In this five-part series of articles we are going to discuss client/server applications and the different protocols involved with them. We are also going to look extensively at parts of the TCP/IP protocol which are critical to these kinds of applications. Then we are going to discuss how and what tools to use when translating these protocols into code.
Client/Server Applications Made Easy - Error and Flow Control (Page 4 of 4 )
So how does TCP/IP actually make sure that a request made by a client application gets to its destination? Through sequence numbers and acknowledgment messages, TCP ensures that your packet is delivered to the destination computer, and at the same time provides delivery information about the packet to the sending computer. You can view the sequence numbers, acknowledgment messages and packet information by using a packet sniffer, if you are interested in seeing how packets (data) are sent by TCP/IP. Here's a snippet of communication between two computers on a network:
|Time | 192.168.0.2 | 192.168.0.1 |
|0.000 | PSH, ACK - Len: 740 |Seq = 0 Ack = 0
| |(1071) ------------------> (2869) |
|0.006 | PSH, ACK - Len: 204 |Seq = 0 Ack = 740
| |(1071) <------------------ (2869) |
|0.006 | PSH, ACK - Len: 958 |Seq = 204 Ack = 740
| |(1071) <------------------ (2869) |
|0.006 | ACK | |Seq = 740 Ack = 1162
| |(1071) ------------------> (2869) |
The above is a conversation between two computers, 192.168 ....1 and 192.168...2
Messages are sent between the two computers through Ack(nowledge)ments and Seq(uences) of numbers.
Conclusion
In the next article we are going to continue discussing TCP/IP. There is only one more part to discuss, which is "Application Support." In that section we will discuss how TCP/IP maps incoming packets to the appropriate server application. We will also look at the text-based intercept of the example shown in this article.
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Delphi-Kylix 
Building a Server Application in Delphi
