From 74d7fc8753a76851db6db556ff0f09de1fbb26d5 Mon Sep 17 00:00:00 2001
From: Richard Scheffenegger <rscheff@FreeBSD.org>
Date: Sat, 19 Jun 2021 19:06:48 +0200
Subject: [PATCH] tcp: Add PRR cwnd reduction for non-SACK loss

This completes PRR cwnd reduction in all circumstances
for the base TCP stack (SACK loss recovery, ECN window reduction,
non-SACK loss recovery), preventing the arriving ACKs to
clock out new data at the old, too high rate. This
reduces the chance to induce additional losses while
recovering from loss (during congested network conditions).

For non-SACK loss recovery, each ACK is assumed to have
one MSS delivered. In order to prevent ACK-split attacks,
only one window worth of ACKs is considered to actually
have delivered new data.

MFC after: 6 weeks
Reviewed By: rrs, #transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29441
---
 sys/netinet/tcp_input.c | 56 ++++++++++++++++++++++++++++++-----------
 1 file changed, 41 insertions(+), 15 deletions(-)

diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
index 1d685b43697..a8fe6d9817d 100644
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -2612,8 +2612,7 @@ tcp_do_segment(struct mbuf *m, struct tcphdr *th, struct socket *so,
 					cc_ack_received(tp, th, nsegs,
 					    CC_DUPACK);
 					if (V_tcp_do_prr &&
-					    IN_FASTRECOVERY(tp->t_flags) &&
-					    (tp->t_flags & TF_SACK_PERMIT)) {
+					    IN_FASTRECOVERY(tp->t_flags)) {
 						tcp_do_prr_ack(tp, th, &to);
 					} else if ((tp->t_flags & TF_SACK_PERMIT) &&
 					    (to.to_flags & TOF_SACK) &&
@@ -2689,8 +2688,16 @@ enter_recovery:
 						 * snd_ssthresh is already updated by
 						 * cc_cong_signal.
 						 */
-						tp->sackhint.prr_delivered =
-						    tp->sackhint.sacked_bytes;
+						if ((tp->t_flags & TF_SACK_PERMIT) &&
+						    (to.to_flags & TOF_SACK)) {
+							tp->sackhint.prr_delivered =
+							    tp->sackhint.sacked_bytes;
+						} else {
+							tp->sackhint.prr_delivered =
+							    imin(tp->snd_max - tp->snd_una,
+							    imin(INT_MAX / 65536,
+								tp->t_dupacks) * maxseg);
+						}
 						tp->sackhint.recover_fs = max(1,
 						    tp->snd_nxt - tp->snd_una);
 					}
@@ -3968,11 +3975,23 @@ tcp_do_prr_ack(struct tcpcb *tp, struct tcphdr *th, struct tcpopt *to)
 	 * (del_data) and an estimate of how many bytes are in the
 	 * network.
 	 */
-	del_data = tp->sackhint.delivered_data;
-	if (V_tcp_do_newsack)
-		pipe = tcp_compute_pipe(tp);
-	else
-		pipe = (tp->snd_nxt - tp->snd_fack) + tp->sackhint.sack_bytes_rexmit;
+	if (((tp->t_flags & TF_SACK_PERMIT) &&
+	    (to->to_flags & TOF_SACK)) ||
+	    (IN_CONGRECOVERY(tp->t_flags) &&
+	     !IN_FASTRECOVERY(tp->t_flags))) {
+		del_data = tp->sackhint.delivered_data;
+		if (V_tcp_do_newsack)
+			pipe = tcp_compute_pipe(tp);
+		else
+			pipe = (tp->snd_nxt - tp->snd_fack) +
+				tp->sackhint.sack_bytes_rexmit;
+	} else {
+		if (tp->sackhint.prr_delivered < (tcprexmtthresh * maxseg +
+					     tp->snd_recover - tp->snd_una))
+			del_data = maxseg;
+		pipe = imax(0, tp->snd_max - tp->snd_una -
+			    imin(INT_MAX / 65536, tp->t_dupacks) * maxseg);
+	}
 	tp->sackhint.prr_delivered += del_data;
 	/*
 	 * Proportional Rate Reduction
@@ -3985,9 +4004,9 @@ tcp_do_prr_ack(struct tcpcb *tp, struct tcphdr *th, struct tcpopt *to)
 			    tp->snd_ssthresh, tp->sackhint.recover_fs) -
 			    tp->sackhint.prr_out;
 	} else {
-		if (V_tcp_do_prr_conservative)
+		if (V_tcp_do_prr_conservative || (del_data == 0))
 			limit = tp->sackhint.prr_delivered -
-			    tp->sackhint.prr_out;
+				tp->sackhint.prr_out;
 		else
 			limit = imax(tp->sackhint.prr_delivered -
 				    tp->sackhint.prr_out, del_data) +
@@ -4001,11 +4020,18 @@ tcp_do_prr_ack(struct tcpcb *tp, struct tcphdr *th, struct tcpopt *to)
 	 * accordingly.
 	 */
 	if (IN_FASTRECOVERY(tp->t_flags)) {
-		tp->snd_cwnd = imax(maxseg, tp->snd_nxt - tp->snd_recover +
-			tp->sackhint.sack_bytes_rexmit + (snd_cnt * maxseg));
+		if ((tp->t_flags & TF_SACK_PERMIT) &&
+		    (to->to_flags & TOF_SACK)) {
+			tp->snd_cwnd = tp->snd_nxt - tp->snd_recover +
+					    tp->sackhint.sack_bytes_rexmit +
+					    (snd_cnt * maxseg);
+		} else {
+			tp->snd_cwnd = (tp->snd_max - tp->snd_una) +
+					    (snd_cnt * maxseg);
+		}
 	} else if (IN_CONGRECOVERY(tp->t_flags))
-		tp->snd_cwnd = imax(maxseg, pipe - del_data +
-				    (snd_cnt * maxseg));
+		tp->snd_cwnd = pipe - del_data + (snd_cnt * maxseg);
+	tp->snd_cwnd = imax(maxseg, tp->snd_cwnd);
 }
 
 /*