If the client and server do not share any common protocol versions, then the sever SHOULD respond with an ERROR frame similar to the following and then close the connection.
In fact, since both ends could not agree, it MUST respond with an ERROR frame.
STOMP 1.2 contains:
If the client and server do not share any common protocol versions, then the sever SHOULD respond with an ERROR frame similar to the following and then close the connection.
In fact, since both ends could not agree, it MUST respond with an ERROR frame.