rfc3533ogg.docx
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rfc3533ogg.docx
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rfc3533ogg
NetworkWorkingGroupS.Pfeiffer
RequestforComments:
3533CSIRO
Category:
InformationalMay2003
TheOggEncapsulationFormatVersion0
StatusofthisMemo
ThismemoprovidesinformationfortheInternetcommunity.Itdoes
notspecifyanInternetstandardofanykind.Distributionofthis
memoisunlimited.
CopyrightNotice
Copyright(C)TheInternetSociety(2003).AllRightsReserved.
Abstract
ThisdocumentdescribestheOggbitstreamformatversion0,whichis
ageneral,freely-availableencapsulationformatformediastreams.
Itisabletoencapsulateanykindandnumberofvideoandaudio
encodingformatsaswellasotherdatastreamsinasinglebitstream.
Terminology
Thekeywords"MUST","MUSTNOT","REQUIRED","SHALL","SHALLNOT",
"SHOULD","SHOULDNOT","RECOMMENDED","MAY",and"OPTIONAL"inthis
documentaretobeinterpretedasdescribedinBCP14,RFC2119[2].
TableofContents
1.Introduction.........................2
2.Definitions.........................2
3.Requirementsforagenericencapsulationformat.......3
4.TheOggbitstreamformat...................3
5.Theencapsulationprocess..................6
6.TheOggpageformat.....................9
7.SecurityConsiderations...................11
8.References..........................12
A.Glossaryoftermsandabbreviations.............13
B.Acknowledgements.......................14
Author'sAddress.......................14
FullCopyrightStatement...................15
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RFC3533OGGMay2003
1.Introduction
TheOggbitstreamformathasbeendevelopedasapartofalarger
projectaimedatcreatingasetofcomponentsforthecodingand
decodingofmultimediacontent(codecs)whicharetobefreely
availableandfreelyre-implementable,bothinsoftwareandin
hardwareforthecomputingcommunityatlarge,includingtheInternet
community.ItistheintentionoftheOggdevelopersrepresentedby
Xiph.Orgthatitbeusablewithoutintellectualpropertyconcerns.
ThisdocumentdescribestheOggbitstreamformatandhowtouseitto
encapsulateoneorseveralmediabitstreamscreatedbyoneorseveral
encoders.TheOggtransportbitstreamisdesignedtoprovide
framing,errorprotectionandseekingstructureforhigher-level
codecstreamsthatconsistofraw,unencapsulateddatapackets,such
astheVorbisaudiocodecortheupcomingTarkinandTheoravideo
codecs.Itiscapableofinterleavingdifferentbinarymediaand
othertime-continuousdatastreamsthatarepreparedbyanencoderas
asequenceofdatapackets.Oggprovidesenoughinformationto
properlyseparatedatabackintosuchencodercreateddatapacketsat
theoriginalpacketboundarieswithoutrelyingondecodingtofind
packetboundaries.
PleasenotethattheMIMEtypeapplication/ogghasbeenregistered
withtheIANA[1].
2.Definitions
FordescribingtheOggencapsulationprocess,asetoftermswillbe
usedwhosemeaningneedstobewellunderstood.Therefore,someof
themostfundamentaltermsaredefinednowbeforewestartwiththe
descriptionoftherequirementsforagenericmediastream
encapsulationformat,theprocessofencapsulation,andtheconcrete
formatoftheOggbitstream.SeetheAppendixforamorecomplete
glossary.
TheresultofanOggencapsulationiscalledthe"Physical(Ogg)
Bitstream".Itencapsulatesoneorseveralencoder-created
bitstreams,whicharecalled"LogicalBitstreams".Alogical
bitstream,providedtotheOggencapsulationprocess,hasa
structure,i.e.,itissplitupintoasequenceofso-called
"Packets".Thepacketsarecreatedbytheencoderofthatlogical
bitstreamandrepresentmeaningfulentitiesforthatencoderonly
(e.g.,anuncompressedstreammayusevideoframesaspackets).They
donotcontainboundaryinformation-strungtogethertheyappearto
bestreamsofrandombyteswithnolandmarks.
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RFC3533OGGMay2003
Pleasenotethattheterm"packet"isnotusedinthisdocumentto
signifyentitiesfortransportoveranetwork.
3.Requirementsforagenericencapsulationformat
ThedesignideabehindOggwastoprovideageneric,linearmedia
transportformattoenablebothfile-basedstorageandstream-based
transmissionofoneorseveralinterleavedmediastreamsindependent
oftheencodingformatofthemediadata.Suchanencapsulation
formatneedstoprovide:
oframingforlogicalbitstreams.
ointerleavingofdifferentlogicalbitstreams.
odetectionofcorruption.
orecaptureafteraparsingerror.
opositionlandmarksfordirectrandomaccessofarbitrarypositions
inthebitstream.
ostreamingcapability(i.e.,noseekingisneededtobuilda100%
completebitstream).
osmalloverhead(i.e.,usenomorethanapproximately1-2%of
bitstreambandwidthforpacketboundarymarking,high-level
framing,syncandseeking).
osimplicitytoenablefastparsing.
osimpleconcatenationmechanismofseveralphysicalbitstreams.
Allofthesedesignconsiderationshavebeentakenintoconsideration
forOgg.Oggsupportsframingandinterleavingoflogical
bitstreams,seekinglandmarks,detectionofcorruption,andstream
resynchronisationafteraparsingerrorwithnomorethan
approximately1-2%overhead.Itisagenericframeworktoperform
encapsulationoftime-continuousbitstreams.Itdoesnotknowany
specificsaboutthecodecdatathatitencapsulatesandisthus
independentofanymediacodec.
4.TheOggbitstreamformat
AphysicalOggbitstreamconsistsofmultiplelogicalbitstreams
interleavedinso-called"Pages".Wholepagesaretakeninorder
frommultiplelogicalbitstreamsmultiplexedatthepagelevel.The
logicalbitstreamsareidentifiedbyauniqueserialnumberinthe
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RFC3533OGGMay2003
headerofeachpageofthephysicalbitstream.Thisuniqueserial
numberiscreatedrandomlyanddoesnothaveanyconnectiontothe
contentorencoderofthelogicalbitstreamitrepresents.Pagesof
alllogicalbitstreamsareconcurrentlyinterleaved,buttheyneed
notbeinaregularorder-theyareonlyrequiredtobeconsecutive
withinthelogicalbitstream.Oggdemultiplexingreconstructsthe
originallogicalbitstreamsfromthephysicalbitstreambytakingthe
pagesinorderfromthephysicalbitstreamandredirectingtheminto
theappropriatelogicaldecodingentity.
EachOggpagecontainsonlyonetypeofdataasitbelongstoone
logicalbitstreamonly.Pagesareofvariablesizeandhaveapage
headercontainingencapsulationanderrorrecoveryinformation.Each
logicalbitstreaminaphysicalOggbitstreamstartswithaspecial
startpage(bos=beginningofstream)andendswithaspecialpage
(eos=endofstream).
Thebospagecontainsinformationtouniquelyidentifythecodectype
andMAYcontaininformationtosetupthedecodingprocess.Thebos
pageSHOULDalsocontaininformationabouttheencodedmedia-for
example,foraudio,itshouldcontainthesamplerateandnumberof
channels.Byconvention,thefirstbytesofthebospagecontain
magicdatathatuniquelyidentifiestherequiredcodec.Itisthe
responsibilityofanyonefieldinganewcodectomakesureitis
possibletoreliablydistinguishhis/hercodecfromallothercodecs
inuse.Thereisnofixedwaytodetecttheendofthecodec-
identifyingmarker.Theformatofthebospageisdependentonthe
codecandthereforeMUSTbegivenintheencapsulationspecification
ofthatlogicalbitstreamtype.Oggalsoallowsbutdoesnotrequire
secondaryheaderpacketsafterthebospageforlogicalbitstreams
andthesemustalsoprecedeanydatapacketsinanylogical
bitstream.Thesesubsequentheaderpacketsareframedintoan
integralnumberofpages,whichwillnotcontainanydatapackets.
So,aphysicalbitstreambeginswiththebospagesofalllogical
bitstreamscontainingoneinitialheaderpacketperpage,followedby
thesubsidiaryheaderpacketsofallstreams,followedbypages
containingdatapackets.
Theencapsulationspecificationforoneormorelogicalbitstreamsis
calleda"mediamapping".Anexampleforamediamappingis"Ogg
Vorbis",whichusestheOggframeworktoencapsulateVorbis-encoded
audiodataforstream-basedstorage(suchasfiles)andtransport
(suchasTCPstreamsorpipes).OggVorbisprovidesthenameand
revisionoftheVorbiscodec,theaudiorateandtheaudioqualityon
theOggVorbisbospage.Italsousestwoadditionalheaderpages
perlogicalbitstream.TheOggVorbisbospagestartswiththebyte
0x01,followedby"vorbis"(atotalof7bytesofidentifier).
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RFC3533OGGMay2003
Oggknowstwotypesofmultiplexing:
concurrentmultiplexing(so-
called"Grouping")andsequentialmultiplexing(so-called
"Chaining").Groupingdefineshowtointerleaveseverallogical
bitstreamspage-wiseinthesamephysicalbitstream.Groupingisfor
exampleneededforinterleavingavideostreamwithseveral
synchronisedaudiotracksusingdifferentcodecsindifferentlogical
bitstreams.Chainingontheotherhand,isdefinedtoprovidea
simplemechanismtoconcatenatephysicalOggbitstreams,asisoften
neededforstreamingapplications.
Ingrouping,allbospagesofalllogicalbitstreamsMUSTappear
togetheratthebeginningoftheOggbitstream.Themediamapping
specifi
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