Internet Core Protocols: The Definitive Guide

Acknowledgment numbers

Acknowledgment numbers and sequence numbers are closely tied, working together to make sure that segments arrive at their destination.

Just as sequence numbers are used to identify the individual bytes of data being sent in a segment, acknowledgment numbers are used to verify that all of the data in that segment was successfully received. However, rather than pointing to the first byte of data in a segment that has just arrived, the acknowledgment number points to the next byte of data that a recipient expects to receive in the next segment.

This process is illustrated earlier in Figure 7.13. In that example, the sender transmits 100 bytes of data, using a sequence number of 1 to identify the first byte of data in the segment. The receiver returns an acknowledgment for the segment, indicating that it’s ready to accept the next segment (starting at sequence number 101). Notice that the acknowledgment does not point to bytes 1 or 100, but instead points to 101, the next byte that the receiver expects to get.

Tip

In truth, acknowledgment numbers are closer in concept to a flow control throttle rather than being explicit acknowledgments. Rather than saying “I got the data,” they say “I’m ready for more data, starting at byte number n.”

This design is commonly referred to as being "cumulatively implicit,” indicating that all of the data up to (but not including) the acknowledgment number has been received successfully, rather than explicitly acknowledging that a particular ...