TheFileTransferProtocol(FTP)isastandardnetworkprotocolusedforthetransferofcomputer(请作答此空)betweenaclientandserveronacomputernetwork.FTPisbuiltona()modelarchitectureusingseparatecontrolanddataconnectionsbetweentheclientandtheserver.FTPusersmayauthenticatethemselveswitha()sign-inprotocol,normallyintheformofausernameandpassword,butcanconnectanonymouslyiftheserverisconfiguredtoallowit.Forsecuretransmissionthatprotectstheusernameandpassword,andencryptsthecontent,FTPisoftensecuredwithSSL/TLS(FTPS)orreplacedwithSSHFileTransferProtocol(SFTP).ThefirstFTPclientapplicationswere()programsdevelopedbeforeoperatingsystemshadgraphicaluserinterfaces,andarestillshippedwithmostWindows,Unix,andLinuxoperatingsystems.ManyFTPclientsandautomationutilitieshavesincebeendevelopedfordesktops,servers,mobiledevices,andhardware,andFTPhasbeenincorporatedintoproductivityapplications,suchasHTMLeditors.FTPmayrunin()mode,whichdetermineshowthedataconnectionisestablished.Inbothcases,theclientcreatesaTCPcontrolconnectionfromarandom,usuallyanunprivileged,portNtotheFTPservercommandport21.Inactivemode,theclientstartslisteningforincomingdataconnectionsfromtheserveronportM.ItsendstheFTPcommandPORTMtoinformtheserveronwhichportitislistening.Theservertheninitiatesadatachanneltotheclientfromitsport20,theFTPserverdataport.InsituationswheretheclientisbehindafirewallandunabletoacceptincomingTCPconnections,passivemodemaybeused.Inthismode,theclientusesthecontrolconnectiontosendaPASVcommandtotheserverandthenreceivesaserverIPaddressandserverportnumberfromtheserver,whichtheclientthenusestoopenadataconnectionfromanarbitraryclientporttotheserverIPaddressandserverportnumberreceived.BothmodeswereupdatedinSeptember1998tosupportIPv6.Furtherchangeswereintroducedtothepassivemodeatthattime,updatingittoextendedpassivemode.
A.network B.programs C.connections D.files正确答案D
