HDLC协议是一个(22)协议，在全双工工作方式中，通过捎带应答减少通信量。若双方地址用X、Y表示，则当X发送了连续2个信息帧＜Y,100,P＞＜Y,110＞，X收到的帧可能是(23)或者(24)，当HDLC的数据中出现与控制字节相同的二进制码时，采取的措施是(25)，x.25中用到的数据链路层协议与HDLC相比，(26)。

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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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