• L7 – Application: User-facing protocols (HTTP, gRPC, DNS)
• L6 – Presentation: Encoding, compression, TLS
• L5 – Session: Connection/session mgmt (mostly implicit today)
• L4 – Transport: Data delivery (TCP, UDP)
• L3 – Network: Routing (IP)
• L2 – Data Link: Local network, frames (Ethernet, Wi-Fi)
• L1 – Physical: Wires, radio, bits

protocols

On top of L7

• REST (style)
  • Layer: L7
  • Runs on: HTTP → TCP
• gRPC
  • Layer: L7
  • Runs on: HTTP/2 → TCP
  • backend to backend, streaming
• WebSockets
  • Layer: L7
  • Runs on: HTTP (upgrade) → TCP
  • web to backend, live chat root, streaming

Application Layer (L7)

• HTTP / HTTPS
  • Layer: L7
  • Runs on: TCP (HTTP/1.1, HTTP/2), UDP (HTTP/3 via QUIC)
• WebRTC
  • Layer: L7
  • Runs on: UDP (SRTP/DTLS), TCP/HTTP for signaling
  • zoom call, streaming
• DNS
  • Layer: L7
  • Runs on: UDP (default), TCP (large responses)
• FTP
  • Layer: L7
  • Runs on: TCP
• SMTP / IMAP / POP3
  • Layer: L7
  • Runs on: TCP

Security / Encoding (L6-ish)

• TLS / SSL
  • Layer: L6 (conceptually)
  • Runs on: TCP (TLS), UDP (DTLS)

Transport Layer (L4)

• TCP
  • Layer: L4
  • Runs on: IP
  • Reliable, ordered, congestion-controlled
• UDP
  • Layer: L4
  • Runs on: IP
  • Fast, unordered, no guarantees
• QUIC
  • Layer: L4-ish
  • Runs on: UDP
  • TCP + TLS + HTTP/3 semantics combined

Network Layer (L3)

• IP (IPv4 / IPv6)
  • Layer: L3
  • Runs on: Ethernet / Wi-Fi
• ICMP
  • Layer: L3
  • Runs on: IP (errors, ping)

Data Link (L2)

• Ethernet
  • Layer: L2
  • Runs on: Physical media
• Wi-Fi (802.11)
  • Layer: L2
  • Runs on: Radio
• ARP
  • Layer: L2/3 bridge
  • Runs on: Ethernet

Common Stacks

• REST API
  REST → HTTP → TCP → IP
• gRPC
  gRPC → HTTP/2 → TCP → IP
• WebSocket
  WS → HTTP → TCP → IP
• HTTP/3
  HTTP/3 → QUIC → UDP → IP
• WebRTC Media
  SRTP → UDP → IP