AVT Working Group P. Kerr
Internet-Draft Xiph.Org
Expires: July 1, 2005 December 31, 2004
draft-kerr-avt-vorbis-rtp-04
RTP Payload Format for Vorbis Encoded Audio
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 1, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes a RTP payload format for transporting Vorbis
encoded audio. It details the RTP encapsulation mechanism for raw
Vorbis data and details the delivery mechanisms for the decoder
probability model, referred to as a codebook, metadata and other
setup information.
Editors Note
Kerr Expires July 1, 2005 [Page 1]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
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 . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 RTP Header . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Payload Header . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Payload Data . . . . . . . . . . . . . . . . . . . . . . . 6
2.4 Example RTP Packet . . . . . . . . . . . . . . . . . . . . 7
3. Frame Packetizing . . . . . . . . . . . . . . . . . . . . . . 8
3.1 Example Fragmented Vorbis Packet . . . . . . . . . . . . . 8
3.2 Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 10
4. Configuration Headers . . . . . . . . . . . . . . . . . . . . 11
4.1 In-band Header Transmission . . . . . . . . . . . . . . . 11
4.2 Session Description for Vorbis RTP Streams . . . . . . . . 14
4.3 Codebook Caching . . . . . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Congestion Control . . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 20
9.2 Informative References . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . 21
Kerr Expires July 1, 2005 [Page 2]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
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 1.0
encoder can encode high-quality CD and DAT rate stereo at below 48k
bits/sec without resampling to a lower rate. Vorbis is also intended
for lower and higher sample rates (from 8kHz telephony to 192kHz
digital masters) and a range of channel representations (monaural,
polyphonic, stereo, quadraphonic, 5.1, ambisonic, or up to 255
discrete channels). Vorbis encoded audio is generally encapsulated
within an Ogg format bitstream [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].
Kerr Expires July 1, 2005 [Page 3]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
2. Payload Format
For RTP based transportation of Vorbis encoded audio the standard RTP
header is followed by a 5 octet payload header, then the payload
data. The payload headers are used to associate the Vorbis data with
its associated decoding codebooks as well as indicating if the
following packet contains fragmented Vorbis data and/or the the
number of whole Vorbis data frames. The payload data contains the
raw Vorbis bitstream information.
2.1 RTP 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
Always set to 0, as audio silence suppression is not used by the
Vorbis codec.
CSRC count (CC): 4 bits
The CSRC count is used in accordance with [3].
Kerr Expires July 1, 2005 [Page 4]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
Marker (M): 1 bit
Set to zero. Audio silence suppression not used. This conforms to
section 4.1 of [9].
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 [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 five 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Codebook Ident |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|F| T |# pkts.|
+-+-+-+-+-+-+-+-+
Codebook Ident: 32 bits
This 32 bit field is used to associate the Vorbis data to a decoding
Codebook. It is created by making a CRC32 checksum of the codebook
required to decode the particular Vorbis audio stream.
Kerr Expires July 1, 2005 [Page 5]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
Continuation (C): 1 bit
Set to one if this is a continuation of a fragmented packet.
Fragmented (F): 1 bit
Set to one if the payload contains complete packets or if it contains
the last fragment of a fragmented packet.
Payload Type (T): 2 bits
This field sets the packet payload type. There are currently four
type of packet payloads.
0 = Raw Vorbis payload
1 = Configuration payload
2 = Codebook payload
3 = Metadata payload
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 [8]
typically produces packets less than ~800 bytes, except for the
codebook header packets which are ~4-12 kilobytes. Within an RTP
context the maximum Vorbis packet size, including the RTP and payload
headers, SHOULD be kept below the path MTU to avoid packet
fragmentation.
Each Vorbis payload packet starts with a one octet length header,
which is used to represent the size of the following data payload,
followed by the raw Vorbis data.
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 octet.
The payload packing of the Vorbis data packets SHOULD follow the
Kerr Expires July 1, 2005 [Page 6]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
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 BS. 775-1 ITU-R.
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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Codebook Ident |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| 0 | 2 pks | len | vorbis data ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. ...vorbis data... ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. data | len | next vorbis packet data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kerr Expires July 1, 2005 [Page 7]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
3. Frame Packetizing
Each RTP packet contains either one complete Vorbis packet, one
Vorbis packet fragment, or an integer number of complete Vorbis
packets (up to a max of 15 packets, since the number of packets is
defined by a 4 bit value).
Any Vorbis data packet that is 256 octets or less SHOULD be bundled
in the RTP packet with as many Vorbis packets as will fit, up to a
maximum of 15.
If a Vorbis packet is larger than 256 octets it MUST be fragmented.
A fragmented packet has a zero in the last four bits of the payload
header. Each fragment after the first will also set the Continued
(C) bit to one in the payload header. The RTP packet containing the
last fragment of the Vorbis packet will have the Fragmented (F) bit
set to one. To maintain the correct sequence for fragmented packet
reception the timestamp field of fragmented packets MUST be the same
as the first packet sent, with the sequence number incremented as
normal for the subsequent RTP packets. Path MTU is detailed in [6]
and [7].
3.1 Example Fragmented Vorbis Packet
Here is an example fragmented Vorbis packet split over three RTP
packets.
Packet 1:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | 1000 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Codebook Ident |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| 0 | 0| len | vorbis data .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..vorbis data.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kerr Expires July 1, 2005 [Page 8]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
In this packet the initial sequence number is 1000 and the timestamp
is xxxxx. The number of packets field is set to 0.
Packet 2:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | 1001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Codebook Ident |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| 0 | 0| len | vorbis data ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..vorbis data.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The C bit is set to 1 and the number of packets field is set to 0.
For large Vorbis fragments there can be several of these type of
payload packets. The maximum packet size SHOULD be no greater than
the path MTU, including all RTP and payload headers. The sequence
number has been incremented by one but the timestamp field remains
the same as the initial packet.
Packet 3:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | 1002 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Codebook Ident |
Kerr Expires July 1, 2005 [Page 9]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1| 0 | 0| len | vorbis data .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..vorbis data.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is the last Vorbis fragment packet. The C and F bits are set
and the packet count remains set to 0. As in the previous packets
the timestamp remains set to the first packet in the sequence and the
sequence number has been incremented.
3.2 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 C and F flags. If we use the fragmented Vorbis
packet example above and the first packet is lost the client SHOULD
detect that the next packet has the packet count field set to 0 and
the C bit is set and MUST drop it. The next packet, which is the
final fragmented packet, SHOULD be dropped in the same manner, or
buffered. Feedback reports on lost and dropped packets MUST be sent
back via RTCP.
Kerr Expires July 1, 2005 [Page 10]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
4. 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 self-contained codebook.
This codebook block also requires additional identification
information detailing the number of audio channels, bitrates and
other information used to initialise the Vorbis stream.
To decode a Vorbis stream three configuration header blocks are
needed. The first header indicates the sample and bitrates, the
number of channels and the version of the Vorbis encoder used. The
second header contains the decoders probability model, or codebook
and the third header details stream metadata.
As the RTP stream may change certain configuration data mid-session
there are two different methods for delivering this configuration
data to a client, in-band and SDP which is detailed below. SDP
delivery is used to set-up an initial state for the client
application and in-band is used to change state during the session.
The changes may be due to different metadata or codebooks as well as
different bitrates of the stream.
Out of the two delivery vectors the use of an SDP attribute to
indicate an URI where the configuration and codebook data can be
obtained is preferred as they can be fetched reliably using TCP. The
in-band codebook delivery SHOULD only be used in situations where the
link between the client is unidirectional or if the SDP-based
information is not available.
Synchronizing the configuration and codebook headers to the RTP
stream is critical. The 32 bit Codebook Ident field is used to
indicate when a change in the stream has taken place. The client
application MUST have in advance the correct configuration and
codebook headers and if the client detects a change in the Ident
value and does not have this information it MUST NOT decode the raw
Vorbis data.
4.1 In-band Header Transmission
The three header data blocks are sent in-band with the packet type
bits set to match the payload type. Normally the codebook and
configuration headers are sent once per session if the stream is an
encoding of live audio, as typically the encoder state will not
change, but the encoder state can change at the boundary of chained
Vorbis audio files. Metadata can be sent at the start as well as any
time during the life of the session. Clients MUST be capable of
dealing with periodic re-transmission of the configuration headers.
Kerr Expires July 1, 2005 [Page 11]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
A Vorbis configuration header is indicated with the payload type
field set to 1. The Vorbis version MUST be set to zero to comply
with this document. The fields Sample Rate, Bitrate Maximum/Nominal/
Minimum and Num Audio Channels are set in accordance with [9] with
the bsz fields above referring to the blocksize parameters. The
framing bit is not used for RTP transportation and so applications
constructing Vorbis files MUST take care to set this if required.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | xxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Codebook Ident |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| 2 | 1| bsz 0 | bsz 1 | Num Audio Channels |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vorbis Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Audio Sample Rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitrate Maximum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitrate Nominal |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitrate Minimum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
If the payload type field is set to 2, this indicates the packet
contains codebook data.
The configuration information detailed below MUST be completely
intact, as a client can not decode a stream with an incomplete or
corrupted codebook set.
A 16 bit codebook length field precedes the codebook datablock. The
length field allows for codebooks to be up to 64K in size. Packet
fragmentation, as per the Vorbis data, MUST be performed if the
codebooks size exceeds path MTU. The Codebook Ident field MUST be
set to match the associated codebook needed to decode the Vorbis
Kerr Expires July 1, 2005 [Page 12]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
stream.
The Codebook Ident is the CRC32 checksum of the codebook and is used
to detect a corrupted codebook as well as associating it with its
Vorbis data stream. This Ident value MUST NOT be set to the value of
the current stream if this header is being sent before the boundary
of the chained file has been reached. If a checksum failure is
detected then this is considered to be a failure and MUST be reported
to the client application.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | xxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Codebook Ident |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| 2 | 1| Codebook Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | Codebook ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Codebook |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
With the payload type flag set to 3, this indicates that the packet
contain the comment metadata, such as artist name, track title and so
on. These metadata messages are not intended to be fully descriptive
but to offer basic track/song information. This message MUST be sent
at the start of the stream, together with the setup and codebook
headers, even if it contains no information. During a session the
metadata associated with the stream may change from that specified at
the start, e.g. a live concert broadcast changing acts/scenes, so
clients MUST have the ability to receive header blocks. Details on
the format of the comments can be found in the Vorbis documentation
[10].
The format for the data takes the form of a 32 bit codec vendors name
length field followed by the name encoded in UTF-8. The next 32 bit
field denotes the number of user comments. Each of the user comments
is prefixed by a 32 bit length field followed by the comment text.
Kerr Expires July 1, 2005 [Page 13]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | xxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Codebook Ident |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1| 3 | 1| Vendor string length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | Vendor string ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User comments list length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. User comment length / User comment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.2 Session Description for Vorbis RTP Streams
Session description information concerning the Vorbis stream SHOULD
be provided if possible and MUST be in accordance with [5].
If the stream comprises chained Vorbis files the configuration and
codebook headers for each file SHOULD be packaged together and passed
to the client using the headers attribute.
Below is an outline of the mandatory SDP attributes.
c=IN IP4/6
m=audio RTP/AVP 98
a=rtpmap:98 VORBIS/44100/2
a=fmtp:98 header=<URI of configuration header>
The Vorbis configuraton specified in the header attribute MUST
contain all of the configuration data and codebooks needed for the
life of the session.
The port value is specified by the server application bound to the
address specified in the c attribute. The bitrate value and channels
specified in the m attribute MUST match the Vorbis sample rate value.
Kerr Expires July 1, 2005 [Page 14]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
4.3 Codebook Caching
Codebook caching allows clients that have previously connected to a
stream to re-use the associated codebooks and configuration data.
When a client receives a codebook it may store it locally and can
compare the CRC32 key with that of the new stream and begin decoding
before it has received any of the headers.
Kerr Expires July 1, 2005 [Page 15]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
5. IANA Considerations
MIME media type name: audio
MIME subtype: vorbis
Required Parameters:
header indicates the URI of the decoding configuration headers.
Optional Parameters:
None.
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 [9] for details.
Applications which use this media type:
Audio streaming and conferencing tools
Additional information: none
Person & email address to contact for further information:
Phil Kerr: <phil@plus24.com>
Intended usage: COMMON
Author/Change controller:
Author: Phil Kerr Change controller: IETF AVT Working Group
Kerr Expires July 1, 2005 [Page 16]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
6. 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.
Kerr Expires July 1, 2005 [Page 17]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
7. 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.
Kerr Expires July 1, 2005 [Page 18]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
8. Acknowledgments
This document is a continuation of draft-moffitt-vorbis-rtp-00.txt.
The MIME type section is a continuation of draft-short-avt-rtp-
vorbis-mime-00.txt
Thanks to the AVT, Ogg Vorbis Communities / Xiph.org including Steve
Casner, Aaron Colwell, Ross Finlayson, Ramon Garcia, Pascal
Hennequin, Ralph Giles, Tor-Einar Jarnbjo, Colin Law, John Lazzaro,
Jack Moffitt, Christopher Montgomery, Colin Perkins, Barry Short,
Mike Smith, Magnus Westerlund.
Kerr Expires July 1, 2005 [Page 19]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
9. References
9.1 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.
9.2 Informative References
[8] "libvorbis: Available from the Xiph website,
http://www.xiph.org".
[9] "Ogg Vorbis I spec: Codec setup and packet decode.
http://www.xiph.org/ogg/vorbis/doc/vorbis-spec-ref.html".
[10] "Ogg Vorbis I spec: Comment field and header specification.
http://www.xiph.org/ogg/vorbis/doc/v-comment.html".
Author's Address
Phil Kerr
Xiph.Org
EMail: phil@plus24.com
URI: http://www.xiph.org/
Kerr Expires July 1, 2005 [Page 20]
Internet-Draft draft-kerr-avt-vorbis-rtp-04 December 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Kerr Expires July 1, 2005 [Page 21]
