Basics Computer Network - Dr. Siddhartha Biswas Sir

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Subject Name : Computer Networks
Subject Code : BTCS603
Branch : B.Tech. (Computer Science & Engineering)
Semester : 6th
Faculty:
Dr. Siddhartha Sankar Biswas
Assistant Professor
Department of Computer Science & Engineering
Jamia Hamdard University, New Delhi
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UNIT - I
Uses of Computer Networks, Network Architecture. Reference Model (ISO-OSI.
TCP/lP-Overview, IP Address Classes, Subneting). Domain Name Registration &
Registrars. The Physical Layer: Theoretical basis for data communication. transmission
media-Magnetic Media, Twisted Pair, Baseband Coaxial Cable. Broadband Coaxial
Cable. Fibre Cable. Structured Cabling, Cable Mounting. Cable Testing. Wireless
transmission. the telephone system, narrowband ISDN, broadband lSDN and ATM.
UNIT - II
The Data Link Layer: Data link layer design issues. error detection and correction, data
link protocols, sliding window protocols. Examples of Data Link Protocols.
UNlT - III
The Medium Access Sublayer: The channel allocation problem. multiple access
protocols, IEEE standard 802 for LANS and MANS. high-speed LANs. satellite
networks. Network devices-repeaters, hubs. Switches and bridges.
UNIT - IV
The Network Layer: Network layer design issues. routing algorithms, congestion
control algorithm, internetworking. the network layer in the internet. the network layer
in ATM networks.
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Unit - 1
1. Uses of Computer Networks.
2. Network Topologies
3. Reference Model
(ISO-OSI , TCP/lP-Overview, IP Address Classes, Subneting).
4. Domain Name Registration & Registrars.
5. The Physical Layer: Theoretical basis for data communication.
6. Transmission media :
i. Magnetic Media
ii. Twisted Pair
iii. Baseband Coaxial Cable
iv. Broadband Coaxial Cable
v. Fiber Cable
vi. Structured Cabling
vii. Cable Mounting
viii. Cable Testing
ix. Wireless transmission
x. The telephone system
xi. Narrowband ISDN
xii. Broadband lSDN
xiii.ATM
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Uses of Computer Networks :
• Introduction to Computer Networks
• Pros and Cons of Computer Network
• Uses of Computer Network
• Network Categories
Unit I / Topic 1.
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A computer network or data network is a communications
network that allows computers to exchange data.
In computer networks, networked computing devices pass
data to each other along data connections.
The connections (network links) between nodes are
established using either cable media or wireless media.
What is a computer network ?
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Pros of Computer Network
(i) Facilitates interpersonal communications
People can communicate efficiently and easily via email,
instant messaging, chat rooms, telephone, video telephone
calls, and video conferencing.
(ii) Allows sharing of files, data, and other types of
information
Authorized users may access information stored on other
computers on the network. Providing access to information
on shared storage devices is an important feature of many
networks.
(iii) Allows sharing of network and computing resources
Users may access and use resources provided by devices on
the network, such as printing a document on a shared
network printer. Distributed computing uses computing
resources across a network to accomplish tasks.02 / 02 / 2016 6

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Cons of Computer Network
(i) May be insecure
A computer network may be used by computer Hackers to
deploy computer viruses or computer worms on devices
connected to the network, or to prevent these devices
from accessing the network (denial of service).
(ii) Interference from other technologies
Power line communication strongly disturbs certain forms of
radio communication, e.g., amateur radio. It may also
interfere with last mile access technologies such as ADSL
and VDSL.
(iii) May be difficult to set up
A complex computer network may be difficult to set up. It
may be costly to set up an effective computer network in a
large organization.02 / 02 / 2016 7
Uses of Computer Networks
1.Business applications.
2.Domestic/Home uses.
3.Social uses.
4.Mobile Applications.
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Network Categories
• Classification based on Transmission Technology.
• Classification based on Implementation.
• Classification based on Scale of network.
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Classification based on Transmission Technology
• Unicast
• Anycast
• Multicast
• Broadcast
• Geocast
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Unicast
Unicast addressing uses a one-to-one association
between destination address and network endpoint,
where each destination address uniquely identifies
a single receiver endpoint.02 / 02 / 2016 11
Anycast
Anycast is a network addressing and routing methodology in
which
datagrams from a single sender are routed to the
topologically nearest node in a group of potential receivers,
though it may be sent to several nodes, all identified by
the same destination address.02 / 02 / 2016 12

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Multicast
In computer networking, multicast (one-to-many
or many-to-many distribution)
is group communication where information is
addressed to a group of destination computers
simultaneously.02 / 02 / 2016 13
Broadcast
In telecommunication and information theory,
broadcasting refers to a method of transferring a message
to all recipients simultaneously.
Broadcasting can be performed as a high level operation in
a program, for example broadcasting Message Passing
Interface, or it may be a low level networking operation,
for example broadcasting on Ethernet.
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Geocast
Geocast refers to the delivery of information to a
group of destinations in a network identified by
their geographical locations.
It is a specialized form of multicast addressing
used by some routing protocols for mobile ad hoc
networks.02 / 02 / 2016 15
Classification based on Implementation :
1. ARPANET
2. Internet
3. Private Networks
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1 . ARPANET
Before we understand the technical details of
ARPANET, lets go through the history from which
it was born :-
In 1950s, the Cold War between USA and USSR
was at its peak.
US wanted a command-and control network that
could survive a nuclear war.
At that time, all military communications used the
public telephone network, which was considered
vulnerable.02 / 02 / 2016 17
Switching Office
Telephone Set
Telephone Set
Telephone Set
Telephone Set
Telephone Set
Telephone SetTelephone Set
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Higher Level
Switching Office
Switching
Office
Switching
Office
Switching
Office
Switching
Office
Switching
Office
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The telephone switching offices, each of which was
connected to thousands of telephones,
were, in turn, connected to higher-level switching
offices to form a national hierarchy with only a
small amount of redundancy.
The vulnerability of the system was that the
destruction of a few key toll offices could
fragment the system into many isolated islands.
Several years went by and still the DoD did not
have a better command-and-control system.
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On 4th October 1957 Soviet Union beat the U.S. into
space race with the launch of the first artificial satellite,
Sputnik.
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At that point of time Gen. Dwight David Eisenhower was the
President of the United States.
Among many other high achievements,
Eisenhower was a five-star general in the United States Army during
World War II
and served as Supreme Commander of the Allied Forces in Europe.
On January 20, 1953 , he became the 34th President of the United
States02 / 02 / 2016 22

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When President Eisenhower tried to find out how this major intelligence
failure for US happened, he was appalled to find the Army, Navy, and
Air Force squabbling over the Pentagon's research budget.
His immediate response was to create a single defence research
organization for.
And so in 1958 ARPA, the Advanced Research Projects Agency was
created
ARPA was renamed to "DARPA" (for Defense) in March 1972.
Then renamed "ARPA" in February 1993,
and then renamed "DARPA" again in March 1996.
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In its infant years, ARPA had no scientists or laboratories; in fact, it
had nothing more than an office and a small (by Pentagon standards)
budget.
It did its work by issuing grants and contracts to universities and
companies whose ideas looked promising to it.
For the first few years, ARPA tried to figure out what its mission
should be.
In 1967, the attention of ARPA's then director, Larry Roberts, turned
to networking.
At that point, there were only a limited number of large, powerful
research computers in USA.
The US Govt. felt that many research investigators who should have
access were geographically separated from them.
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Four computers were the first connected in the original
ARPAnet.
They were located in the respective computer research
labs of
1.UCLA (Honeywell DDP 516 computer).
2.Stanford Research Institute (SDS-940 computer).
3.UC Santa Barbara (IBM 360/75).
4.the University of Utah (DEC PDP-10).
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The first data exchange over this new network occurred
between computers at UCLA and Stanford Research
Institute.
On their first attempt to log into Stanford's computer by
typing "log win", UCLA researchers crashed their computer
when they typed the letter 'g'.02 / 02 / 2016 26

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As the network expanded, different models of computers
were connected, creating compatibility problems.
The solution rested in a better set of protocols called
TCP/IP suite (Transmission Control Protocol/Internet
Protocol suite) designed in 1982.
To send a message on the network, a computer breaks its
data into IP (Internet Protocol) packets, like individually
addressed digital envelopes.
TCP (Transmission Control Protocol) makes sure the packets
are delivered from client to server and reassembled in the
right order.
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Under ARPAnet several major innovations occurred:
 email (or electronic mail), the ability to send simple
messages to another person across the network (1971);
 telnet, a remote connection service for controlling a
computer (1972);
 and file transfer protocol (FTP), which allows information
to be sent from one computer to another in bulk (1973).
As non-military uses for the network increased, more and
more people had access, and it was no longer safe for
military purposes.
As a result, MILnet, network for only military related
purpose was started in 1983.02 / 02 / 2016 28

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2. Internet
The Internet is a global system of interconnected
computer networks that use the standard
Internet protocol suite (TCP/IP) to serve several
billion users worldwide.02 / 02 / 2016 29
It is a network of networks that consists of
millions of private, public, academic, business, and
government networks, of local to global scope,
that are linked by a broad array of electronic,
wireless, and optical networking technologies.
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 the inter-linked hypertext documents of the
World Wide Web (WWW),
 the infrastructure to support email, and
 peer-to-peer networks.
The Internet carries an
extensive range of
information resources
and services,
such as
02 / 02 / 2016 31
Intranet
An intranet is a computer network that uses Internet
Protocol technology to share information, operational
systems, or computing services within an organization.
Intranet
Internet
3. Private Networks
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Extranet
An extranet is a computer network that allows
controlled access to the organization’s internal
network from the outside, for specific business or
educational purposes.
An extranet can be viewed as an extension of an
organization's intranet that is extended to users
outside the organization, usually partners, vendors
and suppliers, in isolation from all other Internet
users.
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Intranet
Internet
Extranet
Relationship/Differnce between Intranet, Extranet and Internet
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Classification based on Scale of network :
1. Local Area Network (LAN)
2. Metropolitan Area Network (MAN)
3. Wide Area Network (WAN)
Assignment # 1
Q1. Explain the Network Classification of based on Scale.
Q2. Explain all the difference between LAN , MAN and WAN.
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Network Topology
Network topology is the arrangement of the
various networking elements (links, nodes, etc.) of
a computer network.
Essentially, it is the structure of a network, and
may be depicted physically or
logically.
Unit I / Topic 2.
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• Design Efficient Protocols
• Solve Internetworking Problems:
- routing
- resource reservation
- administration
• Create Accurate Model for Simulation
• Derive Estimates for Topological Parameters
• Study Fault Tolerance and Anti-Attack Properties
Why Topology is Important ?
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1. Bus Topology
2. Star Topology
3. Ring Topology
4. Tree Topology
5. Mesh / Complete Topology
6. Hybrid Topology
7. Irregular Topology
There are broadly seven topologies in
computer network :-
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1 Bus Topology (sometimes also called Liner Topology)
A bus network is a network topology in which nodes are
connected in a daisy chain by a linear sequence of buses.
In local area networks where bus topology is used, each
node is connected to a single cable, i.e. each computer or
server is connected to the single bus cable.02 / 02 / 2016 39
A signal from the source travels in both directions to all
machines connected on the bus cable until it finds the
intended recipient.
If the machine address does not match the intended
address for the data, the machine ignores the data.
Alternatively, if the data matches the machine address,
the data is accepted.02 / 02 / 2016 40

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Since the bus topology consists of only one wire, it is
rather inexpensive to implement when compared to other
topologies.
Since only one cable is utilized, it can be the single point
of failure.
If the network cable gets damaged or breaks down in
between the network, the entire network will be down.
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Advantages (benefits) of Bus Topology
1) It is easy to set-up and extend bus network.
2) Cable length required for this topology is the least
compared to other networks.
3) Bus topology costs very less.
4) Linear Bus network is very efficient for small networks.
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Disadvantages (Drawbacks) of Linear Bus Topology
1) There is a limit on central cable length and number of nodes
that can be connected.
2) Dependency on central cable in this topology has its
disadvantages. If the main cable (i.e. bus ) encounters some
problem, whole network breaks down.
3) It is difficult to detect and troubleshoot fault at
individual station.
4) Maintenance costs can get higher with time.
5) Efficiency of Bus network reduces, as the number of
devices connected to it increases.
6) It is not suitable for networks with heavy traffic.
7) Security is very low because all the computers receive the
sent signal from the source.
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2 Star Topology
Star networks are one of the most common computer
network topologies.
In its simplest form, a star network consists of a central
node, to which all other nodes are connected; this central
node provides a common connection point for all nodes
through a hub.02 / 02 / 2016 44

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Advantages of Star Topology
1) As compared to Bus topology it gives far much better
performance, signals don’t necessarily get transmitted
to all the workstations. Performance of the network is
dependent on the capacity of central hub.
2) Easy to connect new nodes or devices. In star topology
new nodes can be added easily without affecting rest of
the network. Similarly components can also be removed
easily.
3) Centralized management. It helps in monitoring the
network.
4) Failure of one node or link doesn’t affect the rest of
network. At the same time its easy to detect the
failure and troubleshoot it.02 / 02 / 2016 46

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Disadvantages of Star Topology
1) Too much dependency on central device has its
own drawbacks. If it fails whole network goes
down.
2) The use of hub, a router or a switch as central
device increases the overall cost of the
network.
3) Performance and as well number of nodes which
can be added in such topology is depended on
capacity of central device.
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3 Ring Topology
A ring network is a network topology in which each node
connects to exactly two other nodes, forming a single
continuous pathway for signals through each node - a ring.
Data travel from node to node, with each node along the
way handling every packet.02 / 02 / 2016 48

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Advantages of Ring Topology
1) This type of network topology is very organized. Each
node gets to send the data when it receives an empty
token. This helps to reduces chances of collision.
2) Even when the load on the network increases, its
performance is better than that of Bus topology.
3) There is no need for network server to control the
connectivity between workstations.
4) Additional components do not affect the performance of
network.
5) Each computer has equal access to resources.
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Disadvantages of Ring Topology
1) Each packet of data must pass through all the
computers between source and destination. This makes
it slower than Star topology.
2) If one workstation or port goes down, the entire
network gets affected.
3) Network is highly dependent on the wire which connects
different components.
4) Ring network cards are expensive as compared to
Ethernet cards and hubs.
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4 Tree Topology
In computer networks, a tree network topology is a
combination of two or more star networks connected
together.
In each star network there is a central computer or server
to which all the workstation nodes are directly linked.
The central computers of the star networks are connected
to a main cable called the bus.
Thus in layman’s term we can say that a tree network is a
bus network of star networks.02 / 02 / 2016 51
Advantages of Tree Topology
1. It is an extension of Star and Bus Topologies, so in
networks where these topologies can't be implemented
individually for reasons related to scalability, tree
topology is the best alternative.
2. Expansion of Network is possible and easy.
3. Here, we divide the whole network into segments, which
can be easily managed and maintained.
4. Error detection and correction is easy.
5. Each segment is provided with dedicated point-to-point
wiring to the central hub, therefore even if one segment
is damaged, other segments are not affected.
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Disadvantages of Tree Topology
1. Because of its basic structure, tree topology, relies
heavily on the main bus cable, if it breaks whole
network is crippled.
2. As more and more nodes and segments are added, the
maintenance becomes difficult.
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5 Mesh / Complete Topology
Mesh topology or a mesh network or mesh is a network
topology in which devices are connected with as many
redundant interconnections between network nodes as
possible.
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There are two types of mesh topologies:
full mesh and partial mesh.
Full mesh topology occurs when every node has a circuit
connecting it to every other node in a network.
Full mesh is very expensive to implement but yields the
greatest amount of redundancy, so in the event that one
of those nodes fails, the network traffic can be directed
to any of the other nodes.
Partial mesh topology is less expensive to implement and
yields less redundancy than full mesh topology.
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Advantages of Mesh Topology
1) Data can be transmitted from different devices
simultaneously. This topology can withstand high traffic.
2) Even if one of the components fails there is always an
alternative present. So data transfer doesn’t get
affected.
3) Expansion and modification in topology can be done
without disrupting other nodes.
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Disadvantages of Mesh Topology
1) There are high chances of redundancy in many of the
network connections.
2) Overall cost of this network is way too high as
compared to other network topologies.
3) Set-up and maintenance of this topology is very
difficult. Even administration of the network is tough.
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6 Hybrid Topology
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Hybrid, as the name suggests, is mixture of two different
things.
In this type of topology we integrate two or more different
topologies to form a resultant topology
which has good points as well as weaknesses, of all the
constituent basic topologies rather than having characteristics of
one specific topology.
This combination of topologies is done according to the
requirements of the organization.
For example, if there exists a ring topology in one office
department while a bus topology in another department,
connecting these two will result in Hybrid topology.02 / 02 / 2016 59
Advantages of Hybrid Topology
1) Reliable :
Unlike other networks, fault detection and
troubleshooting is easy in this type of topology.
The part in which fault is detected can be isolated from
the rest of network and required corrective measures
can be taken, WITHOUT affecting the functioning of
rest of the network.
2) Scalable:
Its easy to increase the size of network by adding new
components, without disturbing existing architecture.
3) Flexible:
Hybrid Network can be designed according to the
requirements of the organization and by optimizing the
available resources.02 / 02 / 2016 60

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4) Effective:
Hybrid topology is the combination of two or more
topologies, so we can design it in such a way that
strengths of constituent topologies are maximized while
there weaknesses are neutralized.
Disadvantages of Hybrid Topology
1) There are high chances of redundancy in many of the
network connections.
2) Overall cost of this network is way too high as
compared to other network topologies.
3) Set-up and maintenance of this topology is very
difficult. Even administration of the network is tough.02 / 02 / 2016 61
7 Irregular Topology
Irregular Topology in a network systems are thoses
topology where the interconnections between the nodes has
no specific and defined structure and has been connected
as per ad-hoc or as an where requirements basis.
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Subject Code : BTCS 603
Branch : B.Tech. (Computer Science & Engineering)
Semester : 6th
Faculty:
Assistant Professor
Layering architecture of networks :-
OSI model - Functions of each layer, Services and Protocols of each layer
3. Reference Model
(ISO-OSI , TCP/lP-Overview, IP Address Classes, Subneting).
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User
(Sender)
User
(Receiver)
Data Stream
Why we need Communication Network
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Protocol:
By dictionary meaning protocols means formal agreement
and set of rules defined for any activity.
In the context networking science, a communications
protocol is a system of digital rules for data exchange
within or between computers.
When data is exchanged through a computer network, the
rules system is called a network protocol.
02 / 02 / 2016 65
What is Layering Architecture?
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Telnet HTTP FTP TCP
Coaxial
cable
Fibre
optic
Packet
radio
Applications
Transmission
Media
No Layering
Each new application has to be re-implemented for every
network technology!
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Telnet HTTP FTP TCP
Coaxial
cable
Fibre
optic
Packet
radio
Applications
Transmission
Media
Intermediate
layer
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Telnet HTTP FTP TCP
Coaxial
cable
Fibre
optic
Packet
radio
Applications
Transmission
Media
Layer -1
Layer – 2
Layer - n
Intermediate
layers
Layering
Intermediate layer(s) provide a unique abstraction for various network
technologies
02 / 02 / 2016 69
Benefits of layered architecture in networking
 Layer architecture simplifies the network
design.
 It is easy to debug network applications in a
layered architecture network.
 The network management is easier due to the
layered architecture.
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Open Systems Interconnection (OSI) Reference Model
International Standard Organization (ISO) established a
committee in 1977 to develop an architecture for computer
communication.
Open Systems Interconnection (OSI) reference model is
the result of this effort.
In 1984, the Open Systems Interconnection (OSI)
reference model was approved as an international standard
for communications architecture.
Term “open” denotes the ability to connect any two
systems which conform to the reference model and
associated standards.
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The OSI model is now considered the primary Architectural
model for inter-computer communications.
The OSI model describes how information or data makes
its way
from application programmes (such as spreadsheets)
through a network medium (such as wire)
to another application programme located on another
network.
The OSI reference model divides the problem of moving
information between computers over a network medium into
SEVEN smaller manageable problems .
This separation into smaller more manageable functions is
known as layering.
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Layer No. Layer Name
6 Presentation Layer
5 Session Layer
4 Transport Layer
3 Network Layer
2 Data Link Layer
1 Physical Layer
7 Application Layer
OSI Reference Model
Intelligence
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Application
Layer
Presentation
Layer
Session
Layer
Transport
Layer
Network
Layer
Data Link
Layer
Physical
Layer
Application
Layer
Presentation
Layer
Session
Layer
Transport
Layer
Network
Layer
Data Link
Layer
Physical
Layer
User
(Sender)
User
(Receiver)
Open System
Interconnection
(OSI)
Reference
Model for
Communication
Data
from
the
Sender
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Serial Communication
Serial communication is the process of
sending data one bit at a time, sequentially,
over a communication channel or computer bus.
Parallel Communication
Parallel communication is a method of
conveying multiple binary digits (bits) simultaneously ,
over a communication channel or computer bus.
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Software
Software means computer instructions or data
Software can be divided into two general
classes:
(1) Systems software , and
(2) Applications software.
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System software
Systems software consists of programs that
interact with the computer at a very basic level.
This includes operating systems, compilers, and
utilities for managing computer resources.
Applications software
Applications software (also called end-user
programs) includes database programs, word
processors, spreadsheets, games, etc.
Applications software sits on top of systems
software because it is unable to run without the
operating system and system utilities.02 / 02 / 2016 77
Some of the design issues for the
layering architecture in computer Networking :-
1.Addressing
2.Direction of transmission
3.Error Control.
4.Avoid Loss of Sequencing.
5.To be able to use Multiplexing and
Demultiplexing
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1.Addressing
It is a absolute necessary to have a mechanism to
identify the sender(s) and receiver(s),
and this becomes more necessary because of the simple
fact that there is possibility that a system is
communicating with many other systems simultaneously.
2.Direction of transmission
Based on the
direction of transmission and
weather transmission needs to be simultaneous or
discrete
the following communication systems are classified:
(i) Simplex Communication System.
(ii) Half Duplex Communication System.
(iii) Full Duplex Communication System.02 / 02 / 2016 79
(i) Simplex Communication System
In Simplex Communication System,
the data or signals is transmitted in only and always only in
one particular direction at any point of time from one
station to another,
and there is never a possibility for the vice versa to
happen.
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(ii) Half Duplex Communication System
In Half Duplex Communication System,
the communication nodes are semi bi-directional , i.e.,
though the nodes can do both transmisssion as well as
receiving of data/signals,
they can do only one of the activities at any given point of
time and not simultaneously.
Time
Period Activity
A
A
B
B
T1
to
T2
T2
to
T3
02 / 02 / 2016 81
(ii) Full Duplex Communication System
In Full Duplex Communication System,
the communication nodes are truly bi-directional , i.e.,
all the communicating nodes/systems can transmit as well
as receive data/signal simultaneously from multiple other
nodes/systems at any given point of time.
Internet
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3. Error Control
Error
Unexpected occurrence of an unwanted activity or
operation.
Error message
An error message is information displayed when an
unexpected condition occurs, usually on a computer or other
device.
Types of Errors
(i) Single Bit Error
The term single bit error means that only one
bit of the data unit was changed from 1 to 0
and 0 to 1.
(ii) Burst Error
In term burst error means that two or more
bits in the data unit were changed.02 / 02 / 2016 83
Network is responsible for transmission of data from one
device to another device.
The end to end transfer of data from a transmitting
application to a receiving application involves many steps,
each step is subject to error(s) where Data can be
corrupted during transmission.
For reliable communication, error must be detected and
corrected.
With the error control process, we can be confident that
the transmitted and received data are identical.
Error control is the process of detecting and correcting
both the bit level and packet level errors.
02 / 02 / 2016 84

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43
Error control process/algorithms contains the following two
sub-algorithms
 Error Detection
Error detection is the process of detecting the error
during the transmission between the sender and the
receiver.
Types of error detection methods are as follows
(i) Parity checking.
(ii) Cyclic Redundancy Check (CRC).
(iii) Checksum.
 Error Correction
The part of error control process which allows a
receiver to reconstruct the original information when it
has been corrupted during transmission.
02 / 02 / 2016 85
Example : Hamming Code error correction method.
4. Avoid Loss of Sequencing
Each Data Packet from same Data Stream generated from
the sender’s side may follow different path to reach the
receivers side.
But it is imperative that the data packets gets reorganised
into the same sequence at the receivers side, as they were
sequenced at the sender’s side.
02 / 02 / 2016 86

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44
5. To be able to use
Multiplexing and Demultiplexing
Multiplexing is the set of techniques that allows the
simultaneous transmission of multiple signals across a single
data link.
Demultiplexing is the process via which the individual
signals are retrieved from the combine signal transmission.
A Multiplexer (MUX) is a device that combines several
signals into a single signal.
A Demultiplexer (DEMUX) is a device that performs the
inverse operation.
02 / 02 / 2016 87
Source
Node
1
Source
Node
2
Source
Node
n
Destination
Node
1
Destination
Node
2
Destination
Node
m
....
.
....
.
M
U
X
D
E
M
U
X
Shared
Channel
Sender
Receiver
02 / 02 / 2016 88

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45
Subject Code : BTCS 603
Branch : B.Tech. (Computer Science and Engineering)
Semester : 6th
Faculty:
Assistant Professor
OSI Reference Model
02 / 02 / 2016 89
Application
Layer
Presentation
Layer
Session
Layer
Transport
Layer
Network
Layer
Data Link
Layer
Physical
Layer
Application
Layer
Presentation
Layer
Session
Layer
Transport
Layer
Network
Layer
Data Link
Layer
Physical
Layer
User
(Sender)
User
(Receiver)
Open System
Interconnection
(OSI)
Reference
Model for
Communication
Data
from
the
Sender
02 / 02 / 2016 90

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46
02 / 02 / 2016 91
Layer 7 : Application Layer
The application layer is the seventh level of the seven
layer OSI model.
It’s the “highest layer” and the “most intelligent layer” of
the OSI model.
Everything at this layer is application-specific.
This layer provides application services for e-mail, file
transfers, and other network software services.
This is the layer at which
communication partners are identified
quality of service is identified
user authentication and privacy are considered02 / 02 / 2016 92

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47
Another most important application layer function is file
transfer.
Different file systems have different file naming
conventions, different way of representing text and data,
etc.
Therefore transferring a file between two different
systems require handling these and other incompatibility.
This work is done by the application layer.
For example,
Different e-mail systems have different way of data
handling and application representation.
The compatibility is provided between them by the
application layer.02 / 02 / 2016 93
At Sender’s side
02 / 02 / 2016 94

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48
At Receiver’s side
02 / 02 / 2016 95
Layer 6 : Presentation Layer
The main concern of the Presentation layer is to
deal with the Syntax and Semantics of the data
being transmitted.
Presentation layer is therefore also referred as
Syntax Layer.
The three primary responsibilities of presentation
layer are, viz:
(1) Data Conversion / Translation.
(2) Data Security.
(3) Data Compression.
02 / 02 / 2016 96

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49
(1)Data conversion / translation
In the sender’s computer side
Presentation layer accepts data from the Application Layer
and
converts this data from the receiving format
to a format understood by other layers of the OSI model.
In the receiving computer
Presentation layer is responsible for the conversion of
“the external format” in which data was from the received
to one accepted by the Application Layer in the receiver
side.
02 / 02 / 2016 97
(2) Data Security
One of the critical feature of Presentation Layer
is data security, which it provides using
Cryptography algorithm.
Cryptographic Algorithms is the combined working
process of the two algorithms.
(i) Encryption Algorithm. (on the sender’s side)
(ii) Decryption Algorithm. (on the receiver’s side)
02 / 02 / 2016 98

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50
In the sender’s computer side
Presentation layer does the job Data Encryption.
Encryption is the cryptographic process of encoding
messages (or information) in such a way that only
authorized parties can read it.
In an encryption scheme, the message or information
(referred to as plaintext) is encrypted using an encryption
algorithm, turning it into an unreadable cipher text.
Plai
n
Text
Encryption
Algorithm
Encryptio
n Key
Ciph
er
Text
02 / 02 / 2016 99
In the receiver’s computer side
Presentation layer does the job Data Decryption.
Decryption is the cryptographic process of decoding
messages (or information) in such a way that only
authorized parties can read it.
In an decryption scheme, the cipher text is decrypted
using an decryption algorithm to get back the
original message or information (referred to as plaintext)
Plai
n
Text
Decryption
Algorithm
Decryptio
n Key
Ciph
er
Text
02 / 02 / 2016 100

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51
(3) Data Compression
The goal of data compression algorithm is
to represent an information source (a data file, an
image)
as accurately as possible using the fewest
number of bits.
Its is the duty of the Presentation Layer, to do
data compression at the senders side
and
do the decompression process at the receivers side
during data transmission.
02 / 02 / 2016 101
Translation,
Compression, And
Encryption.
At Sender’s side
02 / 02 / 2016 102

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52
Translation,
Decompression, And
Decryption.
02 / 02 / 2016 103
Layer 5 : Session Layer
Session
In networking science, a session is a semi-permanent
interactive information interchange, also known as a
dialogue, a conversation or a meeting,
between two or more communicating devices, or
between a computer and user (see Login session).
Sessionization
It is the process of establishing a session at any point of
time between any two parties.
(It must be noted that almost all sessions are semi-permanent,
i.e. they are torn down at some later point of time by the
communicating parties)02 / 02 / 2016 104

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53
The session layer is the 5th layer of the Open
Systems Interconnection (OSI) reference model.
In the OSI communications model, the Session
Layer manages the session, i.e. setting up and
taking down of the association/connection between
any two communicating endpoints.
The connection session is maintained while the two
end points are communicating back and forth in a
conversation and terminated when both or even one
of the parties/nodes decides to do so.
02 / 02 / 2016 105
Some connections sessions last only long enough to
send a message in one direction.
However, other sessions may last longer, usually
with one or both of the communicating parties able
to terminate it.
For most of the internet applications, each session
is related to a particular port. (For example, the
HTTP program has port number 80.)
This is the reason that session layer is also
sometimes referred as "port layer“.
02 / 02 / 2016 106

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54
At Sender’s side
02 / 02 / 2016 107
02 / 02 / 2016 108

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55
Layer 4 : Transport Layer
 On the Sender’s Side
The Basic Function of the Transport Layer is
to accept data from the Session layer,
split it into smaller pieces,
ensure that these pieces were properly sequenced,
and pass these data packets to the Network layer.
 On the Receiver’s Side
The Basic Function of the Transport Layer is
to accept data from the Network layer,
combine/rearrange the data pieces according to their
sequence number into the original data stream as was
generated at the sender’s side
and pass the data packets to the Session Layer.
02 / 02 / 2016 109
Depending upon the throughput of the end-to-end
connection Transport Layer decideds/ensures the following:
In simple layman’s term we can say that Transport layer’s
main work is to guarantee the successful and smooth
transmission of data from one end to the other.
(1) If the transmission of a particular data stream
must takes place in serial transmission or by
parallel transmission.
(2) If Multiplexing/Demultiplexing is required, and if
required provides the mechanism to carry out
these processes.
(3) To ensures Flow Control so that a fast host cannot over
run a slow one during transmission thus avoiding
bottleneck problem.
02 / 02 / 2016 110

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56
At Sender’s side
02 / 02 / 2016 111
02 / 02 / 2016 112

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57
Layer 3 : Network Layer
The Basic Function of the Network Layer is to control the
subnet operations and addressing mechanism.
The key design issue for this layer is to determine the routing
mechanism for data packets from source to destination.
Routes can be :
(i) Static
The route to be followed by a data packet is being “wired
into” the network almost permanently and rarely changed.
The data packets under all circumstances and network
conditions travel via the fixed up route only.
(ii) Dynamic
The network route is determined for every individual packet
and this is done keeping in mind the dynamics of the
transmission and current load of the network.
02 / 02 / 2016 113
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58
Another important work of Network Layer is congestion
control.
Network Layer is also responsible for providing the logical
address to the data pieces and translating this address into
physical address at the sender’s side and vice versa at the
receivers side.
Network Layer also does the function of determining the quality
of service requirement by deciding the priority of the message.
As different network follows different set of addressing,
during data transmission from one network to another,
there might arise the problem of protocol and addressing
schemes.
Network Layer helps in synchronizing these dissimilarities.
Routers, Gateway, Switches are some of the Network Layer
devices.02 / 02 / 2016 115
At Sender’s side
Translating
logical address
into
physical address
02 / 02 / 2016 116

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59
Translating
physical address
into
logical address
02 / 02 / 2016 117
Layer 2 : Data Link Layer
Data Link Layer is the 2nd Layer of OSI Reference model
for communication. Its basic job is to provide service to
the Network Layer.
At Sender’s side At Receiver’s side
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60
The critical function of the Data Link Layer is to
take a raw transmission facility and transform it
into a line that appears free of undetected
transmission errors to Network Layer.
The Data-Link layer contains two sub layers
that are described in the IEEE-802 LAN
standards:
Logical Link
Control
Medium Access
Control
DataLink
Layer
to Physical Layer
to Network Layer
(i) Logical Link Control (LLC)
(ii) Media Access Control (MAC)
02 / 02 / 2016 119
Logical Link Control (LLC) Protocol helps to maintain
establish and maintain link between the communication
devices.
Medium Access Control (MAC) Protocol
controls the way the nodes/devices
share the same shared channel for
communication.
In any broadcast network, the stations must ensure that
only one station transmits at a time on the shared
communication channel.
Logical Link
Control
Medium Access
Control
DataLink
Layer
to Physical Layer
to Network Layer
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61
Some important functions of the Data Link Layer are,
viz:
(i) Framing and link access:
(Encapsulate datagram into frame by adding
address header to the data.)
(ii) Ensure Reliable delivery of Data Frames
(By using sender-receiver acknowledgement
system.)
(iii) Error Detection
(iv) Error Correction
02 / 02 / 2016 121
Layer 1 : Physical Layer
The Physical Layer is concerned with transmitting raw bits over
a communication channel.
The design issue have to do with making sure that when one side
sends 1 bit, it is received by other side as a 1 bit, not 0 bit.
This is ensured by prefixing/deciding how many volts should be
used to represent 1 bit and how many volts should be used to
represent 0 bit.
Sender’s side Receiver’s side
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62
Another important function for Physical Layer is
deciding the type of direction for transmission.
Physical Layer must find out if Half-duplex or
Full-duplex transmission is required
and ensure that the same facility is available at
both ends.
In short we can conclude that main issues
pertaining Physical layer are largely confined to
electrical, mechanical, physical media and
procedural interfaces.
The End
02 / 02 / 2016 123
Subject Name : Computer Networks Subject Code : BTCS 603
Faculty:
Assistant Professor
Branch : B.Tech. (Computer Science & Engineering) Semester : 6th
1. TCP / IP Protocol suite.
2. Difference between OSI and TCP/IP
3. Acknowledgement Frame.
4. Maximum Transmission Unit (MTU).
5. IP Datagram Format
6. Internet Protocol Version 4 (IPv4) Header
7. Packet Switching
8. Circuit Switching
9. TCP Datagram
10. TCP Header
11. 3 Way Handshaking Protocol
12. UDP Datagram
13. UDP Header
14. Questions for Assignment – II (LDS : 01st march’ 2016)
Topics
124

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63
Open System Interconnection (OSI) Reference Model for
Communication was just a conceptual model designed by
ISO.
The layers of OSI were proposed before most of the
protocols and applications required for communication and
transmission were actually invented.
As some major protocols like IP, TCP, UDP were invented
and applications such as Telnet, FTP were developed, it
was found that instead of 7 layers the same job can be
managed more efficiently if some layers were combined.
This gave rise to more practical approach of layered
architecture for communication which we call as TCP/IP
Protocol Suite which has 4 layers.
TCP/IP Protocol Suite
31 May 2017 125Dr. Siddhartha Sankar Biswas
Telnet FTP SMTP DNS
TCP UDP
IP
ARPANET SATNET
Packet
Radio LAN
Transport Layer
Internet Layer
Link Layer
(Host-to-Network Layer)
Application Layer
TCP/IP Protocol Suite
HTTP

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Application Layer
Application Layer is the highest layer in TCP/IP suite and
contains all higher-level protocols.
Some of the earlier included applications and protocols
were TELNET in which via virtual terminals communication
devices can access each other.
Later on File Transfer Protocols (FTP) was added which
empowered the users to exchange stored file/data via
network.
More efficient and specialized file transfer mechanism was
developed in form of electronic mail which uses the
philosophy of Simple Mail Transfer Protocol (SMTP).
One of the most recent protocol addition is
Domain Name Service (DNS) and
Hyper Text Transfer Protocol (HTTP) which helped in
transferring web pages over World Wide Web31 May 2017 127Dr. Siddhartha Sankar Biswas
Transport Layer
Depending upon the throughput of the end-to-end
connection Transport Layer decideds/ensures the following:
(1) If the transmission of a particular data stream
must takes place in serial transmission or by
parallel transmission.
(2) If Multiplexing/Demultiplexing is required, and if
required provides the mechanism to carry out
these processes.
(3) To ensures Flow Control so that a fast host cannot over
run a slow one during transmission thus avoiding
bottleneck problem.

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65
In simple layman’s term we can say that Transport layer’s
main work is to guarantee the successful and smooth
transmission of data from one end to the other.
(4) Functions such segmenting or splitting on the data done
by layer four that is transport layer.
Transport layer breaks the message (data) into small
units so that they are handled more efficiently by the
network layer and manages the packets sequence.
Internet Layer
The requirement of the fact that
even in the event of absence or failure of a particular
subnet or network hardware,
the transmission must continue between the end nodes,
as long as the sender node is ready to inject data packets
into the network and the receiver node is ready to accept
those packets,
formed the basis of the designing of the internet layer in
TCP/IP Suite which provides a mechanism for
packet-switching connection-less network architecture.

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The goal of this layer is to provide a mechanism so that
every data packet can independently travel of the network
and reach its intended recipient.
This is done by using Internet Protocol (IP) addressing
system where the addressing related information is
encapsulated to each data packet.
Along with routing mechanism, congestion control is another
important issue being looked up by the Internet Layer in
the TCP/IP protocol suite.
Link Layer (Host-to-Network Layer)
The Link Layer is concerned with transmitting raw bits over a
communication channel.
The design issue have to do with making sure that when one side
sends 1 bit, it is received by other side as a 1 bit, not 0 bit.
This is ensured by prefixing/deciding how many volts should be
used to represent 1 bit and how many volts should be used to
represent 0 bit.
Sender’s side Receiver’s side

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Another important function for Link Layer is
deciding the type of direction for transmission.
Link Layer must find out if
Half-duplex or Full-duplex transmission is required
and commit to the particular transmission direction
only after ensuring if similar facilities are
available at both ends.
In short we can conclude that main issues
pertaining Link layer are largely confined to
electrical, mechanical, physical media and
procedural interfaces.
Transport Layer
Internet Layer
Link Layer
Application Layer
Transport Layer
Internet Layer
Link Layer
Application Layer
User
(Sender)
User
(Receiver)
Application Layer interacts with user and accepts data and
instructions.
Process the instruction with the help of various Application
Layer protocols.
Passes the data down to the Transport Layer.
Transport Layer accepts the data from the Application
Layer.
Port Number is added to the data received from
Application Layer. This is done by using protocols like TCP
or UDP are added as header file.
Passes the data down to the Internet Layer.
Internet Layer accepts the data from the Transport
Layer.
IP address (Logical Address) is assigned to the data
received from the Transport Layer by adding IP header.
Passes the data down to the Link Layer.
Link Layer accepts the data from the Internet Layer.
Physical Address (Link address) is assigned to the data
received from the Internet Layer.
Decides various Physical/Electricals signal issue.
Passes the data to the Link Layer of the receiver side.
Link Layer on the receiver’s side accepts the data from
the Link Layer of the sender’s side.
Processes the Physical Address
and if physical address is found to be correct the
datagram is further processed.
It is then passed up to the Internet Layer.
Internet Layer accepts the data from the Link Layer.
Processes the IP Address
and if IP address is found to be correct the datagram is
processed
and passed up to the Transport Layer.
Transport Layer accepts the data from the Internet
Layer.
Processes the Port Address (Port Number) of the
concerned application requested
Process the data and then passed it up to the Application
Layer.
Application Layer accepts the data from the Transport
Layer.
Processes the data
and then passes it up to the user.
TCP/IP Working

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Layer
No.
Layer
Name
6 Presentation Layer
5 Session Layer
4 Transport Layer
3 Network Layer
2 Data Link Layer
1 Physical Layer
7 Application Layer
OSI Reference Model
Difference between
OSI Reference Model and TCP/IP Suite
Layer
Name
Layer
No.
Transport Layer 3
Internet Layer 2
Link Layer
1
Application Layer 4
TCP/IP Suite
Sl.No OSI Reference Model TCP/IP Suite
It is 7 layers model. It is 4 layers model.
The OSI model is a
reference model
The TCP/IP model is an
practical implementation
of the OSI model
Separate Presentation
Layer.
No presentation layer,
characteristics are provided
by application layer.
Separate Session
Layer.
No session layer,
characteristics are provided
by transport layer.
Network layer provides
both connectionless and
connection oriented
services
Network layer provides
only connection oriented
services.
1
2
3
4
5

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69
Sl.No OSI Reference Model TCP/IP Suite
It defines the services,
interfaces and protocols
very clearly and makes a
clear distinction between
them.
It does not clearly
distinguishes between
service interface and
protocols.
The protocol are better
hidden and can be easily
replaced as the
technology changes
It is not easy to replace
the protocols
Most of the protocols
and applications came
after the OSI
Reference Model was
described
TCP/IP model was
designed according to the
protocols and applications
that were earlier invented
6
7
8
Before we understand :
IP Datagram Format
and
Internet Protocol (IPv4) Header
we must revisit the following concepts:
1.Acknowledgement Frame.
2.Maximum Transmission Unit (MTU).
3.Relation between Bit-Byte-Word.

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Sender Receiver
T1
T2
T3
Tn
Acknowledgement Frame Mechanism in Computer Networks
Normal Case
Time
Time
Sender Receiver
T1
T2
T3
Tn
Fault Case : No Acknowledgement Received
T4
Time
Time

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Sender Receiver
T1
T2
T3
Tn
Fault Case : Acknowledgement Received Late
T4
Time
Time
Host A
Host B
Host A wants to send to Host B an
IP datagram of size = 2500 Bytes
MTU = 1400Router
?
Maximum Transmission Unit (MTU)

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MTU = 1400
Packet Size = 2500 Bytes Packet Size =
1300 Bytes
Packet Size =
1200 Bytes
A maximum transmission unit (MTU) is the largest size
packet or frame, specified in octets (eight-bit bytes),
that can be sent in a packet or frame based network such
as the Internet.
The Transmission Control Protocol (TCP) uses the MTU to
determine the maximum size of each packet in any
transmission.
Too large an MTU size may mean retransmissions if the
packet encounters a router that can't handle that large a
packet.
Too small an MTU size means relatively more header
overhead and more acknowledgements that have to be sent
and handled.31 May 2017 144Dr. Siddhartha Sankar Biswas

31-05-2017
73
The Internet de facto standard MTU is 576, but ISPs
often suggest using much more than that.
Recent Windows systems, is able to sense whether your
connection should use 576 or higher and select the
appropriate MTU for the connection.
Fragmentation is carried out in such manner that no. of
bytes in the individual fragments are always divisible by 8.
the reason we would see later on as we study more about
IP Datagram format , IPv4 header, etc
Rough Notes
IP Datagram Format
As we have know that,
in Transport Layer, data received from Application Layer is
broken into smaller fragmented called data packets
and
when these data packets are passed on to the
Network Layer, an IP header file is encapsulated to each
individual data packets.
This encapsulated data packet, which contains the data as
well as the IP header file, is called IP Datagram.
IP
Header
Data
20 to 60
bytes
20 to 65535 bytes

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IPv4 Packet Header
IP Header Data
Word # 1
Word # 2
Word # 3
Word # 4
Word # 5
Service
Service

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Version
The first header field in an IP packet is Version.
It is a four-bit field.
For IPv4, this has a value of 4. (Hence the name
IPv4).31 May 2017 149Dr. Siddhartha Sankar Biswas
Service
Header Length (HL)
The second field (4 bits size) is the Header
Length (HL),
which is the total numbers of 32-bit words in the
header.31 May 2017 150Dr. Siddhartha Sankar Biswas
Service

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Header Length (HL) cont..
Since an IPv4 header may contain a variable number of
options and padding and HL this field specifies the size of
the header.
Since minimum length of the IP Header is 20 bytes
Therefore,
The minimum value for this field is 5 i.e in binary 0101.
Since maximum length of the IP Header is 60 bytes
Therefore,
The minimum value for this field is 15 i.e in binary 1111.
Type of Service
This field is now known as Differentiated Services
(DiffServ).
It is 8-bits long field.31 May 2017 152Dr. Siddhartha Sankar Biswas

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77
Type of Service (cont..)
The first three bits of this field are known as
precedence bits and are ignored as of today.
The next 4 bits represent type of service and
the last bit is left unused (Last Bit is always taken as 0).
The 4 bits that represent TOS are :
(i) 0000 Minimize delay
(ii) 0100 Maximize throughput
(iii) 0010 Maximize reliability
(iv) 0001 Minimize monetary cost
It may be noted that more ToS can be added if
required in future.
Total Length
This 16-bit field defines the entire packet (fragment)
size, including header and data, in bytes.
In other words we can say this is the combined length of
the header and the data in bytes.31 May 2017 154Dr. Siddhartha Sankar Biswas
Service

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The minimum-length packet is 20 bytes
The maximum is 65,535 bytes,
20  00000000010100
65535  11111111111111
Identification
This field is an identification field and is of 16-bits
length.
It is primarily used for uniquely identifying the group of
fragments of a single IP datagram.31 May 2017 156Dr. Siddhartha Sankar Biswas
Service

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Identification (cont..)
When a datagram is fragmented, the value of the
identification field is copied into all fragmented
datagrams.
The Identification number helps the destination
node in
identifying all the fragments of a a particular
datagram
and
reassembling them as per the sequence.
Flags
It is a three-bit field.
It is used to control whether routers are allowed to
further fragment and to indicate and identify the parts of
a packet to the receiver.31 May 2017 158Dr. Siddhartha Sankar Biswas
Service

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1st Bit = Reserved Bit
It should always be 0. Currently it has no use
2nd Bit = Do not Fragment(DF) Bit
If, DF = 1 , the datagram should not be fragmented
further.
DF = 0 , the datagram should be fragmented further.
3rd Bit = More Fragment(MF) Bit
If, MF = 1 , this datagram is not the last fragment.
MF = 0 , this datagram is the last fragment.
1st Bit
Reserved Bit
2nd Bit
DF Bit
3rd Bit
MF Bit
Flag = 3 Bits length
Fragmentation Offset
It is a 13-bits long field.
Fragmentation Offset shows
the relative positions of the fragment with respect to the
beginning of the original un-fragmented IP datagram,
measured in number of units of eight-byte blocks

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The max size of an IP datagram can be 65535 bytes.
But, many network devices may not allow a packet size
of that many bytes,
for example, in many routers the max packet size
allowed is 1400 to 1500 bytes.
Also in the Link Layer, there might arise some
practical problem while converting bits into
corresponding voltage (and vice-versa) if the data size
is too large
So, the original datagram must sometimes might has to
be fragmented.
When an IP datagram is fragmented, each fragment is
treated as a separate and independent datagram.31 May 2017 161Dr. Siddhartha Sankar Biswas
How Fragmentation Offset
is calculated is done
Byte 0 Byte 3999
Original IP Datagram
Size = 4000 Bytes
(Before Fragmentation)
Lets say MTU = 1400 Bytes
Byte 0 Byte 1399
Offset = 0/8 = 0
Offset = 1400/8 = 175
Offset = 2800/8 = 350
Byte 1400 Byte 2799
Byte 2800 Byte 3999

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When an IP datagram is fragmented, each fragment is
treated as a separate datagram.
All the fragmented datagrams are reassembles at the
final destination, not at a router.
Router does not reassemble because
it may happen that when the fragmented datagram
traverses the network further,
it may has to encounter another router of even smaller
MTU than the routers it has earlier encountered.
Since each datagram fragment is independent and
treated separately, so each has its own header.
The identification number of the original un-fragmented
IP datagram is copied into each of the fragments.
Time To Live (TTL)
In case of IP Datagram, TTL field controls the maximum
number of routers visited by the datagram.
TTL is usually set to 32 or 64.
It gets decremented by each router that processes
the particular datagram.
A router discards the datagram when TTL reaches 0.31 May 2017 164Dr. Siddhartha Sankar Biswas
Service

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Protocol
This field defines the protocol used in the data portion of
the IP datagram.
The Internet Assigned Numbers Authority maintains a list
of IP protocol numbers which was originally defined in RFC
790.31 May 2017 165Dr. Siddhartha Sankar Biswas
Service
Example
Protocol Name
Protocol Number
(in Decimal)
Protocol Number
(in Binary)
Internet Control Message
Protocol (ICMP) 1 00000001
Internet Group
Management Protocol
(IGMP)
2 00000010
Transmission Control
Protocol (TCP) 6 00000110
User Datagram Protocol
(UDP) 17 00010001
Open Shortest Path First
(OSPF) 89 01011001
Stream Control
Transmission Protocol
(SCTP)
132 10000100
Protocol Name/Abbreviation and Number according to RFC 790

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Header Checksum
The 16-bit checksum field is used for error-checking of
the header.
( Error checking is done only for the header part and not
for the data part of the datagram ).
Service
Since some of the fields in the header may change due to
fragmentation process,
therefore
Header Checksum field is recomputed and verified at each
network node (such as router) that the Internet header is
processed during its traversing of the network from sender
to receiver host.
When a packet arrives at a router,
the router calculates the checksum of the header and
compares it to the checksum field.
If the values matches, the router processes the IP
datagram packet.
If the values do not match, the router discards the
packet.

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Source IP Address
This is a 32 Bits long field which contains the IP
address of the original un-fragmented IP
Datagram.31 May 2017 169Dr. Siddhartha Sankar Biswas
Service
Destination IP Address
This is a 32 Bits long field which contains the IP
address of the final destination to which the
original un-fragmented IP Datagram is supposed to
reach
Service

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Optional Field
It is a variable length (0-320 bits) optional field which is
used only for some special purposes.
Most of the time it is used for Network path testing,
error checking, adding some extra security to data, etc..31 May 2017 171Dr. Siddhartha Sankar Biswas
Service
Before we understand :
Packet Switching
And
Circuit Switching
Transmission Control Protocol (TCP)
and
TCP Segment and Header
we must revisit the concepts of:
Packet-switched and circuit-switched networks use
two different technologies for sending messages
and data from one point to another.
Each has its advantages and disadvantages
depending on what you are trying to do.31 May 2017 172Dr. Siddhartha Sankar Biswas

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Packet Switching
In packet-based networks, the message
gets broken into small data packets.
These packets are sent out from the senders computer
and
they travel around the network seeking out the most
efficient route to the receiver as per real-time
situation and availability.
Each packet traverse the network independently and
may go on a different route from the other packets.
Also the individual packets may not necessarily always
follow the shortest route.

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Advantages of Packet Switching
1. Security.
2. Bandwidth used to full potential.
3. Devices of different speeds can communicate.
4. Not affected by line failure (redirects signal).
5. Availability – no waiting for a direct connection to
become available.
6. During a crisis or disaster, when the public
telephone network might stop working, e-mails and
texts can still be sent via packet switching.
Disadvantages of Packet Switching
1. Under heavy use there can be a delay
2. Data packets can get lost or become corrupted
3. Protocols are needed for a reliable transfer
4. Not so good for some types data streams
(e.g. real-time video streams can lose frames due to
the way packets arrive out of sequence)

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Circuit Switching
Circuit Switching
Circuit switching was designed in 1878 in order to send
telephone calls down a dedicated channel.
There are three phases in circuit switching:
1. Establish
2. Transfer
3. Disconnect
The communication channel once established, remains open
and in use throughout the whole call and cannot be used
by any other data or phone calls.
The telephone message is sent all together; it is not
broken up and also the message arrives in the same
order that it was originally sent.31 May 2017 178Dr. Siddhartha Sankar Biswas

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In modern circuit-switched networks, electronic signals
pass through several switches before a connection is
established.
During a transmission no other network traffic can use
those switches and the resources remain dedicated to
the circuit during the entire data transfer and the
entire message follows the same path.
Circuit switching can be analog or digital.
With the expanded use of the Internet for voice and
video, analysts predict a gradual shift away from
circuit-switched networks.
A circuit-switched network is excellent for data that needs
a constant link from end-to-end, for example, real-time
video.
Advantages of Circuit Switching
1. Circuit is dedicated to the call with no
interference, no sharing.
2. It guarantees the full bandwidth for the
duration of the transmission.
3. It guarantees quality of service.

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Disadvantages of Circuit Switching
1. Inefficient – the equipment may be unused for
a lot of the call; if no data is being sent, the
dedicated line still remains open.
2. It takes a relatively long time to set up the
circuit.
3. During a crisis or disaster, the network may
become unstable or unavailable.
4. It was primarily developed for voice traffic
rather than data traffic.31 May 2017 181Dr. Siddhartha Sankar Biswas
One of the important work of Transport Layer is to decide upon which
type of communication service it should offer for the data it is
receiving from Application layer.
Depending upon the requirement of the Application Layer protocols and
user instructions received Transport layer goes for one of the
following services:
1. Connection Oriented Service.
2. Connection Less service.
If the Transport Layer offers Connection Oriented Service for the
data it receives from the Application Layer,
then Transport layer Information like Sequence Number, Post Number,
etc are encapsulated using Transmission Control Protocol (TCP) Header.
If the Transport Layer offers Connection Less Service for the data it
receives from the Application Layer,
then Transport Layer Information like Sequence Number, Post Number,
etc are encapsulated using User Datagram Protocol (UDP) Header.
TCP and UDP

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Transmission Control Protocol
TCP service is obtained by having both the sender and
receiver first create end points called sockets.
TCP provides Connection Oriented services,
i.e. connection must be explicitly established between a
socket on the sending machine and a socket on the
receiving machine,
i.e. the communication channel must be point-to-point.
Each socket has its unique socket number or address.
Socket address consists of two things, viz:-
1.IP address of the host.
2.16-bit port number.
A socket may be used for multiple connections at same
time, i.e more than one connections may get terminated at
same socket and they are identified as (socket1, socket2).31 May 2017 183Dr. Siddhartha Sankar Biswas
When a Application Layer data is passed on to the TCP,
depending upon the situation and requirement,
TCP may process and send it immediately
or
buffer it in and collect a large amount of data to send
all of them at once.
But there is also provision in TCP where
Application Layer may command the TCP
to send a data immediately on urgent basis.
The sending and receiving TCP entities exchange data in
the form of segments.
Transmission Control Protocol

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TCP segment consists of two things, viz:
1.TCP Header.
2.Data field.
TCP Header Data
TCP Segment
20 - 60 Bytes
The size of a particular segment is decided by TCP
software on real-time basis and as per the requirements
of the network.
The size of segment is limited by the following two
factors:-
1.Each TCP segment (TCP Header + Data) must fit
IP Payload.
2. MTU of the network and its devices.
Segment size must be optimal to the network throughput,
otherwise
if the segment size is too large it may get struck and the
network devices such as routers miy have to provide time
to fragment it.
if segment size is too small, it may cause congestion as
many number of segments has to be transmitted.31 May 2017 186Dr. Siddhartha Sankar Biswas

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TCP Header Data
TCP Segment
Source Port Number
(16 Bits)
Destination Port Number
(16 Bits)
Sequence Number
(32 Bits)
Acknowledgement Number
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
(Variable Length from 0 to 40 Bytes)
TCP Header Format
20 - 60 Bytes
Source Port and Destination Port
These are each of 16 bits length field.
Each host may decide for itself how to allocate its port
starting from 256.
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field

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Sequence Number
Sequence number is 32 bits long which helps in identifying every
fragment uniquely.
Initial Sequence Number (ISN) of a connection is set during connection
establishment
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
Acknowledgement number is 32 bits long.
This field plays important role for Acknowledgement purpose.
This field is used to intimidate the sender that receiver has received a
TCP datagram of the mentioned sequence.
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field

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Header Length (HL)
Header Length field is of 4 Bits length.
This field tell the number of 4-Byte words in the header.
TCP Header can be between 20 to 60 bytes.
Therefore, number of 4-Byte words can be between : 5 and 15.
Therefore, Binary value of this field can vary between 0101 to 1111
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
Reserved Feild
This is a 6 Bits long field.
This field is currently unused and
has been reserved for any future use.
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field

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Flag
There are 6 flags of 1 Bit each.
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
Flag : URG – Urgent Pointer Flag
If URG = 1 , The urgent Pointer Field is valid, process it.
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
If URG = 0 , The urgent Pointer Field is invalid, ignore the field and
no need to process it.

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Flag : ACK – Acknowledgement Flag
If ACK = 1 , The value of Acknowledgement Field is valid, therefore it
can be processed.
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
If ACK = 0 , The Acknowledgement Field is invalid, ignore the field
and no need to process it.
Flag : PSH – Push Function Flag
If PSH = 1 , the receiver is requested to pass on the fragment to the
application immediately upon arrival and not wait for the
full buffer to arrive.
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
If PSH = 0 , The fragment can be processed with normal procedure
and receiver may wait for full buffer to arrive if it wants.

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Flag : RST – Reset the Connection Flag
If RST = 1 , the receiver is informed by the sender that sender is
terminating the connection due to some problem.
This may happen due many reason including that sender is facing
congestion, transmission error, etc.
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
If RST = 0 , Receiver continuous with further processing.
Flag : SYN – Synchronize Flag
This flag is used to signify the receiver that sender is trying to
Synchronize a connection with the receiver.
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
This field is most of the time
used by the sender
to inform the intended receiver that
the sender wants to establish a connection with it.
This is done with help of ACK flag.

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Lets see how ACK and SYN Flags helps in TCP connection
establishment between any two host.
It may be noted that TCP connections are Full Duplex,
therefore both the nodes can exchange data with each
other and that too simultaneously.
The method is known as
Three-Way Handshaking
In Three way handshaking,
TCP Segments will be exchanged
between the two hosts for 3 times.
It may be noted that during handshaking phase
TCP segments would not carry any data.
Host : 1 Host : 2
Time
Time
( Ack of receiving M2)
Rough Page

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Host : 1 Host : 2
Time
Time
Host-1 sends “TCP Connection Request” to Host-2:
Step-1
Flag SYN is set 1
Which indicates the Host-2 that
this is a ‘TCP Request’ segment.
It may be noted that
SYN is set to 1 , only for and during TCP Connection establishment
phase.
After connection has been established, for rest of the time,
this flag would always set to 0 (zero).
In Connection Request Step, the TCP segment has no use
of acknowledgement number.
So, ACK field is set to 0 (zero).
It request message has Flag : ACK = 0 , SYN = 1

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Host-2 replies to Host-1’s request
The reply has Flag : ACK = 1 , SYN = 1
SYN = 1, indicated this TCP segment is for synchronization purpose
(i.e. for connection establishment purpose)
ACK = 1, Indicates acknowledgement field is valid.
Host-2 send TCP header with a Initial Sequence number of its choice.
and Sends the Acknowledgement Number by incrementing Host-1’s
sequence Number by 1
Step-2
The reply contains acknowledgement information which indicates Host-1
that Host-2 has received “TCP Request” segment.
The main purpose of this step is for
informing Host-2 that Host-1 has received its reply,
i.e. Host-1 acknowledges receiving Host-2’s reply
ACK is set to 1, which indicates acknowledgement field is valid.
Acknowledgement Number is set bye incrementing Host-2’s sequence
Number by 1
And since this step is just for Acknowledgement purpose and connection
was already established at Step-2,
Therefore SYN is set to 0 (zero)
The step-3 Reply has Flag : ACK = 1 , SYN = 0.
Step-3
In essence SYN flag is used to denote
CONNECTION REQUEST and CONNECTION ACCEPTED,
and ACK flag is used to distinguish between these two.

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Flag : FIN – Finished Transmisssion
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
If FIN = 1 , the sender finished all transmission i.e. it has no more
data for transmission,
the connection may be therefore ended/terminated.
If FIN = 0 , the connection should remain on , sender has more data
for transmission.31 May 2017 205Dr. Siddhartha Sankar Biswas
Host : 1 Host : 2
TCP Termination Operation using FIN
Time
Time

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Window Size
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
It is 16 bits long field.
This field tells the length of the whole TCP segment (TCP Header + Data)
The value of this field indicated the number of total number of Bytes
of the TCP segment.
So its value can vary from 20 to 65535 in binary.
Checksum
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
Checksum field is used for error control
and also to ensure reliability.
This is usually implemented by using the process of
1’s compliment check.31 May 2017 208Dr. Siddhartha Sankar Biswas

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Urgent Pointer
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
This field is considered valid only if Flag URG is set 1.
It contains the number which should be added to the sequence number
so as to get the sequence number of the urgent datagram .
Optional Field
Source Port Number
(16 Bits)
(16 Bits)
Sequence Number
(32 Bits)
(32 Bits)
Window Size
(16 Bits)
HL
(4 Bits)
Reserved
(6 Bits)
U
R
G
A
C
K
P
S
H
R
S
T
S
Y
N
F
I
N
Checksum
(16 Bits)
Urgent Pointer
(16 Bits)
Optional Field
The Optional Field is variable length field.
It can be of size from 0 to 40 Bytes.
This field is used if any extra data (meta data) about the
TCP segment is needed to be passed from the sender to
the receiver.31 May 2017 210Dr. Siddhartha Sankar Biswas

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User Datagram Protocol (UDP)
User Datagram Protocol (UDP) is a connectionless
and unreliable protocol.
Though “reliability factor” is its limitation but it
has its own advantage also.
If a host wants to send a small data to another
host without much caring about reliability, UDP is
the best protocol to implement in such case.
UDP Datagram consists of two things, viz:
1.UDP Header.
2.Data field.
UDP Header Data
UDP Segment
8 Bytes
UDP is a very simple protocol and uses a very small
overhead and therefore it can be processed very fast and
very easily.
Also there is no acknowledgement field therefore UDP
helps in reducing congestion.

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UDP Header Data
UDP Segment
Source Port Number
(16 Bits)
(16 Bits)
Total Length
(16 Bits)
Checksum
(16 Bits)
UDP Header Format
Source Port Number and Destination Port Number
They are 16 Bits length fields.
Therefore their value can be anything between 0 to 65535.
Source Port Number
(16 Bits)
(16 Bits)
Total Length
(16 Bits)
Checksum
(16 Bits)
If the Source or Destination is a client ,
the port number is generally an ephemeral port number
(i.e. the port number is a temporary port number assigned to a port
for only a particular time period)
If the Source or Destination is a server ,
the port number is generally a well known port number of
an well known application.31 May 2017 214Dr. Siddhartha Sankar Biswas

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Source Port Number
(16 Bits)
(16 Bits)
Total Length
(16 Bits)
Checksum
(16 Bits)
Total Length
It is 16 Bits length field.
It is total length of the UDP Datagram (UDP Header + Data)
Its value can be anything between 20 to 65535.
The Length Field of UDP datagram is actually not
necessary.
Since UDP datagram is encapsulated in IP Datagram,
therefore Total length of UDP can easily be calculated
from IP Datagram
UDP Total Length = IP Datagram Length - IP Header’s Length
The designers of the UDP datagram felt that
it will be more efficient for the destination’s UDP to
calculate the total length of UDP datagram from the UDP
Header
rather than ask the IP software to provide it.
We must also remember the fact that when the Network
layer delivers the UDP datagram to the network layer,
IP Header has already been dropped
and therefore again asking the Network Layer to process
the IP header to provide information about the
UDP datagram total length would be more complex and
less efficient.31 May 2017 216Dr. Siddhartha Sankar Biswas

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Source Port Number
(16 Bits)
(16 Bits)
Total Length
(16 Bits)
Checksum
(16 Bits)
Checksum
Checksum field is used for error control
and also to ensure reliability.
This is usually done by using the process of 1’s compliment
check.
Assignment – II
Q1. Explain the following Transmission media :
i. Magnetic Media
ii. Twisted Pair
iii. Baseband Coaxial Cable
iv. Broadband Coaxial Cable
v. Fiber Cable
vi. Structured Cabling
vii.Cable Mounting
viii.Cable Testing
ix. Wireless transmission
x. The telephone system
xi. Narrowband ISDN
xii.Broadband lSDN
xiii.ATM
Q2. Explain difference between TCP and UDP.
(LDS : 01st march’ 2016)
Q3. Explain difference between Connectionless and
Connection oriented services.

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Subject Name : Computer Networks Subject Code : BTCS 603
Faculty:
Assistant Professor
Branch : B.Tech. (Computer Science & Engineering) Semester : 6th
Internet Control Protocols
Topics
219
1 Introduction to MAC Address
2 Introduction to DNS
3 ARP
4 RARP
5 ICMP
31 May 2017
Internet Control Protocols
1 ARP
2 RARP
3 ICMP
Before we understand the above protocols,
we must understand the following :-
MAC Address
By studying about MAC address we will understand
why we need these Internet Control Protocols.
DNS
We will study just introduction.
Later on we will study it in full detail.31 May 2017 220Dr. Siddhartha Sankar Biswas

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A Media Access Control Address (MAC address) is a unique
identifier assigned to network interfaces for
communications on the physical network segment.
MAC addresses are used as a network address for most
network technologies, including Ethernet.
Logically, MAC addresses are used in the Media Access
Control protocol sub layer.
MAC addresses are most often assigned by the
manufacturer of a Network Interface Controller (NIC)
and
is stored in its hardware, such as the card's read-only
memory (ROM) or some other firmware mechanism.
Media Access Control Address (MAC address)
A MAC address is usually encodes the manufacturer's
registered identification number and may be referred to as
the Burned-in Address (BIA).
It is also known as an Ethernet Hardware Address (EHA),
or simply Hardware Address or Physical Address.
A machine connected to a network node may have multiple
NICs and each NIC must have one unique MAC address.
IP addresses are chosen by the local system administrator
to suit the local network.
MAC addresses are built into the interface hardware by
the manufacturer.

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The MAC address is the hardware address, i.e. it is hard
coded in the NIC of the machine. So it cannot be changed.
MAC Addresses do not in any way gives the ability to tell
where a node is on the network
i.e. there is no mechanism via which MAC address can be
used for finding the subnet.
The only information that can be gleaned from them is the
vendor and the node identifier.
Though DNS has many jobs
but the most important work it does is:
DNS translates easily memorized domain names to
the numerical IP addresses
required for the purpose of locating computer
services and devices worldwide.
Domain Name System (DNS)
We will study DNS in full detail later,
but for understanding ARP
the above understanding of DNS must be kept in mind.

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Address Resolution Protocol (ARP)
Address Resolution Protocol (ARP) is a protocol for mapping
Logical Address i.e. Internet Protocol address (IP address) to a
Physical Address that is recognized in the Local network.
Address
Resolution
Protocol
(ARP)
Logical address
(IP Address)
Physical address
(MAC Address)
ARP Cache
Logical Address MAC Address
192.168.0.55 00:eb:24:b2:05:ac
192.168.0.78 10:un:32:t2:76:xb
167.198.10.77 36:cv:44:z7:98:dk
A table, usually called the ARP cache, is used to maintain
a correlation between each MAC address and its
corresponding IP address.
ARP provides the protocol rules for making this correlation
and providing address conversion in both directions.

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Consider the following case:
Two computers Comp-1 and Comp-2 are in an office,
connected to each other on the office Local Area Network (LAN) by
Ethernet cables and network switches, with no intervening routers.
Comp-1 wants to send a packet to Comp-2.
Through DNS, Comp-1 determines that Comp-2's IP address is
192.168.0.55.
But in order to send the message, it also needs to know Comp-2's MAC
address.
Comp-1 searches its cached ARP table to look up
192.168.0.55 for any existing records of Comp-2's MAC
address.
Scenario - 1
ARP Cache of Comp-1
167.198.10.77 36:cv:44:z7:98:dk
192.168.0.78 10:un:32:t2:76:xb
163.129.10.77 36:cv:44:z7:98:dk
192.168.0.55 00:eb:24:b2:05:ac
166.198.10.77 36:cb:24:k7:98:do
155.14.110.79 36:nn:24:k9:67:vi
196.198.10.77 98:cb:24:k7:98:do
If the MAC address is found, Comp-1 sends the IP packet on the link
layer to address 00:eb:24:b2:05:ac via the local network cabling.
......
......
Comp-1 searches its
ARP Cache for
IP of Comp-2: 192.168.0.55
ARP Cache entry for IP of
Comp-2: 192.168.0.55 is found
in the ARP Cache of Comp-1.
MAC address of Comp-2 is
00:eb:24:b2:05:ac

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ARP Cache of Comp-1 don't produce any result for: 192.168.0.55
In this scenario, Comp-1 will send a broadcast ARP Request to all the
nodes on the network, requesting an answer for 192.168.0.55
(destination FF:FF:FF:FF:FF:FF MAC address which is accepted by all
computers)
Comp- 1 Comp- 2
Comp- 3 Comp- 4 Comp- n
192.168.0.55
Scenario - 2
All the nodes discards the ARP Request sent by Comp-1 except the
node which has IP: 192.168.0.55. This node is Comp-2.
ARP Cache of Comp-1 don't produce any result for: 192.168.0.55
In this scenario, Comp-1 will send a broadcast ARP Request to all the
nodes on the network, requesting an answer for 192.168.0.55
Comp- 1 Comp- 2
192.168.0.55
Scenario - 2
All the nodes discards the ARP Request sent by Comp-1 except the
node which has IP: 192.168.0.55. This node is Comp-2.

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Comp2 accepts the ARP Requests of Comp-1 and responds
with its MAC address (and its IP)
i.e. Comp-2 sends an unicast ARP Reply to Comp-1.
Comp- 1 Comp- 2
Comp-2 may insert an entry for Comp-1 into its own ARP
cache table for future use.
The response information is cached in Comp-1's ARP table
and the message can now be sent.31 May 2017 231Dr. Siddhartha Sankar Biswas
Reverse Address Resolution Protocol (ARP)
ARP Cache
192.168.0.55 00:eb:24:b2:05:ac
192.168.0.78 10:un:32:t2:76:xb
167.198.10.77 36:cv:44:z7:98:dk
A table, usually called the ARP cache, is used to maintain a
correlation between each MAC address and its corresponding IP
address.
ARP provides the protocol rules for making this correlation and
providing address conversion in both directions.
Address Resolution Protocol (ARP) is a protocol for mapping
Logical Address i.e. Internet Protocol address (IP address) to a
Physical Address that is recognized in the Local network.

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In the early days of Computers,
when memory was costly, some
computers were created for diskless
booting.
A Diskless clients connected to a
computer network don't have a place
to store there IP number.
RARP (Reverse Address Resolution
Protocol) is an absolute protocol by
which a computer connected in a local
area network can request to learn its
IP address from a gateway server's
Address Resolution Protocol (ARP)
table or cache.
Reverse Address Resolution Protocol (RARP)
Reverse
Address
Resolution
Protocol
(RARP)
Logical address
(IP Address)
Physical address
(MAC Address)
A network administrator
creates a ARP cache table in a
local area network's gateway
router that maps the
physical/machine address
(or Media Access Control -
MAC address) to corresponding
Internet Protocol addresses.
ARP Cache of LAN
at Gateway Router
167.198.10.77 36:cv:44:z7:98:dk
192.168.0.78 10:un:32:t2:76:xb
163.129.10.77 36:cv:44:z7:98:dk
192.168.0.55 00:eb:24:b2:05:ac
166.198.10.77 36:cb:24:k7:98:do
155.14.110.79 36:nn:24:k9:67:vi
196.198.10.77 98:cb:24:k7:98:do
......
......

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When a new machine (diskless machine which don’t had space for
storing its IP Address) is set up, its RARP client program requests
from the RARP server on the router to be sent its IP address.
Since the RARP client don’t have any knowledge of any other’s Physical
or IP address, the RARP is broadcasted.
All the machines, except the RARP server, discards the requests. Only
RARP server accepts the requests and process further.
Comp- 1
RARP Server
ARP Cache of LAN at
Gateway Router (RARP server)
167.198.10.77 36:cv:44:z7:98:dk
192.168.0.78 10:un:32:t2:76:xb
163.129.10.77 4A:6E:A5:57:82:36
192.168.0.55 00:eb:24:b2:05:ac
166.198.10.77 36:cb:24:k7:98:do
155.14.110.79 36:nn:24:k9:67:vi
196.198.10.77 98:cb:24:k7:98:do
..
....
......
This IP address is unicasted to the computer who sent the RARP
request for MAC 4A:6E:A5:57:82:36.
RARP server searches its
ARP Cache for
MAC Address entry of
4A:6E:A5:57:82:36
MAC address entry for
4A:6E:A5:57:82:36 is found
in the ARP Cache of
Gateway Router (RARP Server)
Corresponding IP address is
163.129.10.77

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Assuming that an entry has been set up in the router table, the RARP
server will return the IP address to the machine which can store it for
future use.
Comp- 1
RARP Server
ICMP is a network protocol useful in Internet Protocol (IP)
network management and administration.
ICMP is a control protocol, meaning that it does not carry
application data, but rather information about the status
of the network itself.
ICMP can be used to report:
1.Errors in the underlying communications of network
applications.
2.Availability of remote hosts.
3.Network congestion.
Internet Control Message Protocol - ICMP

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ICMP messages is generated by router R1, in response to
message sent by H0 to H1 and forwarded by R0.
This message could, for instance be generated if the MTU
of the link between R0 and R1 was smaller than size of the
IP packet, etc.
The ICMP message is returned to H0, since this is the
source address specified in the IP packet that suffered
the problem.31 May 2017 239Dr. Siddhartha Sankar Biswas
240
ICMP Header
Type: 8 Code : 8 Checksum : 16
Content specific
0 15 16 31
• Type : Relevant ICMP message
• Code : More details information
• Checksum : It is used for checking error in the ICMP header/data.
31 May 2017 Dr. Siddhartha Sankar Biswas

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241
ICMP Types
Type Code Meaning
0 0 echo reply
3 0 network unreachable
3 1 host is unreachable
3 3 port is unreachable
4 0 source quench
5 0 redirect
8 0 echo request
9/10 0 router discovery/advertisement
11 0 time exceed
12 0 parameter problem
13/14 0 time stamp request
17/18 0 network request/reply
242
ICMP type 0/8 echo request/reply
• PING sends ICMP type 8 echo request to a
node and expects an icmp type 0 echo reply
• identifier and sequence number are used to
identify datagrams.
Type = 0 or 8 code checksum
identifier Sequence number
Optional data

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243
PING : ICMP Echo Request/Reply
• PING sends and ICMP echo request to
a remote host, which then return an
ICMP echo reply to the sender
• All TCP/IP node is supposed to
implement ICMP and respond to ICMP
echo
PING
Reply
244
ICMP type 3 Destination
Unreachable
• Router is unable to deliver datagram, it
can return the ICMP type 3 with failure
code
• Internet header plus 64 bits of original
datagram are used to identify the
datagram caused the problem
Type = 3 code checksum
unused
IP header + 64 bits of original data

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245
ICMP type 4 Source Quench
• Router detected hosts were overload
would send this message to hosts that
were the major cause
• the host would then reduce the rate at
which subsequence message are sent
• RFC recommends that router must not
generate source quench, host must still
accept the message but need take no
action
Unused (must be 0)
246
ICMP type 5 Route Change Request
• Used only by router to suggest a more
suitable route to the originator (also
called ICMP redirect)
IP address of a more suitable router

Basics Computer Network - Dr. Siddhartha Biswas Sir

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