3.6 Interpreting the SACK Option

The data transmitter is assumed to have a retransmission queue that contains the segments that have been transmitted but not yet acknowledged, in sequence-number order. If the data transmitter performs re-packetization before retransmission, the block boundaries in a SACK option that it receives may not fall on boundaries of segments in the retransmission queue; however, this does not pose a serious difficulty for the transmitter.

Let us suppose that for each segment in the retransmission queue there is a (new) flag bit "ACK'd", to be used to indicate that this particular segment has been entirely acknowledged. When a segment is first transmitted, it will be entered into the retransmission queue with its ACK'd bit off. If the ACK'd bit is subsequently turned on (as the result of processing a received SACK option), the data transmitter will skip this segment during any later retransmission. However, the segment will not be dequeued and its buffer freed until the left window edge is advanced over it.

When an acknowledgment segment arrives containing a SACK option, the data transmitter will turn on the ACK'd bits for segments that have been selectively acknowleged. More specifically, for each block in the SACK option, the data transmitter will turn on the ACK'd flags for all segments in the retransmission queue that are wholly contained within that block. This requires straightforward sequence number comparisons.