RFC 2018, "TCP Selective Acknowledgment Options", October 1996

Errata ID: 6602
Status: Rejected
Type: Technical
Publication Format(s) : TEXT
Reported By: Christian Reusch
Date Reported: 2021-06-07
Rejected by: Martin Duke
Date Rejected: 2022-05-13

Section 5.1 says:

   This document does not attempt to specify in detail the congestion
   control algorithms for implementations of TCP with SACK.  However,
   the congestion control algorithms present in the de facto standard
   TCP implementations MUST be preserved [Stevens94].  In particular, to
   preserve robustness in the presence of packets reordered by the
   network, recovery is not triggered by a single ACK reporting out-of-
   order packets at the receiver.  Further, during recovery the data
   sender limits the number of segments sent in response to each ACK.
   Existing implementations limit the data sender to sending one segment
   during Reno-style fast recovery, or to two segments during slow-start
   [Jacobson88].  Other aspects of congestion control, such as reducing
   the congestion window in response to congestion, must similarly be
   preserved.

It should say:

   This document does not attempt to specify in detail the congestion
   control algorithms for implementations of TCP with SACK.  However,
   the congestion control algorithms present in the de facto standard
   TCP implementations MUST be preserved [Stevens94].  In particular, to
   preserve robustness in the presence of packets reordered by the
   network, recovery is not triggered by a single ACK reporting out-of-
   order packets at the receiver.  Further, during recovery the data
   sender limits the number of segments sent in response to each ACK.
   Existing implementations limit the data sender to sending one segment
   during Reno-style recovery upon a timeout , or to two segments during slow-start
   [Jacobson88].  Other aspects of congestion control, such as reducing
   the congestion window in response to congestion, must similarly be
   preserved.

Notes:

RFC 2581, Section 3.1 sets the cndw to a value of 1:
...
Furthermore, upon a timeout cwnd MUST be set to no more than the loss
window, LW, which equals 1 full-sized segment (regardless of the
value of IW). Therefore, after retransmitting the dropped segment
the TCP sender uses the slow start algorithm to increase the window
from 1 full-sized segment to the new value of ssthresh, at which
point congestion avoidance again takes over.

Whereas in the Fast Recovery section(3.2) the cwnd is set by the following formula:
...
The fast retransmit and fast recovery algorithms are usually
implemented together as follows.

1. When the third duplicate ACK is received, set ssthresh to no more
than the value given in equation 3.
2. Retransmit the lost segment and set cwnd to ssthresh plus 3*SMSS.
This artificially "inflates" the congestion window by the number
of segments (three) that have left the network and which the
receiver has buffered.

3. For each additional duplicate ACK received, increment cwnd by
SMSS. This artificially inflates the congestion window in order
to reflect the additional segment that has left the network.

4. Transmit a segment, if allowed by the new value of cwnd and the
receiver's advertised window.

5. When the next ACK arrives that acknowledges new data, set cwnd to
ssthresh (the value set in step 1). This is termed "deflating"
the window.

This ACK should be the acknowledgment elicited by the
retransmission from step 1, one RTT after the retransmission
(though it may arrive sooner in the presence of significant out-
of-order delivery of data segments at the receiver).
Additionally, this ACK should acknowledge all the intermediate
segments sent between the lost segment and the receipt of the
third duplicate ACK, if none of these were lost.
--VERIFIER NOTES--
The sentence in question is not describing cwnd adjustment. When in Fast Recovery, a duplicate ack indeed allows the transmission of one packet to replace the one that left the network. In Slow Start, an ack allows for two packets: one to replace the old one, and one to fill the newly expanded cwnd.

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