TCP Error-Recovery Process

The process is identical to Figure 9-4, but Figure 9-7 shows the TCP header field values. Fred gets segment 1 and 3 successfully, so by implication, he knows that Wilma sent segment 2, but it either didn't make it to Fred, or it had errors in it. So, Fred sends a segment back to Wilma, with ACK set to 2. With forward acknowledgement logic, Wilma knows that Fred expects to get segment 2 next, implying that he got number 1, but not number 2.

In response, Wilma sends segment 2 again. To be able to re-send segment 2, Wilma must have kept a copy of each segment. After a segment has been acknowledged, Wilma can release the memory to be used for other purposes. However, by keeping a copy, Wilma could re-send any lost segments.

Notice that Wilma re-sent segment 2 only, allowing Fred time to send back a segment with ACK of 4. By doing so, Fred acknowledged receipt of the re-sent segment 2, and well as the earlier receipt of segment 3.