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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Reliability and Flow Control Features and Protocol Modifications

TCP Reliability and Flow Control Features and Protocol Modifications 1 2 3 TCP Non-Contiguous Acknowledgment Handling and Selective Acknowledgment (SACK)

Recognizing When a Segment is Fully Acknowledged

One issue we have yet to discuss is how we know when a segment has been fully acknowledged. Retransmissions are handled on a segment basis, but TCP acknowledgments, as we have seen, are done on a cumulative basis using sequence numbers. Each time a segment is sent by Device A to Device B, B looks at the value of the Acknowledgment Number field in the segment. All bytes with sequence numbers lower than the value of this field have been received by A.

Thus, a segment sent by B to A is considered acknowledged when all of the bytes that were sent in the segment have a lower sequence number than the latest Acknowledgment Number sent by B to A. This is determined by calculating the last sequence number of the segment using its first byte number (in the Sequence Number field) and length of the segment’s Data field.

TCP Reliability and Flow Control Features and Protocol Modifications 1 2 3 TCP Non-Contiguous Acknowledgment Handling and Selective Acknowledgment (SACK)