Chapter 2 Part 2 & 3: The Web, HTTP, and Email

• HTTP (HyperText Transfer Protocol)

The application-layer protocol used by the web for transferring web pages and resources between clients and servers. It operates over TCP and follows a request-response model.

• Web Page

A document composed of a base HTML file and multiple embedded objects (like images or scripts), each accessible via a URL.

• URL (Uniform Resource Locator)

A string that specifies the location of a web resource, including the hostname and path to the object.

• Client-Server Model

A communication structure where the client (e.g., browser) requests services and the server responds with the requested data.

• TCP (Transmission Control Protocol)

A reliable transport protocol used by HTTP to ensure ordered and complete delivery of data between client and server.

• Stateless Protocol

A protocol like HTTP where the server does not retain information about previous client requests, simplifying design but limiting session continuity.

• Non-Persistent HTTP

A mode where each object on a web page requires a separate TCP connection, increasing overhead and response time.

• Persistent HTTP

A mode where multiple objects are transferred over a single TCP connection, improving efficiency and reducing latency.

• RTT (Round-Trip Time)

The time it takes for a small packet to travel from client to server and back, used to measure HTTP response time.

• HTTP Request Message

A message sent by the client to request a resource, consisting of a request line, headers, and optionally a body.

• HTTP Response Message

A message sent by the server in reply to a request, containing a status line, headers, and the requested data.

• HTTP Methods

Commands used in HTTP requests, such as GET (retrieve), POST (submit data), PUT (upload), and DELETE (remove).

• Status Codes

Numeric codes in HTTP responses indicating the result of the request, such as 200 (OK), 404 (Not Found), and 301 (Moved Permanently).

• Cookies

Small pieces of data stored on the client and exchanged with the server to maintain state across sessions, enabling features like login, shopping carts, and personalization.

• Web Cache (Proxy Server)

A network entity that stores copies of web objects to satisfy client requests without contacting the origin server. It reduces response time and traffic on access links.

• Conditional GET

A mechanism where the client asks the server to send an object only if it has been modified since a specified date.

• SMTP (Simple Mail Transfer Protocol)

The protocol used to transfer email messages between mail servers using TCP on port 25. It involves handshaking, message transfer, and closure phases.

• User Agent (UA)

A mail reader application like Outlook or Thunderbird that allows users to compose, send, and read email messages.

• Mail Server

A mail server stores incoming messages in a mailbox and outgoing messages in a message queue. It communicates with other servers using SMTP.

• Mail Message Format

Email messages consist of header lines (To, From, Subject) and a body.

• Mail Access Protocols

Protocols like POP3 and IMAP allow users to retrieve email from mail servers.

• POP3 (Post Office Protocol v3)

A mail access protocol that supports authorization and message retrieval. It is stateless across sessions and typically downloads messages to the client.

• IMAP (Internet Mail Access Protocol)

IMAP keeps all messages on the server, supports folder organization, and maintains user state across sessions, allowing access from multiple devices.

• HTTP/2

A major revision of HTTP standardized in 2015, designed to reduce latency and improve performance through features like multiplexing, prioritization, and server push.

• Framing (HTTP/2)

HTTP/2 breaks messages into small frames and interleaves them over a single TCP connection, allowing efficient transmission of multiple objects simultaneously.

• Prioritization (HTTP/2)

Clients can assign weights and dependencies to HTTP/2 messages, allowing servers to optimize response order and improve application performance.

• Server Push (HTTP/2)

HTTP/2 allows servers to send multiple responses for a single client request, reducing latency by eliminating the need for separate requests for each object.

• Head-of-Line (HOL) Blocking

A problem in HTTP/1.1 pipelining where a delay in one response blocks all subsequent responses. HTTP/2 framing solves this by interleaving frames.