Distributed Network Protocol (DNP3) in Remote Systems: Definition & Components

Instructor: Erik Rodriguez

Erik has experience working in Cybersecurity and has a Master's of Science in Information Systems.

In this lesson, we will discuss DNP3 and the layers that comprise it. We will review each layer's purpose in the communication process. Additionally, we will briefly discuss the DNP3 object library.

Rules of Communication

Suppose your college professor assigns you a research paper and requests that it be written in MLA format. Just like there are various standards and rules for how to write a paper (e.g., MLA, APA, etc.), network communication also employs many different protocols that govern how devices communicate with each other and what that entails for the data they send. As industrial processes increasingly become more automated, the need for specialized networking protocols has arisen. DNP3 is one of these protocols, and it has become invaluable in the industrial systems that automate many production processes.

DNP3 Defined

The Distributed Network Protocol, or simply DNP3, is a communication protocol used in industrial automation systems. Specifically, it is used to allow the various components of a SCADA system to easily exchange information amongst one another. For example, DNP3 can be used by SCADA master stations to send instructions to field equipment. In order for this exchange of information to occur, however, DNP3 utilizes three distinct layers.

DNP3 Layers

Like all networking and communication protocols, the DNP3 is comprised of several layers. These include the link layer, transport layer, and the application layer.

Link Layer

The DNP3 link layer is responsible for creating and managing the logical link between the sender and receiver of data. The DNP3 link layer adds two special bytes at the start of the data packet which distinguish it as being DNP3. Additionally, the link layer adds a one-byte length field which is used to inform the receiver of the total length of the data packet. In order to identify the sender and receiver, the link layer also includes two additional bytes with that information. Lastly, the link layer adds a one-byte control field and a two-byte header to the data packet.

Transport Layer

The DNP3 transport layer is used to break apart large DNP3 data link packets and reassemble them into application layer packets. Once broken apart, each segment will contain a maximum of 16 bytes, which the transport layer combines into an 8-bit control header and 16-bit CRC separators. A CRC, or cyclic redundancy check, is specialized code which is designed to detect when raw data has been damaged during transit. Additionally, the transport layer denotes which segments are the first and final ones in the sequence by including a FIR (First) or FIN (Final) bit in the corresponding segment.

Application Layer

The DNP3 application layer is tasked with providing the functionality necessary for the data packet to be transferred and processed. Examples of these functionalities include time synchronization, data group/class assignments, and time stamping. To accomplish this, the application layer is comprised of a one-byte Function Code, which denotes the function that will be performed by the receiver (Confirm, Read, Write, etc.). Additionally, the application layer adds a one-byte Application Control field, which is tasked with indicating whether the packet is a confirmation of a previous message or whether it was unsolicited. In data packets that contain a Response function, the application layer will also include a two-byte Internal Indications field.

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