tcp-tx-buffer.h

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/*
 * Copyright (c) 2010-2015 Adrian Sai-wah Tam
 * Copyright (c) 2016 Natale Patriciello <natale.patriciello@gmail.com>
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 * Original author: Adrian Sai-wah Tam <adrian.sw.tam@gmail.com>
 */
 
#ifndef TCP_TX_BUFFER_H
#define TCP_TX_BUFFER_H
 
#include "tcp-option-sack.h"
#include "tcp-tx-item.h"
 
#include "ns3/object.h"
#include "ns3/sequence-number.h"
#include "ns3/traced-value.h"
 
namespace ns3
{
class Packet;
 
/**
 * \ingroup tcp
 *
 * \brief Tcp sender buffer
 *
 * The class keeps track of all data that the application wishes to transmit to
 * the other end. When the data is acknowledged, it is removed from the buffer.
 * The buffer has a maximum size, and data is not saved if the amount exceeds
 * the limit. Packets can be added to the class through the method Add(). An
 * important thing to remember is that all the data managed is strictly
 * sequential. It can be divided into blocks, but all the data follow a strict
 * ordering. That order is managed through SequenceNumber.
 *
 * In other words, this buffer contains numbered bytes (e.g., 1,2,3), and the
 * class is allowed to return only ordered (using "<" as operator) subsets
 * (e.g. 1,2 or 2,3 or 1,2,3).
 *
 * The data structure underlying this is composed by two distinct packet lists.
 * The first (SentList) is initially empty, and it contains the packets
 * returned by the method CopyFromSequence. The second (AppList) is initially
 * empty, and it contains the packets coming from the applications, but that
 * are not transmitted yet as segments. To discover how the chunks are managed
 * and retrieved from these lists, check CopyFromSequence documentation.
 *
 * The head of the data is represented by m_firstByteSeq, and it is returned by
 * HeadSequence(). The last byte is returned by TailSequence(). In this class,
 * we also store the size (in bytes) of the packets inside the SentList in the
 * variable m_sentSize.
 *
 * SACK management
 * ---------------
 *
 * The SACK information is usually saved in a data structure referred as
 * scoreboard. In this implementation, the scoreboard is developed on top of
 * the existing classes. In particular, instead of keeping raw pointers to
 * packets in TcpTxBuffer we added the capability to store some flags
 * associated with every segment sent. This is done through the use of the
 * class TcpTxItem: instead of storing a list of packets, we store a list of
 * TcpTxItem. Each item has different flags (check the corresponding
 * documentation) and maintaining the scoreboard is a matter of travelling the
 * list and set the SACK flag on the corresponding segment sent.
 *
 * Item properties
 * ---------------
 *
 * An item (that represent a segment in flight) is not considered in flight
 * anymore when it is marked lost or sacked. A sacked item represents an
 * item which is received by the other end, but it is still impossible
 * to delete from the list because other pieces are missing at the other
 * end. A lost item never reached the other end, and retransmission is probably
 * needed. Other properties are retransmitted, that indicates if an item was
 * retransmitted, and the sequence number of the first byte of the packet. When
 * a segment is sent for the first time, only the sequence number is set, and
 * all the remaining properties are set to false. If an item is explicitly
 * sacked by the receiver, we mark it as such. Each time we receive updated
 * sack information from the other end, we perform a check to evaluate the
 * segments that can be lost (\see UpdateLostCount), and we set the flags
 * accordingly.
 *
 * Management of bytes in flight
 * -----------------------------
 *
 * Since this class manages all the output segments and the scoreboard, we can
 * do calculations about the number of bytes in flights. Earlier versions of
 * this class used algorithms copied from RFC 6675. They were inefficient
 * because they required a complete walk into the list of sent segments each
 * time a simple question, such as "Is this sequence lost?" or "How many bytes
 * are in flight?". Therefore, the class has been updated keeping in
 * consideration the RFCs (including RFC 4898) and the Linux operating
 * system. As a reference, we kept the older methods for calculating the
 * bytes in flight and if a segment is lost, renaming them as "RFC" version
 * of the methods. To have a look how the calculations are made, please see
 * BytesInFlight method.
 *
 * Lost segments
 * -------------
 *
 * After the sender receives a new SACK block, it updates the amount of segment
 * that it considers as lost, following the specifications made in RFC 6675
 * (for more detail please see the method UpdateLostCount). In case of SACKless
 * connection, the TcpSocketImplementation should provide hints through
 * the MarkHeadAsLost and AddRenoSack methods.
 *
 * \see BytesInFlight
 * \see Size
 * \see SizeFromSequence
 * \see CopyFromSequence
 */
class TcpTxBuffer : public Object
{
  public:
    /**
     * \brief Get the type ID.
     * \return the object TypeId
     */
    static TypeId GetTypeId();
    /**
     * \brief Constructor
     * \param n initial Sequence number to be transmitted
     */
    TcpTxBuffer(uint32_t n = 0);
    ~TcpTxBuffer() override;
 
    // Accessors
 
    /**
     * \brief Get the sequence number of the buffer head
     * \returns the first byte's sequence number
     */
    SequenceNumber32 HeadSequence() const;
 
    /**
     * \brief Get the sequence number of the buffer tail (plus one)
     * \returns the last byte's sequence number + 1
     */
    SequenceNumber32 TailSequence() const;
 
    /**
     * \brief Returns total number of bytes in this buffer
     * \returns total number of bytes in this Tx buffer
     */
    uint32_t Size() const;
 
    /**
     * \brief Get the maximum buffer size
     * \returns the Tx window size (in bytes)
     */
    uint32_t MaxBufferSize() const;
 
    /**
     * \brief Set the maximum buffer size
     * \param n Tx window size (in bytes)
     */
    void SetMaxBufferSize(uint32_t n);
 
    /**
     * \brief check whether SACK is used on the corresponding TCP socket
     * \return true if SACK is used
     */
    bool IsSackEnabled() const;
 
    /**
     * \brief tell tx-buffer whether SACK is used on this TCP socket
     *
     * \param enabled whether sack is used
     */
    void SetSackEnabled(bool enabled);
 
    /**
     * \brief Returns the available capacity of this buffer
     * \returns available capacity in this Tx window
     */
    uint32_t Available() const;
 
    /**
     * \brief Set the DupAckThresh
     * \param dupAckThresh the threshold
     */
    void SetDupAckThresh(uint32_t dupAckThresh);
 
    /**
     * \brief Set the segment size
     * \param segmentSize the segment size
     */
    void SetSegmentSize(uint32_t segmentSize);
 
    /**
     * \brief Return the number of segments in the sent list that
     * have been transmitted more than once, without acknowledgment.
     *
     * This method is to support the retransmits count for determining PipeSize
     * in NewReno-style TCP.
     *
     * \returns number of segments that have been transmitted more than once, without acknowledgment
     */
    uint32_t GetRetransmitsCount() const;
 
    /**
     * \brief Get the number of segments that we believe are lost in the network
     *
     * It is calculated in UpdateLostCount.
     * \return the number of lost segment
     */
    uint32_t GetLost() const;
 
    /**
     * \brief Get the number of segments that have been explicitly sacked by the receiver.
     * \return the number of sacked segment.
     */
    uint32_t GetSacked() const;
 
    /**
     * \brief Append a data packet to the end of the buffer
     *
     * \param p The packet to be appended to the Tx buffer
     * \return Boolean to indicate success
     */
    bool Add(Ptr<Packet> p);
 
    /**
     * \brief Returns the number of bytes from the buffer in the range [seq, tailSequence)
     *
     * \param seq initial sequence number
     * \returns the number of bytes from the buffer in the range
     */
    uint32_t SizeFromSequence(const SequenceNumber32& seq) const;
 
    /**
     * \brief Copy data from the range [seq, seq+numBytes) into a packet
     *
     * In the following, we refer to the block [seq, seq+numBytes) simply as "block".
     * We check the boundary of the block, and divide the possibilities in three
     * cases:
     *
     * - the block have already been transmitted (managed in GetTransmittedSegment)
     * - the block have not been transmitted yet (managed in GetNewSegment)
     *
     * The last case is when the block is partially transmitted and partially
     * not transmitted. We trick this case by requesting the portion not transmitted
     * from GetNewSegment, and then calling GetTransmittedSegment with the full
     * block range.
     *
     * \param numBytes number of bytes to copy
     * \param seq start sequence number to extract
     * \returns a pointer to the TcpTxItem that corresponds to what requested.
     * Please do not delete the pointer, nor modify Packet data or sequence numbers.
     */
    TcpTxItem* CopyFromSequence(uint32_t numBytes, const SequenceNumber32& seq);
 
    /**
     * \brief Set the head sequence of the buffer
     *
     * Set the head (m_firstByteSeq) to seq. Supposed to be called only when the
     * connection is just set up and we did not send any data out yet.
     * \param seq The sequence number of the head byte
     */
    void SetHeadSequence(const SequenceNumber32& seq);
 
    /**
     * \brief Checks whether the ack corresponds to retransmitted data
     *
     * \param ack ACK number received
     * \return true if retransmitted data was acked
     */
    bool IsRetransmittedDataAcked(const SequenceNumber32& ack) const;
 
    /**
     * \brief Discard data up to but not including this sequence number.
     *
     * \param seq The first sequence number to maintain after discarding all the
     * previous sequences.
     * \param beforeDelCb Callback invoked, if it is not null, before the deletion
     * of an Item (because it was, probably, ACKed)
     */
    void DiscardUpTo(const SequenceNumber32& seq,
                     const Callback<void, TcpTxItem*>& beforeDelCb = m_nullCb);
 
    /**
     * \brief Update the scoreboard
     * \param list list of SACKed blocks
     * \param sackedCb Callback invoked, if it is not null, when a segment has been
     * SACKed by the receiver.
     * \returns the number of bytes newly sacked by the list of blocks
     */
    uint32_t Update(const TcpOptionSack::SackList& list,
                    const Callback<void, TcpTxItem*>& sackedCb = m_nullCb);
 
    /**
     * \brief Check if a segment is lost
     *
     * It does a check on the flags to determine if the segment has to be considered
     * as lost for an external class
     *
     * \param seq sequence to check
     * \return true if the sequence is supposed to be lost, false otherwise
     */
    bool IsLost(const SequenceNumber32& seq) const;
 
    /**
     * \brief Get the next sequence number to transmit, according to RFC 6675
     *
     * \param seq Next sequence number to transmit, based on the scoreboard information
     * \param seqHigh Maximum sequence number to transmit, based on SMSS and/or receiver window
     * \param isRecovery true if the socket congestion state is in recovery mode
     * \return true is seq is updated, false otherwise
     */
    bool NextSeg(SequenceNumber32* seq, SequenceNumber32* seqHigh, bool isRecovery) const;
 
    /**
     * \brief Return total bytes in flight
     *
     * Counting packets in flight is pretty simple:
     *
     * \f$in_flight = sentSize - leftOut + retrans\f$
     *
     * sentsize is SND.NXT-SND.UNA, retrans is the number of retransmitted segments.
     * leftOut is the number of segment that left the network without being ACKed:
     *
     * \f$leftOut = sacked_out + lost_out\f$
     *
     * To see how we define the lost packets, look at the method UpdateLostCount.
     *
     * \returns total bytes in flight
     */
    uint32_t BytesInFlight() const;
 
    /**
     * \brief Set the entire sent list as lost (typically after an RTO)
     *
     * Used to set all the sent list as lost, so the bytes in flight is not counting
     * them as in flight, but we will continue to use SACK information for
     * recovering the timeout.
     *
     * Moreover, reset the retransmit flag for every item.
     * \param resetSack True if the function should reset the SACK flags.
     */
    void SetSentListLost(bool resetSack = false);
 
    /**
     * \brief Check if the head is retransmitted
     *
     * \return true if the head is retransmitted, false in all other cases
     * (including no segment sent)
     */
    bool IsHeadRetransmitted() const;
 
    /**
     * \brief DeleteRetransmittedFlagFromHead
     */
    void DeleteRetransmittedFlagFromHead();
 
    /**
     * \brief Reset the sent list
     *
     */
    void ResetSentList();
 
    /**
     * \brief Take the last segment sent and put it back into the un-sent list
     * (at the beginning)
     */
    void ResetLastSegmentSent();
 
    /**
     * \brief Mark the head of the sent list as lost.
     */
    void MarkHeadAsLost();
 
    /**
     * \brief Emulate SACKs for SACKless connection: account for a new dupack.
     *
     * The method walk the list of the sent segment until it finds a segment
     * that was not accounted in the sackedOut count. The head will never
     * be included. To reset the information added with this function (e.g.,
     * after an RTO) please use ResetRenoSack.
     *
     * The method DiscardUpTo, when invoked, will make sure to properly clean any
     * flag on the discarded item. As example, if the implementation discard an item
     * that is marked as sacked, the sackedOut count is decreased accordingly.
     */
    void AddRenoSack();
 
    /**
     * \brief Reset the SACKs.
     *
     * Reset the Scoreboard from all SACK information. This method also works in
     * case the SACKs are set by the Update method.
     */
    void ResetRenoSack();
 
    /**
     * \brief Set callback to obtain receiver window value
     * \param rWndCallback receiver window callback
     */
    void SetRWndCallback(Callback<uint32_t> rWndCallback);
 
  private:
    friend std::ostream& operator<<(std::ostream& os, const TcpTxBuffer& tcpTxBuf);
 
    typedef std::list<TcpTxItem*> PacketList; //!< container for data stored in the buffer
 
    /**
     * \brief Update the lost count
     *
     * Reset lost to 0, then walk the sent list looking for lost segments.
     * We have two possible algorithms for detecting lost packets:
     *
     * - RFC 6675 algorithm, which says that if more than "Dupack thresh" (e.g., 3)
     * sacked segments above the sequence, then we can consider the sequence lost;
     * - NewReno (RFC6582): in Recovery we assume that one segment is lost
     * (classic Reno). While we are in Recovery and a partial ACK arrives,
     * we assume that one more packet is lost (NewReno).
     *
     * The {New}Reno cases, for now, are managed in TcpSocketBase through the
     * call to MarkHeadAsLost.
     * This function is, therefore, called after a SACK option has been received,
     * and updates the lost count. It can be probably optimized by not walking
     * the entire list, but a subset.
     *
     */
    void UpdateLostCount();
 
    /**
     * \brief Remove the size specified from the lostOut, retrans, sacked count
     *
     * Used only in DiscardUpTo
     *
     * \param item Item that will be discarded
     * \param size size to remove (can be different from pktSize because of fragmentation)
     */
    void RemoveFromCounts(TcpTxItem* item, uint32_t size);
 
    /**
     * \brief Decide if a segment is lost based on RFC 6675 algorithm.
     * \param seq Sequence
     * \param segment Iterator to the sequence
     * \return true if seq is lost per RFC 6675, false otherwise
     */
    bool IsLostRFC(const SequenceNumber32& seq, const PacketList::const_iterator& segment) const;
 
    /**
     * \brief Calculate the number of bytes in flight per RFC 6675
     * \return the number of bytes in flight
     */
    uint32_t BytesInFlightRFC() const;
 
    /**
     * \brief Get a block of data not transmitted yet and move it into SentList
     *
     * If the block is not yet transmitted, hopefully, seq is exactly the sequence
     * number of the first byte of the first packet inside AppList. We extract
     * the block from AppList and move it into the SentList, before returning the
     * block itself. We manage possible fragmentation (or merges) inside AppList
     * through GetPacketFromList.
     *
     * \see GetPacketFromList
     * \param numBytes number of bytes to copy
     *
     * \return the item that contains the right packet
     */
    TcpTxItem* GetNewSegment(uint32_t numBytes);
 
    /**
     * \brief Get a block of data previously transmitted
     *
     * This is clearly a retransmission, and if everything is going well,
     * the block requested is matching perfectly with another one requested
     * in the past. If not, fragmentation or merge are required. We manage
     * both inside GetPacketFromList.
     *
     * \see GetPacketFromList
     *
     * \param numBytes number of bytes to copy
     * \param seq sequence requested
     * \returns the item that contains the right packet
     */
    TcpTxItem* GetTransmittedSegment(uint32_t numBytes, const SequenceNumber32& seq);
 
    /**
     * \brief Get a block (which is returned as Packet) from a list
     *
     * This function extract a block [requestedSeq,numBytes) from the list, which
     * starts at startingSeq.
     *
     * The cases we need to manage are two, and they are depicted in the following
     * image:
     *
     * \verbatim
                         |------|     |----|     |----|
                  list = |      | --> |    | --> |    |
                         |------|     |----|     |----|
 
                         ^      ^
                         | ^ ^  |         (1)
                       seq | |  seq + numBytes
                           | |
                           | |
                        seq   seq + numBytes     (2)
     \endverbatim
     *
     * The case 1 is easy to manage: the requested block is exactly a packet
     * already stored. If one value (seq or seq + numBytes) does not align
     * to a packet boundary, or when both values does not align (case 2), it is
     * a bit more complex.
     *
     * Basically, we have two possible operations:
     *
     *  - fragment : split an existing packet in two
     *  - merge    : merge two existing packets in one
     *
     * and we reduce case (2) to case (1) through sequentially applying fragment
     * or merge. For instance:
     *
     * \verbatim
        |------|
        |      |
        |------|
 
        ^ ^  ^ ^
        | |  | |
    start |  | |
          |  | end
         seq |
             seq + numBytes
     \endverbatim
     *
     * To reduce to case (1), we need to perform two fragment operations:
     *
     * - fragment (start, seq)
     * - fragment (seq + numBytes, end)
     *
     * After these operations, the requested block is exactly the resulting packet.
     * Merge operation is required when the requested block span over two (or more)
     * existing packets.
     *
     * While this could be extremely slow in the worst possible scenario (one big
     * packet which is split in small packets for transmission, and merged for
     * re-transmission) that scenario is unlikely during a TCP transmission (since
     * MSS can change, but it is stable, and retransmissions do not happen for
     * each segment).
     *
     * \param list List to extract block from
     * \param startingSeq Starting sequence of the list
     * \param numBytes Bytes to extract, starting from requestedSeq
     * \param requestedSeq Requested sequence
     * \param listEdited output parameter which indicates if the list has been edited
     * \return the item that contains the right packet
     */
    TcpTxItem* GetPacketFromList(PacketList& list,
                                 const SequenceNumber32& startingSeq,
                                 uint32_t numBytes,
                                 const SequenceNumber32& requestedSeq,
                                 bool* listEdited = nullptr) const;
 
    /**
     * \brief Merge two TcpTxItem
     *
     * Merge t2 in t1. It consists in copying the lastSent field if t2 is more
     * recent than t1. Retransmitted field is copied only if it set in t2 but not
     * in t1. Sacked is copied only if it is true in both items.
     *
     * \param t1 first item
     * \param t2 second item
     */
    void MergeItems(TcpTxItem* t1, TcpTxItem* t2) const;
 
    /**
     * \brief Split one TcpTxItem
     *
     * Move "size" bytes from t2 into t1, copying all the fields.
     * Adjust the starting sequence of each item.
     *
     * \param t1 first item
     * \param t2 second item
     * \param size Size to split
     */
    void SplitItems(TcpTxItem* t1, TcpTxItem* t2, uint32_t size) const;
 
    /**
     * \brief Check if the values of sacked, lost, retrans, are in sync
     * with the sent list.
     */
    void ConsistencyCheck() const;
 
    /**
     * \brief Find the highest SACK byte
     * \return a pair with the highest byte and an iterator inside m_sentList
     */
    std::pair<TcpTxBuffer::PacketList::const_iterator, SequenceNumber32> FindHighestSacked() const;
 
    PacketList m_appList;              //!< Buffer for application data
    PacketList m_sentList;             //!< Buffer for sent (but not acked) data
    uint32_t m_maxBuffer;              //!< Max number of data bytes in buffer (SND.WND)
    uint32_t m_size;                   //!< Size of all data in this buffer
    uint32_t m_sentSize;               //!< Size of sent (and not discarded) segments
    Callback<uint32_t> m_rWndCallback; //!< Callback to obtain RCV.WND value
 
    TracedValue<SequenceNumber32>
        m_firstByteSeq; //!< Sequence number of the first byte in data (SND.UNA)
    std::pair<PacketList::const_iterator, SequenceNumber32> m_highestSack; //!< Highest SACK byte
 
    uint32_t m_lostOut{0};   //!< Number of lost bytes
    uint32_t m_sackedOut{0}; //!< Number of sacked bytes
    uint32_t m_retrans{0};   //!< Number of retransmitted bytes
 
    uint32_t m_dupAckThresh{0}; //!< Duplicate Ack threshold from TcpSocketBase
    uint32_t m_segmentSize{0};  //!< Segment size from TcpSocketBase
    bool m_renoSack{false};     //!< Indicates if AddRenoSack was called
    bool m_sackEnabled{true};   //!< Indicates if SACK is enabled on this connection
    bool m_sackSeen{false};     //!< Indicates if a SACK was received
 
    static Callback<void, TcpTxItem*> m_nullCb; //!< Null callback for an item
};
 
/**
 * \brief Output operator.
 * \param os The output stream.
 * \param tcpTxBuf the TcpTxBuffer to print.
 * \returns The output stream.
 */
std::ostream& operator<<(std::ostream& os, const TcpTxBuffer& tcpTxBuf);
 
/**
 * \brief Output operator.
 * \param os The output stream.
 * \param item the item to print.
 * \returns The output stream.
 */
std::ostream& operator<<(std::ostream& os, const TcpTxItem& item);
 
} // namespace ns3
 
#endif /* TCP_TX_BUFFER_H */