在基于TCP/IP的互联网服务中，传输层的UDP协议提供进程之间(43)报文传输服务，TCP协议提供进程之间(44)报文传送服务；TCP使用三次握手协议建立连接，传输报文，使用修改的三次握手协议来关闭连接。关闭连接时，设甲乙两方发送报文的序号分别为X和Y，甲方发送(45)的报文给乙方，乙方先发送一个(46)确认报文给甲方，并通知应用程序后再发送一个(47)的报文给甲方，最后甲方回送一个ACKY+1确认报文。

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While the Internet is inherently insecure, businesses still need to preserve the privacy of data as it travels over the network. To help do that, the Internet Engineering Task Force an international group of network designers, operators, vendors, and researchers concerned with the evolution of(71)has developed a suite of(72)called Internet Protocol Security(IPSec).

 IPSec creates a standard platform. to develop secure networks and electronic(73)between two machines. Secure tunneling via IPSec creates(74)connections in a network through which data packets can move. It creates these tunnels between remote users and within a local network. It also(75)each data packet in a new packet that contains the information neccessary to set up, maintain and tear down the tunnel when it's no longer needed.

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One of the difficulties in building an SQL-like query lange for the Web is the absence of a database schema for this huge, heterogeneous repository of information. However, if we are interested in HTML documents only, we can construct a virtual(66)from the implicit structure of these files. Thus, at the highest level of(67), every such document is identified by its Uniform. Resource Locator(URL), has a title and a text Also, Web servers provide some additional information such as the type, length, and the last modification date of a document. So, for data mining purposes, we can consider the site of all HTML documents as arelation:

 Document(url,(68), text, type, length, modify)

 Where all the(69)are character strings. In this framework, anindividual document is identified with a(70)in this relation. Of course, if some optional information is missing from the HTML document, the associate fields will de left blank, but this is not uncommon in any database.

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