Code-Memo

TCP/IP Model

TCP/IP vs OSI mapping

• TCP/IP is the practical Internet model; often described as 4 layers:
  • Link (≈ OSI L1–L2)
  • Internet (≈ OSI L3)
  • Transport (≈ OSI L4)
  • Application (≈ OSI L5–L7)

TCP, UDP basics

• TCP: connection-oriented, reliable, ordered byte stream; congestion control; higher overhead.
• UDP: connectionless datagrams; no delivery/order guarantees; used for DNS, VoIP, gaming, QUIC transport (underneath HTTP/3).

IP layer responsibilities

• Addressing (IPv4/IPv6), routing between networks, fragmentation rules, TTL/hop limit.
• Best-effort delivery: IP doesn’t guarantee delivery; reliability is added above (TCP) or by apps.

Real-world internet communication flow

• App resolves name (often DNS) → chooses route (via OS routing table) → opens transport (TCP/UDP) → sends packets across routers → receives response.
• Many “timeouts” are transport/app-level symptoms of lower-layer reachability, filtering, or congestion.