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tcp: disable RFC3168 fallback identifier for CC modules

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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>
This commit is contained in:
Chia-Yu Chang 2026-01-31 23:25:05 +01:00 • committed by Paolo Abeni
parent 100f946b8d
commit e68c28f22f
4 changed files with 24 additions and 8 deletions
Download patch file Download diff file Expand all files Collapse all files

12
include/net/tcp.h
View file

@ -1219,8 +1219,11 @@ 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;
@ -1375,6 +1378,13 @@ 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);

11
include/net/tcp_ecn.h
View file

@ -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);
}

2
net/ipv4/tcp_input.c
View file

@ -7090,7 +7090,7 @@ consume:
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);

7
net/ipv4/tcp_minisocks.c
View file

@ -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);
}
}