code: func (h *receivedPacketTracker) isMissing(p protocol.PacketNumber) bool {
  	if h.lastAck == nil || p < h.ignoreBelow {
  		return false
  	}
  	return p < h.lastAck.LargestAcked() && !h.lastAck.AcksPacket(p)
  }
docs: // isMissing says if a packet was reported missing in the last ACK.