[Cake] ECN not working?

[xnor]

Hello,

after observing mostly drops in cake (instead of marks) I did a little test:

Client is a Linux 5.9.13, server is the same.
Kernel ECN parameters are set identically for both client and server to:
net.ipv4.tcp_ecn = 1
net.ipv4.tcp_ecn_fallback = 1

Using tcpdump on the server I see the following:

The client initiates the IPv4 TCP connection with:
IP Differentiated Services Field: 0x02 (DSCP: CS0, ECN: ECT(0))
TCP Flags: 0x0c2 (SYN, ECN, CWR)
Seq=0 Win=64240 Len=0 MSS=1460 WS=256 SACK_PERM=1

The server responds:
Differentiated Services Field: 0x00 (DSCP: CS0, ECN: Not-ECT)
Flags: 0x012 (SYN, ACK)
Seq=0 Ack=1 Win=64240 Len=0 MSS=1460 SACK_PERM=1 WS=128

Shouldn't the server respond with ECT set in the SYN ACK packet
and possibly also have ECN-related flags set in the TCP header?