code: function probe (mdns, service, cb) {
    var sent = false
    var retries = 0
    var timer
  
    mdns.on('response', onresponse)
    setTimeout(send, Math.random() * 250)
  
    function send () {
      // abort if the service have or is being stopped in the meantime
      if (!service._activated || service._destroyed) return
  
      mdns.query(service.fqdn, 'ANY', function () {
        // This function will optionally be called with an error object. We'll
        // just silently ignore it and retry as we normally would
        sent = true
        timer = setTimeout(++retries < 3 ? send : done, 250)
        timer.unref()
      })
    }
  
    function onresponse (packet) {
      // Apparently conflicting Multicast DNS responses received *before*
      // the first probe packet is sent MUST be silently ignored (see
      // discussion of stale probe packets in RFC 6762 Section 8.2,
      // "Simultaneous Probe Tiebreaking" at
      // https://tools.ietf.org/html/rfc6762#section-8.2
      if (!sent) return
  
      if (packet.answers.some(matchRR) || packet.additionals.some(matchRR)) done(true)
    }
  
    function matchRR (rr) {
      return dnsEqual(rr.name, service.fqdn)
    }
  
    function done (exists) {
      mdns.removeListener('response', onresponse)
      clearTimeout(timer)
      cb(!!exists)
    }
  }
docs: Check if a service name is already in use on the network.
  
  Used before announcing the new service.
  
  To guard against race conditions where multiple services are started
  simultaneously on the network, wait a random amount of time (between
  0 and 250 ms) before probing.
  
  TODO: Add support for Simultaneous Probe Tiebreaking:
  https://tools.ietf.org/html/rfc6762#section-8.2