Interactive applications like Telnet and Character-at-a-time mode are complicated precisely because they are interactive. This means that if you press the B key on your keyboard, the character B is encapsulated in a TCP segment (20+1=21 B), the TCP segment is entered into an IP datagram (20+21=41 B) and this IP datagram then travels over the Internet as segment 1 (see Figure 9.13) until it reaches the server (of course, everything is also entered into link frames from router to router).
Figure 9.13: Press B and Y
The server:
- Confirms receiving the character. In other words, if it does not have any data to send; it sends a non-data segment (40 B), i.e., segment 2.
- Passes character B to the server application for processing. The server application must send character B back (segment 3) so that the client software can display character B on the client's monitor (using remote echo). Echo is necessary to give the user sitting on the client side the feeling that...