@@ -108,3 +108,19 @@ stale_loss_cnt - INTEGER
This is a per-namespace sysctl.
Default: 4
+
+syn_retrans_before_tcp_fallback - INTEGER
+ The number of SYN + MP_CAPABLE retransmissions before falling back to
+ TCP, i.e. dropping the MPTCP options. In other words, if all the packets
+ are dropped on the way, there will be:
+
+ * The initial SYN with MPTCP support
+ * This number of SYN retransmitted with MPTCP support
+ * The next SYN retransmissions will be without MPTCP support
+
+ 0 means the first retransmission will be done without MPTCP options.
+ >= 128 means that all SYN retransmissions will keep the MPTCP options. A
+ lower number might increase false-positive MPTCP blackholes detections.
+ This is a per-namespace sysctl.
+
+ Default: 2
@@ -32,6 +32,7 @@ struct mptcp_pernet {
unsigned int close_timeout;
unsigned int stale_loss_cnt;
atomic_t active_disable_times;
+ u8 syn_retrans_before_tcp_fallback;
unsigned long active_disable_stamp;
u8 mptcp_enabled;
u8 checksum_enabled;
@@ -92,6 +93,7 @@ static void mptcp_pernet_set_defaults(struct mptcp_pernet *pernet)
pernet->mptcp_enabled = 1;
pernet->add_addr_timeout = TCP_RTO_MAX;
pernet->blackhole_timeout = 3600;
+ pernet->syn_retrans_before_tcp_fallback = 2;
atomic_set(&pernet->active_disable_times, 0);
pernet->close_timeout = TCP_TIMEWAIT_LEN;
pernet->checksum_enabled = 0;
@@ -245,6 +247,12 @@ static struct ctl_table mptcp_sysctl_table[] = {
.proc_handler = proc_blackhole_detect_timeout,
.extra1 = SYSCTL_ZERO,
},
+ {
+ .procname = "syn_retrans_before_tcp_fallback",
+ .maxlen = sizeof(u8),
+ .mode = 0644,
+ .proc_handler = proc_dou8vec_minmax,
+ },
};
static int mptcp_pernet_new_table(struct net *net, struct mptcp_pernet *pernet)
@@ -269,6 +277,7 @@ static int mptcp_pernet_new_table(struct net *net, struct mptcp_pernet *pernet)
/* table[7] is for available_schedulers which is read-only info */
table[8].data = &pernet->close_timeout;
table[9].data = &pernet->blackhole_timeout;
+ table[10].data = &pernet->syn_retrans_before_tcp_fallback;
hdr = register_net_sysctl_sz(net, MPTCP_SYSCTL_PATH, table,
ARRAY_SIZE(mptcp_sysctl_table));
@@ -392,17 +401,21 @@ void mptcp_active_enable(struct sock *sk)
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) {
- if (timeouts == 2 || (timeouts < 2 && expired)) {
- MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_MPCAPABLEACTIVEDROP);
+ struct net *net = sock_net(ssk);
+ u8 timeouts, to_max;
+
+ timeouts = inet_csk(ssk)->icsk_retransmits;
+ to_max = mptcp_get_pernet(net)->syn_retrans_before_tcp_fallback;
+
+ if (timeouts == to_max || (timeouts < to_max && expired)) {
+ MPTCP_INC_STATS(net, MPTCP_MIB_MPCAPABLEACTIVEDROP);
subflow->mpc_drop = 1;
mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
} else {
The number of SYN + MPC retransmissions before falling back to TCP was fixed to 2. This is certainly a good default value, but having a fixed number can be a problem in some environments. The current behaviour means that if all packets are dropped, there will be: - The initial SYN + MPC - 2 retransmissions with MPC - The next ones will be without MPTCP. So typically ~3 seconds before falling back to TCP. In some networks where some temporally blackholes are unfortunately frequent, or when a client tries to initiate connections while the network is not ready yet, this can cause new connections not to have MPTCP connections. In such environments, it is now possible to increase the number of SYN retransmissions with MPTCP options to make sure MPTCP is used. Interesting values are: - 0: the first retransmission will be done without MPTCP options: quite aggressive, but also a higher risk of detecting false-positive MPTCP blackholes. - >= 128: all SYN retransmissions will keep the MPTCP options: back to the < 6.12 behaviour. The default behaviour is not changed here. Reviewed-by: Mat Martineau <martineau@kernel.org> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> --- Documentation/networking/mptcp-sysctl.rst | 16 ++++++++++++++++ net/mptcp/ctrl.c | 21 +++++++++++++++++---- 2 files changed, 33 insertions(+), 4 deletions(-) --- base-commit: 7d2eba0f83a59d360ed1e77ed2778101a6e3c4a1 change-id: 20250117-net-next-mptcp-syn_retrans_before_tcp_fallback-5f41bbc8482e Best regards,