HTTP是WWW的核心，它是一个(31)协议，当访问一个URL为http：// www.ccidedu.com.cn/index.htm的网页时，浏览器首先向(32)请求解析http：// www.ccidedu.com.cn的IP地址。获得解析后的IP地址后，浏览器通过(33)端口与服务器建立(34)连接。随后浏览器发送取文件命令(35)。服务器响应并将文件index.htm发送给浏览器，最后释放连接。

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SDLC was invented by IBM to replace the older Bisynchronous protocol for wide area connections between IBM equipment. A varietion of the(71)protocol called NRM is essentially the same as SDLC.

 SDLC is(72)protocol like HDLC, Frame. Relay or X.25.An SDLC network is made up of(73)that controls all communications, and(74).It is capable of(75).operation, but almost all practical applications are strictly half duplex.

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Frame. Relay is simplified form. of(66), similar in principle to(67), in which synchronous, frames of data are routed to different destinations depending on header information.

 Packets are routed throught one or more Virtual Circuits known as(68). Most Virtual Circuits are(69), which means that the network provider sets up all DLCI connections at subscription time.(70)are also part of the Frame. Relay specification. They privide a link that only lasts only as long as the session.