Retain payload behind the scoreboard
04A sender can keep a scoreboard of ranges reported by SACK. This helps it avoid retransmitting received data while it repairs a gap.
ACK 1400 advances the sender’s left window edge over all four ranges. Their payload can now leave this retransmission queue; SACK alone did not free it.
Source for this drawing: RFC 2018 · Read RFC section 5 ↗
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A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
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Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
A SACK flag is bookkeeping; only cumulative acknowledgment permits freeing the payload.
- The sender SHOULD record SACK information for later reference. Section 5 describes one possible implementation: mark queued segments wholly covered by received SACK blocks.
- In that implementation, marked segments are skipped during subsequent retransmission; unmarked segments below the highest SACKed segment are candidates. SACK block boundaries need not match the sender’s segment boundaries.
- The sender MUST NOT discard data until the ordinary TCP Acknowledgment Number acknowledges it. A SACK report is advisory because the receiver may later drop that buffered data.
- Congestion control MUST be preserved. One ACK reporting out-of-order data does not by itself trigger recovery. This drawing assumes a valid recovery trigger and enough congestion/receive-window allowance to send the selected retransmission; RFC 2018 does not define a complete loss-recovery algorithm.
Here the sender has already received ACK 1000 plus SACK [1100,1400). The choice to retransmit A follows the assumed recovery decision, not merely the arrival of that single SACK. SACK neither provides encryption nor authenticates acknowledgments.
Read the RFC section ↗