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 <h1>Page Multiplexing and Ordering in a Physical Ogg Stream</h1>

 <p>The low-level mechanisms of an Ogg stream (as described in the Ogg
 Bitstream Overview) provide means for mixing multiple logical streams
 and media types into a single linear-chronological stream. This
 document specifies the high-level arrangement and use of page
 structure to multiplex multiple streams of mixed media type within a
 physical Ogg stream.</p>

 <h2>Design Elements</h2>

 <p>The design and arrangement of the Ogg container format is governed by
 several high-level design decisions that form the reasoning behind
 specific low-level design decisions.</p>

 <h3>Linear media</h3>

 <p>The Ogg bitstream is intended to encapsulate chronological,
 time-linear mixed media into a single delivery stream or file. The
 design is such that an application can always encode and/or decode a
 full-featured bitstream in one pass with no seeking and minimal
 buffering. Seeking to provide optimized encoding (such as two-pass
 encoding) or interactive decoding (such as scrubbing or instant
 replay) is not disallowed or discouraged, however no bitstream feature
 must require nonlinear operation on the bitstream.</p>

 <h3>Multiplexing</h3>

 <p>Ogg bitstreams multiplex multiple logical streams into a single
 physical stream at the page level. Each page contains an abstract
 time stamp (the Granule Position) that represents an absolute time
 landmark within the stream. After the pages representing stream
 headers (all logical stream headers occur at the beginning of a
 physical bitstream section before any logical stream data), logical
 stream data pages are arranged in a physical bitstream in strict
 non-decreasing order by chronological absolute time as
 specified by the granule position.</p>

 <p>The only exception to arranging pages in strictly ascending time order
 by granule position is those pages that do not set the granule
 position value. This is a special case when exceptionally large
 packets span multiple pages; the specifics of handling this special
 case are described later under 'Continuous and Discontinuous
 Streams'.</p>

 <h3>Seeking</h3>

 <p>Ogg is designed to use an interpolated bisection search to
 implement exact positional seeking.  Interpolated bisection search is
 a spec-mandated mechanism.</p>

 <p><i>An index may improve objective performance, but it seldom
 improves subjective performance outside of a few high-latency use
 cases and adds no additional functionality as bisection search
 delivers the same functionality for both one- and two-pass stream
 types.  For these reasons, use of indexes is discouraged, except in
 cases where an index provides demonstrable and noticable performance
 improvement.</i></p>

 <p>Seek operations are by absolute time; a direct bisection search must
 find the exact time position requested. Information in the Ogg
 bitstream is arranged such that all information to be presented for
 playback from the desired seek point will occur at or after the
 desired seek point. Seek operations are neither 'fuzzy' nor
 heuristic.</p>

 <p><i>Although key frame handling in video appears to be an exception to
 "all needed playback information lies ahead of a given seek",
 key frames can still be handled directly within this indexless
 framework. Seeking to a key frame in video (as well as seeking in other
 media types with analogous restraints) is handled as two seeks; first
 a seek to the desired time which extracts state information that
 decodes to the time of the last key frame, followed by a second seek
 directly to the key frame. The location of the previous key frame is
 embedded as state information in the granulepos; this mechanism is
 described in more detail later.</i></p>

 <h3>Continuous and Discontinuous Streams</h3>

 <p>Logical streams within a physical Ogg stream belong to one of two
 categories, "Continuous" streams and "Discontinuous" streams.
 Although these are discussed in more detail later, the distinction is
 important to a high-level understanding of how to buffer an Ogg
 stream.</p>

 <p>A stream that provides a gapless, time-continuous media type with a
 fine-grained timebase is considered to be 'Continuous'. A continuous
 stream should never be starved of data. Clear examples of continuous
 data types include broadcast audio and video.</p>

 <p>A stream that delivers data in a potentially irregular pattern or with
 widely spaced timing gaps is considered to be 'Discontinuous'. A
 discontinuous stream may be best thought of as data representing
 scattered events; although they happen in order, they are typically
 unconnected data often located far apart. One possible example of a
 discontinuous stream types would be captioning. Although it's
 possible to design captions as a continuous stream type, it's most
 natural to think of captions as widely spaced pieces of text with
 little happening between.</p>

 <p>The fundamental design distinction between continuous and
 discontinuous streams concerns buffering.</p>

 <h3>Buffering</h3>

 <p>Because a continuous stream is, by definition, gapless, Ogg buffering
 is based on the simple premise of never allowing any active continuous
 stream to starve for data during decode; buffering proceeds ahead
 until all continuous streams in a physical stream have data ready to
 decode on demand.</p>

 <p>Discontinuous stream data may occur on a fairly regular basis, but the
 timing of, for example, a specific caption is impossible to predict
 with certainty in most captioning systems. Thus the buffering system
 should take discontinuous data 'as it comes' rather than working ahead
 (for a potentially unbounded period) to look for future discontinuous
 data. As such, discontinuous streams are ignored when managing
 buffering; their pages simply 'fall out' of the stream when continuous
 streams are handled properly.</p>

 <p>Buffering requirements need not be explicitly declared or managed for
 the encoded stream; the decoder simply reads as much data as is
 necessary to keep all continuous stream types gapless (also ensuring
 discontinuous data arrives in time) and no more, resulting in optimum
 implicit buffer usage for a given stream. Because all pages of all
 data types are stamped with absolute timing information within the
 stream, inter-stream synchronization timing is always explicitly
 maintained without the need for explicitly declared buffer-ahead
 hinting.</p>

 <p>Further details, mechanisms and reasons for the differing arrangement
 and behavior of continuous and discontinuous streams is discussed
 later.</p>

 <h3>Whole-stream navigation</h3>

 <p>Ogg is designed so that the simplest navigation operations treat the
 physical Ogg stream as a whole summary of its streams, rather than
 navigating each interleaved stream as a separate entity.</p>

 <p>First Example: seeking to a desired time position in a multiplexed (or
 unmultiplexed) Ogg stream can be accomplished through a bisection
 search on time position of all pages in the stream (as encoded in the
 granule position). More powerful searches (such as a key frame-aware
 seek within video) are also possible with additional search
 complexity, but similar computational complexity.</p>

 <p>Second Example: A bitstream section may consist of three multiplexed
 streams of differing lengths. The result of multiplexing these
 streams should be thought of as a single mixed stream with a length
 equal to the longest of the three component streams. Although it is
 also possible to think of the multiplexed results as three concurrent
 streams of different lengths and it is possible to recover the three
 original streams, it will also become obvious that once multiplexed,
 it isn't possible to find the internal lengths of the component
 streams without a linear search of the whole bitstream section.
 However, it is possible to find the length of the whole bitstream
 section easily (in near-constant time per section) just as it is for a
 single-media unmultiplexed stream.</p>

 <h2>Granule Position</h2>

 <h3>Description</h3>

 <p>The Granule Position is a signed 64 bit field appearing in the header
 of every Ogg page. Although the granule position represents absolute
 time within a logical stream, its value does not necessarily directly
 encode a simple timestamp. It may represent frames elapsed (as in
 Vorbis), a simple timestamp, or a more complex bit-division encoding
 (such as in Theora). The exact encoding of the granule position is up
 to a specific codec.</p>

 <p>The granule position is governed by the following rules:</p>

 <ul>

 <li>Granule Position must always increase forward or remain equal from
 page to page, be unset, or be zero for a header page. The absolute
 time to which any correct sequence of granule position maps must
 similarly always increase forward or remain equal. <i>(A codec may
 make use of data, such as a control sequence, that only affects codec
 working state without producing data and thus advancing granule
 position and time. Although the packet sequence number increases in
 this case, the granule position, and thus the time position, do
 not.)</i></li>

 <li>Granule position may only be unset if there no packet defining a
 time boundary on the page (that is, if no packet in a continuous
 stream ends on the page, or no packet in a discontinuous stream begins
 on the page. This will be discussed in more detail under Continuous
 and Discontinuous streams).</li>

 <li>A codec must be able to translate a given granule position value
 to a unique, deterministic absolute time value through direct
 calculation. A codec is not required to be able to translate an
 absolute time value into a unique granule position value.</li>

 <li>Codecs shall choose a granule position definition that allows that
 codec means to seek as directly as possible to an immediately
 decodable point, such as the bit-divided granule position encoding of
 Theora allows the codec to seek efficiently to key frame without using
 an index. That is, additional information other than absolute time
 may be encoded into a granule position value so long as the granule
 position obeys the above points.</li>

 </ul>

 <h4>Example: timestamp</h4>

 <p>In general, a codec/stream type should choose the simplest granule
 position encoding that addresses its requirements. The examples here
 are by no means exhaustive of the possibilities within Ogg.</p>

 <p>A simple granule position could encode a timestamp directly. For
 example, a granule position that encoded milliseconds from beginning
 of stream would allow a logical stream length of over 100,000,000,000
 days before beginning a new logical stream (to avoid the granule
 position wrapping).</p>

 <h4>Example: framestamp</h4>

 <p>A simple millisecond timestamp granule encoding might suit many stream
 types, but a millisecond resolution is inappropriate to, eg, most
 audio encodings where exact single-sample resolution is generally a
 requirement. A millisecond is both too large a granule and often does
 not represent an integer number of samples.</p>

 <p>In the event that audio frames are always encoded as the same number of
 samples, the granule position could simply be a linear count of frames
 since beginning of stream. This has the advantages of being exact and
 efficient. Position in time would simply be <tt>[granule_position] *
 [samples_per_frame] / [samples_per_second]</tt>.</p>

 <h4>Example: samplestamp (Vorbis)</h4>

 <p>Frame counting is insufficient in codecs such as Vorbis where an audio
 frame [packet] encodes a variable number of samples. In Vorbis's
 case, the granule position is a count of the number of raw samples
 from the beginning of stream; the absolute time of
 a granule position is <tt>[granule_position] /
 [samples_per_second]</tt>.</p>

 <h4>Example: bit-divided framestamp (Theora)</h4>

 <p>Some video codecs may be able to use the simple framestamp scheme for
 granule position. However, most modern video codecs introduce at
 least the following complications:</p>

 <ul>

 <li>video frames are relatively far apart compared to audio samples;
 for this reason, the point at which a video frame changes to the next
 frame is usually a strictly defined offset within the frame 'period'.
 That is, video at 50fps could just as easily define frame transitions
 &lt;.015, .035, .055...&gt; as at &lt;.00, .02, .04...&gt;.</li>

 <li>frame rates often include drop-frames, leap-frames or other
 rational-but-non-integer timings.</li>

 <li>Decode must begin at a 'key frame' or 'I frame'. Keyframes usually
 occur relatively seldom.</li>

 </ul>

 <p>The first two points can be handled straightforwardly via the fact
 that the codec has complete control mapping granule position to
 absolute time; non-integer frame rates and offsets can be set in the
 codec's initial header, and the rest is just arithmetic.</p>

 <p>The third point appears trickier at first glance, but it too can be
 handled through the granule position mapping mechanism. Here we
 arrange the granule position in such a way that granule positions of
 key frames are easy to find. Divide the granule position into two
 fields; the most-significant bits are an absolute frame counter, but
 it's only updated at each key frame. The least significant bits encode
 the number of frames since the last key frame. In this way, each
 granule position both encodes the absolute time of the current frame
 as well as the absolute time of the last key frame.</p>

 <p>Seeking to a most recent preceding key frame is then accomplished by
 first seeking to the original desired point, inspecting the granulepos
 of the resulting video page, extracting from that granulepos the
 absolute time of the desired key frame, and then seeking directly to
 that key frame's page. Of course, it's still possible for an
 application to ignore key frames and use a simpler seeking algorithm
 (decode would be unable to present decoded video until the next
 key frame). Surprisingly many player applications do choose the
 simpler approach.</p>

 <h3>granule position, packets and pages</h3>

 <p>Although each packet of data in a logical stream theoretically has a
 specific granule position, only one granule position is encoded
 per page. It is possible to encode a logical stream such that each
 page contains only a single packet (so that granule positions are
 preserved for each packet), however a one-to-one packet/page mapping
 is not intended to be the general case.</p>

 <p>Because Ogg functions at the page, not packet, level, this
 once-per-page time information provides Ogg with the finest-grained
 time information is can use. Ogg passes this granule positioning data
 to the codec (along with the packets extracted from a page); it is the
 responsibility of codecs to track timing information at granularities
 finer than a single page.</p>

 <h3>start-time and end-time positioning</h3>

 <p>A granule position represents the <em>instantaneous time location
 between two pages</em>. However, continuous streams and discontinuous
 streams differ on whether the granulepos represents the end-time of
 the data on a page or the start-time. Continuous streams are
 'end-time' encoded; the granulepos represents the point in time
 immediately after the last data decoded from a page. Discontinuous
 streams are 'start-time' encoded; the granulepos represents the point
 in time of the first data decoded from the page.</p>

 <p>An Ogg stream type is declared continuous or discontinuous by its
 codec. A given codec may support both continuous and discontinuous
 operation so long as any given logical stream is continuous or
 discontinuous for its entirety and the codec is able to ascertain (and
 inform the Ogg layer) as to which after decoding the initial stream
 header. The majority of codecs will always be continuous (such as
 Vorbis) or discontinuous (such as Writ).</p>

 <p>Start- and end-time encoding do not affect multiplexing sort-order;
 pages are still sorted by the absolute time a given granulepos maps to
 regardless of whether that granulepos represents start- or
 end-time.</p>

 <h2>Multiplex/Demultiplex Division of Labor</h2>

 <p>The Ogg multiplex/demultiplex layer provides mechanisms for encoding
 raw packets into Ogg pages, decoding Ogg pages back into the original
 codec packets, determining the logical structure of an Ogg stream, and
 navigating through and synchronizing with an Ogg stream at a desired
 stream location. Strict multiplex/demultiplex operations are entirely
 in the Ogg domain and require no intervention from codecs.</p>

 <p>Implementation of more complex operations does require codec
 knowledge, however. Unlike other framing systems, Ogg maintains
 strict separation between framing and the framed bitstream data; Ogg
 does not replicate codec-specific information in the page/framing
 data, nor does Ogg blur the line between framing and stream
 data/metadata. Because Ogg is fully data-agnostic toward the data it
 frames, operations which require specifics of bitstream data (such as
 'seek to key frame') also require interaction with the codec layer
 (because, in this example, the Ogg layer is not aware of the concept
 of key frames). This is different from systems that blur the
 separation between framing and stream data in order to simplify the
 separation of code. The Ogg system purposely keeps the distinction in
 data simple so that later codec innovations are not constrained by
 framing design.</p>

 <p>For this reason, however, complex seeking operations require
 interaction with the codecs in order to decode the granule position of
 a given stream type back to absolute time or in order to find
 'decodable points' such as key frames in video.</p>

 <h2>Unsorted Discussion Points</h2>

 <p>flushes around key frames? RFC suggestion: repaginating or building a
 stream this way is nice but not required</p>

 <h2>Appendix A: multiplexing examples</h2>

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	<h1>Page Multiplexing and Ordering in a Physical Ogg Stream</h1>

	<p>The low-level mechanisms of an Ogg stream (as described in the Ogg
	Bitstream Overview) provide means for mixing multiple logical streams
	and media types into a single linear-chronological stream. This
	document specifies the high-level arrangement and use of page
	structure to multiplex multiple streams of mixed media type within a
	physical Ogg stream.</p>

	<h2>Design Elements</h2>

	<p>The design and arrangement of the Ogg container format is governed by
	several high-level design decisions that form the reasoning behind
	specific low-level design decisions.</p>

	<h3>Linear media</h3>

	<p>The Ogg bitstream is intended to encapsulate chronological,
	time-linear mixed media into a single delivery stream or file. The
	design is such that an application can always encode and/or decode a
	full-featured bitstream in one pass with no seeking and minimal
	buffering. Seeking to provide optimized encoding (such as two-pass
	encoding) or interactive decoding (such as scrubbing or instant
	replay) is not disallowed or discouraged, however no bitstream feature
	must require nonlinear operation on the bitstream.</p>

	<h3>Multiplexing</h3>

	<p>Ogg bitstreams multiplex multiple logical streams into a single
	physical stream at the page level. Each page contains an abstract
	time stamp (the Granule Position) that represents an absolute time
	landmark within the stream. After the pages representing stream
	headers (all logical stream headers occur at the beginning of a
	physical bitstream section before any logical stream data), logical
	stream data pages are arranged in a physical bitstream in strict
	non-decreasing order by chronological absolute time as
	specified by the granule position.</p>

	<p>The only exception to arranging pages in strictly ascending time order
	by granule position is those pages that do not set the granule
	position value. This is a special case when exceptionally large
	packets span multiple pages; the specifics of handling this special
	case are described later under 'Continuous and Discontinuous
	Streams'.</p>

	<h3>Seeking</h3>

	<p>Ogg is designed to use an interpolated bisection search to
	implement exact positional seeking. Interpolated bisection search is
	a spec-mandated mechanism.</p>

	<p><i>An index may improve objective performance, but it seldom
	improves subjective performance outside of a few high-latency use
	cases and adds no additional functionality as bisection search
	delivers the same functionality for both one- and two-pass stream
	types. For these reasons, use of indexes is discouraged, except in
	cases where an index provides demonstrable and noticable performance
	improvement.</i></p>

	<p>Seek operations are by absolute time; a direct bisection search must
	find the exact time position requested. Information in the Ogg
	bitstream is arranged such that all information to be presented for
	playback from the desired seek point will occur at or after the
	desired seek point. Seek operations are neither 'fuzzy' nor
	heuristic.</p>

	<p><i>Although key frame handling in video appears to be an exception to
	"all needed playback information lies ahead of a given seek",
	key frames can still be handled directly within this indexless
	framework. Seeking to a key frame in video (as well as seeking in other
	media types with analogous restraints) is handled as two seeks; first
	a seek to the desired time which extracts state information that
	decodes to the time of the last key frame, followed by a second seek
	directly to the key frame. The location of the previous key frame is
	embedded as state information in the granulepos; this mechanism is
	described in more detail later.</i></p>

	<h3>Continuous and Discontinuous Streams</h3>

	<p>Logical streams within a physical Ogg stream belong to one of two
	categories, "Continuous" streams and "Discontinuous" streams.
	Although these are discussed in more detail later, the distinction is
	important to a high-level understanding of how to buffer an Ogg
	stream.</p>

	<p>A stream that provides a gapless, time-continuous media type with a
	fine-grained timebase is considered to be 'Continuous'. A continuous
	stream should never be starved of data. Clear examples of continuous
	data types include broadcast audio and video.</p>

	<p>A stream that delivers data in a potentially irregular pattern or with
	widely spaced timing gaps is considered to be 'Discontinuous'. A
	discontinuous stream may be best thought of as data representing
	scattered events; although they happen in order, they are typically
	unconnected data often located far apart. One possible example of a
	discontinuous stream types would be captioning. Although it's
	possible to design captions as a continuous stream type, it's most
	natural to think of captions as widely spaced pieces of text with
	little happening between.</p>

	<p>The fundamental design distinction between continuous and
	discontinuous streams concerns buffering.</p>

	<h3>Buffering</h3>

	<p>Because a continuous stream is, by definition, gapless, Ogg buffering
	is based on the simple premise of never allowing any active continuous
	stream to starve for data during decode; buffering proceeds ahead
	until all continuous streams in a physical stream have data ready to
	decode on demand.</p>

	<p>Discontinuous stream data may occur on a fairly regular basis, but the
	timing of, for example, a specific caption is impossible to predict
	with certainty in most captioning systems. Thus the buffering system
	should take discontinuous data 'as it comes' rather than working ahead
	(for a potentially unbounded period) to look for future discontinuous
	data. As such, discontinuous streams are ignored when managing
	buffering; their pages simply 'fall out' of the stream when continuous
	streams are handled properly.</p>

	<p>Buffering requirements need not be explicitly declared or managed for
	the encoded stream; the decoder simply reads as much data as is
	necessary to keep all continuous stream types gapless (also ensuring
	discontinuous data arrives in time) and no more, resulting in optimum
	implicit buffer usage for a given stream. Because all pages of all
	data types are stamped with absolute timing information within the
	stream, inter-stream synchronization timing is always explicitly
	maintained without the need for explicitly declared buffer-ahead
	hinting.</p>

	<p>Further details, mechanisms and reasons for the differing arrangement
	and behavior of continuous and discontinuous streams is discussed
	later.</p>

	<h3>Whole-stream navigation</h3>

	<p>Ogg is designed so that the simplest navigation operations treat the
	physical Ogg stream as a whole summary of its streams, rather than
	navigating each interleaved stream as a separate entity.</p>

	<p>First Example: seeking to a desired time position in a multiplexed (or
	unmultiplexed) Ogg stream can be accomplished through a bisection
	search on time position of all pages in the stream (as encoded in the
	granule position). More powerful searches (such as a key frame-aware
	seek within video) are also possible with additional search
	complexity, but similar computational complexity.</p>

	<p>Second Example: A bitstream section may consist of three multiplexed
	streams of differing lengths. The result of multiplexing these
	streams should be thought of as a single mixed stream with a length
	equal to the longest of the three component streams. Although it is
	also possible to think of the multiplexed results as three concurrent
	streams of different lengths and it is possible to recover the three
	original streams, it will also become obvious that once multiplexed,
	it isn't possible to find the internal lengths of the component
	streams without a linear search of the whole bitstream section.
	However, it is possible to find the length of the whole bitstream
	section easily (in near-constant time per section) just as it is for a
	single-media unmultiplexed stream.</p>

	<h2>Granule Position</h2>

	<h3>Description</h3>

	<p>The Granule Position is a signed 64 bit field appearing in the header
	of every Ogg page. Although the granule position represents absolute
	time within a logical stream, its value does not necessarily directly
	encode a simple timestamp. It may represent frames elapsed (as in
	Vorbis), a simple timestamp, or a more complex bit-division encoding
	(such as in Theora). The exact encoding of the granule position is up
	to a specific codec.</p>

	<p>The granule position is governed by the following rules:</p>

	<ul>

	<li>Granule Position must always increase forward or remain equal from
	page to page, be unset, or be zero for a header page. The absolute
	time to which any correct sequence of granule position maps must
	similarly always increase forward or remain equal. <i>(A codec may
	make use of data, such as a control sequence, that only affects codec
	working state without producing data and thus advancing granule
	position and time. Although the packet sequence number increases in
	this case, the granule position, and thus the time position, do
	not.)</i></li>

	<li>Granule position may only be unset if there no packet defining a
	time boundary on the page (that is, if no packet in a continuous
	stream ends on the page, or no packet in a discontinuous stream begins
	on the page. This will be discussed in more detail under Continuous
	and Discontinuous streams).</li>

	<li>A codec must be able to translate a given granule position value
	to a unique, deterministic absolute time value through direct
	calculation. A codec is not required to be able to translate an
	absolute time value into a unique granule position value.</li>

	<li>Codecs shall choose a granule position definition that allows that
	codec means to seek as directly as possible to an immediately
	decodable point, such as the bit-divided granule position encoding of
	Theora allows the codec to seek efficiently to key frame without using
	an index. That is, additional information other than absolute time
	may be encoded into a granule position value so long as the granule
	position obeys the above points.</li>

	</ul>

	<h4>Example: timestamp</h4>

	<p>In general, a codec/stream type should choose the simplest granule
	position encoding that addresses its requirements. The examples here
	are by no means exhaustive of the possibilities within Ogg.</p>

	<p>A simple granule position could encode a timestamp directly. For
	example, a granule position that encoded milliseconds from beginning
	of stream would allow a logical stream length of over 100,000,000,000
	days before beginning a new logical stream (to avoid the granule
	position wrapping).</p>

	<h4>Example: framestamp</h4>

	<p>A simple millisecond timestamp granule encoding might suit many stream
	types, but a millisecond resolution is inappropriate to, eg, most
	audio encodings where exact single-sample resolution is generally a
	requirement. A millisecond is both too large a granule and often does
	not represent an integer number of samples.</p>

	<p>In the event that audio frames are always encoded as the same number of
	samples, the granule position could simply be a linear count of frames
	since beginning of stream. This has the advantages of being exact and
	efficient. Position in time would simply be <tt>[granule_position] *
	[samples_per_frame] / [samples_per_second]</tt>.</p>

	<h4>Example: samplestamp (Vorbis)</h4>

	<p>Frame counting is insufficient in codecs such as Vorbis where an audio
	frame [packet] encodes a variable number of samples. In Vorbis's
	case, the granule position is a count of the number of raw samples
	from the beginning of stream; the absolute time of
	a granule position is <tt>[granule_position] /
	[samples_per_second]</tt>.</p>

	<h4>Example: bit-divided framestamp (Theora)</h4>

	<p>Some video codecs may be able to use the simple framestamp scheme for
	granule position. However, most modern video codecs introduce at
	least the following complications:</p>

	<ul>

	<li>video frames are relatively far apart compared to audio samples;
	for this reason, the point at which a video frame changes to the next
	frame is usually a strictly defined offset within the frame 'period'.
	That is, video at 50fps could just as easily define frame transitions
	<.015, .035, .055...> as at <.00, .02, .04...>.</li>

	<li>frame rates often include drop-frames, leap-frames or other
	rational-but-non-integer timings.</li>

	<li>Decode must begin at a 'key frame' or 'I frame'. Keyframes usually
	occur relatively seldom.</li>

	</ul>

	<p>The first two points can be handled straightforwardly via the fact
	that the codec has complete control mapping granule position to
	absolute time; non-integer frame rates and offsets can be set in the
	codec's initial header, and the rest is just arithmetic.</p>

	<p>The third point appears trickier at first glance, but it too can be
	handled through the granule position mapping mechanism. Here we
	arrange the granule position in such a way that granule positions of
	key frames are easy to find. Divide the granule position into two
	fields; the most-significant bits are an absolute frame counter, but
	it's only updated at each key frame. The least significant bits encode
	the number of frames since the last key frame. In this way, each
	granule position both encodes the absolute time of the current frame
	as well as the absolute time of the last key frame.</p>

	<p>Seeking to a most recent preceding key frame is then accomplished by
	first seeking to the original desired point, inspecting the granulepos
	of the resulting video page, extracting from that granulepos the
	absolute time of the desired key frame, and then seeking directly to
	that key frame's page. Of course, it's still possible for an
	application to ignore key frames and use a simpler seeking algorithm
	(decode would be unable to present decoded video until the next
	key frame). Surprisingly many player applications do choose the
	simpler approach.</p>

	<h3>granule position, packets and pages</h3>

	<p>Although each packet of data in a logical stream theoretically has a
	specific granule position, only one granule position is encoded
	per page. It is possible to encode a logical stream such that each
	page contains only a single packet (so that granule positions are
	preserved for each packet), however a one-to-one packet/page mapping
	is not intended to be the general case.</p>

	<p>Because Ogg functions at the page, not packet, level, this
	once-per-page time information provides Ogg with the finest-grained
	time information is can use. Ogg passes this granule positioning data
	to the codec (along with the packets extracted from a page); it is the
	responsibility of codecs to track timing information at granularities
	finer than a single page.</p>

	<h3>start-time and end-time positioning</h3>

	<p>A granule position represents the <em>instantaneous time location
	between two pages</em>. However, continuous streams and discontinuous
	streams differ on whether the granulepos represents the end-time of
	the data on a page or the start-time. Continuous streams are
	'end-time' encoded; the granulepos represents the point in time
	immediately after the last data decoded from a page. Discontinuous
	streams are 'start-time' encoded; the granulepos represents the point
	in time of the first data decoded from the page.</p>

	<p>An Ogg stream type is declared continuous or discontinuous by its
	codec. A given codec may support both continuous and discontinuous
	operation so long as any given logical stream is continuous or
	discontinuous for its entirety and the codec is able to ascertain (and
	inform the Ogg layer) as to which after decoding the initial stream
	header. The majority of codecs will always be continuous (such as
	Vorbis) or discontinuous (such as Writ).</p>

	<p>Start- and end-time encoding do not affect multiplexing sort-order;
	pages are still sorted by the absolute time a given granulepos maps to
	regardless of whether that granulepos represents start- or
	end-time.</p>

	<h2>Multiplex/Demultiplex Division of Labor</h2>

	<p>The Ogg multiplex/demultiplex layer provides mechanisms for encoding
	raw packets into Ogg pages, decoding Ogg pages back into the original
	codec packets, determining the logical structure of an Ogg stream, and
	navigating through and synchronizing with an Ogg stream at a desired
	stream location. Strict multiplex/demultiplex operations are entirely
	in the Ogg domain and require no intervention from codecs.</p>

	<p>Implementation of more complex operations does require codec
	knowledge, however. Unlike other framing systems, Ogg maintains
	strict separation between framing and the framed bitstream data; Ogg
	does not replicate codec-specific information in the page/framing
	data, nor does Ogg blur the line between framing and stream
	data/metadata. Because Ogg is fully data-agnostic toward the data it
	frames, operations which require specifics of bitstream data (such as
	'seek to key frame') also require interaction with the codec layer
	(because, in this example, the Ogg layer is not aware of the concept
	of key frames). This is different from systems that blur the
	separation between framing and stream data in order to simplify the
	separation of code. The Ogg system purposely keeps the distinction in
	data simple so that later codec innovations are not constrained by
	framing design.</p>

	<p>For this reason, however, complex seeking operations require
	interaction with the codecs in order to decode the granule position of
	a given stream type back to absolute time or in order to find
	'decodable points' such as key frames in video.</p>

	<h2>Unsorted Discussion Points</h2>

	<p>flushes around key frames? RFC suggestion: repaginating or building a
	stream this way is nice but not required</p>

	<h2>Appendix A: multiplexing examples</h2>

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