14.1 Protocols

Protocols are sets of rules that govern how devices communicate with each other over a network. They define the format, order, and timing of data transmission. Understanding different protocols is crucial for comprehending how the internet and various network applications function.

Key Protocols and Their Purposes

This section explores several important network protocols, detailing their functions and typical use cases.

Hypertext Transfer Protocol (HTTP)

HTTP is the foundation of data communication on the World Wide Web. It's used to transfer hypertext, which is text with hyperlinks to other documents.

• Purpose: Transferring web pages, images, videos, and other web content.
• Type: Application Layer Protocol
• Communication Model: Request-Response (Client sends a request, server sends a response).
• Port: Typically uses port 80 (unencrypted) or port 443 (HTTPS - encrypted).
• Example: When you type a URL into your browser, HTTP is used to fetch the corresponding web page from the web server.

File Transfer Protocol (FTP)

FTP is designed for transferring files between a client and a server.

• Purpose: Transferring files (e.g., software updates, documents) between computers.
• Type: Application Layer Protocol
• Communication Model: Uses a control connection (for commands) and a data connection (for file transfer).
• Port: Typically uses port 21 (control) and can use ports 20 (data - active mode) or a range of ports (passive mode).
• Example: Uploading a file to a website's server or downloading a software installation file.

Post Office Protocol version 3 (POP3)

POP3 is a standard protocol used by email clients to retrieve email from a mail server.

• Purpose: Downloading email messages from a mail server to a local device.
• Type: Application Layer Protocol
• Communication Model: Client connects to the mail server, authenticates, and then downloads messages. Messages are typically deleted from the server after download.
• Port: Typically uses port 110 (unencrypted) or port 993 (SSL/TLS encrypted).
• Example: Checking your email inbox on your computer or phone.

Internet Message Access Protocol (IMAP)

IMAP is another protocol used for accessing email, but it offers more features than POP3. It allows you to access and manage email on the server.

• Purpose: Accessing and managing email messages on the mail server.
• Type: Application Layer Protocol
• Communication Model: Client connects to the mail server and can access, organize, and manage messages directly on the server.
• Port: Typically uses port 143 (unencrypted) or port 993 (SSL/TLS encrypted).
• Example: Managing your email folders, reading messages on multiple devices, and synchronizing your inbox across platforms.

Simple Mail Transfer Protocol (SMTP)

SMTP is used for sending email messages from an email client to a mail server.

• Purpose: Sending email messages.
• Type: Application Layer Protocol
• Communication Model: Client connects to the mail server and sends the email message.
• Port: Typically uses port 25 (unencrypted), port 587 (submission - often used with TLS), or port 465 (SSL).
• Example: Sending an email from your email client to a recipient.

BitTorrent Protocol

BitTorrent is a peer-to-peer (P2P) file sharing protocol.

• Purpose: Sharing files directly between multiple users (peers).
• Type: Application Layer Protocol
• Communication Model: Uses a central tracker to locate peers who have pieces of the file being shared. Users download and upload file pieces simultaneously.
• Port: Uses a range of ports, often dynamically assigned.
• Example: Downloading or sharing large files like software, movies, or games.

Protocol	Purpose	Type	Typical Port	Communication Model	Encryption
HTTP	Transferring web content	Application	80/443	Request-Response	Optional (HTTPS)
FTP	File transfer	Application	21/20	Control & Data Connections	Optional (FTPS)
POP3	Downloading email	Application	110/993	Client connects, authenticates, downloads	Optional (SSL/TLS)
IMAP	Managing email on server	Application	143/993	Client connects, manages messages on server	Optional (SSL/TLS)
SMTP	Sending email	Application	25/587/465	Client connects, sends message	Optional (TLS/SSL)
BitTorrent	Peer-to-peer file sharing	Application	Dynamic	Peer-to-Peer (via tracker)	Optional (TLS)