code: func (ta TunnelAttrs) present() TunnelAttrsPresence {
  	// The kernel requires Ipv4Dst and Ttl to be present, so we
  	// always mark those as present, even if we end up wildcarding
  	// them.
  	return TunnelAttrsPresence{
  		TunnelId: !AllBytes(ta.TunnelId[:], 0),
  		Ipv4Src:  !AllBytes(ta.Ipv4Src[:], 0),
  		Ipv4Dst:  true,
  		Tos:      ta.Tos != 0,
  		Ttl:      true,
  		Df:       ta.Df,
  		Csum:     ta.Csum,
  		TpSrc:    ta.TpSrc != 0,
  		TpDst:    ta.TpDst != 0,
  	}
  }
docs: // Extract presence information from a TunnelAttrs mask