xiph.org including Steve
Casner, Aaron Colwell, Ross Finlayson, Fluendo, Ramon Garcia, Pascal
Hennequin, Ralph Giles, Tor-Einar Jarnbjo, Colin Law, John Lazzaro,
Jack Moffitt, Christopher Montgomery, Colin Perkins, Barry Short,
Mike Smith, Phil Kerr, Michael Sparks, Magnus Westerlund, David
Barrett, Silvia Pfeiffer, Politecnico di Torino (LS)^3/IMG Group in
particular Federico Ridolfo, Francesco Varano, Giampaolo Mancini,
Juan Carlos De Martin.
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11. References
11.1. Normative References
[1] Pfeiffer, S., "The Ogg Encapsulation Format Version 0",
RFC 3533.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119.
[3] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for real-time applications",
RFC 3550.
[4] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control.", RFC 3551.
[5] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327.
[6] Mogul et al., J., "Path MTU Discovery", RFC 1063.
[7] McCann et al., J., "Path MTU Discovery for IP version 6",
RFC 1981.
[8] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264.
[9] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548.
[10] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)",
Internet Draft (draft-ietf-avt-rtcp-feedback-11: Work in
progress).
11.2. Informative References
[11] "libvorbis: Available from the Xiph website,
http://www.xiph.org".
[12] "Ogg Vorbis I specification: Codec setup and packet decode.
Available from the Xiph website, http://www.xiph.org".
[13] "Ogg Vorbis I specification: Comment field and header
specification. Available from the Xiph website,
http://www.xiph.org".
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Author's Address
Luca Barbato
Xiph.Org
Email: lu_zero@gentoo.org
URI: http://www.xiph.org/
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Barbato Expires April 24, 2006 [Page 23]
