tailieunhanh - Internetworking with TCP/IP- P57

Internetworking with TCP/IP- P57: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. Simple Mail Transfer Protocol SMTP 519 SMTP is surprisingly straightforward. Communication between a client and server consists of readable ASCH text Although SMTP rigidly defines the command format humans can easily read a transcript of interactions between a client and server. Initially the client establishes a reliable stream connection to the server and waits for the server to send a 220 READY FOR MAIL message. If the server is overloaded it may delay sending the 220 message temporarily. Upon receipt of the 220 message the client sends a HELO command. The end of a line marks the end of a command. The server responds by identifying itself. Once communication has been established the sender can transmit one or more mail messages terminate the connection or request the server to exchange the roles of sender and receiver so messages can flow in the opposite direction. The receiver must acknowledge each message. It can also abort the entire connection or abort the current message transfer. Mail transactions begin with a MAIL command that gives the sender identification as well as a FROM field that contains the address to which errors should be reported. A recipient prepares its data structures to receive a new mail message and replies to a MAIL command by sending the response 250. Response 250 means that all is well. The full response consists of the text 250 OK. As with other application protocols programs read the abbreviated commands and 3-digit numbers at the beginning of lines the remaining text is intended to help humans debug mail software. After a successful MAIL command the sender issues a series of RCPT commands that identify recipients of the mail message. The receiver must acknowledge each RCPT command by sending 250 OK or by sending the error message 550 No such user here. After all RCPT commands have been acknowledged the sender issues a DATA command. In essence a DATA command informs the receiver that the sender is ready to transfer a .

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