问题 单项选择题

虚拟专用网(VPN)用于保证内部数据通过Internet的安全传输。L2TP与PFTP是VPN的两种代表性协议,其区别之一是______。

A.L2TP只适于IP网,传输PPP帧;PPTP既适于IP网,也适于非IP网,传输以太帧
B.L2TP只适于IP网,传输以太帧;PPTP既适于IP网,也适于非IP网,传输PPP帧
C.都传输PPP帧,但PPTP只适于IP网,L2TP既适于IP网,也适于非IP网
D.都传输以太帧,但PPTP只适于IP网,L2TP既适于IP网,也适于非IP网

答案

参考答案:C

解析: 虚拟专用网(VPN)主要采用隧道技术、加解密技术、密钥管理、使用者与设备身份认证技术来保证内部数据通过Internet的安全传输。构建VPN的3要素是认证、加密和封装化,即VPN的工作内容可归纳为:①确认通信对方的VPN设备是约定设备的认证操作;②对数据加密的加密操作;③将要发送的分组进行封装发往Internet的操作。
在TCP/IP协议族中,在数据链路层利用L2F、PPTP、L2TP实现VPN应用:在网络层利用IPSec协议实现VPN应用;在传输层利用SSL/TLS协议实现VPN应用。这些VPN协议的共同特点是,只对要发送的载荷部分进行加密。例如,在PPTP中,PPP有效载荷部分是被加密的,IP头、GRE头和PPP头部分是没有被加密的。
L2TP与PPTP都使用PPP对数据进行封装,然后添加相应的首部用于数据在网络上的传输。但两者存在以下几方面的区别:①PPTP只适用于IP网络,L2TP只要求隧道提供点对点的连接(既适用于IP网,也适用于非IP网);②PPTP只能在两端点间建立单一隧道,L2TP支持在两端点间使用多隧道;③PPTP不支持隧道验证,L2TP可以提供隧道验证;④PPTP不支持首部压缩,L2TP可以提供首部压缩。

单项选择题
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Mainland couples who give birth to a second child in Hong Kong will be fined for breaking the family planning policy,a senior official has warned.

As more women flock to Hong Kong to give birth to their second child,Zhang Feng,family planning department director of Guangdong Province,stressed that this violated China's policies.

“And those who are government employees will even be dismissed from their posts.”he said.

“It doesn’t matter if they give birth to their second child on the mainland or in other countries and regions , they have violated the country’s policies and the probince’s regulations”.

He said that some families had been punished in the past few months after having a second in Hong Kong , but gave no details .

Zhang made his remarks when a Hong Kong newspaper carried a controversial notice claiming residents’ medical services had been affected by the growing number of mainland women who arrive in the city to give birth and gain fight of abode (居住)there.

According to statistics revealed by Hong Kong Special Administrative Region government,about 88,000 babies were born in Hong Kong in 2010,but more than 41,000 or 47 percent,were to mainland couples,including a large number from Guangdong.

Hong Kong has limited the number of mainland women permitted to give birth in the city at 34,000 this year.

The issue also has caused calls for an amendment(修正)to Hong Kong's Basic Law so that babies born to mainland women are no longer granted permanent fight of abode.

“I support Hong Kong government's decision to reduce or limit the number for mainland women giving birth in Hong Kong.”Zhang said.

China introduced its family planning policy in 1979 to limit births in the world's most populous nation,although the rules have been relaxed in recent years.

小题1:Which of the following is true?

A.Those who give birth to a second child in Hong Kong will be fined.

B.Many government employees have been dismissed from their posts.

C.Zhang Feng is family planning department director of Guangdong Province.

D.It doesn't matter if they give birth to their second child on the mainland.小题2:What does the word “violated” mean in the second paragraph?

A.went against

B.was obeyed

C.was for

D.was dismissed小题3:Zhang Feng said that          .

A.few families had been punished after having a second child in China

B.about 88,000 babies were born in Hong Kong in 2010

C.the residents' medical services in Hong Kong had been affected

D.he agreed to reduce or limit the number for mainland women giving birth in Hong Kong小题4:From the passage we can infer       .

A.in 2010 most of the babies born in Hong Kong belonged to mainland couples

B.the number of mainland women permitted to give birth in Hong Kong has been reduced

C.babies born to mainland women in Hong Kong can't get permanent right of abode now

D.the family planning policy in China is as strict as before小题5:Which is NOT the reason why some people want to give birth to a second child in Hong Kong?

A.They want to gain right of abode(居住)there for their babies.

B.They want to escape being punished for breaking the family planning policy.

C.They want to cause calls for an amendment(修正)to Hong Kong's Basic Law.

D.They want their babies to enjoy the good medical services in Hong Kong.