In section 3.1 this is stated:
"AMQP frames received on streams other than stream 0 MUST be buffered until an open performative has been received on stream 0. Implementations SHOULD limit the number of frames that can be buffered and close the connection with an appropriate error code if the buffer is filled and no open has been received."

Is it legal to limit to 0 the number of frames buffered before an open is received?

What error code should be used when closing a connection (stream?) in this case?