On 6/10/25 2:53 PM, chia-yu.chang@xxxxxxxxxxxxxxxxxxx wrote: > From: Ilpo Järvinen <ij@xxxxxxxxxx> > > This change implements Accurate ECN without negotiation and > AccECN Option (that will be added by later changes). Based on > AccECN specifications: > https://tools.ietf.org/id/draft-ietf-tcpm-accurate-ecn-28.txt > > Accurate ECN allows feeding back the number of CE (congestion > experienced) marks accurately to the sender in contrast to > RFC3168 ECN that can only signal one marks-seen-yes/no per RTT. > Congestion control algorithms can take advantage of the accurate > ECN information to fine-tune their congestion response to avoid > drastic rate reduction when only mild congestion is encountered. > > With Accurate ECN, tp->received_ce (r.cep in AccECN spec) keeps > track of how many segments have arrived with a CE mark. Accurate > ECN uses ACE field (ECE, CWR, AE) to communicate the value back > to the sender which updates tp->delivered_ce (s.cep) based on the > feedback. This signalling channel is lossy when ACE field overflow > occurs. > > Conservative strategy is selected here to deal with the ACE > overflow, however, some strategies using the AccECN option later > in the overall patchset mitigate against false overflows detected. > > The ACE field values on the wire are offset by > TCP_ACCECN_CEP_INIT_OFFSET. Delivered_ce/received_ce count the > real CE marks rather than forcing all downstream users to adapt > to the wire offset. > > This patch uses the first 1-byte hole and the last 4-byte hole of > the tcp_sock_write_txrx for 'received_ce_pending' and 'received_ce'. > Also, the group size of tcp_sock_write_txrx is increased from > 91 + 4 to 95 + 4 due to the new u32 received_ce member. Below are > the trimmed pahole outcomes before and after this patch. AFAICS 'received_ce' fills the existing 4 bytes hole, so tcp_sock_write_txrx size should be now (95 + 0), am I missreading something? > @@ -384,17 +387,16 @@ static void tcp_data_ecn_check(struct sock *sk, const struct sk_buff *skb) > if (tcp_ca_needs_ecn(sk)) > tcp_ca_event(sk, CA_EVENT_ECN_IS_CE); > > - if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) { > + if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR) && > + tcp_ecn_mode_rfc3168(tp)) { > /* Better not delay acks, sender can have a very low cwnd */ > tcp_enter_quickack_mode(sk, 2); > tp->ecn_flags |= TCP_ECN_DEMAND_CWR; > } > - tp->ecn_flags |= TCP_ECN_SEEN; It's not clear why you need to move this statement earlier in the code path even for ecn_mode_rfc3168(). Either a comment or if (!tcp_ecn_mode_rfc3168(tp)) break; a few lines aboved could help. > break; > default: > if (tcp_ca_needs_ecn(sk)) > tcp_ca_event(sk, CA_EVENT_ECN_NO_CE); > - tp->ecn_flags |= TCP_ECN_SEEN; Same here. Thanks, Paolo
