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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 < 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 (<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 < 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:ross@ross.net">ross@ross.net</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> + +<div id="copyright"> + The Xiph Fish Logo is a + trademark (™) of Xiph.Org.<br/> + + These pages © 1994 - 2005 Xiph.Org. All rights reserved. +</div> + +</body> +</html> |