Okay, let's briefly review the 2.3.b TCP/IP Model. You don't need exhaustive detail for a system design interview, but understanding the layers and the roles of TCP and UDP is important context.

  • Purpose: The TCP/IP model is a conceptual framework that standardizes how network communication occurs by dividing the process into abstraction layers. It's the foundational protocol suite for the internet.

  • The Layers (Common 4-Layer Model):

    1. Application Layer:

      • Role: Provides network services directly to user applications. This is where protocols that applications use reside.
      • Protocols: HTTP, HTTPS, FTP, SMTP (email), DNS, etc.
      • Focus: Application-specific data exchange.

    2. Transport Layer:

      • Role: Handles end-to-end communication, data segmentation, reliability, and flow control between applications on different hosts.
      • Key Protocols:
        • TCP (Transmission Control Protocol):
          • Connection-Oriented: Establishes a connection via a three-way handshake (SYN, SYN-ACK, ACK) before data transfer.
          • Reliable & Ordered: Guarantees that data arrives in order and without errors (using acknowledgements and retransmissions).
          • Flow Control: Manages transmission rate to prevent overwhelming the receiver.
          • Use Cases: Web Browse (HTTP/HTTPS), email (SMTP), file transfer (FTP) - anywhere reliability is crucial.
        • UDP (User Datagram Protocol):
          • Connectionless: No handshake; just sends packets ("datagrams").
          • Unreliable & Unordered: No guarantees about delivery or order. Packets might be lost or arrive out of sequence.
          • Faster & Lower Overhead: Less protocol overhead compared to TCP.
          • Use Cases: Streaming video/audio, online gaming, DNS lookups - where speed is preferred over perfect reliability, or where occasional loss is acceptable/handled by the application.
      • Focus: Process-to-process communication.

    3. Internet Layer (or Network Layer):

      • Role: Responsible for logical addressing (IP addresses) and routing packets across different networks to their final destination.
      • Key Protocol: IP (Internet Protocol) (IPv4 and IPv6).
      • Devices: Routers operate primarily at this layer.
      • Focus: Packet forwarding and inter-network routing.

    4. Link Layer (or Network Interface / Data Link Layer):

      • Role: Handles the physical transmission of data over a specific network medium (e.g., Ethernet cable, Wi-Fi). Deals with physical addressing (MAC addresses) within a local network segment.
      • Protocols: Ethernet, Wi-Fi (802.11).
      • Devices: Switches operate primarily at this layer.
      • Focus: Communication between adjacent nodes on the same network.

  • Key Takeaways for System Design:

    • The choice between TCP (reliability) and UDP (speed/low overhead) at the Transport Layer depends on the application's needs.
    • Higher-level protocols like HTTP rely on the underlying transport (usually TCP) and internet (IP) layers to function.
    • Understanding layers helps conceptualize where different components (like load balancers - often operating at Layer 4 or Layer 7) fit in.
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