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将某台计算机中的文件通过网络传送到可能相距很远的另一台计算机中,是一项基本的网络应用,即文件传送。文件传送协议FTP
是因特网上使用的最广泛的文件传送协议:
* FTP提供交互式的访问,允许客户指明文件的类型和格式,并允许文件具有存取权限。
* FTP屏蔽了各计算机系统的细节,因而适合于在异构网络中任意计算机之间传送文件。
假设FTP服务器和FTP客户处在同一个网络中,FTP客户可以将文件上传到FTP服务器上,当然FTP客户也可以从FTP服务器中下载文件。根据应用需求不同,FTP服务器可能需要一台高性能、高可靠性的服务器计算机,也可能只需要一台普通的计算机即可。
FTP的常见用途是在计算机之间传输文件,尤其用于批量传输文件。另一个常见用途是让网站设计者将构成网站内容的大批量文件上传到他们的Web服务器。
FTP客户随机选取临时端口号与FTP服务器监听的熟知端口号21建立TCP连接,这个连接用于传送控制命令,在整个会话期间一直保持打开。有数据要传输时,FTP服务器通过自己的熟知端口号20与FTP客户端的随机端口号建立TCP连接,用于文件传输,传输结束后即关闭。
被动模式与主动模式的不同之处在于,有数据要传输时,FTP客户通过命令通道告知FTP服务器开启某个临时通道被动等待TCP连接。