Practical TCP/IP and Ethernet Networking- P32 ppt

Application layer protocols 137 Many operating systems have a GUI-based FTP client such as NetManage’s Chameleon NFS that displays the file systems of the local and the remote machines in two separate windows and allows file transfers from one machine to another by mouse movements on the screen. Most UNIX machines act as FTP servers by default. A daemon process watches the TCP command port (21) continuously for the arrival of a request for a connection and calls the necessary FTP processes when one arrives. Windows 95/98 does not include FTP server software, but it does provide an FTP client program. However, a number of third-party FTP packages have been written for use with Windows. Examples of such software are CuteFTP, an FTP client, and Serv-U-FTP server. 8.2.3 Anonymous FTP Anonymous FTP access allows a client to access publicly available files using the login name ‘anonymous’ and the password ‘guest’. Alternatively the password may be required to be a valid e-mail address. Public files are often placed in a separate directory on the server, and are commonly used by Internet sites such as Network Information Systems, Yellow Pages, etc. 8.3 Trivial file transfer protocol (TFTP) 8.3.1 Introduction TFTP (RFC 1350) is a less sophisticated version of FTP, and caters for situations where the complexity of FTP and the reliability of TCP is neither desired nor required. TFTP does not log on to the remote machine; so it does not provide user access and file permission controls. TFTP is used for simple file transfers and is typically placed in the read-only memory of diskless machines such as PLCs that use it for bootstrapping or to load applications. The absence of authorization controls can be overcome by diligent system administration. For example, on a UNIX system, a file may only be transferred if it is accessible to all users on the remote machine (i.e. both read and write permissions are set). TFTP does not monitor the progress of the file transfer so does not need the reliable stream transport service of TCP. Instead, it uses an unreliable packet delivery system such as UDP, using time-out and retransmission mechanisms to ensure data delivery. The UDP source and destination port fields are used to create the socket at each end, and TFTP transfer identifiers (TIDs) ranging between 0 and 65 535 are created by TFTP and passed to UDP to be placed in the UDP header field as a source port number. The destination (server) port number is set to the well-known port 69, which is reserved for TFTP. The server returns an acknowledgment message, upon which the data transfer commences. Data is then relayed in consecutively numbered blocks of 512 bytes. Each block must be acknowledged, using the block number in the message header, before the next block is transmitted. This system is known as a flip-flop protocol. A block of less than 512 bytes indicates the end of the file. A block is assumed lost and re-sent if an acknowledgment is not received within a certain time period. The receiving end of the connection also sets a 138 Practical TCP/IP and Ethernet Networking timer and if the last block to be received was not the end of file block, on time-out the receiver will re-send the last acknowledgment message. TFTP can fail for many reasons and almost any kind of error encountered during the transfer will cause complete failure of the operation. An error message sent either in place of a block of data or as an acknowledgment terminates the interaction between the client and the server. 8.3.2 Frame types There are five TFTP package types, distinguished by an opcode field. They are: Opcode Operation 1 Read request (RRQ) 2 Write request (WRQ) 3 Data (DATA) 4 Acknowledgment (ACK) 5 Error (ERROR) The frames for the respective operations are constructed as follows: RRQ/WRQ frames Figure 8.1 RRQ/WRQ frame format The various fields are as follows: • Opcode: 2 bytes 1 for RRQ, 2 for WRQ • Filename: variable length Written in Netascii, defined by ANSI X3.4-1968. Terminated by a 0 byte. • Mode: variable length Indicates the type of transfer. Terminated by a 0 byte. The three available modes are: • Netascii • Byte – raw 8-bit bytes and binary information • Mail – indicates destination is a user not a file – information transferred as Netascii DATA frames The filename does not need to be included as the IP address and UDP protocol port number of the client are used as identification. Figure 8.2 Data frame format Application layer protocols 139 The fields are as follows: • Opcode: 2 bytes 3 indicates DATA • Block number: 2 bytes The particular 512-byte block within a specific transfer (allocated sequentially) • Data: Variable, 1–512 bytes. Data is transmitted as consecutive 512-byte blocks, a frame with less than 512 bytes means that it is the last block of a particular transfer ACK frames These frames are sent to acknowledge each block that arrives. TFTP uses a ‘lock-step’ method of acknowledgment, which requires each data packet to be acknowledged before the next can be sent. Figure 8.3 ACK frame format The fields are as follows: • Opcode: 2 bytes 4 indicates acknowledgment • Block number: 2 bytes The number of the block being acknowledged Error frames An error message causes termination of the operation. Figure 8.4 Error frame The fields are: • Opcode: 2 bytes 5 indicates an error • Error code: 2 bytes This field contains a code that describes the problem • 0 Not defined • 1 File not found • 2 Access violation • 3 Disk full/allocation exceeded 140 Practical TCP/IP and Ethernet Networking • 4 Illegal operation • 5 Unknown transfer operation • 6 File already exists • 7 No such user • Error message: Variable length string This is Netascii string, terminated by a 0 byte 8.4 TELNET (telecommunications network) TELNET is a simple remote terminal protocol, included in the TCP/IP suite that enables virtual terminal capability across a network. That is, a user on machine A can log in to another machine B across a network without being aware that he is working across a network. Once connected, the user’s computer emulates the remote computer. When the user types in commands, they are executed on the remote computer. The user’s monitor displays what is taking place on the remote computer during the TELNET session. The procedure for connecting to a remote computer depends on how the user’s Internet access is set up. The process is generally menu driven. Some remote machines require the user to have an account on the machine and will request a username and password. However, many information resources are available to the user without an account and password. TELNET achieves a connection via the well known port number 23, using either the server’s domain name or its IP address, and then passes keystrokes to the remote server and receives output back from it. TELNET treats both ends of the connection similarly, so that software at either end of a connection can negotiate the parameters that will control their interaction. It provides a set of options, such as type of character set to be used (7-bit or 8-bit), type of carriage- return character to be recognized (e.g. CR or LF) etc, which can be negotiated to suit the client and the server. It is possible for a machine to act as both client and server simultaneously, enabling the user to log into other machines while other users log into his machine. In the case of a server capable of managing multiple, concurrent connections, TELNET will listen for new requests and then create a new instantiation (or ‘slave’) to deal with each new connection. The TELNET protocol uses the concept of a network virtual terminal (NVT) to define each end of a connection. NVT uses standard 7-bit US ASCII codes to represent printable characters and control codes such as ‘move right one character’, ‘move down one line’, etc. 8-bit bytes with the high order bit set are used for command sequences. Each end has a virtual keyboard that can generate characters (it could represent the user’s keyboard or some other input stream such as a file) and a logical printer that can display characters (usually a terminal screen). The TELNET programs at either end handle the translation from virtual terminal to physical device. As long as this translation is possible, TELNET can interconnect any type of device. When the connection is first established and the virtual terminals are setup, they are provided with codes that indicate which operations the relevant physical devices can support. An operating system usually reserves certain ASCII keystroke sequences for use as control functions. For example, an application running on UNIX operating systems will not receive the Ctrl-C keystroke sequence as input if it has been reserved for interrupting the currently executing program. TELNET must therefore define such control functions Application layer protocols 141 so that they are interpreted correctly at both ends of the connection. In this case, Ctrl-C would be translated into the TELNET IP command code. TELNET does not use ASCII sequences to represent command codes. Rather, it encodes them using an escape sequence. This uses a reserved octet, called the ‘interpret as command’ (IAC) octet, to indicate that the following octet contains a control code. The actual control code can be represented as a decimal number, as follows: Command Decimal Value Meaning EOR 239 End of record SE 240 End of option sub-negotiation NOP 241 No operation DMARK 242 Data mark – the data stream part of a SYNCH (always marked by TCP as urgent) BRK 243 Break IP 244 Interrupt process – interrupts or terminates the active process AO 245 Abort output – allows the process to run until completion, but does not send the end of record command AYT 246 Are you there – used to check that an application is functioning at the other end EC 247 Erases a character in the output stream EL 248 Erases a line in the output stream GA 249 Go ahead – indicates permission to proceed when using half-duplex (no echo) communications SB 250 Start of option sub-negotiation WILL 251 Agreement to perform the specified option or confirmation that the specified option is now being performed WON’T 252 Refusal to perform the specified option or confirmation that the specified option will no longer be performed DO 253 Asks for the other end to perform the specified option, or acknowledges that the other end will perform the specified option DON’T 254 Demand that the other end stops performing the specified option, or confirmation that the other end is no longer performing the specified option IAC 255 Interpret as command – interpret the next octet as a command. When the IAC octet appears as data the 2-octet sequence that is sent will be IAC-IAC The IAC character to have the above meanings must precede the control code. For example, the two-octet sequence IAC-IP (or 255-244) would induce the server to abort the currently executing program. The following command options are used by TELNET: Option Code Meaning 0 Transmit binary – change transmission to 8-bit binary 1 Echo . lost and re-sent if an acknowledgment is not received within a certain time period. The receiving end of the connection also sets a 138 Practical TCP/IP and Ethernet Networking timer and if. defined • 1 File not found • 2 Access violation • 3 Disk full/allocation exceeded 140 Practical TCP/IP and Ethernet Networking • 4 Illegal operation • 5 Unknown transfer operation • 6 File. such as UDP, using time-out and retransmission mechanisms to ensure data delivery. The UDP source and destination port fields are used to create the socket at each end, and TFTP transfer identifiers