HTTP

4.2. HTTP#

So far, the HTML files we’ve created are just sitting on the computer we made them on. If you wanted to share them with someone, you’d have to send them a copy, like emailing the file or putting it on a USB drive. But that means your HTML files are kind of stuck on their own, unable to connect with anything else.

To solve this problem, the HTTP protocol was invented. It allows computers to share HTML files over a network, like the internet. Thanks to HTTP, your files aren’t trapped — they can be shared across the web, and hyperlinks can connect pages from anywhere in the world. This combination of HTML, hyperlinks, and the internet is what we call the “World Wide Web.”

4.2.1. HTTP Protocol Overview#

The Hypertext Transfer Protocol (HTTP) defines a client-server protocol for the transmission of HTML (and associated) files over standard internet technology.

The term client is interchangably used to mean the user’s physical device and web browser. While the term server is interchangably used to refer to the software which processes the requests then returns appropriate responses and the physical device on which the software is running.

HTTP uses a “request and response” model. Clients send requests for a particular resource and the server provides the resource in the body of the response message.

../../_images/http.png

A client requesting a resource from a server.#

In general data is exchanged over HTTP in the following steps:

  1. server starts and waits for a new TCP connection

  2. client establishes a TCP connection with the server

  3. client sends a request conforming to HTTP protocol over TCP

  4. server processes request

  5. server sends a response conforming to HTTP protocol over TCP

  6. client and server close the TCP connection

The simplest web server hosts static content meaning that it reads the requested HTML files and returns them as the response.

../../_images/static_server.png

A crucial aspect of the HTTP protocol is that it is stateless, meaning that each request is independent of the others and the requests cannot reference any previous requests. However we will see later, that we can add state to our web sites through shared information between client and server inside the request data.

4.2.2. HTTP Requests and Responses#

Request and response messages are sent as plain text but follow a very specific format.

Requests#

Let’s start with an example HTTP request. For example, requesting the Google homepage in your browser would send the following request:

GET / HTTP/1.1
Host: www.google.com.au

Let’s look at each line:

  1. The request line GET / HTTP/1.1 consists of

    • the method or type of request: GET

    • the path to the resource, which in this case is at the root of the server: /

    • the version: HTTP/1.1

  2. The host line Host: www.google.com.au, which is a request header field that specifies the domain name the client is requesting the resource from. This is required since a single server may host many websites!

Request Specification#

METHOD PATH VERSION
Host: DOMAIN_NAME
Header-field-1: value1
Header-field-2: value2
...
Header-field-N: valueN

Breakdown:

  • METHOD, typically one of:

    • GET - request that the server returns the specified resource

    • POST - send data

  • PATH - path on the server to a resource

  • VERSION - normally HTTP/1.1

  • Mandatory Host header field

  • Optional header fields and values, e.g.

    • Accept: text/html

    • Accept-Language: en

Attention

TODO: Add more details about header fields https://en.wikipedia.org/wiki/List_of_HTTP_header_fields#Standard_request_fields

Response#

Continuing the example from earlier, the Google web server would respond with:

HTTP/1.1 200 OK
Date: Monday, 8 Sep 2024 09:00:00 GMT
Content-Type: text/html

<!DOCTYPE html><html><head>...

where we have truncated the HTML to save page space.

Let’s look at each line:

  1. The status line HTTP/1.1 200 OK consists of:

    • the version HTTP/1.1

    • the status code of 200 meaning the request was successful

    • the status code reason phrase OK

  2. Date response header field

  3. Content-type response header field

  4. The body of the response, which contains the HTML of the page

Response Specification#

VERSION STATUS_CODE REASON_PHRASE
Header-field-1: value1
Header-field-2: value2
...
Header-field-N: valueN

BODY

Breakdown:

  • VERSION - normally HTTP/1.1

  • STATUS_CODE REASON_PHRASE - indicates the status of the request, typically one of:

    • 200 OK

    • 404 NOT FOUND

    • 500 INTERNAL SERVER ERROR

  • Optional header fields and values, e.g.

    • Content-Type: text/html

Attention

TODO: Add summary of status codes https://en.wikipedia.org/wiki/List_of_HTTP_status_codes

4.2.3. Glossary#

Client#

TODO

Method#

TODO

Plain text#

TODO

Server#

TODO

Stateless#

TODO

Static#

TODO

Status Code#

TODO

Protocol#

TODO

Resource#

TODO

Request header field#

TODO

Contents