On what open-source engine can you raise your messenger (network)?

Question

Messengers

I want to attach a feature to the project:

instead of the usual PM, so that the user downloads the client, for example, and communicates with other users, like in ICQ)) or something like that.

What to take as a basis? Is there any hassle-free proven engine?

Answer 1

Jabber of course

Answer 2

Maybe just use MyChat Guest? And then the customer will not need to download and install anything at all.

Answer 3

xmpp, irc (ircd)