Application Layer Protocols (Beyond HTTP)

Overview

When HTTP is not used, other application-layer protocols handle communication between applications and transport mechanisms (TCP/UDP). This document covers common application-layer protocols used for different types of communication.

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1. File Transfer & Sharing

• FTP (File Transfer Protocol) – Transfers files over TCP.
• SFTP (Secure File Transfer Protocol) – Secure version of FTP over SSH.
• TFTP (Trivial File Transfer Protocol) – Lightweight, UDP-based file transfer (used for network booting).

📌 Example:

ftp ftp.example.com

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2. Email Protocols

• SMTP (Simple Mail Transfer Protocol) – Sends emails.
• POP3 (Post Office Protocol v3) – Downloads emails from a server.
• IMAP (Internet Message Access Protocol) – Accesses emails stored on a server.

📌 Example:

• Email clients (Gmail, Outlook) use SMTP for sending and IMAP/POP3 for receiving emails.

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3. Remote Access Protocols

• SSH (Secure Shell) – Secure remote server access.
• Telnet – Insecure command-line remote access.

📌 Example:

ssh [email protected]

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4. Domain Name System (DNS)

• DNS (Domain Name System) – Resolves domain names (example.com) to IP addresses.
• Uses UDP (port 53) by default, but can also use TCP.

📌 Example:

nslookup example.com

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5. Voice & Video Communication

• SIP (Session Initiation Protocol) – Used for VoIP calls.
• RTP (Real-time Transport Protocol) – Streams audio/video.
• RTSP (Real-Time Streaming Protocol) – Controls media streaming.

📌 Example:

• Zoom, Skype, and WhatsApp Calls use SIP + RTP instead of HTTP.

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6. Messaging & IoT Protocols

• MQTT (Message Queue Telemetry Transport) – Lightweight IoT messaging.
• AMQP (Advanced Message Queuing Protocol) – Reliable message queuing (e.g., RabbitMQ).

📌 Example:

• IoT devices (smart thermostats, sensors) often use MQTT instead of HTTP.

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7. Database Communication Protocols

• SQL over TCP – MySQL, PostgreSQL communicate directly over TCP.
• Redis RESP (Redis Serialization Protocol) – Binary protocol used by Redis.

📌 Example:

• PostgreSQL clients connect to servers via TCP (port 5432).

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8. Game Networking Protocols

• WebSockets – Bidirectional real-time communication.
• RakNet & ENet – Low-latency game networking.
• QUIC – UDP-based fast communication protocol.

📌 Example:

• Fortnite, Call of Duty, and Apex Legends use custom UDP-based game protocols instead of HTTP.

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Summary

Category	Protocol	Used For
File Transfer	FTP, SFTP, TFTP	Upload/download files
Email	SMTP, IMAP, POP3	Sending & receiving emails
Remote Access	SSH, Telnet	Remote server access
Networking	DNS	Resolving domain names
VoIP & Streaming	SIP, RTP, RTSP	Calls & media streaming
Messaging & IoT	MQTT, AMQP	IoT device communication
Databases	PostgreSQL, MySQL, Redis RESP	Database connections
Gaming	WebSockets, QUIC, RakNet	Multiplayer games

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Final Thought

HTTP is just one of many protocols at the Application Layer. If HTTP is not used, other protocols take its place depending on the application type (email, VoIP, databases, IoT, etc.).