When students start to learn how the Internet operates, it is important for them to be able to experiment with real servers and Internet applications. To encourage the readers of Computer Networking: Principles, Protocols and Practice, we have started to install small basic services on a VPS.

The first service is the echo service defined in RFC862. When a client connects to the server using TCP, the server echoes all the bytes sent by the client over the TCP connection.

Students can test this service using telnet, netcat/nc or even curl.

nc echo.computer-networking.info 7
Hello
Hello
^C

curl telnet://echo.computer-networking.info:7
Hello
Hello

RFC862 also defines this service for UDP, but blindly echoing UDP messages is risky from a security viewpoint.

We also support the Discard protocol defined in RFC863. This service simply accepts TCP connections or UDP messages on port 9 and simply discards them without any reply.

nc discard.computer-networking.info 9
Hello
^C

And for UDP:

nc -u discard.computer-networking.info 9
hello
another

The last service is the daytime protocol specified in RFC867. This service simply accepts TCP connections and returns the time of the day on the server. It uses port 13 by default. For security reasons, we disable the UDP service.

nc daytime.computer-networking.info 13
08 SEP 2025 13:44:50 UTC

These services will be used by examples in the fourth edition of the Computer Networking: Principles, Protocols, and Practice ebook.

This blog aims at encouraging students who read the open Computer Networking: Principles, Protocols, and Practice ebook to explore new networking topics. You can follow this blog by subscribing to its RSS feed or by following @cnp3_ebook on Mastodon. Feel free to share the posts that you find interesting on your preferred social network.