What Is HTTP

Cyrus Yip
Oct 29, 2024

Table of Contents

Introduction #

HTTP (Hypertext Transfer Protocol) is a protocol for transferring information, such as HTML files. It’s the foundation of data communication for the World Wide Web. In a typical HTTP session, the client (such as a browser) sends a request to the server, which sends back a response.

HTTP Request #

A typical HTTP request consists of:

request line (request method, path, protocol version)
request header
empty line
optional message body

Here is an example of HTTP request message (curl --verbose https://cyrusyip.org/en/):

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GET /en/ HTTP/2
Host: cyrusyip.org
User-Agent: curl/8.10.1
Accept: */*

HTTP Request Methods #

HTTP request methods (also called HTTP verbs) indicate the purpose of the request and what to expect. Here are the common request methods:

GET: request the specified resource
POST: sends information to the server

HTTP Response #

A typical HTTP response consists of:

status line (protocol version, status code, optional explanation of status code)
response header
empty line
optional message body

Here is an example of HTTP response message (curl --verbose https://example.com/):

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HTTP/2 200
age: 190635
cache-control: max-age=604800
content-type: text/html; charset=UTF-8
date: Mon, 28 Oct 2024 09:39:38 GMT
etag: "3147526947+gzip+ident"
expires: Mon, 04 Nov 2024 09:39:38 GMT
last-modified: Thu, 17 Oct 2019 07:18:26 GMT
server: ECAcc (sac/2571)
vary: Accept-Encoding
x-cache: HIT
content-length: 1256

<!doctype html>
<html>
<head>
content is stripped...
</head>

<body>
<div>
    <h1>Example Domain</h1>
    <p>This domain is for use in illustrative examples in documents. You may use this
    domain in literature without prior coordination or asking for permission.</p>
    <p><a href="https://www.iana.org/domains/example">More information...</a></p>
</div>
</body>
</html>

HTTP Status Codes #

HTTP status codes are three-digit numbers that indicate whether an HTTP request has been successfully completed. They are grouped in five classes:

1xx (Informational): The request was received, continuing process.
2xx (Successful): The request was successfully received, understood, and accepted.
3xx (Redirection): Further action needs to be taken in order to complete the request.
4xx (Client Error): The request contains bad syntax or cannot be fulfilled.
5xx (Server Error): The server failed to fulfill an apparently valid request.

Here are some commonly used HTTP status codes:

103 Early Hints: This status code lets the user agent preload resources while the server prepares a response.
200 OK: The request succeeded.
301 Moved Permanently: The URL of the requested resource has been permanently changed.
302 Found: The URL of the requested resource has been temporarily changed.
403 Forbidden: The client does not have access permission to the resource.
404 Not Found: The server cannot find the requested resource.
500 Internal Server Error: The server has encountered an unexpected condition that prevented it from fulfilling the request.
503 Service Unavailable: The server is not ready to handle the request. It may be down for maintenance, or it may be overloaded.

References #

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