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Intro to Networking

CS 161 Fall 2025 - Lecture 17

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Last Time: SQL Injection

Web servers interact with databases to store data

Web servers use SQL to interact with databases

SQL injection: Untrusted input is used as parsed SQL

The attacker can construct their own queries to run on the SQL server!
Blind SQL injection: SQLi with little to no feedback from the SQL query
Defense: Input sanitization

Difficult to implement correctly

Defense: Prepared statements

Data only ever treated as data; bulletproof!

Injection: Untrusted input is carelessly included in any parsed language

Defense: Use safe API calls that pass untrusted data separately from trusted commands

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Last Time: CAPTCHAs

CAPTCHA: A challenge that is easy for a human to solve, but hard for a computer to solve

Examples: Reading distorted text, identifying images
Original purpose: Distinguishing between humans and bots
Modern purpose: Forces the attacker to spend some money to solve the CAPTCHAs
Modern purpose: Providing training data for machine learning algorithms

Issues with CAPTCHAs

As computer algorithms get smarter, CAPTCHAs get harder, and not all humans are able to solve them easily
Ambiguity: CAPTCHAs might be so hard that the validator doesn't know the solution either!
Economics: Breaking CAPTCHAs just costs money
Not all bots are bad

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Today: Intro to Networking

Internet: A global network of computers
OSI model: A layered model of protocols

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What’s the Internet?

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Textbook Chapter 25

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What’s the Internet?

Network: A set of connected machines that can communicate with each other

Machines on the network agree on a protocol, a set of rules for communication

Internet: A global network of computers

The web sends data between browsers and servers using the Internet
The Internet can be used for more than the web (e.g., SSH)

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Protocols

A protocol is an agreement on how to communicate that specifies syntax and semantics

Syntax: How a communication is specified and structured (format, order of messages)
Semantics: What a communication means (actions taken when sending/receiving messages)

Example: Protocol for asking a question in lecture?
The student should raise their hand
The student should wait to be called on by the speaker or wait for the speaker to pause
The student should speak the question after being called on or after waiting
If the student has been unrecognized after some time: Vocalize with “Excuse me!”

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Internet Analogy: Mail

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What’s the goal of the Internet?

Move data from one location to another
Analogy: I write a message on a piece of paper. How do I send this message to you?
Solution: Postal system

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Building block 1: Something That Moves Data

Mailman, Pony Express, carrier pigeon, etc.

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Are pigeons faster than the Internet?

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Building block 1: Something That Moves Data

The Internet is built on technology that moves bits across space
Voltages on wires, wireless technology, radio waves, etc.

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An example of packet loss.

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Building block 2: Talking to the Apartment Complex

Using building block 1, we can link up people within a local apartment complex
Forms a local area network (LAN)

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Building block 1: wires

Building block 2: local network

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Building block 3: Post offices

A post office connects two or more apartment complexes
Forms a wide area network

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Building block 1: wires�Building block 2: local network��

Building block 3: wide area network

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Building block 3: The Internet

Connect the entire world using post offices
Messages may pass through multiple post offices before reaching destination

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Building block 1: wires

Building block 3: the Internet

Building block 2: local network

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Building block 3: The Internet

Connect the entire world using post offices
Messages may pass through multiple post offices before reaching destination

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Building block 1: wires

Building block 3: the Internet

Building block 2: local network

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Layers of abstraction

Layer 3: Connect many local networks to form a global network

Layer 2: Create links in a local area

Layer 1: Move bits across space

A change in layer 1 implementation (wireless instead of wires) doesn’t affect the other layers
A change in layer 2 protocols doesn’t affect the other layers

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Layering: The OSI Model

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Layering

Internet design is partitioned into various layers. Each layer…

Has a protocol
Relies on services provided by the layer below it
Provides services to the layer above it

Analogous to the structure of an application and the “services” that each layer relies on and provides

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Code You Write

Run-Time Library

System Calls

Device Drivers

Voltage Levels/Magnetic Domains

Fully isolated from user programs

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OSI Model

OSI model: Open Systems Interconnection model, a layered model of Internet communication

Originally divided into 7 layers

But layers 5 and 6 aren’t used in the real world, so we ignore them

Same reliance upon abstraction

A layer can be implemented in different ways without affecting other layers
A layer’s protocol can be substituted with another protocol without affecting other layers

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Application

Transport

(Inter) Network

Link

Physical

1

2

3

4

7

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Layer 1: Physical Layer

Provides: Sending bits from one device to another

Encodes bits to send them over a physical link

Patterns of voltage levels
Photon intensities
RF modulation

Examples

Wi-Fi radios (IEEE 802.11)
Ethernet voltages (IEEE 802.3)

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Physical

1

Application

Transport

(Inter) Network

Link

2

3

4

7

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Layer 1: Physical Layer

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Physical

1

Application

Transport

(Inter) Network

Link

2

3

4

7

A

B

01110111…01

Physical layer: “How do I transmit this sequence of 0’s and 1’s from A to B?”

Next: How do we talk to more than one device?

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Layer 2: Link Layer

Provides: Sending frames directly from one device to another

Relies upon: Sending bits from one device to another
Encodes messages into groups of bits called “frames”

Examples

Ethernet frames (IEEE 802.3)

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Physical

1

Application

Transport

(Inter) Network

3

4

7

Link

2

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Layer 2: Link Layer

Local area network (LAN): A set of computers on a shared network that can directly address one another

Consists of multiple physical links

Frames must consist of at least 3 things:

Source (“Who is this message coming from?”)
Destination (“Who is this message going to?”)
Data (“What does this message say?”)

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Source: A

Destination: C

“Hello, this is A…”

A

B

D

C

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Layer 2: Link Layer

In reality, computers aren’t all connected to the same wire

Instead, local networks are a set of point-to-point links

However, Layer 2 still allows direct addressing between any two devices on the local-area network

Enabled by transmitting a frame across multiple physical links until it reaches its destination

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Source: A

Dest: C

“Hello, this is A…”

A

B

C

D

E

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Ethernet and MAC Addresses

Ethernet: A common layer 2 protocol that most endpoint devices use
MAC address: A 6-byte address that identifies a piece of network equipment (e.g., your phone’s Wi-Fi antenna)

Typically represented as 6 hex bytes: 13:37:ca:fe:f0:0d
The first 3 bytes are assigned to manufacturers (i.e. who made the equipment)

This is useful in identifying a device

The last 3 bytes are device-specific

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Layer 2: Link Layer

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Physical

1

Application

Transport

(Inter) Network

3

4

7

Link

2

Source: A

Dest: C

“Hello, this is A…”

Link layer: “How do I transmit this frame from A to C, making sure that no one else thinks the message is for them?”

Next: How do we address every device in existence?

A

B

D

C

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Layer 3: Network Layer

Provides: Sending packets from any device to any other device

Relies upon: Sending frames directly from one device to another
Encodes messages into groups of bits called “packets”
Bridges multiple LANs to provide global addressing

Examples

Internet Protocol (IP)

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Physical

1

Application

Transport

4

7

(Inter) Network

3

Link

2

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Layer 3: Network Layer

Recall the ideal layer 2 model: All devices can directly address all other devices

This would not scale to the size of the Internet!

Instead, allow packets to be routed across different devices to reach the destination

Each hop is allowed to use its own physical and link layers!

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A

B

D

C

E

F

G

H

Router

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Layer 3: Network Layer

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A

Router

C

D

E

B

Router

Source: A

Destination: D

“Hello, this is A…”

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Layer 3: Network Layer

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A

Router

C

D

E

B

Router

Source: A

Destination: D

“Hello, this is A…”

This link could be Wi-Fi

And this link could be Ethernet

But the Internet protocol stays the same, end to end

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Layer 3: Network Layer

Packets must consist of at least 3 things:

Source (“Who is this message coming from?”)
Destination (“Who is this message going to?”)
Data (“What does this message say?”)
Similar to frames (layer 2)

Packets may be fragmented into smaller packets

Different links might support different maximum packet sizes
Up to the recipient to reassemble fragments into the original packet

Each router forwards a given packet to the next hop

We will cover how a router knows how to forward—and attacks on it—in the future

Packets are not guaranteed to take a given route

Two packets with the same source and destination may take different routes

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Internet Protocol (IP)

Internet Protocol (IP): The universal layer-3 protocol that all devices use to transmit data over the Internet
IP address: An address that identifies a device on the Internet

IPv4 is 32 bits, typically written as 4 decimal octets, e.g. 35.163.72.93
IPv6 is 128 bits, typically written as 8 groups of 2 hex bytes: 2607:f140:8801::1:23

If digits or groups are missing, fill with 0’s, so 2607:f140:8801:0000:0000:0000:0001:0023

Globally unique from any single perspective

For now, you can think of them as just being globally unique

IP addresses help nodes make decisions on where to forward the packet

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Reliability

Reliability ensures that packets are received correctly or, if random errors occur, not at all

This is implemented with a checksum
However, there is no cryptographic MAC, so there are no guarantees if an attacker modifies packets

IP is unreliable and only provides a best effort delivery service, which means:

Packets may be lost (“dropped”)
Packets may be corrupted
Packets may be delivered out of order

It is up to higher level protocols to ensure that the connection is reliable

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Layer 3: Network Layer

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A

Router

C

D

E

B

Router

Source: A

Destination: D

“Hello, this is A…”

Layer 3: “How do I get this packet from A to D?”

Next: How do we reliably send any length of data, not just packets?

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Layer 4: Transport Layer

Provides: Transportation of variable-length data from any point to any other point

Relies upon: Sending packets from any device to any other device
Builds abstractions that are useful to applications on top of layer 3 packets

Useful abstractions

Reliability: Transmit data reliably, in order
Ports: Provide multiple “addresses” per real IP address

Examples

TCP: Provides reliability and ports
UDP: Provides ports, but no reliability

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Physical

1

Application

7

Link

2

(Inter) Network

3

Transport

4

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Layer 4: Transport Layer

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A

D

I am now sending an arbitrary length message that will probably be broken into several packets…

Unreliable Internet

Layer 4: “How do I transport this arbitrary data over an unreliable medium?”

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Layer 7: Application Layer

Provides: Applications and services to users

Relies upon: Transportation of variable-length data from any point to any other point

Every online application is Layer 7

Web browsing
Online video games
Messaging services
Video calls (Zoom)

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Physical

1

Link

2

(Inter) Network

3

Transport

4

Application

7

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Headers

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Example: Sending Mail

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Alice

Bob

I am hungry.

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Example: Sending Mail

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Alice

Bob

Send to: Bob

I am hungry.

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Example: Sending Mail

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Alice

Bob

Mail to: 123 Bob St

Send to: Bob

I am hungry.

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Example: Sending Mail

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Alice

Bob

Mail to: 123 Bob St

Send to: Bob

I am hungry.

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Example: Sending Mail

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Alice

Bob

Send to: Bob

I am hungry.

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Example: Sending Mail

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Alice

Bob

I am hungry.

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Example: Sending Mail

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Alice

Bob

Each layer communicates with each other, relying on abstractions below them!

Relies upon: Sending messages to people

Provides: Sending messages to people

Relies upon: Sending messages to addresses

Provides: Sending messages to addresses

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Layers of Abstraction and Headers

As you move to lower layers, we wrap additional headers around the message
As you move to higher layers, you peel off headers around the message

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Mail to: 123 Bob St

I’m hungry.

Send to: Bob

I’m hungry.

Send to: Bob

I’m hungry.

Lower layer, more headers

Higher layer, fewer headers

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Layers of Abstraction and Headers

As you move to lower layers, you wrap additional headers around the message
As you move to higher layers, you peel off headers around the message
When sending a message we go from the highest to the lowest layer
When receiving a message we go from the lowest to highest layer

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Layer 2 Header

Ethernet and MAC address headers

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Source MAC Address (6 bytes)
Destination MAC Address (6 bytes)
VLAN Tag (4 bytes)				Type (2 bytes)
Data (variable-length)

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Layer 3 Header

IPv4 header

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Version (4 bits)	Header Length (4 bits)	Type of Service (6 bits)	ECN (2 bits)	Total Length (16 bits)
Identification (16 bits)				Flags (3 bits)	Fragment Offset (13 bits)
Time to Live (8 bits)		Protocol (8 bits)		Header Checksum (16 bits)
Source Address (32 bits)
Destination Address (32 bits)
Options (variable length)
Data (variable length)

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

GET / HTTP/1.1

...

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

From: Port 1234

To: Port 80

GET / HTTP/1.1

...

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

From: 1.2.3.4

To: 5.6.7.8

From: Port 1234

To: Port 80

GET / HTTP/1.1

...

Final destination

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

From: 20:61:84:3a:a9:52

To: 6d:36:ff:4a:32:92

From: 1.2.3.4

To: 5.6.7.8

From: Port 1234

To: Port 80

GET / HTTP/1.1

...

Address of next hop

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

From: 20:61:84:3a:a9:52

To: 6d:36:ff:4a:32:92

From: 1.2.3.4

To: 5.6.7.8

From: Port 1234

To: Port 80

GET / HTTP/1.1

...

Converted into bits and transmitted

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

From: 89:8d:33:25:47:24

To: d5:a9:20:68:e0:80

From: 1.2.3.4

To: 5.6.7.8

From: Port 1234

To: Port 80

GET / HTTP/1.1

...

Received over the physical medium

Notice: The MAC addresses changed because the recipient is on a different network

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

From: 89:8d:33:25:47:24

To: d5:a9:20:68:e0:80

From: 1.2.3.4

To: 5.6.7.8

From: Port 1234

To: Port 80

GET / HTTP/1.1

...

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

From: 1.2.3.4

To: 5.6.7.8

From: Port 1234

To: Port 80

GET / HTTP/1.1

...

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

From: Port 1234

To: Port 80

GET / HTTP/1.1

...

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

GET / HTTP/1.1

...

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Example: HTTP Request

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HTTP

TCP

IP

Ethernet

Wires

HTTP

TCP

IP

Ethernet

Wires

Relies upon: Transport of data

Provides: Transport of data

Relies upon: Global packet delivery

Provides: Global packet delivery

Relies upon: Local frame delivery

Provides: Local frame delivery

Relies upon: Communication of bits

Provides: Communication of bits

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Intro to Networking: Summary

Internet: A global network of computers

Protocols: Agreed-upon systems of communication

OSI model: A layered model of protocols

Layer 1: Communication of bits
Layer 2: Local frame delivery

Ethernet: The most common Layer 2 protocol
MAC addresses: 6-byte addressing system used by Ethernet

Layer 3: Global packet delivery

IP: The universal Layer 3 protocol
IP addresses: 4-byte (or 16-byte) addressing system used by IP

Layer 4: Transport of data (more on this next time)
Layer 7: Applications and services (the web)

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Application

Transport

(Inter) Network

Link

Physical

1

2

3

4

7

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