Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Ethernet and TCP/IP networks 337
• Transmission control protocol (TCP), a connection-oriented protocol that
offers a very reliable method of transferring a stream of data in byte format
between applications (RFC 793)
The process and application layer
This layer provides the user or application programs with interfaces to the TCP/IP stack.
Protocols at this level include (but are not limited to) file transfer protocol (FTP), trivial
file transfer protocol (TFTP), simple mail transfer protocol (SMTP), telecommunications
network (TELNET), post office protocol (POP3), remote procedure calls (RPC), remote
login (RLOGIN), hypertext transfer protocol (HTTP) and network time protocol (NTP).
12.12.2 Internet layer protocols (packet transport)
This section will deal with the Internet protocol (IP) and the Internet control message
protocol (ICMP).
IP version 4 (IPv4)
IP (RFC 791) is responsible for the delivery of packets (‘datagrams’) between hosts. It is
analogous to the postal system, in that it forwards (routes) and delivers datagrams on the
basis of IP addresses attached to the datagrams, in the same way the postal service would
process a letter based on the postal address. The IP address is a 32-bit entity containing
both the network address (the ‘zip code’) and the host address (the ‘street address’).
IP also breaks up (fragments) datagrams that are too large. This is often necessary
because the LANs and WANs that a datagram may have to traverse on its way to its
destination may have different frame size limitations. For example, Ethernet can handle
1500 bytes but X.25 can handle only 576 bytes. IP on the sending side will fragment
a datagram if necessary, attach an IP header to each fragment, and send them off consecutively.
On the receiving side, IP will again rebuild the original datagram.
The IPv4 header
The IP header is appended to the information that IP accepts from higher-level protocols,
before passing it around the network. This information could, within itself, contain the
headers appended by higher level protocols such as TCP. The header consists of at least
five 32-bit (4 byte) ‘long words’ i.e. 20 bytes total and is made up as follows.
Figure 12.14
IPv4 header