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Intro to CS 9-10 / Week 11 / Tuesday
2/6
Week 11 · Systems and the Internet

Tuesday

Packets and protocols
// Packets, paths and a network that keeps working
⏱ about 20 min

Tuesday: Packets and Protocols

Wren sends a long Beacon report to mission control. It arrives as a jumble.

"Need at noon tomatoes water," Comet reads. "That is not what you wrote."

"The report was split into pieces," Wren says. "What do you notice? The pieces arrived in the wrong order."

Comet frowns. "Then how does mission control ever put it back together?"

Nova circles the jumbled screen. "Look at what each piece carries besides the words," she says.

Wren zooms in. Every piece has a tiny label: where it came from, where it is going, and a number.

"Packet 3 of 4," Comet reads. "Oh. The numbers are the instructions."

Packets

Data cross the Internet as a stream. The stream is cut into chunks, and each chunk travels inside a packet.

A packet holds a chunk of data plus metadata. The metadata are used for routing the packet and for putting the data back together.

Packets may arrive in order, out of order, or not at all.

PacketToFromNumberData
AMission ControlGreenhouse2 of 4NEED
BMission ControlGreenhouse4 of 4AT NOON
CMission ControlGreenhouse1 of 4TOMATOES
DMission ControlGreenhouse3 of 4WATER

These packets arrived in the order A, B, C, D. They are invented for our story.

PUT THE PACKETS BACK IN ORDER
  • Tap a card.
  • Then tap its spot.
1Packet 1
2Packet 2
3Packet 3
4Packet 4
READ THE PACKETS
  • Read the question.
  • Tap your answer.
Which part of each packet tells mission control the right order?
Which metadata help routers send the packet the right way?
Packet D never arrives. What does mission control know?

Protocols, the Internet and the Web

A protocol is an agreed set of rules that say how a system behaves.

The Internet is a network of networks. It uses open, standard protocols. IP, TCP and UDP are common Internet protocols.

Each device on a network has an address that identifies it. Routers compare IP addresses to choose the way a packet should go.

Routing on the Internet is usually dynamic: the path is not set in advance. The Internet was designed to scale, to grow very large.

The World Wide Web is a system of linked pages, programs and files. It uses the Internet, and HTTP is one of its protocols.

StatementTrue or false?
The Internet and the World Wide Web are exactly the same thing.?
The World Wide Web uses the Internet.?
HTTP is a protocol used by the World Wide Web.?
On the Internet, every path is fixed before the packet leaves.?
Routers compare IP addresses to choose a packet's way.?
WHY THIS EXERCISETelling the Internet from the Web, and rules from paths, keeps your picture of networks accurate.
An agreed set of rules for how a system behaves is a ____.
A chunk of data plus its routing and reassembly metadata is a ____.
Name one common Internet protocol from today's reading that starts with T or U.
What kind of data, carried in every packet, holds the addresses and the packet number? Type one word.
WHY THIS EXERCISEMetadata let packets travel separately and still become one message.
Try it
Write a short sentence on a strip of paper and cut it into four word groups.
Number each piece, shuffle them, and ask someone to rebuild the sentence using only the numbers.
On paper, sketch one packet card. Label the data chunk and each piece of metadata.

Sharp work. Tomorrow in the Beacon Lab, you will run the relay network on paper.

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