Client-Server Applications

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Client-Server Applications
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Indirect Top Level Interaction
Application
Application
Application
Application
Application
Application
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Client asks server to connect
Application Client
Application Server
Connect request
Connect request
Socket provides a STREAM based interaction
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Server accepts
Application Client
Application Server
Connect accepted
Connect accepted
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Client asks for a specific service
Application Client
Application Server
Service Request
Service Request
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Server answers
Application Client
Application Server
Service Response
Service Response
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Client disconnects
Application Client
Application Server
Disconnect
Disconnect
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Server acknowledges disconnected
Application Client
Application Server
Disconnect Acknowledged
Disconnect Acknowledged
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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

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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

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How do clients and serversidentify each other?
Client
Server

• Each end has
• IP address
• port

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Ftp Exampleserver at 137.155.2.10
Client
Server

• 137.155.2.10
• Port 21

• 137.155.2.23
• Port 12345

Servers run at a well-known port

• Each end has
• IP address
• port

Client use a randomly assigned port
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Addresses

• IP -gt 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

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FTP
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FTP

• A program for copying files from one machine to
  another
• Lets you do a FEW unix-type commands in the
  command-line version

FTP
SERVER
CLIENT
General Exchange of Messages
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Sample client interaction w/ftp
dgame_at_cambria 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.edudgame) anonymous 331 Guest
login ok, send ident as password. Password (your
id) 230 Guest login ok, access restrictions
apply. ftpgt cd pub/dgame/335 250 CWD command
successful. ftpgt binary 200 Type set to I. ftpgt
mget mget AaronsFavCar.bmp? y 200 PORT command
successful. 150 Binary data connection for
AaronsFavCar.bmp (137.155.2.17,33136) (177562
bytes). 226 Binary Transfer complete. local
AaronsFavCar.bmp remote AaronsFavCar.bmp 32768
bytes received in 0.017 seconds (1880.25
Kbytes/s) ftpgt bye 221 Goodbye. dgame_at_cambria
Connect request
Service interaction
Disconnect
Client to Server Server to Client
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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

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USER commands
USER
Client
Server
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Table 8.3 CLIENT commands

• Exchanges that actually go into the socket
• Not what the user enters
• This is the protocol.

From Fig 8.16 Session Connection
USER
ftp ftp.pcs.cnu.edu (1)
Client
SYN (2)
SYNACK (3)
Server
ACK (4)
220 FTP server ready (5)
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Figure 8.16user enters username
USER
anonymous (1)
Client
Server
USER anonymous(2)
331 password required for anonymous(3)
331 password required for anonymous(3)
USER anonymous(2)
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Figure 8.16user enters password
USER
dgame_at_pcs.cnu.edu (1)
Client
220 user anonymous logged in(3)
Server
PASS dgame_at_pcs.cnu.edu (2)
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System OS.. No user input Clients and Servers
interact on their own
USER
Unix System V Release 4.0 (3)
Client
Server
SYST(1)
215 Unix System V Release 4.0 (2)
In the previous example this exchange is done
prior to the login, not after.
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Telnet
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Terminal - little local intelligence

Application
Mainframe
Terminal
NO local processing per se. Only a conduit to the
Mainframe.
Here the PC has the program instead of the
mainframe! In previously graphic, terminal is a
relay.
Application
PC
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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

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Telnet example
telnet cambria_at_pcs.cnu.edu SunOS 5.6 login
dgame Password(.) Last login Tue Jan 25
190704 from cx81733-c.nwptn1 You have new
mail. dgame_at_cambria pwd /home/faculty1/dgame
dgame_at_cambria exit dgame_at_cambria
logout (back on the original machine)
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How is telnet organized?
Computer to Use
Computer now using
Internet
Telnet Server
Telnet Client
Unix Shell
Keyboard
Display and xmit
Unix Shell
Display
Mainframe
Terminal
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Mail

• smtp/imap/pop

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SMTP
SERVER mail.pcs.cnu.edu
SERVER mail.uva.edu
SMTP
POP or IMAP
POP or IMAP
pine
Whatever client the user chooses
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.

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WWW/http
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How to Program

• Sockets