TheTTLfieldwasoriginallydesignedtoholdatimestamp,whichwasdecrementedbyeachvisitedrouter.Thedatagramwas()whenthevaluebecamezero.However,forthisscheme,allthemachinesmusthavesynchronizedclocksandmustknowhowlongittakesforadatagramtogofromonemachinetoanother.Today,thisfieldisusedmostlytocontrolthe()numberofhops(routers)visitedbythedatagram.Whenasourcehostsendsthedatagram,it(请作答此空)anumberinthisfield.Eachrouterthatprocessesthedatagramdecrementsthisnumberby1.Ifthisvalue,afterbeingdecremented,iszero,therouterdiscardsthedatagram.ThisfieldisneededbecauseroutingtablesintheInternetcanbecomecorrupted.Adatagrammaytravelbetweentwoormoreroutersforalongtimewithoutevergettingdeliveredtothe().Thisfieldlimitsthe()ofadatagram.
A.controls
B.transmits
C.stores
D.receives
正确答案是C