code: func (tap TunnelAttrsPresence) mask() (res TunnelAttrs) {
  	if tap.TunnelId {
  		res.TunnelId = [8]byte{
  			0xff, 0xff, 0xff, 0xff,
  			0xff, 0xff, 0xff, 0xff,
  		}
  	}
  
  	if tap.Ipv4Src {
  		res.Ipv4Src = [4]byte{0xff, 0xff, 0xff, 0xff}
  	}
  
  	if tap.Ipv4Dst {
  		res.Ipv4Dst = [4]byte{0xff, 0xff, 0xff, 0xff}
  	}
  
  	if tap.Tos {
  		res.Tos = 0xff
  	}
  
  	if tap.Ttl {
  		res.Ttl = 0xff
  	}
  
  	res.Df = tap.Df
  	res.Csum = tap.Csum
  
  	if tap.TpSrc {
  		res.TpSrc = 0xffff
  	}
  
  	if tap.TpDst {
  		res.TpDst = 0xffff
  	}
  
  	return
  }
docs: // Convert a TunnelAttrsPresence to a mask