TCP and UDP

TCP delivers reliable, ordered byte streams with connection setup, acknowledgments, retransmission, and congestion control. UDP sends connectionless datagrams with no delivery guarantees — lower overhead, suitable for real-time or loss-tolerant traffic. HTTP/3 runs over QUIC, which uses UDP with built-in encryption and multiplexing.

					Client                    Server
  |---- SYN -------------->|
  |<--- SYN-ACK -----------|
  |---- ACK -------------->|
  |==== data stream ======>|
				

| Property | TCP | UDP | |----------|-----|-----| | Reliability | Retransmits lost segments | Best effort | | Ordering | In-order delivery | Application handles order | | Connection | 3-way handshake | None | | Typical use | HTTP/1.1, HTTP/2, databases | DNS, QUIC, VoIP |

On interviews: explain why a Node service opening many outbound TCP connections can hit ephemeral port limits; when UDP is acceptable; and how QUIC changes head-of-line blocking compared to TCP.

Common pitfalls: assuming "async HTTP" removes TCP handshake cost; ignoring TIME_WAIT socket accumulation; using UDP where ordering matters without application logic.

The trade-off is reliability and fairness versus latency and simplicity — pick the transport that matches your failure tolerance.

Checklist:

• Name the TCP handshake and what RTT adds.
• Contrast UDP datagrams with TCP streams.
• Mention QUIC/HTTP/3 over UDP.
• Tie connection reuse to latency in Node agents.