Delivery Confirmation, No Errors

In Figure 9-6, TCP confirms delivery of each segment using the sequence and acknowledgment number fields. However, the acknowledgment number looks a little funny. Wilma numbered the segments 1, 2, and 3, but notice that Fred sent back an ACK field of 4. The ACK field means that this number is the number of the next segment Fred expects to receive. So, Fred is really meaning, "I got everything up to number 4, so send me 4 next." By implication, Fred got 1, 2, and 3, respectively, because he set the acknowledgment field to the next number4 in this case. The use of the acknowledgment field to imply the next segment that should be received is called forward acknowledgment.

By using the sequence number field in the segments it sends and watching the acknowledgment number field in the segment it receives, Wilma's TCP software can know that all three segments were delivered successfully to Fred. Next, you'll see how Wilma deals with the loss of a segment.