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The TCP/IP Model

While the OSI model is a theoretical framework, the Transmission Control Protocol/Internet Protocol (TCP/IP) model is the practical implementation that actually powers the Internet and most modern networks.

Architecture of the TCP/IP Model​

The TCP/IP model is more concise than the OSI model, condensing the seven layers down to four.

4. Application Layer​

(Corresponds to OSI Layers 5, 6, and 7) This layer defines the protocols used by most applications for providing user services or exchanging data.

  • Protocols: HTTP, HTTPS, FTP, SSH, DNS.
  • Function: Handles high-level protocols, data representation, and session control.

3. Transport Layer​

(Corresponds to OSI Layer 4) Responsible for providing communication session management between host computers. Defines the level of service and status of the connection used when transporting data.

  • Protocols: TCP, UDP.
  • Function:
    • TCP (Transmission Control Protocol): Connection-oriented, ensures all data arrives intact and in order. Slower but reliable.
    • UDP (User Datagram Protocol): Connectionless, sends data without checking if it arrived. Fast but unreliable (used for video streaming, gaming).

2. Internet Layer​

(Corresponds to OSI Layer 3) Responsible for routing packets across independent networks.

  • Protocols: IP (IPv4, IPv6), ICMP, ARP.
  • Function: Attaches IP addresses to packets and determines the best path to the destination (Routing).

(Corresponds to OSI Layers 1 and 2) Defines the protocols and hardware required to deliver data across some physical network.

  • Protocols: Ethernet, Wi-Fi (802.11).
  • Function: Handles MAC addressing and physical transmission of data.

TCP/IP in Linux​

In Linux, the TCP/IP stack is implemented directly in the kernel. You can interact with these layers using various commands:

  • Application: curl http://example.com
  • Transport: ss -tuna (Shows active TCP/UDP ports)
  • Internet: ip route (Shows routing table) or ping (Uses ICMP)
  • Network Access: ip link (Shows physical interfaces and MAC addresses)

Encapsulation​

When data is sent, it goes down the model. Each layer wraps the data with its own header information (like putting a letter in an envelope, then putting that envelope in a box).

  1. Application Data
    • TCP Header = Segment
    • IP Header = Packet
    • Ethernet Header/Trailer = Frame
  5. Converted to bits and sent across the wire.