Lecture 17-DNS,Email, Telnet, FTP, SNMP

8/2/2019 Lecture 17-DNS,Email, Telnet, FTP, SNMP

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Dept. of EEE CS65-Computer Netowrk

S.Muralidharan 1

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Why DNS?Why DNS?

To identify an entity, TCP/IP uses IP address, which

uniquely identify the connection of a host to the

internet. But remembering a name is more

convenient than numbers. Hence a system is

needed to map the names into numbers. In TCP/IP,

it is taken care by Domain Name System(DNS)

To have a hierarchical name space, a domain name

space was designed. In this design the names are

defined in an inverted-tree structure with theroot at the top. The tree can have only 128 levels:

level 0 (root) to level 127.

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DNS in InternetDNS in Internet

In internet, the Domain Name Space is divided

into three : generic domains, country domains and

inverse domains

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Generic Domains :

Define registered hosts according to their generic

behavior. Each node in the tree defines a domain, which

is an index to the domain name space data base.

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The first level in the generic domain section allows

seven possible three character labels. These labels

describe the organization types.

Label Description

co m Co mmer cial o rganizatio n

ed u Ed ucatio nal institu tio n

gov Go vernm ent institutio n

int Inter natio na l o rganization

mil Military groups

net Network support centers

o rg No npr ofit o rganizations

Label Description

Arts Cultural organization

Firm Businesses or firms

Info Informat ion se rv ice providers

Nom Personal nomenclatures

Rec Recreat ion / entertainment organizat ion

Store Businesses offering goods to purchase

We b Web- relate d organizatio ns

Existing Generic

domain labels

Proposed Generic

domain labels

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Country Domains

Uses two-character country name abbreviations.

Second levels can be organizational or more specific

When referring De AnzaCollege in Cupertino in

California in the US

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Inverse Domain

Used to map an address to a name.

This will happen when a serverwants to verify the authentication

of a clients identity with its list of

authorized clients list.

For instance, if a server receives a

request from a client and this server has

only the ip addresses of the clients in its

list then the server needs to find out if

this client is on its authorized client list.

In order to determine if the client is on

the authorized client list,server asks itsresolver to query to the DNS server to

map an address to name.

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Domain names and labelsDomain names and labels

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Name servers

The Domain Name System is maintained by

a distributed database system, which uses

the client-server model. The nodes of this

database are the name servers. Each domain hasat least one authoritative DNS server that

publishes information about that domain and the

name servers of any domains subordinate to it.

The top of the hierarchy is served by the root

name servers, the servers to query when looking

up (resolving) a Top Level Domain.


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Working of DNS ServerWorking of DNS Server

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TelnetTelnet

Users of internet wants to run several application

programs in a remote site and create results that

can be transferred to their local site.

But it would be impossible to write a specific

client-server programs for each demand. Hence a general purpose client-server program is

needed which lets the user to access any

application program on remote computer.

TELNET (TErminaL NETwork) is a general-

purpose client/server application program.

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Local LoginLocal Login When a user logs into a local time sharing system it is

called local login.

As the user types at a terminal, the key strokes are

accepted by the terminal driver and passed it to

operating system. The operating system, in turn,

interprets the combination of characters and invokesthe desired application program or utility.

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Remote LoginRemote Login When a user wants to access an application program or utility

located on a remote machine, then it requires remote login.

The user sends the keystroke to the terminal driver where the

local operating system accepts the characters but does not

interpret them.

The characters are sent to the TELNET client, which transforms

the characters to a universal character set called network terminal

characters and delivers them to the local TCP/IP stack.

These commands or text, in Network Virtual Terminal (N VT) form,

travel through the internet and arrive at the TCP/IP stack at the

remote machine.

Here the characters are delivered to the operating system andpassed to the TELNET server, which changes the characters to the

corresponding characters understandable by the remote computer.

Since the remote operating system may not be designed to receive

the characters from TELNET server, it is designed to receive the

characters from a pseudo terminal driver, which pretends that

the characters are coming from its own terminal. 17

Remote logRemote log--inin

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Concept of NVTConcept of NVT

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Some NVT control charactersSome NVT control characters

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File Transfer Protocol (FTP)File Transfer Protocol (FTP)

FTP is a mechanism provided by TCP/IP for

copying a file from one host to another.

FTP helps to resolve the problems like

When two hosts follow different file name conventions

When two hosts follow different ways to represent text

and data.

When two systems have different directory structures.

FTP establishes two connections between the

hosts : one for data transfer and other for controlinformation(commands and responses).

Trivial File Transfer Protocol (TFTP) is designed to

copy a file without the need for all of the

functionalities of the FTP protocol. 22

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Electronic MailElectronic Mail

One of the most popular Internet services isOne of the most popular Internet services is

electronic mail (eelectronic mail (e--mail). TCP/IP protocol thatmail). TCP/IP protocol that

supports electronic mail on the internet is calledsupports electronic mail on the internet is called

Simple Mail Transfer Protocol (SMTP).Simple Mail Transfer Protocol (SMTP).

SMTP provides mail exchange between user onSMTP provides mail exchange between user on

the same or different computers and supports :the same or different computers and supports :

Sending a single message to one or more recipientsSending a single message to one or more recipients

Sending message that include text, voice, video orSending message that include text, voice, video or

graphicsgraphics Sending message to users on networks outside theSending message to users on networks outside the

internet.internet.

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User Agent(UA)User Agent(UA) User Agent (UA) is a software program used to

send and receive mail.

Some examples of command-driven user agents

are mail, pine, and elm.

Some examples of GUI-based user agents areEudora,Outlook, and Netscape.

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SMTPSMTP

SMTP clients andservers have twomain components

User AgentsPrepares the message,

encloses it in anenvelope. (Eudora forexample)

Mail Transfer Agent(MTA) Transfers themail across theinternet

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EE--mail addressmail address

To deliver a mail, mail handling system must use a

unique addressing system. SMTP addressing

consists of the following two parts.

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Mail Transfer Agent(MTA)Mail Transfer Agent(MTA)

Mail exchange takes place via MTA

To send a mail, a system must have a client MTA,

and to receive a mail, a system must have a server

MTA.

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First scenario in electronic mailFirst scenario in electronic mail

When the sender and the receiver of an e-mail are

on the same system, we need only two user

agents

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Second scenario in electronic mailSecond scenario in electronic mail

When the sender and the receiver of an e-mail are

on different systems, we need two UAs and a pair

of MTAs (client and server).

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Third scenario in electronic mailThird scenario in electronic mail

When the sender is connected to the mail server

via a LAN or a WAN,

we need two UAs and two pairs

of MTAs (client and server).

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Fourth scenario in electronic mailFourth scenario in electronic mailWhen both sender and receiver are connected to

the mail server via a LAN or a WAN, we needtwo UAs, two pairs of MTAs and a pair of MAAs.

This is the most common situation today.

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Format of an eFormat of an e--mailmail

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SMTP rangeSMTP range

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Dept of EEE CS65 Computer Netowrk


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Connection EstablishmentConnection Establishment

TCP Connection

Establishment

Message ProgressMessage Progress

Connection TerminationConnection Termination

TCP Connection Termination

Lecture 17-DNS,Email, Telnet, FTP, SNMP

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