From: Chia-Yu Chang <redacted>

For Accurate ECN, the first SYN/ACK sent by the TCP server shall set the
ACE flag (see Table 1 of RFC9768) and the AccECN option to complete the
capability negotiation. However, if the TCP server needs to retransmit such
a SYN/ACK (for example, because it did not receive an ACK acknowledging its
SYN/ACK, or received a second SYN requesting AccECN support), the TCP server
retransmits the SYN/ACK without the AccECN option. This is because the
SYN/ACK may be lost due to congestion, or a middlebox may block the AccECN
option. Furthermore, if this retransmission also times out, to expedite
connection establishment, the TCP server should retransmit the SYN/ACK with
(AE,CWR,ECE) = (0,0,0) and without the AccECN option, while maintaining
AccECN feedback mode.

This complies with Section 3.2.3.2.2 of the AccECN specification (RFC9768).

Signed-off-by: Chia-Yu Chang <redacted>

---
v6:
- Use new synack_type TCP_SYNACK_RETRANS and num_retrans.
---
 include/net/tcp_ecn.h | 20 ++++++++++++++------
 net/ipv4/tcp_output.c |  4 ++--
 2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/include/net/tcp_ecn.h b/include/net/tcp_ecn.h
index a709fb1756eb..57841dfa6705 100644
--- a/include/net/tcp_ecn.h
+++ b/include/net/tcp_ecn.h

@@ -649,12 +649,20 @@ static inline void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
 }
 
 static inline void
-tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
-{
-	if (tcp_rsk(req)->accecn_ok)
-		tcp_accecn_echo_syn_ect(th, tcp_rsk(req)->syn_ect_rcv);
-	else if (inet_rsk(req)->ecn_ok)
-		th->ece = 1;
+tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th,
+		    enum tcp_synack_type synack_type)
+{
+	// num_retrans will be incresaed after tcp_ecn_make_synack()