- tc->cwnd -= tc->bytes_acked;
- if (tc->bytes_acked > tc->snd_mss)
- tc->bytes_acked += tc->snd_mss;
+ /* RFC 6582 Sec. 3.2 */
+ if (!tcp_opts_sack_permitted (&tc->rcv_opts))
+ {
+ /* Deflate the congestion window by the amount of new data
+ * acknowledged by the Cumulative Acknowledgment field.
+ * If the partial ACK acknowledges at least one SMSS of new data,
+ * then add back SMSS bytes to the congestion window. This
+ * artificially inflates the congestion window in order to reflect
+ * the additional segment that has left the network. This "partial
+ * window deflation" attempts to ensure that, when fast recovery
+ * eventually ends, approximately ssthresh amount of data will be
+ * outstanding in the network.*/
+ tc->cwnd = (tc->cwnd > tc->bytes_acked + tc->snd_mss) ?
+ tc->cwnd - tc->bytes_acked : tc->snd_mss;
+ if (tc->bytes_acked > tc->snd_mss)
+ tc->cwnd += tc->snd_mss;
+ }