title: How do I determine user ID of MQTT clients?
body: <p>Currently I have an MQTT system where one client publishes, and one subscribes. Now I want a system where many clients publish. The susbcriber must understand which client is sending information.</p>
  
  <p>While one obvious way to do this is to simply append a client ID in the publishing message, I wanted to know if there is a way of getting client ID without explicitly adding it to the message.</p>
  
  <p>To elaborate, suppose the topic is "/hello/world", and client 1 publshes "OK", client 2 publishes "ERR". Is there a way to determine which client sent what message?</p>