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TCP/IP 51
3.3.4 The TCP segment
The data to be sent are transmitted by the application layer to the transport layer. The
transport layer packs the information in the data field and then adds a TCP header. The whole
is then transmitted to the internet layer for further processing. The sending of information
by means of the TCP protocol takes place on the basis of TCP segments.
Figure 3.12: Building of a TCP segment
In order to get a good idea of the functions of the TCP protocol, the TCP header is further
explained. Figure 3.12 shows the different fields within the TCP header. The header consists
of 20 bytes.
• Source, Destination Port: for the different upper-layer applications, TCP is accessible
via different port numbers. Ports is a unique 16-bit address. The combination of a port
and an internet address is called a socket, in accordance with the original definition of
a socket as defined by ARPA (1971). The use of port numbers is essential to build up
a communication between the different applications. This is discussed in further detail
elsewhere in the chapter. This course also includes table 3.6 that gives an overview of
often used ports within the automation.
• Sequence number: Every byte of TCP has a number. The sequence number is the
number of the first data byte in the TCP segment after the TCP header.
• Acknowledgement number: this field contains the next sequence number that is expected
from the partner.
• Header Length: length of the TCP header in 32-bit words
• Code bits: different bits with which a number of statuses can be included.
– the RST bit to initialise the communication again.
– the SYN bit that is used to start a communication