// AppendSocketControlMessage appends a single socket control message to b,
// returning a new slice and the subslice of the message data.
func AppendSocketControlMessage(b []byte, level, typ int32, size int) (_, data []byte) {
        off := len(b)
        b = append(b, make([]byte, unix.CmsgSpace(size))...)
        h := (*unix.Cmsghdr)(unsafe.Pointer(&b[off]))
        h.Level = level
        h.Type = typ
        h.SetLen(unix.CmsgLen(size))
        return b, b[off+unix.CmsgSpace(0):][:size]
}

var control []byte
control, data = AppendSocketControlMessage(control, unix.IPPROTO_IP, unix.IP_TOS)
data[0] = 0b000000_01 // set ECN field to ECT1
conn.WriteMessageUDPAddrPort(datagram, control, address)

func PktInfo4(info *Inet4Pktinfo) []byte {
        b := make([]byte, CmsgSpace(SizeofInet4Pktinfo))
        h := (*Cmsghdr)(unsafe.Pointer(&b[0]))
        h.Level = SOL_IP
        h.Type = IP_PKTINFO
        h.SetLen(CmsgLen(SizeofInet4Pktinfo))
        *(*Inet4Pktinfo)(h.data(0)) = *info
        return b
}

func PktInfo4(info *Inet4Pktinfo) []byte {
        b, data := AppendSocketControlMessage(nil, SOL_IP, IP_PKTINFO, SizeofInet4Pktinfo)
        *(*Inet4Pktinfo)(unsafe.Pointer(&data[0])) = *info
        return b
}