Hi All,
Please find below an updated Vorbis-RTP Internet Draft document for
review and discussion at the Xiph IRC meeting on Saturday.
The changes in this version have been:
Codebook caching mechanism
Expanded SDP parameters
Expanded MIME section
Expanded introduction
Packet loss section
Minor tweaks and clarity changes to text
There are probably some minor tweaks to the formatting needed which will
be done before the final submission.
Open issues concern:
Bitrate peeling for congestion control needs to be firmed up
A clearer definition of the path MTU is probably needed
Feedback and comments welcomed.
All being well I will submit this to the IETF early next week with a
request to move the document to AVT WG status (a step closer to RFC).
Regards
Phil
<p><p>-------------------8<-------------------------------------8<------------
<p>Network Working Group Phil Kerr
Internet-Draft Ogg Vorbis Community
June 10, 2003 OpenDrama
Expires: December 10, 2003
<p> RTP Payload Format for Vorbis Encoded Audio
<draft-kerr-avt-vorbis-rtp-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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.
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.
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<p>Table of Contents
1. Introduction ........................................ 2
1.1 Terminology ......................................... 3
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
11. Full Copyright Statement ............................ 15
12. Authors Address ..................................... 15
1 Introduction
The Xiph.org Foundation creates and defines codecs for use in
multimedia that are not encumbered by patents and thus may be freely
implemented by any individual or organization.
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.
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<p>1.1 Terminology
The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED",
"MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].
2 Payload Format
For RTP based transportation of Vorbis encoded audio the standard
RTP header is followed by an 8 bit payload header, then the payload
data.
<p>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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<p> 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
If the padding bit is set, the packet contains one or more
additional padding octets at the end which are not part of
the payload. P is set if the total packet size is less than
the MTU.
Extension (X): 1 bit
If the extension, X, bit is set, the fixed header MUST be
followed by exactly one header extension, with a format defined
in Section 5.3.1. of [4],
CSRC count (CC): 4 bits
The CSRC count contains the number of CSRC identifiers.
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<p> 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.
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].
<p>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 SHOULD be 0.
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<p>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 [2]
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 octet is the length of the data block
minus one.
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.
<p>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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<p> 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... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<p>3 Frame Packetizing
Each RTP packet contains either one complete Vorbis packet, one
Vorbis packet fragment, or an integer number of complete Vorbis
packets (upto 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
Final Fragment (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.
<p><p><p>3.1 Example Fragmented Vorbis Packet
Here is an example fragmented Vorbis packet split over three RTP
packets.
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<p> 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.
<p> 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 MTU of 1500 octets, 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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<p> 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.
<p>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, MUST be dropped
in the same manner. Feedback reports on lost and dropped packets
MUST be sent back via RTCP.
<p>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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<p> 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 6. 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.
<p>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.
VORB RTCP packets MUST set the padding (P) flag and add the
appropriate padding octets needed to conform with section 6.6
of [3]. 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 MUST 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 MUST 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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<p> 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
<p> 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.
This setup information MUST be completely intact and 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 upto 64k in size. The checksum is used to
detect a corrupted codebook.
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<p>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.
<p>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 6.1 below.
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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<p> Below is an outline of the mandatory SDP attributes.
u=<URI of Vorbis header file>
m=audio <port> RTP/AVP 98
c=IN IP4/6 <URI of Vorbis stream>
a=rtpmap:98 vorbis/<sample rate>
The contents of the Vorbis Header file referred to in the
u attribute MUST contain all three of the config header blocks
as specified in section 5. The overflow bit of the header packet
MUST not be set.
The port value is specified by the server application bound to
the URI specified in the c attribute. The bitrate value specified
in the a attribute MUST match the Vorbis sample rate value.
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=md5key:<MD5 key of codebook>
a=bitrate_min:<Bitrate Minimum>
a=bitrate_norm:<Bitrate Normal>
a=bitrate_max:<Bitrate Maximum>
a=bsz0:<Block Size 0>
a=bsz1:<Block Size 1>
a=channels:<Num Audio Channels>
a=meta_vendor:<Vendor Name>
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 in the u attribute.
The md5key requires other attributes which detail bitrates, channels
and metadata associated with the RTP stream. The attributes
following the md5key example above MUST all be present.
The metadata attribute, meta_vendor, provides the bare minimum
information required for decoding but does not convey any
meaningfull 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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<p> a=meta_vendor:Xiph.Org libVorbis I 20020717
a=meta_artist:Honest Bob and the Factory-to-Dealer-Incentives
a=meta_title:I'm Still Around
a=meta_tracknumber:5
<p>6 IANA Considerations
MIME media type name: audio
MIME subtype: vorbis
Required Parameters: none
Optional Parameters: none
Encoding considerations:
This type is only defined for transfer via RTP as specified in
a Work in Progress.
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
Intended usage: COMMON
Author/Change controller:
Author: Phil Kerr
Change controller: IETF AVT Working Group
<p>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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Internet Draft draft-kerr-avt-vorbis-rtp-02.txt June 10, 2003
<p>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.
<p>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.
<p>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.
<p><p><p><p><p>Kerr Expires
December 10, 2003 [Page 14]
Internet Draft draft-kerr-avt-vorbis-rtp-02.txt June 10, 2003
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
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.
<p>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/
<p><p>--- >8 ----
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