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Networking and the Internet

Computers are often linked together to share information and resources.
Such linked systems are called computer networks.
In a network, data can be transferred from one computer to another.
Users in a network can exchange messages and share resources like:
Printers
Software packages
Data storage devices
To manage these tasks, network software is used.

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Networking and the Internet

4.1 Network Fundamentals

Network Classifications:

1. Personal Area Network (PAN)

Used for short-range communication (a few meters).
Connects personal devices such as:
Smartphone ↔ Wireless headset
PC ↔ Wireless mouse
Example: Bluetooth connection between phone and earbuds.

2. Local Area Network (LAN)

Connects computers within a limited area (in a single building or campus).
Used in schools, offices, or factories.
Example: Network connecting computers in a university campus.

3. Metropolitan Area Network (MAN)

Covers a larger area than a LAN, such as a city or community.
Used to connect multiple LANs within a city.
Example: City-wide cable TV network.

4. Wide Area Network (WAN)

Covers large geographical areas—from cities to countries.
Connects computers and LANs across long distances.
Example: The Internet.

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Networking and the Internet

Network Classifications:

Classification Based on Network Design

1. Open Network

Based on publicly domain designs.
Designs are freely available for anyone to use or modify.
Example: The Internet — governed by the TCP/IP protocol suite.

2. Closed / Proprietary Network

Based on privately owned designs controlled by a company or individual.
Usage of applications is restricted by licenses or contract conditions.
The company earns income by selling or leasing these proprietary systems.
Example: Novell Inc. developed proprietary systems.

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Networking and the Internet

Network Classifications:

Classification Based on Network Topology

Network Topology:

Refers to the pattern in which computers are connected.

Bus Topology
All machines are connected to a common communication line called a bus.
Data travels along the bus, shared by all machines.
Popularized in the 1990s with Ethernet standards.
Ethernet networks are still one of the most widely used networking systems today.

Star Topology:
All machines are connected to a central device (like a hub or Access Point (AP)), which acts as a focal point.
The central machine (Access Point, AP) coordinates all communication.
The central device relays signals to all machines connected to it, sometimes amplifying them.
Communication occurs via radio broadcast.
Popular in modern wireless networks (Wi-Fi).

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Network Protocols:

Definition: Rules that govern how activities are conducted in a network.
Purpose: Ensure reliable and coordinated communication between devices.
Without protocols:
Multiple computers might send messages at the same time → collisions.
Computers may fail to assist each other when required.
Protocol standards allow vendors to build products for network applications that are compatible with other vendors’ products.
Protocols are essential for the development of networking technologies.

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Network Protocols:

CSMA/CD Protocol in Bus Networks:

Full Name: Carrier Sense, Multiple Access with Collision Detection (CSMA/CD)
Purpose: CSMA/CD controls which machine can transmit messages on an Ethernet bus.
How It Works:
Every message is broadcast to all machines on the bus.
Each machine monitors all messages but keeps only those addressed to itself.
To transmit a message, a machine waits until the bus is silent. and at this time it begins transmitting message.

If two machines transmit simultaneously → both detect the collision.
Both machines pause for a short random time before trying to transmit again.
Analogy:
Similar to a group conversation: if two people start talking at once, both stop and try again later.

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Networking and the Internet

Limitations of CSMA/CD Protocol:

CSMA/CD is not compatible with wireless star networks in which all machines communicate through a central AP.

Reason CSMA/CD fails in wireless networks:
A machine may not detect collisions with another machine.
Its own signal can drown out the signal of others.
Another reason CSMA/CD fails in wireless networks:
Signals from different machines may be blocked by objects or distance.
Machines may not detect each other’s transmissions, even though they can communicate with the same Access Point (AP).
Machines may transmit at the same time, causing their signals to collide at the AP.
This is called the hidden terminal problem.

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Networking and the Internet

Network Protocols:

CSMA/CA Protocol in Wireless Networks

Full Name: Carrier Sense, Multiple Access with Collision Avoidance (CSMA/CA)
Wireless networks avoid collisions rather than detecting them. This approach is called (CSMA/CA)
CSMA/CA Protocols are commonly known as WiFi protocols.
Collision avoidance protocols aim to reduce collisions but may not eliminate them completely.
If collisions occur, messages must be retransmitted.

Collision Avoidance Strategy in Wireless Networks:

When the channel is silent:
The machine waits for a short time after finding the channel silent.
It transmits only if the channel remains silent during that waiting period.
When the channel is busy:
The machine waits for a random time before trying again.
After this time, if the channel is free, it transmits immediately.

Benefit:

Collisions are avoided between new messages (“newcomers”) and messages that have already been waiting.
Messages that have been waiting longer get priority to claim the channel.

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Network Protocols:

Handling the Hidden Terminal Problem in WiFi

CSMA/CA protocol cannot solve the hidden terminal problem.
Solution in WiFi:
Each machine sends a short request to the Access Point (AP).
Machine waits for AP acknowledgment before sending the entire message.
If the AP is busy (handling a hidden terminal), it ignores the request, so the machine waits.

If the AP acknowledges, the machine can transmit safely.
Benefit:
All machines hear AP acknowledgments, helping them know if the AP is busy, even if they cannot hear other transmissions.

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Networking and the Internet

Combining Networks

Sometimes, we need to connect existing networks to form an extended communication system.
This can be done by linking networks of the same type (for example, connecting multiple Ethernet bus networks).
The connection is achieved using network devices such as:
Repeaters
Bridges
Switches

Repeater

The simplest device used to connect two existing networks.
It passes signals back and forth between the two buses.
It does not interpret the signals.

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Combining Networks

Bridge

A bridge is more complex than a repeater.
A bridge connects two buses like a repeater but does not forward all messages.
Unlike a repeater, it examines the destination address of each message.
It forwards messages only if the destination is on the other side of the bridge.
Machines on the same side of a bridge can communicate without affecting the other side.
This makes the network more efficient than a repeater.

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Combining Networks

Switch

A switch is like a bridge but with multiple connections, allowing it to connect several buses.
A switch creates a network where several buses extend from it like spokes on a wheel.
Like a bridge, a switch checks destination addresses of all messages and forwards messages only to the correct bus.
This minimizes traffic on each connected bus.

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Combining Networks

When multiple networks are connected using repeaters, bridges, or switches, they all combine to form one big network.
Even after connecting, the overall network behaves just like the original networks — it follows the same protocol for sending and receiving messages.

internet:

Sometimes, networks to be connected have incompatible characteristics.
Example: The characteristics of WiFi network are not compatible with Ethernet network.
In such cases, networks are connected to form a network of networks, called an internet.

In an internet
Each original network maintains its individuality
Each original network continues to function as an autonomous network.
Note:
The generic term internet ≠ Internet (the worldwide system).
There are many internets, e.g., traditional telephone networks were early examples.

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Networking and the Internet

Combining Networks

Routers

Routers are devices that connect networks to form an internet.
They are special purpose computers used for forwarding messages.
Unlike repeaters, bridges, or switches, routers:
Provide links between networks
Allow each network to maintain its unique internal characteristics.

Example: Routers connecting two WiFi networks and an Ethernet network

A machine in a WiFi network sends a message to its Access Point (AP).

From there, the AP forwards the message to its router.
The router sends the message to the router of the Ethernet network.
The message finally reaches the target machine in the Ethernet network.

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Combining Networks

Why Routers are so named?

Routers forward messages in the correct direction.
The message forwarding process is based on an internet-wide addressing system.
All devices in an internet (machines + routers) has a unique internet address.
Each machine in the network has two addresses:
Local address – within its own network.
Internet address – used across the network.
When a machine wants to send a message to another machine in a distant network,
The machine attaches the internet address of the destination.

The message is then sent to its local router.
The router checks its forwarding table to find the correct path.
The router forwards the message in the proper direction toward the destination network.
Each router maintains a forwarding table that stores information about where to send messages based on their destination addresses.

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Combining Networks

Gateways

The point where a network is linked to an internet is called a gateway.
A gateway serves as a passageway between the network and the outside world.
Gateways can take different forms:
Often, a network’s gateway is just the router used to communicate with the rest of the internet.
In some cases, a gateway includes more than just a router.
Example: In residential WiFi networks, the gateway can refer to the combined network AP + router unit.

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Networking and the Internet

Methods of Process Communication

Processes running on different computers within a network (or the same computer) often need to communicate with each other.
Communication helps them coordinate their actions and to perform their assigned tasks.
This type of communication between processes is called interprocess communication (IPC).

Client/Server Model

In interprocess communication (IPC), one commonly used method is the client-server model.
Client: is a process (or program) that makes requests of other processes.
Server: is a process which satisfies the requests made by the clients.

Example of Client/Server Model

Early use of the client/server model was in networks within offices.
A single high-quality printer attached to the network was shared by all machines in the network.
Printer played the role of a server (called a print server).
Other machines acted as clients that sent print requests to the print server.

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Networking and the Internet

Methods of Process Communication

Client/Server Model – File Server Example

Another early use of the client/server model was used to reduce the cost of data storage.
It also helped to avoid duplicate copies of records.
One machine in a network had a high-capacity storage system (magnetic disk) that stored all organization’s records.
Other machines on the network requested access to the records as needed.
Server: Thus the machine that actually contained the records played the role of a server (called a file server)
Clients: The other machines played the role of clients that requested access to the files that were stored at the file server.

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Networking and the Internet

Methods of Process Communication

Peer-to-Peer (P2P) Model

Client/server model is widely used today in network applications, but it’s not the only model for interprocess communication. Another model is the peer-to-peer (P2P) model.
In the client/server model, one process (i.e., server) provides services to many processes (i.e., clients.)
In the peer-to-peer (P2P) model, all processes provide and receive services from each other.
In the client/server model, a server runs continuously to serve its clients at anytime.
In contrast, P2P processes usually run on temporary basis.
Examples of P2P applications:

- Instant messaging for online text conversations

- Interactive or competitive online games

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Networking and the Internet

Methods of Process Communication

Peer-to-Peer (P2P) Model

P2P is popular for distributing files like music recordings and movies over the Internet.
In this case, one peer may receive a file from another and then provide that file to other peers.
The collection of peers distributing files in a P2P system is called a swarm.
Swarm-based file distribution differs from client/server, which uses a central server for clients to download files.
P2P is replacing client/server for file sharing because it distributes the service task among many peers instead of relying on a single server.
The absence of a central server in P2P makes the system more efficient.

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Networking and the Internet

Methods of Process Communication

Peer-to-Peer (P2P) Model

Legal Issues in P2P File Sharing
Lack of a central server makes it difficult to enforce copyright laws in illegal file sharing.
But still, many individuals have faced serious legal consequences for copyright violations.
A process might use the peer-to-peer model to communicate with another process and later use the client/server model to communicate with another process over the same network.

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Networking and the Internet

Client/Server model

Server provides services; clients request services
Server runs continuously to serve clients
Relies on a central server
Server can get overwhelmed when handling too many clients
Clients download files from the server
Web servers, email servers, databases
A central server makes it simpler to monitor and enforce copyright rules

Peer to Peer (P2P) model

All processes both provide and receive services from one another.
Peers run temporarily, only when needed
No central server; tasks are shared among peers
Efficiency improves as tasks and resources are shared among multiple peers.
Files are shared among peers in a swarm
Instant messaging, online games, file sharing (music, movies)
Harder to enforce copyright laws

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Networking and the Internet

Distributed Systems

Distributed systems consist of software units that run as processes on different computers.
Modern software systems include:
Global information retrieval systems
Company-wide accounting & inventory systems
Computer games
Network infrastructure software
Distributed systems rely on network communication between computers to function.

Types of distributed computing systems:

1. Cluster computing:

Cluster computing: Many independent computers work closely together to provide computation /services like a larger machine.
Cost-effective: The cost of these individual machines plus the high-speed network to connect them, can be less than a higher-priced supercomputer.
Advantages:
High availability: At least one computer of the cluster will be able to answer requests if others fail.
Load balancing: Workload can be automatically shifted from busy cluster members to idle ones.
Lower maintenance costs.

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Networking and the Internet

Distributed Systems

Types of distributed computing systems:

2. Grid computing

Grid computing: distributed systems that are more loosely coupled than clusters can work together to accomplish large tasks.
Grid computing may use specialized software to distribute data and algorithms to machines participating in the grid.
Examples include University of Wisconsin’s Condor system, or Berkeley’s Open Infrastructure for Network Computing (BOINC).
These two systems often run on regular PCs at work or home, which volunteer computing power when the machine is idle.

This type of voluntary distributed grid computing, powered by the Internet connectivity, allows millions of home PCs to solve complex mathematical and scientific problems.

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Distributed Systems

Types of distributed computing systems:

2. Cloud computing

Cloud computing is a distributed system where large pools of shared computers can be allocated to clients as needed.
The Internet enables entities to store data and run computations on the Cloud.
The Cloud provides enormous computing resources available on the network.
Amazon Elastic Compute Cloud: lets clients rent virtual computers by the hour without worrying about where the computer hardware is actually located.

Google Drive and Google Apps: let users to collaborate on information or create web services without worrying about how many computers are involved or where the data is stored.
Cloud computing services offer reliability and scalability, but raise privacy and security concerns since the owners and operators of the computers may be unknown.

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The Internet

The Internet is a commercial system used worldwide.
It connects many networks such as PANs, LANs, MANs, and WANs, involving millions of computers.

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The Internet

Internet Architecture

The Internet is a collection of connected networks.

In general, these networks are constructed and maintained by organizations called Internet Service Providers (ISPs).
We often say we “connect to an ISP.” This means we are actually connecting to the network provided by the ISP.
The networks run by ISPs are arranged in a hierarchy based on their role in the Internet.
Tier-1 ISPs
Tier-2 ISPs
Tier-3 ISPs
End systems

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The Internet

Internet Architecture

Tier-1 ISPs
At the top of the hierarchy are few tier-1 ISPs.
Tier-1 ISPs consist of very high-speed, high-capacity, international WANs.
These networks form the backbone of the Internet.
They are typically operated by large companies that are in the communications business.
Tier-2 ISPs
Tier-2 ISPs connect to the tier-1 ISPs.
They usually cover regional areas and have lower capabilities than tier-1 ISPs.
Tier-2 ISPs are also run by communication companies.
Tier-1 ISPs and tier-2 ISPs are mainly made up of networks of routers.
These network of routers collectively provide the communication infrastructure of the Internet.
As such they are considered as the core of the Internet.

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The Internet

Tier-3 ISPs/Access ISPs
Access to the core of the Internet is given through tier-3 ISPs, also called access ISPs.
It is run by a single authority whose job is to provide Internet access to homes and businesses.
Example: Cable and telephone companies that charge for their service.
End Systems / Hosts:
The devices used by individual users to connect to the access ISP are called end systems or hosts.
These end systems may be laptops or PCs, telephones, video cameras, automobiles, and home appliances.
The Internet is a communication system, so any device that needs to communicate with other devices can become an end system.

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The Internet

Internet Addressing

An internet needs a common addressing system that assigns a unique identifying address to each computer in the system.
In the Internet these addresses are known as IP (Internet Protocol) addresses.
Earlier, each IP address was a pattern of 32 bits. To support more devices, the Internet is moving to 128-bit addresses, called IPv6.
Blocks of consecutively numbered IP addresses are awarded to ISPs by the Internet Corporation for Assigned Names and Numbers (ICANN).
ICANN is a nonprofit corporation that coordinates the Internet’s operation.
The ISPs then allocate the addresses within their awarded blocks to machines within their region of service.
This ensures that each machine on the Internet has a unique IP address.

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The Internet

Internet Addressing

IP addresses are written in a form called dotted decimal notation.
In this notation, each byte of the IP is expressed as an integer represented in base 10 notation.
The bytes of the address are separated by periods (dots).
Example:
The dotted decimal 5.2 represents the two-byte pattern 00000101 00000010
(5 is 00000101 in binary, and 2 is 00000010 in binary).
The dotted decimal 17.12.25 represents three byte pattern:
17 → 00010001, 12 → 00001100, 25 → 00011001
A normal 32-bit IP address appears as 192.207.177.133 in dotted decimal notation.

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The Internet

Internet Addressing

IP addresses in bit pattern form or even dotted decimal notation are hard for humans to remember.
For this reason, the Internet has an alternative addressing system in which machines are identified by mnemonic names.
This addressing system is based on the concept of a domain. A domain is like a “region” of the Internet operated by one organization, such as a: university, company, club, government agency.
The region does not refer to a physical area.
Every domain must be registered with ICANN. The registration is handled by companies called registrars (assigned by ICANN).
As a part of this registration process, the domain is assigned a unique mnemonic domain name,(for example: rnsit.ac.in).
Domain names usually describe the organization that registers them. This makes them useful and easy for humans to understand.
Example: The domain name of Marquette University is mu.edu. The part after the dot (edu) shows the type of domain, edu – educational.
These suffixes are called Top-Level Domains (TLDs)

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The Internet

Internet Addressing

Common TLDs
edu – educational institutions
com – commercial organizations
gov – U.S. government
org – nonprofit organizations
museum – museums
info – general information sites
net – originally for ISPs, but now used widely
Two letter TLDs
Two-letter TLDs are used for countries
Examples:
au – Australia
ca – Canada

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Internet Applications

Early Internet applications were simple standalone programs that each followed a specific network protocol.
Examples include NNTP(Network News Transfer Protocol) for news reading, FTP(File Transfer Protocol ) for file transfers, and Telnet or SSH(Secure Shell) for remote computer access.
These applications directly communicated with servers using their designated protocols.
As web browsers and servers grew more advanced, many traditional applications became integrated into webpages using HTTP(Hyper Text Transfer Protocol (HTTP)).
Studying simpler protocols first helps build understanding before examining more complex protocols like HTTP.

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Internet Applications

Early Internet applications were simple standalone programs that each followed a specific network protocol.
Examples include NNTP(Network News Transfer Protocol) for news reading, FTP(File Transfer Protocol ) for file transfers, and Telnet or SSH(Secure Shell) for remote computer access.
These applications directly communicated with servers using their designated protocols.
As web browsers and servers grew more advanced, many traditional applications became integrated into webpages using HTTP(Hyper Text Transfer Protocol (HTTP)).
Studying simpler protocols first helps build understanding before examining more complex protocols like HTTP.

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Internet Applications

Electronic Mail :

Electronic communication systems include IM, online chat, tweets, and social media posts, but email remains one of the oldest and most widely used Internet services.
Users typically read and compose email through browsers or applications like Outlook, Apple Mail, or Thunderbird.
Despite modern interfaces, email transmission between computers still relies on fundamental network protocols such as SMTP.
SMTP (Simple Mail Transfer Protocol) defines how two networked computers interact to send an email message from one host to another.
In an example, a mail process on mail.skaro.gov contacts the mail server on mail.tardis.edu to send a message from user “dalek” to user “doctor.”

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Internet Applications

Electronic Mail :

SMTP was originally designed only for ASCII text, requiring additional protocols to handle other data types.
MIME (Multipurpose Internet Mail Extensions) converts non-ASCII data—such as images, audio, and attachments—into a format compatible with SMTP.
Email retrieval after it reaches a user’s mail server is handled by access protocols such as POP3 and IMAP(Internet Mail Access Protocol).
POP3 (Post Office Protocol version 3) is the simpler protocol used for downloading messages to the user’s local computer.
With POP3, downloaded messages can be read, stored, organized, and edited locally using the user’s own device storage.

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Internet Applications

VoIP:

VoIP (Voice over Internet Protocol) provides Internet-based voice communication similar to traditional telephony.
Basic VoIP audio transfer uses the P2P (peer-to-peer) model between two processes on different machines.
VoIP introduces challenges such as call setup, call reception, 911 emergency support, and connection to traditional telephone systems.
Some governments tax or restrict VoIP because it competes with state-owned telephone companies.
VoIP soft phones like Skype use P2P software allowing PC-to-PC calls with only a microphone and speaker.
Skype’s proprietary design raises concerns because users must trust its closed operational structure.

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Internet Applications

VoIP:

Skype requires users to stay connected, and their PC may unknowingly support other Skype communications.
ATAs (Analog Telephone Adapters) enable regular telephones to connect to VoIP service from an access ISP, often bundled with Internet or TV.
Embedded VoIP phones connect directly to a TCP/IP network and are increasingly replacing internal copper-wire phone systems in organizations.
Modern 4G smartphones use wireless VoIP because 4G is fully IP-based, making the phone function as another host on the global Internet.

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Internet Applications

Internet Multimedia Streaming :

A large portion of Internet traffic is real-time multimedia streaming, with major services like Netflix and YouTube consuming massive bandwidth.
N-unicast streaming sends separate streams to each client, creating heavy load on the server and nearby network infrastructure.
Alternatives such as P2P distribution and multicast shift the distribution burden to peers or routers, though multicast is not widely deployed on the global Internet.
On-demand streaming requires individualized unicast streams so users can start, pause, and control content independently.
Large-scale streaming relies on CDNs (Content Delivery Networks) and anycast routing so users connect to the nearest server for fast, efficient media delivery.

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The World Wide Web

The World Wide Web (WWW) was created by Tim Berners-Lee.
He combined internet technology with the concept of linked documents, called hypertext.
Hypertext allows documents to contain hyperlinks (links to other documents).
The first Web software was released in December 1990.
The early version did not support multimedia (no images, audio, or video).
But it had the main parts of today’s Web:
A hypertext document format for adding hyperlinks
A protocol to transfer hypertext across the network
A web server that sends web pages when a user requests them
After this beginning, the Web quickly added images, audio, and video.
By the mid-1990s, the Web became the most important application on the Internet and helped the Internet grow rapidly.

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The World Wide Web

Software used to access hypertext on the Internet is of two types:
Browsers
Webservers

Browsers

A browser is a software on the user’s computer.
Its main jobs are:
To get the web pages the user requests
To display these pages in an organized way
Examples of browsers: Firefox, Safari, Internet Explorer.

Webservers

A webserver is a computer that stores hypertext documents (web pages).
Its job is to send the requested documents to clients (browsers).

Communication

Hypertext documents are transferred between browsers and webservers using HTTP (Hypertext Transfer Protocol).

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The World Wide Web

In order to locate and retrieve documents on the Web, each document is given a unique address called a Uniform Resource Locator (URL).
A URL gives all the information needed by a browser to:
Find the correct webserver
Request the desired document
To view a webpage:
The user enters the URL of the document in the browser.
The browser then retrieves the document.
Finally, the browser displays the document to the user.

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The World Wide Web

A URL has four segments. These segments help the browser find the correct document on the Web.

Four segments of a URL:

Protocol – tells the browser how to communicate with the server�(Example: HTTP)
Machine address – the mnemonic address (name) of the computer that has the webserver�(Example: eagle.mu.edu)
Directory path – the path showing the folders where the document is stored�(Example: authors/Shakespeare/)
Document name – the actual file name of the webpage�(Example: Julius_Caesar.html)

A typical URL is presented in Figure 4.8.

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Internet Protocols

Messages are transferred over the Internet. This transfer process requires the cooperation of all the computers in the system.
Therefore software for controlling this transfer process is installed on every computer connected to the Internet.

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Internet Protocols

The Layered Approach to Internet Software

Messages are transferred over the Internet. This transfer process requires the cooperation of all the computers in the system.
Therefore software for controlling this transfer process is installed on every computer connected to the Internet.
A principal task of networking software is to provide the infrastructure required for transferring messages from one machine to another.
In the Internet, this message transfer happens through a hierarchy of software layers.
Example: You want to send a gift in a package from the West Coast of the United States to a friend on the East Coast.

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Internet Protocols

The Layered Approach to Internet Software

You would first wrap the gift as a package and write the address on the outside of the package.
You would take the package to a shipping company such as the U.S. Postal Service.
The shipping company might place the package along with others in a large container.
The shipping company might then deliver the container to an airline.
The airline would place the container in an aircraft.
The airline would then transfer it to the destination city, perhaps with intermediate stops along the way.
At the final destination, the airline would remove the container from the aircraft.
The airline would then give the container to the shipping company’s office at the destination.
The shipping company would then take your package out of the container.
The shipping company would then deliver it to the addressee.

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Internet Protocols

The Layered Approach to Internet Software

In short, the transportation of the gift would be carried out by a three-level hierarchy:
the user level (consisting of you and your friend),
the shipping company, and
the airline.
Each level uses the next lower level as an abstract tool.
You are not concerned with the details of the shipping company.
The shipping company is not concerned with the internal operations of the airline.
Each level in the hierarchy has representatives at both the ends. (origin and the destination).
At the destination, the representatives do the opposite work of what the representatives had done at the origin

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Internet Protocols

The Layered Approach to Internet Software

In the Internet, communication is controlled by software.
The Internet software has four layers, not three. The four layers are known as:
The application layer,
The transport layer,
The network layer, and
The link layer
Each layer consists of a collection of software routines (rather than people and businesses).
At the origin:
A message typically originates in the application layer.
From there it is passed down through the transport and network layers as it is prepared for transmission.
Finally it is transmitted by the link layer.
At the destination:
The message is received by the link layer.
Then the message is passed up through the layers.
Finally, the message is delivered to the application layer

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Internet Protocols

The Layered Approach to Internet Software

We will trace how a message moves through the Internet.

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Internet Protocols

Application layer
The application layer uses the transport layer to send and receive messages over the Internet.
This is similar to how you use a shipping company to send and receive packages.
It is your job to write an address in the format that the shipping company accepts.
It is the application layer’s responsibility to provide an address that is compatible with the Internet infrastructure.
For this, the application layer may use the services of the name servers within the internet.
The Name servers translate mnemonic addresses used by humans into Internet-compatible IP addresses.

The Layered Approach to Internet Software

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Internet Protocols

Transport layer
An important task of the transport layer is to accept messages from the application layer.
It ensures the messages are properly formatted before transmitting them over the Internet.
The transport layer divides long messages into small segments. These segments are called packets.
These small segments are transmitted as individual units over the Internet.
This division is needed because one long message can block the flow of other messages at the Internet routers.
Small message segments can mix and move through these routers easily.
A long message forces others to wait while it passes. . (Like cars waiting for a long train to pass at a railroad crossing).
The transport layer adds sequence numbers to the small segments it produces.
This helps the receiver put the segments back together in the correct order.
The transport layer then sends the packets to the network layer.

The Layered Approach to Internet Software

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Internet Protocols

Network layer
After this, each packet is treated as an individual, unrelated messages until it reaches the transport layer at the destination.
Packets from the same original message may travel through different paths in the Internet before arriving.
It is the network layer’s job to decide in which direction a packet should be sent at each step as it moves through the Internet.
The combination of the network layer and the link layer below it constitutes the software residing on the Internet routers.
The network layer maintains the router’s forwarding table. It uses this table to choose the correct direction to forward packets.
The link layer at the router is in charge of receiving and transmitting the packets.
At the origin, the network layer receives the packet from the transport layer.
It looks at its forwarding table to decide the correct direction for the packet to begin its journey.
After choosing the direction, the network layer passes the packet to the link layer. The link layer then transmits the packet.

The Layered Approach to Internet Software

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Internet Protocols

Link layer
The link layer has the responsibility of transferring the packet.
It must handle the communication details of the specific local network where the computer is connected.
If the computer is on an Ethernet network, the link layer uses CSMA/CD.
If the computer is on a WiFi network, the link layer uses CSMA/CA.

The Layered Approach to Internet Software

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Internet Protocols

At the Intermediate nodes:

When a packet is transmitted, it is received by the link layer at the other end of the connection.
The link layer
then passes the packet to the network layer.
The network layer checks its forwarding table.
It compares the packet’s final destination with the table.
It then decides the next direction the packet should go.
After deciding the direction, the network layer returns the packet to the link layer.
The link layer forwards the packet to the next machine.
In this way, each packet hops from one machine to another�until it reaches its final destination.
During a packet’s journey, only the link layer and network layer work at the intermediate machines (routers).

The Layered Approach to Internet Software

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Internet Protocols

At the destination:

At the final destination, the network layer identifies that the packet has reached its end point.
Instead of forwarding it, the network layer passes the packet to the transport layer.
The transport layer takes the packets it receives from the network layer and extracts the message segments.
Using the sequence numbers provided by the transport layer at the origin, it reconstructs the original message in the correct order.
After rebuilding the complete message, the transport layer hands it over to the correct application in the application layer.
Once the message reaches the application layer, the message transmission is complete.

The Layered Approach to Internet Software

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Internet Protocols

The TCP/IP Protocol Suit

Open networks require published standards so that equipment and software from different companies can work together.
One such standard is the Open System Interconnection (OSI) reference model. It was produced by the International Organization for Standardization (ISO).
The OSI model is based on seven-level hierarchy, which is different from the four-level hierarchy used in the Internet.
The OSI model is often mentioned because it is approved by an international organization.
However, OSI model has not replaced the four-layer model. This is mainly because the OSI model was created later, after the four-layer model had already become the commonly used standard for the Internet.

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Internet Protocols

TCP/IP protocol suite: The TCP/IP protocol suite is a collection of protocol standards used by the Internet to implement the four-level communication hierarchy.
The names TCP (Transmission Control Protocol) and IP (Internet Protocol) are only two protocols in this large collection.
So, calling the entire collection “TCP/IP” is somewhat misleading.
Transport layer versions:
TCP defines one version of the transport layer.
The TCP/IP suite actually provides more than one way to implement the transport layer.
Another option is the User Datagram Protocol (UDP).
Analogy: This is similar to choosing different shipping companies. They all ship packages, but each offers different features.
So, based on the type of service needed, an unit in the application layer can choose to send data via TCP or UDP version of the transport layer

The TCP/IP Protocol Suit

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UDP does not create any connection before sending data.
A transport layer based on UDP does not establish such a connection prior to sending a message.
It simply sends the message to the given address and does not check whether the destination received it or not. For this reason, UDP is called a connectionless protocol.

The TCP/IP Protocol Suit

TCP

Before sending the actual message from the application layer, a transport layer based on TCP first sends its own control message to the destination’s transport layer to inform it that message will be sent.
TCP then waits for an acknowledgment from the destination.
Only after receiving this acknowledgment does TCP start sending the application layer’s message.
In this manner, a TCP transport layer is said to establish a connection before sending a message.
Because TCP creates this connection before data transfer, TCP is called a connection-oriented protocol

UDP

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UDP is simpler and faster than TCP.
If an application is prepared to handle the potential consequences of UDP, then UDP may be the better option.
For example, DNS lookups and VoIP use UDP because they need speed.

The TCP/IP Protocol Suit

TCP

For Example: Because email is less time sensitive, mail servers use TCP to transfer email.

UDP

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Internet Protocols

The IP standard includes protocols that help routers share routing information with each other so they can update their tables correctly.
When an IP network layer creates a packet, it adds a value called the hop count or time to live (TTL).
This value sets the maximum number of times the packet can be forwarded through the Internet.
Every time a router forwards the packet, it reduces the hop count by one.
This prevents packets from circling forever in the network if something goes wrong.
Even though the Internet is very large, a starting hop count of 64 is usually enough for a packet to reach its destination.

The TCP/IP Protocol Suit

Internet Protocol (IP)

IP is the Internet’s standard for implementing the tasks assigned to the network layer.
The network layer performs two main tasks:
Forwarding – sending packets from one router to the next until they reach the destination.
Routing – Routing means updating the forwarding table based on changes in the network.
The routing table must be updated when:

A router stops working, so packets should not be sent through it.

A part of the network becomes congested, so traffic should be sent through a different path.

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The TCP/IP Protocol Suit

Internet Protocol (IP): IPv4 and IPv6

For many years, the Internet has used IPv4 (IP version 4) to implement the network layer.
IPv4 uses 32-bit addresses, but the Internet has grown so much that these addresses are no longer enough.
To solve this problem and to support new features like multicast, a new version called IPv6 has been developed.
IPv6 uses 128-bit addresses, which provide a much larger address space.
The Internet is currently transitioning from IPv4 to IPv6.
It is expected that the use of 32-bit IPv4 addresses will eventually disappear.

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Cybersecurity

The National Institute of Standards and Technology (NIST) was founded in 1901 and is part of the U.S. Department of Commerce.
NIST defines Cybersecurity as:

Prevention of damage to, protection of, and restoration of computers, electronic communications systems, electronic communications services, wire communication, and electronic communication, including information contained therein, to ensure its availability, integrity, authentication, confidentiality, and nonrepudiation

U.S. law (44 USC 3552) defines information security as

protecting information and information systems from:

Unauthorized access
Unauthorized use
Unauthorized disclosure
Disruption
Modification
Destruction

in order to provide—integrity…, confidentiality…, and availability…

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Cybersecurity

Overview—What is Cybersecurity?

In cybersecurity, information security is often explained using the CIA triad:
C – Confidentiality: keeping information private
I – Integrity: keeping information correct and unchanged
A – Availability: making sure information and systems are available when needed
The main goal of cybersecurity is to protect information and the systems that manage that information.
Today, our information and systems face many attacks.
Attackers want to steal information for profit or other benefits.
So, understanding cybersecurity is important to protect people, resources, and society.
To understand today’s cybersecurity situation, we must look at the past, understand the present, and prepare for the future.

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Cybersecurity

Overview—What is Cybersecurity?

Confidentiality

According to 44 USC 3552(b)(3)(B):

Confidentiality…means preserving authorized restrictions on access and disclosure, including means for

protecting personal privacy and proprietary information.

The law recognizes every individual’s right to privacy. This right extends to any information that, if made public, could harm the person.
Therefore, it is the responsibility of the custodians of the information to safeguard that privacy for the individuals whose information they hold.
For example, people rely on banks to protect the privacy of their credit card information. The expectation is that banks—the custodians of this information - should not allow their customer account information to be exposed to those who do not require its access.
Modern cybersecurity services help banks protect the privacy of their customers’ information.
For example, banks now provide services such as fraud alerts to customers when credit cards are used in unusual manner. These alerts give customers the option to immediately freeze their card so that it cannot be misused by anyone who has accessed their confidential information.

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Cybersecurity

Overview—What is Cybersecurity?

Integrity:

According to 44 USC 3552(b)(3)(A):

Integrity…means guarding against improper information modification or destruction, and includes ensuring information non-repudiation and authenticity.

When you access information from an information system, for example, your grades from school, or the monthly statement from your bank account, you trust that the information provided is reliable.
For example, when the bank reports the balance in your checking account, you do not think it necessary to calculate the totals of credits, debits, and interest yourself to verify the amount. Rather, you trust that the bank has made the right calculations.
Life would become complex if the information received from IT systems could not be trusted to be accurate. Integrity is the aspect of information security that prevents that from happening.

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Cybersecurity

Overview—What is Cybersecurity?

Availability:

According to 44 USC 3552(b)(3)(C):

Availability…means ensuring timely and reliable access to and use of information.

For example, when you log in to an online class, you expect the system to be up and running—that is availability.
If an information system is not available, it becomes useless. Most viruses affect availability. They typically delete important files, causing a loss of availability.
Even if the files can be recovered from backups or other sources, time is still lost during the recovery process. This is the time that cannot be used for doing useful work, which means lack of availability.

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Cybersecurity

The Basic Information Security Model

Information security is a very broad subject area. This is because most information security incidents happen when attackers find new weaknesses in an organization.
To maintain information security, an organization must take care of many different aspects of the organization—not just one.
To manage all the activities used to maintain information security, it is helpful to organize them into one unified model.
A model is used to represent a concept that exists in the real world.
Using a model makes it easier to understand why cybersecurity incidents happen and what motivates them.

Fig. shows a simple framework to understand information security. This model has four main parts:

i)assets, ii) vulnerabilities, iii) threats, and iv) controls.

These four parts are very important because they help us understand any cybersecurity event where information is attacked or at risk.

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Cybersecurity

The Basic Information Security Model

Assets:

At the center of Fig. are assets.
In the context of information security, an asset is defined as

a resource or information that is to be protected.

Every security situation (whether related to information security or simply related to securing one’s own property), starts with an asset that is considered valuable.
We protect assets because they are valuable and require effort to protect them from harm.
Information security is no different. If some information or resource is valuable, the organization must make special efforts to secure it.
There are two main differences between conventional assets and information assets:
Invisibility
Duplicability

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Cybersecurity

The Basic Information Security Model

Assets

Information assets are invisible and can be copied easily. These two features—invisibility and duplicability—make information security more difficult than conventional security.
Conventional security methods like locks and guards do not work well for information security.

For example, a conventional lock cannot stop someone from stealing data over the network.

A conventional asset like gold can usually be recovered and returned to its owner. But stolen data can be copied many times and stored in many places.
Even if some copies are deleted, the thief may still keep other copies.

Information security controls must focus on preventing data theft before it happens. They must also detect and stop thefts while they are happening.
This is done through continuous monitoring of systems and networks.

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Cybersecurity

The Basic Information Security Model

Threats

A threat is defined as

the capabilities, intentions, and attack methods of adversaries to exploit or cause harm to assets.

Example: In the Excel example, a worker may want to exploit the lack of password protection on the file and modify their hourly rate. These threats are shown in the Fig. as arrows.

MITRE ATT&CK: Today, cybersecurity professionals commonly use a framework created by MITRE called “MITRE ATT&CK.

MITRE is a non-profit organization funded by the government for research and development.

The MITRE ATT&CK framework is based on real-world observations.

It provides a global knowledge base of the tactics and techniques used by cybercriminals.

It shows how attackers exploit vulnerabilities in IT systems.

The MITRE ATT&CK knowledge inventory is a tremendous source for what is called threat intelligence.

NIST (National Institute of Standards and Technology) defines threat intelligence as:

Threat information that has been aggregated, transformed, analyzed, interpreted, or enriched to provide the necessary context for decision-making processes.

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Cybersecurity

The Basic Information Security Model

Threats

There are two types of threat intelligence sources:

Public sources – Free and open to everyone

Private sources – Paid or available only to specific organizations

Cybersecurity professionals use these sources to collect important information about:

Current global cyber threats

Hackers who are trying to attack systems

Vulnerability in IT assets that attackers may exploit

A threat intelligence report from MITRE ATT&CK warns about three specific threats:

Axiom

GOLD SOUTHFIELD

Hikit

The report explains that these threats have been used to carry out phishing attacks to gain access to victim IT systems.

Axiom is believed to be a Chinese cyber espionage group.

GOLD SOUTHFIELD is motivated for financial gain.

Both groups are global cybersecurity threats.

Hikit, is classified as malware that can be used to remotely connect to an IT system for the purpose of creating a persistent threat

Persistent threats are used to cause damage to an IT system at any time a malicious actor chooses.

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Cybersecurity

The Basic Information Security Model

Threats

The most popular types of threats observed in cybersecurity include

viruses,

worms,

phishing, and

malware.

VIruses and Worms:

are computer programs that adversely affect computers and propagate through the network without the user’s consent.

The difference between a virus and a worm is that:

a virus uses other programs (e.g., the user’s email client) to spread, whereas

the worm can propagate all by itself.

Malware (malicious software):

is a general term used to describe software or code specifically designed to exploit a computer, or the data it contains, without the user’s consent.

A common way for malware to reach computers is through free downloads. Malware author creates a computer software that appears to be very useful and distributes it for free.
Unsuspecting users download and install the useful-looking software. Along with it, malware gets installed without their knowledge.
This method is called the Trojan horse technique.

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Threats

Phishing:

is an attempt to compromise a user by masquerading as a trustworthy entity in electronic communication.

Early phishing attacks attempted to steal information such as usernames, passwords, and credit card details.
Most people receive at least one or two of these emails every week.
The emails appear to originate from banks and lead users to visit a website that looks like the bank’s website.
At the website, users are asked to provide their username and password in order to make some correction at the bank.
While the emails and target website appear to be legitimate, they really aren’t.
A close look at the URL will show that the website is actually hosted on a compromised server.

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Cybersecurity

The Basic Information Security Model

Vulnerabilities

Information security becomes important because all systems have vulnerabilities.

A vulnerability is a weakness in an information system that gives a threat the opportunity to compromise an asset.

A vulnerability in an asset allows threats to attack it.
In the Excel example, a worker might try to take advantage of the file not being password-protected and change their hourly rate.

In this Excel-based IT system, vulnerabilities include:

Unauthorized access, which can cause loss of confidentiality or integrity.

Hard drive failures, which can cause loss of availability.

If IT systems had no vulnerabilities at all, we would not need to study information security, and we would not need professionals who work in information security.
However, modern software products are very large.

For example, Microsoft Windows has millions of lines of code. It is difficult to predict and eliminate every possible vulnerability in such big software systems.

Common Vulnerabilities and Exposures (CVE) list:

To handle vulnerabilities, the software industry (in collaboration with the federal government) has created a list of known software vulnerabilities. This list is called the Common Vulnerabilities and Exposures (CVE) list.
The CVE list gives common names and ID numbers for all publicly known software vulnerabilities. The list is maintained by MITRE.

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Cybersecurity

The Basic Information Security Model

Vulnerabilities

Vulnerabilities can be classified into specialized categories such as software vulnerabilities.

A software vulnerability is an error in the specification, development, or configuration of software such that its execution can violate the security policy.

For example, a software developer may create a website that requires user input within a textbox prior to submitting a form.

If the developer does not write code to validate the user’s input, a hacker can type SQL commands into the textbox and submit the form. This type of attack is called SQL Injection.

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Cybersecurity

The Basic Information Security Model

Controls

All IT systems will continue to have vulnerabilities for the foreseeable future.
During this time, there will always be dedicated attackers who try to exploit these vulnerabilities for personal gain or for other motives.
What does a system administrator do to defend the computers?
The role of information security is to reduce the harm caused by threats.
This is done by deploying security controls around the vulnerable IT system.

Security controls are safeguards used to minimize the impact of threats.

Within the framework illustrated in Fig. these controls are shown as the ring around the IT system.

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The Basic Information Security Model

Controls

Most threats are blocked by the controls commonly adopted by organizations.
For example:

Most operating systems now include a firewall with some default settings.

They also encourage users to create a strong password to protect the administrative user account.

Even the smallest businesses backup their important files on external storage appliances or other Internet services.

They also keep their computers locked to stop unauthorized access.

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The Basic Information Security Model

Controls

Even basic controls like firewalls and passwords can block most of the threats that organizations face.
However, as shown in Fig, even the best security controls have holes.
For example:

Users often prefer memorable passwords over secure passwords.

They also do not back up their data regularly, even if they have spent a lot of money to purchase backup systems.

Threats exploit these weaknesses in security controls to attack the vulnerable IT systems.
The arrow on the right shows the threats breaching the controls and reaching the IT system.
Fortunately, many of these threats do not cause harm because, as shown by the arrow on the right in Figure 250, they cannot reach the IT system.

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The Basic Information Security Model

Controls

Cybersecurity controls can be classified as:

i) physical, ii) procedural, and iii) technical.

Physical controls: use traditional non-technical methods of preventing harm. Typically, they prevent unauthorized users from being able to enter technical facilities.

Examples of such controls include:

locks, fire extinguishers, background checks, and doors.

Procedural controls: are prescribed plans of action that guide how computer resources should be used.

Examples of procedural controls include:

Procedures for obtaining computer accounts

Procedures for escalating privileges

Procedures for modifying programs

Procedures for hiring

Requirements that users change their passwords periodically

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The Basic Information Security Model

Controls

Cybersecurity controls can be classified as:

i) physical, ii) procedural, and iii) technical.

Technical controls: are the security measures built into the information system itself.

Examples include

passwords, firewalls, intrusion detection systems, system updates, and antivirus software.

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Cybersecurity

Cyber Hygiene

Definition

Defined by CISA (Cybersecurity & Infrastructure Security Agency) as practices that reduce the risk of a successful cyber-attack.
Similar to personal hygiene (like brushing teeth), cyber hygiene is about regular habits that keep your digital life safe.

Why Important?

People rely on IT systems (phones, laptops, social media, banking, etc.).
Hackers often exploit human negligence (like weak passwords or ignoring updates).
Good cyber hygiene = protection of personal information, privacy, and financial security.

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Key Cyber Hygiene Practices

Keep Devices Updated
Install software and security updates when notified.
Prevents hackers from exploiting old vulnerabilities.

b. End-point Protection (Antivirus Software)

Tools like McAfee, Norton, Kaspersky.
Protect identity, privacy, and devices from malware.
Password Management
Strongest defense against unauthorized access.
Best practices:

Use unique passwords (not same across accounts).

Change passwords regularly.

Safe Social Media Practices
Don’t post private info (address, phone number).
Review privacy settings.
Data Backup
Store files in cloud or external drives.
Protects against ransomware or accidental loss.

f. Secure Network Practices 

Change default name of Wi-Fi router.
Use a Guest Network for visitors.

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Key Cyber Hygiene Practices

g. Device Security

Lock devices with PIN or password.
Encrypt sensitive data (USBs, laptops).

Weakest Link: Humans

Most cyber-attacks target people, not technology.

Example: Social Engineering → hackers trick people into sharing passwords, OTPs, or private data.

Poor cyber hygiene can lead to:

Identity theft

Bank fraud

Privacy violations

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Cybersecurity

Teams in Cybersecurity

Cybersecurity is not a one-person job — it’s more like a team sport where multiple groups work together to protect systems and respond to attacks. These teams study the TTPs (Tactics, Techniques, and Procedures) used by hackers.

What are TTPs?

Tactics → What the hacker wants to achieve (e.g., gaining access, stealing data).
Techniques → How the hacker does it (e.g., phishing email, malware, password cracking).
Procedures → Step-by-step execution plan (e.g., sending phishing mail → tricking user → installing malware → stealing data).

Types of Teams

Red Teams (Offense)
Act like real hackers (simulate cyberattacks).
Use hacker tools & techniques to test the system’s weaknesses.
Purpose: Show companies how attackers can break in → help them fix vulnerabilities.
Think of them as the "attackers in a practice match".

Example: A Red Team might run a phishing campaign inside a company to see how many employees fall for it.

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2. Blue Teams (Defense)

Constantly monitor and defend systems.
Work in Security Operation Centers (SOC).
Tasks: Detect intrusions, investigate attacks, analyze logs, and respond quickly.
Aim: Prevent and minimize damage from attacks.
Think of them as the "defenders in a practice match".

Example: If malware infects a server, the Blue Team checks how it happened, removes the malware, and adds stronger controls to prevent repeat attacks.

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Ethical Issues in Information

Ethical responsibility applies not just to users but also to:

Site owners (companies, admins).

Content posters and monitors.

Must ensure that content is reliable, respectful, and truthful.

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A type of intellectual property that protects original works once they are fixed in a tangible form (writing, photo, video, software, etc.). Owned by creator unless work-for-hire
Expression protected, not idea
Duration: life + 70 years

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Open-Source Software (OSS)

Source code publicly available
Motivations: altruism, standards, collaboration
Risks: license violations in commercial work

Ex:Popular in industry (e.g., Linux OS powers much of the internet

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Public Domain

Definition: Creative works with no intellectual property rights (IPR).
Reasons a work enters public domain:
Never qualified for copyright

Ex:Aristotle’s Nicomachean Ethics.

Public domain works can be used freely in your own creations, but you cannot claim ownership over them

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Fair Use

Definition: Limited and transformative use of copyrighted work without permission.

Permitted for:

Commentary

Criticism

Teaching

Research

Parody

Balances copyright owner’s rights with freedom of expression.

👉 Example:

Quoting a few lines from a book in your research paper = Fair Use.

Uploading the entire book for free distribution = Copyright violation.

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Plagiarism

Definition: Using someone else’s words or ideas without giving credit (no attribution or acknowledgment).
Why unethical?:

It disrespects intellectual creativity.

Society thrives on building ideas

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Piracy

Unauthorized copying/distribution of copyrighted works

Plagiarism = Passing off someone’s work as your own.

Piracy = Sharing/distributing without claiming it as your own.

Examples: movies, software sharing

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Online Privacy

Right to control personal information

sector-specific laws:

HIPAA (healthcare)

FERPA (education).

Responsibilities:

Collect personal data only for the intended purpose.

Protect against misuse, fraud, and identity theft.

Teachers must safeguard student academic records.