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MODULE 02 · LESSON 2 OF 5

Transport: TCP, UDP, and QUIC

12 min read · Sources linked below

Theory and industry practice are explained below. Cloudflare-specific behavior links to official documentation. Customer examples are hypothetical.

Learning objectives

  • Separate IP, transport, and HTTP responsibilities
  • Explain why QUIC can use UDP reliably
  • Localize connection failures before application debugging

The mental model

An IP address identifies a destination, but applications still need a way to exchange data. TCP provides a reliable ordered byte stream and handles retransmission and congestion control. UDP provides datagrams without those reliability guarantees at its own layer. That does not make every application using UDP unreliable: QUIC builds a secure, reliable transport over UDP. HTTP/3 uses QUIC, while HTTP/1.1 and HTTP/2 commonly use TCP with TLS for HTTPS. The distinction matters during troubleshooting because a firewall may allow TCP on a port while blocking UDP. A customer can therefore reach a service using one protocol and experience fallback or failure using another.

How it works

Separate protocol layers. IP moves packets between addresses; transport identifies application endpoints with ports and establishes the semantics of delivery. TCP connection setup commonly uses SYN, SYN-ACK, and ACK before application bytes flow. Packet loss, retransmission, congestion, and round-trip time can affect performance even when the origin application is fast. A successful TCP connection confirms a narrower fact than a successful HTTP response.

QUIC multiplexes streams and integrates TLS-based security while operating over UDP. Loss affecting one stream does not impose TCP's connection-wide ordered-byte delivery dependency on all the other streams, although congestion and network capacity still affect the connection. QUIC is not a promise of zero latency or immunity to loss. During diagnosis, identify the negotiated HTTP version and transport, then compare controlled requests over supported versions. Check client and server support, network filtering, and fallback behavior before attributing a difference to the application. Avoid changing several protocol and security settings at once. Capture timing and errors at the connection boundary; if the handshake never completes, application logs may correctly show no request at all.

Applied example · hypothetical

Hypothetical example: office users report a delay on first page load, while home users do not. The application server is healthy. A controlled browser test shows attempts to use HTTP/3 followed by fallback, and network policy blocks outbound UDP on the relevant port. You test the same URL over TCP and compare connection timings. The corrective action belongs to the network policy and deployment decision, not to a database index. The evidence also prevents a misleading claim that HTTP/3 itself is slower for every customer.

In a customer engagement

When discussing Cloudflare performance, verify the client-to-edge protocol separately from the edge-to-origin connection. The two legs need not use the same HTTP version or transport. Ask where the latency occurs and whether an enterprise network policy changes behavior. A clear two-leg diagram helps application and network teams investigate together without assuming one end-to-end connection.

Interview relevance

An interview answer should distinguish reliability supplied by TCP from reliability implemented by QUIC over UDP. Then explain a practical consequence, such as filtering or loss behavior. Avoid claiming that UDP means unreliable HTTP/3 or that QUIC removes all head-of-line effects in every layer of a system.

Common mistakes

  • Calling all UDP-based applications unreliable
  • Assuming edge and origin use identical protocols
  • Interpreting no application logs as proof of no network problem

Key terminology

TCP — A transport protocol providing a reliable ordered byte stream with congestion control.

UDP — A datagram transport without built-in delivery or ordering guarantees.

QUIC — A secure multiplexed transport built over UDP, used by HTTP/3.

Explain it without the page

  • What would a successful TCP handshake rule out?
  • How would you compare HTTP versions without changing the workload?

Further reading

Sources explain the underlying concepts; examples and practice answers are original course material. Recheck current limits and plan eligibility before a real proposal.

Compare recursive and authoritative views

dig example.test A
dig example.test AAAA
dig @ns1.example.test example.test A +norecurse

Replace the reserved example.test names with an authorized test zone. Capture answer, TTL, server and timestamp. A successful DNS query does not establish HTTP health.