Figure 9-4. Assuring Data Delivery
In Figure 9-4, Wilma's computer is sending three TCP segments to Fred. The second segment experienced some errors during transmission, and Fred noticed that errors had occurred. How did he know errors had occurred? Well, you might recall the frame check sequence (FCS) field in Ethernet that tells you whether something transmitted over a LAN had errors. Although this chapter doesn't consider Ethernet much, imagine that an Ethernet is in use, and that Fred does indeed notice that the frame had an error.
The process is simple: The sender (Wilma) numbered the segments so that if one got lost, Fred could not only know that one segment had errors, but he could ask for the missing numbered segment. So, Fred asks Wilma to resend segment 2. In the next two sections, you'll read about how TCP determines which segments were delivered successfully, and then how TCP notices that segments were lost and need to be recovered.