Lesson Notes: Network Protocol Layers

A network layer model helps break down communication into smaller, more manageable functions. The most common model is the OSI (Open Systems Interconnection) Model, which has seven layers, each responsible for a specific role in data transmission.

1. Application Layer – User-facing services (e.g., web browsing, email).
   • Protocols: HTTP, HTTPS, FTP, SMTP, POP3
2. Presentation Layer – Formats and encrypts data for applications.
   • Protocols: SSL/TLS
3. Session Layer – Manages connections and sessions between devices.
   • Protocols: NetBIOS, RPC
4. Transport Layer – Ensures reliable data delivery (e.g., error checking).
   • Protocols: TCP (reliable), UDP (fast but unreliable)
5. Network Layer – Routes data between networks using IP addresses.
   • Protocols: IP, ICMP, ARP
6. Data Link Layer – Manages data transfer between directly connected devices.
   • Protocols: Ethernet, Wi-Fi (802.11), MAC addresses
7. Physical Layer – Transmits raw bits over physical media (cables, radio waves).
   • Examples: Ethernet cables, Fiber optics, Wireless signals

TCP/IP Model (Simplified Version)

The TCP/IP Model is a simpler version of OSI with four layers:

1. Application Layer – Combines OSI’s top three layers (e.g., HTTP, HTTPS, FTP).
2. Transport Layer – Manages connections (e.g., TCP, UDP). TCP has error checking, UDP does not. Data is split into packets (datagrams) which includes
3. Internet Layer – Routes data (e.g., IP, ICMP). ICMP is Internet Connection Management Protocol and includes traceroute and ping.
4. Network Access Layer – Physical and data link functions (e.g., Ethernet, Wi-Fi). How the packet is managed when received at rhe Lan of its destination. Packets are turned into frames and the switch forwards the right frames to the right IP address. Switch knows which MAC address has requested data and returns the data to the right one. The frame

Network Protocols

Protocols are rules and standards that define how data is transmitted across networks.

• HTTP/HTTPS – Used for accessing websites.
• FTP – Transfers files between computers.
• SMTP/POP3/IMAP – Email communication. (Also X500)
• TCP – Ensures reliable data transmission.
• UDP – Fast, but no error checking.
• IP – Assigns addresses to devices.
• Ethernet/Wi-Fi – Used for local network communication.

Real-World Analogy for Network Layers – The Royal Mail Postal System

Think of network communication like sending a letter through a postal service. Each network layer has a role similar to steps in delivering a letter.

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1. Physical Layer (Sending the Letter)

• Represents the physical means of communication (wires, Wi-Fi signals, fibre optics).
• In our analogy, this is like the postman’s satchel, postal vans, roads, and planes that physically transport your letter.

🔹 Example: Your letter is put into a satchel and carried by a van to the sorting office.

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2. Data Link Layer (Stamping & Addressing)

• Ensures that data moves correctly between directly connected devices.
• Like adding a stamp and return address, ensuring your letter is accepted by the postal system.
• Also checks for errors in transmission.

🔹 Example: If the letter is missing a stamp, it gets rejected.

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3. Network Layer (Routing the Letter)

• Determines the best route for data using an IP address.
• Like a postal sorting office deciding which city or country to send the letter to.

🔹 Example: Your letter is sent from London to New York based on the address.

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4. Transport Layer (Ensuring Delivery)

• Manages the reliability of communication (e.g., TCP ensures delivery, UDP sends quickly but without confirmation).
• Like choosing regular mail (UDP – fast but no tracking) or tracked mail (TCP – slower but reliable).

🔹 Example: If using TCP (tracked mail), the sender gets a notification when the letter arrives.

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5. Session Layer (Managing the Conversation)

• Establishes and maintains a session between devices.
• Like a customer service call that stays connected while you get information.

🔹 Example: You keep sending letters back and forth in an ongoing conversation.

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6. Presentation Layer (Translating the Message)

• Converts data into a usable format (e.g., encryption, compression).
• Like translating a letter from English to French if the recipient speaks a different language.

🔹 Example: If the message is encrypted (TLS), only the intended recipient can read it.

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7. Application Layer (Receiving & Reading the Letter)

• Directly interacts with the user (e.g., websites, emails, file transfers).
• Like the recipient opening and reading the letter.

🔹 Example: The recipient reads a web page (HTTP) or opens an email (SMTP/IMAP).

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Summary of the Postal System Analogy

OSI Layer	Postal System Equivalent
Application	Reading the letter (Web, Email)
Presentation	Translating the letter (Encryption, Formatting)
Session	Keeping communication open (Back-and-forth letters)
Transport	Ensuring delivery (Regular vs. Tracked Mail)
Network	Choosing the best route (Sorting Office)
Data Link	Addressing & Stamping (MAC Address, Error Checking)
Physical	Delivering the letter (Postman’s Satchel, Roads, Aeroplanes)

Lesson Plan: Teaching Cybersecurity without touching technology

This is the lesson plan which got me the job. I couldn’t be sure of the resources I would have available, so I opted to teach about the CIA Model (set by the College) without anything more than a PowerPoint and a bunch of envelopes and cards. Use freely. Expand as you wish. Post improvements in the comments.

File Name: Microteach-CIA-model.pptx

Download File

The CIA Triad

Triad means 3, so there are 3 parts to the CIA triad model

Confidentiality, Integrity and Availability.

Let me illustrate with a bunch of envelopes…

Confidentiality:

Access Control

I have a list of people who are allowed the data. I ask a requesting student for their name and password which I check on a clipboard and if they are correct I give them the envelope.

I give another student the wrong password card and they ask 3 times which I reject and throw them out, showing that brute force password attacks can be foiled.

Authentication

Another student has a clipboard and a piece of paper with “Permission” on it. The requesting student gives their name and password to the clipboard student who gives me the “Permission” paper and I give the requesting student the envelope.

One issue can be the Man in the Middle attack with these two, so a student stands in between us and takes the envelope. He might not pass it on if he is a bad router and open it himself, so we must ensure the data is safe if it falls into the wrong hands with encryption.

Encryption

I have another envelope which I show as having something in code inside it. A student who is unauthorised asks for the envelope and I give them the one without the solution inside it. It doesn’t make sense. An authorised requesting student asks me for the envelope, and I give the one with the decrypted side to them, which they open and turn over which has the message on it decrypted.

Integrity:

Checksums

The requesting student asks for the envelope, I give them an envelope and tell them that the checksum of this data should be 1024, but on the outside of this envelope the checksum says “63”. I ask them if the checksum is correct and they will say no, so I give them an envelope with “1024” on it and ask if it is correct. If it is, they can open the envelope and get the data confident that it is correct.

Malware such as viruses and trojans often affect the checksum of a file, so the discarded envelopes could contain infected files.

Digital Signatures

I give the requesting student an envelope when they ask for it which I have signed. The first envelope isn’t my name. I ask them if it is correct and they say no, so they should discard it as it isn’t properly signed. I sign another one with my own signature and they can confirm it is correct and again can open it in the knowledge that it is correct.

Availability:

Backups

I create two envelopes. A student is given an envelope, and another student is encouraged to take it off them and throw it away. That is a cyber-attack. I ask him for the envelope, but it is lost. I have a second envelope somewhere, but I have to search for a moment before I hand it to them. This is restoring from a backup.

Redundancy

I create two envelopes. A student is given an envelope and then is told to throw it away. I ask him for the envelope, but it is lost. Luckily, I have a second envelope, and I give it to them immediately. This is redundancy.

These two deal with disaster recovery after systems failure, malicious damage or user error.

Denial of Service Prevention

The requesting student asks for the envelope, but all the other students start asking for the envelope at the same time, and I don’t know who to give it to. I ask the requesting student for a token, and they give me the card which says “Permission” on it and so I can give it to them, explaining that without the right token, a firewall or rate limiter will ignore all the other requests and only give the data to the right person.

Worksheet

Resources

Envelopes:

• Basic x 5
• Encrypted x 1
• Encrypted with solution x 1
• Checksum 63
• Checksum 3794
• Checksum 1024
• Envelope for wrong signature
• Envelope for right signature   = 12 envelopes in total.

Cards:

• Password card
• Wrong password card
• Permission
• Example of my signature   = 4 cards