单选题
The TTL field was originally designed to hold a time stamp．which was decremented by each visited router．The datagram Was(71)________________ when the value became zero．However，for this scheme，all the machines must have synchronized clocks and must know how long it takes for a datagram to go from one machine to another．Today,this field is used mostly to control the(72)________________ number of hops(reuters)visited by the datagram．When a source host sends the datagram，it(73)________________ a number in this field．Each router that processes the datagram decrements this number by 1．Ifthis value，after being decremented，is zero，the router discards the datagram．This field is needed because routing tables in the Internet can become corrupted． A datagram may travel between two or more reuters for a long time without ever getting delivered to the (74)________________．This field limits the(75)________________ of a datagram．