CSEN2021 COMPUTER

NETWORKS
UNIT I
What’s the Computer Network
PC • Is connecting of millions of computing devices,
server running network apps.
wireless
• Computing devices are hosts or end systems
laptop namely PC, server, laptop, smartphone
smartphone

 They are connected by communication links

wireless Namely fiber, copper, radio, satellite cables
links with different
wired
links transmission rate: bandwidth

 forwardpackets (chunks of data) using

router switches using routers and switches
HOST
“Fun” Internet appliances

Web-enabled toaster +
weather forecaster

IP picture frame
http://www.ceiva.com/

Tweet-a-watt:
monitor energy use

Slingbox: watch,
control cable TV remotely
Internet
refrigerator Internet phones
• Internet: is connecting“network of
networks”, by interconnecting ISP(Internet
Service Provider)
• protocols control sending, receiving of
messages
• e.g., TCP, IP,UDP, HTTP, Skype, 802.11
• Internet standards
• RFC: Request for comments
• IETF: Internet Engineering Task Force
Internet Service Providers (ISPs)
•ISP is like a gateway that allows our devices connect to the Internet and
access online resources.
•ISPs operate in a hierarchy:
• Tier-1 ISPs: Large backbone networks that interconnect globally.
• Tier-2 ISPs: Regional providers that connect to Tier-1 ISPs.
• Tier-3 ISPs: Local providers that connect end-users to the Internet.
mobile network

global ISP

home
network
regional ISP

institutional
network
 Internet structure: network of
networks
Tier 1 ISP Tier 1 ISP Google

IXP IXP IXP

Regional ISP Regional ISP

access access access access access access access access

ISP ISP ISP ISP ISP ISP ISP ISP

• tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national &
international coverage
• content provider network (e.g, Google): private network that connects it
data centers to Internet
• An Internet Exchange Point (IXP) is a physical location where different Internet
Level 3 Communications (now part of
CenturyLink/Lumen):A major Tier-1
network provider offering global IP
transit and backbone connectivity.
Sprint (now part of T-Mobile):A
significant telecom and Internet
provider with strong peering and transit
services.
AT&T:A large telecommunications and
Internet service provider with its global
network infrastructure.
NTT Communications:A Japanese-
based global network provider with
extensive peering and IP transit
services.
Tier 1 ISPs in India include: Bharti, Reliance, Tata, and VSNL. Tier 2 ISPs are similar to Tier 1s, however they aren't big enough
to connect to every network and so have to purchase transit from Tier 1.Example of Tier2 are Comcast, Cox, Charter
Communications, China Telecom, British Telecom, Vodafone, Virgin Media, and Softbank Japan. Tier 3 internet service
providers (ISPs) in India: Jio, Airtel, BSNL, and Vi.
IP (Internet Protocol):
• IP (Internet Protocol) is a set of rules that govern how
data packets are addressed, routed, and delivered across
interconnected networks.
• Two versions: IPv4 and IPv6.
•IPv4: 32-bit address (e.g., 192.168.1.1).
• Supports about 4.3 billion addresses.
•IPv6: 128-bit address (e.g.,
2001:0db8:85a3:0000:0000:8a2e:0370:7334).
• Supports 3.4×10³⁸ addresses, designed to handle the
growing number of Internet-connected devices.
IPV6 FORMAT
Domain Name System (DNS)
•DNS maps domain names (e.g., www.google.com) to IP addresses.
•Example:
• Domain: www.google.com
• IP Address: 142.250.190.14
MAC Address
•The MAC (Media Access Control) address is a hardware address
assigned to a network interface card (NIC).
•NIC is a hardware component that connects devices to a network.
• IP addresses are logical and changeable, MAC addresses are
permanent and unique for each device.
How Data Travels on the Internet
1.Packet Creation:
1. Data is split into packets, each containing a header (addressing information) and a
payload (data).
2.Routing:
1. Packets travel from the source to the destination via routers using routing algorithms
(e.g., Link State, Distance Vector).
3.Reassembly:
1. At the destination, packets are reassembled into the original data.
•TCP (Transmission Control Protocol): 3 way handshake protocol
• Ensures reliable delivery of data packets with error checking.

SYN (Synchronize):
•The client sends a TCP segment with the
SYN flag set and an initial sequence
number (ISN).
SYN-ACK (Synchronize-
Acknowledge):
•The server responds with a TCP segment
with both SYN and ACK flags set.
ACK (Acknowledge):
•The client responds with an ACK
segment, acknowledging the server’s

ISN(Initial Sequence Number

represents starting point for a sequence
of bytes, ensuring order of data
segment and error recovery through
retransmission).
•UDP (User Datagram Protocol):
• Used for faster, connectionless communication (e.g., video streaming).
Client Request:
•The client sends a UDP packet to the server with a request (e.g., a query
or command).
Server Response:
•The server processes the received request.
• Infrastructure that provides services to applications:
• Web, VoIP(Voice over Internet Protocol ), email, games, e-
commerce, social nets, …
• An application programming interface (API) is a set of
rules that allows software applications to communicate with
each other.
What’s a protocol? protocols are set of rules, define format, order of
messages actions taken on message transmission, receipt among network
entities

a human protocol and a computer network protocol:

Hi TCP connection
request
Hi TCP connection
response
Got the
time? Get http://www.awl.com/kurose-ross
2:00
<file>
time

Q: other human
protocols?
 Physical media
refers to the physical medium through which data is transmitted from one device to another within a network.

• bit: propagates between transmitter/ Twisted pair (TP)

receiver pairs Twisted Pair cables are used in
telephone lines to provide data and
• physical link: what lies between voice channels.
transmitter & receiver
Example two insulated copper wires
• guided media: (wired communication or • Category 5: 100 Mbps, 1 Gpbs
bounded transmission media) Ethernet
• The signal energy propagates through • Category 6: 10Gbps
wires in guided media. : copper,
fiber, coax
• unguided media: (wireless
communication or unbounded
transmission media)
• The signal energy propagates through
the air in unguided media e.g., radio
coaxial cable: fiber optic cable:
 is made of glass fiber and
 is a type of guided media made
of Plastics, and copper wires which transmit data signals as light
transmit the signal in electrical  used for long-distance and
form high-performance data
Example:two concentric copper networking.
conductors  high-speed operation:
 high-speed point-to-point
• bidirectional
transmission (e.g., 10’s-100’s
• broadband: Gpbs transmission rate)
• multiple channels on cable  low error rate
• HFC
 NETWORK EDGE
• Network edge: network edge is where your device connects
to the Internet to send and receive information.
• Devices are Hosts(end system) namely clients and servers

 access networks, physical media

are either wired, wireless
communication links

 Network Core:
 interconnecting routers to routers from
network of networks
Q: How to connect end systems
to edge router?
• residential access nets
• institutional access networks
(school, company)
• mobile access networks

• Bandwidth of access network?

• Is maximum amount of data that
can be transmitted over a network. It
is measured in bits per
second(Mbps or Gbps)or Mega bits
per second or Giga bits per second
How the Network Edge Works
•The network edge works by connecting end devices (like laptops,
smartphones, and IoT devices) to the Internet through access networks
(e.g., Wi-Fi, DSL, 4G/5G). It enables these devices to send data (e.g., web
requests) and receive responses (e.g., website content) by communicating
with the network core(routers), which routes the data to its destination.
 Access net: digital subscriber line (DSL)
DSL is used to transmit digital data over traditional telephone lines
(copper wires).
central office telephone
network

DSL splitter
modem DSLAM

ISP
voice, data transmitted
at different frequencies over DSL access
dedicated line to central office multiplexer

DSLAM(acts as a central point where multiple customer DSL lines (from homes or
businesses) are aggregated and connected to a larger backbone network, ISP)
 data over DSL phone line goes to Internet
 voice over DSL phone line goes to telephone net
 < 2.5 Mbps upstream indicates direction of data transfer(Data flows from the client to the
server), transmission rate (typically < 1 Mbps)
 < 24 Mbps downstream(Data flows from the server to the client) ,transmission rate
(typically < 10 Mbps)
 Transmission Rate is the speed at which data is transmitted across a network
 Access net: cable
network
cable headend

cable splitter
modem

C
O
V V V V V V N
I I I I I I D D T
D D D D D D A A R
E E E E E E T T O
O O O O O O A A L

1 2 3 4 5 6 7 8 9

Channels

frequency division multiplexing: is a method of dividing a communication channel's

bandwidth into smaller channels. Different channels transmitted in different frequency
bands.
An example is Cable Television (CATV).
 Access net: cable
network
cable headend

cable splitter cable modem

modem CMTS termination system

data, TV transmitted at different

frequencies over shared cable ISP
distribution network

 HFC: hybrid fiber coax is a network technology that combines fiber optic cables
and coaxial cables to provide high-speed internet and other services
 Asymmetric(communication type where the data transmission rates for upstream and
downstream traffic are different. ): up to 30Mbps downstream transmission rate, 2
Mbps upstream transmission rate
 network of cable, fiber attaches homes to ISP router
 homes share access network to cable headend
 CMTS allow cable modem to send and receive packets over internet
 Access net: home
network
wireless
devices

to/from headend or
central office
often combined
in single box

cable or DSL modem

wireless access router, firewall, NAT

point (54 Mbps)
wired Ethernet (100 Mbps)
Enterprise access networks (Ethernet)

institutional link to
ISP (Internet)
institutional router

Ethernet institutional mail,

switch web servers

• typically used in companies, universities, etc

 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates
 end systems connect into Ethernet switch
 Wireless access networks
• shared wireless access network connects end
system to router
wide-area wireless access
via base station or “access point”
wireless LANs: 
 within building (100 ft) provided by telco (cellular)
operator, 10’s km( distances on
 802.11b/g (WiFi): 11, 54 the order of tens of kilometers e.g., 10
Mbps transmission rate km, 20 km, 30 km, etc.).
 between 1 and 10 Mbps
1 , 000  3G, 4G: LTE(Long Term
s =
1 kbp Evolution)
bps
s = 1,000
p
1 Mb
kbps
to Internet

to Internet
Network core
• mesh of
interconnected
routers
• packet-switching:In
application-layer, host
breaks messages into
packets and forward
packets from one router
to the next, across links
on path from source to
destination
• each packet
transmitted at full link
capacity
Network-core functions
routing: determines forwarding: move
source-destination route packets from router’s
taken by packets input to appropriate
 routing algorithms router output

routing algorithm

local forwarding table

header value output link
0100 3 1
0101 2
0111 2 3 2
1001 1
1
011

dest address in arriving

packet’s header
Difference Between Network Edge and
Network Core
Aspect Network Edge Network Core

Connects end-users to the Transports data between

Purpose
network networks

End systems (laptops,

Devices Routers, switches
phones)

Lower speeds (e.g., DSL, High-speed (fiber-optic

Speed
Wi-Fi) links)

Transfers between
Data Flow Starts or ends at the edge
networks
 How delay occur?
• Delay refers to the time taken for a packet of data to travel from the
source to the destination across a network
• Types of Delay:1.Transmission Delay 2.Propogation delay 3.Queuing delay 4.
Processing Delay

packet being transmitted (delay)

B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
• 1.Transmission Delay: Time it takes for a sender to push the packet onto the link.
• 2.Propagation Delay: Time for the signal to travel through the physical medium to the
next device.
transmission
A propagation

B
nodal
processing queueing

dnodal = dproc + dqueue + dtrans +

dprop
dtrans: transmission dprop: propagation delay:
delay:  d: length of physical link
 L: packet length (bits)  s: propagation speed in
 R: link bandwidth (bps) medium (~2x108 m/sec)
 dtrans = L/R dtrans and dprop  dprop = d/s
very different
3.Queueing delay
Packets wait in the router's buffer (queue) until
they can be forwarded.

average queueing
• R: link bandwidth

delay
(bps)
• L: packet length
(bits)
• a: average packet traffic intensity
arrival rate = La/R
 La/R ~ 0: avg. queueing delay small La/R ~ 0
 La/R -> 1: avg. queueing delay large
 La/R > 1: more “work” arriving
than can be serviced, average delay
infinite!
La/R -> 1
Caravan analogy
100 km 100 km
ten-car toll toll
caravan booth booth

• cars “propagate” at  time to “push” entire

100 km/hr caravan through toll booth
onto highway = 12*10 =
• toll booth takes 12 sec to 120 sec
service car (bit transmission
 time for last car to
time)
propagate from 1st to 2nd
• car~bit; caravan ~ packet toll both:
• Q: How long until caravan is  Time=Distance/speed
lined up before 2nd toll 100km/(100km/hr)= 1 hr
booth?  A: 60 minutes
4. Processing Delay: refers to the time a network device (such as a
router or switch) takes to process the header of a packet, perform
error checking, and determine the next hop for the packet before
forwarding it.

Example of Processing Delay

Suppose a packet arrives at a router. The router takes:

•0.2 ms to inspect the header.
•0.1 ms to check for errors (CRC).
•0.3 ms to perform route lookup.
The total processing delay at this router is:

Processing Delay=0.2+0.1+0.3=0.6 ms
“Real” Internet delays and routes

• traceroute program: provides

delay measurement from source
to router along end-end Internet
path towards destination.

3 probes 3 probes

3 probes
cmd prompt, C:\Users\sylaj>tracert 172.24.10.5

Tracing route to XXX.GITAM.EDU [172.24.10.5]

over a maximum of 30 hops:

1 <1 ms <1 ms <1 ms SYLAJASAJU.GITAM.EDU [172.22.162.22]

Trace complete.

Note:

Hop Number (1): Indicates this is the first step (or hop) in the network path to the
destination.

<1 ms <1 ms <1 ms: These are the round-trip times (RTT is the time taken for a signal or
data packet to travel from the sender to the receiver and back to the sender. ) for three
packets sent to the first hop.
 Loss

Packet loss occurs when packets arrive at the

router but cannot be processed or forwarded
because the router's buffer is full(OR )arrival
rate to link (temporarily) exceeds output link
capacity

buffer
(waiting area) packet being transmitted
A

B
packet arriving to
full buffer is lost
 Throughput

• throughput: rate (bits/time unit) at which bits

transferred between sender/receiver
Let
• Rc: Capacity of the Receiver's Network Link
• Rs: Capacity of the Sender's Network Link

server, with
server sends link capacity
pipe that can carry link capacity
pipe that can carry
file ofbits
F bits fluid at rate
Rs bits/sec fluid at rate
Rc bits/sec
to(fluid)
send into
to client
pipe Rs bits/sec) Rc bits/sec)
A bottleneck occurs when the capacity of a network
component (e.g., link, router, or server) is lower than the
demand placed on it.

In a network

 If Rs < Rc, the sender's link speed becomes the bottleneck,

meaning the sender cannot send data as fast as the
receiver can process it.

 If Rc < Rs, the receiver’s link speed becomes the

bottleneck, meaning the receiver cannot process incoming
data as fast as the sender sends it.
Per-connection end-end throughput=
min(Rc, Rs, R/10)
This assumes that the total available Rs
bandwidth is evenly divided among Rs
10 simultaneous connections. Rs

R
R: link bandwidth. It represents the
maximum transmission capacity of Rc Rc
the shared network link
Rc
Rc: Receiver Network Link

Rs: Sender Network Link

10 connections (fairly) share
backbone bottleneck link R bits/sec
Layering of airline
functionality
ticket (purchase) ticket (complain) ticket

baggage (check) baggage (claim baggage

gates (load) gates (unload) gate

runway (takeoff) runway (land) takeoff/landing

airplane routing airplane routing airplane routing airplane routing airplane routing

departure intermediate air-traffic arrival

airport control centers airport

layers: each layer implements a service

• via its own internal-layer actions
• relying on services provided by layer below
Protocol Layers and Their Service
Models
• The communication process is
divided in to layers, as the OSI Model
or TCP/IP Model.
• Each layer defines a service model
that specifies what services it
provides to the upper layer.
• For example, the Transport Layer
provides reliable data delivery to the
Application Layer.
Internet protocol
stack(TCP/IP)
• application: supporting network
applications
• FTP, SMTP, HTTP application
• transport: process-process data
transfer transport
• TCP, UDP
• network: routing of datagrams from network
source to destination
• IP, routing protocols link
• link: data transfer between
neighboring network elements physical
• Ethernet, 802.111 (WiFi), PPP
• physical: bits “on the wire”
OSI(Open System Interconnect)Model
The OSI Model (Open Systems Interconnection) is a conceptual framework that divides network
communication into 7 layers
ISO/OSI reference
model
• presentation: allow applications
to interpret meaning of data, application
e.g., encryption, compression, presentation
machine-specific conventions
session
• session: synchronization,
checkpointing, recovery of data transport
exchange network
link
physical
Real-World Example: Loading a Web Page
1.Application Layer: You type a URL in your browser.
2.Presentation Layer: Data is formatted into HTTP requests.
3.Session Layer: The connection to the web server is established.
4.Transport Layer: The data packets are managed using TCP.
5.Network Layer: The packets are routed to the server via IP.
6.Data Link Layer: The packets are sent over a single link using MAC
addresses.
7.Physical Layer: The bits are transmitted over your Wi-Fi or cable.
History of computer networking and the Internet
8. Early Days (1950s-1960s): Standalone ComputersComputers were
big and used for individual tasks only. Scientists started thinking about
connecting computers to share information. Example: Imagine two
researchers wanting to share data but having to physically exchange
punch cards.
9. ARPANET: The First Network (1969)The U.S. Department of Defense
created ARPANET, the first network to connect multiple computers
using packet switching. Initially connected four universities. Example:
3. The Birth of TCP/IP (1970s)A new communication protocol, TCP/IP,
was developed to allow computers on different networks to talk to each
other.This became the foundation of the modern Internet.Example:
Computers in different cities exchanging emails seamlessly.
4. Local Area Networks (1980s)Companies and universities began
setting up smaller networks like Ethernet within their buildings.The
Domain Name System (DNS) was created, so people could use easy
names (e.g., www.google.com) instead of numbers (IP
addresses).Example: Offices using Ethernet to connect computers and
share printers.
5. The World Wide Web (1990s)Tim Berners-Lee invented the World
Wide Web (WWW), which allowed people to access information through
hyperlinks and browsers like Mosaic.Internet became available to the
public.Example: Visiting a website to read news or send emails for the
first time.
6. Mobile and Fast Internet (2000s)Internet speeds increased with
broadband and mobile networks (like 3G and 4G).People started using
smartphones to browse the Internet.Example: Watching YouTube videos or
Internet history
1961-1972: Early packet-switching
principles
• 1961: Kleinrock - • 1972:
queueing theory • ARPAnet public demo
shows effectiveness • NCP (Network Control
of packet-switching Protocol) first host-host
• 1964: Baran - protocol
packet-switching in • first e-mail program
military nets • ARPAnet has 15 nodes
• 1967: ARPAnet
conceived by
Advanced Research
Projects Agency
• 1969: first ARPAnet
node operational
 1972-1980: Internetworking, new and proprietary
nets
• 1970: ALOHAnet satellite
network in Hawaii Cerf and Kahn’s
• 1974: Cerf and Kahn - internetworking
architecture for principles:
interconnecting networks • minimalism, autonomy
• 1976: Ethernet at Xerox PARC - no internal changes
required to
• late70’s: proprietary interconnect networks
architectures: DECnet, SNA,
• best effort service
XNA
model
• late 70’s: switching fixed • stateless routers
length packets (ATM • decentralized control
precursor)
• 1979: ARPAnet has 200 nodes define today’s Internet
architecture
1980-1990: new protocols, a proliferation of
networks
• 1983: deployment of • new national
TCP/IP networks: Csnet,
• 1982: smtp e-mail BITnet, NSFnet,
protocol defined Minitel
• 1983: DNS defined for
• 100,000 hosts
name-to-IP-address connected to
translation confederation of
networks
• 1985: ftp protocol
defined
• 1988: TCP congestion
control
 1990, 2000’s: commercialization, the Web,
new apps
• early 1990’s: ARPAnet late 1990’s – 2000’s:
decommissioned
• more killer apps:
• 1991: NSF lifts restrictions on instant messaging,
commercial use of NSFnet
(decommissioned, 1995)
P2P file sharing
• network security to
• early 1990s: Web
• hypertext [Bush 1945, forefront
Nelson 1960’s] • est. 50 million host,
• HTML, HTTP: Berners-Lee 100 million+ users
• 1994: Mosaic, later • backbone links
Netscape
running at Gbps
• late 1990’s:
commercialization of the
Web
2005-present
• ~750 million hosts
• Smartphones and tablets
• Aggressive deployment of broadband access
• Increasing ubiquity of high-speed wireless access
• Emergence of online social networks:
• Facebook: soon one billion users
• Service providers (Google, Microsoft) create their
own networks
• Bypass Internet, providing “instantaneous”
access to search, emai, etc.
• E-commerce, universities, enterprises running their
services in “cloud” (eg, Amazon EC2)
Network security
• Network Security refers to practices
and technologies designed to
protect data, devices, and networks
from unauthorized access, attacks,
and damage.
• Example: Firewalls and antivirus
software are common tools used to
enhance network security by
blocking malicious traffic and
detecting threats.
Bad guys: put malware into hosts via
Internet
• malware can get in host from:
• virus: self-replicating infection by
receiving/executing object (e.g., e-mail
attachment)
• worm: self-replicating infection by passively
receiving object that gets itself executed
• spyware malware can record keystrokes,
web sites visited, upload info to
collection site
• infected host can be enrolled in botnet,
used for spam. DDoS attacks
Bad guys: attack server, network
infrastructure
Denial of Service (DoS): attackers make resources
(server, bandwidth) unavailable to legitimate
traffic by overwhelming resource with bogus traffic

1. select target
2. break into hosts
around the network
(see botnet)
3. send packets to target
from compromised
hosts target
 Bad guys can sniff packets
packet “sniffing”:
• broadcast media (shared ethernet, wireless)
• promiscuous network interface reads/records all
packets (e.g., including passwords!) passing by

A C

src:B dest:A payload

B
 wireshark software used for end-of-chapter
labs is a (free) packet-sniffer
 Bad guys can use fake
addresses
IP spoofing: send packet with false source address
A C

src:B dest:A payload

… lots more on security (throughout, Chapter 8)