[theora-dev] Ogg IETF standard (was: Re: [vorbis-dev] application/ogg status update)
Silvia.Pfeiffer at csiro.au
Silvia.Pfeiffer at csiro.au
Thu Nov 14 19:54:53 PST 2002
Hi all,
orry for cross-posting, but I believe both developer communities should
be interested in this.
In order to get Ogg accepted as application/ogg MIME type, there was a
requirement by the IETF to get an RFC written on the Ogg encapsulation
format. I have now authored a draft of that document and would like to
get input from the community and especially from Monty. Linus and I
would like to submit it asap to IETF (their next meeting is next week).
Please could everybody help me to make sure I have not made any
mistakes!
In addition, as I have copied many words and paragraphs from the online
documents, which I presume were authored by you, Monty, I would like to
suggest to include you as author in this RFC.
Let me know what you think ...
Cheers,
Silvia.
<p>Silvia Pfeiffer wrote:
>
> Hi Linus,
>
> I'm just wondering how much further you got with writing the RFC on the
> Ogg stream/file format (and of course the RFC on the application/ogg
> MIME type).
>
> I would like to offer my help in writing an RFC on the "Ogg media
> encapsulation format" and in pursuing it with the IETF for
> standardisation. We could e.g. write up a document in RFC-compliant
> format (see RFC2629) rather quickly by using the "xml2rfc" tools
> provided by IETF.
>
> I believe this effort is necessary and an excellent way to further the
> publicity of Ogg and all the related media encoders. I'd be very happy
> to get involved as I'd like to support the excellent efforts of this
> community.
>
> Cheers,
>
> Silvia.
>
> Linus Walleij wrote:
> >
> > On Mon, 26 Aug 2002 Silvia.Pfeiffer at csiro.au wrote:
> >
> > > are you planning to write an RFC on the Ogg file format or on the ogg
> > > vorbis encoding format? Also, are you thinking about getting
> > > audio/ogg-vorbis and video/ogg-vp3 registered as mime formats?
> >
> > I have written an RFC for the application/ogg MIME type, so to support
> > this I might have to write an RFC for the Ogg stream/file format. I yet
> > don't know if I have to do that though, as my aim was primarily to get the
> > mimetype registered.
> >
> > There is a definition for audio/vorbis that will be common for both RTP
> > streamed "raw" vorbis and Ogg-packaged vorbis. However it is not yet
> > finished, some guy in Finland is working with it.
> >
> > Noone has thought about video/theora (as I think it should be named) and
> > with the extreme complexity that has surrounded the RTP version of Vorbis
> > I really don't know which way to go with that. Any suggestions?
> >
> > Linus Walleij
<p>
<p>Network Working Group S. Pfeiffer
Internet-Draft Xiph.Org
Expires: May 16, 2003 November 15, 2002
<p> The Ogg encapsulation format version 0
ogg_rfc
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.
This Internet-Draft will expire on May 16, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes the Ogg bitstream format version 1.0, which
is a general, freely-available encapsulation format for media
streams. It is capable to encapsulate any kind and number of video
and audio encoding formats as well as other data streams in a single
bitstream.
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<p>1. Introduction
The Ogg bitstream format has been developed as a part of a larger
project aimed at creating a set of components for the coding and
decoding of multimedia content (codecs) which are to be freely
available and freely re-implementable both in software and in
hardware for the computing community at large, including the Internet
community. It is the intention of the Ogg developers represented by
Xiph.Org that it be usable without intellectual property concerns.
This document describes the Ogg bitstream format and how to use it to
encapsulate one or several media bitstreams created by one or several
encoders. The Ogg transport bitstream is designed to provide
framing, error protection and seeking structure for higher-level
codec streams that consist of raw, unencapsulated data packets, such
as the Vorbis audio codec or the upcoming Tarkin and Theora video
codecs. It is capable to interleave different binary media and other
time-continuous data streams that are prepared by an encoder as a
sequence of data packets. Ogg provides enough information to
properly separate data back into such encoder created data packets at
the original packet boundaries without relying on decoding to find
packet boundaries.
Please note that there is a related document containing all required
information to register application/ogg as MIME type. It is
currently being processed as Internet-Draft: application/ogg MIME
type I-D [1].
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<p>2. Definitions
For describing the Ogg encapsulation process, a set of terms will be
used whose meaning needs to be well understood. Therefore, some of
the most fundamental terms are defined now before we start with the
description of the requirements for a generic media stream
encapsulation format, the process of encapsulation, and the concrete
format of the Ogg bitstream. See the Appendix for a more complete
glossary.
The result of an Ogg encapsulation is called the "Physical (Ogg)
Bitstream". It encapsulates one or several encoder-created
bitstreams, which are called "Logical Bitstreams". A logical
bitstream which is provided to the Ogg encapsulation process has a
structure, i.e. it is split up into a sequence of so-called
"Packets". The packets are created by the encoder of that logical
bitstream and represent meaningful entities for that encoder only
(e.g. an uncompressed stream may use video frames as packets). They
do not contain boundary information - strung together they appear to
be streams of random bytes with no landmarks.
Please note that the term "packet" is not used in this document to
signify entities for transport over a network.
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<p>3. Requirements for a generic encapsulation format
The design idea behind Ogg was to provide a generic, linear media
transport format to enable both file-based storage and stream-based
transmission of one or several interleaved media streams independent
of the encoding format of the media data. Such an encapsulation
format needs to provide:
o framing for logical bitstreams.
o interleaving of different logical bitstreams.
o detection of corruption.
o recapture after a parsing error.
o position landmarks for direct random access.
o streaming capability (i.e. no seeking is required to build a 100%
complete bitstream).
o small overhead.
o simplicity to be fast.
o simple concatenation mechanism.
All of these design considerations have been taken into consideration
for Ogg. Ogg supports framing and interleaving of logical
bitstreams, seeking landmarks, detection of corruption, and stream
resynchronisation after a parsing error with no more than
approximately 1-2% overhead. It is a generic framework to perform
encapsulation of time-continuous bitstreams. It does not know any
specifics about the codec data that it encapsulates and is thus
independent of any media codec.
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<p>4. The Ogg bitstream format
A physical Ogg bitstream consists of multiple logical bitstreams
interleaved in so-called "Pages". Whole pages are taken in order
from multiple logical bitstreams multiplexed at the page level. The
logical bitstreams are identified by a unique serial number in the
header of each page of the physical bitstream. This unique serial
number is created randomly and does not have any connection to the
content or encoder of the logical bitstream it represents. Pages of
all logical bistreams are concurrently interleaved, but they need not
be in a regular order - they only require to be consecutive within
the logical bitstream. Ogg demultiplexing reconstructs the original
logical bitstreams from the physical bitstream by taking the pages in
order from the physical bitstream and redirecting them into the
appropriate logical decoding entity.
Each Ogg page contains only one type of data as it belongs to one
logical bitstream only. Pages are of variable size and have a page
header containing encapsulation and error recovery information. Each
logical bitstream in a physical Ogg bitstream starts with a special
start page (bos=beginning of stream) and ends with a special page
(eos=end of stream). The bos page contains information to identify
the codec type and any additional information to set up the decoding
process. The format of that page is therefore dependent on the codec
and therefore must be given in the encoding specification of that
logical bitstream type. An example for such a media mapping is "Ogg
Vorbis", which provides the name and revision of the Vorbis codec,
the audio rate and the audio quality on the Ogg Vorbis bos page.
Ogg knows two types of multiplexing: concurrent multiplexing (so-
called "Grouping") and sequential multiplexing (so-called
"Chaining"). Grouping defines how to interleave several logical
bitstreams page-wise in the same physical bitstream. Grouping is for
example required for interleaving a video stream with several
synchronised audio tracks using different codecs in different logical
bitstreams. Chaining on the other hand is defined to provide a
simple mechanism to concatenate physical Ogg bitstreams as is often
required for streaming applications.
In grouping, all bos pages of all logical bitstreams must appear
together at the beginning of the Ogg bitstream. The media mapping
specifies the order of the initial pages. For example, grouping of a
specific Ogg video and Ogg audio bitstream may specify that the
physical bitstream must begin with the bos page of the logical video
bitstream followed by the bos page of the audio bitstream. Unlike
bos pages eos pages for the logical bitstreams need not all occur
contiguously. Eos pages may be 'nil' pages, that is, pages
containing no content but simply a page header with position
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<p> information and the eos flag set in the page header. Each grouped
logical bitstream must have a unique serial number within the scope
of the physical bitstream.
In chaining, complete logical bitstreams are concatenated. The
bitstreams do not overlap, i.e. the eos page of a given logical
bistream is immediately followed by the bos page of the next. Each
chained logical bitstream must have a unique serial number within the
scope of the physical bitstream.
It is possible to consecutively chain groups of concurrently
multiplexed bitstreams. The groups, when unchained, must stand on
their own as a valid concurrently multiplexed bitstream. The
following diagram shows a schematic example of such a physical
bitstream that obeys all the rules of both grouped and chained
multiplexed bitstreams.
physical bitstream with pages of
different logical bitstreams grouped and chained
-------------------------------------------------------------
|*A*|*B*|*C*|A|A|C|B|A|B|#A#|C|...|B|C|#B#|#C#|*D*|D|...|#D#|
-------------------------------------------------------------
bos bos bos eos eos eos bos eos
<p> In this example, there are two chained physical bitstreams, the first
of which is a grouped stream of three logical bitstreams A, B, and C.
The second physical bitstream is chained after the end of the grouped
bitstream, which ends after the last eos page of all its grouped
logical bitstreams. As can be seen, grouped bitstreams begin
together - all of the eos pages must appear before any data pages.
It can also be seen that pages of concurrently multiplexed bitstreams
need not conform to a regular order. And it can be seen that a
grouped bitstream can end long before the other bitstreams in the
group end.
Ogg does not know any specifics about the codec data except that each
logical bitstream belongs to a different codec, the data from the
codec comes in order and has position markers (so-called "Granule
positions"). Ogg does not have a concept of 'time': it only knows
about sequentially increasing, unitless position markers. An
application can only get temporal information through higher layers
which have access to the codec APIs to assign and convert granule
positions or time.
A specific definition of a media mapping using Ogg may put further
constraints on its specific use of the Ogg bitstream format. For
example, a specific media mapping may require that all the eos pages
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<p> for all grouped bitstreams need to appear in direct sequence. An
example for a media mapping is the specification of "Ogg Vorbis"
which uses the Ogg framework to encapsulate Vorbis-encoded audio data
for stream-based storage (such as files) and transport (such as TCP
streams or pipes). Ogg Vorbis puts a further constraint onto Ogg by
specifying that concurrent multiplexing is not allowed in Ogg Vorbis
files. Another example is the upcoming Ogg Theora specification.
Ogg Theora encapsulates a Vorbis-encoded audio bitstream and a
Tarkin-encoded video bitstream in a single physical Ogg bitstream.
As Ogg does not specify temporal relationships between the
encapsulated concurrently multiplexed bitstreams, the temporal
synchronisation between the audio and video stream will be specified
in this media mapping.
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<p>5. The encapsulation process
The process of multiplexing different logical bitstreams happens at
the level of pages as described above. The bitstreams provided by
encoders are however handed over to Ogg as so-called "Packets" with
packet boundaries dependent on the encoding format. The process of
encapsulating packets into pages will be described now.
From Ogg's perspective, packets can be of any arbitrary size,
although a nominal size of approximately 4-8 kByte is recommended.
As Ogg pages have a maximum size of about 64 kByte, sometimes a
packet has to be distributed over several pages. To simplify that
process, Ogg divides each packet into 255 byte long chunks plus a
final usually shorter chunk. These chunks are called "Ogg Segments".
They are only a logical construct and do not have a header for
themselves.
A group of contiguous segments is wrapped into a variable lenght page
preceeded by a header. A segment table in the page header tells
about the "Lacing values" (sizes) of the segments included in the
page. A flag in the page header tells whether a page contains a
packet continued from a previous page. Note that a lacing value of
255 implies that a second lacing value follows in the packet, and a
value of less than 255 marks the end of the packet after that many
additional bytes. A packet of 255 bytes (or a multiple of 255 bytes)
is terminated by a lacing value of 0. Note also that a 'nil' (zero
length) packet is not an error; it consists of nothing more than a
lacing value of zero in the header.
The encoding is optimised for speed and the expected case of the
majority of packets being between 50 and 200 bytes large. This
encoding both avoids imposing a maximum packet size as well as
imposing minimum overhead on small packets. In contrast, e.g.
simply using two bytes at the head of every packet and having a max
packet size of 32 kBytes would always penalize small packets (< 255
bytes, the typical case) with twice the segmentation overhead. Using
the lacing values as suggested, small packets see the minimum
possible byte-aligned overhead (1 byte) and large packets (>512
bytes) see a fairly constant ~0.5% overhead on encoding space.
The following diagram shows a schematic example of a media mapping
using Ogg and grouped logical bitstreams:
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<p> logical bitstream with packet boundaries
-----------------------------------------------------------------
> | packet_1 | packet_2 | packet_3 | <
-----------------------------------------------------------------
|segmentation (logically only)
v
packet_1 (5 segments) packet_2 (4 segs) p_3 (2 segs)
------------------------------ -------------------- ------------
.. |seg_1|seg_2|seg_3|seg_4|s_5 | |seg_1|seg_2|seg_3|| |seg_1|s_2 | ..
------------------------------ -------------------- ------------
| page encapsulation
v
page_1 (of packet_1) page_2 (of packet_1) page_3 (of packet_2)
------------------------ ---------------- ------------------------
|H|------------------- | |H|----------- | |H|------------------- |
|D||seg_1|seg_2|seg_3| | |D|seg_4|s_5 | | |D||seg_1|seg_2|seg_3| | ...
|R|------------------- | |R|----------- | |R|------------------- |
------------------------ ---------------- ------------------------
|
pages of |
other --------| |
logical -------
bitstreams | MUX |
-------
|
v
page_1 page_2 page_3
------ ------ ------- ----- -------
... || | || | || | || | || | ...
------ ------ ------- ----- -------
physical Ogg bitstream
In this example we take a snapshot of the encapsulation process of
one logical bitstream. We can see part of that bitstream's
subdivision into packets as provided by the codec. The Ogg
encapsulation process chops up the packets into segments. The
packets in this example are rather large such that packet_1 is split
into 5 segments - 4 segments with 255 bytes and a final smaller one.
Packet_2 is split into 4 segments - 3 segments with 255 bytes and a
final very small one - and packet_3 is split into two segments. The
encapsulation process then creates pages, which are quite small in
this example. Page_1 consists of the first three segments of
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<p> packet_1, page_2 contains the remaining 2 segments from packet_1, and
page_3 contains the first three pages of packet_2. Finally, this
logical bitstream is multiplexed into a physical Ogg bitstream with
pages of other logical bitstreams.
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<p>6. The Ogg page format
A physical Ogg bitstream consists of a sequence of concatenated
pages. Pages are of variable size, usually 4-8 kB, maximum 65307
bytes. A page header contains all the required information to
demultiplex the logical bitstreams out of the physical bitstream and
to perform basic error recovery and landmarks for seeking. Each page
is a self-contained entity such that the page decode mechanism can
recognize, verify, and handle single pages at a time without
requiring the overall bitstream.
The Ogg page header has the following format:
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| Byte
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| capture_pattern: Magic number for page start "OggS" | 0-3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| version | header_type | granule_position | 4-7
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | 8-11
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | bitstream_serial_number | 12-15
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | page_sequence_number | 16-19
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | CRC_checksum | 20-23
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |page_segments | segment_table | 24-27
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | 28-
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The LSb (least significant bit) comes first in the Bytes. Fields
with more than one byte length are encoded LSB (least significant
byte) first.
The fields in the page header have the following meaning:
1. capture_pattern: a 4 Byte field that signifies the beginning of a
page. It contains the magic numbers:
0x4f 'O'
0x67 'g'
0x67 'g'
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<p> 0x53 'S'
It helps a decoder to find the page boundaries and regain
synchronisation after parsing a corrupted stream. Once the
capture pattern is found, the decoder verifies page sync and
integrity by computing and comparing the checksum.
2. stream_structure_version: 1 Byte signifying the version number of
the Ogg file format used in this stream (this document specifies
version 0).
3. header_type_flag: the bits in this 1 Byte field identify the
specific type of this page.
* bit 0x01
set: page contains data of a media frame continued from the
previous page
unset: page contains a fresh media frame
* bit 0x02
set: this is the first page of a logical bitstream (bos)
unset: this page is not a first page
* bit 0x04
set: this is the last page of a logical bitstream (eos)
unset: this page is not a last page
4. granule_position: a 8 Byte field containing position information.
For example, for an audio stream it contains the total number of
PCM samples encoded after including all frames finished on this
page. For a video stream it contains the total number of video
frames encoded after this page. This is a hint for the decoder
and gives it some timing and position information. It's meaning
is dependent on the codec for that logical bitstream and
specified in a specific media mapping.
5. bitstream_serial_number: a 4 Byte field containing the serial
number by which the logical bitstream is identified.
6. page_sequence_number: a 4 Byte field containing the sequence
number of the page so the decoder can identify page loss.
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<p> 7. CRC_checksum: a 4 Byte field containing a 32 bit CRC checksum of
the page (including header with zero CRC field and page content).
The generator polynomial is 0x04c11db7.
8. number_page_segments: 1 Byte giving the number of segment entries
encoded in the segment table.
9. segment_table: number_page_segments Bytes containing the lacing
values of all segments in this page. Each Byte contains one
lacing value.
The total header size in bytes is given by:
header_size = number_page_segments + 27 [Byte]
The total page size in Bytes is given by:
page_size = header_size + sum(lacing_values: 1..number_page_segments)
[Byte]
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<p>7. Security considerations
The Ogg encapsulation format is a container format and only
encapsulates content (such as Vorbis-encoded audio). It does not
provide for any generic encryption or signing of itself or its
contained content bitstreams. It however encapsulates any kind of
content bitstream as long as there is a codec for it, and is thus
capable to contain encrypted and signed content data. It is also
possible to add an external security mechanism that encrypts or signs
an Ogg physical bistream and thus provides content confidentiality
and authenticity.
As Ogg enapsulates binary data, it is possible to include executable
content in an Ogg bitstream. This may be an issue with applications
that are implemented using the Ogg format, especially when Ogg is
used for streaming or file transfer in a networking scenario. Ogg as
such does not pose a threat there. However, an application decoding
Ogg and its encapsulated content bitstreams has to ensure correct
handling of manipulated bitstreams, of buffer overflows and the like.
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<p>References
[1] Walleij, L., "Internet-Draft: The application/ogg Media Type",
Internet-Draft XXXX, May 2002.
<p>Author's Address
Silvia Pfeiffer
For Xiph.Org
EMail: Silvia.Pfeiffer at csiro.au
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<p>Appendix A. Glossary of terms and abbreviations
bos page: The initial page (beginning of stream) of a logical
bitstream which contains information to identify the codec type
and other decoding-relevant information.
chaining (or sequential multiplexing): Concatenation of two or more
complete physical Ogg bitstreams.
eos page The final page (end of stream) of a logical bitstream.
granule position An increasing position number for a specific logical
bitstream stored in the page header. It's meaning is dependent on
the codec for that logical bitstream and specified in a specific
media mapping.
grouping (or concurrent multiplexing): Interleaving of pages of
several logical bitstreams into one complete physical Ogg bistream
under the restriction that all bos pages of all grouped logical
bitstreams must appear before any data pages.
lacing value: An entry in the segment table of a page header
representing the size of the related segment.
logical bistream: A sequence of bits being the result of an encoded
media stream.
media mapping: A specific use of the Ogg encapsulation format
together with a specific (set of) codec(s).
(Ogg) packet: A subpart of a logical bitstream that is created by the
encoder for that bitstream and represents a meaningful entity for
the encoder, but only a sequence of bits to the Ogg encapsulation.
(Ogg) page: A physical bitstream consists of a sequence of Ogg pages
containing data of one logical bitstream only. It usually
contains a group of contiguous segments of one packet only, but
sometimes packets are too large and must be split over several
pages.
physical (Ogg) bitstream: The sequence of bits resulting from an Ogg
encapsulation of one or several logical bitstreams. It consists
of a sequence of pages from the logical bitstreams with the
restriction that the pages of one logical bitstream must come in
their correct temporal order.
(Ogg) segment: The Ogg encapsulation process splits each packet into
chunks of 255 bytes plus a last fractional chunk of less than 255
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<p> bytes. These chunks are called segments.
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<p>Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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<p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p><p>Pfeiffer Expires May 16, 2003 [Page 18]
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