问题 问答题

【问题1】 SSL是一个协议独立的加密方案,在网络信息分组的应用层和传输层之间提供了安全的通道。SSL主要包括SSL修改密文协议、SSL握手协议、SSL告警协议、SSL记录协议等,其协议栈见图7-16。请根据SSL协议栈结构,将(1)~(4)处空缺的协议名称填写完整。

答案

参考答案:

解析:(1) SSL握手协议 (2) SSL记录协议 (3) TCP协议 (4) TP协议这是一道要求读者熟悉SSL协议栈的基本概念题。本题所涉及的知识点有: 1)SSL是一个协议独立的加密方案,在网络信息包的应用层和传输层之间提供了安全的通道。安全的HTTPS协议是SSL应用的一个范例,该应用协议使用TCP协议的443端口来安全发送和接受报文。 2)SSL主要包括SSL修改密文协议、SSL握手协议、SSL告警协议、SSL记录协议等,其协议栈见图7-22。

3)由图7-22可知,SSL协议包括两个子协议:SSL记录协议和SSL握手协议,其中记录协议位于握手协议之下,主要为SSL连接提供两种服务——机密性和报文完整性。 SSL记录协议字段主要包括4个必选字段,分别是①内容类型(占8b),用于指明所封装的高层协议类型;②主要版本(占8b),即所使用的SSL主要版本。对于SSLv3.0,值为3;③次要版本(占8b),即所使用的SSL次要版本。对于SSLv3.0,值为0;④压缩长度(占16b),即明文数据或压缩数据的长度,以字节为单位。 4)同理由图7-22可知,SSL握手协议被封装在SSL记录协议中,它允许服务器与客户机在应用程序传输和接收数据之前互相认证、协商加密算法和密钥。其报文头部包括3个字段:①类型字段(占1字节),用于指明所使用的SSL握手协议报文类型;②长度字段(占3字节),即SSL握手协议报文的长度,以字节为单位。③内容字段(≥1字节),用于设置所使用的SSL握手报文的有关参数。

阅读理解

Below is a housing guide for students going to London.

University accommodation(住所) office

Many university accommodation offices have their own list of registered landlords (房东). Others also provide information on accommodation agencies and other housing organizations. The advantage of using your university accommodation office is that you can get some support if you have 'a problem. The disadvantage is that they are unlikely to have enough registered landlords to house all their students.

Property papers: Loot and Renting

Loot is an important source (来源) of information about private housing for co-renters. The offers are from private landlords, agencies and individuals looking for other co-renters. They also have a website: www. loot. com. The advantage of using Loot is that there are some excellent bargains. The disadvantage is that there is no quality control over the offers.

Renting is another useful paper. The offers in this paper are mainly from accommodation agencies. Their website is at www. Renting.co. uk.

Accommodation agencies

The majority of rented accommodation in London is probably advertised through accommodation agencies. The advantage of using accommodation agencies is that you will have access to a large number of accommodations. A good agent will listen to your requirements and can save you time in looking for the right accommodation. The disadvantage is that they will make a range of charges to potential renters.

Noticeboards

Around the universities you will find a number of noticeboards where offers of accommodation will be posted. These will either be from landlords or from students. Some universities will also have online noticeboards where students can advertise to other students.  Advertisements from students can be an excellent way to find                                                             accommodation. However, advertisements from landlords can be problematic.

Word of mouth

Some of the best housing in London is never advertised but is passed on from one group of students to another by word of mouth. It might be that you can find out about good offers from final year students. However, don't suppose that just because you have found out about housing from a friend it is necessarily, going to be better than that found through any other source.

Family

Faced with the very high rents charged in London, some students and their parents will consider buying as an alternative. In some cases this might be a good choice.

1. What is the advantage of using Loot?

A. It has more offers from accommodation agencies than Renting.    

B. It gives you personal information about other co-renters.

C. Their website is designed mainly for students.    

D. There are some good bargains.

2. A good agent can help you      .

A. know more people                         

B. find cheap accommodation

C. get the right accommodation quickly

D. get free information about most accommodations

3. The information passed on by word of mouth is important because      .

A. it is better than that found through any other source

B. it helps you find some of the best housing never advertised    

C. the final year students always offer better information

D. the landlords have little valuable information

4. For students going to London for the first time, which of the following provides the most reliable information?

A. University accommodation offices.              

B. Loot and Renting.    

C. Noticeboards.                                 

D. Family.

单项选择题