On Fri, Aug 15, 2025 at 1:39 AM <chia-yu.ch...@nokia-bell-labs.com> wrote:
>
> From: Ilpo Järvinen <i...@kernel.org>
>
> Accurate ECN negotiation parts based on the specification:
> https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt
>
> Accurate ECN is negotiated using ECE, CWR and AE flags in the
> TCP header. TCP falls back into using RFC3168 ECN if one of the
> ends supports only RFC3168-style ECN.
>
> The AccECN negotiation includes reflecting IP ECN field value
> seen in SYN and SYNACK back using the same bits as negotiation
> to allow responding to SYN CE marks and to detect ECN field
> mangling. CE marks should not occur currently because SYN=1
> segments are sent with Non-ECT in IP ECN field (but proposal
> exists to remove this restriction).
>
> Reflecting SYN IP ECN field in SYNACK is relatively simple.
> Reflecting SYNACK IP ECN field in the final/third ACK of
> the handshake is more challenging. Linux TCP code is not well
> prepared for using the final/third ACK a signalling channel
> which makes things somewhat complicated here.
>
> tcp_ecn sysctl can be used to select the highest ECN variant
> (Accurate ECN, ECN, No ECN) that is attemped to be negotiated and
> requested for incoming connection and outgoing connection:
> TCP_ECN_IN_NOECN_OUT_NOECN, TCP_ECN_IN_ECN_OUT_ECN,
> TCP_ECN_IN_ECN_OUT_NOECN, TCP_ECN_IN_ACCECN_OUT_ACCECN,
> TCP_ECN_IN_ACCECN_OUT_ECN, and TCP_ECN_IN_ACCECN_OUT_NOECN.
>
> After this patch, the size of tcp_request_sock remains unchanged
> and no new holes are added. Below are the pahole outcomes before
> and after this patch:
>
>
> Signed-off-by: Ilpo Järvinen <i...@kernel.org>
> Co-developed-by: Olivier Tilmans <olivier.tilm...@nokia.com>
> Signed-off-by: Olivier Tilmans <olivier.tilm...@nokia.com>
> Co-developed-by: Chia-Yu Chang <chia-yu.ch...@nokia-bell-labs.com>
> Signed-off-by: Chia-Yu Chang <chia-yu.ch...@nokia-bell-labs.com>
> Acked-by: Paolo Abeni <pab...@redhat.com>
>
> + if (tp->ecn_flags & TCP_ECN_MODE_ACCECN) {
> + TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_ACE;
> + TCP_SKB_CB(skb)->tcp_flags |=
> + tcp_accecn_reflector_flags(tp->syn_ect_rcv);
> + tp->syn_ect_snt = inet_sk(sk)->tos & INET_ECN_MASK;
> + }
> }
>
> /* Packet ECN state for a SYN. */
> @@ -125,8 +377,20 @@ static inline void tcp_ecn_send_syn(struct sock *sk,
> struct sk_buff *skb)
> {
> struct tcp_sock *tp = tcp_sk(sk);
> bool bpf_needs_ecn = tcp_bpf_ca_needs_ecn(sk);
> - bool use_ecn = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn) == 1 ||
> - tcp_ca_needs_ecn(sk) || bpf_needs_ecn;
> + bool use_ecn, use_accecn;
> + u8 tcp_ecn = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn);
> +
> + /* +================+===========================+
> + * | tcp_ecn values | Outgoing connections |
> + * +================+===========================+
> + * | 0,2,5 | Do not request ECN |
> + * | 1,4 | Request ECN connection |
> + * | 3 | Request AccECN connection |
> + * +================+===========================+
> + */
You have nice macros, maybe use them ?
TCP_ECN_IN_NOECN_OUT_NOECN = 0,
TCP_ECN_IN_ECN_OUT_ECN = 1,
TCP_ECN_IN_ECN_OUT_NOECN = 2,
TCP_ECN_IN_ACCECN_OUT_ACCECN = 3,
TCP_ECN_IN_ACCECN_OUT_ECN = 4,
TCP_ECN_IN_ACCECN_OUT_NOECN = 5,
This can be done later, no need to respin.
Reviewed-by: Eric Dumazet <eduma...@google.com>
