tcp: add rfc3168, section 6.1.1.1. fallback

This work as a follow-up of commit f7b3bec6f516 ("net: allow setting ecn
via routing table") and adds RFC3168 section 6.1.1.1. fallback for outgoing
ECN connections. In other words, this work adds a retry with a non-ECN
setup SYN packet, as suggested from the RFC on the first timeout:

[...] A host that receives no reply to an ECN-setup SYN within the
normal SYN retransmission timeout interval MAY resend the SYN and
any subsequent SYN retransmissions with CWR and ECE cleared. [...]

Schematic client-side view when assuming the server is in tcp_ecn=2 mode,
that is, Linux default since 2009 via commit 255cac91c3c9 ("tcp: extend
ECN sysctl to allow server-side only ECN"):

1) Normal ECN-capable path:

SYN ECE CWR ----->
<----- SYN ACK ECE
ACK ----->

2) Path with broken middlebox, when client has fallback:

SYN ECE CWR ----X crappy middlebox drops packet
(timeout, rtx)
SYN ----->
<----- SYN ACK
ACK ----->

In case we would not have the fallback implemented, the middlebox drop
point would basically end up as:

SYN ECE CWR ----X crappy middlebox drops packet
(timeout, rtx)
SYN ECE CWR ----X crappy middlebox drops packet
(timeout, rtx)
SYN ECE CWR ----X crappy middlebox drops packet
(timeout, rtx)

In any case, it's rather a smaller percentage of sites where there would
occur such additional setup latency: it was found in end of 2014 that ~56%
of IPv4 and 65% of IPv6 servers of Alexa 1 million list would negotiate
ECN (aka tcp_ecn=2 default), 0.42% of these webservers will fail to connect
when trying to negotiate with ECN (tcp_ecn=1) due to timeouts, which the
fallback would mitigate with a slight latency trade-off. Recent related
paper on this topic:

Brian Trammell, Mirja Kühlewind, Damiano Boppart, Iain Learmonth,
Gorry Fairhurst, and Richard Scheffenegger:
"Enabling Internet-Wide Deployment of Explicit Congestion Notification."
Proc. PAM 2015, New York.
http://ecn.ethz.ch/ecn-pam15.pdf

Thus, when net.ipv4.tcp_ecn=1 is being set, the patch will perform RFC3168,
section 6.1.1.1. fallback on timeout. For users explicitly not wanting this
which can be in DC use case, we add a net.ipv4.tcp_ecn_fallback knob that
allows for disabling the fallback.

tp->ecn_flags are not being cleared in tcp_ecn_clear_syn() on output, but
rather we let tcp_ecn_rcv_synack() take that over on input path in case a
SYN ACK ECE was delayed. Thus a spurious SYN retransmission will not prevent
ECN being negotiated eventually in that case.

Reference: https://www.ietf.org/proceedings/92/slides/slides-92-iccrg-1.pdf
Reference: https://www.ietf.org/proceedings/89/slides/slides-89-tsvarea-1.pdf
Signed-off-by: Daniel Borkmann <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Mirja Kühlewind <[e-mail]>
Signed-off-by: Brian Trammell <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Dave That <[e-mail]>
Acked-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/dctcp.txt

···
 To enable it on end hosts:
 
   sysctl -w net.ipv4.tcp_congestion_control=dctcp
+  sysctl -w net.ipv4.tcp_ecn_fallback=0 (optional)
 
 All switches in the data center network running DCTCP must support ECN
 marking and be configured for marking when reaching defined switch buffer

Documentation/networking/ip-sysctl.txt

···
 		  but do not request ECN on outgoing connections.
 	Default: 2
 
+tcp_ecn_fallback - BOOLEAN
+	If the kernel detects that ECN connection misbehaves, enable fall
+	back to non-ECN. Currently, this knob implements the fallback
+	from RFC3168, section 6.1.1.1., but we reserve that in future,
+	additional detection mechanisms could be implemented under this
+	knob. The value	is not used, if tcp_ecn or per route (or congestion
+	control) ECN settings are disabled.
+	Default: 1 (fallback enabled)
+
 tcp_fack - BOOLEAN
 	Enable FACK congestion avoidance and fast retransmission.
 	The value is not used, if tcp_sack is not enabled.

include/net/netns/ipv4.h

···
 	struct local_ports ip_local_ports;
 
 	int sysctl_tcp_ecn;
+	int sysctl_tcp_ecn_fallback;
+
 	int sysctl_ip_no_pmtu_disc;
 	int sysctl_ip_fwd_use_pmtu;
 	int sysctl_ip_nonlocal_bind;

include/net/tcp.h

···
 #define TCPHDR_ECE 0x40
 #define TCPHDR_CWR 0x80
 
+#define TCPHDR_SYN_ECN	(TCPHDR_SYN | TCPHDR_ECE | TCPHDR_CWR)
+
 /* This is what the send packet queuing engine uses to pass
  * TCP per-packet control information to the transmission code.
  * We also store the host-order sequence numbers in here too.

net/ipv4/sysctl_net_ipv4.c

···
 		.proc_handler	= proc_dointvec
 	},
 	{
+		.procname	= "tcp_ecn_fallback",
+		.data		= &init_net.ipv4.sysctl_tcp_ecn_fallback,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec
+	},
+	{
 		.procname	= "ip_local_port_range",
 		.maxlen		= sizeof(init_net.ipv4.ip_local_ports.range),
 		.data		= &init_net.ipv4.ip_local_ports.range,

net/ipv4/tcp_ipv4.c

···
 			goto fail;
 		*per_cpu_ptr(net->ipv4.tcp_sk, cpu) = sk;
 	}
+
 	net->ipv4.sysctl_tcp_ecn = 2;
+	net->ipv4.sysctl_tcp_ecn_fallback = 1;
+
 	net->ipv4.sysctl_tcp_base_mss = TCP_BASE_MSS;
 	net->ipv4.sysctl_tcp_probe_threshold = TCP_PROBE_THRESHOLD;
 	net->ipv4.sysctl_tcp_probe_interval = TCP_PROBE_INTERVAL;
-	return 0;
 
+	return 0;
 fail:
 	tcp_sk_exit(net);
 

net/ipv4/tcp_output.c

···
 	}
 }
 
+static void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
+{
+	if (sock_net(sk)->ipv4.sysctl_tcp_ecn_fallback)
+		/* tp->ecn_flags are cleared at a later point in time when
+		 * SYN ACK is ultimatively being received.
+		 */
+		TCP_SKB_CB(skb)->tcp_flags &= ~(TCPHDR_ECE | TCPHDR_CWR);
+}
+
 static void
 tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th,
 		    struct sock *sk)
···
 			tcp_adjust_pcount(sk, skb, oldpcount - tcp_skb_pcount(skb));
 		}
 	}
+
+	/* RFC3168, section 6.1.1.1. ECN fallback */
+	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
+		tcp_ecn_clear_syn(sk, skb);
 
 	tcp_retrans_try_collapse(sk, skb, cur_mss);