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