code: func (kcp *KCP) SetMtu(mtu int) int {
  	if mtu < 50 || mtu < IKCP_OVERHEAD {
  		return -1
  	}
  	if kcp.reserved >= int(kcp.mtu-IKCP_OVERHEAD) || kcp.reserved < 0 {
  		return -1
  	}
  
  	buffer := make([]byte, mtu)
  	if buffer == nil {
  		return -2
  	}
  	kcp.mtu = uint32(mtu)
  	kcp.mss = kcp.mtu - IKCP_OVERHEAD - uint32(kcp.reserved)
  	kcp.buffer = buffer
  	return 0
  }
docs: // SetMtu changes MTU size, default is 1400