--- c3a9f033ef3456c5d910d77d26f60b34d77590c4
+++ 681429987f61ab1ac7f7bca6ee376adc96783270
@@ -2843,9 +2843,13 @@ static int tcp_try_undo_loss(struct sock
 static inline void tcp_complete_cwr(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
-	/* Do not moderate cwnd if it's already undone in cwr or recovery */
-	if (tp->undo_marker && tp->snd_cwnd > tp->snd_ssthresh) {
-		tp->snd_cwnd = tp->snd_ssthresh;
+
+	/* Do not moderate cwnd if it's already undone in cwr or recovery. */
+	if (tp->undo_marker) {
+		if (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR)
+			tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_ssthresh);
+		else /* PRR */
+			tp->snd_cwnd = tp->snd_ssthresh;
 		tp->snd_cwnd_stamp = tcp_time_stamp;
 	}
 	tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
@@ -2963,6 +2967,38 @@ void tcp_simple_retransmit(struct sock *
 }
 EXPORT_SYMBOL(tcp_simple_retransmit);
 
+/* This function implements the PRR algorithm, specifcally the PRR-SSRB
+ * (proportional rate reduction with slow start reduction bound) as described in
+ * http://www.ietf.org/id/draft-mathis-tcpm-proportional-rate-reduction-01.txt.
+ * It computes the number of packets to send (sndcnt) based on packets newly
+ * delivered:
+ *   1) If the packets in flight is larger than ssthresh, PRR spreads the
+ *	cwnd reductions across a full RTT.
+ *   2) If packets in flight is lower than ssthresh (such as due to excess
+ *	losses and/or application stalls), do not perform any further cwnd
+ *	reductions, but instead slow start up to ssthresh.
+ */
+static void tcp_update_cwnd_in_recovery(struct sock *sk, int newly_acked_sacked,
+					int fast_rexmit, int flag)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	int sndcnt = 0;
+	int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
+
+	if (tcp_packets_in_flight(tp) > tp->snd_ssthresh) {
+		u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
+			       tp->prior_cwnd - 1;
+		sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out;
+	} else {
+		sndcnt = min_t(int, delta,
+			       max_t(int, tp->prr_delivered - tp->prr_out,
+				     newly_acked_sacked) + 1);
+	}
+
+	sndcnt = max(sndcnt, (fast_rexmit ? 1 : 0));
+	tp->snd_cwnd = tcp_packets_in_flight(tp) + sndcnt;
+}
+
 /* Process an event, which can update packets-in-flight not trivially.
  * Main goal of this function is to calculate new estimate for left_out,
  * taking into account both packets sitting in receiver's buffer and
@@ -2974,7 +3010,8 @@ EXPORT_SYMBOL(tcp_simple_retransmit);
  * It does _not_ decide what to send, it is made in function
  * tcp_xmit_retransmit_queue().
  */
-static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked, int flag)
+static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
+				  int newly_acked_sacked, int flag)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
@@ -3124,13 +3161,17 @@ static void tcp_fastretrans_alert(struct
 
 		tp->bytes_acked = 0;
 		tp->snd_cwnd_cnt = 0;
+		tp->prior_cwnd = tp->snd_cwnd;
+		tp->prr_delivered = 0;
+		tp->prr_out = 0;
 		tcp_set_ca_state(sk, TCP_CA_Recovery);
 		fast_rexmit = 1;
 	}
 
 	if (do_lost || (tcp_is_fack(tp) && tcp_head_timedout(sk)))
 		tcp_update_scoreboard(sk, fast_rexmit);
-	tcp_cwnd_down(sk, flag);
+	tp->prr_delivered += newly_acked_sacked;
+	tcp_update_cwnd_in_recovery(sk, newly_acked_sacked, fast_rexmit, flag);
 	tcp_xmit_retransmit_queue(sk);
 }
 
@@ -3644,6 +3685,8 @@ static int tcp_ack(struct sock *sk, stru
 	u32 prior_in_flight;
 	u32 prior_fackets;
 	int prior_packets;
+	int prior_sacked = tp->sacked_out;
+	int newly_acked_sacked = 0;
 	int frto_cwnd = 0;
 
 	/* If the ack is older than previous acks
@@ -3715,6 +3758,9 @@ static int tcp_ack(struct sock *sk, stru
 	/* See if we can take anything off of the retransmit queue. */
 	flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una);
 
+	newly_acked_sacked = (prior_packets - prior_sacked) -
+			     (tp->packets_out - tp->sacked_out);
+
 	if (tp->frto_counter)
 		frto_cwnd = tcp_process_frto(sk, flag);
 	/* Guarantee sacktag reordering detection against wrap-arounds */
@@ -3727,7 +3773,7 @@ static int tcp_ack(struct sock *sk, stru
 		    tcp_may_raise_cwnd(sk, flag))
 			tcp_cong_avoid(sk, ack, prior_in_flight);
 		tcp_fastretrans_alert(sk, prior_packets - tp->packets_out,
-				      flag);
+				      newly_acked_sacked, flag);
 	} else {
 		if ((flag & FLAG_DATA_ACKED) && !frto_cwnd)
 			tcp_cong_avoid(sk, ack, prior_in_flight);