Practical TCP/IP and Ethernet Networking for Industry

Host-to-host (transport) layer protocols 127

Figure 7.3

TCP connection establishment

As mentioned before, TCP generates pseudo-random sequence numbers by means of a

32-bit software counter that resets at boot-up and then increments every 4 microseconds.

The host establishing the connection reads a value ‘x’ from the counter where x can vary

between 0 and 2

32

–1) and inserts it in the sequence number field. It then sets the SYN

flag = 1 and transmits the header (no data yet) to the appropriate IP address and port

number. Assuming that the chosen sequence number was 132, this action would then be

abbreviated as SYN 132.

The receiving host (e.g. ...