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-<h1><font color=#000070>
-Ogg logical bitstream framing
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-<em>Last update to this document: July 14, 2002</em><br>
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+
+<h1>Ogg logical bitstream framing</h1>
<h2>Ogg bitstreams</h2>
-The Ogg transport bitstream is designed to provide framing, error
+<p>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 Tarkin video codec.
+codec or Theora video codec.</p>
<h2>Application example: Vorbis</h2>
-Vorbis encodes short-time blocks of PCM data into raw packets of
-bit-packed data. These raw packets may be used directly by transport
+
+<p>Vorbis encodes short-time blocks of PCM data into raw packets of
+bit-packed data. These raw packets may be used directly by transport
mechanisms that provide their own framing and packet-separation
-mechanisms (such as UDP datagrams). For stream based storage (such as
+mechanisms (such as UDP datagrams). For stream based storage (such as
files) and transport (such as TCP streams or pipes), Vorbis uses the
Ogg bitstream format to provide framing/sync, sync recapture
after error, landmarks during seeking, and enough information to
properly separate data back into packets at the original packet
-boundaries without relying on decoding to find packet boundaries.<p>
+boundaries without relying on decoding to find packet boundaries.</p>
<h2>Design constraints for Ogg bitstreams</h2>
-<ol><li>True streaming; we must not need to seek to build a 100%
- complete bitstream.
-
-<li> Use no more than approximately 1-2% of bitstream bandwidth for
- packet boundary marking, high-level framing, sync and seeking.
-
-<li> Specification of absolute position within the original sample
- stream.
-
-<li> Simple mechanism to ease limited editing, such as a simplified
- concatenation mechanism.
-
-<li> Detection of corruption, recapture after error and direct, random
- access to data at arbitrary positions in the bitstream.
+<ol>
+<li>True streaming; we must not need to seek to build a 100%
+ complete bitstream.</li>
+<li>Use no more than approximately 1-2% of bitstream bandwidth for
+ packet boundary marking, high-level framing, sync and seeking.</li>
+<li>Specification of absolute position within the original sample
+ stream.</li>
+<li>Simple mechanism to ease limited editing, such as a simplified
+ concatenation mechanism.</li>
+<li>Detection of corruption, recapture after error and direct, random
+ access to data at arbitrary positions in the bitstream.</li>
</ol>
<h2>Logical and Physical Bitstreams</h2>
-A <em>logical</em> Ogg bitstream is a contiguous stream of
-sequential pages belonging only to the logical bitstream. A
+<p>A <em>logical</em> Ogg bitstream is a contiguous stream of
+sequential pages belonging only to the logical bitstream. A
<em>physical</em> Ogg bitstream is constructed from one or more
than one logical Ogg bitstream (the simplest physical bitstream
-is simply a single logical bitstream). We describe below the exact
-formatting of an Ogg logical bitstream. Combining logical
+is simply a single logical bitstream). We describe below the exact
+formatting of an Ogg logical bitstream. Combining logical
bitstreams into more complex physical bitstreams is described in the
-<a href="oggstream.html">Ogg bitstream overview</a>. The exact
+<a href="oggstream.html">Ogg bitstream overview</a>. The exact
mapping of raw Vorbis packets into a valid Ogg Vorbis physical
-bitstream is described in <a href="vorbis-stream.html">Vorbis
-bitstream mapping</a>.
+bitstream is described in the Vorbis I Specification.</p>
<h2>Bitstream structure</h2>
-An Ogg stream is structured by dividing incoming packets into
+<p>An Ogg stream is structured by dividing incoming packets into
segments of up to 255 bytes and then wrapping a group of contiguous
packet segments into a variable length page preceded by a page
-header. Both the header size and page size are variable; the page
+header. Both the header size and page size are variable; the page
header contains sizing information and checksum data to determine
-header/page size and data integrity.<p>
+header/page size and data integrity.</p>
-The bitstream is captured (or recaptured) by looking for the beginning
-of a page, specifically the capture pattern. Once the capture pattern
+<p>The bitstream is captured (or recaptured) by looking for the beginning
+of a page, specifically the capture pattern. Once the capture pattern
is found, the decoder verifies page sync and integrity by computing
and comparing the checksum. At that point, the decoder can extract the
-packets themselves.<p>
+packets themselves.</p>
<h3>Packet segmentation</h3>
-Packets are logically divided into multiple segments before encoding
+<p>Packets are logically divided into multiple segments before encoding
into a page. Note that the segmentation and fragmentation process is a
logical one; it's used to compute page header values and the original
page data need not be disturbed, even when a packet spans page
-boundaries.<p>
+boundaries.</p>
-The raw packet is logically divided into [n] 255 byte segments and a
-last fractional segment of < 255 bytes. A packet size may well
+<p>The raw packet is logically divided into [n] 255 byte segments and a
+last fractional segment of < 255 bytes. A packet size may well
consist only of the trailing fractional segment, and a fractional
-segment may be zero length. These values, called "lacing values" are
-then saved and placed into the header segment table.<p>
+segment may be zero length. These values, called "lacing values" are
+then saved and placed into the header segment table.</p>
-An example should make the basic concept clear:<p>
+<p>An example should make the basic concept clear:</p>
<pre>
<tt>
</tt>
</pre>
-We simply add the lacing values for the total size; the last lacing
+<p>We simply add the lacing values for the total size; the last lacing
value for a packet is always the value that is less than 255. Note
that this encoding both avoids imposing a maximum packet size as well
as imposing minimum overhead on small packets (as opposed to, eg,
simply using two bytes at the head of every packet and having a max
-packet size of 32k. Small packets (<255, the typical case) are
+packet size of 32k. Small packets (<255, the typical case) are
penalized with twice the segmentation overhead). Using the lacing
values as suggested, small packets see the minimum possible
byte-aligned overheade (1 byte) and large packets, over 512 bytes or
-so, see a fairly constant ~.5% overhead on encoding space.<p>
+so, see a fairly constant ~.5% overhead on encoding space.</p>
-Note that a lacing value of 255 implies that a second lacing value
-follows in the packet, and a value of < 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:<p>
+<p>Note that a lacing value of 255 implies that a second lacing value
+follows in the packet, and a value of < 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:</p>
<pre><tt>
raw packet:
lacing values: 255, 0
</tt></pre>
-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.<p>
+<p>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.</p>
<h3>Packets spanning pages</h3>
-Packets are not restricted to beginning and ending within a page,
+<p>Packets are not restricted to beginning and ending within a page,
although individual segments are, by definition, required to do so.
Packets are not restricted to a maximum size, although excessively
large packets in the data stream are discouraged; the Ogg
bitstream specification strongly recommends nominal page size of
approximately 4-8kB (large packets are foreseen as being useful for
-initialization data at the beginning of a logical bitstream).<p>
+initialization data at the beginning of a logical bitstream).</p>
-After segmenting a packet, the encoder may decide not to place all the
+<p>After segmenting a packet, the encoder may decide not to place all the
resulting segments into the current page; to do so, the encoder places
the lacing values of the segments it wishes to belong to the current
-page into the current segment table, then finishes the page. The next
+page into the current segment table, then finishes the page. The next
page is begun with the first value in the segment table belonging to
the next packet segment, thus continuing the packet (data in the
packet body must also correspond properly to the lacing values in the
spanned pages. The segment data in the first packet corresponding to
the lacing values of the first page belong in that page; packet
segments listed in the segment table of the following page must begin
-the page body of the subsequent page).<p>
+the page body of the subsequent page).</p>
-The last mechanic to spanning a page boundary is to set the header
+<p>The last mechanic to spanning a page boundary is to set the header
flag in the new page to indicate that the first lacing value in the
segment table continues rather than begins a packet; a header flag of
-0x01 is set to indicate a continued packet. Although mandatory, it
+0x01 is set to indicate a continued packet. Although mandatory, it
is not actually algorithmically necessary; one could inspect the
preceding segment table to determine if the packet is new or
-continued. Adding the information to the packet_header flag allows a
+continued. Adding the information to the packet_header flag allows a
simpler design (with no overhead) that needs only inspect the current
-page header after frame capture. This also allows faster error
+page header after frame capture. This also allows faster error
recovery in the event that the packet originates in a corrupt
preceding page, implying that the previous page's segment table
-cannot be trusted.<p>
+cannot be trusted.</p>
-Note that a packet can span an arbitrary number of pages; the above
-spanning process is repeated for each spanned page boundary. Also a
+<p>Note that a packet can span an arbitrary number of pages; the above
+spanning process is repeated for each spanned page boundary. Also a
'zero termination' on a packet size that is an even multiple of 255
must appear even if the lacing value appears in the next page as a
-zero-length continuation of the current packet. The header flag
+zero-length continuation of the current packet. The header flag
should be set to 0x01 to indicate that the packet spanned, even though
-the span is a nil case as far as data is concerned.<p>
+the span is a nil case as far as data is concerned.</p>
-The encoding looks odd, but is properly optimized for speed and the
+<p>The encoding looks odd, but is properly optimized for speed and the
expected case of the majority of packets being between 50 and 200
bytes (note that it is designed such that packets of wildly different
sizes can be handled within the model; placing packet size
restrictions on the encoder would have only slightly simplified design
-in page generation and increased overall encoder complexity).<p>
+in page generation and increased overall encoder complexity).</p>
-The main point behind tracking individual packets (and packet
+<p>The main point behind tracking individual packets (and packet
segments) is to allow more flexible encoding tricks that requiring
explicit knowledge of packet size. An example is simple bandwidth
limiting, implemented by simply truncating packets in the nominal case
if the packet is arranged so that the least sensitive portion of the
-data comes last.<p>
+data comes last.</p>
<h3>Page header</h3>
-The headering mechanism is designed to avoid copying and re-assembly
+<p>The headering mechanism is designed to avoid copying and re-assembly
of the packet data (ie, making the packet segmentation process a
logical one); the header can be generated directly from incoming
-packet data. The encoder buffers packet data until it finishes a
+packet data. The encoder buffers packet data until it finishes a
complete page at which point it writes the header followed by the
-buffered packet segments.<p>
+buffered packet segments.</p>
<h4>capture_pattern</h4>
- A header begins with a capture pattern that simplifies identifying
- pages; once the decoder has found the capture pattern it can do a more
- intensive job of verifying that it has in fact found a page boundary
- (as opposed to an inadvertent coincidence in the byte stream).<p>
+<p>A header begins with a capture pattern that simplifies identifying
+pages; once the decoder has found the capture pattern it can do a more
+intensive job of verifying that it has in fact found a page boundary
+(as opposed to an inadvertent coincidence in the byte stream).</p>
<pre><tt>
byte value
<h4>stream_structure_version</h4>
- The capture pattern is followed by the stream structure revision:
+<p>The capture pattern is followed by the stream structure revision:</p>
<pre><tt>
byte value
<h4>header_type_flag</h4>
- The header type flag identifies this page's context in the bitstream:
+<p>The header type flag identifies this page's context in the bitstream:</p>
<pre><tt>
byte value
<h4>absolute granule position</h4>
- (This is packed in the same way the rest of Ogg data is packed; LSb
- of LSB first. Note that the 'position' data specifies a 'sample'
- number (eg, in a CD quality sample is four octets, 16 bits for left
- and 16 bits for right; in video it would likely be the frame number.
- It is up to the specific codec in use to define the semantic meaning
- of the granule position value). The position specified is the total
- samples encoded after including all packets finished on this page
- (packets begun on this page but continuing on to the next page do not
- count). The rationale here is that the position specified in the
- frame header of the last page tells how long the data coded by the
- bitstream is. A truncated stream will still return the proper number
- of samples that can be decoded fully.
-<p>
- A special value of '-1' (in two's complement) indicates that no packets
- finish on this page.
+<p>(This is packed in the same way the rest of Ogg data is packed; LSb
+of LSB first. Note that the 'position' data specifies a 'sample'
+number (eg, in a CD quality sample is four octets, 16 bits for left
+and 16 bits for right; in video it would likely be the frame number.
+It is up to the specific codec in use to define the semantic meaning
+of the granule position value). The position specified is the total
+samples encoded after including all packets finished on this page
+(packets begun on this page but continuing on to the next page do not
+count). The rationale here is that the position specified in the
+frame header of the last page tells how long the data coded by the
+bitstream is. A truncated stream will still return the proper number
+of samples that can be decoded fully.</p>
+
+<p>A special value of '-1' (in two's complement) indicates that no packets
+finish on this page.</p>
<pre><tt>
byte value
<h4>stream serial number</h4>
- Ogg allows for separate logical bitstreams to be mixed at page
- granularity in a physical bitstream. The most common case would be
- sequential arrangement, but it is possible to interleave pages for
- two separate bitstreams to be decoded concurrently. The serial
- number is the means by which pages physical pages are associated with
- a particular logical stream. Each logical stream must have a unique
- serial number within a physical stream:
+<p>Ogg allows for separate logical bitstreams to be mixed at page
+granularity in a physical bitstream. The most common case would be
+sequential arrangement, but it is possible to interleave pages for
+two separate bitstreams to be decoded concurrently. The serial
+number is the means by which pages physical pages are associated with
+a particular logical stream. Each logical stream must have a unique
+serial number within a physical stream:</p>
<pre><tt>
byte value
<h4>page sequence no</h4>
- Page counter; lets us know if a page is lost (useful where packets
- span page boundaries).
+<p>Page counter; lets us know if a page is lost (useful where packets
+span page boundaries).</p>
<pre><tt>
byte value
<h4>page checksum</h4>
- 32 bit CRC value (direct algorithm, initial val and final XOR = 0,
- generator polynomial=0x04c11db7). The value is computed over the
- entire header (with the CRC field in the header set to zero) and then
- continued over the page. The CRC field is then filled with the
- computed value.<p>
-
- (A thorough discussion of CRC algorithms can be found in <a
- href="ftp://ftp.rocksoft.com/clients/rocksoft/papers/crc_v3.txt">"A
- Painless Guide to CRC Error Detection Algorithms"</a> by Ross
- Williams <a
- href="mailto:ross@guest.adelaide.edu.au">ross@guest.adelaide.edu.au</a>.)
+<p>32 bit CRC value (direct algorithm, initial val and final XOR = 0,
+generator polynomial=0x04c11db7). The value is computed over the
+entire header (with the CRC field in the header set to zero) and then
+continued over the page. The CRC field is then filled with the
+computed value.</p>
+
+<p>(A thorough discussion of CRC algorithms can be found in <a
+href="http://www.ross.net/crc/download/crc_v3.txt">"A
+Painless Guide to CRC Error Detection Algorithms"</a> by Ross
+Williams <a href="mailto:ross@ross.net">ross@ross.net</a>.)</p>
<pre><tt>
byte value
<h4>page_segments</h4>
- The number of segment entries to appear in the segment table. The
- maximum number of 255 segments (255 bytes each) sets the maximum
- possible physical page size at 65307 bytes or just under 64kB (thus
- we know that a header corrupted so as destroy sizing/alignment
- information will not cause a runaway bitstream. We'll read in the
- page according to the corrupted size information that's guaranteed to
- be a reasonable size regardless, notice the checksum mismatch, drop
- sync and then look for recapture).<p>
+<p>The number of segment entries to appear in the segment table. The
+maximum number of 255 segments (255 bytes each) sets the maximum
+possible physical page size at 65307 bytes or just under 64kB (thus
+we know that a header corrupted so as destroy sizing/alignment
+information will not cause a runaway bitstream. We'll read in the
+page according to the corrupted size information that's guaranteed to
+be a reasonable size regardless, notice the checksum mismatch, drop
+sync and then look for recapture).</p>
<pre><tt>
byte value
<h4>segment_table (containing packet lacing values)</h4>
- The lacing values for each packet segment physically appearing in
- this page are listed in contiguous order.
+<p>The lacing values for each packet segment physically appearing in
+this page are listed in contiguous order.</p>
<pre><tt>
byte value
n 0x00-0xff (0-255, n=page_segments+26)
</tt></pre>
-Total page size is calculated directly from the known header size and
+<p>Total page size is calculated directly from the known header size and
lacing values in the segment table. Packet data segments follow
-immediately after the header.<p>
+immediately after the header.</p>
-Page headers typically impose a flat .25-.5% space overhead assuming
-nominal ~8k page sizes. The segmentation table needed for exact
+<p>Page headers typically impose a flat .25-.5% space overhead assuming
+nominal ~8k page sizes. The segmentation table needed for exact
packet recovery in the streaming layer adds approximately .5-1%
nominal assuming expected encoder behavior in the 44.1kHz, 128kbps
-stereo encodings.<p>
-
-<hr>
-<a href="http://www.xiph.org/">
-<img src="white-xifish.png" align=left border=0>
-</a>
-<font size=-2 color=#505050>
-
-Ogg is a <a href="http://www.xiph.org">Xiph.org Foundation</a> effort
-to protect essential tenets of Internet multimedia from corporate
-hostage-taking; Open Source is the net's greatest tool to keep
-everyone honest. See <a href="http://www.xiph.org/about.html">About
-the Xiph.org Foundation</a> for details.
-<p>
-
-Ogg Vorbis is the first Ogg audio CODEC. Anyone may freely use and
-distribute the Ogg and Vorbis specification, whether in a private,
-public or corporate capacity. However, the Xiph.org Foundation and
-the Ogg project (xiph.org) reserve the right to set the Ogg Vorbis
-specification and certify specification compliance.<p>
-
-Xiph.org's Vorbis software CODEC implementation is distributed under a
-BSD-like license. This does not restrict third parties from
-distributing independent implementations of Vorbis software under
-other licenses.<p>
-
-Ogg, Vorbis, Xiph.org Foundation and their logos are trademarks (tm)
-of the <a href="http://www.xiph.org/">Xiph.org Foundation</a>. These
-pages are copyright (C) 1994-2002 Xiph.org Foundation. All rights
-reserved.<p>
+stereo encodings.</p>
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