One ofthe main resposibilities ofICMP is to report(71). Although technology hasproduced increasingly relaible transmission media

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问题 One ofthe main resposibilities ofICMP is to report(71). Although technology hasproduced increasingly relaible transmission media, errors still exist and must be handled. IP isan(72) protocol. This means that error checking and error control are not a concem ofIP. ICMP was designed, in part, to compensate for this shortcoming. Howerver, ICMP doesnot(73)errors, it simply report them. Error correction is left to the higher-levelpotocols. Error messages are always sent to the original(74)because the only informationavailable in the datagram about the route is the source and destination IP addresses. ICMP usethe source IP address to sent the error massage to the source ofthe  (75).
(75)

选项 A、datagram
B、frame
C、service
D、protocol

答案A

解析  ICMP的主要责任之一就是报告错误。虽然技术的发展产生了日益可靠的传输介质,但是错误仍然会出现,所以必须进行处理。IP是一个不可靠的协议。这意味着错误检查和错误控制与IP无关。ICMP的设计就是为了弥补这个缺陷。然而,ICMP并不纠正错误,只是报告错误。纠正错误的功能留给高层协议。错误报文总是被发送给源站,因为在数据报中有关路由的信息就是IP源地址和目标地址。ICMP使用源IP地址把错误报文发送给数据报的源结点。
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