[PATCH v12 net-next 05/15] tcp: disable RFC3168 fallback identifier for CC modules
From: <hidden>
Date: 2026-01-28 14:34:56
Also in:
bpf, linux-doc, linux-kselftest
Subsystem:
networking [general], networking [tcp], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Neal Cardwell, Linus Torvalds
From: Chia-Yu Chang <redacted> 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 <redacted> Acked-by: Paolo Abeni <pabeni@redhat.com> Reviewed-by: Eric Dumazet <edumazet@google.com> --- v3: - Add empty line between variable declarations and code. --- 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 ba347b87a63c..7965d4bfb4d7 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h@@ -1208,8 +1208,11 @@ 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;
@@ -1364,6 +1367,13 @@ 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 fdde1c342b35..2e1637edf1d3 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 cbc8e28fe492..aef6f2bbcd0c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c@@ -7015,7 +7015,7 @@ 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 bd5462154f97..9776c921d1bb 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c@@ -485,9 +485,10 @@ 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); } }
--
2.34.1