רשתות מחשבים Computer Networks

קורס רשתות ללימוד עצמי - באנגלית

סדרת סרטונים ללימוד עצמי של רשתות מחשבים המתמקדת במודל חמש השכבות. הסדרה נוצרה על ידי עומר רוזנבוים, הכותב הראשי של הספר רשתות מחשבים של המרכז לחינוך סייבר. הסדרה בהתהוות. סרטונים נוספים יעלו מעת לעת.

סדרת הסרטונים הינה בשיטת "מלמטה למעלה" (bottom up) בעוד הספר כתוב בשיטת "מלמעלה למטה" (top down). כלומר סדרת הסרטונים תתחיל בשכבה 2 (שכבה 1 איננה מכוסה) ותעלה בשכבות, בעוד הספר מתחיל משכבה 5 ויורד בשכבות.

אל הפלייליסט של הערוץ של עומר רוזנבויים.

A self-study video series of computer networks focusing on the five-layer model. The series was created by Omer Rosenbaum, the lead author of the Computer Networks book of the Cyber Education Center. The series is in progress. More videos will be uploaded from time to time.

The video series is made "bottom-up", while the book is written "top-down". In other words, the video series starts from layer 2 (layer 1 is not covered) and goes up in layers, while the book starts from layer 5 and goes down in layers.

To the playlist at Omer Rosenbaum channel.

1.1 - Intro - How does the internet work?

General introduction to this video series.
In this video we will ask how does Facebook work?

Length of video 1:53 minutes.
Chapter #1 on the book.

1.2 - About this course

No requirements are needed for this course.

Length of video 1:49 minutes.
Chapter #1 on the book.

1.3 - Basic Terms

Defining basic terms.

Concepts:
Communication - Exchanging data between two or more hosts.
Client - Initiates a request from the Server
Server - Provides some services for the client
Protocol - A language used by hosts to communicate

Length of video 3:17 minutes.
Chapter #2 on the book.

2.1 - What do we need layers for?

An introduction to the layers model.
Communication is separated into layers, each layer is responsible for a specific subject and every layer rely on the layer before.

Length of video 2:53 minutes.
Chapter #3 on the book after page 64.

2.2 - Five Layers Model - part 1

Introduction to the five layers model.
In part 1 we shall cover the bottom three layers : (1) the physical layer; (2) the data link layer; (3) the network layer.
Since this is an introduction video, we shall focus on the division of responsibility between the layers, not on specific details.
We will dive deeper in future videos.

Concepts:
First Layer - Physical layer - Responsible for transferring a single bit.
&nbsp&nbsp&nbsp&nbspBit rate - number of bits transfer per unit of time.
Second Layer - Data Link layer - Transmit data between directly linked hosts and responsible to fix errors caused by physical layer.
&nbsp&nbsp&nbsp&nbspHop - A rough measure of distance between two hosts.
&nbsp&nbsp&nbsp&nbspFrames - Datagrams (a basic transfer unit) in the second layer
&nbsp&nbsp&nbsp&nbspMAC Address - Physical Address. Each host has a unique mac address. (MAC - Media Access Control)

Third Layer - Network layer - Routing - Determining the path where the data will travel.
&nbsp&nbsp&nbsp&nbspPacket- Datagrams (a basic transfer unit) in the third layer.
&nbsp&nbsp&nbsp&nbspIP Address - Logical Address.

Length of video 9:05 minutes.
Chapter #3 on the book after page 64.

2.3 - Five Layers Model - part 2

Introduction to the five layers model.
In part 2 we shall cover the top two layers - (4) the transportation layer; (5) the application layer; and summarize the five layers model.
Since this is an introduction video, we shall focus on the division of responsibility between the layers, not on specific details. We will dive deeper in future videos.

Concepts:
Transport layer - End to end communication for specific service. may support reliability and explain what is it. Introduce segments.
&nbsp&nbsp&nbsp&nbspMultiplexing - To allow communication over different services
&nbsp&nbsp&nbsp&nbspReliability - Making sure that packets arrive without errors and in the correct order.
&nbsp&nbsp&nbsp&nbspSegments - Datagrams (a basic transfer unit) in the fourth layer
Application layer - Provides the service to the user's application. Different protocols use for different applications / services.
&nbsp&nbsp&nbsp&nbspHTTP - Protocol for surfing pages on the WWW.
&nbsp&nbsp&nbsp&nbspSMTP - Protocol for emails.
&nbsp&nbsp&nbsp&nbspFTP - Protocol for exchanging files.

Length of video 5:29 minutes.
Chapter #3 on the book after page 64.

2.4 - Five Layers Model - part 3

Introduction to the five layers model.
In part 3 we shall talk about encapsulation, and then have a short recap on what we've learnt.

Concepts:
Encapsulation - A process in which each layer adds its one header to the packet.

Pop Quiz:
At the end of this video will'll have a pop quiz (with the answers)

Length of video 4:01 minutes.
Chapter #3 on the book after page 64.

2.5 - Wireshark - tutorial

A tutorial to one of the most important tool that we'll get to know - Wireshark.
Open source, free and widely used sniffer.
A sniffer is helpful for:
&nbsp&nbsp- debugging network issues
&nbsp&nbsp- programmers
The tutorial:
&nbsp&nbsp- Demonstrate how to start and stop sniffing using wireshark
&nbsp&nbsp- Introduce the packet list pane and the packet detail windows
&nbsp&nbsp- Explain the display filter, show "apply as filter" and "add to filter"

Concepts:
Sniffer- A packet analyzer

Links:
Downloading wireshark: https://www.wireshark.org/download.html
About the hexadecimal number system: https://www.khanacademy.org/math/alge...
More about Wireshark: https://www.wireshark.org/#learnWS

Keyboard shortcut:
<Ctrl>+K - Start sniffing
<Ctrl>+E - Stop sniffing

Length of video 13:17 minutes.
Chapter #3 on the book after page 64.

2.6 - Wireshark - Exercise 1 + Guided Solution - ping

First exercise for Wireshark.
Get the exercise here: https://goo.gl/bEkdnd and try to solve it yourself!
The exercise asks to analyze ping traffic to google. This is the first real world demonstrate for how to really use wireshark.
In order to solve this exercise only need some common sense and to know basic wireshark from the former video.
After solving by yourself watch the guided solution in the video.

Length of video 11:14 minutes.
Chapter #3 on the book after page 64.

2.7 - Wireshark - Exercise 2 + Guided Solution - pcap

Second exercise for Wireshark.
Get the exercise here: https://goo.gl/yCkVm2 . It is a pcap file. Try to solve the exercise yourself!
The exercise aim is to get familiar with IP address and to use filter to show ARP frames and TCP packets.
After solving by yourself watch the guided solution in the video.

Length of video 4:43 minutes.
Chapter #3 on the book after page 64.

2.8 - Standards

In this video we shall learn about standards - why they're important in the world of computer networks, and what RFCs are.
All vendors need to implement standards in order to communicate with each others.

Concepts:
RFC - Request for Comments
IP - Internet Protocol

Length of video 3:23 minutes.
Chapter #3 on the book after page 64.

3.1 - Data Link Layer Goals; Framing

In this video we begin exploring the second layer - that is, the data link layer.
Remind of this layer's goals: transmit data between directly linked hosts.
Data link layer separate the data into frames. Why? In order to get reliability, easier validation and avoid starvation.

Concepts:
Frame - one chunk of the data, separated by the layer.
Starvation - One device using a shared resource and preventing access to it to other devices.
Reliability - Making sure that packets arrive without errors and in the correct order.

Length of video 3:16 minutes.
Chapter #8 on the book after page 280.

3.2 Framing Techniques

In this video we shall continue exploring the data link layer by learning about some framing techniques - how we may split data into frames.
Framing techniques: Character count; Flag Byte with Byte Stuffing;

Concepts:
Character count - Determines the start and end of each frame by the count of remaining characters in the frame header.
Flag Byte - Determines the start and end of each frame by precedes the frame with a special byte sequence.

Links:
Additional references about framing techniques: *
http://www.ece.ubc.ca/~edc/3525.jan20... *
https://web.cs.wpi.edu/~rek/Undergrad... *
https://ocw.mit.edu/courses/aeronauti...

Length of video 3:16 minutes.
Chapter #8 on the book after page 280.

3.3 Handling Errors - Error Detection, Error Correction and Overhead

In this video we shall learn about handing errors. We will get to know two different approaches - detection, and correction.
First, we will look at error detection, where we only know whether a frame is valid or not. We introduce the terms Checksum and CRC.
Second, we will consider error correction, where the receiver can restore the correct value of an erroneous frame. We will also understood why we use error detection in the internet, and introduce the important term - overhead.

Concepts:
Detection - Approach to handing errors. Detect if the chunk is valid or not. If valid read the chunk, if not drop the chunk.
&nbsp&nbsp- Checksum - Error detection code, which is the result of a pre-known function that is calculated on the original data.
&nbsp&nbsp- CRC - Cyclic Redundancy Check.
Correction - Approach to handing errors. Detect if the chunk is valid or not. If valid read the chunk, if not find the error and fix it. than read the chunk.
Overhead - Data or time needed to convey the actual message.
Payload - The actual data or massage.

Links:
For more info regarding CRCs read here: https://en.wikipedia.org/wiki/Cyclic_...

Pop Quiz:
At the end of this video will'll have a pop quiz (with the answers)

Length of video 14:29 minutes.
Not covered in the book.

3.4 Ethernet Frame

Now, we are finally ready to meet our first protocol - Ethernet!
Ethernet is a datalink protocol.
We will talk about the history of Ethernet, and then consider the Ethernet frame format.

Ethernet Frame Format:
Preamble (SOF) | Destination address | Source address | Type/Length | Data (PAD) | CRC32
8 Bytes | 6 Bytes | 6 Bytes | 2 Bytes |46-1500 Bytes | 4 Bytes

Concepts:
Handler - The code handles the protocol.
Padding - padding the data to reach the minimum length of 46 bytes.
Interpacket Gap - Idle time between frames.

Length of video 12:51 minutes.
Chapter #8 on the book after page 292.

3.5 Ethernet Addresses

In this video we cover the structure of Ethernet Addresses, and mention the two special bits they contain.

Ethernet Frame Format:
Preamble (SOF) | Destination add. | Source add. | Type/Length | Data (PAD) | CRC32
8 Bytes | 6 Bytes | 6 Bytes | 2 Bytes |46-1500 Bytes | 4 Bytes

The 6 bytes of address are consist of 3 bytes of Vendor ID (OUI) followed by 3 bytes of host ID.

Concepts:
OUI - Organizationally Unique Identifier (Vendor ID).
MSB - Most Significant Bit / Byte
LSB - List Significant Bit / Byte
Special Bits-
&nbsp&nbsp- LSBit of the MSByte - indicate whether the address is uni-cast or multicast.
&nbsp&nbsp- Second LSBit of the MSByte - indicate whether the address is globally unique.
Broadcast Address - Multicast address that contains all the machines.

Length of video 6:32 minutes.
Chapter #8 on the book after page 292.

3.7 Network Devices - Hub and Switch

In this video we cover two network devices - Hub and Switch.
What are the differences between a Hub and a Switch?
Why are they needed?
How does a Hub operate?
How does a Switch operate?
How does a Switch learn and manage mac address?

Concepts:
Hub - A device with multiple ports that single Ethernet cables are connected to. Solve the extensibility issue. Doesn't know the Ethernet protocol, hence, forwards every frame to all ports but the source.
Switch - A device with multiple ports that multiple Ethernet cables are connected to. Familiar with the Ethernet protocol, maps MAC addresses into ports, based on source addresses.

Exercise:
After watching the video try to solve the exercise found here: https://drive.google.com/file/d/1WeHT...
If you have any questions about the exercise - post them in the comments of the YouTube video. Good luck!

Length of video 12:00 minutes.
Chapter #10 on the book.

3.8 Hacking the Switch: Promiscuous Mode and Switch Security

In this video we learn how and when someone can see traffic sent between two other devices, even if a Switch is in the network.
By understanding how a switch operates, we are able to estimate security issues that stem from its way of operation, as well as relevant countermeasures.
In the next video, we shall have a short recap on what we have covered in this module, and allow you to test yourself and make sure you’ve really understood everything we talked about.

Concepts:
MAC Spoofing - A technique for changing a factory-assigned MAC address of a network interface on a networked device.

Length of video 5:04 minutes.
Not covered in the book.


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