Question 1

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 blocking.** HTTP/2 allowed many streams over one TCP connection — but TCP guarantees ordered delivery of **everything**, so **one lost packet stalls every stream**, including those whose data arrived fine. The multiplexing was real and the benefit was undermined at the layer below.

**QUIC moves stream management above the transport**, so streams are genuinely independent: a lost packet affecting one stream does not block the others.

**Why it uses UDP.** Not because UDP is better, but because **TCP cannot be changed in practice.** Middleboxes — firewalls, NAT devices, carrier equipment — inspect and enforce assumptions about TCP, so a new transport would be blocked or mangled across the internet. UDP passes through, so QUIC builds reliability, ordering and congestion control **on top of it**, in user space, where it can be updated with an application rather than an operating system. **That deployability argument is the whole reason it exists.**

**The other improvements:**

**Faster connection establishment.** QUIC integrates the cryptographic handshake with the transport handshake, so a connection is typically established in **one round trip**, and **zero** for a resumed connection — against TCP plus TLS separately.

**Encryption is mandatory** and covers almost the whole transport header, which prevents middlebox interference and ossification.

**Connection migration.** A connection is identified by a **connection ID rather than by the IP and port**, so moving from Wi-Fi to mobile does not break it — genuinely valuable on phones, where connections previously had to be re-established.

**Better loss recovery and modern congestion control.**

**The trade-offs:**

**Higher CPU cost**, since processing happens in user space, which matters at scale.

**Some networks block or throttle UDP**, requiring a TCP fallback.

**Harder to inspect and debug**, which is a feature for users and a problem for network operators.

Question 2

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 3

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:

Question 4

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 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 net neutrality, and what does it actually require?

Accepted Answer

The principle that networks should carry all traffic equally, without favouring or discriminating between sources, destinations or applications on commercial grounds. What it prohibits, specifically:

Question 7

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

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