tcp: make the first N SYN RTO backoffs linear

Currently the SYN RTO schedule follows an exponential backoff
scheme, which can be unnecessarily conservative in cases where
there are link failures. In such cases, it's better to
aggressively try to retransmit packets, so it takes routers
less time to find a repath with a working link.

We chose a default value for this sysctl of 4, to follow
the macOS and IOS backoff scheme of 1,1,1,1,1,2,4,8, ...
MacOS and IOS have used this backoff schedule for over
a decade, since before this 2009 IETF presentation
discussed the behavior:
https://www.ietf.org/proceedings/75/slides/tcpm-1.pdf

This commit makes the SYN RTO schedule start with a number of
linear backoffs given by the following sysctl:
* tcp_syn_linear_timeouts

This changes the SYN RTO scheme to be: init_rto_val for
tcp_syn_linear_timeouts, exp backoff starting at init_rto_val

For example if init_rto_val = 1 and tcp_syn_linear_timeouts = 2, our
backoff scheme would be: 1, 1, 1, 2, 4, 8, 16, ...
Signed-off-by: David Morley <morleyd@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Tested-by: David Morley <morleyd@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230509180558.2541885-1-morleyd.kernel@gmail.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/ip-sysctl.rst

@@ -881,9 +881,10 @@ tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs
 tcp_syn_retries - INTEGER
 	Number of times initial SYNs for an active TCP connection attempt
 	will be retransmitted. Should not be higher than 127. Default value
-	is 6, which corresponds to 63seconds till the last retransmission
-	with the current initial RTO of 1second. With this the final timeout
-	for an active TCP connection attempt will happen after 127seconds.
+	is 6, which corresponds to 67seconds (with tcp_syn_linear_timeouts = 4)
+	till the last retransmission with the current initial RTO of 1second.
+	With this the final timeout for an active TCP connection attempt
+	will happen after 131seconds.
 
 tcp_timestamps - INTEGER
 	Enable timestamps as defined in RFC1323.
@@ -946,6 +947,16 @@ tcp_pacing_ca_ratio - INTEGER
 
 	Default: 120
 
+tcp_syn_linear_timeouts - INTEGER
+	The number of times for an active TCP connection to retransmit SYNs with
+	a linear backoff timeout before defaulting to an exponential backoff
+	timeout. This has no effect on SYNACK at the passive TCP side.
+
+	With an initial RTO of 1 and tcp_syn_linear_timeouts = 4 we would
+	expect SYN RTOs to be: 1, 1, 1, 1, 1, 2, 4, ... (4 linear timeouts,
+	and the first exponential backoff using 2^0 * initial_RTO).
+	Default: 4
+
 tcp_tso_win_divisor - INTEGER
 	This allows control over what percentage of the congestion window
 	can be consumed by a single TSO frame.

include/net/netns/ipv4.h

@@ -194,6 +194,7 @@ struct netns_ipv4 {
 	int sysctl_udp_rmem_min;
 
 	u8 sysctl_fib_notify_on_flag_change;
+	u8 sysctl_tcp_syn_linear_timeouts;
 
 #ifdef CONFIG_NET_L3_MASTER_DEV
 	u8 sysctl_udp_l3mdev_accept;

net/ipv4/sysctl_net_ipv4.c

@@ -34,6 +34,7 @@ static int ip_ttl_min = 1;
 static int ip_ttl_max = 255;
 static int tcp_syn_retries_min = 1;
 static int tcp_syn_retries_max = MAX_TCP_SYNCNT;
+static int tcp_syn_linear_timeouts_max = MAX_TCP_SYNCNT;
 static int ip_ping_group_range_min[] = { 0, 0 };
 static int ip_ping_group_range_max[] = { GID_T_MAX, GID_T_MAX };
 static u32 u32_max_div_HZ = UINT_MAX / HZ;
@@ -1470,6 +1471,15 @@ static struct ctl_table ipv4_net_table[] = {
 		.extra1         = SYSCTL_ZERO,
 		.extra2         = &tcp_plb_max_cong_thresh,
 	},
+	{
+		.procname = "tcp_syn_linear_timeouts",
+		.data = &init_net.ipv4.sysctl_tcp_syn_linear_timeouts,
+		.maxlen = sizeof(u8),
+		.mode = 0644,
+		.proc_handler = proc_dou8vec_minmax,
+		.extra1 = SYSCTL_ZERO,
+		.extra2 = &tcp_syn_linear_timeouts_max,
+	},
 	{ }
 };
 

net/ipv4/tcp_ipv4.c

@@ -3275,6 +3275,7 @@ static int __net_init tcp_sk_init(struct net *net)
 	else
 		net->ipv4.tcp_congestion_control = &tcp_reno;
 
+	net->ipv4.sysctl_tcp_syn_linear_timeouts = 4;
 	return 0;
 }
 

net/ipv4/tcp_timer.c

@@ -234,14 +234,19 @@ static int tcp_write_timeout(struct sock *sk)
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct net *net = sock_net(sk);
 	bool expired = false, do_reset;
-	int retry_until;
+	int retry_until, max_retransmits;
 
 	if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
 		if (icsk->icsk_retransmits)
 			__dst_negative_advice(sk);
 		retry_until = icsk->icsk_syn_retries ? :
 			READ_ONCE(net->ipv4.sysctl_tcp_syn_retries);
-		expired = icsk->icsk_retransmits >= retry_until;
+
+		max_retransmits = retry_until;
+		if (sk->sk_state == TCP_SYN_SENT)
+			max_retransmits += READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts);
+
+		expired = icsk->icsk_retransmits >= max_retransmits;
 	} else {
 		if (retransmits_timed_out(sk, READ_ONCE(net->ipv4.sysctl_tcp_retries1), 0)) {
 			/* Black hole detection */
@@ -577,8 +582,12 @@ void tcp_retransmit_timer(struct sock *sk)
 	    icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
 		icsk->icsk_backoff = 0;
 		icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
-	} else {
-		/* Use normal (exponential) backoff */
+	} else if (sk->sk_state != TCP_SYN_SENT ||
+		   icsk->icsk_backoff >
+		   READ_ONCE(net->ipv4.sysctl_tcp_syn_linear_timeouts)) {
+		/* Use normal (exponential) backoff unless linear timeouts are
+		 * activated.
+		 */
 		icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
 	}
 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,