DNP 3 protocol - what?

[tcpdump 0]

Hi, Does anyone know anything about DNP3.0? 1. Is this a protocol as such, like TCP, UDP, IP ... 2. Is it a framework, e.g. like OSI 3. If it is a protocol, does it live in a layer such as network layer etc... From reading the description on dnp.org, it seems like it a protocol that works at many layers... Can anyone find the actual format of the protocols, in terms of headers and payloads. Why does it pass both the raw and engineering range to the Master Station?

[Mark- 17]

Hello, >Does anyone know anything about DNP3.0? Yes, many people do. ;) >1. Is this a protocol as such, like TCP, UDP, IP ... It is a protocol. This might help you: http://www.dnp.org/About/DNP3%20Primer%20Rev%20A.pdf > 2. Is it a framework, e.g. like OSI DNP has two layers. Well 2.01. It has a link layer and an application layer. It also has a 1 byte 'transport layer.' > 3. If it is a protocol, does it live in a layer such as network layer etc... > From reading the description on dnp.org, it seems like it a protocol that works at many layers... See above answers. >Can anyone find the actual format of the protocols, in terms of headers and payloads. Sure. The specification is published. >Why does it pass both the raw and engineering range to the Master Station? Not sure what you mean by this. There are many different objects in the DNP protocol. Good luck, Mark http://www.peakhmi.com/

[tcpdump 0]

"...DNP has two layers. Well 2.01. It has a link layer and an application layer. It also has a 1 byte 'transport layer.' ..." How can a single protocol have more than one layer? A protocol resides in a layer, such as TCP resides in Transport layer, IP resides in the network layer, ...

[tcpdump 0]

A PLC/RTU communicates with a Master station. The PLC, from the 4mA to 20mA signals, converts this to a raw range (say 0 to 10, 000); then to a scaled range (say 0 to 100 %). If the signal is 12mA, this is a raw value of 5,000 and equivalent to 50%. Is the values 5,000 and 50 both passed to the MAster station in the DNP 3.0 protocol????

[Mark- 17]

Hello, protocol: standard procedure for regulating data transmission between computers. I do not know of any definition of 'protocol' that limits a protocol in any manner. Your second question; it depends on what the out-station publishes. DNP uses the 'objects' model to pass data. All the objects are defined in DNP3 Specification, Volume 6, Part 2, Objects. DNP does not know the connection of the three data points you mentioned. Good luck, Mark http://www.peakhmi.com/ Edited 13 Nov 2009 by Mark-

[tcpdump 0]

Thanks MArk, You have been a great help so far... 1. In terms of encapsulation of the DNP 3 protocol on a SCADA system running on a TCP/IP Ethernet network, Does the the DNP 3 packet get placed into the payload of the TCP segment, then into the payload of the IP packet, then into the Ethernet frame.... I thought the DNP 3 protocl exists at the application layer and also link layer, so is there another layer of encapsulation between the IP packet and the Ethernet frame? 2. If the DNP 3 protocol does not know about the raw value and the scaled value, how is this information passed to the MAster station for display? Thanks, again Mark. Regards, tcpdump.

[Mark- 17]

Hello, Your welcome. > 1. In terms of encapsulation of the DNP 3 protocol on a SCADA system runni... It seems this is becoming much more complicated then it is. The application layer, link layer and one octet of transport layer are part of DNP regardless of any other protocol. DNP does not know anything about protocols that transport DNP information and the transport protocols do not need to know anything about DNP. Look at it as a train made up of many cars. The cars are connected and go from point A to point B based on the rules of the train. DNP puts information in the cars in a defined order (the protocol). How DNP segments that data does not concern the train and its cars. For example: HTTP, FTP, STMP are all protocols that can be implemented and TCP can transport the data. TCP does not know anything about the protocols it carriers. TCP does not know what physical media carriers its packets and what its rules are. From the bottom to the top, all the layers ‘wrap’ the data and generally no nothing of the data they ‘wrap’. > If the DNP 3 protocol does not know about the raw value and the scaled value, how is > this information passed to the Master station for display? DNP does not know what the data represents or how it relates to other data and it does not know what the master is going to do with the data. DNP is a protocol and only specifies how the data is encapsulated. From the primer document: Protocols define the rules by which devices talk with each other, and DNP3 is a protocol for transmission of data from point A to point B using serial and IP communications. DNP3 provides the rules for remotely located computers and master station computers to communicate data and control commands. As you see it uses the word ‘data’. DNP does not know what the data represents only its size and in the case of events the time. Good luck, Mark http://www.peakhmi.com/

[tcpdump 0]

I think this is getting clearer now. So in terms of the OSI REference model, DNP 3 is an application layer protocol? The meaning of the DNP transport, link layer and application layers, are all of these DNP specific? i.e. not related to the OSI definitions of transport, link layer and application?

[Mark- 17]

Hello, > I think this is getting clearer now. Good. > So in terms of the OSI Reference model, DNP 3 is an application layer protocol? Except DNP has and uses an application, link layer and a one byte transport layer. ;) >The meaning of the DNP transport, link layer and application layers, are all of these DNP specific? i.e. not related to the OSI definitions of transport, link layer and application? No. The application layer and link layer perform the same task. The transport layer is a bit different. From the spec: “The ISO (International Organization for Standardization) defines a communication architecture that separates functions into seven layers called the Open System Interconnection (OSI) reference model. DNP3 protocol is based upon a simplified model termed the Enhanced Performance Architecture (EPA) that consists of only three layers: Application, Data Link and Physical.” Using the OSI model, many protocols use 1 or more layers. Ethernet/IP uses all 7. MODBUS uses 1. Good luck, Mark http://www.peakhmi.com/