GCSE · Computer Science · AQA · Spec 8525

HTTP

Click a link and your browser has a lightning-fast chat with a faraway computer: one asks, one answers. What do they say, and who else could be listening?

Computer Science · Networks

One click, two journeys

Step through what happens when you click a link to a page that exists, then a link to a page that doesn't.

Before you step through: on click 2 the page doesn't exist. Will the server stay silent, or will it still answer? And what will the answer carry?

Message: Click 1: www.example.com/cats.html — Click 2: www.example.com/dragons.html

Browser: HTTP (application layer)→Browser: TCP (transport layer)→Web server

Click=1 of 2: a page that existsMessage=RequestProtocol=HTTP (Hypertext Transfer Protocol)Asking for=the page at www.example.com/cats.html

Output

Step 1: You click. Your browser writes an HTTP request: a message asking the web server for the page at that URL (the web address of the thing you want). HTTP is the set of rules for how that asking, and the answering, is worded. It works at the application layer.

1 / 11

Press Next to follow the click. Run 1 ends at step 6; run 2 starts at step 7.

Step 1 of 11: You click. Your browser writes an HTTP request: a message asking the web server for the page at that URL (the web address of the thing you want). HTTP is the set of rules for how that asking, and the answering, is worded. It works at the application layer..

Exam line: A browser sends an HTTP request for a URL. The server's HTTP response carries the page if it exists, or an error status if it doesn't. HTTP works at the application layer and passes its request down to TCP at the transport layer.

Watch out: HTTP only words the request and the response. Getting them there complete and in order is TCP's job, and neither of them hides the data from anyone listening.

Who does what?

Which idea is closest to yours?

A web page has just arrived on your screen. Every part of it got there, and in the right order.

Which idea is closest to what you think made that happen?

HTTP is what makes sure every part of the page arrives complete and in order.HTTP is the web page itself, or the web address you type in.The browser makes sure the page arrives complete, so no protocol is needed for that.HTTP sets the rules for the request and response, and TCP underneath it gets the data across complete and in order.

How sure are you?

I'm sureI think soI'm guessing

HTTP is what makes sure every part of the page arrives complete and in order.

This is a common misconception

Understandable, because HTTP is the name everyone sees. But HTTP only defines how the request and the response are worded between browser and server. It hands its request down to TCP at the transport layer, and TCP is what makes sure the data arrives complete and in order.

HTTP is the web page itself, or the web address you type in.

This is a common misconception

The page is the thing being asked for (the resource, sent as HTML), and the URL is its address. HTTP is neither of those. It is the set of rules for requesting that page and returning it, like the rules of a conversation rather than the thing the conversation is about.

The browser makes sure the page arrives complete, so no protocol is needed for that.

This is a common misconception

The browser forms the request and displays what comes back. Making sure the data gets across complete and in order is a protocol's job, and the protocol that does it here is TCP at the transport layer, working underneath HTTP.

HTTP sets the rules for the request and response, and TCP underneath it gets the data across complete and in order.

This matches the evidence

Exactly. HTTP works at the application layer: it words the request for a URL and the response that carries the page or an error status. TCP at the transport layer does the delivering. Two jobs, two protocols.

HTTP asks and answers. TCP delivers. The page is what's being asked for. Keep those three apart and half the confusion in this topic disappears.

Exam line: HTTP is the rulebook for asking and answering. TCP is the delivery that arrives complete and in order. The page is the thing being asked for.

Predict, then check

You are logging in to a website that uses plain HTTP. Someone is secretly listening in on the connection.

Your login form is sent in a plain HTTP request. Can the person listening in read your username and password?

Your turn

Say it in your own words

Describe how HTTP is used when a user opens a web page. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

The browser sends an HTTP request to the web server (1 mark)
The request asks for the page identified by the URL (1 mark)
HTTP works at the application layer and passes the request down to TCP at the transport layer (1 mark)
The web server sends back an HTTP response that carries the page if it exists (1 mark)
If the page does not exist, the response carries an error status instead (1 mark)

Model answer

When a user opens a web page, the browser sends an HTTP request to the web server, asking for the page at the URL. HTTP is an application layer protocol, and it passes the request down to TCP at the transport layer. The web server replies with an HTTP response. If the page exists the response carries the page, and if it doesn't exist the response carries an error status instead.

Request out, response back, with the page or an error status in the response. HTTP at the application layer, passing down to TCP at the transport layer.

Exam line: Cover the whole journey: the request for the URL, the response carrying the page or an error status, the application layer, and the hand-down to TCP.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Every page you open is a request and an answer. Follow one click out and back, then watch what happens when the page doesn't exist.

What you need to know

HTTP is the set of rules a web browser and a web server use to ask for and send web pages.
The browser sends an HTTP request for a URL. The server sends back an HTTP response: the page if it exists, or an error status if it doesn't.
HTTP works at the application layer. It passes its request down to TCP at the transport layer, which makes sure the data arrives complete and in order.
HTTP does not encrypt, so data sent with it could be read if intercepted. HTTPS, which encrypts, is used for private data.

The big picture

HTTP is the set of rules a web browser and a web server follow to ask for and send web pages. The browser sends a request for a URL, the server sends back a response carrying the page or an error status, and the request is handed down to TCP, which delivers it complete and in order. HTTP does not encrypt anything, which is why HTTPS is used for private data.

Key points

1HTTP = the rules for requesting and returning web pages. It is not the page, not the address and not the delivery.

2Request out, response back. The response carries the resource if it exists, or an error status if it doesn't.

3Application layer: HTTP. Transport layer: TCP. HTTP hands the request down, and TCP gets it there complete and in order.

4No encryption in HTTP. For private data, use HTTPS.

.we-_R_18slubsnpfiv5ubrb_ [data-we-step]{display:flex !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-solution]{display:block !important}.we-_R_18slubsnpfiv5ubrb_ [data-we-reveal]{display:none !important}

Worked example

Problem

Opening www.example.com/maps.html involves four separate jobs. For each one, decide whether it is HTTP's job, TCP's job, or something plain HTTP cannot do: (1) asking the server for maps.html; (2) making sure the request arrives complete and in order; (3) answering with an error status when maps.html isn't there; (4) scrambling a password so a listener can't read it.

⚠ Watch out

Giving HTTP every networking job: saying HTTP makes sure data arrives complete and in order (that's TCP), or that HTTP keeps data private (plain HTTP doesn't encrypt anything), or that HTTP is the web page or the address itself (it is the set of rules for requesting and returning pages).

🧠

Memory hook

HTTP is the conversation, TCP is the courier, and plain HTTP is a postcard: it arrives, but anyone who handles it can read it. (Like all analogies, it stops there: the courier doesn't write the message, and a postcard is paper, not data.)

✓

Check yourself

In one breath: what does the browser send, what comes back if the page exists, and what if it doesn't? Which protocol carries the request once HTTP hands it down?

Flashcards

(9)

What is HTTP?

The set of rules a web browser and web server use to request and return web pages. It works at the application layer.

What is a URL?

The web address of the resource the browser is asking for, for example a particular page on a server.

What does a browser's HTTP request ask for?

The resource at a URL, such as a web page.

What does the HTTP response carry when the resource exists?

The page itself, written in HTML.

What does the HTTP response carry when the resource does not exist?

An error status instead of the page.

At which layer does HTTP work?

The application layer.

Which protocol makes sure the data arrives complete and in order?

TCP, at the transport layer. HTTP passes its request down to it.

Does HTTP encrypt the data it carries?

No. Data sent with HTTP could be read by anyone who intercepts it.

Why is HTTPS used for private data?

HTTPS encrypts the data. Plain HTTP does not.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

One click, two journeys

Which idea is closest to yours?

Say it in your own words

More AQA GCSE Computer Science topics