The DNP3 protocol was developed by Westronic based on the early versions of the IEC 60870-5 standard telecontrol protocol specifications. Now the protocol specification is controlled by the DNP3 Users Group at http://www.dnp.org.
The ISO/OSI based model supported by this protocol specifies physical, data link and application layers only. This reduced protocol stack is referred to as EPA. However, to support advanced RTU functions and messages larger than the maximum frame length as defined by the IEC document 60870-5-1, the DNP3 data link is intended to be used with a transport pseudo-layer. As a minimum, this transport layer implements message assembly and disassembly services.