USING CONNECTIONS

When using the TCP protocol with processes, a connection must be first established and maintained in order to provide for the flow of data. By establishing a connection between two stations, the proper buffer area is provided for the impending data. When station A wants to communicate to station B, it must first establish a connection which allows for the synchronization of sequence numbers and acknowledgments. This process is shown in Figure 3. One of the flags within the TCP header is the SYN bit, which is used to indicate the initial sequence number when a connection is established. This informs the receiver to synchronize its error checking means to this sequence number. Therefore, station A sends a TCP segment with SYN set and its sequence number, which in this case is 10, to station B. Station B responds by sending an acknowledgment with the value 11 to indicate that is the value of the sequence number it expects to receive next. Station B has its own sequence number that it sends to station A. In this case it is 30 which station A acknowledges by sending out a 31. This establishes two connections. Once the connections are established, data is sent from station A to station B with station A incrementing sequence numbers and station B acknowledging them. This is what is called a full-duplex connection since two connections were established; one from A to B and another from B to A. These connections remain established until terminated.
Once the required data transfer has been completed, the two connections should be terminated in order to free up buffer space in the two stations. There is a flag called FIN, which is used to terminate the connection. Station A sends a TCP segment with FIN flag sent along with a sequence number. Station B acknowledges the request and once the acknowledgment is received at Station A, the connection from A to B is terminated. This does not mean that the connection from B to A is terminated. This must be done in a like fashion in order to terminate the full-duplex connection.
Figure 3 — A full-duplex connection must first be established before data can be transferred using TCP.


FLOW CONTROL

Flow control is the management of data transfer between two stations. Depending upon the role of the station, be it client or server, or its processing power, a station may not be able to keep up with the network traffic. In order to slow down events, the TCP header has a field called window. The receiving station sets a value in the window field informing the sender how many bytes of data it will accept. The window is dynamic and the window can be increased as buffer space in the receiver becomes available. The window can also be zero halting transmission. If the sender still needs to communicate important information while in this condition, it can send out a segment with the URG (urgent) flag set along with a sequence number in the urgent pointer field that indicates the first byte of data following the urgent data. The receiver should always allow room for urgent data.
SUMMARY

Although TCP is more reliable than UDP, UDP can be quite effective if the application layer can handle error checking and retransmission. For applications that require secure communication, TCP is the best choice. Both TCP and UDP use port numbers and when used with IP addresses create unique socket definitions across the network. These socket definitions facilitate the processes between the stations on the control network.

References

Illustrated TCP/IP, Matthew Naugle, 1998, Wiley Computer Publishing

TCP/IP Clearly Explained, Pete Loshin, 1997, Academic Press