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.
问题1选项
A.received
B.discarded
C.rejected
D.transferred
问题2选项
A.maximum
B.minimum
C.exact
D.certain
问题3选项
A.controls
B.transmits
C.stores
D.receives
问题4选项
A.switch
B.router
C.source host
D.destination host
问题5选项
A.lifetime
B.moving time
C.receiving time
D.transmitting time
正确答案及解析
正确答案
解析
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