多媒体传输网络层的需求是高带宽

多媒体传输网络层的需求是高带宽、多点发送、资源保留和QoS保证，有支持数据流有新路由协议，及支持集成式服务的新高容量路由的前提条件。Therequirementsonthenetworklayerformultimediatransmissionareaprovisionofhighbandwidth,multicasting,resourcereservationandQoSguarantees,newroutingprotocolswithsupportforstreamingcapabilitiesandnewhigher-capacityrouterswithsupportofintergatedservices.1．Internet服务和协议⑴Internet协议（IP）IP提供从源端点到目标端点之间的非可靠的数据文传输，在处理过程中可能经过一个或几个网关（路由器）和网络。1.InternetServicesandProtocols(1).InternetProtocol(IP)IPprovidesfortheunreliablecarriageofdatagramsfromsourcehosttodestinationhosts,possiblypassingthroughoneormoregateways(routers)andnetworksintheprocess.⑵Internet（IGMP，InternetGroupManagementProtocol）Internet组管理协议（IGMP）是用于管理Internet多点发送组的协议。它可让会议应用加入和离开特定的多点发送组。(2)InternetGroupManagementProtocol(IGMP)InternetGroupManagementprotocol(IGMP)isaprotocolformanagingInternetmulticastinggroups.Itisusedbyconferencingapplicationstojoinandleaveparticularmulticastgroup.基本的服务是让源发送数据包给多点发送组的所有成员。它不保证数据发送能到达组中的任一目标或所有目标。Thebasicservicepermitsasourcetosenddatagramstoallmembersofamulticastgroup.Thesearenoguaranteesofthedeliverytoanyoralltargetsinthegroup.⑶ResourcereSerVationProtocol（RSVP）RSVP协议用于传输保留资源，并在中间节点保留状态。它没有数据传输组件。RSVP消息以IP数据报文的形式发送，路由器保留“软状态”，由周期性保留消息进行刷新。如果没有刷新消息，则路由器在某一时间之后保留。RSVPisaprotocolwhichtransfersreservationsandkeepsastareattheintermediatenodes.Itdoesnothaveadatatransfercomponent.RSVPmessagesaresentasIPdatagrams,andtherouterkeeps“softstate”,whichisrefreshedbyperiodicreservationmessages.Intheabsenceoftherefreshmessages,theroutersdeletethereservationafteracertaintimeout.1Stream协议，版本2（ST-II）ST-II提供基于流模型的面向连接的、性能得到期保证的数据传输服务。尽管也可建立双向连接。但一般以单向连接方式建立发送者与几个接收者之间的连接。1.Streamprotocol,Version2(ST-II)ST-IIprovidesaconnection-oriented,guaranteedservicefordatatransportbasedonthestreammodel[Top90].Theconnectionsbetweenthesenderandseveralreceiversaresetupasuni-directionalconnections,althoughalsoduplexconnectionscanbesetup.ST-II是原ST协议的扩展[FOR79]。它由两个组件组成：ST控制消息协议（SCMP），它支持可靠的面向非连接的协议消息的传输；另一组件是ST协议本身，它支持非可靠的数据传输。ST-IIisanextensionoftheoriginalSTprotocol[For79].Itconsistsoftwocomponents:theSTControlMessageProtocol(SCMP),whichisareliable,connectionlesstransportfortheprotocolmessagesandtheSTprotocolitself,whichisanunreliabletransportforthedata.ST-II在连接建立时提供资源保留，资源保留来自己于源。用流说明发送SCMP消息。它以数据包大小、数据位率描述流需求（QoS）。如果目标接收呼叫，则返回最终的流说明给源。11.3.3节详细说明资源保留机制。ST-IIprovidesaresourcereservationduringtheconnectionsetup.Thereservationsoriginatedfromthesource.ItsendsaSCMPmessagewithaflowspecification,whichdescribesthestreamrequirements(QoS)intermsofpacketsize,datarate,etc.Ifthedestinationacceptsthecall,itreturnsthefinalflowspecificationtothesource.TheresourcereservationschemeispresentedinmoredetailinSection11.3.3.ST-II非常适合于多媒体传输，因为它从发送者到接收者沿路都进行资源保留。ST-IIissuitableformultimediatransmissionbecauseofitsresourcereservationalongthepathbetweenthesenderandreceiver.1．实时Internet协议(RTIP)Tenet机制中使用RTIP（实时Internet协议）服务。RTIP提供面向连接的、保证性能的、非可靠的数据包发送。就像IP在Internet协议本栈中占有的地位那样[VZ91]。1.Real-TimeInternetProtocol(RTIP)IntheTenetscheme,theservicesofRTIP(Real-TimeInternetProtocol)areused.RTIPprovidesforconnection-oriented,performance-guaranteed,unreliabledeliveryofpackets[VZ91].服务质量的数由国际标准化组织（ISO）通过QoS（QualityofService）这一术语来定义。QoS确定了网络服务质量的优劣，QoS有一组参数来标定。BasicConceptsParameterizationoftheservicesisdefinedinISO(InternationalStandardOrganization)standardsthroughthenotionofQualityofService(QoS).TheISOstandarddefinesQoSasaconceptforspecifyinghow“good”theofferednetworkingservicesare.QoScanbecharacterizedbyanumberofspecificparameters.ThereareseveralimportantissueswhichneedtobeconsideredwithrespecttoQoS.1．QoS分层传统的QoS在通信系统工程的网络层中定义，通过把QoS概念要引入到传输层，可加强对QoS的控制。1.QoSLayeringTraditionalQoS(ISOstandards)wasprovidedbythenetworklayerofthecommunicationsystem.AnenhancementofQoSwasachievedthroughinducingQoSintotransportservices.2，服务对象各种服务在不同的对象（如媒体源、媒体信息接收点、连接和虚拟线路等）上，因此，在QoS的描述集中在处理传输层/网络连接的服务上。2.ServiceObjectsServicesareperformedondifferentobjects,forexample,mediasources,mediasinks,connectionsandVirtualCircuits(VCs),hencetheQoSparameterizationspecifiestheseserviceobjects.InISOstandards,theQoSdescriptionismeanttobeforservices,processingatransport/networkconnection.在Telnet协议组中，服务操作在分组交换网的实时通道上进行[FV90]。InTenetprotocolsuite,theservicesoperateovera“real-time”channelofapacketswitchednetwork[FV90].