AVT Working Group L. Barbato
Internet-Draft Xiph
Expires: August 20, 2008 Feb 17, 2008
RTP Payload Format for Vorbis Encoded Audio
draft-ietf-avt-rtp-vorbis-09
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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 this memo are media type registrations, and the
details necessary for the use of Vorbis with the 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. Conformance and Document Conventions . . . . . . . . . . . 3
2. Payload Format . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. RTP Header . . . . . . . . . . . . . . . . . . . . . . . . 4
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 . . . . . . . . . . . . . . . . . 11
3.2.1. Packed Headers . . . . . . . . . . . . . . . . . . . . 11
3.3. Loss of Configuration Headers . . . . . . . . . . . . . . 12
4. Comment Headers . . . . . . . . . . . . . . . . . . . . . . . 12
5. Frame Packetization . . . . . . . . . . . . . . . . . . . . . 13
5.1. Example Fragmented Vorbis Packet . . . . . . . . . . . . . 14
5.2. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
6.1. Packed Headers IANA Considerations . . . . . . . . . . . . 18
7. SDP related considerations . . . . . . . . . . . . . . . . . . 19
7.1. Mapping Media Type Parameters into SDP . . . . . . . . . . 20
7.1.1. SDP Example . . . . . . . . . . . . . . . . . . . . . 20
7.2. Usage with the SDP Offer/Answer Model . . . . . . . . . . 21
8. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 21
9. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
9.1. Stream Radio . . . . . . . . . . . . . . . . . . . . . . . 21
10. Security Considerations . . . . . . . . . . . . . . . . . . . 22
11. Copying Conditions . . . . . . . . . . . . . . . . . . . . . . 22
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
13.1. Normative References . . . . . . . . . . . . . . . . . . . 23
13.2. Informative References . . . . . . . . . . . . . . . . . . 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-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).
Vorbis encoded audio is generally encapsulated within an Ogg format
bitstream [11], 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. Conformance and Document Conventions
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, [1] and
indicate requirement levels for compliant implementations.
Requirements apply to all implementations unless otherwise stated.
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.
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".
2. Payload Format
For RTP based transport of Vorbis encoded audio the standard RTP
header is followed by a 4 octets 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 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.
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2.1. RTP Header
The format of the RTP header is specified in [2] and shown in Figure
Figure 1. This payload format uses the fields of the header in a
manner consistent with that specification.
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 [2] and [3] 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 [2].
Extension (X): 1 bit
The Extension bit is used in accordance with [2].
CSRC count (CC): 4 bits
The CSRC count is used in accordance with [2].
Marker (M): 1 bit
Set to zero. Audio silence suppression not used. This conforms to
section 4.1 of [10].
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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.
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 [2].
Timestamp: 32 bits
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).
SSRC/CSRC identifiers:
These two fields, 32 bits each with one SSRC field and a maximum of
16 CSRC fields, are as defined in [2].
2.2. Payload Header
The 4 octets following the RTP Header section 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. It is stored as network byte order integer.
Fragment type (F): 2 bits
This field is set according to the following list
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0 = Not Fragmented
1 = Start Fragment
2 = Continuation Fragment
3 = End Fragment
Vorbis Data Type (VDT): 2 bits
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)
0 = Raw Vorbis payload
1 = Vorbis Packed Configuration payload
2 = Legacy Vorbis Comment payload
3 = Reserved
The packets with a VDT of value 3 MUST be ignored
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.
2.3. Payload Data
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 [12] 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.
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 in bytes of the following data
payload, followed by the raw Vorbis data padded to the nearest byte
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boundary, as explained by the vorbis specification [10]. The length
value is stored as network byte order integer.
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 include the length field.
The payload packing of the Vorbis data packets MUST follow the
guidelines set-out in [3] where the oldest Vorbis packet occurs
immediately after the RTP packet header. Subsequent Vorbis packets,
if any, MUST follow in temporal order.
Channel mapping of the audio is in accordance with the Vorbis I
Specification [10].
2.4. Example RTP Packet
Here is an example RTP payload containing two Vorbis 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Figure 4: Example Raw Vorbis Packet
The payload data section of the RTP packet begins 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 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.
3. Configuration Headers
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 along 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 nominally included as
special "header" packets at the start of the compressed data. In
addition, the Vorbis I specification [10] requires the presence of a
comment header packet which gives simple metadata about the stream,
but this information is not required for decoding the frame sequence.
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.
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. In order to
set up an initial state for the client application the configuration
MUST be conveyed via the signalling channel used to setup the
session. One example of such signalling is SDP [5] with the Offer/
Answer Model [8]. Changes to the configuration MAY be communicated
via a re-invite, conveying new SDP, or sent in-band in the RTP
channel. Implementations MUST support 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.
For non chained streams, the recommended Configuration delivery
method is inline the Packed Configuration (Section 3.1.1) in the SDP
as explained in the IANA considerations (Section 7.1).
The 24 bit Ident field is used to map which Configuration will be
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used to decode a packet. When the Ident field changes, it indicates
that a change in the stream has taken place. The client application
MUST have in advance the correct configuration and if the client
detects a change in the Ident value and does not have this
information it MUST NOT decode the raw Vorbis data associated until
it fetches the correct Configuration.
3.1. In-band Header Transmission
The Packed Configuration (Section 3.1.1) Payload is sent in-band with
the packet type bits set to match the Vorbis Data Type. Clients MUST
be capable of dealing with fragmentation and periodic re-transmission
of [14] the configuration headers. The RTP timestamp value MUST
reflect the transmission time of the first data packet for which this
configuration applies.
3.1.1. Packed Configuration
A Vorbis Packed Configuration is indicated with the Vorbis Data Type
field set to 1. Of the three headers defined in the Vorbis I
specification [10], the Identification and the Setup MUST be packed
as they are, while the comment header MAY be replaced with a dummy
one.
The packed configuration follows a generic way to store Xiph codec
configurations: 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), the headers immediately follow the list
of length fields. The size of the last header is implicit.
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 flag and the following 7 used to store the
value. The first N bit are to be taken, where N is number of bits
needed to represent the value, taken modulo 7, and stored in the
first byte. If there are more bits, the flag bit is set to 1 and the
subsequent 7bit 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. In order to decode it is
enough to iterate over the bytes until the flag bit set to 0, for
every byte the data is added to the accumulated value multiplied by
128.
The headers are packed in the same order they are present in ogg:
Identification, Comment, Setup.
The 2 byte length tag defines the length of the packed headers as the
sum of the Configuration, Comment and Setup lengths.
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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 | 1 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | n. of headers | length1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length2 | Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification | Comment ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Comment ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Comment ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Comment ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Comment | Setup ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Setup ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Setup ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Packed Configuration Figure
The Ident field is set with the value that will be used by the Raw
Payload Packets to address this Configuration. The Fragment type is
set to 0 since the packet bears the full Packed configuration, the
number of packet is set to 1.
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3.2. Out of Band Transmission
The following packet definition 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 (network byte ordered) 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 6: Packed Headers Overview
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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 ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. | n. of headers | length1 | length2 ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. | Identification Header ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.................................................................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. | Comment Header ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.................................................................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Comment Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Setup Header ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.................................................................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Setup Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Packed Headers Detail
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.
3.3. Loss of Configuration Headers
Unlike the loss of raw Vorbis payload data, loss of a configuration
header lead to a situation where it will not be possible to
successfully decode the stream. Implementations MAY try to recover
from 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 be
either reset or end the RTP session.
4. Comment Headers
With the Vorbis Data 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 [10].
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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 8: Comment Packet
The 2 bytes length field is necessary since this packet could be
fragmented.
5. Frame Packetization
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).
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
[6] and [7].
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 payloads 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
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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.
5.1. Example Fragmented Vorbis Packet
Here is an example fragmented Vorbis packet split over three RTP
payloads. Each of them contains the standard RTP headers as well as
the 4 octets 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 12345 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ident | 1 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | vorbis data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. vorbis data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Example Fragmented Packet (Packet 1)
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.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 12345 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ident | 2 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | vorbis data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. vorbis data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: 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 this
type 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.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 12345 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ident | 3 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | vorbis data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. vorbis data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Example Fragmented Packet (Packet 3)
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.
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 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 received two fragments will be
kept and the incomplete Vorbis packet decoded.
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.
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6. IANA Considerations
Type name: audio
Subtype name: vorbis
Required parameters:
rate: indicates the RTP timestamp clock rate as described in RTP
Profile for Audio and Video Conferences with Minimal Control.
[3]
channels: indicates the number of audio channels as described in
RTP Profile for Audio and Video Conferences with Minimal
Control. [3]
configuration: the base64 [9] representation of the Packed
Headers (Section 3.2.1).
Encoding considerations:
This media type is framed and contains binary data.
Security considerations:
See Section 10 of RFC XXXX.
Interoperability considerations:
None
Published specification:
RFC XXXX [RFC Editor: please replace by the RFC number of this
memo, when published]
Ogg Vorbis I specification: Codec setup and packet decode.
Available from the Xiph website, http://xiph.org
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> IETF Audio/Video Transport
Working Group
Intended usage:
COMMON
Restriction on usage:
This media type depends on RTP framing, and hence is only defined
for transfer via RTP [2]
Author:
Luca Barbato
Change controller:
IETF AVT Working Group delegated from the IESG
6.1. Packed Headers IANA Considerations
The following IANA considerations refers to the split configuration
Packed Headers (Section 3.2.1) used within RFC XXXX.
Type name: audio
Subtype name: vorbis-config
Required parameters:
None
Optional parameters:
None
Encoding considerations:
This media type contains binary data.
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Security considerations:
See Section 10 of RFC XXXX.
Interoperability considerations:
None
Published specification:
RFC XXXX [RFC Editor: please replace by the RFC number of this
memo, when published]
Applications which use this media type:
Vorbis encoded audio, configuration data.
Additional information:
None
Person & email address to contact for further information:
Luca Barbato: <lu_zero@gentoo.org>
IETF Audio/Video Transport Working Group
Intended usage: COMMON
Restriction on usage:
This media type doesn't depend on the transport.
Author:
Luca Barbato
Change controller:
IETF AVT Working Group delegated from the IESG
7. SDP related considerations
The following paragraphs define the mapping of the parameters
described in the IANA considerations section and their usage in the
Offer/Answer Model [8]. In order to be forward compatible the
implementation MUST ignore unknown parameters.
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7.1. Mapping Media Type Parameters into SDP
The information carried in the 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 type name ("audio") goes in SDP "m=" as the media name.
o The subtype name ("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 "configuration" MUST be included in the
SDP "a=fmtp" 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 port value is specified by the server application bound to the
address specified in the c= line. The channel count value specified
in the rtpmap attribute SHOULD match the current Vorbis stream or
considered the maximum number of channels to be expected. The
timestamp clock rate MUST be a multiple of the sample rate, different
payload number MUST be used if the clock rate changes. The
Configuration payload delivers the exact information, thus the SDP
information SHOULD be considered as a hint. An example is found
below.
7.1.1. SDP Example
The following example shows a basic SDP single stream. The first
configuration packet is inlined in the SDP, other configurations
could be fetched at any time from the URIs provided. The inline
base64 [9] configuration string is folded in this example due to RFC
line length limitations.
c=IN IP4 192.0.2.1
m=audio RTP/AVP 98
a=rtpmap:98 vorbis/44100/2
a=fmtp:98 configuration=AAAAAZ2f4g9NAh4aAXZvcmJpcwA...;
Note that the payload format (encoding) names are commonly shown in
upper case. Media Type subtypes are commonly shown in lower case.
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These names are case-insensitive in both places. Similarly,
parameter names are case-insensitive both in Media Type types and in
the default mapping to the SDP a=fmtp attribute. The a=fmtp line is
a single line even if it is shown as multiple lines in this document
for clarity.
7.2. Usage with the SDP Offer/Answer Model
The are no negotiable parameters. All the of them are declarative.
8. Congestion Control
The general congestion control considerations for transporting RTP
data apply to vorbis audio over RTP as well. See the RTP
specification [2] and any applicable RTP profile (e.g., [3]). Audio
data can be encoded using a range of different bit rates, so it is
possible to adapt network bandwidth by adjusting the encoder bit rate
in real time or by having multiple copies of content encoded at
different bit rates.
9. Example
The following example shows a common usage pattern that MAY be
applied in such situation, the main scope of this section is to
explain better usage of the transmission vectors.
9.1. Stream Radio
This 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 webjockey's
voice, and stored streams as the music she plays.
In this situation we don't know in advance how many codebooks we will
use. 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 so that the decoding
will start immediately after.
When the streamed content changes the new Configuration is sent in-
band before the actual stream and the Configuration that has to be
sent inline in the SDP updated. Since the in-band method is
unreliable, an out of band fallback is provided.
The client may choose to fetch the Configuration from the alternate
source as soon as it discovers a Configuration packet got lost in-
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band or use selective retransmission [13], if the server supports the
feature.
A serverside optimization would be to keep an hash list of the
Configurations per session to avoid packing all of them and send the
same Configuration with different Ident tags
A clientside optimization would be to keep a tag list of the
Configurations per session and don't process configuration packets
already known.
10. Security Considerations
RTP packets using this payload format are subject to the security
considerations discussed in the RTP specification [2], the base64
specification [9] and the URI Generic syntax specification [4].
Among other considerations, this implies that the confidentiality of
the media stream is archieved 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.
11. Copying Conditions
The authors agree to grant third parties the irrevocable right to
copy, use and distribute the work, with or without modification, in
any medium, without royalty, provided that, unless separate
permission is granted, redistributed modified works do not contain
misleading author, version, name of work, or endorsement information.
12. Acknowledgments
This document is a continuation of draft-moffitt-vorbis-rtp-00.txt
and draft-kerr-avt-vorbis-rtp-04.txt. The Media Type declaration is
a continuation of draft-short-avt-rtp-vorbis-mime-00.txt.
Thanks to the AVT, Vorbis Communities / Xiph.Org Foundation 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, Stefan Ehmann, Alessandro Salvatori.
Politecnico di Torino (LS)^3/IMG Group in particular Federico
Ridolfo, Francesco Varano, Giampaolo Mancini, Dario Gallucci, Juan
Carlos De Martin.
13. References
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13.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119.
[2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for real-time applications", STD 64,
RFC 3550.
[3] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control.", STD 65, RFC 3551.
[4] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986.
[5] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[6] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[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] "Ogg Vorbis I specification: Codec setup and packet decode.
Available from the Xiph website,
http://xiph.org/vorbis/doc/Vorbis_I_spec.html".
13.2. Informative References
[11] Pfeiffer, S., "The Ogg Encapsulation Format Version 0",
RFC 3533.
[12] "libvorbis: Available from the dedicated website,
http://vorbis.com".
[13] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol
Extended Reports (RTCP XR)", RFC 3611, November 2003.
[14] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenberg,
"RTP Retransmission Payload Format", RFC 4588, July 2006.
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Author's Address
Luca Barbato
Xiph.Org Foundation
EMail: lu_zero@gentoo.org
URI: http://xiph.org/
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