code: function msgReceived(msg, rinfo) {
      var self = this, now = Date.now(), pkt, entry;
  
      if (msg.length === 0) {
          // Not sure why we sometimes receive an empty message.
          // As far as I'm concerned it shouldn't happen, but we'll ignore it
          // and if it's necessary a retransmission of the request will be
          // made later.
          return;
      }
  
      // Parse the packet, or call the informative
      // parse error display if we fail.
      try {
          pkt = parse(msg);
      } catch (error) {
          return self.emit('error', error);
      }
  
      // If this message's request id matches one we've sent,
      // cancel any outstanding timeout and call the registered
      // callback.
      entry = self.reqs[pkt.pdu.reqid];
      if (entry) {
          clearRequest(self.reqs, pkt.pdu.reqid);
  
          if (typeof entry.callback === 'function') {
              if (pkt.pdu.error !== 0) {
                  // An error response should be reported as an error to the callback.
                  // We try to find the error description, or in worst case call it
                  // just "Unknown Error <number>".
                  var errorDescr = Object.keys(asn1ber.errors).filter(function (key) {
                      return asn1ber.errors[key] === pkt.pdu.error;
                  })[0] || 'Unknown Error ' + pkt.pdu.error;
                  return entry.callback(new Error(errorDescr));
              }
  
              pkt.pdu.varbinds.forEach(function (vb) {
                  vb.receiveStamp = now;
                  vb.sendStamp = entry.sendStamp;
              });
  
              entry.callback(null, pkt.pdu.varbinds);
          }
      }
  }
docs: Communication functions ----- This is called for when we receive a message.