Does there exist any protocol which follows only request model?

HTTP follows the request-response model, i.e. for every request from a client there will be a response from the server.

Does there exist any protocol that follows only request model (there will be only requests from client)?

I have read about SMTP. Can I consider SMTP as request only model as we are sending the mail but not receiving any response from server?

If there exists any such protocol, please explain about it. I searched in Google but I didn't find any question related to my specific query.