code: func (h *receivedPacketTracker) IgnoreBelow(p protocol.PacketNumber) {
  	if p <= h.ignoreBelow {
  		return
  	}
  	h.ignoreBelow = p
  	h.packetHistory.DeleteBelow(p)
  	if h.logger.Debug() {
  		h.logger.Debugf("\tIgnoring all packets below %#x.", p)
  	}
  }
docs: // IgnoreBelow sets a lower limit for acking packets.
  // Packets with packet numbers smaller than p will not be acked.