●Thenetworklayerprovidesservicestothetransportlayer.Itcanbebasedoneither(请作答此空).Inbothcases,itsmainjobis(72)packetsfromthesourcetothedestination.Innetworklayer,subnetscaneasilybecomecongested，increasingthedelayand(73)forpackets.Networkdesignersattempttoavoidcongestionbyproperdesign.Techniquesinclude(74)policy,caching,flowcontrol,andmore.Thenextstepbeyondjustdealingwithcongestionistoactuallytrytoachieveapromisedqualityofservice.Themethodsthatcanbeusedforthisincludebufferingattheclient,trafficshaping,resource(75)，andadmissioncontrol.Approachesthathavebeendesignedforgoodqualityofserviceincludeintegratedservices(includingRSVP),differentiatedservices，andMPLS.
A.virtual circuits or datagrams B.TCP or UDP C.TCP or IP D.IP or ARP正确答案A
