Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

336 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
TCP/IP, or rather – the TCP/IP protocol suite - is not limited to the TCP and IP
protocols, but consists of a multitude of interrelated protocols that occupy the upper three
layers of the ARPA model. TCP/IP does NOT include the bottom network interface layer,
but depends on it for access to the medium.
As depicted in the following figure (‘Internet frame’), an Internet transmission frame
originating on a specific host (computer) would contain the local network (e.g. Ethernet)
header and trailer applicable to that host. As the message proceeds along the Internet, this
header and trailer could be replaced depending on the type of network on which the packet
finds itself – be that X.25, frame relay or ATM. The IP datagram itself would remain
untouched, unless it has to be fragmented and reassembled along the way.
Figure 12.13
Internet frame
Note: Any Internet-related specification is referenced as a request for comments or RFC.
RFCs can be obtained from various sources on the Internet such as www.rfc-editor.org.
The Internet layer
This layer is primarily responsible for the routing of packets from one host to another.
Each packet contains the address information needed for its routing through the
internetwork to the destination host. The dominant protocol at this level is the Internet
protocol (IP).
There are, however, several other additional protocols required at this level such as:
• Address resolution protocol (ARP), RFC 826. This is used for the translation
of an IP address to a hardware (MAC) address, such as required by Ethernet
• Reverse address resolution protocol (RARP), RFC 903. This is the complement
of ARP and translates a hardware address to an IP address
• Internet control message protocol (ICMP), RFC 792. This is a protocol used
for exchanging control or error messages between routers or hosts
The host-to-host layer
This layer is primarily responsible for data integrity between the sender host and receiver
host regardless of the path or distance used to convey the message. It has two protocols
associated with it, namely:
• User data protocol (UDP), a connectionless (unreliable) protocol used for
higher layer port addressing with minimal protocol overhead (RFC 768)