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TCP/IP Model

Context & Purpose​

The TCP/IP Model (Internet Protocol Suite) is the practical implementation of networking. Unlike OSI which is a reference model, TCP/IP is the actual stack of protocols that powers the modern Internet. It prioritizes connectivity and robustness over strict theoretical layers.

Mechanism / Architecture​

It condenses networking into 4 Pragmatic Layers.

Layer Breakdown​

  1. Network Access (Link Layer):

    • Corresponds to OSI L1 (Physical) + L2 (Data Link).
    • Handles placing TCP/IP packets on the network medium.
    • Protocols: Ethernet, Wi-Fi (802.11), ARP.

  2. Internet:

    • Corresponds to OSI L3 (Network).
    • Handles logical addressing and routing across multiple networks.
    • Protocols: IP (IPv4, IPv6), ICMP (Ping).

  3. Transport:

    • Corresponds to OSI L4 (Transport).
    • Handles host-to-host communication and flow control.
    • Protocols: TCP (Reliable), UDP (Fast/Unreliable).

  4. Application:

    • Corresponds to OSI L5 (Session) + L6 (Presentation) + L7 (Application).
    • Handles process-to-process communication.
    • Protocols: HTTP, DNS, SSH, SMTP.

Analysis & Trade-offs​

Pros​

  • Proven Scale: It scales to the size of the global internet.
  • Robustness: Designed to survive high loss and dynamic routing changes (originally for DARPA).
  • Loose Coupling: Does not enforce strict presentation/session handling, leaving that to the application developer.

Cons​

  • Security: Originally designed without built-in security (IPSec and SSL/TLS were added later).
  • Complexity: The boundaries between layers can be blurry in implementation (e.g., where does SSL sit? Technically App layer, but behaves like Transport).

Real-world Usage​

  • Kernel Space: The Transport and Internet layers live in the OS Kernel (Linux Networking Stack).
  • User Space: The Application layer lives in your code (Node.js, Go, Python).
  • Hardware: The Network Access layer lives in the Network Interface Card (NIC) and Driver.

Interview Check​

  1. Why do we use TCP/IP instead of OSI?
    • OSI is too complex and prescriptive. TCP/IP won the "Protocol War" because it was bundled with UNIX and was simpler to implement.
  2. Where does the Linux Kernel fit in?
    • The Kernel handles L2 (Internet) and L3 (Transport). Your app handles L4 (Application).
  3. Is ARP part of L2 (Internet) or L1 (Link)?
    • In TCP/IP, ARP is strictly the glue between Layer 2 (Internet/IP) and Layer 1 (Link/MAC), but it lives in the Link Layer.

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