TCP vs UDP

TCP and UDP are the two transport protocols almost everything is built on. TCP gives you a reliable, ordered byte stream at the cost of handshakes and waiting; UDP gives you raw speed and leaves every guarantee up to you.

The analogy

TCP is registered mail: every package is numbered, the recipient signs for it, missing packages are re-sent, and they are handed over in order. UDP is dropping postcards in a mailbox: cheap and quick, but some may get lost or arrive out of order, and nobody tells you.

How TCP works

• Connection setup: a three-way handshake (SYN, SYN-ACK, ACK) costs one round trip before data flows.
• Reliability: every byte has a sequence number; the receiver acknowledges, and the sender retransmits anything not acknowledged in time.
• Ordering: bytes are delivered to the application in order. If packet 5 is lost, packets 6–10 wait in a buffer until 5 is retransmitted. This is head-of-line blocking.
• Flow control: the receiver advertises a window so a fast sender cannot overwhelm a slow reader.
• Congestion control: algorithms such as CUBIC or BBR probe for available bandwidth and back off when loss signals congestion. New connections start slow (slow start), which is another reason to reuse connections.

How UDP works

UDP adds only ports and a checksum on top of IP. There is no handshake, no retransmission, no ordering, and no congestion control. Each datagram stands alone. Applications that use UDP rebuild whatever guarantees they need: sequence numbers for ordering, selective retransmission for important data, forward error correction for media.

Comparison

TCP stream with a lost packet:  sent:      [1][2][3][4][5][6]  received:  [1][2][ ][4][5][6]  delivered: [1][2] ...wait... (retransmit 3) [3][4][5][6]UDP with a lost packet:  sent:      [1][2][3][4][5][6]  delivered: [1][2]   [4][5][6]   (app decides whether 3 matters)

Why real-time media prefers UDP

In a video call, a frame that arrives 300 ms late is worthless; you would rather skip it and show the next one. TCP would stall everything behind the lost packet and retransmit data that is already stale. UDP-based protocols (RTP, WebRTC) let the application drop, conceal, or partially recover loss instead.

QUIC: the best of both

QUIC runs over UDP but implements reliability, congestion control, and encryption in user space, with independent streams so a loss on one stream does not block the others. It merges the transport and TLS handshakes, so a new connection needs one round trip and a resumed one can need zero. It also identifies connections by an ID rather than the IP/port pair, so a phone moving from Wi-Fi to cellular keeps its connection.

Common mistakes

• Picking UDP for "speed" in a system that actually needs every message, then reinventing TCP badly.
• Ignoring the cost of new TCP connections. Handshakes plus slow start make short-lived connections expensive; use pools.
• Forgetting that some corporate networks block or throttle UDP. Always keep a TCP fallback.
• Assuming TCP means end-to-end delivery to the application. A TCP acknowledgement only means the kernel received the bytes, not that your service processed them, so you still need application-level acknowledgements.

In the interview

Q: Which transport would you use for a multiplayer game's position updates? UDP, because each update supersedes the previous one; resending a stale position is wasteful. Critical events such as purchases go over a reliable channel, either TCP or a reliable layer on top of UDP.

Q: Why does HTTP/3 use UDP if it needs reliability? Building on UDP lets QUIC implement per-stream reliability, avoiding TCP's connection-wide head-of-line blocking, and lets it ship improvements in user space without waiting for operating system kernels to change.

Q: Does a TCP ACK mean the server processed my message? No. It means the receiving kernel buffered the bytes. Application-level acknowledgements (for example a broker confirming a publish) are what tell you the work was accepted.

Key takeaways

• TCP provides ordered, reliable delivery with handshakes, flow control, and congestion control.
• UDP is minimal and is preferred when late data is useless, such as live audio, video, and games.
• Head-of-line blocking is TCP's core cost; QUIC removes it while keeping reliability.
• Transport-level delivery is not application-level success; design your own acknowledgements.