TCP IP Illustrated

The file transfer provided by ftp copies a complete

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Unformatted text preview: om client to server using Telnet linemode. If we compare this with the same command typed to Rlogin (<a href="tcp_int.htm file:///D|/Documents%20and%20Settings/bigini/Docum...ti/homenet2run/tcpip/tcp-ip-illustrated/telnet.htm (26 of 26) [12/09/2001 14.47.45] Chapter 27. FTP: File Transfer Protocol FTP: File Transfer Protocol 27.1 Introduction FTP is another commonly used application. It is the Internet standard for file transfer. We must be careful to differentiate between file transfer, which is what FTP provides, and file access, which is provided by applications such as NFS (Sun's Network File System, Chapter 29). The file transfer provided by FTP copies a complete file from one system to another system. To use FTP we need an account to login to on the server, or we need to use it with a server that allows anonymous FTP (which we show an example of in this chapter). Like Telnet, FTP was designed from the start to work between different hosts, running different operating systems, using different file structures, and perhaps different character sets. Telnet, however, achieved heterogeneity by forcing both ends to deal with a single standard: the NVT using 7-bit ASCII. FTP handles all the differences between different systems using a different approach. FTP supports a limited number of file types (ASCII, binary, etc.) and file structures (byte stream or record oriented). RFC 959 [Postel and Reynolds 1985] is the official specification for FTP. This RFC contains a history of the evolution of file transfer over the years. 27.2 FTP Protocol FTP differs from the other applications that we've described because it uses two TCP connections to transfer a file. 1. The control connection is established in the normal client-server fashion. The server does a passive open on the well-known port for FTP (21) and waits for a client connection. The client does an active open to TCP port 21 to establish the control connection. The control connection stays up for the entire time that the client communicates with...
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