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Networking

Understanding the pipes that connect our systems.

The OSI Model (Mental Model vs Reality)

Layer Name Protocol Examples Unit Function
7 Application HTTP, DNS, SSH Data Network process to application
4 Transport TCP, UDP, QUIC Segment Host-to-host communication
3 Network IP, ICMP, BGP Packet Path determination and IP (Logical addressing)
2 Data Link Ethernet, Wi-Fi Frame Physical addressing (MAC)
1 Physical Fiber, Copper Bit Media, signal, and binary transmission

TCP vs UDP

TCP (Transmission Control Protocol)

  • Reliable: Guarantees delivery and order.
  • Connection-oriented: 3-way handshake (SYN, SYN-ACK, ACK).
  • Flow Control: Prevents sender from overwhelming receiver.
  • Congestion Control: Prevents sender from overwhelming the network.
  • Use Case: Web (HTTP), Email, File Transfer.

UDP (User Datagram Protocol)

  • Unreliable: Fire and forget. No guarantees.
  • Connectionless: No handshake.
  • Fast: Low overhead.
  • Use Case: Video streaming, VoIP, Gaming, DNS.

DNS (Domain Name System)

The phonebook of the internet.

  1. Browser: Checks local cache.
  2. OS: Checks /etc/hosts and OS cache.
  3. Resolver (ISP): Asks Root Server -> TLD Server (.com) -> Authoritative Server (google.com).
  4. A Record: Maps name to IPv4.
  5. CNAME: Maps name to name (alias).

HTTP/HTTPS

Versions

  • HTTP/1.1: Text-based, Keep-Alive, Head-of-Line blocking.
  • HTTP/2: Binary, Multiplexing (multiple requests over one connection), Header Compression (HPACK).
  • HTTP/3 (QUIC): Built on UDP. Solves TCP Head-of-Line blocking. Faster handshake.

HTTPS (TLS/SSL)

  • Encryption: Nobody can read the data.
  • Integrity: Nobody can modify the data.
  • Authentication: You are talking to who you think you are.
  • Handshake: Exchange keys to establish a symmetric session key.