func (c *genericOpt) TrafficClass() (int, error) {
	if !c.ok() {
		return 0, errInvalidConn
	}
	so, ok := sockOpts[ssoTrafficClass]
	if !ok {
		return 0, errOpNoSupport
	}
	return so.GetInt(c.Conn)
}
TrafficClass returns the traffic class field value for outgoing
packets.
package ipv6