Re: [PATCH v9 net-next 09/15] tcp: accecn: retransmit SYN/ACK without AccECN option or non-AccECN SYN/ACK

On Mon, Jan 19, 2026 at 7:59 PM [off-list ref] wrote:

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.

Reviewed-by: Eric Dumazet <edumazet@google.com>