What is the difference between TCP and UDP?

Question

IT Interview Practice · Accepted Answer

Both are Transport-layer protocols, but they make opposite trade-offs. **TCP** is connection-oriented and reliable — it guarantees that every byte arrives, in order, exactly once. **UDP** is connectionless and "fire-and-forget" — it just sends packets (datagrams) with no delivery or ordering guarantees. You trade TCP's reliability for UDP's low overhead and speed.

```text
             TCP                          UDP
   ┌───────────────────────┐    ┌───────────────────────┐
   │ connection (handshake)│    │ no connection         │
   │ reliable (retransmit) │    │ best-effort (no retry)│
   │ ordered delivery      │    │ unordered             │
   │ flow + congestion ctrl│    │ none                  │
   │ higher overhead       │    │ minimal overhead      │
   └───────────────────────┘    └───────────────────────┘
```

| Property        | TCP                        | UDP                         |
|-----------------|----------------------------|-----------------------------|
| Connection      | Yes (3-way handshake)      | No                          |
| Reliability     | Guaranteed, retransmits    | Best-effort, may drop       |
| Ordering        | In-order                   | No ordering                 |
| Speed/overhead  | Slower, more overhead      | Faster, lean header (8 B)   |
| Use cases       | Web, email, file transfer  | DNS, video/voice, gaming    |

**Use TCP** when correctness matters more than a few milliseconds: HTTP, database connections, file downloads — losing bytes is unacceptable. **Use UDP** when speed and low latency beat perfection: live video/voice calls (a dropped frame is better than a stalled stream), online gaming, and DNS lookups (a single small query/response). Notably, **HTTP/3 runs over UDP** (via QUIC), rebuilding reliability itself to avoid TCP's head-of-line blocking.

## Why it matters

This is a foundational question, and the interviewer is really probing whether you can reason about **trade-offs** rather than recite two lists. The mature answer is "it depends on what the application needs": reliability and ordering (TCP) versus latency and overhead (UDP). Being able to justify a choice — and to name the interesting exception, HTTP/3 choosing UDP and rebuilding reliability on top — signals you understand *why* the protocols differ, not just *that* they differ.

Property	TCP	UDP
Connection	Yes (3-way handshake)	No
Reliability	Guaranteed, retransmits	Best-effort, may drop
Ordering	In-order	No ordering
Speed/overhead	Slower, more overhead	Faster, lean header (8 B)
Use cases	Web, email, file transfer	DNS, video/voice, gaming

Why it matters

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