mptcp: fallback to TCP after SYN+MPC drops

Some middleboxes might be nasty with MPTCP, and decide to drop packets
with MPTCP options, instead of just dropping the MPTCP options (or
letting them pass...).

In this case, it sounds better to fallback to "plain" TCP after 2
retransmissions, and try again.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/477Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20240909-net-next-mptcp-fallback-x-mpc-v1-2-da7ebb4cd2a3@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/mptcp.h

@@ -223,6 +223,8 @@ static inline __be32 mptcp_reset_option(const struct sk_buff *skb)
 
 	return htonl(0u);
 }
+
+void mptcp_active_detect_blackhole(struct sock *sk, bool expired);
 #else
 
 static inline void mptcp_init(void)
@@ -307,6 +309,8 @@ static inline struct request_sock *mptcp_subflow_reqsk_alloc(const struct reques
 }
 
 static inline __be32 mptcp_reset_option(const struct sk_buff *skb)  { return htonl(0u); }
+
+static inline void mptcp_active_detect_blackhole(struct sock *sk, bool expired) { }
 #endif /* CONFIG_MPTCP */
 
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)

net/ipv4/tcp_timer.c

@@ -282,6 +282,7 @@ static int tcp_write_timeout(struct sock *sk)
 		expired = retransmits_timed_out(sk, retry_until,
 						READ_ONCE(icsk->icsk_user_timeout));
 	tcp_fastopen_active_detect_blackhole(sk, expired);
+	mptcp_active_detect_blackhole(sk, expired);
 
 	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RTO_CB_FLAG))
 		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RTO_CB,

net/mptcp/ctrl.c

@@ -12,6 +12,7 @@
 #include <net/netns/generic.h>
 
 #include "protocol.h"
+#include "mib.h"
 
 #define MPTCP_SYSCTL_PATH "net/mptcp"
 
@@ -277,6 +278,25 @@ static void mptcp_pernet_del_table(struct mptcp_pernet *pernet) {}
 
 #endif /* CONFIG_SYSCTL */
 
+/* Check the number of retransmissions, and fallback to TCP if needed */
+void mptcp_active_detect_blackhole(struct sock *ssk, bool expired)
+{
+	struct mptcp_subflow_context *subflow;
+	u32 timeouts;
+
+	if (!sk_is_mptcp(ssk))
+		return;
+
+	timeouts = inet_csk(ssk)->icsk_retransmits;
+	subflow = mptcp_subflow_ctx(ssk);
+
+	if (subflow->request_mptcp && ssk->sk_state == TCP_SYN_SENT &&
+	    (timeouts == 2 || (timeouts < 2 && expired))) {
+		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_MPCAPABLEACTIVEDROP);
+		mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
+	}
+}
+
 static int __net_init mptcp_net_init(struct net *net)
 {
 	struct mptcp_pernet *pernet = mptcp_get_pernet(net);

net/mptcp/mib.c

@@ -15,6 +15,7 @@ static const struct snmp_mib mptcp_snmp_list[] = {
 	SNMP_MIB_ITEM("MPCapableACKRX", MPTCP_MIB_MPCAPABLEPASSIVEACK),
 	SNMP_MIB_ITEM("MPCapableFallbackACK", MPTCP_MIB_MPCAPABLEPASSIVEFALLBACK),
 	SNMP_MIB_ITEM("MPCapableFallbackSYNACK", MPTCP_MIB_MPCAPABLEACTIVEFALLBACK),
+	SNMP_MIB_ITEM("MPCapableSYNTXDrop", MPTCP_MIB_MPCAPABLEACTIVEDROP),
 	SNMP_MIB_ITEM("MPFallbackTokenInit", MPTCP_MIB_TOKENFALLBACKINIT),
 	SNMP_MIB_ITEM("MPTCPRetrans", MPTCP_MIB_RETRANSSEGS),
 	SNMP_MIB_ITEM("MPJoinNoTokenFound", MPTCP_MIB_JOINNOTOKEN),

net/mptcp/mib.h

@@ -10,6 +10,7 @@ enum linux_mptcp_mib_field {
 	MPTCP_MIB_MPCAPABLEPASSIVEACK,	/* Received third ACK with MP_CAPABLE */
 	MPTCP_MIB_MPCAPABLEPASSIVEFALLBACK,/* Server-side fallback during 3-way handshake */
 	MPTCP_MIB_MPCAPABLEACTIVEFALLBACK, /* Client-side fallback during 3-way handshake */
+	MPTCP_MIB_MPCAPABLEACTIVEDROP,	/* Client-side fallback due to a MPC drop */
 	MPTCP_MIB_TOKENFALLBACKINIT,	/* Could not init/allocate token */
 	MPTCP_MIB_RETRANSSEGS,		/* Segments retransmitted at the MPTCP-level */
 	MPTCP_MIB_JOINNOTOKEN,		/* Received MP_JOIN but the token was not found */