对照ISO/OSI参考模型中的网络安全服务，在物理层可以采用(39)加强通信线路的安全；在数据链路层，可以采用(40)进行链路加密；在网络层可以采用(41)来处理信息内外网络边界流动和建立透明的安全加密信道；在传输层主要解决进程到进程间的加密，最常见的传输层安全技术有(42)等；为了将低层安全服务进行抽象和屏蔽，最有效的一类做法是可以在传输层和应用层之间建立中间层实现通用的安全服务功能，通过定义统一的安全服务接口向应用层提供(43)安全服务。

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Ipv6 is(71)for "Internet Protocol Version 6"。Ipv6 is the "next generation" protocol design by the IETF to(72)the current version Inernet Protocol, IP Version 4. Most of today's internet uses Ipv4, which is now nearly twenty years old. Ipv4 has been remarkably resilient in spite of its age, but it is beginging to have problems. Most importantly, there is a growing(73)of Ipv4 addresses, which are needed by all new machines added to the Internet. Ipv6 fixs a number of problems in Ipv4, shch as the(74)number of available Ipv4 addresses. It also adds many improvements to Ipv4 in areas such as souting and network autoconfiguration. Ipv6 is expected to gradually replace Ipv4, with the two coexing for a number of years during a transition(75)A.

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In the following essay, each blank has four choices. Choose the most suitable one from the four choices and write down in the answer sheet.

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