Barrapunto

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.

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.

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.