A2.1.5 Function of the TCP/IP Model (HL only) Notes

Application Layer

It defines protocols that applications use to exchange data, such as:

1. HTTP for web browsing
2. SMTP for email
3. DNS for domain name resolution

Transport Layer

It uses protocols like:

1. TCP (Transmission Control Protocol): Provides reliable, ordered, and error-checked delivery.
2. UDP (User Datagram Protocol): Offers faster, but less reliable, communication.

Internet Layer

Its primary protocol is IP (Internet Protocol), which:

1. Assigns unique IP addresses to devices
2. Routes packets across networks

Network Interface Layer

It includes:

1. Data framing: Encapsulating packets into frames
2. MAC addressing: Ensuring data reaches the correct device on a local network

Interaction Between Layers

Each layer of the TCP/IP model performs a specific function, but they work together to ensure reliable data transmission:

1. Application Layer: Prepares data for transmission (e.g., an HTTP request).
2. Transport Layer: Segments data and adds error-checking information.
3. Internet Layer: Encapsulates segments into packets with IP addresses.
4. Network Interface Layer: Converts packets into signals for physical transmission.
   

Step-by-Step: Sending a File from Computer A to B

On Computer A

1. Application Layer (User level)
   1. This is where the file transfer starts.
   2. The user on Computer A uses a program (e.g. a browser, FTP client, or email app) to send the file.
   3. The application adds its own header with information like:
      1. The type of data (e.g. file, text, video)
      2. The protocol used (e.g. HTTP, FTP, SMTP)
2. Transport Layer (Process-to-process communication)
   1. The data from the application is passed to the Transport Layer.
   2. TCP (Transmission Control Protocol) is used for reliable delivery.
   3. The data is broken into smaller segments.
   4. Each segment gets:
      1. A TCP header with source and destination port numbers.
      2. A sequence number so segments can be reassembled correctly.
      3. Error-checking info.
   5. TCP also starts a handshake between A and B to establish a connection.
3. Internet Layer (Host-to-host communication)
   1. Each segment is passed to the Internet Layer.
   2. This layer uses the IP protocol (Internet Protocol).
   3. Adds an IP header to create a packet.
   4. Contains source and destination IP addresses.
   5. If the packet is too large for the next network, it may be fragmented.
4. Network Interface Layer / Link Layer (Hardware-to-hardware, local delivery)
   1. The packet is passed to the Network Interface Layer.
   2. This layer deals with actual hardware and transmission.
   3. The packet is turned into frames and sent over the network.
   4. Each frame includes:
      1. MAC addresses (unique IDs for physical devices).
      2. Start and end markers.
   5. The data travels over Ethernet, Wi-Fi, fibre optic, etc.

On the way to Computer B…

1. Routers use the IP address to forward packets toward Computer B.
2. Each router strips and rebuilds the Link Layer frame as needed.

On Computer B (the receiving end)…

1. The Network Interface Layer (Link Layer) reads the MAC address, removes the frame header, and passes the packet up.
2. The Internet Layer checks the IP address, removes the IP header, and passes the segment up.
3. The Transport Layer reassembles the segments using sequence numbers, checks for errors, and sends the data to the correct application using the port number.
4. The Application Layer receives the reassembled data and makes the file available to the user.
5. Connection and handshake is terminated.