/*
 * Copyright (c) 2007, 2014 University of Washington
 *               2015 Universita' degli Studi di Napoli Federico II
 *
 * SPDX-License-Identifier: GPL-2.0-only
 */

#ifndef QUEUE_DISC_H
#define QUEUE_DISC_H

#include "packet-filter.h"

#include "ns3/object.h"
#include "ns3/queue-fwd.h"
#include "ns3/queue-item.h"
#include "ns3/queue-size.h"
#include "ns3/traced-callback.h"
#include "ns3/traced-value.h"

#include <functional>
#include <map>
#include <string>
#include <vector>

namespace ns3
{

class QueueDisc;
class NetDeviceQueueInterface;

/**
 * @ingroup traffic-control
 *
 * QueueDiscClass is the base class for classes that are included in a queue
 * disc. It has a single attribute, QueueDisc, used to set the child queue disc
 * attached to the class. Classful queue discs needing to set parameters for
 * their classes can subclass QueueDiscClass and add the required parameters
 * as attributes.
 */
class QueueDiscClass : public Object
{
  public:
    /**
     * @brief Get the type ID.
     * @return the object TypeId
     */
    static TypeId GetTypeId();

    QueueDiscClass();
    ~QueueDiscClass() override;

    /**
     * @brief Get the queue disc attached to this class
     * @return the queue disc attached to this class.
     */
    Ptr<QueueDisc> GetQueueDisc() const;

    /**
     * @brief Set the queue disc attached to this class
     * @param qd The queue disc to attach to this class
     */
    void SetQueueDisc(Ptr<QueueDisc> qd);

  protected:
    /**
     * @brief Dispose of the object
     */
    void DoDispose() override;

  private:
    Ptr<QueueDisc> m_queueDisc; //!< Queue disc attached to this class
};

/**
 * @ingroup traffic-control
 * @brief Enumeration of the available policies to handle the queue disc size.
 *
 * - SINGLE_INTERNAL_QUEUE is intended to handle the maxSize attribute
 * of queue discs having a single internal queue. If no internal queue is
 * yet attached to the queue disc, setting/getting this attribute involves
 * setting/getting the member variable of the queue disc; otherwise, the
 * corresponding attribute of the internal queue is set/get.
 * - SINGLE_CHILD_QUEUE_DISC is intended to handle the maxSize attribute
 * of queue discs having a single child queue disc. If no child queue disc is
 * yet attached to the queue disc, setting/getting this attribute involves
 * setting/getting the member variable of the queue disc; otherwise, the
 * corresponding attribute of the child queue disc is set/get.
 * - MULTIPLE_QUEUES is intended to handle the maxSize attribute of queue
 * discs having multiple internal queues or child queue discs. Setting/getting
 * this attribute always involves setting/getting the member variable of the
 * queue disc. Queue discs should warn the user if a packet is dropped by an
 * internal queue/child queue disc because of lack of space, while the queue
 * disc limit is not exceeded.
 */
enum QueueDiscSizePolicy
{
    SINGLE_INTERNAL_QUEUE,   //!< Used by queue discs with single internal queue
    SINGLE_CHILD_QUEUE_DISC, //!< Used by queue discs with single child queue disc
    MULTIPLE_QUEUES, //!< Used by queue discs with multiple internal queues/child queue discs
    NO_LIMITS        //!< Used by queue discs with unlimited size
};

/**
 * @ingroup traffic-control
 *
 * QueueDisc is an abstract base class providing the interface and implementing
 * the operations common to all the queueing disciplines. Child classes
 * need to implement the methods used to enqueue a packet (DoEnqueue),
 * dequeue a single packet (DoDequeue), get a copy of the next packet
 * to extract (DoPeek), check whether the current configuration is correct
 * (CheckConfig).
 *
 * As in Linux, a queue disc may contain distinct elements:
 * - queues, which actually store the packets waiting for transmission
 * - classes, which allow to reserve a different treatment to different packets
 * - filters, which determine the queue or class which a packet is destined to
 *
 * Notice that a child queue disc must be attached to every class and a packet
 * filter is only able to classify packets of a single protocol. Also, while in Linux
 * some queue discs (e.g., fq-codel) use an internal classifier and do not make use of
 * packet filters, in ns-3 every queue disc including multiple queues or multiple classes
 * needs an external filter to classify packets (this is to avoid having the traffic-control
 * module depend on other modules such as internet).
 *
 * Queue disc configuration vary from queue disc to queue disc. A typical taxonomy divides
 * queue discs in classful (i.e., support classes) and classless (i.e., do not support
 * classes). More recently, after the appearance of multi-queue devices (such as Wifi),
 * some multi-queue aware queue discs have been introduced. Multi-queue aware queue discs
 * handle as many queues (or queue discs -- without using classes) as the number of
 * transmission queues used by the device on which the queue disc is installed.
 * An attempt is made, also, to enqueue each packet in the "same" queue both within the
 * queue disc and within the device.
 *
 * The traffic control layer interacts with a queue disc in a simple manner: after
 * requesting to enqueue a packet, the traffic control layer requests the qdisc to
 * "run", i.e., to dequeue a set of packets, until a predefined number ("quota")
 * of packets is dequeued or the netdevice stops the queue disc. A netdevice shall
 * stop the queue disc when its transmission queue does not have room for another
 * packet. Also, a netdevice shall wake the queue disc when it detects that there
 * is room for another packet in its transmission queue, but the transmission queue
 * is stopped. Waking a queue disc is equivalent to make it run.
 *
 * Every queue disc collects statistics about the total number of packets/bytes
 * received from the upper layers (in case of root queue disc) or from the parent
 * queue disc (in case of child queue disc), enqueued, dequeued, requeued, dropped,
 * dropped before enqueue, dropped after dequeue, queued in the queue disc and
 * sent to the netdevice or to the parent queue disc. Note that packets that are
 * dequeued may be requeued, i.e., retained by the traffic control infrastructure,
 * if the netdevice is not ready to receive them. Requeued packets are not part
 * of the queue disc. The following identities hold:
 * - dropped = dropped before enqueue + dropped after dequeue
 * - received = dropped before enqueue + enqueued
 * - queued = enqueued - dequeued
 * - sent = dequeued - dropped after dequeue (- 1 if there is a requeued packet)
 *
 * Separate counters are also kept for each possible reason to drop a packet.
 * When a packet is dropped by an internal queue, e.g., because the queue is full,
 * the reason is "Dropped by internal queue". When a packet is dropped by a child
 * queue disc, the reason is "(Dropped by child queue disc) " followed by the
 * reason why the child queue disc dropped the packet.
 *
 * The QueueDisc base class provides the SojournTime trace source, which provides
 * the sojourn time of every packet dequeued from a queue disc, including packets
 * that are dropped or requeued after being dequeued. The sojourn time is taken
 * when the packet is dequeued from the queue disc, hence it does not account for
 * the additional time the packet is retained within the traffic control
 * infrastructure in case it is requeued.
 *
 * The design and implementation of this class is heavily inspired by Linux.
 * For more details, see the traffic-control model page.
 */
class QueueDisc : public Object
{
  public:
    /// @brief Structure that keeps the queue disc statistics
    struct Stats
    {
        /// Total received packets
        uint32_t nTotalReceivedPackets;
        /// Total received bytes
        uint64_t nTotalReceivedBytes;
        /// Total sent packets -- this value is not kept up to date, call GetStats first
        uint32_t nTotalSentPackets;
        /// Total sent bytes -- this value is not kept up to date, call GetStats first
        uint64_t nTotalSentBytes;
        /// Total enqueued packets
        uint32_t nTotalEnqueuedPackets;
        /// Total enqueued bytes
        uint64_t nTotalEnqueuedBytes;
        /// Total dequeued packets
        uint32_t nTotalDequeuedPackets;
        /// Total dequeued bytes
        uint64_t nTotalDequeuedBytes;
        /// Total dropped packets
        uint32_t nTotalDroppedPackets;
        /// Total packets dropped before enqueue
        uint32_t nTotalDroppedPacketsBeforeEnqueue;
        /// Packets dropped before enqueue, for each reason
        std::map<std::string, uint32_t, std::less<>> nDroppedPacketsBeforeEnqueue;
        /// Total packets dropped after dequeue
        uint32_t nTotalDroppedPacketsAfterDequeue;
        /// Packets dropped after dequeue, for each reason
        std::map<std::string, uint32_t, std::less<>> nDroppedPacketsAfterDequeue;
        /// Total dropped bytes
        uint64_t nTotalDroppedBytes;
        /// Total bytes dropped before enqueue
        uint64_t nTotalDroppedBytesBeforeEnqueue;
        /// Bytes dropped before enqueue, for each reason
        std::map<std::string, uint64_t, std::less<>> nDroppedBytesBeforeEnqueue;
        /// Total bytes dropped after dequeue
        uint64_t nTotalDroppedBytesAfterDequeue;
        /// Bytes dropped after dequeue, for each reason
        std::map<std::string, uint64_t, std::less<>> nDroppedBytesAfterDequeue;
        /// Total requeued packets
        uint32_t nTotalRequeuedPackets;
        /// Total requeued bytes
        uint64_t nTotalRequeuedBytes;
        /// Total marked packets
        uint32_t nTotalMarkedPackets;
        /// Marked packets, for each reason
        std::map<std::string, uint32_t, std::less<>> nMarkedPackets;
        /// Total marked bytes
        uint32_t nTotalMarkedBytes;
        /// Marked bytes, for each reason
        std::map<std::string, uint64_t, std::less<>> nMarkedBytes;

        /// constructor
        Stats();

        /**
         * @brief Get the number of packets dropped for the given reason
         * @param reason the reason why packets were dropped
         * @return the number of packets dropped for the given reason
         */
        uint32_t GetNDroppedPackets(std::string reason) const;
        /**
         * @brief Get the amount of bytes dropped for the given reason
         * @param reason the reason why packets were dropped
         * @return the amount of bytes dropped for the given reason
         */
        uint64_t GetNDroppedBytes(std::string reason) const;
        /**
         * @brief Get the number of packets marked for the given reason
         * @param reason the reason why packets were marked
         * @return the number of packets marked for the given reason
         */
        uint32_t GetNMarkedPackets(std::string reason) const;
        /**
         * @brief Get the amount of bytes marked for the given reason
         * @param reason the reason why packets were marked
         * @return the amount of bytes marked for the given reason
         */
        uint64_t GetNMarkedBytes(std::string reason) const;
        /**
         * @brief Print the statistics.
         * @param os output stream in which the data should be printed.
         */
        void Print(std::ostream& os) const;
    };

    /**
     * @brief Get the type ID.
     * @return the object TypeId
     */
    static TypeId GetTypeId();

    /**
     * @brief Constructor
     * @param policy the policy to handle the queue disc size
     */
    QueueDisc(QueueDiscSizePolicy policy = QueueDiscSizePolicy::SINGLE_INTERNAL_QUEUE);

    /**
     * @brief Constructor
     * @param policy the policy to handle the queue disc size
     * @param unit The fixed operating mode of this queue disc
     */
    QueueDisc(QueueDiscSizePolicy policy, QueueSizeUnit unit);

    ~QueueDisc() override;

    // Delete copy constructor and assignment operator to avoid misuse
    QueueDisc(const QueueDisc&) = delete;
    QueueDisc& operator=(const QueueDisc&) = delete;

    /**
     * @brief Get the number of packets stored by the queue disc
     * @return the number of packets stored by the queue disc.
     *
     * The requeued packet, if any, is counted.
     */
    uint32_t GetNPackets() const;

    /**
     * @brief Get the amount of bytes stored by the queue disc
     * @return the amount of bytes stored by the queue disc.
     *
     * The requeued packet, if any, is counted.
     */
    uint32_t GetNBytes() const;

    /**
     * @brief Get the maximum size of the queue disc.
     *
     * @returns the maximum size of the queue disc.
     */
    QueueSize GetMaxSize() const;

    /**
     * @brief Set the maximum size of the queue disc.
     *
     * Trying to set a null size has no effect.
     *
     * @param size the maximum size.
     * @returns true if setting the size succeeded, false otherwise.
     */
    bool SetMaxSize(QueueSize size);

    /**
     * @brief Get the current size of the queue disc in bytes, if
     *        operating in bytes mode, or packets, otherwise.
     *
     * Do not call this method if the queue disc size is not limited.
     *
     * @returns The queue disc size in bytes or packets.
     */
    QueueSize GetCurrentSize() const;

    /**
     * @brief Retrieve all the collected statistics.
     * @return the collected statistics.
     */
    const Stats& GetStats();

    /**
     * @param ndqi the NetDeviceQueueInterface aggregated to the receiving object.
     *
     * Set the pointer to the NetDeviceQueueInterface object aggregated to the
     * object receiving the packets dequeued from this queue disc.
     */
    void SetNetDeviceQueueInterface(Ptr<NetDeviceQueueInterface> ndqi);

    /**
     * @return the NetDeviceQueueInterface aggregated to the receiving object.
     *
     * Get the pointer to the NetDeviceQueueInterface object aggregated to the
     * object receiving the packets dequeued from this queue disc.
     */
    Ptr<NetDeviceQueueInterface> GetNetDeviceQueueInterface() const;

    /// Callback invoked to send a packet to the receiving object when Run is called
    typedef std::function<void(Ptr<QueueDiscItem>)> SendCallback;

    /**
     * @param func the callback to send a packet to the receiving object.
     *
     * Set the callback used by the Transmit method (called eventually by the Run
     * method) to send a packet to the receiving object.
     */
    void SetSendCallback(SendCallback func);

    /**
     * @return the callback to send a packet to the receiving object.
     *
     * Get the callback used by the Transmit method (called eventually by the Run
     * method) to send a packet to the receiving object.
     */
    SendCallback GetSendCallback() const;

    /**
     * @brief Set the maximum number of dequeue operations following a packet enqueue
     * @param quota the maximum number of dequeue operations following a packet enqueue.
     */
    virtual void SetQuota(const uint32_t quota);

    /**
     * @brief Get the maximum number of dequeue operations following a packet enqueue
     * @return the maximum number of dequeue operations following a packet enqueue.
     */
    virtual uint32_t GetQuota() const;

    /**
     * Pass a packet to store to the queue discipline. This function only updates
     * the statistics and calls the (private) DoEnqueue function, which must be
     * implemented by derived classes.
     * @param item item to enqueue
     * @return True if the operation was successful; false otherwise
     */
    bool Enqueue(Ptr<QueueDiscItem> item);

    /**
     * Extract from the queue disc the packet that has been dequeued by calling
     * Peek, if any, or call the private DoDequeue method (which must be
     * implemented by derived classes) to dequeue a packet, otherwise.
     *
     * @return 0 if the operation was not successful; the item otherwise.
     */
    Ptr<QueueDiscItem> Dequeue();

    /**
     * Get a copy of the next packet the queue discipline will extract. This
     * function only calls the (private) DoPeek function. This base class provides
     * a default implementation of DoPeek, which dequeues the next packet but
     * retains it into the queue disc.
     * @return 0 if the operation was not successful; the item otherwise.
     */
    Ptr<const QueueDiscItem> Peek();

    /**
     * Modelled after the Linux function __qdisc_run (net/sched/sch_generic.c)
     * Dequeues multiple packets, until a quota is exceeded or sending a packet
     * to the device failed.
     */
    void Run();

    /// Internal queues store QueueDiscItem objects
    typedef Queue<QueueDiscItem> InternalQueue;

    /**
     * @brief Add an internal queue to the tail of the list of queues.
     * @param queue the queue to be added
     */
    void AddInternalQueue(Ptr<InternalQueue> queue);

    /**
     * @brief Get the i-th internal queue
     * @param i the index of the queue
     * @return the i-th internal queue.
     */
    Ptr<InternalQueue> GetInternalQueue(std::size_t i) const;

    /**
     * @brief Get the number of internal queues
     * @return the number of internal queues.
     */
    std::size_t GetNInternalQueues() const;

    /**
     * @brief Add a packet filter to the tail of the list of filters used to classify packets.
     * @param filter the packet filter to be added
     */
    void AddPacketFilter(Ptr<PacketFilter> filter);

    /**
     * @brief Get the i-th packet filter
     * @param i the index of the packet filter
     * @return the i-th packet filter.
     */
    Ptr<PacketFilter> GetPacketFilter(std::size_t i) const;

    /**
     * @brief Get the number of packet filters
     * @return the number of packet filters.
     */
    std::size_t GetNPacketFilters() const;

    /**
     * @brief Add a queue disc class to the tail of the list of classes.
     * @param qdClass the queue disc class to be added
     */
    void AddQueueDiscClass(Ptr<QueueDiscClass> qdClass);

    /**
     * @brief Get the i-th queue disc class
     * @param i the index of the queue disc class
     * @return the i-th queue disc class.
     */
    Ptr<QueueDiscClass> GetQueueDiscClass(std::size_t i) const;

    /**
     * @brief Get the number of queue disc classes
     * @return the number of queue disc classes.
     */
    std::size_t GetNQueueDiscClasses() const;

    /**
     * Classify a packet by calling the packet filters, one at a time, until either
     * a filter able to classify the packet is found or all the filters have been
     * processed.
     * @param item item to classify
     * @return -1 if no filter able to classify the packet has been found, the value
     * returned by first filter found to be able to classify the packet otherwise.
     */
    int32_t Classify(Ptr<QueueDiscItem> item);

    /**
     * @enum WakeMode
     * @brief Used to determine whether the queue disc itself or its children must
     *        be activated when a netdevice wakes a transmission queue
     */
    enum WakeMode
    {
        WAKE_ROOT = 0x00,
        WAKE_CHILD = 0x01
    };

    /**
     * When setting up the wake callbacks on the netdevice queues, it is necessary to
     * determine which queue disc (the root queue disc or one of its children) should
     * be activated when the netdevice wakes one of its transmission queues. The
     * implementation of this method for the base class returns WAKE_ROOT, i.e., the
     * root queue disc is activated. Subclasses implementing queue discs adopting
     * a different strategy (e.g., multi-queue aware queue discs such as mq) have
     * to redefine this method.
     *
     * @return the wake mode adopted by this queue disc.
     */
    virtual WakeMode GetWakeMode() const;

    // Reasons for dropping packets
    static constexpr const char* INTERNAL_QUEUE_DROP =
        "Dropped by internal queue"; //!< Packet dropped by an internal queue
    static constexpr const char* CHILD_QUEUE_DISC_DROP =
        "(Dropped by child queue disc) "; //!< Packet dropped by a child queue disc
    static constexpr const char* CHILD_QUEUE_DISC_MARK =
        "(Marked by child queue disc) "; //!< Packet marked by a child queue disc

  protected:
    /**
     * @brief Dispose of the object
     */
    void DoDispose() override;

    /**
     * @brief Check whether the configuration is correct and initialize parameters
     *
     * This method is not virtual to prevent subclasses from redefining it.
     * Subclasses must instead provide the implementation of the CheckConfig
     * and InitializeParams methods (which are called by this method).
     * \sa QueueDisc::InitializeParams
     * \sa QueueDisc::CheckConfig
     */
    void DoInitialize() override;

    /**
     * @brief Perform the actions required when the queue disc is notified of
     *        a packet dropped before enqueue
     * @param item item that was dropped
     * @param reason the reason why the item was dropped
     * This method must be called by subclasses to record that a packet was
     * dropped before enqueue for the specified reason
     */
    void DropBeforeEnqueue(Ptr<const QueueDiscItem> item, const char* reason);

    /**
     * @brief Perform the actions required when the queue disc is notified of
     *        a packet dropped after dequeue
     * @param item item that was dropped
     * @param reason the reason why the item was dropped
     * This method must be called by subclasses to record that a packet was
     * dropped after dequeue for the specified reason
     */
    void DropAfterDequeue(Ptr<const QueueDiscItem> item, const char* reason);

    /**
     * @brief Marks the given packet and, if successful, updates the counters
     *        associated with the given reason
     * @param item item that has to be marked
     * @param reason the reason why the item has to be marked
     * @return true if the item was successfully marked, false otherwise
     */
    bool Mark(Ptr<QueueDiscItem> item, const char* reason);

  private:
    /**
     * This function actually enqueues a packet into the queue disc.
     * @param item item to enqueue
     * @return True if the operation was successful; false otherwise
     */
    virtual bool DoEnqueue(Ptr<QueueDiscItem> item) = 0;

    /**
     * This function actually extracts a packet from the queue disc.
     * @return 0 if the operation was not successful; the item otherwise.
     */
    virtual Ptr<QueueDiscItem> DoDequeue() = 0;

    /**
     * @brief Return a copy of the next packet the queue disc will extract.
     *
     * The implementation of this method is based on the qdisc_peek_dequeued
     * function of the Linux kernel, which dequeues a packet and retains it in the
     * queue disc as a requeued packet. The packet is not traced as requeued, nor
     * is the total count of requeued packets increased. The packet is still
     * considered to be part of the queue disc and the dequeue trace is fired
     * when Dequeue is called and the packet is actually extracted from the
     * queue disc.
     *
     * This approach is especially recommended for queue discs for which it is not
     * obvious what is the next packet that will be dequeued (e.g., queue discs
     * having multiple internal queues or child queue discs or queue discs that
     * drop packets after dequeue). Subclasses can however provide their own
     * implementation of this method that overrides the default one.
     *
     * @return 0 if the operation was not successful; the packet otherwise.
     */
    virtual Ptr<const QueueDiscItem> DoPeek();

    /**
     * Check whether the current configuration is correct. Default objects (such
     * as internal queues) might be created by this method to ensure the
     * configuration is correct.  This method is automatically called at
     * simulation initialization time, and it is called before
     * the InitializeParams () method.  It is appropriate to promote parameter
     * initialization to this method if it aids in checking for correct
     * configuration.
     * \sa QueueDisc::InitializeParams
     * @return true if the configuration is correct, false otherwise
     */
    virtual bool CheckConfig() = 0;

    /**
     * Initialize parameters (if any) before the first packet is enqueued.
     * This method is automatically called at simulation initialization time,
     * after the CheckConfig() method has been called.
     * \sa QueueDisc::CheckConfig
     */
    virtual void InitializeParams() = 0;

    /**
     * Modelled after the Linux function qdisc_run_begin (include/net/sch_generic.h).
     * @return false if the qdisc is already running; otherwise, set the qdisc as running and return
     * true.
     */
    bool RunBegin();

    /**
     * Modelled after the Linux function qdisc_run_end (include/net/sch_generic.h).
     * Set the qdisc as not running.
     */
    void RunEnd();

    /**
     * Modelled after the Linux function qdisc_restart (net/sched/sch_generic.c)
     * Dequeue a packet (by calling DequeuePacket) and send it to the device (by calling Transmit).
     * @return true if a packet is successfully sent to the device.
     */
    bool Restart();

    /**
     * Modelled after the Linux function dequeue_skb (net/sched/sch_generic.c)
     * @return the requeued packet, if any, or the packet dequeued by the queue disc, otherwise.
     */
    Ptr<QueueDiscItem> DequeuePacket();

    /**
     * Modelled after the Linux function dev_requeue_skb (net/sched/sch_generic.c)
     * Requeues a packet whose transmission failed.
     * @param item the packet to requeue
     */
    void Requeue(Ptr<QueueDiscItem> item);

    /**
     * Modelled after the Linux function sch_direct_xmit (net/sched/sch_generic.c)
     * Sends a packet to the device if the device queue is not stopped, and requeues
     * it otherwise.
     * @param item the packet to transmit
     * @return true if the device queue is not stopped and the queue disc is not empty
     */
    bool Transmit(Ptr<QueueDiscItem> item);

    /**
     * @brief Perform the actions required when the queue disc is notified of
     *        a packet enqueue
     * @param item item that was enqueued
     */
    void PacketEnqueued(Ptr<const QueueDiscItem> item);

    /**
     * @brief Perform the actions required when the queue disc is notified of
     *        a packet dequeue
     * @param item item that was dequeued
     */
    void PacketDequeued(Ptr<const QueueDiscItem> item);

    /// Default quota (as in /proc/sys/net/core/dev_weight)
    static const uint32_t DEFAULT_QUOTA = 64;

    std::vector<Ptr<InternalQueue>> m_queues;   //!< Internal queues
    std::vector<Ptr<PacketFilter>> m_filters;   //!< Packet filters
    std::vector<Ptr<QueueDiscClass>> m_classes; //!< Classes

    TracedValue<uint32_t> m_nPackets; //!< Number of packets in the queue
    TracedValue<uint32_t> m_nBytes;   //!< Number of bytes in the queue
    TracedCallback<Time> m_sojourn;   //!< Sojourn time of the latest dequeued packet
    QueueSize m_maxSize;              //!< max queue size

    Stats m_stats;    //!< The collected statistics
    uint32_t m_quota; //!< Maximum number of packets dequeued in a qdisc run
    Ptr<NetDeviceQueueInterface> m_devQueueIface; //!< NetDevice queue interface
    SendCallback m_send;           //!< Callback used to send a packet to the receiving object
    bool m_running;                //!< The queue disc is performing multiple dequeue operations
    Ptr<QueueDiscItem> m_requeued; //!< The last packet that failed to be transmitted
    bool m_peeked;                 //!< A packet was dequeued because Peek was called
    std::string m_childQueueDiscDropMsg; //!< Reason why a packet was dropped by a child queue disc
    std::string m_childQueueDiscMarkMsg; //!< Reason why a packet was marked by a child queue disc
    QueueDiscSizePolicy m_sizePolicy;    //!< The queue disc size policy
    bool m_prohibitChangeMode;           //!< True if changing mode is prohibited

    /// Traced callback: fired when a packet is enqueued
    TracedCallback<Ptr<const QueueDiscItem>> m_traceEnqueue;
    /// Traced callback: fired when a packet is dequeued
    TracedCallback<Ptr<const QueueDiscItem>> m_traceDequeue;
    /// Traced callback: fired when a packet is requeued
    TracedCallback<Ptr<const QueueDiscItem>> m_traceRequeue;
    /// Traced callback: fired when a packet is dropped
    TracedCallback<Ptr<const QueueDiscItem>> m_traceDrop;
    /// Traced callback: fired when a packet is dropped before enqueue
    TracedCallback<Ptr<const QueueDiscItem>, const char*> m_traceDropBeforeEnqueue;
    /// Traced callback: fired when a packet is dropped after dequeue
    TracedCallback<Ptr<const QueueDiscItem>, const char*> m_traceDropAfterDequeue;
    /// Traced callback: fired when a packet is marked
    TracedCallback<Ptr<const QueueDiscItem>, const char*> m_traceMark;

    /// Type for the function objects notifying that a packet has been dropped by an internal queue
    typedef std::function<void(Ptr<const QueueDiscItem>)> InternalQueueDropFunctor;
    /// Type for the function objects notifying that a packet has been dropped by a child queue disc
    typedef std::function<void(Ptr<const QueueDiscItem>, const char*)> ChildQueueDiscDropFunctor;
    /// Type for the function objects notifying that a packet has been marked by a child queue disc
    typedef std::function<void(Ptr<const QueueDiscItem>, const char*)> ChildQueueDiscMarkFunctor;

    /// Function object called when an internal queue dropped a packet before enqueue
    InternalQueueDropFunctor m_internalQueueDbeFunctor;
    /// Function object called when an internal queue dropped a packet after dequeue
    InternalQueueDropFunctor m_internalQueueDadFunctor;
    /// Function object called when a child queue disc dropped a packet before enqueue
    ChildQueueDiscDropFunctor m_childQueueDiscDbeFunctor;
    /// Function object called when a child queue disc dropped a packet after dequeue
    ChildQueueDiscDropFunctor m_childQueueDiscDadFunctor;
    /// Function object called when a child queue disc marked a packet
    ChildQueueDiscMarkFunctor m_childQueueDiscMarkFunctor;
};

/**
 * @brief Stream insertion operator.
 *
 * @param os the stream
 * @param stats the queue disc statistics
 * @returns a reference to the stream
 */
std::ostream& operator<<(std::ostream& os, const QueueDisc::Stats& stats);

} // namespace ns3

#endif /* QueueDisc */
