tcp: properly handle stretch acks in slow start

Slow start now increases cwnd by 1 if an ACK acknowledges some packets,
regardless the number of packets. Consequently slow start performance
is highly dependent on the degree of the stretch ACKs caused by
receiver or network ACK compression mechanisms (e.g., delayed-ACK,
GRO, etc).  But slow start algorithm is to send twice the amount of
packets of packets left so it should process a stretch ACK of degree
N as if N ACKs of degree 1, then exits when cwnd exceeds ssthresh. A
follow up patch will use the remainder of the N (if greater than 1)
to adjust cwnd in the congestion avoidance phase.

In addition this patch retires the experimental limited slow start
(LSS) feature. LSS has multiple drawbacks but questionable benefit. The
fractional cwnd increase in LSS requires a loop in slow start even
though it's rarely used. Configuring such an increase step via a global
sysctl on different BDPS seems hard. Finally and most importantly the
slow start overshoot concern is now better covered by the Hybrid slow
start (hystart) enabled by default.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -267,17 +267,6 @@ tcp_max_orphans - INTEGER
 	more aggressively. Let me to remind again: each orphan eats
 	up to ~64K of unswappable memory.
 
-tcp_max_ssthresh - INTEGER
-	Limited Slow-Start for TCP with large congestion windows (cwnd) defined in
-	RFC3742. Limited slow-start is a mechanism to limit growth of the cwnd
-	on the region where cwnd is larger than tcp_max_ssthresh. TCP increases cwnd
-	by at most tcp_max_ssthresh segments, and by at least tcp_max_ssthresh/2
-	segments per RTT when the cwnd is above tcp_max_ssthresh.
-	If TCP connection increased cwnd to thousands (or tens of thousands) segments,
-	and thousands of packets were being dropped during slow-start, you can set
-	tcp_max_ssthresh to improve performance for new TCP connection.
-	Default: 0 (off)
-
 tcp_max_syn_backlog - INTEGER
 	Maximal number of remembered connection requests, which have not
 	received an acknowledgment from connecting client.

include/net/tcp.h

@@ -275,7 +275,6 @@ extern int sysctl_tcp_mtu_probing;
 extern int sysctl_tcp_base_mss;
 extern int sysctl_tcp_workaround_signed_windows;
 extern int sysctl_tcp_slow_start_after_idle;
-extern int sysctl_tcp_max_ssthresh;
 extern int sysctl_tcp_thin_linear_timeouts;
 extern int sysctl_tcp_thin_dupack;
 extern int sysctl_tcp_early_retrans;
@@ -797,7 +796,7 @@ struct tcp_congestion_ops {
 	/* lower bound for congestion window (optional) */
 	u32 (*min_cwnd)(const struct sock *sk);
 	/* do new cwnd calculation (required) */
-	void (*cong_avoid)(struct sock *sk, u32 ack, u32 in_flight);
+	void (*cong_avoid)(struct sock *sk, u32 ack, u32 acked, u32 in_flight);
 	/* call before changing ca_state (optional) */
 	void (*set_state)(struct sock *sk, u8 new_state);
 	/* call when cwnd event occurs (optional) */
@@ -824,12 +823,12 @@ void tcp_get_available_congestion_control(char *buf, size_t len);
 void tcp_get_allowed_congestion_control(char *buf, size_t len);
 int tcp_set_allowed_congestion_control(char *allowed);
 int tcp_set_congestion_control(struct sock *sk, const char *name);
-void tcp_slow_start(struct tcp_sock *tp);
+int tcp_slow_start(struct tcp_sock *tp, u32 acked);
 void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w);
 
 extern struct tcp_congestion_ops tcp_init_congestion_ops;
 u32 tcp_reno_ssthresh(struct sock *sk);
-void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight);
+void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked, u32 in_flight);
 u32 tcp_reno_min_cwnd(const struct sock *sk);
 extern struct tcp_congestion_ops tcp_reno;
 

net/ipv4/sysctl_net_ipv4.c

@@ -700,13 +700,6 @@ static struct ctl_table ipv4_table[] = {
 		.mode		= 0644,
 		.proc_handler   = proc_allowed_congestion_control,
 	},
-	{
-		.procname	= "tcp_max_ssthresh",
-		.data		= &sysctl_tcp_max_ssthresh,
-		.maxlen		= sizeof(int),
-		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
-	},
 	{
 		.procname       = "tcp_thin_linear_timeouts",
 		.data           = &sysctl_tcp_thin_linear_timeouts,

net/ipv4/tcp_bic.c

@@ -140,7 +140,8 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
 		ca->cnt = 1;
 }
 
-static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+			      u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct bictcp *ca = inet_csk_ca(sk);
@@ -149,7 +150,7 @@ static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 		return;
 
 	if (tp->snd_cwnd <= tp->snd_ssthresh)
-		tcp_slow_start(tp);
+		tcp_slow_start(tp, acked);
 	else {
 		bictcp_update(ca, tp->snd_cwnd);
 		tcp_cong_avoid_ai(tp, ca->cnt);

net/ipv4/tcp_cong.c

@@ -15,8 +15,6 @@
 #include <linux/gfp.h>
 #include <net/tcp.h>
 
-int sysctl_tcp_max_ssthresh = 0;
-
 static DEFINE_SPINLOCK(tcp_cong_list_lock);
 static LIST_HEAD(tcp_cong_list);
 
@@ -299,35 +297,24 @@ bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight)
 }
 EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited);
 
-/*
+/* Slow start is used when congestion window is no greater than the slow start
- * Slow start is used when congestion window is less than slow start
+ * threshold. We base on RFC2581 and also handle stretch ACKs properly.
- * threshold. This version implements the basic RFC2581 version
+ * We do not implement RFC3465 Appropriate Byte Counting (ABC) per se but
- * and optionally supports:
+ * something better;) a packet is only considered (s)acked in its entirety to
- * 	RFC3742 Limited Slow Start  	  - growth limited to max_ssthresh
+ * defend the ACK attacks described in the RFC. Slow start processes a stretch
- *	RFC3465 Appropriate Byte Counting - growth limited by bytes acknowledged
+ * ACK of degree N as if N acks of degree 1 are received back to back except
+ * ABC caps N to 2. Slow start exits when cwnd grows over ssthresh and
+ * returns the leftover acks to adjust cwnd in congestion avoidance mode.
  */
-void tcp_slow_start(struct tcp_sock *tp)
+int tcp_slow_start(struct tcp_sock *tp, u32 acked)
 {
-	int cnt; /* increase in packets */
+	u32 cwnd = tp->snd_cwnd + acked;
-	unsigned int delta = 0;
-	u32 snd_cwnd = tp->snd_cwnd;
 
-	if (unlikely(!snd_cwnd)) {
+	if (cwnd > tp->snd_ssthresh)
-		pr_err_once("snd_cwnd is nul, please report this bug.\n");
+		cwnd = tp->snd_ssthresh + 1;
-		snd_cwnd = 1U;
+	acked -= cwnd - tp->snd_cwnd;
-	}
+	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);
-
+	return acked;
-	if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)
-		cnt = sysctl_tcp_max_ssthresh >> 1;	/* limited slow start */
-	else
-		cnt = snd_cwnd;				/* exponential increase */
-
-	tp->snd_cwnd_cnt += cnt;
-	while (tp->snd_cwnd_cnt >= snd_cwnd) {
-		tp->snd_cwnd_cnt -= snd_cwnd;
-		delta++;
-	}
-	tp->snd_cwnd = min(snd_cwnd + delta, tp->snd_cwnd_clamp);
 }
 EXPORT_SYMBOL_GPL(tcp_slow_start);
 
@@ -351,7 +338,7 @@ EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);
 /* This is Jacobson's slow start and congestion avoidance.
  * SIGCOMM '88, p. 328.
  */
-void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked, u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
@@ -360,7 +347,7 @@ void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 
 	/* In "safe" area, increase. */
 	if (tp->snd_cwnd <= tp->snd_ssthresh)
-		tcp_slow_start(tp);
+		tcp_slow_start(tp, acked);
 	/* In dangerous area, increase slowly. */
 	else
 		tcp_cong_avoid_ai(tp, tp->snd_cwnd);

net/ipv4/tcp_cubic.c

@@ -304,7 +304,8 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
 		ca->cnt = 1;
 }
 
-static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+			      u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct bictcp *ca = inet_csk_ca(sk);
@@ -315,7 +316,7 @@ static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 	if (tp->snd_cwnd <= tp->snd_ssthresh) {
 		if (hystart && after(ack, ca->end_seq))
 			bictcp_hystart_reset(sk);
-		tcp_slow_start(tp);
+		tcp_slow_start(tp, acked);
 	} else {
 		bictcp_update(ca, tp->snd_cwnd);
 		tcp_cong_avoid_ai(tp, ca->cnt);

net/ipv4/tcp_highspeed.c

@@ -109,7 +109,7 @@ static void hstcp_init(struct sock *sk)
 	tp->snd_cwnd_clamp = min_t(u32, tp->snd_cwnd_clamp, 0xffffffff/128);
 }
 
-static void hstcp_cong_avoid(struct sock *sk, u32 adk, u32 in_flight)
+static void hstcp_cong_avoid(struct sock *sk, u32 ack, u32 acked, u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct hstcp *ca = inet_csk_ca(sk);
@@ -118,7 +118,7 @@ static void hstcp_cong_avoid(struct sock *sk, u32 adk, u32 in_flight)
 		return;
 
 	if (tp->snd_cwnd <= tp->snd_ssthresh)
-		tcp_slow_start(tp);
+		tcp_slow_start(tp, acked);
 	else {
 		/* Update AIMD parameters.
 		 *

net/ipv4/tcp_htcp.c

@@ -227,7 +227,7 @@ static u32 htcp_recalc_ssthresh(struct sock *sk)
 	return max((tp->snd_cwnd * ca->beta) >> 7, 2U);
 }
 
-static void htcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void htcp_cong_avoid(struct sock *sk, u32 ack, u32 acked, u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct htcp *ca = inet_csk_ca(sk);
@@ -236,7 +236,7 @@ static void htcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 		return;
 
 	if (tp->snd_cwnd <= tp->snd_ssthresh)
-		tcp_slow_start(tp);
+		tcp_slow_start(tp, acked);
 	else {
 		/* In dangerous area, increase slowly.
 		 * In theory this is tp->snd_cwnd += alpha / tp->snd_cwnd

net/ipv4/tcp_hybla.c

@@ -85,7 +85,8 @@ static inline u32 hybla_fraction(u32 odds)
  *     o Give cwnd a new value based on the model proposed
  *     o remember increments <1
  */
-static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+			     u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct hybla *ca = inet_csk_ca(sk);
@@ -102,7 +103,7 @@ static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 		return;
 
 	if (!ca->hybla_en) {
-		tcp_reno_cong_avoid(sk, ack, in_flight);
+		tcp_reno_cong_avoid(sk, ack, acked, in_flight);
 		return;
 	}
 

net/ipv4/tcp_illinois.c

@@ -256,7 +256,8 @@ static void tcp_illinois_state(struct sock *sk, u8 new_state)
 /*
  * Increase window in response to successful acknowledgment.
  */
-static void tcp_illinois_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void tcp_illinois_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+				    u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct illinois *ca = inet_csk_ca(sk);
@@ -270,7 +271,7 @@ static void tcp_illinois_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 
 	/* In slow start */
 	if (tp->snd_cwnd <= tp->snd_ssthresh)
-		tcp_slow_start(tp);
+		tcp_slow_start(tp, acked);
 
 	else {
 		u32 delta;

net/ipv4/tcp_input.c

@@ -2934,10 +2934,10 @@ static void tcp_synack_rtt_meas(struct sock *sk, const u32 synack_stamp)
 		tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, seq_rtt, -1);
 }
 
-static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 acked, u32 in_flight)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);
-	icsk->icsk_ca_ops->cong_avoid(sk, ack, in_flight);
+	icsk->icsk_ca_ops->cong_avoid(sk, ack, acked, in_flight);
 	tcp_sk(sk)->snd_cwnd_stamp = tcp_time_stamp;
 }
 
@@ -3454,7 +3454,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 
 	/* Advance cwnd if state allows */
 	if (tcp_may_raise_cwnd(sk, flag))
-		tcp_cong_avoid(sk, ack, prior_in_flight);
+		tcp_cong_avoid(sk, ack, acked, prior_in_flight);
 
 	if (tcp_ack_is_dubious(sk, flag)) {
 		is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));

net/ipv4/tcp_lp.c

@@ -115,12 +115,13 @@ static void tcp_lp_init(struct sock *sk)
  * Will only call newReno CA when away from inference.
  * From TCP-LP's paper, this will be handled in additive increasement.
  */
-static void tcp_lp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void tcp_lp_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+			      u32 in_flight)
 {
 	struct lp *lp = inet_csk_ca(sk);
 
 	if (!(lp->flag & LP_WITHIN_INF))
-		tcp_reno_cong_avoid(sk, ack, in_flight);
+		tcp_reno_cong_avoid(sk, ack, acked, in_flight);
 }
 
 /**

net/ipv4/tcp_scalable.c

@@ -15,7 +15,8 @@
 #define TCP_SCALABLE_AI_CNT	50U
 #define TCP_SCALABLE_MD_SCALE	3
 
-static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+				    u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
@@ -23,7 +24,7 @@ static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 		return;
 
 	if (tp->snd_cwnd <= tp->snd_ssthresh)
-		tcp_slow_start(tp);
+		tcp_slow_start(tp, acked);
 	else
 		tcp_cong_avoid_ai(tp, min(tp->snd_cwnd, TCP_SCALABLE_AI_CNT));
 }

net/ipv4/tcp_vegas.c

@@ -163,13 +163,14 @@ static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp)
 	return  min(tp->snd_ssthresh, tp->snd_cwnd-1);
 }
 
-static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+				 u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct vegas *vegas = inet_csk_ca(sk);
 
 	if (!vegas->doing_vegas_now) {
-		tcp_reno_cong_avoid(sk, ack, in_flight);
+		tcp_reno_cong_avoid(sk, ack, acked, in_flight);
 		return;
 	}
 
@@ -194,7 +195,7 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 			/* We don't have enough RTT samples to do the Vegas
 			 * calculation, so we'll behave like Reno.
 			 */
-			tcp_reno_cong_avoid(sk, ack, in_flight);
+			tcp_reno_cong_avoid(sk, ack, acked, in_flight);
 		} else {
 			u32 rtt, diff;
 			u64 target_cwnd;
@@ -243,7 +244,7 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 
 			} else if (tp->snd_cwnd <= tp->snd_ssthresh) {
 				/* Slow start.  */
-				tcp_slow_start(tp);
+				tcp_slow_start(tp, acked);
 			} else {
 				/* Congestion avoidance. */
 
@@ -283,7 +284,7 @@ static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 	}
 	/* Use normal slow start */
 	else if (tp->snd_cwnd <= tp->snd_ssthresh)
-		tcp_slow_start(tp);
+		tcp_slow_start(tp, acked);
 
 }
 

net/ipv4/tcp_veno.c

@@ -114,13 +114,14 @@ static void tcp_veno_cwnd_event(struct sock *sk, enum tcp_ca_event event)
 		tcp_veno_init(sk);
 }
 
-static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+				u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct veno *veno = inet_csk_ca(sk);
 
 	if (!veno->doing_veno_now) {
-		tcp_reno_cong_avoid(sk, ack, in_flight);
+		tcp_reno_cong_avoid(sk, ack, acked, in_flight);
 		return;
 	}
 
@@ -133,7 +134,7 @@ static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 		/* We don't have enough rtt samples to do the Veno
 		 * calculation, so we'll behave like Reno.
 		 */
-		tcp_reno_cong_avoid(sk, ack, in_flight);
+		tcp_reno_cong_avoid(sk, ack, acked, in_flight);
 	} else {
 		u64 target_cwnd;
 		u32 rtt;
@@ -152,7 +153,7 @@ static void tcp_veno_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 
 		if (tp->snd_cwnd <= tp->snd_ssthresh) {
 			/* Slow start.  */
-			tcp_slow_start(tp);
+			tcp_slow_start(tp, acked);
 		} else {
 			/* Congestion avoidance. */
 			if (veno->diff < beta) {

net/ipv4/tcp_yeah.c

@@ -69,7 +69,8 @@ static void tcp_yeah_pkts_acked(struct sock *sk, u32 pkts_acked, s32 rtt_us)
 	tcp_vegas_pkts_acked(sk, pkts_acked, rtt_us);
 }
 
-static void tcp_yeah_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
+static void tcp_yeah_cong_avoid(struct sock *sk, u32 ack, u32 acked,
+				u32 in_flight)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct yeah *yeah = inet_csk_ca(sk);
@@ -78,7 +79,7 @@ static void tcp_yeah_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 		return;
 
 	if (tp->snd_cwnd <= tp->snd_ssthresh)
-		tcp_slow_start(tp);
+		tcp_slow_start(tp, acked);
 
 	else if (!yeah->doing_reno_now) {
 		/* Scalable */