问题 单项选择题

若路由器R因为拥塞丢弃IP分组,则此时R可向发出该IP分组的源主机发送的ICMP报文的类型是______。

A.路由重定向
B.目标不可到达
C.源抑制
D.超时

答案

参考答案:C

解析: 若路由器或目的主机缓冲资源耗尽而必须丢弃数据报,则每丢弃一个数据报就向源主机发送一个ICMP源抑制报文,此时,源主机必须减小发送速度。另外一种情况是系统的缓冲区已用完,并预感到将发生拥塞,则发送源抑制报文。但是与前一种情况不同,涉及的数据报尚能提交给目的主机。
当一台主机向自己的默认网关发送一个需要转发的数据包时,如果该网关路由器查找路由表发现有更好的路由,就会向源主机发出“重定向”的ICMP报文。
如果在IP数据报的传送过程中,路由器发现网络出现拥塞,则路由器将向源主机发出“目标不可到达”的ICMP报文。
一个IP包从源节点出发时,其TTL值被设定一个初始值(比如255),经过一跳一跳的传输,如果这个IP包的TTL降低到零,路由器就会丢弃此包。此时,该路由器上的ICMP便会发出一个“超时(time exceeded)”的ICMP报文。

解答题
阅读理解

In 1986, when Monty Reed was a member of the Army Rangers (黑鹰突击 队), he had an accident. "It was a night jump, and we were jumping low," he recalls. "We were training. Somebody got too close to my parachute (降落伞)."The parachute below blocked Reed’s air and his parachute failed to open. Reed crashed a hundred feet to the ground, breaking his back in five places.

The next morning, Reed could not move. "Doctors said it was likely that I would never walk again." Reed stared out of his hospital window, wondering what the future would be like. It was the saddest moment of his life. "The experts are telling me my body doesn’t work! What am I supposed to do?" he thought.

To distract (转移注意力) himself, he picked up a book: Robert Heinlein’s Starship Troopers. The science-fiction describes a set of man-made muscles that would allow people to carry 2,000 pounds. He thought that if he could build something like that, it might be able to lift him out of his wheelchair. So he began his project and finally succeeded in building a set of robot legs. Several years later, he made an amazing recovery and was able to get up and walk away from his wheelchair. He even jumped out of an airplane again to celebrate.

Reed wanted to do something for others with the second chance that life had given him. He has now perfected the robot legs that can get the injured, the elderly and the paralyzed (瘫痪的) out of their wheelchairs. Now he has started his own company to make the legs. "I’ve seen them compete in marathons and go swimming and mountain climbing," he said. "So never give up!"

小题1:Monty had an accident in 1986 because_______.

A.his feet hit the ground too hard

B.his parachute was under another person’s

C.another person’s parachute stopped his from opening

D.he was training to jump at night小题2:Monty started to read Starship Troopers to ______.

A.create a new kind of wheelchair for himself

B.discover how to make man-made muscles

C.learn how to jump out of an airplane

D.stop himself from thinking about his pain小题3:Which of the following is TRUE about Monty?

A.He didn’t lose heart in the face of difficulty.

B.He is fond of reading science fiction.

C.He spent the rest of his life in a wheelchair.

D.He didn’t believe what the doctors said.小题4:We can learn from the passage that Monty’s goal is to ______.

A.become a famous writer like Robert Heinlein

B.return to the Army Rangers and continue with his jumps

C.compete in games and marathons and go swimming

D.help more patients in wheelchairs to recover