Chapter 4: HYPERTEXT TRANSFER PROTOCOL (HTTP)

Hypertext Transfer Protocol (HTTP) Overview

• HTTP is a client-server protocol used by web browsers to interact with servers over TCP/IP networks.

• It follows a request/response paradigm with message headers and a body.

• Stateless: Each request-response is treated independently.

• Some transactions requiring state maintenance can use proxies and cookies.

HTTP Versions

• HTTP uses a <major>.<minor> version numbering scheme.

• HTTP/1.1, defined in RFC 2616, is widely supported and focuses on persistent connections.

• HTTP/2.0 (2015) introduced binary format, multiplexing, and header compression.

• Persistent connections in HTTP/1.1 reduce Round Trip Times (RTT) compared to non-persistent.

HTTP Connections

• HTTP connections can be persistent or non-persistent.

• Non-persistent (HTTP/1.0) requires a new TCP connection for each object.

• Persistent (HTTP/1.1) keeps the connection open for multiple objects, reducing RTT.

• Persistent connections save CPU time, reduce latency, and support pipelining.

HTTP Communication Model

1. Handshaking: Opening a TCP connection to the web server.

2. Client request: HTTP client sends an HTTP Request message specifying the resource.

3. Server response: Server reads the request, takes action, and creates an HTTP response.

4. Closing: Optional step for closing the connection.

HTTP Request Message

• Start Line: Request method, Request URI, HTTP version.

• Headers: General, request, and entity headers.

• Request methods include GET, POST, PUT, DELETE, etc.

HTTP Response Message

• Status Line: HTTP version, Status Code, Status Phrase.

• Headers: General, response, and entity headers.

• Status codes categorized into 1xx, 2xx, 3xx, 4xx, and 5xx groups.

HTTPS (Secure HTTP)

• HTTPS combines HTTP with SSL/TLS protocols for secure communication.

• SSL layer verifies direct communication and ensures data confidentiality.

• Used for secure transactions, HTTPS encrypts and decrypts data during transmission.

Cookies for State Retention

• Cookies are small pieces of information sent from the server to the client.

• They retain state in the stateless HTTP protocol.

• Cookies have a lifespan, are stored on the client, and sent back to the server with each request.

Creating Cookies

• Set-Cookie header is sent from the server to create cookies.

• Cookies are sent back with subsequent requests to the same server.

• Cookies can have expiration, domain, and path specifications.

HTTP Cache

• Caching optimizes performance by storing reusable responses.

• Types of caching include browser cache, intermediary caching proxies, and reverse cache.

• Advantages include decreased network costs, improved responsiveness, and increased performance.

Cache Consistency

• Cache consistency ensures cached copies are eventually updated.

• Pull method: Cached pages have expiration times; fresh copies obtained on expiration.

• Push method: Server sends update requests to web proxies when a page is modified.