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.Acontrols B.Btransmits C.Cstores D.Dreceives正确答案C
