Network Working Group Phil Kerr
Internet-Draft Ogg Vorbis Community
October 27, 2003 OpenDrama
Expires: April 27, 2003
RTP Payload Format for Vorbis Encoded Audio
<draft-kerr-avt-vorbis-rtp-03.txt>
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
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.
[Note to RFC Editor: All references to RFC XXXX are to be replaced
by references to the RFC number of this memo, when published.]
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Table of Contents
1. Introduction ........................................ 2
1.1 Terminology ......................................... 2
2. Payload Format ...................................... 3
2.1 RTP Header .......................................... 3
2.2 Payload Header ...................................... 4
2.3 Payload Data ........................................ 5
2.4 Example RTP Packet .................................. 5
3. Frame Packetizing ................................... 6
3.1 Example Fragmented Vorbis Packet .................... 6
3.2 Packet Loss ......................................... 8
4. Configuration Headers ............................... 8
4.1 RTCP Based Config Header Transmission ............... 9
4.2 Codebook Caching .................................... 11
5. Session Description ................................. 11
5.1 SDP Based Config Header Transmission ................ 12
6. IANA Considerations ................................. 13
7. Congestion Control .................................. 13
8. Security Considerations ............................. 14
9. Acknowledgements .................................... 14
10. Normative References ................................ 14
10.1 Informative References ................................ 15
11. Full Copyright Statement ............................ 15
11.1 IPR Statement ....................................... 15
12. Authors Address ..................................... 15
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].
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2 Payload Format
For RTP based transportation of Vorbis encoded audio the standard
RTP header is followed by an 8 bit payload header, then the payload
data. The payload header is used to signify 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 [4] and [5] 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].
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Marker (M): 1 bit
Set to zero. Audio silence suppression not used. This conforms
to section 4.1 of [6].
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 next octet is the Payload Header.
This octet is split into a number of bitfields detailing the format
of the following Payload Data packets.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| C | F | R | # of packets |
+---+---+---+---+---+---+---+---+
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.
Reserved (R): 1 bit
Reserved, MUST be set to zero by senders, and ignored by
receivers.
The last 5 bits are the number of complete packets in this payload.
This provides for a maximum number of 32 Vorbis packets in the
payload. If C is set to one, this number MUST be 0.
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2.3 Payload Data
Vorbis packets are unbounded in length currently. At some future
point there will likely be a practical limit placed on packet
length.
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
header packets which are ~4-12 kilobytes.
Within a RTP context the maximum Vorbis packet SHOULD be kept below
the MTU size, typically 1500 octets, including the RTP and payload
headers, to avoid fragmentation. For the delivery of Vorbis audio
using RTP the maximum size of the header block is limited to 64K.
If the payload contains a single Vorbis packet or a Vorbis packet
fragment, the Vorbis packet data follows the payload header.
For payloads which consist of multiple Vorbis packets, payload data
consists of one octet representing the packet length followed by
the packet data for each of the Vorbis packets in the payload.
The Vorbis packet length field is the length of the Vorbis data
block minus one octet.
The payload packing of the Vorbis data packets SHOULD follow the
guidelines set-out in section 4.4 of [5] 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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1|0| # pks: 2| len | vorbis data ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...vorbis data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | len | next vorbis packet data... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 32 packets, since the number of packets
is defined by a 5 bit value).
Any Vorbis packet that is larger than 256 octets and less than the
path-MTU MUST be placed in a RTP packet by itself.
Any Vorbis packet that is 256 bytes or less SHOULD be bundled in the
RTP packet with as many Vorbis packets as will fit, up to a maximum
of 32.
If a Vorbis packet will not fit within the network MTU, it SHOULD be
fragmented. A fragmented packet has a zero in the last five 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 [9] and [10].
3.1 Example Fragmented Vorbis Packet
Here is an example fragmented Vorbis packet split over three RTP
packets.
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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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0| 0| len | vorbis data .. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..vorbis data.. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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.
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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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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.
4 Configuration Headers
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
codebooks and the third header details stream metadata.
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As the RTP stream may change certain configuration data mid-session
there are two different methods for delivering this configuration
data to a client, RTCP which is detailed below and SDP which is
detailed in section 5. SDP delivery is used to set-up an initial
state for the client application and RTCP 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.
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, bit rates and
other information used to initialise the Vorbis stream.
4.1 RTCP Based Header Transmission
The three header data blocks are sent out-of-band as an APP defined
RTCP message with the 4 octet name field set to VORB.
Synchronizing the configuration headers to the RTP stream is
critical. A 32 bit timestamp field is used to indicate the
timepoint when a VORB header MUST be applied to the RTP stream.
VORB RTCP packets SHOULD be sent just ahead of the change in the
RTP stream. As the reception loss of the RTCP header will mean
the RTP stream will fail to decode properly the freqency of their
periodic retransmission SHOULD be high enough to minimize the
stream disturbance whilst remaining under the RTCP bandwidth
allocation.
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| subtype | PT=APP=204 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC/CSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VORB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timestamp (in sample rate units) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vorbis Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Audio Sample Rate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitrate Maximum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitrate Nominal |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitrate Minimum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| bsz 0 | bsz 1 | Num Audio Channels |c|m|o|x|x|x|x|x|
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Codebook length | Codebook checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Codebook |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. URI string |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Vendor string length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor string ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User comments list length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. User comment length / User comment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first Vorbis config header defines the Vorbis stream
attributes. 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 [6] 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.
The next 8 bits are used to indicate the presence of the two
other Vorbis stream config headers and the size overflow header.
The c flag indicates the presence of a codebook header block, the
m flag indicates the presence of a comment metadata block. The o
flag indicates if the size of either of the c and m headers would
make the VORB packet greater than that allowed for a RTCP message.
The remaining five bits, indicated with an x, are reserved/unused
and MUST be set to 0 for this version of the document.
If the c flag is set then the next header block will contain the
codebook configuration data.
The configuration information detailed above 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 and a 16 bit 1's complement checksum
of the codebook precedes the codebook datablock. The length field
allows for codebooks to be up to 64K in size. The checksum is used
to detect a corrupted codebook.
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If a checksum failure is detected then a new config header file
SHOULD be obtained from SDP, if the codebook has not changed since
the session has started. If no SDP value is set and no other method
for obtaining the config headers exists then this is considered to
be a failure and SHOULD be reported to the client application.
If the m flag is set then the next header block will 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 m header blocks. Details
on the format of the comments can be found in the Vorbis
documentation [7].
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
field denotes the number of user comments and then the user comments
length and text field pairs, up to the number indicated by the user
comment list length.
If the o, overflow, bit is set then the URI of a whole header block
is specified in an overflow URI field, which is a null terminated
UTF-8 string. The header file specified at the URI MUST NOT have
the overflow flag set, otherwise a loop condition will occur.
4.2 Codebook Caching
Codebook caching allows clients that have previously connected to a
stream to re-use the codebooks and thus begin the playback of the
session faster. When a client receives a codebook it may store
it, together with the MD5 key, locally and can compare the MD5 key
of locally cached codebooks with the key it receives via SDP, which
is detailed in section 5.
5 Session Description for Vorbis RTP Streams
Session description information concerning the Vorbis stream
SHOULD be provided if possible and MUST be in accordance with [8].
The SDP information is split into two sections, a mandatory
section detailing the RTP stream and an optional section used to
convey information needed for codebook caching.
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Below is an outline of the mandatory SDP attributes.
c=IN IP4/6 <Vorbis stream>
m=audio <port> RTP/AVP 98
a=rtpmap:98 vorbis/<sample rate>
a=fmtp:98 header=<URI of Vorbis codebooks>
a=fmtp:98 md5key=<MD5 key of codebook>
The port value is specified by the server application bound to
the address specified in the c attribute. The bitrate value
specified in the a attribute MUST match the Vorbis sample rate
value.
The Vorbis codebook specified in the header attribute MUST contain
all of the configuration data. If the codebook MD5 attribute,
md5key, is set the key is compared to a locally held cache and
if found the associated local codebook is used, if not the
client MUST use the configuration headers specified with the
header attribute.
5.1 SDP Based Config Header Transmission
The optional SDP attributes are used to convey details of the
Vorbis stream which are required for codebook caching. If the
following attributes are set they take precedent over values
specified in the u attribute detailed above. The maximum size
of the mandatory and optional SDP attributes MUST be less than
1K in size to conform to section 4.1 of [8].
a=fmtp:98 bitrate_min=<Bitrate Minimum>
a=fmtp:98 bitrate_norm=<Bitrate Normal>
a=fmtp:98 bitrate_max=<Bitrate Maximum>
a=fmtp:98 bsz0=<Block Size 0>
a=fmtp:98 bsz1=<Block Size 1>
a=fmtp:98 channels=<Num Audio Channels>
a=fmtp:98 meta_vendor=<Vendor Name>
The metadata attribute, meta_vendor, provides the bare minimum
information required for decoding but does not convey any
meaningful stream metadata information. As outlined in the Vorbis
comment field and header specification documentation, [7], a number
of predefined field names are available which SHOULD be used. An
example would be:
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a=fmtp:98 meta_vendor=Xiph.Org libVorbis I 20020717
a=fmtp:98 meta_artist=Honest Bob and the Factory-to-Dealer-Incentives
a=fmtp:98 meta_title=I'm Still Around
a=fmtp:98 meta_tracknumber=5
6 IANA Considerations
MIME media type name: audio
MIME subtype: vorbis
Required Parameters:
header indicates the URI of the decoding codebook.
md5key indicates the MD5 key of the codebooks.
Optional Parameters:
bitrate_min, bitrate_norm and bitrate_max indicate the
minimum, nominal and maximum bitrates. bsz0 and bsz1
indicate the blocksize values. channels indicates the
number of audio channels in the stream. meta_vendor
indicates the encoding codec vendor.
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 [2] 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
philkerr@elec.gla.ac.uk/phil@plus24.com
Intended usage: COMMON
Author/Change controller:
Author: Phil Kerr
Change controller: IETF AVT Working Group
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.
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8 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.
9 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, Ramon Garcia, Pascal Hennequin, Ralph Jiles,
Tor-Einar Jarnbjo, Colin Law, John Lazzaro, Jack Moffitt,
Colin Perkins, Barry Short, Mike Smith, Magnus Westerlund.
10 Normative References
1. The Ogg Encapsulation Format Version 0 (RFC 3533), S. Pfeiffer.
2. Key words for use in RFCs to Indicate Requirement Levels
(RFC 2119), S. Bradner.
3. RTP: A Transport Protocol for Real-Time Applications (RFC 1889),
Schulzrinne, et al.
4. RTP: A transport protocol for real-time applications. Work
in progress, draft-ietf-avt-rtp-new-11.txt.
5. RTP Profile for Audio and Video Conferences with Minimal Control.
Work in progress, draft-ietf-avt-profile-new-12.txt.
6. Ogg Vorbis I spec: Codec setup and packet decode.
http://www.xiph.org/ogg/vorbis/doc/vorbis-spec-ref.html
7. Ogg Vorbis I spec: Comment field and header specification.
http://www.xiph.org/ogg/vorbis/doc/v-comment.html
8. SDP: Session Description Protocol (RFC 2327), Handley, M. and
V. Jacobson.
9. Path MTU Discovery (RFC 1063), Mogul & Deering
10. Path MTU Discovery for IP version 6 (RFC 1981), McCann, J. et al.
Kerr Expires April 27, 2003 [Page 14]
Internet Draft draft-kerr-avt-vorbis-rtp-03.txt October 27, 2003
10.1 Informative References
11. libvorbis: Available from the Xiph website, http://www.xiph.org
11 Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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10.1 IPR Statement
"The IETF takes no position regarding the validity or scope of
any intellectual property 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; neither does
it represent that it has made any effort to identify any such
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rights in standards-track and standards-related documentation
can be found in BCP-11. Copies of claims of rights made
available for publication 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 implementors or users of this
specification can be obtained from the IETF Secretariat."
Further IPR details on the Vorbis bitstream may be found on the
Xiph website: http://www.xiph.org
12 Authors Address
Phil Kerr
Centre for Music Technology
University of Glasgow
Glasgow, Scotland
UK, G12 8LT
Phone: +44 141 330 5740
Email: philkerr@elec.gla.ac.uk
phil@plus24.com
WWW: http://www.xiph.org/
Kerr Expires April 27, 2003 [Page 15]
