work(),systemsthateffectivelyisolateanorganization'sinternalnetworkstructurefroman()network,suchastheINTERNETisbecomingincreasinglypopular.Thesefirewallsystemstypicallyactasapplication-layer()betweennetworks,usuallyofferingcontrolledTELNET,FTP,andSMTPaccess.Withtheemergenceofmoresophisticated(请作答此空)layerprotocolsdesignedtofacilitateglobalinformationdiscovery,thereexistsaneedtoprovideageneral()fortheseprotocolstotransparentlyandsecurelytraverseafirewall.
A.network B.session C.transmission D.application正确答案D
