## 5 Discussion  
The practice of the presented methods in PicknPack line achieves production line-scale ubiquitous interaction and interoperable understanding. The performance of the system based on the presented technical solutions is encouraging. The lessons learned are summarised as follows. A significant technical challenge is the integration of multiple levels applied technologies. This CoM2M messaging mechanism introduces platform-specific designs to comply with the  
Figure 7: Quantity and Total Size of Messages Received and Sent by Machines (M1-M4) for Dynamic Label Printing. Zyre-based messaging protocol in order to gain higher level interoperability. For end machines, APIs for the mainstream platforms C++, C#, and LabVIEW are developed for Ubuntu and Windows 7 as the major operating systems. If machines do not support third-party development, extra controllers need to be introduced as the intermediaries. Once all machines are compliant with the Zyre-based messaging protocol, machines can identify all peer machines in the network and initiate P2P or one-to-multiple messaging for data and events interactions. Some configurations, notifications, and requests can be accessed by all concerned machines with this messaging mechanism. In addition, the machines of PicknPack line are within a LAN, where ubiquitous interaction between machines is the major concern. With internet connection, manufacturing configuration and data maintenance can be deployed in a remote cloud service. For inter-enterprise interactive manufacturing which exceeds the scope of a LAN, further intermediary protocol adaptors need to be adopted. Another crucial issue is the lack of common modelling formalism for the interoperable understanding between machines. The system supports flexible machine interactions, and therefore potentially promises flexibly manufacturing. However, the machine understanding, machine collaborations and the production flexibility need to be described with appropriate modelling, such as manufacturing policies and machine configurations. The PicknPack defines machine profiles, message formats, and machine configurations, and so forth, with a global description model named 'world model'. All machines access the 'world model' for communal understanding to gain interoperability. A further crucial issue in terms of flexible manufacturing based on CoM2M is the appropriate use of domain knowledge. Based on the interoperable technologies, some manufacturing functions fulfilled are application specific. Therefore, application specific knowledge such as the size and shape of packages, robot grippers, sensors, and quality evaluation methods for different food categories are all significant to the success of highly adaptive manufacturing. Such application-specific customizations are challenges for system developers. For some IoT application scenarios, such as smart meter, smart home, smart healthcare, internet of vehicles, centralised system structure for lightweight devices and network connections might be of interest. The M2M solutions for these application scenarios may use publish/subscribe mode, and focus on their lightweight, mobility, and interoperability characteristics. Typical applications are lightweight XMPP publish/subscribe scheme [27], CoAP/Observer-based WSN [28], MQTT protocol for mobility in IoT [29], and AMQP/MQTT-based Message Oriented Middleware (MOM) protocols [30]. However, machines in manufacturing industrial networks require ubiquity of M2M messaging and machines interact with each other in an intertwined mode. It distinguishes with the others mainly in two aspects: (1) machines communicate with one another in an interactive and highly frequent way; (2) machines need data from peer machines to perform manufacturing operations. Therefore, it is a pivotal task to discovery and identify the machines, and talk to any machine in the network efficiently.