Reporting ranges beyond the cumulative frontier
Cumulative acknowledgment advances the contiguous receive frontier. When later bytes arrive beyond a gap, that one number does not describe the whole queue; SACK adds left and right edges for received noncontiguous ranges.
The left edge is the first sequence number and the right edge is the one after the last byte. Thus [1000,1500) reports 500 bytes. These describe byte sequence space, not necessarily the sender’s original packet boundaries.
References: [1]
Permission differs from a range report
SACK-Permitted in SYN signals ability to receive and process SACK, while later SACK options carry actual ranges. Limited option space means one report need not list every queued block, so senders combine reports into state.
The RFC 6675 recovery method uses a scoreboard of cumulative and selective information to estimate in-flight data, infer loss and choose transmissions. That is a specified recovery algorithm, whereas the SACK option chiefly provides feedback format.
Recovery and congestion constraints remain
SACK is advisory because a receiver can discard previously reported data. A sender must retain retransmission data until cumulative acknowledgment advances, and timeout recovery remains relevant.
Reordering alone does not prove loss, nor does one gap report license unlimited retransmission. Applicable loss criteria, receive windows and congestion control still constrain recovery; better information does not remove network limits or all delay.