RECVTOS support (including macOS)

[past-due]

Saw a commit about disabling IP_RECVTOS support on macOS (ref: 1736cfa), and I thought I’d make a note of some findings from taking a brief look into it.

Useful references

Configuring UDP Sockets for ECN for Common Platforms:
https://www.ietf.org/archive/id/draft-duke-tsvwg-udp-ecn-01.html

macOS support

The documentation I’ve found (including the above) would seem to indicate that this line:

GameNetworkingSockets/src/steamnetworkingsockets/clientlib/steamnetworkingsockets_lowlevel.cpp

Line 2487 in 725e273

if ( unlikely( cmsg->cmsg_level != IPPROTO_IP || cmsg->cmsg_type != IP_TOS ) )

Should be checking cmsg->cmsg_type != IP_RECVTOS when defined(__APPLE__) (IP_RECVTOS does differ in value from IP_TOS in the macOS SDK headers).

If a UDP message (UDP/IPv4) is received on an IPv4 socket, the ancillary data will contain a cmsg of level IPPROTO_IP and type IP_RECVTOS. The cmsg data contains an unsigned char.

IPv6 support

See the reference document above, which details how to implement this across Linux + macOS + Windows.