+ +

Ogg bitstream overview

+ +

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 index page +give distilled technical descriptions and references of the container +mechanisms. This document is intended to aid understanding. + +

Container format design points

+ +

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 Skeleton or Annodex/CMML). + +

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. + +

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 metadata design +flux. + +

Streaming

+ +

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. + +

Variable Bit Rate, Variable Payload Size

+ +

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. + +

Simple pagination

+ +

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). + +

Capture

+ +

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. + +

Seeking

+ +

Ogg implements simple coarse- and fine-grained seeking by design. + +

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. + +

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. + +

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. + +

Seeking without use of an index is a major point of the Ogg +design. There two primary reasons why Ogg transport forgoes an index: + +

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. + +
'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. + +

+ +

In addition, it must be possible to create an Ogg stream in a +single pass. Although an optional index can simply be tacked on the +end of the created stream, some software groups object to +end-positioned indexes and claim to be unwilling to support indexes +not located at the stream beginning. + +

All this said, it's become clear that an optional index is a +demanded feature. For this reason, the OggSkeleton now defines a +proposed index. + +

Simple multiplexing

+ +

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). + +

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. + +

Continuous and Discontinuous Media

+ +

Ogg streams belong to one of two categories, "Continuous" streams and +"Discontinuous" streams. + +

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. + +

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 Ogg Kate. 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. + +

The fundamental reason for distinction between continuous and +discontinuous streams concerns buffering. + +

Buffering

+ +

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. + +

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. + +

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. + +

Codec metadata

+ +

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 centralized interface, +left to the implementation, for this data when it is needed. + +

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 code becomes more independent, but the +specifications become logically less independent. A codec can't do +what a container hasn't already provided for. Novel 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. +Such a splitter is limited to splitting at keyframes only, or building +yet another new mechanism into the container layer to mark what frames +to skip displaying). + +

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. + +

Stream structure metadata

+ +

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 by the Ogg container rather than being built into the Ogg +container itself. 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. + +

Frame accurate absolute position

+ +

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). + +

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. + +

Ogg stream arrangement

+ +

Packets, pages, and bitstreams

+ +

Ogg codecs place raw compressed data into packets. +Packets are octet payloads 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 generally have any framing +information; strung together, the unframed packets form a logical +bitstream of codec data with no internal landmarks. + +

+ +

Packets of raw codec data are not typically internally framed. + When they are strung together into a stream without any container to + provide framing, they lose their individual boundaries. Seek and + capture are not possible within an unframed stream, and for many + codecs with variable length payloads and/or early-packet termination + (such as Vorbis), it may become impossible to recover the original + frame boundaries even if the stream is scanned linearly from + beginning to end. + +

+ +

Logical bitstream packets are grouped and framed into Ogg pages +along with a unique stream serial number to produce a +physical bitstream. An elementary stream is a +physical bitstream containing only 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. + +

+ +

The primary purpose of a container is to provide framing for raw + packets, marking the packet boundaries so the exact packets can be + retrieved for decode later. The container also provides secondary + functions such as capture, timestamping, sequencing, stream + identification and so on. Not all of these functions are represented in the diagram. + +

In the Ogg container, pages do not necessarily contain + integer numbers of packets. Packets may span across page boundaries + or even multiple pages. This is necessary as pages have a maximum + possible size in order to provide capture guarantees, but packet + size is unbounded. +

+ + +

Ogg Bitstream Framing specifies +the page format of an Ogg bitstream, the packet coding process +and elementary bitstreams in detail. + +

Multiplexed bitstreams

+ +

Multiple logical/elementary bitstreams can be combined into a single +multiplexed bitstream 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 Ogg Skeleton) 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. + +

+ +

Multiple media types are mutliplexed into a single Ogg stream by +interleaving the pages from each elementary physical stream. + +

+ +

Ogg Bitstream Multiplexing specifies +proper multiplexing of an Ogg bitstream in detail. + +

Chaining

+ +

Multiple Ogg physical bitstreams may be concatenated into a single new +stream; this is chaining. The bitstreams do not overlap; the +final page of a given logical bitstream is immediately followed by the +initial page of the next.

+ +

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 link or segment of the chain.

+ +

Continuous and discontinuous streams

+ +

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. + +

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' at the proper time when +buffering the continuous streams. See 'Examples' for an illustration +of the buffering mechanism. + +

Multiplexing Requirements

+ +

Multiplexing requirements within Ogg are straightforward. When +constructing a single-link (unchained) physical bitstream consisting +of multiple elementary streams: + +

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. + +
All auxiliary headers for all streams must follow. Order +is unspecified. The final auxiliary header of each stream must flush +its page. + +
Data pages for each stream follow, interleaved in time order. + +
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. +

+ +

Each grouped bitstream must have a unique serial number within the +scope of the physical bitstream.

+ +

chaining and multiplexing

+ +

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 or interleave; a new segment may not begin until all streams +in the preceding segment have terminated.

+ +

Codec Mapping Requirements

+ +

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 mapping. Ogg imposes a few mapping requirements +on any codec. + +

The framing specification 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.
+ +
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. + +
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. Vorbis comments and detailed codec setup appears in the larger +auxiliary headers.
+ +
Granule positions must be translatable to an exact absolute +time value. As described above, the mux layer is permitted to query a +codec or codec stub plugin to perform this mapping. It is not +necessary for an absolute time to be mappable into a single unique +granule position value. + +
Codecs are not required to use a fixed duration-per-packet (for +example, Vorbis does not). the mux layer is permitted to query a +codec or codec stub plugin for the time duration of a packet. + +
Although an absolute time need not be translatable to a unique +granule position, a codec must be able to determine the unique granule +position of the current packet using the granule position of a +preceeding packet. + +
Packets and pages must be arranged in ascending +granule-position and time order. + +

+ +

Examples

+ +[More to come shortly; this section is currently being revised and expanded] + +

Below, we present an example of a multiplexed and chained bitstream:

+ +

stream

+ +

In this example, we see pages from five total logical bitstreams +multiplexed into a physical bitstream. Note the following +characteristics:

+ +

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.
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.

+ +