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 18 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, two fundamental outcomes.

First see a packet arrive. Then lose the same packet and notice what the internet layer does—and does not—promise.

The transport protocols come next.

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.

Name & route

Before the packet moves, it needs an address.

Humans begin with a name such as example.com. The network needs an address, then a sequence of useful next hops.

01 · Resolve the name

DNS is the address book.

You remember a domain name. A recursive DNS resolver finds the corresponding record—or returns a cached answer—and gives your device an IP address it can place on the packet.

Person or company↔example.com

Address book↔DNS resolver

Street address↔203.0.113.42

01Your deviceasks its recursive resolver
02Rootpoints toward .com
03.compoints to the authoritative server
04Authoritativereturns the domain's record
05203.0.113.42answer returned and cached

This is the uncached path. A recursive resolver can answer immediately when it already has a valid cached record.

02 · Choose the next hop

Each sorting center only needs the next useful direction.

A router reads the destination prefix, looks in its forwarding table, and sends the packet to a next hop. The process repeats; no ordinary router needs to carry a single master plan for the entire trip.

At every hop destination prefix → forwarding table → next hop

Adevice

→

AS 65001access network

→

AS 65010transit network

→

AS 65020destination network

→

Bserver

03 · Cross network boundaries

BGP and SCION answer a larger question.

How should traffic cross independently operated networks? BGP and SCION approach that problem with very different routing architectures.

BGProute advertisements

“I can reach that postcode.”

Autonomous systems advertise reachable IP prefixes and path attributes to neighboring systems. Each network applies policy and selects routes for its own forwarding tables.

Inter-AS, destination-based routing
Policy matters—not simply the shortest path
The source usually delegates path choice to the network

BGP-4 standard

SCIONpath segments

“Choose from authorized route cards.”

SCION organizes ASes into isolation domains. Endpoints obtain and combine up, core, and down path segments; the chosen forwarding path travels in the packet header.

Explicit end-to-end path choice
Multipath and failure isolation
Isolation domains make trust relationships visible

SCION architecture

The complete chain

Name → address → next hop → network path → destination.

How packets movethrough the internet

A packet is a little like a letter.

One route, two fundamental outcomes.

The internet layer is deliberately modest.

Three protocols, three different stories.

Three ways to send the letter.

Drop each message in the box.

Account for every page, in order.