Network Working Group S. Pfeiffer Internet-Draft CSIRO Expires: May 2, 2003 November 2002 The Ogg encapsulation format version 0 draft-pfeiffer-ogg-fileformat-01 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 2, 2003. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document describes the Ogg bitstream format version 0, which is a general, freely-available encapsulation format for media streams. It is capable to encapsulate any kind and number of video and audio encoding formats as well as other data streams in a single bitstream. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [2]. Pfeiffer Expires May 2, 2003 [Page 1] Internet-Draft OGG November 2002 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Requirements for a generic encapsulation format . . . . . . . 5 4. The Ogg bitstream format . . . . . . . . . . . . . . . . . . . 6 5. The encapsulation process . . . . . . . . . . . . . . . . . . 9 6. The Ogg page format . . . . . . . . . . . . . . . . . . . . . 12 7. Security considerations . . . . . . . . . . . . . . . . . . . 15 References . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 16 A. Glossary of terms and abbreviations . . . . . . . . . . . . . 17 B. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19 Full Copyright Statement . . . . . . . . . . . . . . . . . . . 20 Pfeiffer Expires May 2, 2003 [Page 2] Internet-Draft OGG November 2002 1. Introduction The Ogg bitstream format has been developed as a part of a larger project aimed at creating a set of components for the coding and decoding of multimedia content (codecs) which are to be freely available and freely re-implementable both in software and in hardware for the computing community at large, including the Internet community. It is the intention of the Ogg developers represented by Xiph.Org that it be usable without intellectual property concerns. This document describes the Ogg bitstream format and how to use it to encapsulate one or several media bitstreams created by one or several encoders. The Ogg transport bitstream is designed to provide framing, error protection and seeking structure for higher-level codec streams that consist of raw, unencapsulated data packets, such as the Vorbis audio codec or the upcoming Tarkin and Theora video codecs. It is capable to interleave different binary media and other time-continuous data streams that are prepared by an encoder as a sequence of data packets. Ogg provides enough information to properly separate data back into such encoder created data packets at the original packet boundaries without relying on decoding to find packet boundaries. Please note that there is a related document containing information to register application/ogg as MIME type. It is currently being processed as Internet-Draft: application/ogg MIME type I-D [1]. Pfeiffer Expires May 2, 2003 [Page 3] Internet-Draft OGG November 2002 2. Definitions For describing the Ogg encapsulation process, a set of terms will be used whose meaning needs to be well understood. Therefore, some of the most fundamental terms are defined now before we start with the description of the requirements for a generic media stream encapsulation format, the process of encapsulation, and the concrete format of the Ogg bitstream. See the Appendix for a more complete glossary. The result of an Ogg encapsulation is called the "Physical (Ogg) Bitstream". It encapsulates one or several encoder-created bitstreams, which are called "Logical Bitstreams". A logical bitstream which is provided to the Ogg encapsulation process has a structure, i.e. it is split up into a sequence of so-called "Packets". The packets are created by the encoder of that logical bitstream and represent meaningful entities for that encoder only (e.g. an uncompressed stream may use video frames as packets). They do not contain boundary information - strung together they appear to be streams of random bytes with no landmarks. Please note that the term "packet" is not used in this document to signify entities for transport over a network. Pfeiffer Expires May 2, 2003 [Page 4] Internet-Draft OGG November 2002 3. Requirements for a generic encapsulation format The design idea behind Ogg was to provide a generic, linear media transport format to enable both file-based storage and stream-based transmission of one or several interleaved media streams independent of the encoding format of the media data. Such an encapsulation format needs to provide: o framing for logical bitstreams. o interleaving of different logical bitstreams. o detection of corruption. o recapture after a parsing error. o position landmarks for direct random access. o streaming capability (i.e. no seeking is needed to build a 100% complete bitstream). o small overhead. o simplicity to be fast. o simple concatenation mechanism. All of these design considerations have been taken into consideration for Ogg. Ogg supports framing and interleaving of logical bitstreams, seeking landmarks, detection of corruption, and stream resynchronisation after a parsing error with no more than approximately 1-2% overhead. It is a generic framework to perform encapsulation of time-continuous bitstreams. It does not know any specifics about the codec data that it encapsulates and is thus independent of any media codec. Pfeiffer Expires May 2, 2003 [Page 5] Internet-Draft OGG November 2002 4. The Ogg bitstream format A physical Ogg bitstream consists of multiple logical bitstreams interleaved in so-called "Pages". Whole pages are taken in order from multiple logical bitstreams multiplexed at the page level. The logical bitstreams are identified by a unique serial number in the header of each page of the physical bitstream. This unique serial number is created randomly and does not have any connection to the content or encoder of the logical bitstream it represents. Pages of all logical bistreams are concurrently interleaved, but they need not be in a regular order - they only require to be consecutive within the logical bitstream. Ogg demultiplexing reconstructs the original logical bitstreams from the physical bitstream by taking the pages in order from the physical bitstream and redirecting them into the appropriate logical decoding entity. Each Ogg page contains only one type of data as it belongs to one logical bitstream only. Pages are of variable size and have a page header containing encapsulation and error recovery information. Each logical bitstream in a physical Ogg bitstream starts with a special start page (bos=beginning of stream) and ends with a special page (eos=end of stream). The bos page contains information to identify the codec type and any additional information to set up the decoding process. The format of that page is therefore dependent on the codec and therefore MUST be given in the encoding specification of that logical bitstream type. An example for such a media mapping is "Ogg Vorbis", which provides the name and revision of the Vorbis codec, the audio rate and the audio quality on the Ogg Vorbis bos page. Ogg knows two types of multiplexing: concurrent multiplexing (so- called "Grouping") and sequential multiplexing (so-called "Chaining"). Grouping defines how to interleave several logical bitstreams page-wise in the same physical bitstream. Grouping is for example needed for interleaving a video stream with several synchronised audio tracks using different codecs in different logical bitstreams. Chaining on the other hand is defined to provide a simple mechanism to concatenate physical Ogg bitstreams as is often needed for streaming applications. In grouping, all bos pages of all logical bitstreams MUST appear together at the beginning of the Ogg bitstream. The media mapping specifies the order of the initial pages. For example, grouping of a specific Ogg video and Ogg audio bitstream may specify that the physical bitstream MUST begin with the bos page of the logical video bitstream followed by the bos page of the audio bitstream. Unlike bos pages eos pages for the logical bitstreams need not all occur contiguously. Eos pages may be 'nil' pages, that is, pages containing no content but simply a page header with position Pfeiffer Expires May 2, 2003 [Page 6] Internet-Draft OGG November 2002 information and the eos flag set in the page header. Each grouped logical bitstream MUST have a unique serial number within the scope of the physical bitstream. In chaining, complete logical bitstreams are concatenated. The bitstreams do not overlap, i.e. the eos page of a given logical bistream is immediately followed by the bos page of the next. Each chained logical bitstream MUST have a unique serial number within the scope of the physical bitstream. It is possible to consecutively chain groups of concurrently multiplexed bitstreams. The groups, when unchained, MUST stand on their own as a valid concurrently multiplexed bitstream. The following diagram shows a schematic example of such a physical bitstream that obeys all the rules of both grouped and chained multiplexed bitstreams. physical bitstream with pages of different logical bitstreams grouped and chained ------------------------------------------------------------- |*A*|*B*|*C*|A|A|C|B|A|B|#A#|C|...|B|C|#B#|#C#|*D*|D|...|#D#| ------------------------------------------------------------- bos bos bos eos eos eos bos eos In this example, there are two chained physical bitstreams, the first of which is a grouped stream of three logical bitstreams A, B, and C. The second physical bitstream is chained after the end of the grouped bitstream, which ends after the last eos page of all its grouped logical bitstreams. As can be seen, grouped bitstreams begin together - all of the eos pages MUST appear before any data pages. It can also be seen that pages of concurrently multiplexed bitstreams need not conform to a regular order. And it can be seen that a grouped bitstream can end long before the other bitstreams in the group end. Ogg does not know any specifics about the codec data except that each logical bitstream belongs to a different codec, the data from the codec comes in order and has position markers (so-called "Granule positions"). Ogg does not have a concept of 'time': it only knows about sequentially increasing, unitless position markers. An application can only get temporal information through higher layers which have access to the codec APIs to assign and convert granule positions or time. A specific definition of a media mapping using Ogg may put further constraints on its specific use of the Ogg bitstream format. For example, a specific media mapping may require that all the eos pages Pfeiffer Expires May 2, 2003 [Page 7] Internet-Draft OGG November 2002 for all grouped bitstreams need to appear in direct sequence. An example for a media mapping is the specification of "Ogg Vorbis" which uses the Ogg framework to encapsulate Vorbis-encoded audio data for stream-based storage (such as files) and transport (such as TCP streams or pipes). Another example is the upcoming "Ogg Theora" specification. "Ogg Theora" multiplexes a Vorbis-encoded audio bitstream and a VP3-encoded video bitstream in a single physical Ogg bitstream. As Ogg does not specify temporal relationships between the encapsulated concurrently multiplexed bitstreams, the temporal synchronisation between the audio and video stream will be specified in this media mapping. Pfeiffer Expires May 2, 2003 [Page 8] Internet-Draft OGG November 2002 5. The encapsulation process The process of multiplexing different logical bitstreams happens at the level of pages as described above. The bitstreams provided by encoders are however handed over to Ogg as so-called "Packets" with packet boundaries dependent on the encoding format. The process of encapsulating packets into pages will be described now. From Ogg's perspective, packets can be of any arbitrary size. A specific media mapping will define how to group or break up packets from a specific media encoder. A nominal page size of approximately 4-8 kByte is RECOMMENDED for latency reasons. As Ogg pages have a maximum size of about 64 kByte, sometimes a packet has to be distributed over several pages. To simplify that process, Ogg divides each packet into 255 byte long chunks plus a final usually shorter chunk. These chunks are called "Ogg Segments". They are only a logical construct and do not have a header for themselves. A group of contiguous segments is wrapped into a variable length page preceeded by a header. A segment table in the page header tells about the "Lacing values" (sizes) of the segments included in the page. A flag in the page header tells whether a page contains a packet continued from a previous page. Note that a lacing value of 255 implies that a second lacing value follows in the packet, and a value of less than 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. 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. The encoding is optimised for speed and the expected case of the majority of packets being between 50 and 200 bytes large. This is a design justification rather than a recommendation. This encoding both avoids imposing a maximum packet size as well as imposing minimum overhead on small packets. In contrast, e.g. simply using two bytes at the head of every packet and having a max packet size of 32 kBytes would always penalize small packets (< 255 bytes, the typical case) 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 (>512 bytes) see a fairly constant ~0.5% overhead on encoding space. The following diagram shows a schematic example of a media mapping using Ogg and grouped logical bitstreams: Pfeiffer Expires May 2, 2003 [Page 9] Internet-Draft OGG November 2002 logical bitstream with packet boundaries ----------------------------------------------------------------- > | packet_1 | packet_2 | packet_3 | < ----------------------------------------------------------------- |segmentation (logically only) v packet_1 (5 segments) packet_2 (4 segs) p_3 (2 segs) ------------------------------ -------------------- ------------ .. |seg_1|seg_2|seg_3|seg_4|s_5 | |seg_1|seg_2|seg_3|| |seg_1|s_2 | .. ------------------------------ -------------------- ------------ | page encapsulation v page_1 (packet_1 data) page_2 (pket_1 data) page_3 (packet_2 data) ------------------------ ---------------- ------------------------ |H|------------------- | |H|----------- | |H|------------------- | |D||seg_1|seg_2|seg_3| | |D|seg_4|s_5 | | |D||seg_1|seg_2|seg_3| | ... |R|------------------- | |R|----------- | |R|------------------- | ------------------------ ---------------- ------------------------ | pages of | other --------| | logical ------- bitstreams | MUX | ------- | v page_1 page_2 page_3 ------ ------ ------- ----- ------- ... || | || | || | || | || | ... ------ ------ ------- ----- ------- physical Ogg bitstream In this example we take a snapshot of the encapsulation process of one logical bitstream. We can see part of that bitstream's subdivision into packets as provided by the codec. The Ogg encapsulation process chops up the packets into segments. The packets in this example are rather large such that packet_1 is split into 5 segments - 4 segments with 255 bytes and a final smaller one. Packet_2 is split into 4 segments - 3 segments with 255 bytes and a final very small one - and packet_3 is split into two segments. The encapsulation process then creates pages, which are quite small in this example. Page_1 consists of the first three segments of Pfeiffer Expires May 2, 2003 [Page 10] Internet-Draft OGG November 2002 packet_1, page_2 contains the remaining 2 segments from packet_1, and page_3 contains the first three pages of packet_2. Finally, this logical bitstream is multiplexed into a physical Ogg bitstream with pages of other logical bitstreams. Pfeiffer Expires May 2, 2003 [Page 11] Internet-Draft OGG November 2002 6. The Ogg page format A physical Ogg bitstream consists of a sequence of concatenated pages. Pages are of variable size, usually 4-8 kB, maximum 65307 bytes. A page header contains all the information needed to demultiplex the logical bitstreams out of the physical bitstream and to perform basic error recovery and landmarks for seeking. Each page is a self-contained entity such that the page decode mechanism can recognize, verify, and handle single pages at a time without requiring the overall bitstream. The Ogg page header has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1| Byte +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | capture_pattern: Magic number for page start "OggS" | 0-3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | version | header_type | granule_position | 4-7 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | 8-11 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | bitstream_serial_number | 12-15 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | page_sequence_number | 16-19 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | CRC_checksum | 20-23 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |page_segments | segment_table | 24-27 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | 28- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The LSb (least significant bit) comes first in the Bytes. Fields with more than one byte length are encoded LSB (least significant byte) first. The fields in the page header have the following meaning: 1. capture_pattern: a 4 Byte field that signifies the beginning of a page. It contains the magic numbers: 0x4f 'O' 0x67 'g' 0x67 'g' Pfeiffer Expires May 2, 2003 [Page 12] Internet-Draft OGG November 2002 0x53 'S' It helps a decoder to find the page boundaries and regain synchronisation after parsing a corrupted stream. Once the capture pattern is found, the decoder verifies page sync and integrity by computing and comparing the checksum. 2. stream_structure_version: 1 Byte signifying the version number of the Ogg file format used in this stream (this document specifies version 0). 3. header_type_flag: the bits in this 1 Byte field identify the specific type of this page. * bit 0x01 set: page contains data of a media frame continued from the previous page unset: page contains a fresh media frame * bit 0x02 set: this is the first page of a logical bitstream (bos) unset: this page is not a first page * bit 0x04 set: this is the last page of a logical bitstream (eos) unset: this page is not a last page 4. granule_position: a 8 Byte field containing position information. For example, for an audio stream it contains the total number of PCM samples encoded after including all frames finished on this page. For a video stream it contains the total number of video frames encoded after this page. This is a hint for the decoder and gives it some timing and position information. It's meaning is dependent on the codec for that logical bitstream and specified in a specific media mapping. 5. bitstream_serial_number: a 4 Byte field containing the serial number by which the logical bitstream is identified. 6. page_sequence_number: a 4 Byte field containing the sequence number of the page so the decoder can identify page loss. This sequence number is increasing on each logical bitstream Pfeiffer Expires May 2, 2003 [Page 13] Internet-Draft OGG November 2002 separately. 7. CRC_checksum: a 4 Byte field containing a 32 bit CRC checksum of the page (including header with zero CRC field and page content). The generator polynomial is 0x04c11db7. 8. number_page_segments: 1 Byte giving the number of segment entries encoded in the segment table. 9. segment_table: number_page_segments Bytes containing the lacing values of all segments in this page. Each Byte contains one lacing value. The total header size in bytes is given by: header_size = number_page_segments + 27 [Byte] The total page size in Bytes is given by: page_size = header_size + sum(lacing_values: 1..number_page_segments) [Byte] Pfeiffer Expires May 2, 2003 [Page 14] Internet-Draft OGG November 2002 7. Security considerations The Ogg encapsulation format is a container format and only encapsulates content (such as Vorbis-encoded audio). It does not provide for any generic encryption or signing of itself or its contained content bitstreams. It however encapsulates any kind of content bitstream as long as there is a codec for it, and is thus capable to contain encrypted and signed content data. It is also possible to add an external security mechanism that encrypts or signs an Ogg physical bistream and thus provides content confidentiality and authenticity. As Ogg enapsulates binary data, it is possible to include executable content in an Ogg bitstream. This can be an issue with applications that are implemented using the Ogg format, especially when Ogg is used for streaming or file transfer in a networking scenario. Ogg as such does not pose a threat there. However, an application decoding Ogg and its encapsulated content bitstreams has to ensure correct handling of manipulated bitstreams, of buffer overflows and the like. Pfeiffer Expires May 2, 2003 [Page 15] Internet-Draft OGG November 2002 References [1] Walleij, L., "Internet-Draft: The application/ogg Media Type", Internet-Draft XXXX, May 2002. Author's Address Silvia Pfeiffer CSIRO, Australia Locked Bag 17 North Ryde, NSW 2113 Australia Phone: +61 2 9325 3141 EMail: Silvia.Pfeiffer@csiro.au URI: http://www.cmis.csiro.au/Silvia.Pfeiffer/ Pfeiffer Expires May 2, 2003 [Page 16] Internet-Draft OGG November 2002 Appendix A. Glossary of terms and abbreviations bos page: The initial page (beginning of stream) of a logical bitstream which contains information to identify the codec type and other decoding-relevant information. chaining (or sequential multiplexing): Concatenation of two or more complete physical Ogg bitstreams. eos page: The final page (end of stream) of a logical bitstream. granule position: An increasing position number for a specific logical bitstream stored in the page header. It's meaning is dependent on the codec for that logical bitstream and specified in a specific media mapping. grouping (or concurrent multiplexing): Interleaving of pages of several logical bitstreams into one complete physical Ogg bistream under the restriction that all bos pages of all grouped logical bitstreams MUST appear before any data pages. lacing value: An entry in the segment table of a page header representing the size of the related segment. logical bistream: A sequence of bits being the result of an encoded media stream. media mapping: A specific use of the Ogg encapsulation format together with a specific (set of) codec(s). (Ogg) packet: A subpart of a logical bitstream that is created by the encoder for that bitstream and represents a meaningful entity for the encoder, but only a sequence of bits to the Ogg encapsulation. (Ogg) page: A physical bitstream consists of a sequence of Ogg pages containing data of one logical bitstream only. It usually contains a group of contiguous segments of one packet only, but sometimes packets are too large and need to be split over several pages. physical (Ogg) bitstream: The sequence of bits resulting from an Ogg encapsulation of one or several logical bitstreams. It consists of a sequence of pages from the logical bitstreams with the restriction that the pages of one logical bitstream MUST come in their correct temporal order. (Ogg) segment: The Ogg encapsulation process splits each packet into chunks of 255 bytes plus a last fractional chunk of less than 255 Pfeiffer Expires May 2, 2003 [Page 17] Internet-Draft OGG November 2002 bytes. These chunks are called segments. Pfeiffer Expires May 2, 2003 [Page 18] Internet-Draft OGG November 2002 Appendix B. Acknowledgements The author gratefully acknowledges the work that Monty and the Xiph.Org foundation (also known as Xiphophorus) have done in defining the Ogg multimedia project and as part of it the open file format described in this document. The author hopes that providing this document to the Internet community will help in promoting the Ogg multimedia project at http://www.xiph.org/. Pfeiffer Expires May 2, 2003 [Page 19] Internet-Draft OGG November 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Pfeiffer Expires May 2, 2003 [Page 20]