Question 1

What is the difference between TCP and UDP?

Accepted Answer

**TCP guarantees delivery; UDP does not** — and that single trade-off determines which one every internet application uses.

**TCP (Transmission Control Protocol).** A **connection-oriented, reliable** protocol:

**Establishes a connection** first, via a three-way handshake, before any data flows.

**Numbers every segment** and requires acknowledgement. Anything unacknowledged is **retransmitted**.

**Delivers in order**, buffering out-of-order arrivals until the gap is filled.

**Controls flow and congestion**, slowing down when the network or receiver cannot keep up — which is how the internet avoids collapsing under load.

The cost is **latency and overhead**: handshakes before data, retransmission delays, and **head-of-line blocking**, where one lost packet stalls everything behind it even if those packets already arrived.

**UDP (User Datagram Protocol).** Connectionless and minimal:

**No handshake** — send immediately.

**No acknowledgement, no retransmission, no ordering guarantee.** Packets may arrive out of order, duplicated, or not at all, and nothing tells you.

**Almost no overhead**, with a very small header.

**Why lossy delivery is sometimes better.** In a live voice or video call, a packet that arrives 200ms late is **useless** — the moment it represented has passed. Retransmitting it wastes bandwidth and delays everything after it. Dropping it and continuing produces a brief artefact and keeps the conversation real time. **For real-time media, timeliness beats completeness.**

**What uses which:**

**TCP** — web pages, email, file transfer, and anything where a missing byte corrupts the result.

**UDP** — voice and video calls, live streaming, online game state updates, DNS queries, and DHCP.

**Applications build their own reliability on UDP** where they need some. Game engines resend important state and discard stale position updates; media protocols use forward error correction.

**QUIC** is the significant modern development — a transport built **on UDP** that provides TCP-like reliability and encryption while eliminating head-of-line blocking between streams and reducing handshake round trips. It underpins **HTTP/3**, which means a large share of web traffic now travels over UDP.

Question 2

What is TCP congestion control, and why does one slow connection affect everything?

Accepted Answer

The mechanism by which senders discover how fast the network can carry their data without overwhelming it — and it matters because the internet has no central traffic controller. Capacity is

Question 3

What is actually inside a network packet?

Accepted Answer

Layers of headers wrapped around your data, each added by a different part of the networking stack and each read by a different piece of equipment along the way. Why layers exist. Each layer solves

Question 4

What is QUIC, and what did HTTP/3 change?

Accepted Answer

QUIC is a transport protocol built on UDP rather than TCP, designed to fix problems that could not be fixed in TCP itself. HTTP/3 is HTTP running over QUIC. The problem it solves: head-of-line

Question 5

What are jitter and packet loss, and why do they matter more than speed?

Accepted Answer

Because a video call does not need much bandwidth — it needs packets arriving consistently, and these two measures describe whether they do. Latency is how long a packet takes to travel, measured as

Question 6

What is Quality of Service on a router?

Accepted Answer

A set of techniques for deciding which traffic gets priority when there is not enough capacity for everything — because the only time prioritisation matters is when a link is full. The problem. A

Question 7

What is NAT, and why does every home network use it?

Accepted Answer

Network Address Translation lets many devices share one public IP address, by rewriting addresses and ports as traffic passes through the router. Why it exists. IPv4 provides around 4.3 billion

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