How you would have implemented backward compatibility? I am interested to hear the general technical details of how this could have been possible. I am mostly

How you would have implemented backward compatibility? I am interested to hear the general technical details of how this could have been possible. I am mostly interested in two basic scenarios. With expectation that only on one side is any changes made. Host from new addressing scheme connecting to old one and receiving data back. Host from old addressing scheme connecting to one in new one and receiving data back.