Internet Core Protocols: The Definitive Guide

Delayed acknowledgments

Figure 7.15 earlier in this chapter shows the receiver sending an acknowledgment every time it receives a segment from the sender. However, this is not necessarily an effective use of resources. For one thing, the receiver has to spend CPU cycles on calculating the acknowledgment, as does the sender when it gets the acknowledgment. Furthermore, the use of frequent acknowledgments also generates excessive amounts of network traffic, thereby consuming bandwidth that could otherwise be used by the sender to transmit data.

Rather than acknowledging every segment, it is better for the receiver to only send acknowledgments on a periodic basis. A mechanism called Delayed Acknowledgment is used for this purpose, allowing multiple segments to be acknowledged simultaneously. Remember that acknowledgments are implicit, stating that “all data up to n has been received.” It is therefore possible for a recipient to acknowledge multiple segments simultaneously by simply setting the Acknowledgment Identifier to a higher inclusive value, rather than sending multiple distinct acknowledgments. Not only does this consume less network resources, but it also requires less computational resources on the part of the two endpoints.

This concept is illustrated in Figure 7.16. In that example, the recipient only sends an acknowledgment after receiving two segments. This approach not only generates less traffic, but it also allows the sender to increment their sliding window by two segment ...