2018下半年网络工程师考试已经结束，编程学习网小编为大家汇总了2018下半年网络工程师考试上午真题，以下为第八部分，供考生们参考。
● The TTL field was originally designed to hold a time stamp, which was decremented by each visited router. The datagram was ( ) 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 ( ) number of hops (routers) visited by the datagram. When a source host sends the datagram, it ( ) a number in this field. Each router that processes the datagram decrements this number by 1. If this 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 routers for a long time without ever getting delivered to the ( ).This field limits the ( ) of a datagram.
(71)A.received
B.discarded
C.rejected
D.tansferred
(72)A.maximum
B.minimum
C.exact
D.certain
(73)A.controls
B.transmits
C.stores
D.receives
(74)A.switch
B.router
C.source host
D.destination host
(75)A.lifetime
B.moving time
C.receiving time
D.transmitting time
