code: func (b IPv6) SetTOS(t uint8, l uint32) {
  	vtf := (6 << 28) | (uint32(t) << 20) | (l & 0xfffff)
  	binary.BigEndian.PutUint32(b[versTCFL:], vtf)
  }
docs: // SetTOS sets the "traffic class" and "flow label" fields of the ipv6 header.