Summary

  • RACK asks whether an older outstanding transmission has exceeded a time threshold after a newer transmission is known to have arrived.
  • A bounded, DSACK-informed reordering window protects that inference from ordinary reordering. RFC 8985 permits at most one extra loss probe beyond the congestion-window limit; it counts in flight accounting and the next ACK removes the temporary overcommit, while RTO fallback remains operative.

Traditional TCP loss detection often reasons from duplicate acknowledgments and sequence-space gaps. That evidence remains useful, but packet reordering can make it ambiguous. RFC 8985 specifies RACK-TLP, where RACK uses transmission-time evidence and TLP can send one probe to seek acknowledgment evidence before the ordinary retransmission timeout.

The distinction matters: detecting loss is not the same as receiving permission to transmit. Congestion-control rules, the congestion window, flight accounting, and the retransmission timeout remain separate constraints.

Imagine two transmissions still represented in a TCP sender's state. The receiver reports that the newer transmission arrived, while an older transmission remains unacknowledged. RACK can compare their transmission times. If the older one has remained undelivered beyond an estimated round-trip interval plus a bounded reordering window, the sender has time-based evidence for declaring it lost.

That is a different question from simply counting duplicate acknowledgments. Sequence numbers describe where bytes belong; transmission timestamps help describe when the corresponding transmissions were sent. A later-delivered transmission can therefore establish a temporal reference for judging an older outstanding one.

RACK-TLP thus represents a protocol-design shift in the form of evidence used by loss recovery. Its standardization also makes the boundaries explicit: time can inform a loss judgment, reordering must be accommodated, probing is limited, and established congestion-control and timeout mechanisms still govern the sender.

The governing specification is RFC 8985; the congestion-control, SACK-recovery, and retransmission-timer boundaries are established by RFC 5681, RFC 6675, and RFC 6298. These standards define protocol behavior, not current implementation defaults, adoption, deployment, or performance.