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<h1>Ogg bitstream overview</h1>
This document serves as starting point for understanding the design
and implementation of the Ogg container format. If you're new to Ogg
or merely want a high-level technical overview, start reading here.
Other documents linked from the <a href="index.html">index page</a>
give distilled technical descriptions and references of the container
mechanisms. This document is intended to aid understanding.
<h2>Container format design points</h2>
<p>Ogg is intended to be a simplest-possible container, concerned only
with framing, ordering, and interleave. It can be used as a stream delivery
mechanism, for media file storage, or as a building block toward
implementing a more complex, non-linear container (for example, see
the <a href="skeleton.html">Skeleton</a> or <a
href="http://en.wikipedia.org/wiki/Annodex">Annodex/CMML</a>).
<p>The Ogg container is not intended to be a monolithic
'kitchen-sink'. It exists only to frame and deliver in-order stream
data and as such is vastly simpler than most other containers.
Elementary and multiplexed streams are both constructed entirely from a
single building block (an Ogg page) comprised of eight fields
totalling twenty-eight bytes (the page header) a list of packet lengths
(up to 255 bytes) and payload data (up to 65025 bytes). The structure
of every page is the same. There are no optional fields or alternate
encodings.
<p>Stream and media metadata is contained in Ogg and not built into
the Ogg container itself. Metadata is thus compartmentalized and
layered rather than part of a monolithic design, an especially good
idea as no two groups seem able to agree on what a complete or
complete-enough metadata set should be. In this way, the container and
container implementation are isolated from unnecessary design flux.
<h3>Streaming</h3>
<p>The Ogg container is primarily a streaming format,
encapsulating 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 all features of a 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 container feature requires nonlinear access of the bitstream.
<h3>Variable Bit Rate, Variable Payload Size</h3>
<p>Ogg is designed to contain any size data payload with bounded,
predictable efficiency. Ogg packets have no maximum size and a
zero-byte minimum size. There is no restriction on size changes from
packet to packet. Variable size packets do not require the use of any
optional or additional container features. There is no optimal
suggested packet size, though special consideration was paid to make
sure 50-200 byte packets were no less efficient than larger packet
sizes. The original design criteria was a 2% overhead at 50 byte
packets, dropping to a maximum working overhead of 1% with larger
packets, and a typical working overhead of .5-.7% for most practical
uses.
<h3>Simple pagination</h3>
<p>Ogg is a byte-aligned container with no context-dependent, optional
or variable-length fields. Ogg requires no repacking of codec data.
The page structure is written out in-line as packet data is submitted
to the streaming abstraction. In addition, it is possible to
implement both Ogg mux and demux as MT-hot zero-copy abstractions (as
is done in the Tremor sourcebase).
<h3>Capture</h3>
<p>Ogg is designed for efficient and immediate stream capture with
high confidence. Although packets have no size limit in Ogg, pages
are a maximum of just under 64kB meaning that any Ogg stream can be
captured with confidence after seeing 128kB of data or less [worst
case; typical figure is 6kB] from any random starting point in the
stream.
<h3>Seeking</h3>
<p>Ogg implements simple coarse- and fine-grained seeking by design.
<p>Coarse seeking may be performed by simply 'moving the tone arm' to a
new position and 'dropping the needle'. Rapid capture with
accompanying timecode from any location in an Ogg file is guaranteed
by the stream design. From the acquisition of the first timecode,
all data needed to play back from that time code forward is ahead of
the stream cursor.
<p>Ogg implements full sample-granularity seeking using an
interpolated bisection search built on the capture and timecode
mechanisms used by coarse seeking. As above, once a search finds
the desired timecode, all data needed to play back from that time code
forward is ahead of the stream cursor.
<p>Both coarse and fine seeking use the page structure and sequencing
inherent to the Ogg format. All Ogg streams are fully seekable from
creation; seekability is unaffected by truncation or missing data, and
is tolerant of gross corruption. Seek operations are neither 'fuzzy' nor
heuristic.
<p>Seeking without use of an index is a major point of the Ogg
design. There are several reasons why Ogg forgoes an index:
<ul>
<li>It must be possible to create an Ogg stream in a single pass, and
an index requires either two passes to create, or the index must be
tacked onto the end of a live stream after the stream is finished.
Both methods run afoul of other design constraints.
<li>An index is only marginally useful in Ogg for the complexity
added; it adds no new functionality and seldom improves performance
noticeably. Empirical testing shows that indexless interpolation
search does not require many more seeks in practice than using an
index would.
<li>'Optional' indexes encourage lazy implementations that can seek
only when indexes are present, or that implement indexless seeking
only by building an internal index after reading the entire file
beginning to end. This has been the fate of other containers that
specify optional indexing.
</ul>
<h3>Simple multiplexing</h3>
<p>Ogg multiplexes streams by interleaving pages from multiple elementary streams into a
multiplexed stream in time order. The multiplexed pages are not
altered. Muxing an Ogg AV stream out of separate audio,
video and data streams is akin to shuffling several decks of cards
together into a single deck; the cards themselves remain unchanged.
Demultiplexing is similarly simple (as the cards are marked).
<p>The goal of this design is to make the mux/demux operation as
trivial as possible to allow live streaming systems to build and
rebuild streams on the fly with minimal CPU usage and no additional
storage or latency requirements.
<h3>Continuous and Discontinuous Media</h3>
<p>Ogg streams belong to one of two categories, "Continuous" streams and
"Discontinuous" streams.
<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. Examples of continuous data
types include broadcast audio and video.
<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 example of a
discontinuous stream types would be captioning such as <a
href="http://wiki.xiph.org/OggKate">Ogg Kate</a>. 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>The fundamental reason for distinction between continuous and
discontinuous streams concerns buffering.
<h3>Buffering</h3>
<p>A continuous stream is, by definition, gapless. Ogg buffering is based
on the simple premise of never allowing an active continuous stream
to starve for data during decode; buffering works ahead until all
continuous streams in a physical stream have data ready and no further.
<p>Discontinuous stream data is not assumed to be predictable. The
buffering design takes discontinuous data 'as it comes' rather than
working ahead to look for future discontinuous data for a potentially
unbounded period. Thus, the buffering process makes no attempt to fill
discontinuous stream buffers; their pages simply 'fall out' of the
stream when continuous streams are handled properly.
<p>Buffering requirements in this design need not be explicitly
declared or managed in the encoded stream. The decoder simply reads as
much data as is necessary to keep all continuous stream types gapless
and no more, with discontinuous data processed as it arrives in the
continuous data. Buffering is implicitly optimal for the given
stream. Because all pages of all data types are stamped with absolute
timing information within the stream, inter-stream synchronization
timing is always maintained without the need for explicitly declared
buffer-ahead hinting.
<h3>Codec metadata</h3>
<p>Ogg does not replicate codec-specific metadata into the mux layer
in an attempt to make the mux and codec layer implementations 'fully
separable'. Things like specific timebase, keyframing strategy, frame
duration, etc, do not appear in the Ogg container. The mux layer is,
instead, expected to query a codec through a standardized interface,
left to the implementation, for this data when it is needed.
<p>Though modern design wisdom usually prefers to predict all possible
needs of current and future codecs then embed these dependencies and
the required metadata into the container itself, this strategy
increases container specification complexity, fragility, and rigidity.
The mux and codec implementations become more independent, but the
specifications become less independent. A codec can't do what a
container hasn't already provided for. New codecs are harder to
support, and you can do fewer useful things with the ones you've
already got (eg, try to make a good splitter without using any codecs.
You're stuck splitting at keyframes only, or building yet another new
mechanism into the container layer to mark what frames to skip
displaying).
<p>Ogg's design goes the opposite direction, where the specification
is to be as simple, easy to understand, and 'proofed' against novel
codecs as possible. When an Ogg mux layer requires codec-specific
information, it queries the codec (or a codec stub). This trades a
more complex implementation for a simpler, more flexible
specification.
<h3>Stream structure metadata</h3>
<p>The Ogg container itself does not define a metadata system for
declaring the structure and interrelations between multiple media
types in a muxed stream. That is, the Ogg container itself does not
specify data like 'which steam is the subtitle stream?' or 'which
video stream is the primary angle?'. This metadata still exists, but
is stored in the Ogg container rather than being built into the Ogg
container. Xiph specifies the 'Skeleton' metadata format for Ogg
streams, but this decoupling of container and stream structure
metadata means it is possible to use Ogg with any metadata
specification without altering the container itself, or without stream
structure metadata at all.
<h3>Frame accurate absolute position</h3>
<p>Every Ogg page is stamped with a 64 bit 'granule position' that
serves as an absolute timestamp for mux and seeking. A few nifty
little tricks are usually also embedded in the granpos state, but
we'll leave those aside for the moment (strictly speaking, they're
part of each codec's mapping, not Ogg).
<p>As previously mentioned above, granule positions are mapped into
absolute timestamps by the codec, rather than being a hard timestamp.
This allows maximally efficient use of the available 64 bits to
address every sample/frame position without approximation while
supporting new and previously unknown timebase encodings without
needing to extend or update the mux layer. When a codec needs a novel
timebase, it simply brings the code for that mapping along with it.
This is not a theoretical curiosity; new, wholly novel timebases were
deployed with the adoption of both Theora and Dirac. "Rolling INTRA"
(keyframeless video) also benefits from novel use of the granule
position.
<h2>Ogg stream arrangement</h2>
<h3>Packets, pages, and bitstreams</h3>
<p>Ogg codecs use <em>packets</em>. Packets are octet payloads of
raw, compressed data, containing the data needed for a single
decompressed unit, eg, one video frame. Packets have no maximum size
and may be zero length. They do not have any high-level structure or
boundary information; strung together, the unframed packets form a
<em>logical bitstream</em> of apparently random bytes with no internal
landmarks.
<p>Logical bitstream packets are grouped and framed into Ogg pages
along with a unique stream <em>serial number</em> to produce a
<em>physical bitstream</em>. An <em>elementary stream</em> is a
physical bitstream containing only the pages framing a single logical
bitstream. Each page is a self contained entity, although a packet may
be split and encoded across one or more pages. The page decode
mechanism is designed to recognize, verify and handle single pages at
a time from the overall bitstream.
<p><a href="framing.html">Ogg Bitstream Framing</a> specifies
the page format of an Ogg bitstream, the packet coding process
and elementary bitstreams in detail.
<h3>Multiplexed bitstreams</h3>
<p>Multiple logical/elementary bitstreams can be combined into a single
<em>multiplexed bitstream</em> by interleaving whole pages from each
contributing elementary stream in time order. The result is a single
physical stream that multiplexes and frames multiple logical streams.
Each logical stream is identified by the unique stream serial number
stamped in its pages. A physical stream may include a 'meta-header'
(such as the <a href="skeleton.html">Ogg Skeleton</a>) comprising its
own Ogg page at the beginning of the physical stream. A decoder
recovers the original logical/elementary bitstreams out of the
physical bitstream by taking the pages in order from the physical
bitstream and redirecting them into the appropriate logical decoding
entity.
<p><a href="ogg-multiplex.html">Ogg Bitstream Multiplexing</a> specifies
proper multiplexing of an Ogg bitstream in detail.
<h3>Chaining</h3>
<p>Multiple Ogg physical bitstreams may be concatenated into a single new
stream; this is <em>chaining</em>. The bitstreams do not overlap; the
final page of a given logical bitstream is immediately followed by the
initial page of the next.</p>
<p>Each logical bitstream in a chain must have a unique serial number
within the scope of the full physical bitstream, not only within a
particular <em>link</em> or <em>segment</em> of the chain.</p>
<h3>Continuous and discontinuous streams</h3>
<p>Within Ogg, each stream must be declared (by the codec) to be
continuous- or discontinuous-time. Most codecs treat all streams they
use as either inherently continuous- or discontinuous-time, although
this is not a requirement. A codec may, as part of its mapping, choose
according to data in the initial header.
<p>Continuous-time pages are stamped by end-time, discontinuous pages
are stamped by begin-time. Pages in a multiplexed stream are
interleaved in order of the time stamp regardless of stream type.
Both continuous and discontinuous logical streams are used to seek
within a physical stream, however only continuous streams are used to
determine buffering depth; because discontinuous streams are stamped
by start time, they will always 'fall out' in time when buffering
tracks only the continuous streams. See 'Examples' for an
illustration of the buffering mechanism.
<h2>Mapping Requirements</h2>
<p>Each codec is allowed some freedom in deciding how its logical
bitstream is encapsulated into an Ogg bitstream (even if it is a
trivial mapping, eg, 'plop the packets in and go'). This is the
codec's <em>mapping</em>. Ogg imposes a few mapping requirements
on any codec.
<p>The <a href="framing.html">framing specification</a> defines
'beginning of stream' and 'end of stream' page markers via a header
flag (it is possible for a stream to consist of a single page). A
correct stream always consists of an integer number of pages, an easy
requirement given the variable size nature of pages.</p>
<p>The first page of an elementary Ogg bitstream consists of a single,
small 'initial header' packet that must include sufficient information
to identify the exact CODEC type. From this initial header, the codec
must also be able to determine its timebase and whether or not it is a
continuous- or discontinuous-time stream. The initial header must fit
on a single page. If a codec makes use of auxiliary headers (for
example, Vorbis uses two auxiliary headers), these headers must follow
the initial header immediately. The last header finishes its page;
data begins on a fresh page.
<p>As an example, Ogg Vorbis places the name and revision of the
Vorbis CODEC, the audio rate and the audio quality into this initial
header. Comments and detailed codec setup appears in the larger
auxiliary headers.</p>
<h2>Multiplexing Requirements</h2>
<p>Multiplexing requirements within Ogg are straightforward. When
constructing a single-link (unchained) physical bitstream consisting
of multiple elementary streams:
<ol>
<li> The initial header for each stream appears in sequence, each
header on a single page. All initial headers must appear with no
intervening data (no auxiliary header pages or packets, no data pages
or packets). Order of the initial headers is unspecified. The
'beginning of stream' flag is set on each initial header.
<li> All auxiliary headers for all streams must follow. Order
is unspecified. The final auxiliary header of each stream must flush
its page.
<li>Data pages for each stream follow, interleaved in time order.
<li>The final page of each stream sets the 'end of stream' flag.
Unlike initial pages, terminal pages for the logical bitstreams need
not occur contiguously; indeed it may not be possible for them to do so.
</oL>
<p>Each grouped bitstream must have a unique serial number within the
scope of the physical bitstream.</p>
<h3>chaining and multiplexing</h3>
<p>Multiplexed and/or unmultiplexed bitstreams may be chained
consecutively. Such a physical bitstream obeys all the rules of both
chained and multiplexed streams. Each link, when unchained, must
stand on its own as a valid physical bitstream. Chained streams do
not mix; a new segment may not begin until all streams in the
preceding segment have terminated. </p>
<h2>Examples</h2>
<em>[More to come shortly; this section is currently being revised and expanded]</em>
<p>Below, we present an example of a multiplexed and chained bitstream:</p>
<p><img src="stream.png" alt="stream"/></p>
<p>In this example, we see pages from five total logical bitstreams
multiplexed into a physical bitstream. Note the following
characteristics:</p>
<ol>
<li>Multiplexed bitstreams in a given link begin together; all of the
initial pages must appear before any data pages. When concurrently
multiplexed groups are chained, the new group does not begin until all
the bitstreams in the previous group have terminated.</li>
<li>The ordering of pages of concurrently multiplexed bitstreams is
goverened by timestamp (not shown here); there is no regular
interleaving order. Pages within a logical bitstream appear in
sequence order.</li>
</ol>
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