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class7 - 1 CSCI 233 Internet Protocols Class 7 David C....

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Unformatted text preview: 1 CSCI 233 Internet Protocols Class 7 David C. Roberts Copyright 2006, David C. Roberts, all rights reserved 2 Agenda • Client-Server • The Socket Interface • Bootstrap and autoconfiguration • The Domain Name System • TELNET • Secure Shell 3 Client-Server • Server—any program that offers a service that can be reached over a network – Server accepts the request – Performs the service – Returns the result • Client—a program that sends a request to a server and waits for a response 4 Servers • Typically implemented as application programs • Allows flexibility in where they run, how many are run • Servers usually do not terminate; clients usually terminate after some number of server interactions • Servers must enforce access and protection policies, are generally harder to build than application programs 5 UDP Echo Server • Server starts, uses port 7 • Server enters loop – Wait for request datagram at port 7 – Reverse source and destination addresses, and port IDs – Return datagram to its sender • Server starts, continues to provide service without terminating • Client makes a request, awaits a response, usually terminates after using a server some number of times. Question: Who would use such a server? When? 6 An Example: UDP Echo Server 7 RARP Server • An unusual server • Does not use IP; communicates at the physical level • Cannot be an application program, because it needs access to physical level communications 8 Server Complexity • Servers generally don’t wait to complete one request before starting another • They start a separate copy of themselves for each request • This concurrency typically makes servers much more complex to program than clients 9 Summary • Distributed programs often interact as client and server • Server may be application program; or not • Server may use IP communications; or it may not • Client-server communication may be directly addressed between two machines, or it may be broadcast 10 The Socket Abstraction • Socket—a mechanism that provides an endpoint for communication • Program asks that a socket be created, and the system provides a socket number • For TCP, both endpoints must be specified • For UDP, remote endpoint can be passed as an argument with each message 11 Application Interface • How programs interface with TCP/IP • APIs are not specified in protocol standards • We won’t deal with the definition, either • Socket used in Berkeley UNIX is the model for Windows Socket (WinSock) • To a program, socket I/O looks much like file I/O 12 Open—Read—Write—Close • Open – Specifies file or device, obtain permission – Returns integer file descriptor • Read – Transfers data into user process • Write – Transfers data from user process to file or device • Close – Informs operating system that the program is finished using the object that was opened 13...
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This note was uploaded on 09/03/2010 for the course CS 233 taught by Professor Davidc.roberts during the Fall '10 term at GWU.

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class7 - 1 CSCI 233 Internet Protocols Class 7 David C....

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