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OSI Model

Room information

Type: Walkthrough
Difficulty: Info
Tags: -
Meta Tags: Walkthrough, Walk-through, Write-up, Writeup
Subscription type: Premium
Description:
Learn about the fundamental networking framework that determines the various stages in 
which data is handled across a network

Room link: https://tryhackme.com/room/osimodelzi

Solution

Task 1: What is the OSI Model?

The OSI model (or Open Systems Interconnection Model) is an essential model used in networking.
This critical model provides a framework dictating how all networked devices will send, receive
and interpret data.

One of the main benefits of the OSI model is that devices can have different functions and designs
on a network while communicating with other devices. Data sent across a network that follows the
uniformity of the OSI model can be understood by other devices.

The OSI model consists of seven layers which are illustrated in the diagram below. Each layer has
a different set of responsibilities and is arranged from Layer 7 to Layer 1.

The seven OSI layers are:

  1. Physical
  2. Data link
  3. Network
  4. Transport
  5. Session
  6. Presentation
  7. Application

What does the "OSI" in "OSI Model" stand for?

Answer: Open Systems Interconnection

How many layers (in digits) does the OSI model have?

Answer: 7

What is the key term for when pieces of information get added to data?

Answer: encapsulation

Task 2: Layer 1 - Physical

This layer is one of the easiest layers to grasp. Put simply, this layer references the physical components
of the hardware used in networking and is the lowest layer that you will find. Devices use electrical signals
to transfer data between each other in a binary numbering system (1's and 0's).

What is the name of this Layer?

Answer: Physical

What is the name of the numbering system that is both 0's and 1's?

Answer: Binary

What is the name of the cables that are used to connect devices?

Answer: Ethernet Cables

Task 3: Layer 2 - Data Link

The data link layer focuses on the physical addressing of the transmission. It receives a packet from the
network layer (including the IP address for the remote computer) and adds in the physical MAC (Media Access
Control) address of the receiving endpoint. Inside every network-enabled computer is a Network Interface Card
(NIC) which comes with a unique MAC address to identify it.

What is the name of this Layer?

Answer: Data Link

What is the name of the piece of hardware that all networked devices come with?

Hint: NIC

Answer: Network Interface Card

Task 4: Layer 3 - Network

The third layer of the OSI model (network layer) is where the magic of routing & re-assembly of data takes place
(from these small chunks to the larger chunk). Firstly, routing simply determines the most optimal path in which
these chunks of data should be sent.

What is the name of this Layer?

Answer: Network

Will packets take the most optimal route across a network? (Y/N)

Answer: Y

What does the acronym "OSPF" stand for?

Answer: Open Shortest Path First

What does the acronym "RIP" stand for?

Answer: Routing Information Protocol

What type of addresses are dealt with at this layer?

Hint: Devices on a network use these. For example, 192.168.1.100

Answer: IP Addresses

Task 5: Layer 4 - Transport

Layer 4 of the OSI model plays a vital part in transmitting data across a network and can be a little bit difficult
to grasp. When data is sent between devices, it follows one of two different protocols that are decided based upon
several factors:

  • TCP
  • UDP

Let's begin with TCP. The Transmission Control Protocol (TCP). Potentially hinted by the name, this protocol is
designed with reliability and guarantee in mind. This protocol reserves a constant connection between the two devices
for the amount of time it takes for the data to be sent and received.

Now let's move onto the User Datagram Protocol (or UDP for short). This protocol is not nearly as advanced as its
brother - the TCP protocol. It doesn't boast the many features offered by TCP, such as error checking and reliability.
In fact, any data that gets sent via UDP is sent to the computer whether it gets there or not. There is no
synchronisation between the two devices or guarantee; just hope for the best, and fingers crossed.

What is the name of this Layer?

Answer: Transport

What does TCP stand for?

Answer: Transmission Control Protocol

What does UDP stand for?

Answer: User Datagram Protocol

What protocol guarantees the accuracy of data?

Answer: TCP

What protocol doesn't care if data is received or not by the other device?

Answer: UDP

What protocol would an application such as an email client use?

Answer: TCP

What protocol would an application that downloads files use?

Answer: TCP

What protocol would an application that streams video use?

Answer: UDP

Task 6: Layer 5 - Session

Once data has been correctly translated or formatted from the presentation layer (layer 6), the session layer
(layer 5) will begin to create and maintain the connection to other computer for which the data is destined.
When a connection is established, a session is created. Whilst this connection is active, so is the session.

What is the name of this Layer?

Answer: Session

What is the technical term for when a connection is succesfully established?

Hint: These are unique per connection

Answer: Session

Task 7: Layer 6 - Presentation

Layer 6 of the OSI model is the layer in which standardisation starts to take place. Because software developers
can develop any software such as an email client differently, the data still needs to be handled in the same way
— no matter how the software works.

This layer acts as a translator for data to and from the application layer (layer 7).

What is the name of this Layer?

Answer: Presentation

What is the main purpose that this Layer acts as?

Hint: This layer translates data from one format to another

Answer: Translator

Task 8: Layer 7 - Application

The application layer of the OSI model is the layer that you will be most familiar with. This familiarity is because
the application layer is the layer in which protocols and rules are in place to determine how the user should interact
with data sent or received.

What is the name of this Layer?

Answer: Application

What is the technical term that is given to the name of the software that users interact with?

Answer: Graphical User Interface

Task 9: Practical - OSI Game

Escape the dungeon to retrieve the flag. What is the flag?

Answer: THM{<REDACTED>}

For additional information, please see the references below.

References