OSI Model

• Open Systems Interconnection Model
• dictating how all networked devices will send, receive and interpret data
• consists of 7 layers

The model (layers)

• 7. Application
  • determine how the user interact with the data
  • Email clients, file browsers or chat application belongs here
  • 💡 Users interact with data trough GUI (Graphical user interface)
  • DNS (Domain Name System) protocol also belongs here
    • Translates web addresses to the IP addresses
• 6. Presentation
  • standardization - the data of one application needs to be handled the same way
  • 💡acts as translator for data and from the application layer
  •  data encryption occur here
• 5. Session
  • create and maintain the connection to other computer (When a connection is established, a session is created)
  • responsible for closing the connection
  • session can contain “checkpoints”
    • save bandwidth by only requiring send newest pieces of data
  • data cannot travel over different sessions
• 4. Transport
  • plays important part in transmitting data across network
  • Data sent between devices follows 2 protocols
    • TCP
    • UDP
• 3. Network
  • routing & re-assembly of data is part of this layer
  • 💡protocol of this layer determine what’s the “optimal” path for data to reach device
  • 💡 e.g. OSPF (Open Shortest Path First) and RIP (Routing Information Protocol)
  • Deciding is based on the following
    • What path is the shortest? I.e. has the least amount of devices that the packet needs to travel across.
    • What path is the most reliable? I.e. have packets been lost on that path before?
    • Which path has the faster physical connection? I.e. is one path using a copper connection (slower) or a fibre (considerably faster)?
  • 💡 everything here is dealt with via IP addresses
• 2. Data Link
  • focuses of the physical adresing of the transmission
  • adds MAC address to the packet
    • Mac addresses are unique, set by manufacturer and can be spoofed
    • 💡 MAC address belongs to the network cards. Devices need network cards to connect to the network
  • present the data in format suitable for transmission
• 1. Physical
  • physical components of hardware
  • 💡 Devices uses electrical signals to transfer data in a binary system
  • 💡 For example, ethernet cables are part of this layer

Transport Protocols

TCP

• 💡Transmission Control protocol
• designed with reliabailit and guarantee in mind
• reserves constant connection between devices for the ammout of time it takes for the data to be send and received
• incorporates error checking
• can guarantee that data sent from the small chunks in the session layer (layer 5) has then been received and reassembled in the same order.
• stateful - 3-way handshake

Pros

• Guarantees the accuracy of data.
• Capable of synchronising two devices to prevent each other from being flooded with data.
• Performs a lot more processes for reliability.

Cons

• Requires a reliable connection between the two devices.
• A slow connection can bottleneck another device as the connection will be reserved on the receiving computer the whole time.
• is significantly slower than UDP because more work has to be done by the devices using this protocol

Used in situation as file share, internet browsing or sending email.

UDP

• 💡User Datagram Protocol
• much simpler than TCP
• does not include error checking
• synchronization between devices is not guaranted, hope is what you left
• 💡 stateless

Pros

• much faster than TCP
• leaves the application layer (user software) to decide if there is any control over how quickly packets are sent.
• does not reserve a continuous connection on a device.

Cons

• doesn’t care if the data is received.
• It is quite flexible to software developers in this sense.
•  unstable connections result in a terrible experience for the user.

This protocol is used in situation where you sending and receive small data. e.g. Discovering protocols - ARP, DHCP

Also using for video streaming (packet loss result in pixelating).

UDP packet consist of following headers:

Packet vs Frame

• Packet
  • part of layer 3
  • contain information such and IP address and payload
  • efficient ways to communicate data across network
  • have different structures that are dependant upon the type of packet that is being sent
• Frame
  • part of layer 2
  • Adds MAC address to the packet in process called encapsulation
  • analogy is envelope

Some headers that packets can include