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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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Connection EstablishmentConnection Establishment
TCP Connection
Establishment
Message ProgressMessage Progress
Connection TerminationConnection Termination
TCP Connection Termination
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