Question: Explain how DNP3 can ensure data packets are received correctly when they are sent over a LAN usi……

If the packet will be sent over a LAN, the three DNP3 layers are rolled up into the application layer. The assembled packet is wrapped in the Transport Control Protocol (TCP) by the transport layer. This in turn is wrapped in the Internet Protocol (IP) by the internet layer. The User Datagram Protocol (UDP) can also be used but presents some extra issues related to reliable delivery in congested networks.

The fourth layer is the Network Interface layer. This is where the assembled packet is actually interfaced to some kind of transport media (for example, RG58 co-axial or fiber). While this multi-layer model may seem a bit confusing, it effectively isolates the tasks of communication and ultimately assists in designing and implementing a network.

To illustrate the function of this layered model, let’s look at a single DNP3 Read request over a LAN. The DNP3 Master wants to know the current status of the Remote’s power and prepares a Read request message for the appropriate object. After passing through all three DNP3 layers, the message is passed to the TCP/UDP transport layer. The transport layer adds a data block that IDs the Master port from which the request is sent. Then it finds the port on which it expects the Remote DNP3 process to be listening for messages. The packet thus formed is then passed to the IP layer. Here, a data block is added. It contains the IP and Media Access addresses of the Master and the Remote. Then, the entire assembled packet gets passed to the Network Interface layer. The Network Interface layer verifies media access and availability and places the packet on the media for transmission.

After working its way across bridges and through routers based on the IP information, the packet finally arrives at the Remote. Here it passes through the same four layers in exactly the opposite order as it did at the Master. First, it is pulled off the media by the Network Interface layer. After confirming that the packet is intact and valid, the Network Interface layer simply passes it to the IP layer. The IP layer verifies the Media Access and IP address and passes it on to the TCP/UDP layer. There, the target port is checked for joined applications. If an application is listening at the target port, the packet is passed to the Application layer. If the listening application is the Remote DNP3 process, the Read request is passed through its three layers to validate the request and identify what information needs to be collected. The Remote response then follows the identical path in reverse to reach the Master.