code: func announceUDP(opt Announce, _url *url.URL) (AnnounceResponse, error) {
  	ua := udpAnnounce{
  		url: *_url,
  		a:   &opt,
  	}
  	defer ua.Close()
  	return ua.Do(opt.Request)
  }
docs: // TODO: Split on IPv6, as BEP 15 says response peer decoding depends on
  // network in use.