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  3. The TTL field was originally designed to

The TTL field was originally designed to

资格题库2022-08-02  57
问题 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.receivedB.discardedC.rejectedD.transferred问题2选项A.maximumB.minimumC.exactD.certain问题3选项A.controlsB.transmitsC.storesD.receives问题4选项A.switchB.routerC.source hostD.destination host问题5选项A.lifetimeB.moving timeC.receiving timeD.transmitting time
选项
答案 BACDA
解析 ttl字段最初被设计用来存放时间戳，每个被访问的路由器都会对时间戳进行降级处理。当ttl值为0时，数据报被丢弃。然而对于这个方案，所有的机器都必须有同步时钟，并且必须知道从一台机器到另一台机器需要多长时间。现在，这个字段主要用于查询数据报访问的（路由器）最大跳数。当源主机发送数据报时，它在这个字段中存储一个数字。每个处理数据报的路由器将这个数字减1。如果这个值在被递减之后为零，路由器就会丢弃数据报。
这个字段是必需的，因为因特网上的路由表可能会损坏。一个数据报可能在两个或多个路由器之间传输很长时间，而始终没有到达目标主机。所以这个字段限制了一个数据报的生命周期
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中级 网络工程师

软件水平考试初中高级

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