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
正确答案是C