Networking for Developers
Understanding the Pipes of the Internet

Most backend issues are actually network issues. "Why can't Service A talk to Service B?" "Why is the connection timing out?" Understanding the fundamental layers of networking will save you hours of debugging.

The Models: OSI vs TCP/IP

You don't need to memorize every detail, but you do need to know where things happen.

Note: Layers 5 (Session) and 6 (Presentation) are often handled within the Application layer in modern web development.

TCP vs UDP: Reliability vs Speed

TCP (Transmission Control Protocol)

TCP is the polite, reliable protocol. It cares about feelings.

• Connection-oriented: Must establish a connection before sending data.
• Reliable: Ensures packets arrive. If one is lost, it resends it.
• Ordered: Ensures packets arrive in the correct order.
• Use cases: Web (HTTP/2), Email (SMTP), File Transfer (FTP).

UDP (User Datagram Protocol)

UDP is the honey badger of protocols. It doesn't care.

• Connectionless: Just starts yelling (sending data).
• Unreliable: Fire and forget. If packets are dropped, they are gone.
• Fast: Low overhead, no handshakes, no acknowledgments.
• Use cases: Video streaming, Voice over IP (VoIP), Gaming, DNS looks.

The TCP 3-Way Handshake

Before any data is exchanged in TCP (like loading this webpage), your computer and the server perform a specific ritual.

Step 4 (Often forgotten): The 4-Way Handshake (Teardown). Connections must be closed properly (FIN packets) or you risk "Address Already In Use" errors (TIME_WAIT state).

Everything is a Socket

When you write app.listen(3000) in Node.js or Python, you are creating a Socket.

A socket is defined by: {Protocol, Local IP, Local Port, Remote IP, Remote Port}.

Socket States

• LISTEN: Server is waiting for a connection (port is open).
• ESTABLISHED: Active connection between client and server.
• TIME_WAIT: Connection closed, but waiting to ensure all packets arrived before releasing the port.
• CLOSE_WAIT: A common error state where the application hasn't properly closed the socket.

Where HTTP and TLS Fit In

HTTP is an application-layer protocol that usually runs over TCP. HTTPS = HTTP + TLS encryption.

• HTTP/1.1: One request per connection (keep-alive helps reuse).
• HTTP/2: Multiplexing multiple streams over one TCP connection.
• HTTPS: Adds a TLS handshake before HTTP starts, which can add latency but secures traffic.

Latency, MTU, and Packet Loss

Speed issues are often caused by network fundamentals, not your code.

• Latency: Time for a packet to travel. High latency causes slow APIs.
• Packet Loss: Dropped packets force retransmissions (TCP) and cause retries/timeouts.
• MTU: Maximum packet size. If mismatched, packets get fragmented or dropped.

Decoding Network Errors

ECONNREFUSED (Connection Refused)

Result of a TCP RST packet. Usually means: Nothing is listening on that IP:Port. (Server is down or wrong port).

ETIMEDOUT (Connection Timed Out)

The establishing packet (SYN) was sent, but no reply ever came. Usually means: Firewall dropped the packet or the IP doesn't exist.

EADDRINUSE (Address Already in Use)

Another process is already bound to this port. Kill the other process.

ECONNRESET (Connection Reset by Peer)

The connection was established, but the other side abruptly closed it (crashed or panic).

Retries, Timeouts, and Backoff

Robust apps assume networks fail. Use timeouts and retries carefully.

• Timeouts: Always set them for outbound HTTP calls.
• Retries: Use exponential backoff to avoid thundering herds.
• Idempotency: Only retry requests that are safe (GET, PUT with idempotency keys).

Essential Commands

Pro tip: Combine curl -v with --resolve to debug DNS vs IP behavior.