L4 — Transport

Per-connection semantics. Ports identify services.

What lives at L4

• TCP — reliable, connection-oriented
• UDP — fire-and-forget
• QUIC — UDP-based, blurs L4/L7
• SCTP

L4 devices

• L4 load balancers (AWS NLB, Azure LB)
• Stateful firewalls (track connection state)

Port ranges

• 0–1023 well-known
• 1024–49151 registered
• 49152–65535 ephemeral