tyt06fi.htm

快速学习TCP/IP协议

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<P>18

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<P>Logout

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<P>19

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<P>Bytes macro

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<P>20

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<P>Data entry terminal

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<P>21

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<P>SUPDUP

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<P>22

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<P>SUPDUP output

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<P>23

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<P>Send location

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<P>24

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<P>Terminal type

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<P>25

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<P>End of Record

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<P>26

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<P>TACACS user identification

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<P>27

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<P>Output marking

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<P>28

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<P>Terminal location number

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<P>29

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<P>3270 regime

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<P>30

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<P>X.3 PAD (Packet assembly and disassembly)

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<P>31

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<P>Window size</FONT>

</TABLE></CENTER><BR>

<P>If you refer to the previous code listing with the options toggled on, some of the commands can be understood more clearly now. For example, will ECHO (which would be transmitted as values 255 251 1) instructs the other end to begin echoing back characters it receives. The command won't ECHO (the command would be 255 252 1) indicates that the sender will not echo back characters or wants to stop echoing.

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>The use of ASCII characters and small tables of commands and options make it relatively easy to follow Telnet communications.</NOTE>

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<FONT SIZE=4 COLOR="#FF0000"><B>TN3270</B></FONT></CENTER></H4>

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<P>Many mainframes use EBCDIC, whereas most smaller machines rely on ASCII. This can cause a problem when trying to Telnet from EBCDIC-based machines to ASCII-based machines and vice-versa, because the codes being transferred are not accurate. To correct this, a Telnet application called TN3270 was developed, which provides translation between the two formats.

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<P>When TN3270 is used to connect between two machines, Telnet itself establishes the initial connection, and then one end sets itself up for translation. If an ASCII machine is calling an EBCDIC machine, the translation between the two formats is conducted at the EBCDIC (server) end unless there is a gateway between them, in which case the gateway can perform the translation.

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<P>Many TCP/IP application suites that include a Telnet program also include a TN3270 program. For example, Figure 6.6 shows a TN3270 window from the NetManage ChameleonNFS suite in the process of connecting to a mainframe EBCDIC-based machine. The mainframe's IP address is used to initiate the connection.

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<P><B><A HREF="06tyt06.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/06tyt06.gif">Figure 6.6. TN3270 provides conversion between </B><B>ASCII and EBCDIC.</A></B>

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<FONT SIZE=5 COLOR="#FF0000"><B>File Transfer Protocol (FTP)</B></FONT></CENTER></H3>

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<P><I>File Transfer Protocol,</I> usually called FTP, is a utility for managing files across machines without having to establish a remote session with Telnet. FTP enables you to transfer files back and forth, manage directories, and access electronic mail. FTP is not designed to enable access to another machine to execute programs, but it is the best utility for file transfers.

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<P>FTP uses two TCP channels. TCP port 20 is the data channel, and port 21 is the command channel. FTP is different from most other TCP/IP application programs in that it does use two channels, enabling simultaneous transfer of FTP commands and data. It also differs in one other important aspect: FTP conducts all file transfers in the foreground, instead of the background. In other words, FTP does not use spoolers or queues, so you are watching the transfer process in real time. By using TCP, FTP eliminates the need to worry about reliability or connection management, because FTP can rely on TCP to perform these functions properly.

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<P>In FTP parlance, the two channels that exist between the two machines are called the <I>protocol interpreter, </I>or PI, and the <I>data transfer </I><I>process,</I> or DTP. The PI transfers instructions between the two implementations using TCP command channel 21, and the DTP transfers data on TCP data channel 20. This is shown in Figure 6.7.

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<P><B><A HREF="06tyt07.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/06tyt07.gif">Figure 6.7. FTP channel connections.</A></B>

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<P>FTP is similar to Telnet in that it uses a server program that runs continuously and a separate program that is executed on the client. On UNIX systems, these programs are named ftpd and ftp, respectively (similar to telnetd and telnet).

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<FONT SIZE=4 COLOR="#FF0000"><B>FTP Commands</B></FONT></CENTER></H4>

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<P>Before looking at how you can use FTP to transfer files, you should look at the commands behind the protocol itself. As with Telnet's commands, these are for the protocol's use only and should not be used by a user (although administrators sometimes use the FTP commands for debugging and diagnostic purposes).

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<P>FTP's internal protocol commands are four-character ASCII sequences terminated by a newline character. Some of the codes require parameters after them. One primary advantage to using ASCII characters for commands is that a user can observe the command flow and understand it easily. This helps considerably in the debugging process. Also, it enables a knowledgeable user to communicate directly with the FTP server component (ftpd).

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<P>FTP commands used by the protocol are summarized in Table 6.3. These commands provide for the connection process, password checking, and the actual file transfers. These are not to be confused with the commands available to a user.

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<FONT COLOR="#000080"><B>Table 6.3. FTP internal commands.</B></FONT></CENTER>

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<CENTER><TABLE  BORDERCOLOR=#000040 BORDER=1 CELLSPACING=2 CELLPADDING=3 >

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<P><B><I>Command</I></B>

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<P><B><I>Description</I></B>

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<P>ABOR

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<P>Abort previous command

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<P>ACCT

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<P>User account ID

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<P>ALLO

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<P>Allocate storage for forthcoming operation

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<P>APPE

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<P>Append incoming data to an existing file

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<P>CDUP

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<P>Change to parent directory

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<P>CWD

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<P>Change working directory

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<P>DELE

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<P>Delete file

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<P>HELP

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<P>Retrieve information

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<P>LIST

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<P>Transfer list of directories

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<P>MKD

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<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Make a directory

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<P>MODE

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<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Set transfer mode

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<P>NLST

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<P>Transfer a directory listing

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<P>NOOP

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<P>No operation

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<P>PASS

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<P>User password

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