Course 01 · Networking

How packets movethrough the internet

From a letter in an envelope to independent QUIC streams—a visual guide to what the network promises, what it does not, and how transport protocols fill the gaps.

Beginner 12 min 5 experiments No prerequisites

01 · Start with an analogy

A packet is a little like a letter.

You put a message inside, write a destination on the outside, and hand it to a system that knows the next useful direction—not necessarily the whole route.

The analogy is imperfect, and that is useful. Unlike a postal letter, a large digital message is split into many packets. They may take different routes, arrive out of order, or not arrive at all.

Postal systemInternet

✉01

Street address↔IP address

Sorting center↔Router

Delivery time↔Latency

Lost letter↔Packet loss

02 · Make the network move

One route, five different stories.

Run each experiment. The physical route stays the same; the protocol changes what happens around it.

Tip: switch experiments at any time.

A packet goes from A to B

simulation · slowed down

Routers inspect the destination and forward the packet one hop closer.

Network state Ready. Choose an experiment.

idle Waiting at A

03 · Notice the promises

The internet layer is deliberately modest.

IP tries to move each packet toward its destination. It does not promise delivery, order, or a fixed travel time. That small contract keeps the network flexible.

✓

Best-effort forwardingUse the destination address to choose a next hop.

Guaranteed arrivalCongestion, failures, or full queues can drop packets.

Guaranteed orderPackets can follow different paths and arrive rearranged.

The idea to keep

IP moves packets. Transport protocols decide what to do when reality gets messy.

Protocol shelf

Same packets. Different promises.

UDP, TCP, and QUIC all sit above IP, but they expose very different contracts to an application.

UDPDatagrams

Send it and keep moving.

UDP adds ports and a checksum, then leaves delivery strategy to the application. There is no built-in handshake, retransmission, or ordering.

Low overheadMessage boundariesApplication-controlled

Useful when fresh data matters more than recovering every old packet, or when a protocol wants to build its own behavior.

TCPByte stream

Deliver every byte, in order.

TCP establishes a connection, detects missing data, retransmits it, and presents one ordered byte stream to the application.

ReliableOrderedCongestion controlled

The trade-off a missing segment can hold back later bytes in the same stream until the gap is repaired.

QUICStreams over UDP

Reliability, with independent lanes.

QUIC builds secure, reliable connections in user space over UDP. Multiple streams share one connection without one stream's loss blocking the others.

TLS built inMultiplexedConnection migration

Why UDP? it gives QUIC a widely deployable packet substrate while QUIC defines the connection behavior above it.

Transport protocol comparison
Property	UDP	TCP	QUIC
Connection setup	None	Handshake	Secure handshake
Reliability	Application decides	Built in	Built in
Data model	Datagrams	One byte stream	Multiple streams
Encryption	Not inherent	Usually TLS above	TLS integrated
Loss isolation	Application decides	Single stream waits	Other streams continue

Field kit

Ask the network one precise question.

A tool is easiest to remember when it is attached to the question it answers.

ping

“Can I reach that host, and roughly how long is the round trip?”

$ ping example.com

Uses ICMP echo messages. A failed ping is a clue, not proof that the host is down—networks can filter ICMP.

traceroute

“Which visible hops does traffic cross on the way there?”

$ traceroute example.com

Changes the IP hop limit to provoke responses along the path. Some hops stay silent, so the result is a partial view.

tcpdump

“Which packets are crossing this interface right now?”

$ tcpdump -ni any port 443

A compact command-line packet capture tool. Filters keep a busy machine from becoming a wall of packets.

Wireshark

“What is inside this exchange, field by field?”

filter tcp.port == 443

Turns captures into a layered, searchable view of frames, packets, streams, timing, retransmissions, and more.

curl

“What did the application protocol negotiate and return?”

$ curl -v https://example.com

Verbose mode exposes DNS, connection setup, TLS negotiation, request headers, and response headers in one trace.

rule

Observe in layers

Start broad, then move closer to the wire.

Is the name resolving?
Is the host reachable?
Is the port accepting?
What do the packets say?