--- title: "Across the internet" subtitle: "Networks · Keeping it safe · Lesson 4" --- # Across the internet **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** You press Buy. Less than a second later, a shop on the other side of the world has your order, and nobody on the way could read your card number. What happened in between? :::teacher-note **In one sentence:** Students follow an online purchase from laptop to shop on a board and its step log, connecting wired and wireless links, packets, TCP and HTTPS encryption into one end-to-end account, then put the journey in order themselves (AC9TDI8K02 node 5). **Students should be able to say:** I can describe the journey of data across the internet. I can say where packets, a protocol and encryption each play a part. I can point out which parts of the trip are wired and which are wireless. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **Your card number travels from your laptop to an online shop. Where does it get encrypted?** ☐ At the internet provider ☐ At the shop, when it arrives ☐ At your home router ☐ On your laptop, before it leaves My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** On your laptop, by the browser (HTTPS), before it leaves. Anywhere later would leave the first part of the trip, including the Wi-Fi, readable. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: the whole chain :::keep-together ### Follow the purchase 1. $\text{encrypt (HTTPS)}$ — On the laptop, before anything is sent (Who can see your data?). 2. $\text{split into packets}$ — Numbered packets (Packets). 3. $\text{Wi-Fi, then fibre}$ — Wireless first, then wired links (Wired or wireless). 4. $\text{routers, different routes}$ — Each packet is passed on towards the shop. 5. $\text{lost packet sent again}$ — TCP: the shop never confirmed it, so it was resent. 6. $\text{reorder, decrypt}$ — The shop rebuilds the message by number and decrypts it with its key. Now pick out the wireless part. **Which part of this journey is wireless?** ☐ None of it ☐ Home router to the provider ☐ Provider to the shop ☐ Laptop to the home router (Wi-Fi) ::: :::teacher-note **Answer:** Laptop to the home router (Wi-Fi) Only the first hop is wireless here. Most of the internet is cables and fibre, including under the sea. ::: :::keep-together ### Find the mistake **Sam describes how the card number is protected. Click the line where it first goes wrong.** ``` 1 I type my card number and press Buy. 2 My laptop splits it into packets. 3 My home router encrypts the packets. 4 The packets travel across the internet to the shop. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 3 Line 3: the browser encrypts the data on the laptop (HTTPS) before it is sent. If the router did it, the packets would cross the Wi-Fi as plain text, where anyone nearby could copy them. ::: :::keep-together ### HTTPS off **With HTTPS **off**, who could read the card number?** ☐ Only the shop ☐ Anyone on the Wi-Fi or any network along the way ☐ Nobody, because it is split into packets ☐ Only the internet provider ::: :::teacher-note **Answer:** Anyone on the Wi-Fi or any network along the way Without encryption, every link and router on the way handles readable packets. ::: :::typical ## Vary: change one thing **HTTPS off.** **HTTPS on.** **Slow at the far end.** **Without TCP.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## The internet :::keep-together ### Mobile video **You watch a video on the school bus on mobile data. Which part of the trip is wireless?** ☐ The video company's servers ☐ The fibre between cities ☐ Phone to the mobile tower ☐ None ::: :::teacher-note **Answer:** Phone to the mobile tower Phone to tower is wireless; from the tower onwards it is mostly fibre. ::: :::keep-together ### Which protocol? **Which protocol puts a web page's packets back in order and asks for lost ones?** ☐ Fibre ☐ TCP ☐ Wi-Fi ☐ HTTPS ::: :::teacher-note **Answer:** TCP TCP numbers, acknowledges and resends packets. ::: :::keep-together ### Review: decrypt **Review: a message was encrypted with key 2. You receive **JK**. What did it say?** ☐ IJ ☐ JK ☐ LM ☐ HI ::: :::teacher-note **Answer:** HI J → H and K → I: HI. ::: ## Make: put the journey in order Then: which steps would change if you used mobile data instead of home Wi-Fi? Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Describe the journey of a password from your phone to a website in four steps, naming where encryption, packets and TCP each come in.** {{lines-3}} ::: :::teacher-note **Answer:** Encrypted on the phone (HTTPS); split into packets; sent over wireless then wired links through routers, with TCP resending lost packets; put back in order and decrypted by the website. :::