+ * Returns OFPUTIL_P_NONE, which is not a valid protocol, if 'ofconn' hasn't
+ * completed version negotiation. This can't happen if at least one OpenFlow
+ * message, other than OFPT_HELLO, has been received on the connection (such as
+ * in ofproto.c's message handling code), since version negotiation is a
+ * prerequisite for starting to receive messages. This means that
+ * OFPUTIL_P_NONE is a special case that most callers need not worry about. */