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Client-Server Applications. Indirect Top Level Interaction. Application. Application. Application. Application. Application. Application. Client asks server to connect. Application Client. Application Server. Connect request. Connect request.

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indirect top level interaction
Indirect Top Level Interaction

Application

Application

Application

Application

Application

Application

client asks server to connect
Client asks server to connect

Application

Client

Application

Server

Connect request

Connect request

Socket provides a STREAM based interaction

slide4

Server accepts

Application

Client

Application

Server

Connect accepted

Connect accepted

slide5

Client asks for a specific service

Application

Client

Application

Server

Service Request

Service Request

slide6

Server answers

Application

Client

Application

Server

Service Response

Service Response

client disconnects
Client disconnects

Application

Client

Application

Server

Disconnect

Disconnect

slide8

Server acknowledges

… disconnected

Application

Client

Application

Server

Disconnect Acknowledged

Disconnect Acknowledged

most interactions
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
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
How do clients and serversidentify each other?

Client

Server

  • Each end has
  • IP address
  • port
ftp example server at 137 155 2 10
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

addresses
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
slide15

connect

request login

login

request file

send file

logoff

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

sample client interaction w ftp
Sample client interaction w/ftp

{dgame@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.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/335

250 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 (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)

ftp> bye

221 Goodbye.

{dgame@cambria}

Connect request

Service interaction

Disconnect

Client to ServerServer to Client

slide17

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
USER commands

USER

Client

Server

table 8 3 client commands
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)

SYN+ACK (3)

Server

ACK (4)

220 FTP server ready (5)

figure 8 16 user enters username
Figure 8.16user enters username

USER

anonymous (1)

Client

USER anonymous(2)

331 password required

for anonymous(3)

Server

331 password required

for anonymous(3)

USER anonymous(2)

slide21

Figure 8.16user enters password

USER

dgame@pcs.cnu.edu (1)

Client

220 user anonymous

logged in(3)

Server

PASS dgame@pcs.cnu.edu (2)

slide22

System OS.. No user input

Clients and Servers interact on their own

USER

Unix System V Release 4.0 (3)

Client

SYST(1)

215 Unix System V Release 4.0 (2)

Server

In the previous example this exchange is done prior to the login, not after.

terminal little local intelligence

Display

Keyboard

(Display and) xmit

Display

Keyboard

Terminal - little local intelligence

Application

Mainframe

Terminal

NO local processing per se.

Only a conduit to the Mainframe.

Application

Here the PC has the program instead

of the mainframe!

In previously graphic, terminal is a relay.

PC

remote terminal
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 example

telnet cambria@pcs.cnu.edu

SunOS 5.6

login: dgame

Password:(….)

Last login: Tue Jan 25 19:07:04 from cx81733-c.nwptn1

You have new mail.

{dgame@cambria:~ } pwd

/home/faculty1/dgame

{dgame@cambria:~ }exit

{dgame@cambria:~ }logout

(back on the original machine)

how is telnet organized
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

slide28

Mail

smtp/imap/pop

slide29
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.