While writing a simple UDP server, I had the idea to focus on the differences with a TCP one.

The standard TCP Server flow is the following:

1. create TCP socket()
2. bind() it to an ip:port address
3. listen() for incoming connection
4. accept() them

The 1. is done via a socket(.., STREAM, TCP) system call. Replacing STREAM, TCP with DATAGRAM, UDP, and going thru point 2. to 4. we’ll get the following errors:

• listen(): Operation not supported
• accept(): Operation not supported

What happened? Yep, easy: as UDP is a connection-less protocol, we cannot “listen for” or “accept” connection.

While TCP establishes a fixed connection between client and server, an UDP server just gets every single packet which arrives from a client.

You can check this behavior running netcat in udp mode:

#nc -u localhost 12345

This command is successful even if there’s nothing listening on that port: it’s like a cannon ready to fire everything against a castle’s port.

Each time you send something, it fires. And you need to switch it off to “close” it!