Internet DRAFT - draft-barbato-avt-rtp-theora
draft-barbato-avt-rtp-theora
AVT Working Group L. Barbato
Internet-Draft Xiph.Org
Expires: December 28, 2006 June 26, 2006
draft-barbato-avt-rtp-theora-01
RTP Payload Format for Theora Encoded Video
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a RTP payload format for transporting Theora
encoded video. It details the RTP encapsulation mechanism for raw
Theora data and configuration headers necessary to configure the
decoder.
Also included within the document are the necessary details for the
use of Theora with MIME and Session Description Protocol (SDP).
Editors Note
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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 . . . . . . . . . . . . . . . . . . . . . . . 4
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. Configuration Headers . . . . . . . . . . . . . . . . . . . . 8
3.1. In-band Header Transmission . . . . . . . . . . . . . . . 9
3.1.1. Packed Configuration . . . . . . . . . . . . . . . . . 9
3.2. Out of Band Transmission . . . . . . . . . . . . . . . . . 10
3.2.1. Packed Headers . . . . . . . . . . . . . . . . . . . . 11
3.3. Loss of Configuration Headers . . . . . . . . . . . . . . 13
4. Comment Headers . . . . . . . . . . . . . . . . . . . . . . . 13
5. Frame Packetizing . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Example Fragmented Theora Packet . . . . . . . . . . . . . 15
5.2. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
6.1. Mapping MIME Parameters into SDP . . . . . . . . . . . . . 19
6.1.1. SDP Example . . . . . . . . . . . . . . . . . . . . . 20
6.2. Usage with the SDP Offer/Answer Model . . . . . . . . . . 20
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Stream Video . . . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
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1. Introduction
Theora is a general purpose, lossy video codec. It is based on the
VP3 video codec produced by On2 Technologies and has been donated to
the Xiph.org Foundation.
Theora I is a block-based lossy transform codec that utilizes an 8 x
8 Type-II Discrete Cosine Transform and block-based motion
compensation. This places it in the same class of codecs as MPEG-1,
MPEG-2, MPEG-4, and H.263. The details of how individual blocks are
organized and how DCT coefficients are stored in the bitstream differ
substantially from these codecs, however. Theora supports only intra
frames (I frames in MPEG) and inter frames (P frames in MPEG).
Theora provides none of its own framing, synchronization, or
protection against transmission errors. Instead, the codec expects
to receive a discrete sequence of data packets. Theora is a free-
form variable bit rate (VBR) codec, and these packets have no minimum
size, maximum size, or fixed/expected size. Theora packets are thus
intended to be used with a transport mechanism that provides free-
form framing, synchronization, positioning, and error correction in
accordance with these design assumptions, such as Ogg [1] or RTP/AVP
[3].
Theora I currently supports progressive video data of arbitrary
dimensions at a constant frame rate in one of several Y'CbCr color
spaces. Three different chroma subsampling formats are supported:
4:2:0, 4:2:2, and 4:4:4. The Theora I format does not support
interlaced material, variable frame rates, bit-depths larger than 8
bits per component, nor alternate color spaces such as RGB or
arbitrary multi-channel spaces. Black and white content can be
efficiently encoded, however, because the uniform chroma planes
compress well. Arbitrary frame size will be encoded rounding to the
upper multiple of 16 both dimension for performance reason. The
original width and height will be encoded in the header and the
decoder will use this information to clip the decoded frame to the
right dimensions.
Theora is similar to the Vorbis audio [10] in that it the decoder
reads the probability model for the entropy coder and all
quantization parameters from special "header" packets at the start of
the compressed data. It is therefore impossible to decode any video
data without having previously fetched the codec info and codec setup
headers, although Theora can initiate decode at an arbitrary intra-
frame packet so long as the codec has been initialized with the
associated headers.
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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 Theora encoded video the standard RTP
header is followed by a 4 octets payload header, then the payload
data. The payload headers are used to associate the Theora data with
its associated decoding codebooks as well as indicating if the
following packet contains fragmented Theora data and/or the number of
whole Theora data frames. The payload data contains the raw Theora
bitstream information.
For RTP based transport of Theora encoded video the standard RTP
header is followed by a 4 octets payload header, then the payload
data.
2.1. RTP Header
The format of the RTP header is specified in [3] and shown in Figure
1. This payload format uses the fields of the header in a manner
consistent with that specification.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP Header
The RTP header begins with an octet of fields (V, P, X, and CC) to
support specialized RTP uses (see [3] and [4] for details). For
Theora RTP, the following values are used.
Version (V): 2 bits
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This field identifies the version of RTP. The version used by this
specification is two (2).
Padding (P): 1 bit
Padding MAY be used with this payload format according to section 5.1
of [3].
Extension (X): 1 bit
The Extension bit is used in accordance with [3].
CSRC count (CC): 4 bits
The CSRC count is used in accordance with [3].
Marker (M): 1 bit
The Marker bit is used in accordance with [3].
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 Theora.
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 presentation time of the first sample of
the first Theora packet in the RTP packet. The clock frequency MUST
be set to 90kHz.
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
The 4 octets following the RTP Header section are the Payload Header.
This header is split into a number of bitfields detailing the format
of the following Payload Data packets.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Configuration Ident | F |TDT|# pkts.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+
Figure 2: Payload Header
Configuration Ident: 24 bits
This 24 bit field is used to associate the Theora data to a decoding
Packed Configuration.
Fragment type (F): 2 bit
This field is set according to the following list
0 = Not Fragmented
1 = Start Fragment
2 = Continuation Fragment
3 = End Fragment
This field must be zero if the number of packets field is non-zero.
Theora Data Type (TDT): 2 bits
This field sets the packet payload type for the Theora data. There
are currently three Theora payload types.
0 = Raw Theora payload
1 = Theora Packed Configuration payload
2 = Legacy Theora Comment payload
3 = Reserved
The packets with a TDT of value 3 MUST be ignored
The last 4 bits represent the number of complete packets in this
payload. This provides for a maximum number of 15 Theora packets in
the payload. If the packet contains fragmented data the number of
packets MUST be set to 0.
2.3. Payload Data
Each Theora payload section starts with a two octets length header
that is used to represent the size of the following data payload,
followed by the raw Theora packet data.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | Theora Data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Payload Data
The Theora codec uses relatively unstructured raw packets containing
binary integer fields of arbitrary width that often do not fall on an
octet boundary. When a Theora encoder produces packets, unused space
in the last byte of a packet is always zeroed during the encoding
process. Thus, should this unused space be read, it will return
binary zeros.
For payloads which consist of multiple Theora packets the payload
data consists of the payload length field followed by the first
Theora packet's data, then the payload length followed by the second
Theora packet, and so on for each of the Theora packets in the
payload.
2.4. Example RTP Packet
Here is an example RTP packet containing two Theora 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronisation source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Example RTP Packet
Payload Data:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Configuration Ident | 0 | 0 | 2 pks |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Theora data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. data | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Theora data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Example Theora Payload Packet
The payload portion of the packet begins with the 24 bit
Configuration ident field followed by 8 bits describing the payload.
The Fragment type field is set to 0, indicating that this packet
contains whole Theora frame data. The Data type field is set to 0
since it is theora raw data. The number of whole Theora data packets
is set to 2.
Each of the payload blocks starts with the two octets length field
followed by the variable length Theora packet data.
3. Configuration Headers
To decode a Theora stream three configuration header packets are
needed. The first, called the Identification Header, indicates the
frame dimensions, quality, blocks used and the version of the Theora
encoder used. The second, called the Comment Header, contains stream
metadata and the third, called the Setup Header, contains details of
the dequantization and Huffman tables.
Since this information must be transmitted reliably, and as the RTP
stream may change certain configuration data mid-session, there are
different methods for delivering this configuration data to a client,
both in-band and out-of-band which are detailed below. SDP delivery
is used to set up an initial state for the client application. The
changes may be due to different dequantization and Huffman tables as
well as different bitrates of the stream.
The delivery vectors in use are specified by an SDP attribute to
indicate the method and the optional URI where the Theora Packed
Configuration (Section 3.1.1) Packets could be fetched. Different
delivery methods MAY be advertised for the same session. The in-band
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codebook delivery SHOULD be considered as baseline, out-of-band
delivery methods that don't use RTP will not be described in this
document. For non chained streams, the Configuration delivery method
RECOMMENDED is inline the Packed Configuration (Section 3.1.1) in the
SDP as explained in the IANA considerations (Section 6.1)
The 24 bit Ident field is used to map which Configuration will be
used to decode a packet. When the Ident field changes, it indicates
that a change in the stream has taken place. The client application
MUST have in advance the correct configuration and if the client
detects a change in the Ident value and does not have this
information it MUST NOT decode the raw data associated until it
fetches the correct Configuration.
3.1. In-band Header Transmission
The Packed Configuration (Section 3.1.1) Payload is sent in-band with
the packet type bits set to match the payload type. Clients MUST be
capable of dealing with periodic re-transmission of the configuration
headers.
3.1.1. Packed Configuration
A Theora Packed Configuration is indicated with the payload type
field set to 1. Of the three headers, defined in the Theora I
specification [16], the identification and the setup will be packed
together, the comment header is completely suppressed. It is up to
the client to provide a minimal size comment header to the decoder if
required by the implementation.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | xxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Configuration Ident | 0 | 1 | 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. | Setup ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Setup ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Setup |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Packed Configuration Figure
The Ident field is set with the value that will be used by the Raw
Payload Packets to address this Configuration. The Fragment type is
set to 0 since the packet bears the full Packed configuration, the
number of packet is set to 1. In practice, Packed Headers usually
need to be fragmented to fit the path MTU.
3.2. Out of Band Transmission
This section, as stated above, does not cover all the possible out-
of-band delivery methods since they rely on different protocols and
be linked to specific applications. The following packet definition
SHOULD be used in out-of-band delivery and MUST be used when
Configuration is inlined in the SDP.
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3.2.1. Packed Headers
As mentioned above, the recommended delivery vector for Theora
configuration data is via a retrieval method that can be performed
using a reliable transport protocol. As the RTP headers are not
required for this method of delivery the structure of the
configuration data is slightly different. The packed header starts
with a 32 bit count field which details the number of packed headers
that are contained in the bundle. Next is the Packed header payload
for each setup id.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Number of packed headers |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packed header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Packed header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Packed Headers Overview
Since the Configuration Ident and the Identification Header are fixed
length there is only a 16bit Length tag to define the length of the
packed headers.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Configuration Ident | ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Length | Identification Header ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Identification Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Setup Header ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Setup Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Packed Headers Detail
The key difference from the in-band format is that there is no need
for the payload header octet.
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3.2.1.1. Packed Headers IANA Considerations
The following IANA considerations MUST only be applied to the packed
headers.
MIME media type name: audio
MIME subtype: theora-config
Required Parameters:
None
Optional Parameters:
None
Encoding considerations:
This media type contains binary data.
Security Considerations:
See Section 6 of RFC XXXX.
Interoperability considerations:
None
Published specification:
RFC XXXX [RFC Editor: please replace by the RFC number of this
memo, when published]
Applications which use this media type:
Theora encoded video, configuration data.
Additional information:
None
Person & email address to contact for further information:
Luca Barbato: <lu_zero@gentoo.org>
IETF Audio/Video Transport Working Group
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Intended usage: COMMON
Restriction on usage:
This media type does not depend on the transport.
Author:
Luca Barbato
Change controller:
IETF AVT Working Group
3.3. Loss of Configuration Headers
Unlike the loss of raw Theora payload data, the loss of a
configuration header can lead to a situation where it will not be
possible to successfully decode the stream.
A loss of a Configuration Packet results in the halting of stream
decoding and SHOULD be reported to the client as well as a loss
report sent via RTCP.
4. Comment Headers
When the payload type is set to 2, the packet contains 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
title information. Clients MAY ignore them completely. The details
on the format of the comments can be found in the Theora
documentation [16].
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | xxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Configuration Ident | 0 | 2 | 1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| length | Comment ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Comment ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Comment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Comment Packet
The 2 byte length field is necessary since this Theora packet could
be fragmented.
5. Frame Packetizing
Each RTP packet contains either one complete Theora packet, one
Theora packet fragment, or an integer number of complete Theora
packets (up to a maximum of 15 packets, since the number of packets
is defined by a 4 bit value).
Any Theora data packet that is less than path MTU SHOULD be bundled
in the RTP packet with as many Theora packets as will fit, up to a
maximum of 15. Path MTU is detailed in [7] and [8].
A fragmented packet has a zero in the last four bits of the payload
header. The RTP packet containing the first fragment will set the
Fragment type to 1. Each RTP packet after the first will set the
Fragment type to 2 in the payload header. The RTP packet containing
the last fragment of the Theora packet will have the Fragment type
set to 3. If the fragmented Theora packet spans only two RTP
packets, the first will set the Fragment type field to 1 and the
second will set it to 2. To maintain the correct sequence for
fragmented packet reception the timestamp field of fragmented packets
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MUST be the same as the first packet sent, with the sequence number
incremented as normal for the subsequent RTP packets.
5.1. Example Fragmented Theora Packet
Here is an example fragmented Theora packet split over three RTP
packets. Each packet contains the standard RTP headers as well as
the 4 octets Theora headers.
Packet 1:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | 1000 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Configuration Ident | 1 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | Theora data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Theora data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Example Fragmented Packet (Packet 1)
In this packet the initial sequence number is 1000 and the timestamp
is xxxxx. The Fragment type field is set to one, indicating it is
the start packet of a serie of fragments. The number of packets
field is set to 0, and as the payload is raw Theora data the Theora
payload type field is set to 0.
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Packet 2:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M| PT | 1001 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| xxxxx |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Configuration Ident | 2 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Theora data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Example Fragmented Packet (Packet 2)
The Fragment type field is set to 2 and the number of packets field
is set to 0. For large Theora fragments there can be several of
these type of payload packets. The maximum RTP 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 |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Configuration Ident | 3 | 0 | 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.. Theora data ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Example Fragmented Packet (Packet 3)
This is the last Theora fragment packet. The Fragment type filed is
set to 3 and the packet count remains set to 0. As in the previous
packets the timestamp remains set to the first packet in the sequence
and the sequence number has been incremented.
5.2. Packet Loss
As there is no error correction within the Theora stream, packet loss
will result in a loss of signal. Packet loss is more of an issue for
fragmented Theora packets as the client will have to cope with the
handling of the Fragment type field. If we use the fragmented Theora
packet example above and the first packet is lost the client MUST
detect that the next packet has the packet count field set to 0 and
the Fragment type is set to 2 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.[note: reordering]
If a particular multicast session has a large number of participants
care must be taken to prevent an RTCP feedback implosion, [9], in the
event of packet loss from a large number of participants.
Loss of any of the Configuration fragment will result in the loss of
the full Configuration packet as detailed in the Loss of
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Configuration Headers (Section 3.3) section.
6. IANA Considerations
MIME media type name: video
MIME subtype: theora
Required Parameters:
sampling: Determines the chroma subsampling format.
width: Determines the number of pixels per line. This is an
integer between 1 and 1048561 and MUST be in multiples of 16.
height: Determines the number of lines per frame encoded. This is
an integer between 1 and 1048561 and MUST be in multiples of
16.
delivery-method: indicates the delivery methods in use, the
possible values are: inline, in_band, out_band/specific_name
Where "specific_name" is the name of the out of band delivery
method.
configuration: the base16 [11] (hexadecimal) representation of the
Packed Headers (Section 3.2.1).
Optional Parameters:
configuration-uri: the URI of the configuration headers in case of
out of band transmission. In the form of
"protocol://path/to/resource/". Depending on the specific
method the single ident packets could be retrived by their
number or aggregated in a single stream, aggregates MAY be
compressed using gzip [12] or bzip2 [14] and an sha1 [13]
checksum MAY be provided in the form of
"protocol://path/to/resource/aggregated.bz2!sha1hash"
Encoding considerations:
This media type is framed and contains binary data.
Security Considerations:
See Section 6 of RFC XXXX.
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Interoperability considerations:
None
Published specification:
RFC XXXX [RFC Editor: please replace by the RFC number of this
memo, when published]
Ogg Theora I specification: Codec setup and packet decode.
Available from the Xiph website, http://www.xiph.org
Applications which use this media type:
Audio streaming and conferencing tools
Additional information:
None
Person & email address to contact for further information:
Luca Barbato: <lu_zero@gentoo.org>
IETF Audio/Video Transport Working Group
Intended usage:
COMMON
Restriction on usage:
This media type depends on RTP framing, and hence is only defined
for transfer via RTP [3]
Author:
Luca Barbato
Change controller:
IETF AVT Working Group
6.1. Mapping MIME Parameters into SDP
The information carried in the MIME media type specification has a
specific mapping to fields in the Session Description Protocol (SDP)
[6], which is commonly used to describe RTP sessions. When SDP is
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used to specify sessions the mapping are as follows:
o The MIME type ("video") goes in SDP "m=" as the media name.
o The MIME subtype ("theora") goes in SDP "a=rtpmap" as the encoding
name.
o The clock rate in the "a=rtpmap" line MUST be 90000
o The mandated parameters "delivery-method" and "configuration" MUST
be included in the SDP "a=fmpt" attribute.
o The optional parameter "configuration-uri", when present, MUST be
included in the SDP "a=fmpt" attribute and MUST follow the
delivery-method that applies.
If the stream uses multiple decoder setup configurations and all of
them are known in advance, the Configuration Packet for each file
SHOULD be packaged together and passed to the client using the
configuration attribute.
The URI specified in the configuration-uri attribute MUST point to a
location where all of the Configuration Packets needed for the life
of the session reside.
6.1.1. SDP Example
The following example shows a basic SDP for a single stream. The
first configuration packet is inlined in the sdp, other
configurations could be fetched at any time from the first provided
uri using or all the known configuration could be downloaded using
the second uri. The inline base16 [11] configuration string is
omitted because of the lenght.
c=IN IP4 192.0.0.1
m=video RTP/AVP 98
a=rtpmap:98 theora/90000
a=fmtp:98 sampling=YCbCr-4:2:2; width=1280; height=720; delivery-
method=inline; configuration=base16string1; delivery-
method=out_band/rtsp; delivery-method=out_band/rtsp;
configuration-uri=rtsp://path/to/resource/; delivery-
method=out_band/http; configuration-uri=http://another/path/to/
resource/aggregate.bz2!sha1hash;
6.2. Usage with the SDP Offer/Answer Model
The offer, as described in An Offer/Answer Model Session Description
Protocol [5], may contain a large number of delivery methods per
single fmtp attribute, the answerer MUST remove every delivery-method
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and configuration-uri not supported. All the parameters MUST not be
altered on answer otherwise.
7. Examples
The following examples are common usage patterns that MAY be applied
in such situations, the main scope of this section is to explain
better usage of the transmission vectors.
7.1. Stream Video
This is one of the most common situation: one single server streaming
content in multicast, the clients may start a session at random time.
The content itself could be a mix of live stream, as the wj's voice
or studio scenes, and stored streams, as the music she plays.
In this situation we don't know in advance how many codebooks we will
use. The clients can join anytime and users expect to start the
fruition of the content in a short time.
On join the client will receive the current Configuration necessary
to decode the current streams inlined in the SDP so that the decoding
will start immediately after.
When the streamed content changes the new Configuration is sent in-
band before the actual stream, and the Configuration that has to be
sent inline in the SDP updated. Since the inline method is
unreliable, an out of band fallback is provided.
The client could choose to fetch the Configuration from the alternate
source as soon it discovers a Configuration packet got lost inline or
use selective retransmission [17], if the server supports the
feature.
A serverside optimization would be to keep an hash list of the
Configurations per session to avoid packing all of them and send the
same Configuration with different Ident tags
A clientside optimization would be to keep a tag list of the
Configurations per session and don't process configuration packets
already known.
8. Security Considerations
RTP packets using this payload format are subject to the security
considerations discussed in the RTP specification [3]. This implies
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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-kerr-avt-theora-rtp-00.txt
Thanks to the AVT, Ogg Theora Communities / Xiph.org, Fluendo, Ralph
Giles, Mike Smith, Phil Kerr, Timothy Terriberry, Stefan Ehmann,
Alessandro Salvatori, Politecnico di Torino (LS)^3/IMG Group in
particular Federico Ridolfo, Francesco Varano, Giampaolo Mancini,
Juan Carlos De Martin.
10. References
10.1. 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 video and Video
Conferences with Minimal Control.", RFC 3551.
[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264.
[6] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327.
[7] Mogul et al., J., "Path MTU Discovery", RFC 1063.
[8] McCann et al., J., "Path MTU Discovery for IP version 6",
RFC 1981.
[9] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)",
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Internet Draft (draft-ietf-avt-rtcp-feedback-11: Work in
progress).
[10] Barbato, L., "RTP Payload Format for Vorbis Encoded Audio -
draft-ietf-avt-vorbis-rtp-00", Internet Draft (Work in
progress).
[11] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548.
[12] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952.
[13] National Institute of Standards and Technology, "Secure Hash
Standard", May 1993.
[14] Seward, J., "libbz2 and bzip2".
10.2. Informative References
[15] "libTheora: Available from the Xiph website,
http://www.xiph.org".
[16] "Theora I specification: Codec setup and packet decode.
http://www.xiph.org/theora/doc/Theora_I_spec.pdf".
[17] Friedman, T., Caceres, R., and A. Clark, "RTP Control Protocol
Extended Reports (RTCP XR)", RFC 3611, November 2003.
[18] "ITU-T Recommendation V.42, 1994, Rev. 1. Error-correcting
Procedures for DCEs Using Asynchronous-to-Synchronous
Conversion. International Telecommunications Union. Available
from the ITU website, http://www.itu.int".
[19] "ISO 3309, October 1984, 3rd Edition. Information Processing
Systems--Data Communication High-Level Data Link Control
Procedure--Frame Structure. International Organization for
Standardization.".
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Author's Address
Luca Barbato
Xiph.Org
Email: lu_zero@gentoo.org
URI: http://www.xiph.org/
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Barbato Expires December 28, 2006 [Page 25]
