[PATCH 6.18 295/641] tcp: disable RFC3168 fallback identifier for CC modules

Greg Kroah-Hartman
2026-02-25T06:55:57Z

6.18-stable review patch. If anyone has any objections, please let me know.

------------------

From: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>

[ Upstream commit e68c28f22f46ecfdec3656ae785dd8ccbb4d557d ]

When AccECN is not successfully negociated for a TCP flow, it defaults
fallback to classic ECN (RFC3168). However, L4S service will fallback
to non-ECN.

This patch enables congestion control module to control whether it
should not fallback to classic ECN after unsuccessful AccECN negotiation.
A new CA module flag (TCP_CONG_NO_FALLBACK_RFC3168) identifies this
behavior expected by the CA.

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260131222515.8485-6-chia-yu.chang@nokia-bell-labs.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: c5ff6b837159 ("tcp: accecn: handle unexpected AccECN negotiation feedback")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/net/tcp.h | 12 +++++++++++-
include/net/tcp_ecn.h | 11 ++++++++---
net/ipv4/tcp_input.c | 2 +-
net/ipv4/tcp_minisocks.c | 7 ++++---
4 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index c1db8a851243d..3c84d95cdba81 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1194,8 +1194,11 @@ enum tcp_ca_ack_event_flags {
#define TCP_CONG_NEEDS_ACCECN BIT(2)
/* Use ECT(1) instead of ECT(0) while the CA is uninitialized */
#define TCP_CONG_ECT_1_NEGOTIATION BIT(3)
+/* Cannot fallback to RFC3168 during AccECN negotiation */
+#define TCP_CONG_NO_FALLBACK_RFC3168 BIT(4)
#define TCP_CONG_MASK (TCP_CONG_NON_RESTRICTED | TCP_CONG_NEEDS_ECN | \
- TCP_CONG_NEEDS_ACCECN | TCP_CONG_ECT_1_NEGOTIATION)
+ TCP_CONG_NEEDS_ACCECN | TCP_CONG_ECT_1_NEGOTIATION | \
+ TCP_CONG_NO_FALLBACK_RFC3168)

union tcp_cc_info;

@@ -1341,6 +1344,13 @@ static inline bool tcp_ca_ect_1_negotiation(const struct sock *sk)
return icsk->icsk_ca_ops->flags & TCP_CONG_ECT_1_NEGOTIATION;
}

+static inline bool tcp_ca_no_fallback_rfc3168(const struct sock *sk)
+{
+ const struct inet_connection_sock *icsk = inet_csk(sk);
+
+ return icsk->icsk_ca_ops->flags & TCP_CONG_NO_FALLBACK_RFC3168;
+}
+
static inline void tcp_ca_event(struct sock *sk, const enum tcp_ca_event event)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
index fdde1c342b35c..2e1637edf1d3c 100644
--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h
@@ -507,7 +507,9 @@ static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct sk_buff *skb
* | ECN | AccECN | 0 0 1 | Classic ECN |
* +========+========+============+=============+
*/
- if (tcp_ecn_mode_pending(tp))
+ if (tcp_ca_no_fallback_rfc3168(sk))
+ tcp_ecn_mode_set(tp, TCP_ECN_DISABLED);
+ else if (tcp_ecn_mode_pending(tp))
/* Downgrade from AccECN, or requested initially */
tcp_ecn_mode_set(tp, TCP_ECN_MODE_RFC3168);
break;
@@ -531,9 +533,11 @@ static inline void tcp_ecn_rcv_synack(struct sock *sk, const struct sk_buff *skb
}
}

-static inline void tcp_ecn_rcv_syn(struct tcp_sock *tp, const struct tcphdr *th,
+static inline void tcp_ecn_rcv_syn(struct sock *sk, const struct tcphdr *th,
const struct sk_buff *skb)
{
+ struct tcp_sock *tp = tcp_sk(sk);
+
if (tcp_ecn_mode_pending(tp)) {
if (!tcp_accecn_syn_requested(th)) {
/* Downgrade to classic ECN feedback */
@@ -545,7 +549,8 @@ static inline void tcp_ecn_rcv_syn(struct tcp_sock *tp, const struct tcphdr *th,
tcp_ecn_mode_set(tp, TCP_ECN_MODE_ACCECN);
}
}
- if (tcp_ecn_mode_rfc3168(tp) && (!th->ece || !th->cwr))
+ if (tcp_ecn_mode_rfc3168(tp) &&
+ (!th->ece || !th->cwr || tcp_ca_no_fallback_rfc3168(sk)))
tcp_ecn_mode_set(tp, TCP_ECN_DISABLED);
}

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index f920fa44c3d39..ede266463d5d1 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6817,7 +6817,7 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
tp->snd_wl1 = TCP_SKB_CB(skb)->seq;
tp->max_window = tp->snd_wnd;

- tcp_ecn_rcv_syn(tp, th, skb);
+ tcp_ecn_rcv_syn(sk, th, skb);

tcp_mtup_init(sk);
tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 2ec8c6f1cdccc..1fade94813c62 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -488,9 +488,10 @@ static void tcp_ecn_openreq_child(struct sock *sk,
tp->accecn_opt_demand = 1;
tcp_ecn_received_counters_payload(sk, skb);
} else {
- tcp_ecn_mode_set(tp, inet_rsk(req)->ecn_ok ?
- TCP_ECN_MODE_RFC3168 :
- TCP_ECN_DISABLED);
+ if (inet_rsk(req)->ecn_ok && !tcp_ca_no_fallback_rfc3168(sk))
+ tcp_ecn_mode_set(tp, TCP_ECN_MODE_RFC3168);
+ else
+ tcp_ecn_mode_set(tp, TCP_ECN_DISABLED);
}
}

--
2.51.0