Tested Platforms

The programs have been tested on

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

Test Programs

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 packet 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.

Compilation

FreeBSD and MacOS

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

Linux

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

Execution

Output of receiver4

FreeBSD

sender4 127.0.0.1	sender6 ::ffff:127.0.0.1
tos = 0x00	tos = 0x00
tos = 0x89	tos = 0x89
tos = 0x93	tos = 0x93
tos = 0x89	tos = 0x89

Linux

sender4 127.0.0.1	sender6 ::ffff:127.0.0.1
tos = 0x00	tos = 0x00
tos = 0x89	tos = 0x89
tos = 0x93	tos = 0x93
tos = 0x89	tos = 0x89

macOS

sender4 127.0.0.1	sender6 ::ffff:127.0.0.1
tos = 0x00	tos = 0x00
tos = 0x89	tos = 0x01
tos = 0x93	tos = 0x93
tos = 0x89	tos = 0x01

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.

Output of receiver6

FreeBSD

sender6 ::1	sender6 ::ffff:127.0.0.1	sender4 127.0.0.1
tclass = 0x00	tclass = 0x00	tclass = 0x00
tclass = 0x89	tclass = 0x89	tclass = 0x89
tclass = 0x93	tclass = 0x93	tclass = 0x93
tclass = 0x89	tclass = 0x89	tclass = 0x89

Linux

sender6 ::1	sender6 ::ffff:127.0.0.1	sender4 127.0.0.1
tclass = 0x00	tos = 0x00	tos = 0x00
tclass = 0x89	tos = 0x89	tos = 0x89
tclass = 0x93	tos = 0x93	tos = 0x93
tclass = 0x89	tos = 0x89	tos = 0x89

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.

macOS

sender6 ::1	sender6 ::ffff:127.0.0.1	sender4 127.0.0.1
tclass = 0x00	tclass = 0x00	tclass = 0x00
tclass = 0x89	tclass = 0x01	tclass = 0x89
tclass = 0x93	tclass = 0x93	tclass = 0x93
tclass = 0x89	tclass = 0x01	tclass = 0x89

The observation is that on macOS setting the DSCP value using the IPPROTO_IPV6-level socket option IPV6_TCLASS for UDP/IPv4 packets is not possible. Only the ECN value is set.