WebRTC - APIs and RTCWEB Protocols of the HTML5 Real-Time Web原版完整文件

WebRTC:

APIsandRTCWEBProtocolsoftheHTML5Real-TimeWeb

AlanB.JohnstonDanielC.Burnett

SecondEditionJune2013

C:

DigitalCodexLLC

ØØØ1

©Copyright2013DigitalCodexLLC

P.O.Box9131,St.Louis,MO63117,

USA

SmashwordsEdition

Allrightsreserved.

Alltrademarksarethepropertyoftheirrespectiveholders.

ISBN-13:978-0-9859788-5-3

ISBN-10:0985978856

DEDICATION

ForAidan&Nora,andCraig&Fiona

ACKNOWLEDGMENTS

WewouldliketothankourtechnicalreviewersAlexAgranovsky,CarolDavids,EmilIvov,DavidKemp,HenrySinnreich,HarveyWaxman,andDanYork.WewouldalsoliketothankMarinaBurnettandChrisComfortfortheirproofreadingandcomments.Wewouldalsoliketothankourfamiliesfortheirencouragementandsupport.

Andfinally,wewouldliketoacknowledgeourcolleaguesinW3CandtheIETFwhoareworkingincrediblyhardatcreatingtheWebRTCstandards.

iii

PREFACE

WebRTCcontinuestoevolveandgrowinthehandfulofmonthssincewepublishedthefirsteditionofWebRTC:APIsandProtocolsoftheHTML5Real-TimeWeb.

TherehasbeenrealprogressinmanyareasintheIETFandW3Cstandards,althoughmuchworkremains.Eventing,streamrepresentationattheprotocollevel,andeventhesyntaxforsomecallbacksareallstillverymuchunderdiscussion,whiledetailssuchashowmultiplevideotrackswithinasingleMediaStreamworkandwhatshouldhappenwhenaMediaStreamattachedtoanHTMLelementhastracksaddedorremovedareonlynowbeginningtobeconsidered.Nevertheless,thecoreAPIsarefirmingup.Ontheusageside,conferences,meetups,andstartupshavesprungupandaregrowingrapidlyinsize,withtheWorldseekingtounderstandWebRTC’simpactandopportunities.

Othertrendshavebecomeclearaswell.Thedisagreementaboutcodecs,especiallyvideocodecsisescalatingfromafight,toawar,toMutuallyAssuredPatentDestruction.Theauthorssincerelyhopethatthestandardsandindustrycometoagreementquicklyonthemandatorytoimplementvideocodecverysoon.

WhilethebasicpartsofWebRTCareworkingwellindemosandapplicationstoday,manyoftheconcernsandissueswithWebRTCrevolvearoundsecurityandthesignalingchannel,whichhavecompletenewchaptersinthissecondedition.Additionally,thesamplecodeoutthereontheWebtodayisofteneithertoocomplexorinsufficientlyexplained,motivatingthecompleterunningandcompletelyexplainedexampleinthisnewedition.Thisdemocode,runningonbothChromeandFirefox,isalsoonourwebsiteat

.

Notethatallcodeandlistswilllookbestwiththesmallestfontsizeofyourdevice.

WehopethatthisnewupdatededitionkeepsyouexcitedandinformedaboutWebRTC.Happyreading!

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PREFACETOTHEFIRSTEDITION

TheInternetandtheWorldWideWebhavechangedourworld.Whenthehistoryofthisperiodiswritten,muchwillbesaidabouttheimpactofthesetechnologiesonlifeinthelate20thandearly21stcenturies.Thewebhaschangedthewaywereceiveinformation,interactwithothers,work,andplay.Now,thewebisabouttodramaticallychangethewaywecommunicateusingvoiceandvideo.Thisbookgivesanup-to-the-minutesnapshotofthestandardsandindustryeffortknownasWebRTC,whichisshortforWebReal-TimeCommunications.Thistechnology,alongwithotheradvancesinHTML5browsers,hasthepotentialtorevolutionizethewayweallcommunicate,inbothpersonalandbusinessspheres.

TheauthorshavebeeninvolvedintheInternetCommunicationsindustryformanyyears,andhaveseentheadvancesandimpactoftheInternetonvoiceandvideocommunications.WehaveworkedonsignalingprotocolssuchasSessionInitiationProtocol(SIP),SessionDescriptionProtocol(SDP),andsecurityprotocolssuchasZRTPforvoiceandvideocommunicationsystemsthatwillformthebasisofwhatwillinevitablyreplacethetelephonesystem(calledthePublicSwitchedTelephoneNetworkorPSTN).TheseInternetCommunicationstechnologieshavebroughtanamazingwaveofdisruption,butwebelieveWebRTChasthepotentialtocreateevengreaterdisruption.

Thisbookprovidesinformationforwebdevelopersandtelephonydeveloperswhowanttocatchthisnewwavewhileitisstillbuilding.ThestandardsandprotocolsneededforWebRTCarestillbeingdevelopedandinvented.BrowsersarestartingtosupportWebRTCfunctionality,littlebylittle.However,theauthorshaveseentheneedforabooktoexplainthisstill-developingtechnology.Thisbookwillexplainthetechnicalgoals,architectures,protocols,andApplicationProgrammingInterfaces(APIs)ofWebRTC.Inapublishingexperiment,weplantoproducefrequenteditionsofthisbook,perhapsasoftenasthreeperyear,andfocusondigitaldeliveryandon-demandpublishingtokeepcostsdownandformaximumhyperlinkingusefulness.Forinformationonthelatesteditionandforalistofupdatesandchanges,visit

.

ThisbookbeginswithanintroductiontoWebRTCanddiscusseswhatisnewaboutit.TheuniqueaspectsofWebRTCpeer-to-peermediaflowsareexplored,andNetworkAddressTranslation(NAT)traversalexplained.WethendiscusstheworkingdocumentsandfinalizeddocumentsthattogethercomprisetheWebRTCstandards-in-

progressinboththeWorldWideWebConsortium(W3C)andtheInternetEngineeringTaskForce(IETF).Eachchapterendswithareferencessection,listingthevariousstandardsdocuments.Referencesoftheform[RFC…]areIETFRequestForCommentsdocuments.Referencesoftheform[draft-…]areIETFInternet-Drafts,workingdocuments,whosecontentmayhavebeenupdatedorchangedsincethepublicationofthisbook.Thehyperlinksprovidedwill,inmostcases,retrievethemostrecentversionofthedocument.ReferencestoW3Cdraftsincludealinktothelatestpublicworkingdraft,andalsoalinktothelatesteditor’sdraft.ForthoseofyouunfamiliarwiththestandardizationprocessesofW3CandtheIETF,wehaveprovidedareferenceinAppendicesAandB.Finally,wediscussthecurrentstateofdeploymentinpopularbrowsers.

Ifyouareawebdeveloper,welcometotheworldofInternetCommunications!Youruserswillgreatlyenjoytheabilitytointeractwitheachotherusingyourapplication’sreal-timevoiceandvideocapabilities.TounderstandourdescriptionsofAPIs,youwillneedaworkingknowledgeofHTMLandJavaScript,andsomeexperienceinwebapplications.SeeAppendixDforsomeusefulreferencesforthis.

IfyouareaVoIPortelephonydeveloper,welcometothewebworld!Youruserswillenjoythecapabilitiesofhighqualityaudioandvideocommunication,andrich,web-powereduserinterfaces.Tounderstandourdescriptionsoftheon-the-wireprotocolsfortransportingvoice,video,anddata,youwillneedabasicunderstandingoftheInternet.KnowledgeofanotherInternetCommunicationsignalingprotocolsuchasSIPorJingleisalsouseful.SeeAppendixDforsomeadditionalusefulreferencereading.

Inmanyways,WebRTCisamergingofworldsbetweenthewebandtelephony.Tohelpbridgethegapbetweenthewebandtelephonyworld,wehavealsoincludedaGlossaryinAppendixCtobrieflyexplainsomecommontermsandconceptsfromeachworld.

TheauthorslookforwardtoparticipatinginthenextwaveofdisruptionandinnovationthatWebRTCwilllikelyunleash.

WewouldlovetohearfromyouandinteractwithyouonTwitter(@alanbjohnstonand@danielcburnett)oronGoogle+

(alanbjohnston@

anddanielcburnett@).

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1INTRODUCTIONTOWEBREAL-TIMECOMMUNICATIONS

WebReal-TimeCommunications(RTC),orWebRTC,addsnewfunctionalitytothewebbrowser.Forthefirsttime,browserswillinteractdirectlywithotherbrowsers,resultinginanumberofarchitecturesincludingatriangleandtrapezoidmodel.ThemediacapabilitiesofWebRTCarestate-of-the-art,withmanynewfeatures.TheunderlyingstandardsofWebRTCarebeingdevelopedbytheWorldWideWebConsortium(W3C)andtheInternetEngineeringTaskForce(IETF).

WebRTCIntroduction

WebRTCisanindustryandstandardsefforttoputreal-timecommunicationscapabilitiesintoallbrowsersandmakethesecapabilitiesaccessibletowebdevelopersviastandard[HTML5]tagsandJavaScriptAPIs(ApplicationProgrammingInterfaces).Forexample,considerfunctionalitysimilartothatofferedbySkype™[SKYPE]butwithouthavingtoinstallanysoftwareorplug-ins.Forawebsiteorwebapplicationtoworkregardlessofwhichbrowserisused,standardsarerequired.Also,standardsarerequiredsothatbrowserscancommunicatewithnon-browsers,includingenterpriseandserviceprovidertelephonyandcommunicationsequipment.

TheWebBrowsingModel

ThebasicmodelofwebapplicationsisshowninFigure1.1.TransportofinformationbetweenthebrowserandthewebserverisprovidedbytheHyper-TextTransportProtocol,HTTP(Section6.2.1),whichrunsoverTransmissionControlProtocol,TCP(Section6.2.9),orinsomenewimplementations,overtheWebSocketprotocol(seeSection6.2.2).ThecontentorapplicationiscarriedinHyper-TextMarkupLanguage,HTML,whichtypicallyincludesJavaScriptandCascadingStyleSheets[CSS].Inthesimplecase,thebrowsersendsanHTTPrequesttothewebserverforcontent,andthewebserversendsaresponsecontainingthedocumentorimageorotherinformationrequested.Inthemorecomplexcase,theserversendsJavaScriptwhichrunsonthebrowser,interactingwiththebrowserthroughAPIsandwiththeuserthroughclicksandselects.ThebrowserexchangesinformationwiththeserverthroughanopenHTTPorWebSocketschannel.

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WebRTC:APIsandRTCWEBProtocolsoftheReal-TimeWeb

Figure1.1WebBrowserModel

Infiguresinthisbook,wewillshowanarrowbetweenthewebbrowserandthewebservertoindicatethewebsessionbetweenthem.SinceWebRTCcanutilizeanywebtransport,thedetailsofthisconnection,andwhetheritisHTTPorWebSocketsisnotdiscussed.

TheReal-TimeCommunicationFunctionintheBrowser

Figure1.2showsthebrowsermodelandtheroleofthereal-timecommunicationfunction.Thelighterblockcalled“BrowserRTCFunction”isthefocusofthisbook.Theuniquenatureandrequirementsofreal-timecommunicationsmeansthataddingandstandardizingthisblockisnon-trivial.TheRTCfunctioninteractswiththewebapplicationusingstandardAPIs.ItcommunicateswiththeOperatingSystemusingthebrowser.AnewaspectofWebRTCistheinteractionthatoccursbrowser-to-browser,knownasa“PeerConnection”,wheretheRTCFunctioninonebrowsercommunicatesusingon-the-wirestandardprotocols(notHTTP)withtheRTCFunctioninanotherbrowserorVoiceoverIP(VoIP)orvideoapplication.WhilewebtrafficusesTCPfortransport,theon-the-wireprotocolbetweenbrowserscanuseothertransportprotocolssuchasUserDatagramProtocol,UDP.AlsonewistheSignalingServer,whichprovidesthesignalingchannelbetweenthebrowserandtheotherendofthePeerConnection.

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1IntroductiontoWebRTC

Figure1.2Real-TimeCommunicationintheBrowser

ElementsofaWebRTCSystem

Figure1.3showsatypicalsetofelementsinaWebRTCsystem.Thisincludeswebservers,browsersrunningvariousoperatingsystemsonvariousdevicesincludingdesktopPCs,tablets,andmobilephones,andotherservers.AdditionalelementsincludegatewaystothePublicSwitchedTelephoneNetwork(PSTN)andotherInternetcommunicationendpointssuchasSessionInitiationProtocol(SIP)phonesandclientsorJingleclients.WebRTCenablescommunicationamongallthesedevices.Thefiguresinthisbookwillusetheseiconsandelementsasexamples.

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WebRTC:APIsandRTCWEBProtocolsoftheReal-TimeWeb

Figure1.3ElementsinaWebRTCEnvironment

TheWebRTCTriangle

Initially,themostcommonscenarioislikelytobewherebothbrowsersarerunningthesameWebRTCwebapplication,downloadedfromthesamewebpage.ThisproducestheWebRTC“Triangle”showninFigure

1.4.Thisarrangementiscalledatriangleduetotheshapeofthesignaling(sidesoftriangle)andmediaordataflows(baseoftriangle)betweenthethreeelements.APeerConnectionestablishesthetransportforvoiceandvideomediaanddatachannelflowsdirectlybetweenthebrowsers.

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1IntroductiontoWebRTC

Figure1.4TheWebRTCTriangle

Notethatwhilewesometimesrefertotheconnectionbetweenthebrowserandserverassignaling,itisnotreallysignalingasusedintelephonysystems.SignalingisnotstandardizedinWebRTCasitisjustconsideredpartoftheapplication.ThissignalingmayrunoverHTTPorWebSocketstothesamewebserverthatservesHTMLpagestothebrowser,ortoacompletelydifferentwebserverthatjusthandlesthesignaling.

TheWebRTCTrapezoid

Figure1.5showstheWebRTCTrapezoid[draft-ietf-rtcweb-overview],basedontheSIPTrapezoid[RFC3261].ThetwowebserversareshowncommunicatingusingastandardsignalingprotocolsuchasSessionInitiationProtocol(SIP),usedbymanyVoIPandvideoconferencingsystems,orJingle[XEP-0166],usedtoaddvoiceandvideocapabilitytoJabber[RFC6120]instantmessagingandpresencesystems.Alternatively,aproprietarysignalingprotocolcouldbeused.Notethatinthesemorecomplicatedcases,themediamaynotflowdirectlybetweenthetwobrowsers,butmaygothroughmediarelaysandotherelements,asdiscussedinChapter3.

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WebRTC:APIsandRTCWEBProtocolsoftheReal-TimeWeb

Figure1.5TheWebRTCTrapezoid

WebRTCandtheSessionInitiationProtocol(SIP)

Figure1.6showsWebRTCinteroperatingwithSIP.TheWebServerhasabuilt-inSIPsignalinggatewaytoallowthecallsetupinformationtobeexchangedbetweenthebrowserandtheSIPclient.TheresultingmediaflowisdirectlybetweenthebrowserandtheSIPclient,asthePeerConnectionestablishesastandardReal-timeTransportProtocol(RTP)mediasession(Section6.2.3)withtheSIPUserAgent.OtherwaysofinteroperatingwithSIParecoveredinSection2.2.6.

Figure1.6WebRTCInteroperatingwithSIP

WebRTCandJingle

Figure1.7showshowWebRTCcaninteroperatewithJingle.TheWeb

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1IntroductiontoWebRTC

Serverhasabuilt-inExtensibleMessagingandPresenceProtocol,XMPP[RFC6120],alsoknownasJabber,serverwhichtalksthroughanotherXMPPservertoaJingleclient.

Figure1.7WebRTCInteroperatingwithJingle

WebRTCandthePublicSwitchedTelephoneNetwork(PSTN)

Figure1.8showshowWebRTCcaninteroperatewiththePublicSwitchedTelephoneNetwork(PSTN).ThePSTNGatewayterminatestheaudio-onlymediastreamandconnectsthePSTNtelephonecallwiththemedia.SomesortofsignalingisneededbetweentheWebServerandthePSTNGateway.ItcouldbeSIP,oramaster/slavecontrolprotocol.

Figure1.8WebRTCInteroperatingwiththePSTN

ItisnotexpectedthatbrowserswillbeassignedtelephonenumbersorbepartofthePSTN.Instead,anInternetCommunicationservicecouldassignatelephonenumbertoauser,andthatusercoulduseWebRTCto

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WebRTC:APIsandRTCWEBProtocolsoftheReal-TimeWeb

accesstheservice.Asaresult,atelephonecalltothatPSTNnumberwould“ring”thebrowserandanansweredcallwouldresultinanaudiosessionacrosstheInternetconnectedtothePSTNcaller.Otherservicescouldincludetheabilityto“dial”atelephonenumberinaWebRTCapplicationwhichwouldresultintheaudiopathacrosstheInternettothePSTN.

NotethatthephoneinFigure1.8couldbeanormalPSTNphone(“landline”or“blackphone”)oramobilephone.ThefactthatitmightberunningVoLTE(VoiceoverLongTermEvolution)orotherVoIP(VoiceoverInternetProtocol)protocoldoesn’tchangethispicture,asthePeerConnectionwillterminatewithaVoIPgateway.

AnotherinterestingareaistheroleofWebRTCinprovidingemergencyservices.WhileaWebRTCservicecouldsupportemergencycallinginthesamewayasVoIPInternetCommunicationservices,thereisthepotentialthatthePublicServiceAnsweringPoint(PSAP)couldbecomeaWebRTCapplication,andansweremergency“calls”directlyfromotherbrowsers,completelybypassingthePSTN.Ofcourse,thisraisesallkindsofinterestingsecurity,privacy,andjurisdictionissues.

MultipleMediaStreamsinWebRTC

Devicestodaycangenerateandconsumemultiplemediatypesandmultiplestreamsofeachtype.Eveninthesimplepoint-to-pointexampleshowninFigure1.9,amobilephoneandadesktopPCcouldgenerateatotalofsixmediastreams.Formultipartysessions,thisnumberwillbemuchhigher.Asaresult,WebRTChasbuilt-incapabilitiesfordealingwithmultiplemediastreamsandsources.

Figure1.9MultipleMediaStreamsinaPoint-to-PointWebRTCSession

Multi-PartySessionsinWebRTC

Theprecedingexampleshavebeenpoint-to-pointsessionsbetweentwobrowsers,orbetweenabrowserandanotherendpoint.WebRTCalsosupportsmulti-partyorconferencingsessionsinvolvingmultiplebrowsers.

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1IntroductiontoWebRTC

OnewaytodothisistohaveeachbrowserestablishaPeerConnectionwiththeotherbrowsersinthesession.ThisisshowninFigure1.10.Thisissometimesreferredtoasa“fullmesh”or“fullydistributed”conferencingarchitecture.EachbrowserestablishesafullmeshofPeerConnectionswiththeotherbrowsers.Foraudiomedia,thismightmeanmixingthemediareceivedfromeachbrowser.Forvideo,thismightmeanrenderingthevideostreamsfromotherbrowserstodifferentwindowswithappropriatelabeling.Asnewbrowsersjointhesession,newPeerConnectionsareestablishedtosendandreceivethenewmediastreams.

Figure1.10MultiplePeerConnectionsBetweenBrowsers

Analternativearchitecturetothefull-meshmodelofFigure1.10isalsopossiblewithWebRTC.Foramultiplebrowserconference,acentralizedmediaserver/mixer/selectorcanbeused;thisrequiresonlyasinglePeerConnectiontobeestablishedbetweeneachbrowserandthemediaserver.ThisisshowninFigure1.11.Thisissometimesreferredtoasa“centrallymixed”conferencingarchitecture.Eachbrowsersendsmediatotheserver,whichdistributesittotheotherbrowsers,withorwithoutmixing.FromtheperspectiveofbrowserM,mediastreamsfrombrowserLandTarereceivedoverasinglePeerConnectionfromtheserver.Asnewbrowsersjointhesession,nonewPeerConnectionsinvolvingbrowserMneedtobeestablished.Instead,newmediastreamsarereceivedovertheexistingPeerConnectionbetweenbrowserMandthemediaserver.

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WebRTC:APIsandRTCWEBProtocolsoftheReal-TimeWeb

Thefull-mesharchitectureofFigure1.10hastheadvantagesofnomediaserverinfrastructure,andlowestmedialatencyandhighestquality.However,thisarchitecturemaynotbesuitableforalargemulti-partyconferencebecausethebandwidthrequiredateachbrowsergrowswitheachnewparticipant.ThecentralizedarchitectureofFigure1.11hastheadvantageofbeingabletoscaletoverylargesessionswhilealsominimizingtheamountofprocessingneededbyeachbrowserwhenanewparticipantjoinsthesession,althoughitisperhapsinefficientwhenonlyoneorasmallnumberofbrowsersareinvolved,suchasinpeer-to-peergaming.

Figure1.11SinglePeerConnectionwithMediaServer

WebRTCStandards

TheWebRTCstandardsarecurrentlyunderjointdevelopmentbytheWorldWideWebConsortium(W3C)[W3C]andtheInternetEngineeringTaskForce(IETF)[IETF].W3CisworkingondefiningtheAPIsneededforJavaScriptwebapplicationstointeractwiththebrowserRTCfunction.TheseAPIs,suchasthePeerConnectionAPI,aredescribedinChapter5.TheIETFisdevelopingtheprotocolsusedbythebrowserRTCfunctiontotalktoanotherbrowserorInternetCommunicationsendpoint.Theseprotocols,forexample,extensionstotheReal-timeTransportProtocol,aredescribedinChapter8.

Therearepre-standardimplementationsofmanyofthecomponentsofWebRTCinsomebrowserstoday.SeeChapter8fordetails.

Notethatthereisanimportantdistinctionbetween‘pre-standard’and‘proprietary’implementations.Pre-standardimplementations

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emergeduringthedevelopmentstageofstandards,andarecriticaltogainexperienceandinformationbeforestandardsarefinalizedandlockeddown.Pre-standardimplementationsoftenfollowanearlyordraftversionofthestandards,orpartiallyimplementstandardsasa‘proofofconcept’.Oncethestandardhasbeenfinalized,thesepre-standardimplementationsmustmovetowardsthestandards,orelsetheyriskbecomingaproprietaryimplementation.Proprietaryimplementationsfragmenttheuseranddevelopmentbase,whichinanareasuchascommunicationscangreatlyreducethevalueoftheservices.

TheW3CworkiscenteredaroundtheWEBRTCWorkingGroupandtheIETFworkiscenteredaroundtheRTCWEB(Real-TimeCommunicationsWeb)WorkingGroup.Thetwogroupsareindependent,butcloselycoordinatetogetherandhavemanycommonparticipants,includingtheauthors.

TheprojectedtimeframeforpublicationofthefirstversionofstandardsintheIETFandW3Cisin2014.However,thesedatesaremostlikelyoverlyoptimistic.(Whendoengineerseverrealisticallyestimatelevelofeffort?)Standards-compliantWebRTCbrowsersareexpectedtobegenerallyavailablesometimeinlate2014.

WhatisNewinWebRTC

TherearemanynewandexcitingcapabilitiesinWebRTCthatarenotavailableevenintoday’sVoIPandvideoconferencingsystems.SomeofthesefeaturesarelistedinTable1.1.TherestofthisbookwillexplainhowtheseareachievedusingtheWebRTCAPIsandprotocols.

ImportantTerminologyNotes

Inthisbook,whenwerefertotheentireefforttoaddstandardizedcommunicationcapabilitiesintobrowsers,weshalluseWebRTC.WhenwearereferringtotheW3CWorkingGroup,wewilluseWEBRTC.WhenwearereferringtotheIETFWorkingGroup,wewilluseRTCWEB.NotethatWebRTCisalsousedtodescribetheGoogle/Mozillaopensourcemediaengine[WEBRTC.ORG],whichisanimplementationofWebRTC.

Inaddition,becausethemainW3Cspecificationistitled“TheWebRTCSpecification”[WEBRTC1.0],weuseitsfulltitletoreferencethisparticularW3Cdocument,whichisakeypartofWebRTC,butbynomeanstheentirespecification.

AlsonotethattheWorldWideConsortiumreferstoitselfas“W3C”

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WebRTC:APIsandRTCWEBProtocolsoftheReal-TimeWeb

andnot“theW3C”.Wehaveadoptedthisconventionthroughoutthisbook.

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Table1.1NewFeaturesofWebRTC

References

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WebRTC:APIsandRTCWEBProtocolsoftheReal-TimeWeb

[HTML5]

/TR/html5

[SKYPE]

[CSS]

/Style/CSS

[draft-ietf-rtcweb-overview]

/html/draft-ietf-rtcweb-

overview

[RFC3261]

/html/rfc3261

[XEP-0166]

/extensions/xep-0166.html

[RFC6120]

/html/rfc6120

[W3C]

[IETF]

[WEBRTC.ORG]

[WEBRTC1.0]

/TR/webrtc

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2HOWTOUSEWEBRTC

WebRTCiseasytouse,withjustafewstepsnecessarytoestablishmediasessions.Anumberofmessagesflowbetweenthebrowserandtheserver,whileothersflowdirectlybetweenthetwobrowsers,knownaspeers.WebRTCcanevenestablishsessionswithSIP,Jingle,andPSTNendpoints.TherearemanystandardsinvolvedintheWebRTCeffort–somanythatitcanbedifficulttoknowwheretostartwhenlearningaboutit.SincemanyreadersofthistextwilllikelybedevelopersofWebRTCapplications,thefollowingsectionsgiveanoverviewofhowtosetupaWebRTCsession,whatcanbedonewhilethesessionisrunning,andhowtoclosedownthesession.WebRTCcanbeusedinanumberofdifferentarchitectures.ExamplepseudocodeillustratestheoperationoftheWebRTCAPIs.

SettingUpaWebRTCSession

Asanapplicationdeveloper,thefourmainactionstotakewhensettingupaWebRTCsessionare:

Obtainlocalmedia,

Setupaconnectionbetweenthebrowserandthepeer(otherbrowserorendpoint),

Attachmediaanddatachannelstotheconnection,and

Exchangesessiondescriptions

ThesefourstepsareshowninFigure2.1. NotethatthisfigurecorrespondswithFigure1.4.

Figure2.1WebRTCSessionEstablishment,APIView

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WebRTC:APIsandRTCWEBProtocolsoftheReal-TimeWeb

Figure2.2WebRTCAPIViewwithSignaling

Figure2.2showsanotherviewofthesteps,thistimewithsignalingstepsshown.

Thefollowingsubsectionsbrieflydescribeeachofthesesteps,aswellastheprocessforclosingdownasessionwhencomplete.

ObtainingLocalMedia

Thereareavarietyofwaystoobtainmedia,thecompletelistofwhichisoutofscopeforthisbook.However,oneofthemostcommonwaysisdefinedbytheWebRTCeffort:getUserMedia()(Section5.3.2).ThismethodcanbeusedtoobtainasinglelocalMediaStream.Onceyouhaveoneormoremediastreams,youcanpiecethemtogetherintothestreamsyouwantusingtheMediaStreamAPI(ApplicationProgrammingInterface).Forprivacyreasons,awebapplication’srequestforaccesstoauser’smicrophoneorcamerawillonlybegrantedafterthebrowserhasobtainedpermissionfromtheuser.

SettingupthePeerConnection

AnotherimportantstepistosetupthePeerConnectionusingtheAPIbythesamename.ThecoreofWebRTCistheRTCPeerConnectionAPI,which,asitsnamesuggests,setsupaconnectionbetweentwoPeers.Inthiscontext,“peers”meanstwocommunicationendpointsontheWorldWideWeb,asinthephrase“peer-to-peerfilesharing”.Insteadofrequiringcommunicationthroughaserver,thecommunicationisdirectbetweenthetwoentities.InthespecificcaseofWebRTC,aPeerConnectionisadirectmediaconnectionbetweentwowebbrowsers.

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2HowtoUseWebRTC

Thisisparticularlyrelevantwhenamulti-waycommunicationsuchasaconferencecallissetupamongthreeormorebrowsers.EachpairofbrowserswillrequireasinglePeerConnectiontojointhem,allowingforaudioandvideomediatoflowdirectlybetweenthetwo,asshowninFigure1.10.Thus,threebrowserscommunicatingwouldneedatotalofthreeconnectionsamongthem.AnapplicationdeveloperwillneedtosetuponePeerConnectionperpairofbrowsers(orabrowserandanotherendpointsuchasanexistingcommunicationsnetwork)beingconnected.Thealternativearchit