问题 问答题 简答题

为了防止数据丢失,TCP采用了重发机制。请说明TCP的重发定时器为什么不能采用一个固定的值。

答案

参考答案:

IP互联网可以将任意类型的物理网络互联在一起,这些物理网络的传输延时有可能差别很大。

在重发定时器采用固定值时,如果选择的值小于数据在互联网上传输的往返时间,则会造成数据的无为重发。如果选择的值大于数据在互联网上传输的往返时间,则又会造成出错后重传不及时。因此必须采用自适应方法决定重传定时器的值。

阅读理解

阅读理解。

     About 21,000 young people in 17 American states do not attend classes in school buildings. Instead,

they receive their elementary and high school education by working at home on computers. The center

for Education Reform says the United States has 67 public "cyberschools" and that is about twice as many

as two years ago.

     The money for students to attend a cybersehool cotes from the governments of the states where they

live. Some educators say cyberschools receive money that should support traditional public schools. They

also say it is difficult to know if students are learning well.

     Other educators praise this new form of education for letting students work at their own speed. These

people say cyberschools help students who were unhappy or unsuccessful in traditional schools. They say

learning at home by computer ends long bus rides for children who live far from school.

     Whatever the judgment of cyberschools, they are getting more and more popular. For example, a new

cyberschool called Commonwealth Connections Academy will take in students this fall. It will serve children

in the state of Pennsylvania from ages five through thirteen.

     Children get free equipment for their online education. This includes a computer, a printer, books and

technical services. Parents and students talk with teachers by telephone or by sending emails through their

computers when.necesary.

     Students at cyberschools usually do not know one another. But 56 such students finished studies at

Western Pennsvlváni? Cyber Charter School recently met for the first time. They were guests of honour at

their graduation.

1. What do we know from the text about students of a cyberschool?

A. They have to take long bus rides to school.

B. They study at home rather than in classrooms.

C. They receive money from traditional public schools.

D. They do well in traditional school programs.

2. What is a problem with cyberschoois?

A. Their equipment costs a lot of money.

B. They get little support from the state government.

C. It is hard to know students' progress in learning.

D. The students find it hard to make friends.

3. Cyberschools are getting popular became _____.

A. they are less expensive for students

B. their students can work at their own speed

C. their graduates are more successful in society

D. they serve students in a wider age range

4. We can infer that the author of the text is _____.

A. unprejudiced in his description of cyberschools

B. excited about the future of cyberschools

C. doubtful about the quality of cyberschoois

D. disappointed at the development of cyberschools

单项选择题