Understanding HTTP: A Comprehensive Guide

Posted on Nov 9, 2024 in Computers

What is HTTP?

HTTP, or Hypertext Transfer Protocol, is the foundation of data exchange on the World Wide Web (WWW). It’s the most common method for exchanging information online, as described in RFC 1945. The secure version, HTTPS, utilizes various encryption methods.

A Brief History of HTTP and the WWW

In 1990, the World Wide Web (WWW), or simply the Web, was developed. This system, based on hypertext, revolutionized information sharing. Initially a simple multimedia navigation system, the Web evolved to encompass images, sounds, videos, and more.

The WWW: A Client-Server Model

The Web operates on a client-server model within a TCP/IP network. Users request HTML pages from a client (web browser) to a server, which may or may not be on the same network. The browser interprets HTML tags to display formatted content. If the content isn’t a text document, the browser either activates an external application or prompts the user.

The Role of the HTTP Client

The HTTP client, or web browser, requests and receives web pages from the server. These pages are plain text documents structured with HTML tags. The client interprets these tags to display the content correctly.

The Role of the HTTP Server

The server’s primary function is to respond to client requests for pages and documents. HTTP is a stateless protocol, meaning it doesn’t retain memory of past connections. If the requested document exists, the server sends it; otherwise, it returns an error code. The connection is then closed.

Cookies and Session Management

To address the stateless nature of HTTP, cookies are used. These text files, exchanged between client and server, store information about the session. This allows for persistent information across multiple interactions.

HTTP Server Examples
  • NCSA HTTPd: One of the first free servers, but limited by its inability to act as a proxy or handle secure transactions.
  • Apache: Building upon NCSA, Apache became dominant due to its configurability and cross-platform performance.
  • Internet Information Server (IIS): Developed by Microsoft for Windows servers, IIS supports a significant portion of web servers.

Transfer of Web Pages

Requesting a web page involves typing its URL into a browser. The URL structure typically includes the protocol, server address, and resource path.

  1. The user enters the URL in the HTTP client.
  2. The client decodes the URL.
  3. The client connects to the server and requests the page.
  4. The server sends the page, and the client interprets the HTML.
  5. The connection closes.

Characteristics of HTTP

Key features of HTTP include client-server communication with 8-bit characters, three basic request methods, and stateless communication.

HTTP Communication

HTTP communication consists of requests from client to server and responses from server to client. These are text lines with commands and parameters defined by the HTTP protocol.

Request Methods

An HTTP request follows the syntax: method URI version. HTTP 1.0 defines GET, HEAD, and POST. HTTP 1.1 adds PUT, DELETE, OPTIONS, and others.

Response Methods

An HTTP response includes a status line with the version, error code, and text. Status codes are three-digit numbers categorized by information (1xx), success (2xx), redirection (3xx), client error (4xx), and server error (5xx).

HTTP Headers

Headers provide crucial information exchanged between clients and servers, adding flexibility to the protocol. They are categorized by:

  • General: Used by both clients and servers.
  • Request: Sent by the client to the server.
  • Response: Sent by the server to the client.
  • Entity: Related to the resource being provided.

MIME Types (Internet Media Types)

Initially used for email, MIME types now classify data types on the internet. They consist of a type and subtype, registered by IANA. HTTP uses MIME types to:

  • Inform the client about the data type.
  • Enable content negotiation.
  • Encapsulate objects within the message body.

A Communication Scheme

A typical HTTP communication process:

  1. The client establishes a connection to the server.
  2. The client sends a request with the method, URI, and protocol version.
  3. The server responds with a status line including the protocol version.
  4. The server closes the connection.

Secure Protocols and Keys (SSL)

SSL (Secure Sockets Layer) encrypts communication between web clients and servers. It allows for digital signatures and message digest algorithms. SSL, designed by Netscape, operates at the session level of the OSI model.

How SSL Works

SSL provides authentication and confidentiality. A session key encrypts data exchanged with the secure server. The process is as follows:

  1. The client requests an SSL-supported URL.
  2. The SSL connection is established.
  3. The client and server verify each other and agree on a cryptography algorithm.
  4. The server sends its identification.
  5. The client verifies the server’s identity and generates a session key, encrypted with the server’s public key.

Once both parties have the session key, secure communication begins. If the SSL session is abandoned, a warning message is typically displayed.

HTTPS Protocol

A common use of SSL is to secure web communication via HTTPS. Browsers typically warn users when transitioning between HTTP and HTTPS, indicating a change in security level.