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