Notes: OSI Model | Pre Security | THM

What is the OSI Model

OSI Model = Open Systems Interconnection Model

- fundamental model

- networking

- framework: how all networked devices send, receive and interpret data

- Benefits: devices different functions/design - communicate with other devices - data sent across network that follow OSI model = understood by other devices

- 7 layers (7-1): each layer different set of responsibilities

- Each layer: data travel through - specific process happens, pieces of info added to data = encapsulation

The OSI Model (Open Systems Interconnection Model)

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data link

1. Physical

Layer 1 - Physical

- physical components of hardware used in networking

- lowest layer

- devices use electrical signals to transfer data - binary numbering system (1's and 0's)

- Ex.: ethernet cables connecting devices

Layer 2 - Data Link

- physical addressing of the transmission

- receives a packet from layer 3 network (incl IP adr for remote computer) and adds physical MAC adr of receiving endpoint

- present data in a format suitable for transmission

Layer 3 - Network

- routing and re-assembly of data (from small chunks to larger chunk)

- routing: determine most optimal path

- OSPF (Open Shortest Path First)

- RIP (Routing Information Protocol)

- delivering packets using IP adr

- Layer 3 devices: routers

- Factors to device which route:

- shortest path: least amount of devices the packet needed to travel across

- most reliable path: have packets been lost on path before

- faster physical connection: path using copper connection (=slower) or fibre (=faster)

Layer 4 - Transport

- vital part in transmitting data across network

TCP (= Transmission Control Protocol)

- constant connection for the time it takes for the data to be sent and received

- error checking: guarantee data sent from small chunks in layer 5 session has been received and reassembled in the same order

- used for file sharing, internet browsing, sending email <- require data to be accurate and complete

- small pieces of data = packets

Advantages:

- guarantee accuracy of data

- prevent flooding by synchronising 2 devices

- performs processes for reliability

Disadvantages:

- require reliable connected between 2 devices. If a small chunk of data is not received - entire chunk of data can't be used

- slow connection can cause bottleneck on another device as connection is constant with the receiving computer

- slower than UDP because more work needs to be done on the devices using TCP

UDP (= User Datagram Protocol)

- data is sent to computer whether is gets there or not

- no synchronization

- useful: for small pieces of data being sent, f.ex.: protocols used for discovering devices (ARP, DHCP), larger fles like video streaming (ok if parts of video is pixelated - pixels = lost pieces of data)

Advantages:

- faster than TCP

- leave application layer (user software) - decide if any control over how quickly packets are sent

- don't reserve a continuous connection on a device as TCP does

Disadvantages:

- doesn't care if data is received

- flexible to software developers

- unstable connections = terrible user experience

Layer 5 - Session

- receive correctly translated/formatted data from layer 6 presentation

- create and maintain connection with destination computer - when connection established = session

- responsible for closing the connection when unused or lost

- can contain checkpoints = if data is lost, only newest pieces of data are required to be sent = saving bandwitdh

- sessions are unique - data can't travel over different sessions - only across each session

Layer 6 - Presentation

- standardisation starts to take place

- translator for data to/from layer 7 application

- receiving computer will understand data sent to a computer in a format destined for another format (f.ex.: different email programs need to display emails the same)

- data encryption occur at this layer (f.ex.: HTTPS)

Layer 7 - Application

- protocols and rules in place to determine how user should interact with data sent/received

- applications like email clients, browsers, file server browsing software provide a friendly Graphical User Interface (GUI) for users to interact with data sent or received - or protocols like Domain Name System (DNS) (= how websites addresses are translated into IP adr)