Client-Server Applications

Indirect Top Level Interaction Application

Client asks server to connect Application Client Application Server Socket provides a STREAM based interaction Connect request

Server accepts Application Client Application Server Connect accepted

Client asks for a specific service Application Client Application Server Service Request

Server answers Application Client Application Server Service Response

Client disconnects Application Client Application Server Disconnect

Server acknowledges … disconnected Application Client Application Server Disconnect Acknowledged

Most interactions Connection request Service interaction Disconnect Give an illusion of looking like a single program when running effectively –Network/server delay is frequently obvious

Common CLIENT-SERVERs FTP for transferring files from one machine to another (even if no account at both machines). TELNET/SSH for providing remote command-line (shell) access when you have an account on both machines. TELNET also will provide other functionality as we will see later. MAIL(smtp/pop/imap) transfer agents WWW(http) (WorldWideWeb) servers/browsers

How do clients and servers identify each other? ClientServer Each end has 1.IP address 2.port

Ftp Example server at ClientServer Each end has 1.IP address 2.port Port Port Servers run at a well-known port Client use a randomly assigned port

Addresses IP -> unique to machine (actually to the interface/network card) –Only gets the message to the right machine Port –Service uses a standard port for connection –Clients are randomly allocated –A port is unique to a connection. –Must identify a connection for a specific application. –Makes sure the correct application receives the data

FTP

A program for copying files from one machine to another Lets you do a FEW unix-type commands in the command-line version CLIENT SERVER connect request login login request file send file logoff General Exchange of Messages FTP

Sample client interaction w/ftp ftp ftp.pcs.cnu.edu Connected to ftp.pcs.cnu.edu. 220 america3 FTP server (UNIX(r) System V Release 4.0) ready. Name (ftp.pcs.cnu.edu:dgame): anonymous 331 Guest login ok, send ident as password. Password: (your id) 230 Guest login ok, access restrictions apply. ftp> cd pub/dgame/ CWD command successful. ftp> binary 200 Type set to I. ftp> mget * mget AaronsFavCar.bmp? y 200 PORT command successful. 150 Binary data connection for AaronsFavCar.bmp ( ,33136) ( bytes). 226 Binary Transfer complete. local: AaronsFavCar.bmp remote: AaronsFavCar.bmp bytes received in seconds ( Kbytes/s) ftp> bye 221 Goodbye. Client to Server Server to Client Connect request Service interaction Disconnect

USER Commands to use (not CLIENT) Not like a shell, but a few of the commands –cd –close –pwd –lcd (change on the client side!) –binary –get –put –mget –prompt

USER commands Client Server USER

Exchanges that actually go into the socket Not what the user enters This is the protocol. Table 8.3 CLIENT commands Client Server USER ftp ftp.pcs.cnu.edu (1)ftp.pcs.cnu.edu SYN (2)SYN+ACK (3) ACK (4)220 FTP server ready (5) From Fig 8.16 Session Connection

Figure 8.16 user enters username Client Server USER anonymous (1) USER anonymous(2)331 password required for anonymous(3) USER anonymous(2) 331 password required for anonymous(3)

Figure 8.16 user enters password Client Server USER (1) PASS (2) 220 user anonymous logged in(3)

System OS.. No user input Clients and Servers interact on their own Client Server USER SYST(1) 215 Unix System V Release 4.0 (2) Unix System V Release 4.0 (3) In the previous example this exchange is done prior to the login, not after.

Telnet

Terminal - little local intelligence Terminal NO local processing per se. Only a conduit to the Mainframe. Keyboard Here the PC has the program instead of the mainframe! In previously graphic, terminal is a relay. Mainframe Application Keyboard (Display and) xmit PC Application Display

Remote terminal Run program remotely Like a long cable to a keyboard at your house Hyperterminal on Windows 95/98 Typically requires a modem Only run command shell, not windows apps. What if you want a terminal access through the internet? Telnet is your answer

Telnet example telnet SunOS 5.6 login: dgame Password:(….) Last login: Tue Jan 25 19:07:04 from cx81733-c.nwptn1 You have new mail. } pwd /home/faculty1/dgame }exit }logout (back on the original machine)

How is telnet organized? Computer to Use Telnet Server Computer now using Telnet Client Internet Keyboard Terminal Display and xmit Display Mainframe Unix Shell

Mail smtp/imap/pop

SMTP SERVER mail.pcs.cnu.edu SERVER mail.uva.edu pine Whatever client the user chooses POP or IMAP SMTP Here a local server and remote server in addition to a remote and local client. For SMTP, the servers also function as clients depending on which “server” makes the request.

WWW/http

How to Program Sockets