TCP/IP (OCR A Level Computer Science)

TCP/IP

What is Protocol layering & the TCP/IP stack?

• Protocol layering is the way network protocols are divided into layers, each of which performs specific functions

• This allows for modular design, simplifies troubleshooting, and promotes interoperability

• This means that protocols that operate at the network layer can be altered independently of application layer protocols

• TCP/IP stack is the suite of protocols that the internet is based on

• Network protocols are organised into layers to handle different aspects of communication tasks

Why use Layering?

• Modularity: By breaking the complex process of networking into more manageable layers, it's easier to design, implement, and troubleshoot networks

• Interoperability: Layering allows different technologies to work together seamlessly. E.g. an application can send data to another application on a different device without knowing the details of how the network structure in between works

• Ease of Updates: With a layered model, changes can be made to one layer without affecting others. This makes updates and improvements easier to implement

• Specialisation: Each layer can be specialised to perform its functions without worrying about the specifics of other layers. This allows for more effective and efficient design

The 4 layer TCP/IP model

• TCP/IP, or the Transmission Control Protocol/Internet Protocol, is a suite of communication protocols used to interconnect network devices on the internet

• This model splits the various protocols into four layers:

  • Application

  • Transport

  • Internet

  • Link

Application layer

• This is the layer where the communication process begins

• The application layer interacts directly with software applications, such as web browsers and email clients

• The application layer prepares data for transmission over the network by converting it into a format that can be sent and received over the network (known as encapsulation)

Transport layer

• The transport layer receives data from the application layer

• The transport layer is responsible for end-to-end communication between the source and destination

• The transport layer breaks the data it receives down into smaller units called packets

• Each packet is assigned a port number (so the data can be reassembled in the correct order at the destination)

• Each packet is also labelled with a header containing information (e.g. the packet number) 

Internet layer

• The internet layer receives packets from the transport layer

• It adds a header to each packet, including the sender's IP address and the receiver's IP address

• The internet layer is responsible for routing each packet across the network using the IP addresses in the headers

Link layer

• Also known as the network interface layer

• The link layer receives packets from the Internet layer and prepares them for transmission over the physical network

• The link layer translates the digital packets into an electrical, optical, or wireless signal that can be sent over the network

• Once the signal reaches the receiving end, the network layer translates it back into digital packets

TCP/IP stack layers

Key points

• At each layer of the TCP/IP model, specific tasks are performed to prepare data for transmission over the network

• The process is reversed at the receiving end, with each layer removing its specific header and performing its specific tasks to get the data back into a format that the receiving application can use

Data transmission over the Internet

• The Internet relies on packet-switched networks, where data is broken down into packets, each of which can take its own route to the destination

• Packets are units of data. They are small and easier to manage

• Each packet contains:

  • The payload (the data)

  • A header (with metadata like source and destination IP addresses)

  • A footer (to signify the end of the packet)

IP addresses

• An IP address is a unique identifier for a device on a network

• IP addresses are used to deliver packets to the correct destination

• Two versions are in use: IPv4 (e.g. 104.22.74.202) and IPv6 (e.g. 0000:0000:0000:0000:0000:ffff:6816:4aca)