Client Server Aplication

8/11/2019 Client Server Aplication

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Client Server Concepts,

DNS, Telnet, FTP

Prof. Indranil Sen Gupta

Dept. of Computer Science & Engg.

I.I.T. Kharagpur, INDIA

Indian Institute of Technology Kharagpur

Lecture 9: Client server concepts,DNS, Telnet, FTP

On completion, the student wil l be able to:

1. Explain how the client-server concept works.

2. Explain and illustrate the name resolution

process used by DNS servers.

3. Explain how the Telnet and FTP protocol works.4. Demonstrate usage of Telnet and FTP

commands.


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Client-server Model

Standard model for developing networkapplications.

Notion of client and server.

A server is a process that is offer ing some

service.

A cl ient is a process that is requesting the

service.

Server or client may be running ondifferent machines.

Server waits for requests from cl ient(s).

CLIENT

CLIENT

CLIENT

SERVER

Client-server Model (contd.)


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Typical scenario:

The server process starts on some

computer system.

Initializes itself, then goes to sleep

waiting for a client request.

A cl ient process starts , either on the

same system or on some other system.

Sends a request to the server.

When the server process has finished

provid ing its service to the client, the

server goes back to sleep, waiting for

the next client request to arrive.

The process repeats.


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Roles of the client and the serverprocesses are asymmetric.

Two types of servers:

Iterative servers.

Concurrent servers.

Iterative Servers

Used when the server process knowsin advance how long it takes to

handle each request and it handles

each request itself.

Single copy of server runs at all times.

A cl ient may have to wait i f the server is

busy.


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Develop a Network Application

The best way is to use somestandard and well-accepted protocol.

At the data link layer level, use Ethernet.

At the network layer level, use IP.

At the transport layer level, use TCP.

At the application layer level, use a

standard API like the Berkeley Socket

Interface.

What is a Socket?

The socket is the BSD method forachieving inter-process communication

(IPC).

It is used to allow one process to speakto another (on same or different

machine).

Analogy: Like the telephone is used toallow one person to speak to another.


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Basic Idea

When two processes located on twomachines communicate, we define

association and socket.

Association: basically a 5-tuple

Protocol

Local IP address

Local port number

Remote IP addressRemote port number

Socket: also called half-association

(a 3-tuple)

Protocol, local IP address, local port

number

Protocol, remote IP address, remote

port number


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Domain Name System (DNS)

What is DNS?

The global database system forInternet addressing, mail and other

information.

Much easier to use and memorize.

Concept of domains and sub-domains.

Domain management is distributed.

DNS servers translate domain names to IP

addresses.


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Top Level Domains

com Commercial

org Non-profit

net Network serviceprovider

gov US govt.

mil military

edu Education

au Australian

at Austrian

ca Canadian

dk Dutch

f r French

de German

in Indian

it Italian

jp Japanese

kr Korean

nz New Zealand

es Spanish

tw Taiwanese uk British or Irish

us U.S.

Domain Name Structure

Domain names are arranged in a

hierarchical tree-like structure.

in

ernet

iitkgp

iitb

ece

cse

math

cse

mech

nicEg: cse.iitkgp.ernet.in


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Some Statistics

2507002503001,7002,4001995

1751251002501,0751,2001994

7525301507007001993

5010201004503501992

50520752502001991

ORGNETMILGOVEDUCOMYear

In thousands

Name Resolution Process

The commonly used server is BIND(Berkeley Internal Name Domain).

Runs under UNIX as a process called

named.

When an application needs someinformation from the server, it invokes

the DNS name resolver.DNS translates a fully qualif ied domain

name into the cor responding IP address.

Using the command nslookup.


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TELNET Protocol

TELNET

Allows a person sitting on onecomputer to work on another computer.

Starts a remote session on anothermachine.

Requires a person to supply login nameand password to gain entry.

Command:

telnet

telnet

A B


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TELNET (contd.)

Why required?

For using software only available on the

remote host.

For using devices (like printer) connected

to the remote host.

Typical scenario

Many users do a telnet to a remote server,

and work there.Server is a bigger and faster computer.

TELNET (contd.)

By default connection is establishedover port 23.

Any other port number can also bespecified.

telnet cse.iitkgp.ernet.in 25


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A Sample TELNET Session

SUNNY>> telnet cseTrying 144.16.192.57...Connected to cse.Escape character is '^]'.Linux 1.2.8 (cse) (ttyp0)

cse login: isgPassword:Last login: Wed Jun 11 12:17:55 from144.16.202.210

Linux 1.2.8. (POSIX).You have new mail.cse:~$

FTP Protocol


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File Transfer Protocol (FTP)

Allows a person to transfer filesbetween two machines.

Requires a person to supply loginname and password to gain entry.

Command:

ftp

ftp

Anonymous FTP

A special form of FTP which hasbecome very popular.

Does not require a person to know login

name and password.

In place of login name, type the word

anonymous.

In place of password, type the emailaddress.

Huge amount of resources areavailable in anonymous FTP sites.


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Typical FTP Commands

put, mputSend one (or more) file(s).

get, mgetReceive one (or more) file(s).

hash Indicates progress of file transfer.

promptPrompts user before actual transfer of each fi le.

bin, ascSelects binary or ascii mode of transfer.

exitTerminates connection with f tp server.

Sample FTP Sessions

SUNNY>> ftp cseindigoConnected to cseindigo.220 cseindigo FTP server ready.Name (cseindigo:isg): isg331 Password required for isg.Password:230 User isg logged in.

Remote system type is UNIX.Using binary mode to transfer files.ftp>


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Sample FTP Sessions (contd.)

ftp> dir200 PORT command successful.150 Opening ASCII mode data connection for '/bin/ls'.total 97drwxr-xr-x 2 isg faculty 512 Oct 8 1995 PROJECTS-rw-r--r-- 1 isg faculty 7693 Mar 31 1995 lan-cse.fig-rw-r--r-- 1 isg faculty 6578 Apr 26 1995 lan-cse.fig-rw-r--r-- 1 isg faculty 6721 Jul 12 1995 newlan.fig226 Transfer complete.

Sample FTP Sessions (contd.)

ftp> get lan-cse.fig200 PORT command successful.150 Opening BINARY mode data connection for 'lan-cse.fig'(6721 bytes).226 Transfer complete.6721 bytes received in 0.0389 secs (1.7e+02 Kbytes/sec)

ftp> quit

221 Goodbye.

SUNNY>>


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SOLUTIONS TO QUIZ

QUESTIONS ON

LECTURE 8


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Quiz Solutions on Lecture 8

1. What are the four types of BGP messages?

OPEN, UPDATE, KEEPALIVE and

NOTIFICATION.

2. How is a BGP connection between two

routers maintained?

By periodically sending the KEEPALIVE

message.


3. What are the main functional procedures inBGP?

Neighbor acquisition, networkacquisition, and network reachability.

4. Why is IPv4 not suitable for supportingreal-time traffic?

IPv4 does not guarantee packets tofol low the same path. This leads tounequal delays and out-of-orderdelivery.


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5. How are the (multiple) headers in a IPv6

packet kept track of?

Al l extension headers after the

mandatory base header are chained

in a linked list.

6. How does IPv6 attempt to support real-

time traffic?

IPv6 allows a pair of stations to

establish a path before sending

packets.


7. What does the Payload length field inthe IPv6 header signi fy?

Total length of the extension headersand the transport level protocol dataunit (PDU).

8. What does the Next Header field in the

IPv6 header s ignify?

Type of information that immediatelyfollows the current header.


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9. In IPv6, how are the fragments kept track

of, in case a packet is fragmented?

Base header is copied into all the

fragments. Presence of fragment

headers in all the fragments.

10. What is anycast addressing?

Packet will be delivered to exactly oneof a set of computers.


11. How can we use tunneling to send a IPv6

packet over a IPv4 network?

The enti re IPv6 packet can be

encapsulated as payload inside an

INv4 packet.


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QUIZ QUESTIONS ON

LECTURE 9

Quiz Questions on Lecture 9

1. What is an iterative server?

2. What is a concurrent server?

3. Which of client or server must start first?

4. What are the components of a socket?

5. What is a domain?

6. What are named and nslookup?

7. How does recursive name resolutionworks?


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Client Server Aplication