• Computer Network is a system
which allows communication
among different computers
• It is used for resource sharing, to
save money, to provide reliability,
provides a powerful
• A system with one control unit
(master computer) and many
slaves is not called a computer
network, it is called distributed
system.• Types of transmission technology
• Broadcast networks : In broadcast
networks all machines on the network
use or share single communication
• Point-to-point networks : In point to
point networks there exits many
connections between individual pairs
What are protocol
• Protocol hierarchies are series of layers
which reduces the design complexity.
• The number of layers, the name of each
layer, the contents of each layer and
functions of each layer differ from network
• In network architecture various tasks and
functions are grouped into related and
manageable sets called LAYERS.
• The problem with the layered
protocol is that we lose touch
with the reality.
• Layering is a kind of hiding
• Layered architecture can
sometimes result in poor
What are various
design issues for the
• Some of the design issues
• Direction of Transmission :
simple, half or full duplex.
• Error control
• Avoid loss of sequencing
• Ability to receive long
• To use multiplexing and
Explain OSI model.
• OSI and TCP/IP are reference
• An OSI model is a layered
framework for the design of
network systems that allows
communication across all
types of computer systems.
• Layer n on one machine will
communicate with layer n on
another machine with layer n
What is the difference
between OSI and
TCP/IP model ?
OSI Model TCP/IP
Has 7 layers Has 4 layers
Transport layer guarantees delivery of
Transport layer does not guarantee
delivery of packets.
Separate presentation & session layer. No presentation and session layer.
Network layer provides both
connectionless and connection
Network layer provides only
• In both the models the
functions of the layer is
• Both models use the
concept of layered
What is an
application layer ?
• The application layer is the
topmost layer of the internet
model. This is the layer where all
the interesting applications are
• People can use the Internet due to
presence of application layer.
• The application layer receives the
service from the transport layer.
• For the real applications in the
application layer to function, there is
a need for support protocols
• The three areas or protocols
required for such support are:
1. Network security
2. Domain Name Service(DNS)
3. Network management
• Security is not a single protocol but it
contains a large number of concepts
and protocols used for providing
• DNS is used to handle naming or
addressing within the Internet.
• The third support protocol is
What is Domain Name
System (DNS) ?
• For communication to take
place, the sender and receiver
both should have addresses and
they should know each other.
• It is important to note that there
is an alias name for address of
remote host. The application
program uses an alias name
instead of an IP address.
• So the alias address has to be mapped
to the IP address. For this application
program needs service of another
• This entity is an application program
• Note that DNS is not used directly by
the user.Resolving NS
What is an Electronic
Mail (E-mail) ?
• One of the most popular network
service is electronic mail (e-mail).
• SMTP is the standard mechanism for
electronic mail in the internet.
• The first e-mail systems simply
consisted of file transfer protocol.
• But there were limitations:
1. It is difficult to send a message to
group of people.
2. Message did not have any internal
structure. So its computer processing
3. It was not possible to create and
send messages containing an image,
drawing or voice.
FUNCTIONS 1. Composition 2. Transfer 3.
Reporting 4. Displaying 5. Disposition
Why do we
use MIME ?
• MIME adds
structure to the
No Header Name Meaning
1 MIMI-Version Indicates the MIME-Version
2 Content-Desc Tells what is in the message
3 Content ID Identifier
How the body is wrapped for
5 Content-Type Type of message
NEW HEADER IN MIME
Why do we use POP 3
and IMAP protocol?
• The SMTP is used in the first &
second stages of mail delivery. But
SMTP is not used in the third stage
because SMTP is a push protocol
which is meant for pushing the
message from client to server.
• The third stage needs a pull protocol
because client has to pull the
message from the server.
• Two message access agents available
are: POP 3 & IMAP 4
• MODES OF POP 3
1. Delete mode : In this mode the
mail is deleted from the mailbox
after each retrieval.
2. Keep mode : The mail remains in
the mailbox after retrieval.
• Disadvantages : POP 3 does not
allow organization of emails on
• The user can not create different
folders on the server. It can create
them only on his own computer.
• The user can not partially check
contents of Email before
• Internet Mail Access
Protocol Version 4 is
another mail access
protocol which is very
similar to POP 3 but has
• This makes IMAP 4 more
powerful and more
complex as compared to
• Extra features of IMAP 4
• It is possible for user to
check header before
• It is possible to partially
• It is possible to create,
rename or delete
mailboxes on the mail
What is FTP (File Transfer
• A standard mechanism
provided by the internet
which helps in copying a file
from one host to other is
known as the File Transfer
1. Two systems may use
different file name
2. Two systems may
represent text & data
in different ways.
3. The directory
structures of the two
system may be
FTP provides solution
What is the difference
between FTP and TFTP ?
Sr. No Parameter FTP TFTP
1 Operation Transferring files Transferring files
2 Authentication Yes No
3 Control & data Separated Not Separated
4 Protocol TCP UDP
5 Ports 21-control 20-data Port 3214,69,4012
6 Data transfer Reliable Unreliable
• www is an architectural framework
for accessing documents which are
spread over a number of machines
• Web and Internet are not the same
things. The web is a collection of
standard protocols or instructions
which are sent back and forth over
the internet to gain access to internet.
• Internet on the other hand, is a
network of networks –Physical
• Web from user side :
• Pages, links or pointers
• Web from the server side :
• Every website has a server process.
It is listening to port 80 on which
incoming clients are connected
• The protocol used for defining the
legal request is called HTTP
Explain HTTP Protocol.
• The principle of HTTP is
very simple. A client sends
a request. The server
sends a response. The
request and response
message carries data in the
form of a letter with a
MIME like format.
• HTTP is an application layer protocol.
• It is implemented in two programs Client
program & Server program.
• HTTP is using TCP as its underlying protocol.
• TCP provides a reliable data transfer service to
HTTP. That means each HTTP request
message, transmitted by a client will
eventually arrive at the server.
What is telnet?
• Telnet is an old terminal emulation
program to login into remote system. It
uses TCP/IP protocol and underlying
communication can take place through
PSTN, Satellites etc.
• Telnet allows to login in the system for
any operation whereas FTP is used only
for file transfer.
• It can be used for Telneting, checking
emails and other online services.
• Telnet is a network protocol used
on the Internet or local area
networks to provide bidirectional
communication facility using a
virtual terminal connection.
• Historically, Telnet provided access
to command-line interface on a
• Telnet was developed in 1968
beginning with RFC 15 extended in
• Telnet is not encrypted.
• Hence now we use SSH.
What is DHCP ?
• DHCP is a protocol that is used
for communication here
manage and automate assign
of Internet Protocol (IP)
addresses in an organizational
• If we cannot use DHCP, then it
is compulsory to enter the IP
address to each computer and
a new IP.
UDP Port 67 –Connection to Server
Port 68 –Connection to Client
DHCP server can be installed
on Router or it can be a
What is transport layer?
Transport layer is the core
of the internet model. It
provides service to the
application layer and
takes service from
provides service to
takes service from
What are the
2. Connection control
4. Flow & Error control
5. Congestion control &
6. Quality of Service
• The data link layer provides node to node delivery.
• The network layer carries out datagram delivery
between two hosts(host to host delivery).
• The transport layer takes care of the process-to-
process delivery. Packet from one process is delivered
to the other process.
in transport layer.
• Addressing requirements at
different layer are different.
• At data link layer we need
• At transport layer we need
• In the Internet model, the port numbers are 16
• Hence total port numbers will be 2^16 = 65,535
• The client program identifies itself with a port
number which is chosen randomly.
• The server process uses well known port
numbers which are not chosen randomly.
1. Well known ports 0-1023
2. Registered ports 1024-49,151
3. Dynamic ports 49,152-65,535
Then what is socket
The combination of IP
address and port
number is known as
• Process to process delivery (transport layer
communication) has to use two addresses, one is IP
and other port number at each end to make a
• These four pieces go to the IP header and transport
layer protocol header.
• The IP header contains the IP address and TCP and
UDP header contains the port number.
• Connection oriented : It
establishes a dedicated
connection between the
users before data is
• Connection establishment
may take place on physical or
logical level and needs some
kind of signalling.
• TCP• Connectionless : Information
is transferred as independent
• Each data units contains
complete destination address.
• The packets are not numbered
• They get delayed, lost or can
arrive at sequence .
• UDP is a connectionless
Why error control at
both transport layer and
data link layer?
We need reliability at
transport layer event
though data link layer is
reliable because data link
can provide reliability for
only the node to node
delivery.• If the application layer program needs
reliability then the reliable transport layer
protocol is used which implements the flow
and error control mechanism .
• But some application layer program do not
need reliability because they have their own
error and flow control mechanism. Such
programs use an unreliable service.
How to establish
Connection Request A to B
Connection Request B to A
• Problems : Due to
congestion on a subnet,
the acknowledgement do
not get back in time or
take longer time.
• The same connection
getting re-established due
to duplication of packets.
• Solution : We should
ensure that no packet
lives longer than some
• Like time stamping or
putting hop counter in
How to solve
• The first TPDU is a delayed
duplicate connection request
from an old connection. The
host A does not about it.
• Host B receives TPDU and sends
to host A
• But host A is not trying to
establish any connection, so it
sends a reject along with ACK
• Simple unreliable and
• A unit of data sent using UDP
is called datagram.
• UDP adds four 16 bit header
to all the data sent i.e a length
field, checksum, source and
destination port numbers.
• Although UDP is not reliable, it is still a
preferred choice for many applications like
audio and video where if data is lost, it is
better to send it again.
• It is also by protocols like Simple Network
• UDP is used in RIP and multicasting application
Source port number
number 16 bits
• TCP is a connection-oriented
protocol and provides reliable
• Reliability, efficient flow
control, full duplex operation.
• TCP provides multiplexing
means that numerous
conversation can be
multiplexed over a single
• TCP message gets encapsulated or inserted in IP
datagram and sent to destination.
• At the destination IP hands over the message to
TCP software running on the destination computer.
• Thus TCP deals with the actual data to be
transferred and IP takes care of transfer of that data
Source Port Destination Port
Checksum Urgent Pointer
Options(0-32 bit) Padding
What is TCP sliding
Application reads 2k
does a 2k write
does a 2k write
Sender is blocked
send up to 2k
• If extremely large number of
packets are present in a part of a
subnet, the performance degrades.
• Congestion may occur due to
overloading of network.
• At high traffic, the performance
collapses completely and almost all
packets are lost. This is the worst
Congestion Control :
Open loop solution
design to prevent
on current status of
Closed loop solution
the congestion to
place where action
can be taken.
3. Adjust the system
operations to correct
• A socket acts as an end point.
• Two processes communicate
through socket if they have
socket at their ends.
• STREAM SOCKET, DATAGRAM
SOCKET, RAW SOCKET
Family Type Protocol
Local Socket Address
Remote Socket Structure
• Family : This field is used for defining the protocol
group such as IPv4 or IPv6
• Type : This field is used for defining the type of socket
such as stream socket, datagram socket or raw socket.
• Protocol : This field is usually zero for TCP and UDP
• Local Socket Address : It is used for defining the local
socket address which is a combination of IP address and
port address of the local application program.
• Remote Socket Address : It is used for defining the
remote socket address which is a combination of IP
address and port address of the remote application
SERVER SIDE :
• Server creates a socket and checks
for error using SOCKET.
• Assign address to newly created
socket using BIND
• Use LISTEN to allocate space for
queue which is used for incoming
• Execute an ACCEPT for blocking the
waiting incoming connections.
CLIENT SIDE :
• Create a socket using SOCKET
• Use CONNECT to initiate
• Establish the connection.
Why do we use
• Real-time protocol is concerned
with timing information.
• TCP is quite unsuitable for real-
• TCP is a point to point protocol
without direct support for
• There is no timing information
carried, which is needed by most
• Hence IETF adopted RTP
• Assuming transport protocol as a RTP is bit
confusing because it is typically used with
UDP that can say as a connection less
• Real-time transport protocol is very closely
coupled to the application it carries. So, RTP is
best viewed as a framework that applications
can use to implement a new single-protocol.
• RTP does not guarantee timely release of
packets, nor does it maintain the packets in
• RTP provide the responsibility for improving
lost segment and re-sequencing of packets for
What is Stream
Protocol (SCTP) ?
• SCTP is referred to as next
generation TCP to hold a telephone
connection over the internet.
• SCTP is standard protocol define RFC
2960 and invented by IETF
• Like TCP, SCTP handle reliable
• Contrasting to TCP, SCTP guarantee
the complete simultaneous
transmission of several streams of
data between connected terminals.
• SCTP’s multi-streaming allows data
to be delivered in independent
streams, so if there is loss of data in
one stream, delivery will not be
affected for other streams.
• Acknowledged error free non-
duplicated transfer of user data.
• Sequenced delivery of user message
within multiple streams.
• Network level fault tolerance
through supporting of multi-homing
at either or both ends of an
• Traffic shaping
• Leaky bucket algorithm
• Token bucket algorithm
• Resource reservation
• Admission control
• Proportional routing
• Packet scheduling
Fixed size packet Variable size packet
What is Token Bucket
• Token bucket algorithm is a
variant of leaky bucket
algorithm, here the buckets
are filled with tokens.
• A packet which grabs the
token is allowed to leave the
bucket due to which packets
never get lost.
• Token bucket can be easily
implemented with a counter. The
counter is initialised to zero.
• Every time a token is added counter is
incremented by 1 and every time a
packet is dispatched, the counter is
decremented by 1.
Explain Scheduling and
• The good QoS can be ensured
by following some mechanism
like Scheduling and Policing.
• FIFO, Priority Queueing, Round
Robin and Weighted Fair
• In order to regulate this flow
certain rate aspects should be
• Following are the important
policing criteria Average rate,
Peak rate, Burst size
What are Integrated &
• These are the two architectures
Intserv is defined as a framework
which is developed within IEFT for
providing QoS guarantees to invidual
• The goal of Diffserv is to provide the
ability to handle different classes of
traffic in different ways within the
• Intserv architecture has two key features: 1.
Reserved resources and 2. Call set up
• Reserved resources : a router must know
about how much of its resources such as
buffer, link bandwidth have already reserved
for the current ongoing session.
• The call setup is also called as call admission.
Each router has to determine the local
resources required by the session and also it
should find amount of resources already
• Problems: 1. Scalability-Per flow reservation
can increase the overhead in large networks
to a great extent.
• 2 Intserv framework provides for a small
number of pre specified service classes. Class
A service is preferred over Class B service and
• The diffserv architecture consists of two sets
of functional elements
• Edge functions
• Core functions
• Edge functions :packet classification and traffic conditioning
• For example in figure packets which are sent from H1 to H3
travel through the routers R1,R3,R5 and R7 may be marked
• Such type of mark received by a packet will identify the class
of traffic which it belongs. Different classes of service will
then receive different service within the core network.
• Core functions: Forwarding
• The DS marked packet arrives at the DS capable router. The
router understands the class of received packet.
• The per hop behaviour of the router is associated with the
packet class. The router will forward the packet onto their
hop according to the per hop behaviour.
• The per hop behaviour is important because it decides how
a router’s buffer and link bandwidth are shared among
various service class.
What is RSVP ?
• Resource ReserVation Protocol is a
signalling protocol which allows
running in the host computer to
reserve resource in the Internet.
• The resource in the Internet are : link
bandwidth and router buffer.
• To implement RSVP, the RSVP software
has to be present in the receivers,
senders and routers.
• The transmitting source advertises its
contents by sending the RSVP path message
through the multicast tree.
• These messages indicate the bandwidth
required for the contents, time out interval,
upstream path to the sender etc.
• Each receiver sends an RSVP reservation
message upstream to the sender.
• When the reservation message reaches a
router, it will adjust its packet scheduler for
accommodating the reservation. It then
sends the reservation upstream.