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Index: sys/netinet/tcp_input.c
===================================================================
--- sys/netinet/tcp_input.c
+++ sys/netinet/tcp_input.c
@@ -3337,22 +3337,36 @@
tcp_xmit_timer(struct tcpcb *tp, int rtt)
{
int delta;
+ int expected_samples, expected_shift, shift;
INP_WLOCK_ASSERT(tp->t_inpcb);
+ /* RFC 7323 Appendix G RTO Calculation Modification */
+ /* ExpectedSamples = ceiling(FlightSize / (SMSS * 2)) */
+ expected_samples = ((tcp_compute_pipe(tp) + (tp->t_maxseg - 1)) / (tp->t_maxseg << 1));
+ /* alpha' = alpha / ExpectedSamples */
+ expected_shift = min(max(fls(expected_samples + 1) - 1, 0), TCP_DELTA_SHIFT);
+ shift = TCP_DELTA_SHIFT - expected_shift;
TCPSTAT_INC(tcps_rttupdated);
tp->t_rttupdated++;
if (tp->t_srtt != 0) {
/*
- * srtt is stored as fixed point with 5 bits after the
- * binary point (i.e., scaled by 8). The following magic
+ * srtt is stored as fixed point with 16 bits after the
+ * binary (decimal) point (i.e., scaled by 1^16). The following magic
* is equivalent to the smoothing algorithm in rfc793 with
* an alpha of .875 (srtt = rtt/8 + srtt*7/8 in fixed
- * point). Adjust rtt to origin 0.
+ * point) when FlightSize is 1. Adjust rtt to origin 0.
*/
- delta = ((rtt - 1) << TCP_DELTA_SHIFT)
- - (tp->t_srtt >> (TCP_RTT_SHIFT - TCP_DELTA_SHIFT));
+ /* delta = (rtt-1)*4 - (srtt*32)/8 = 4*rtt - 4*srtt - 4 */
+ /* old: tp_srtt += delta => 32*srtt = 32*srtt - 4*srtt + 4*rtt - 4 */
+ /* srtt = 7/8 * srtt + 1/8 * rtt - 1 / 8 */
+ delta = ((rtt - 1) << shift)
+ - (tp->t_srtt >> (TCP_RTT_SHIFT - shift));
+ /*
+ * essential logic unchanged, but this now equates to
+ * srtt = (1/expected_samples)*1/8*rtt + (1 - (1/expect_samples)*7/8)*srtt
+ */
if ((tp->t_srtt += delta) <= 0)
tp->t_srtt = 1;
@@ -3368,7 +3382,7 @@
*/
if (delta < 0)
delta = -delta;
- delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - TCP_DELTA_SHIFT);
+ delta -= tp->t_rttvar >> (TCP_RTTVAR_SHIFT - shift);
if ((tp->t_rttvar += delta) <= 0)
tp->t_rttvar = 1;
if (tp->t_rttbest > tp->t_srtt + tp->t_rttvar)
Index: sys/netinet/tcp_var.h
===================================================================
--- sys/netinet/tcp_var.h
+++ sys/netinet/tcp_var.h
@@ -383,15 +383,15 @@
* are stored as fixed point numbers scaled by the values below.
* For convenience, these scales are also used in smoothing the average
* (smoothed = (1/scale)sample + ((scale-1)/scale)smoothed).
- * With these scales, srtt has 3 bits to the right of the binary point,
- * and thus an "ALPHA" of 0.875. rttvar has 2 bits to the right of the
+ * With these scales, srtt has 13 bits to the right of the binary point,
+ * and thus an "ALPHA" of 0.875. rttvar has 13 bits to the right of the
* binary point, and is smoothed with an ALPHA of 0.75.
*/
-#define TCP_RTT_SCALE 32 /* multiplier for srtt; 3 bits frac. */
-#define TCP_RTT_SHIFT 5 /* shift for srtt; 3 bits frac. */
-#define TCP_RTTVAR_SCALE 16 /* multiplier for rttvar; 2 bits */
-#define TCP_RTTVAR_SHIFT 4 /* shift for rttvar; 2 bits */
-#define TCP_DELTA_SHIFT 2 /* see tcp_input.c */
+#define TCP_RTT_SCALE (1<<16) /* multiplier for srtt; 14 bits frac. */
+#define TCP_RTT_SHIFT 16 /* shift for srtt; 14 bits frac. */
+#define TCP_RTTVAR_SCALE (1<<15) /* multiplier for rttvar; 2 bits */
+#define TCP_RTTVAR_SHIFT 15 /* shift for rttvar; 2 bits */
+#define TCP_DELTA_SHIFT 13 /* see tcp_input.c */
/*
* The initial retransmission should happen at rtt + 4 * rttvar.

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