libogg/doc/framing.html

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<h1>Ogg logical bitstream framing</h1>

<h2>Ogg bitstreams</h2>

<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 Theora video codec.</p>

<h2>Application example: Vorbis</h2>

<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
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>

<h2>Design constraints for Ogg bitstreams</h2>

<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>

<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
bitstreams into more complex physical bitstreams is described in the
<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 the Vorbis I Specification.</p>

<h2>Bitstream structure</h2>

<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 contains sizing information and checksum data to determine
header/page size and data integrity.</p>

<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>

<h3>Packet segmentation</h3>

<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>

<p>The raw packet is logically divided into [n] 255 byte segments and a
last fractional segment of &lt; 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>

<p>An example should make the basic concept clear:</p>

<pre>
<tt>
raw packet:
  ___________________________________________
 |______________packet data__________________| 753 bytes

lacing values for page header segment table: 255,255,243
</tt>
</pre>

<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 (&lt;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 overhead (1 byte) and large packets, over 512 bytes or
so, see a fairly constant ~.5% overhead on encoding space.</p>

<p>Note that a lacing value of 255 implies that a second lacing value
follows in the packet, and a value of &lt; 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:
  _______________________________
 |________packet data____________|          255 bytes

lacing values: 255, 0
</tt></pre>

<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>

<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.</p>

<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 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>

<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
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
simpler design (with no overhead) that needs only inspect the current
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>

<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
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>

<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>

<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>

<a name="page_header"></a>
<h3>Page header</h3>

<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
complete page at which point it writes the header followed by the
buffered packet segments.</p>

<h4>capture_pattern</h4>

<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

  0  0x4f 'O'
  1  0x67 'g'
  2  0x67 'g'
  3  0x53 'S'
</tt></pre>

<h4>stream_structure_version</h4>

<p>The capture pattern is followed by the stream structure revision:</p>

<pre><tt>
 byte value

  4  0x00
</tt></pre>

<h4>header_type_flag</h4>

<p>The header type flag identifies this page's context in the bitstream:</p>

<pre><tt>
 byte value

  5  bitflags: 0x01: unset = fresh packet
	               set = continued packet
	       0x02: unset = not first page of logical bitstream
                       set = first page of logical bitstream (bos)
	       0x04: unset = not last page of logical bitstream
                       set = last page of logical bitstream (eos)
</tt></pre>

<h4>absolute granule position</h4>

<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

  6  0xXX LSB
  7  0xXX
  8  0xXX
  9  0xXX
 10  0xXX
 11  0xXX
 12  0xXX
 13  0xXX MSB
</tt></pre>

<h4>stream serial number</h4>

<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

 14  0xXX LSB
 15  0xXX
 16  0xXX
 17  0xXX MSB
</tt></pre>

<h4>page sequence no</h4>

<p>Page counter; lets us know if a page is lost (useful where packets
span page boundaries).</p>

<pre><tt>
 byte value

 18  0xXX LSB
 19  0xXX
 20  0xXX
 21  0xXX MSB
</tt></pre>

<h4>page checksum</h4>

<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:[email protected]">[email protected]</a>.)</p>

<pre><tt>
 byte value

 22  0xXX LSB
 23  0xXX
 24  0xXX
 25  0xXX MSB
</tt></pre>

<h4>page_segments</h4>

<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

 26 0x00-0xff (0-255)
</tt></pre>

<h4>segment_table (containing packet lacing values)</h4>

<p>The lacing values for each packet segment physically appearing in
this page are listed in contiguous order.</p>

<pre><tt>
 byte value

 27 0x00-0xff (0-255)
 [...]
 n  0x00-0xff (0-255, n=page_segments+26)
</tt></pre>

<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>

<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>

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