The programs have been tested on

• FreeBSD 15.0
• macOS 15.7.1
• macOS 26.0.1
• Ubuntu 24.04

The sender sends initially a UDP packet with the a DSCP code point CS0 and Not-ECT. Then a socket option is used to set the DSCP to AF41 and ECT(1) which corresponds to 0x89. The second UDP is sent using this setting. When sending the third UDP packet, a cmsg is used to specifiy a DSCP of AF42 and CE which corresponds to 0x93. The fourth UDP packet is then send with a cmsg and therefore use again AF41 and ECT(1) coresponding to 0x89.

cc -o receiver4 receiver4.c
cc -o receiver6 receiver6.c
cc -o sender4 sender4.c
cc -o sender6 sender6.c

gcc -o receiver4 receiver4.c
gcc -o receiver6 receiver6.c
gcc -o sender4 sender4.c
gcc -o sender6 sender6.c

The observation is that on macOS setting the DSCP value using the IPPROTO_IPV6-level socket option IPV6_TCLASS is not possible. Only the ECN value is set.

So the observation is that on Linux an IPPROTO_IP-cmsg_level cmsg with cmsg_type of IP_TOS is used to report the DSCP and ECN fields on a AF_INET6 socket, if a UDP/IPv4 packet has been received.

The observation is that on macOS setting the DSCP value using the IPPROTO_IPV6-level socket option IPV6_TCLASS is not possible. Only the ECN value is set.