D.5
Date Code 20170927 Instruction Manual SEL-751 Relay
DNP3 Communications
Introduction to DNP3
The DNP3 technical bulletin (DNP Confirmation and Retry Guidelines 9804-
002) on confirmation processes recommends against using data link
confirmations because these processes can add to traffic in situations where
communications are marginal. The increased traffic reduces connection
throughput further, possibly preventing the system from operating properly.
Network Medium Contention
When more than one device requires access to a single (serial) network
medium, you must provide a mechanism to resolve the resulting network
medium contention. For example, unsolicited reporting results in network
medium contention if you do not design your serial network as a star topology
of point-to point connections or use carrier detection on a multidrop network.
To avoid collisions among devices trying to send messages, DNP3 includes a
collision avoidance feature. Before sending a message, a DNP3 device listens
for a carrier signal to verify that no other node is transmitting data. The device
transmits if there is no carrier or waits for a random time before transmitting.
However, if two nodes both detect a lack of carrier at the same instant, these
two nodes could begin simultaneous transmission of data and cause a data
collision. If your serial network allows for spontaneous data transmission
including unsolicited event data transmissions, you also must use application
confirmation to provide a retry mechanism for messages lost as a result of data
collisions.
DNP3 LAN/WAN
Overview
The main process for carrying DNP3 over an Ethernet Network (LAN/WAN)
involves encapsulating the DNP3 data link layer data frames within the
transport layer frames of the Internet Protocol (IP) suite. This allows the IP
stack to deliver the DNP3 data link layer frames to the destination in place of
the original DNP3 physical layer.
The DNP User Group Technical Committee has recommended the following
guidelines for carrying DNP3 over a network:
NOTE: Link layer confirmations are
explicitly disabled for DNP3
LAN/WAN. The IP suite provides a
reliable delivery mechanism, which is
backed up at the application layer by
confirmations when necessary.
➤ DNP3 shall use the IP suite to transport messages over a
LAN/WAN
➤ Ethernet is the recommended physical link, though others may
be used
➤ TCP must be used for WANs
➤ TCP is strongly recommended for LANs
➤ User Datagram Protocol (UDP) may be used for highly reliable
single segment LANs
➤ UDP is necessary if you need broadcast messages
➤ The DNP3 protocol stack shall be retained in full
➤ Link layer confirmations shall be disabled
The Technical Committee has registered a standard port number, 20000, for
DNP3 with the Internet Assigned Numbers Authority (IANA). Use this port
for either TCP or UDP.
TCP/UDP Selection
The Committee recommends the selection of TCP or UDP protocol as per the
guidelines in Table D.4.
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