Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

348 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
Sending a UDP datagram involves very little overhead in that there are no synchronization
parameters, no priority options, no sequence numbers, no timers, and no retransmission of
packets. The header is small, the protocol is streamlined functionally. The only major
drawback is that delivery is not guaranteed. UDP is therefore used for communications
that involve broadcasts, for general network announcements, or for real-time data.
The UDP header
The UDP header is significantly smaller than the TCP header and only contains four
fields.
Figure 12.20
UDP header format
UDP source port (16 bits) is an optional field. When meaningful, it indicates the port
of the sending process, and may be assumed to be the port to which a reply should be
addressed in the absence of any other information. If not used, a value of zero is inserted.
UDP destination port has the same meaning as for the TCP header, and indicates the
process on the destination host to which the data is to be sent. UDP message length is the
length in bytes of the datagram including the header and the data.
The 16-bit UDP checksum is used for validation purposes.