The Application Layer of Internet
When you open a browser, send an email, or stream a video, you’re using network applications.
What Is a Network Application?
A network application is any software program that runs on end systems (laptops, phones, or servers) and communicates over the Internet.
Examples -
- Web browsers and servers (using HTTP)
- Email clients (using SMTP, POP3, IMAP)
- Messaging apps (like WhatsApp)
- File-sharing services (like BitTorrent)
Each application is designed to communicate using a specific set of rules known as application-layer protocols.
How Network Applications Work
Here are the main building blocks and design choices behind every network application.
Application Architecture
- Client-Server Model - One machine (the server) provides services, and other machines (the clients) request those services. Example- A web browser (client) sends a request to a web server, which responds with the web page.
- Peer-to-Peer (P2P) Model - ****All participating devices (called peers) both request and provide services to each other. Example- BitTorrent, you download and upload pieces of files from/to other peers.
Processes Communicating
A process is a program running on a device. When network applications communicate, it’s really the processes that are talking and not the computers themselves. Each process is identified by -
- IP address (which host it’s on)
- Port number (which application on that host)
Example: Your web browser might talk to a web server at IP 192.0.2.1 on port 80 (HTTP).
The Role of Sockets
A socket is like a door through which data flows in and out of a process. It serves as the interface between the application and the transport layer (TCP or UDP).
Transport Services Needed by Applications
| Service | What It Means | Who Needs It |
|---|---|---|
| Reliable Data Transfer | Ensures no data is lost or duplicated | Email, file transfer |
| Throughput | Needs guaranteed data rate | Video streaming |
| Timing | Low delay between messages | Online gaming, voice calls |
| Security | Data should be encrypted/authenticated | Banking apps, messaging apps |
What Is an Application-Layer Protocol ?
An application-layer protocol defines message types and Message format (what fields are in the message) and the rules for sending/receiving messages (when to respond, what happens on error, etc.)
Examples of Application Protocols -
| Protocol | Used For | Transport Layer Used |
|---|---|---|
| HTTP | Web browsing | TCP |
| SMTP | Sending email | TCP |
| POP3/IMAP | Retrieving email | TCP |
| DNS | Translating names to IPs | UDP (mostly) |
| BitTorrent | File sharing (P2P) | TCP |
Designing Your Own Protocol ?
If you’re building a custom network app, and want to create your own protocol then you define -
- The type of messages (e.g., HELLO, DATA, BYE)
- The format of the messages (e.g., JSON, XML, plain text)
- The rules for how messages are exchanged
- Whether you want TCP or UDP as your transport protocol
Example - a multiplayer game might send position updates using UDP every 50ms, while a messaging app might use TCP to ensure messages aren’t lost.