Practical TCP/IP and Ethernet Networking- P31 ppt

132 Practical TCP/IP and Ethernet Networking Checksum: 16 bits This is the 16-bit one’s complement of the one’s complement sum of a pseudo header of information from the IP header, the UDP header, and the data, padded with ‘0’ bytes at the end (if necessary) to make a multiple of two bytes. The pseudo header, conceptually prefixed to the UDP header, contains the source address, the destination address, the protocol, and the UDP length. As in the case of TCP, this header is used for computational purposes only, and is NOT transmitted. This information gives protection against misrouted datagrams. This checksum procedure is the same as is used in TCP. Figure 7.8 UDP pseudo header format If the computed checksum is zero, it is transmitted as all ones (the equivalent in one’s complements arithmetic). An all zero transmitted checksum value means that the transmitter generated no checksum (for debugging or for higher level protocols that don’t care). UDP is numbered protocol 17 (21 octal) when used with the Internet protocol. 8 Application layer protocols Objectives When you have completed study of this chapter you should have a basic understanding of the application and operation of the following application layer protocols: • FTP • TFTP • TELNET • RLOGIN • NFS • DNS • WINS • SNMP • SMTP • POP3 • HTTP • BOOTP • DHCP 8.1 Introduction This chapter examines the process/application layer of the TCP/IP model. Protocols at this layer act as intermediaries between some user application (external to the TCP/IP communication stack) and the lower-level protocols such as TCP or UDP. An example is SMTP, which acts as an interface between an e-mail client or server and TCP. Note that the list of protocols supplied here is by no means complete, as new protocols are developed all the time. Using a developer’s toolkit such as WinSock, software developers can interface their own application protocols at this level to the TCP/IP protocol stack. 134 Practical TCP/IP and Ethernet Networking 8.2 File transfer protocol (FTP) File transfer requires a reliable transport mechanism, and therefore TCP connections are used. The FTP process running on the host that is making the file transfer request is called the FTP client, while the FTP process running on the host that is receiving the request is called the FTP server. The process involved in requesting a file is as follows: • The FTP client opens a control connection to port 21 of the server • The FTP client forwards user name and password to the FTP server for authentication. The server indicates whether authentication was successful • The FTP client sends commands indicating file name, data type, file type, transmission mode and direction of data flow (i.e. to or from the server) to the server. The server indicates whether the transfer options are acceptable • The server establishes another connection for data flow, using port 20 on the server • Data packages are now transferred utilizing the standard TCP flow control, error checking, and retransmission procedures. Data is transferred using the basic NVT format as defined by the TELNET network virtual terminal protocol (no option negotiation is provided for) • When the file has been transferred, the sending FTP process closes the data connection, but retains the control connection The control connection can now be used for another data transfer, or it can be closed 8.2.1 Internal FTP commands These commands are exchanged between the FTP client and FTP server. Each internal protocol command comprises a four-character ASCII sequence terminated by a new-line (<CRLF>) character. Some commands also require parameters. The use of ASCII character sequences for commands allows the user to observe and understand the command flow, and aids the debugging process. The user can communicate directly with the server program by using these codes, but in general this is not advisable. FTP commands can be divided into three categories, namely service commands, transfer parameter commands and access control commands. There is also a series of reply codes. Here follows a brief summary of the commands and reply codes. Service commands These commands define the operation required by the requester. The format of the pathname depends on the specific FTP server being used. RETR<SP><pathname><CRLF> Retrieve a copy of the file from the server STOR<SP><pathname><CRLF> Store data at the server STOU<CRLF> Store unique APPE<SP><pathname><CRLF> Append ALLO<SP><decimal integer> Allocate storage [<SP>R<SP><decimal integer>]<CRLF> REST<SP><marker><SP> Restart transfer at checkpoint RNFR<SP><pathname><CRLF> Rename from RNTO<SP><pathname><CRLF> Rename to ABOR<CRLF> Abort previous service command DELE<SP><pathname><CRLF> Delete file at server Application layer protocols 135 RMD<SP><pathname><CRLF> Remove directory MKD<SP><pathname><CRLF> Make directory PWD<CRLF> Print working directory LIST<SP><pathname><CRLF> List files or text NLST<SP><pathname><CRLF> Name list SITE<SP><string><CRLF> Site parameters SYST<CRLF> Determine operating system STAT<SP><pathname><CRLF> Status HELP[<SP><string>]CRLF Help information NOOP<CRLF> No operation Transfer parameter commands These commands are used to alter the default parameters used to transfer data on an FTP connection. PORT<SP><host-port><CRLF> Specifies the data port to be used. PASV<CRLF> Request server DTP to listen on a data port TYPE<SP><type code><CRLF> Representation type: ASCII, EBCDIC, image, or local. STRU<SP><structure code><CRLF> File structure: file, record or page. MODE<SP><mode code><CRLF> Transmission mode: stream, block or compressed Access control commands These commands are invoked by the server and determine which users may access a particular file. USER<SP><username> <CRLF> User name PASS<SP><password><CRLF> User password ACCT<SP><acc. information><CRLF> User account CWD<SP><pathname><CRLF> Change working directory CDUP<CRLF> Change to parent directory SMNT<SP><pathname><CRLF> Structure mount REIN<CRLF> Terminate user and re-initialize QUIT<CRLF> Logout <SP> Space character <CRLF> Carriage return, line feed characters Reply codes FTP uses a three-digit return code ‘xyz’ followed by a space to indicate transfer conditions. The first digit (value 1–5) indicates whether a response is good, bad or incomplete. The second and third digits are encoded to provide additional information about the reply. The values for the first digit are: Value Description 1yz Action initiated. Expect another reply before sending a new command. 2yz Action completed. Can send a new command. 3yz Command accepted but on hold due to lack of information. 4yz Command not accepted or completed. Temporary error condition exists. Command can be reissued. 5yz Command not accepted or completed. Don’t reissue – reissuing the command will result in the same error. 136 Practical TCP/IP and Ethernet Networking The second digit provides more detail about the condition indicated by the first digit: Value Description X0z Syntax error or illegal command X1z Reply to request for information X2z Reply that refers to connection management X3z Reply for authentication command X5z Reply for status of server The third digit of the reply code also provides further information about the condition, but the meanings vary between implementations. 8.2.2 FTP user commands Although designed for use by applications, FTP software usually also provides interactive access to the user, with a range of commands that can be used to control the FTP session. There are several dozen commands available to the user, but for normal file transfer purposes very few of them ever need to be used. Command Description ASCII Switch to ASCII transfer mode Binary Switch to binary transfer mode Cd Change directory on the server Cdup Change remote working directory to parent directory Close Terminate the data connection Del Delete a file on the server Dir Display the server directory Get Get a file from the server Help Display help Ls List contents of remote directory Lcd Change directory on the client Mget Get several files from the server Mput Send several files to the server Open Connect to a server Put Send a file to the server Pwd Display the current server directory Quote Supply a file transfer protocol (FTP) command directly Quit Terminate the file transfer protocol (FTP) session Trace Display protocol codes Verbose Display all information To execute a command, the user types the commands at the ftp prompt, e.g. ftp>close A list of available user commands can be viewed by typing help at the ftp prompt, e.g. ftp> help close After logging into another machine using FTP, the user is still logically connected to the (local) client machine. This is different to TELNET, where the user is logically connected to the (remote) server machine. References to directories and movements of files are relative to the client machine. For example, getting a file involves moving it from the server to the client; putting a file involves moving it from the client to the server. It may be wise to create a special directory on the client computer just for the transfer of files into and out of the client’s system. This helps guard against accidental file deletion, and allows easier screening of incoming files for viruses. . condition exists. Command can be reissued. 5yz Command not accepted or completed. Don’t reissue – reissuing the command will result in the same error. 136 Practical TCP/IP and Ethernet Networking. (<CRLF>) character. Some commands also require parameters. The use of ASCII character sequences for commands allows the user to observe and understand the command flow, and aids the debugging process this level to the TCP/IP protocol stack. 134 Practical TCP/IP and Ethernet Networking 8.2 File transfer protocol (FTP) File transfer requires a reliable transport mechanism, and therefore TCP