tcp-rx-buffer.cc

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/*
 * Copyright (c) 2010 Adrian Sai-wah Tam
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 * Author: Adrian Sai-wah Tam <adrian.sw.tam@gmail.com>
 */
 
#include "tcp-rx-buffer.h"
 
#include "ns3/log.h"
#include "ns3/packet.h"
 
namespace ns3
{
 
NS_LOG_COMPONENT_DEFINE("TcpRxBuffer");
 
NS_OBJECT_ENSURE_REGISTERED(TcpRxBuffer);
 
TypeId
TcpRxBuffer::GetTypeId()
{
    static TypeId tid = TypeId("ns3::TcpRxBuffer")
                            .SetParent<Object>()
                            .SetGroupName("Internet")
                            .AddConstructor<TcpRxBuffer>()
                            .AddTraceSource("NextRxSequence",
                                            "Next sequence number expected (RCV.NXT)",
                                            MakeTraceSourceAccessor(&TcpRxBuffer::m_nextRxSeq),
                                            "ns3::SequenceNumber32TracedValueCallback");
    return tid;
}
 
/* A user is supposed to create a TcpSocket through a factory. In TcpSocket,
 * there are attributes SndBufSize and RcvBufSize to control the default Tx and
 * Rx window sizes respectively, with default of 128 KiByte. The attribute
 * RcvBufSize is passed to TcpRxBuffer by TcpSocketBase::SetRcvBufSize() and in
 * turn, TcpRxBuffer:SetMaxBufferSize(). Therefore, the m_maxBuffer value
 * initialized below is insignificant.
 */
TcpRxBuffer::TcpRxBuffer(uint32_t n)
    : m_nextRxSeq(n),
      m_gotFin(false),
      m_size(0),
      m_maxBuffer(32768),
      m_availBytes(0)
{
}
 
TcpRxBuffer::~TcpRxBuffer()
{
}
 
SequenceNumber32
TcpRxBuffer::NextRxSequence() const
{
    return m_nextRxSeq;
}
 
void
TcpRxBuffer::SetNextRxSequence(const SequenceNumber32& s)
{
    m_nextRxSeq = s;
}
 
uint32_t
TcpRxBuffer::MaxBufferSize() const
{
    return m_maxBuffer;
}
 
void
TcpRxBuffer::SetMaxBufferSize(uint32_t s)
{
    m_maxBuffer = s;
}
 
uint32_t
TcpRxBuffer::Size() const
{
    return m_size;
}
 
uint32_t
TcpRxBuffer::Available() const
{
    return m_availBytes;
}
 
void
TcpRxBuffer::IncNextRxSequence()
{
    NS_LOG_FUNCTION(this);
    // Increment nextRxSeq is valid only if we don't have any data buffered,
    // this is supposed to be called only during the three-way handshake
    NS_ASSERT(m_size == 0);
    m_nextRxSeq++;
}
 
// Return the lowest sequence number that this TcpRxBuffer cannot accept
SequenceNumber32
TcpRxBuffer::MaxRxSequence() const
{
    if (m_gotFin)
    { // No data allowed beyond FIN
        return m_finSeq;
    }
    else if (!m_data.empty() && m_nextRxSeq > m_data.begin()->first)
    { // No data allowed beyond Rx window allowed
        return m_data.begin()->first + SequenceNumber32(m_maxBuffer);
    }
    return m_nextRxSeq + SequenceNumber32(m_maxBuffer);
}
 
void
TcpRxBuffer::SetFinSequence(const SequenceNumber32& s)
{
    NS_LOG_FUNCTION(this);
 
    m_gotFin = true;
    m_finSeq = s;
    if (m_nextRxSeq == m_finSeq)
    {
        ++m_nextRxSeq;
    }
}
 
bool
TcpRxBuffer::Finished()
{
    return (m_gotFin && m_finSeq < m_nextRxSeq);
}
 
bool
TcpRxBuffer::Add(Ptr<Packet> p, const TcpHeader& tcph)
{
    NS_LOG_FUNCTION(this << p << tcph);
 
    uint32_t pktSize = p->GetSize();
    SequenceNumber32 headSeq = tcph.GetSequenceNumber();
    SequenceNumber32 tailSeq = headSeq + SequenceNumber32(pktSize);
    NS_LOG_LOGIC("Add pkt " << p << " len=" << pktSize << " seq=" << headSeq
                            << ", when NextRxSeq=" << m_nextRxSeq << ", buffsize=" << m_size);
 
    // Trim packet to fit Rx window specification
    if (headSeq < m_nextRxSeq)
    {
        headSeq = m_nextRxSeq;
    }
    if (!m_data.empty())
    {
        SequenceNumber32 maxSeq = m_data.begin()->first + SequenceNumber32(m_maxBuffer);
        if (maxSeq < tailSeq)
        {
            tailSeq = maxSeq;
        }
        if (tailSeq < headSeq)
        {
            headSeq = tailSeq;
        }
    }
    // Remove overlapped bytes from packet
    auto i = m_data.begin();
    while (i != m_data.end() && i->first <= tailSeq)
    {
        SequenceNumber32 lastByteSeq = i->first + SequenceNumber32(i->second->GetSize());
        if (lastByteSeq > headSeq)
        {
            if (i->first > headSeq && lastByteSeq < tailSeq)
            { // Rare case: Existing packet is embedded fully in the new packet
                m_size -= i->second->GetSize();
                m_data.erase(i++);
                continue;
            }
            if (i->first <= headSeq)
            { // Incoming head is overlapped
                headSeq = lastByteSeq;
            }
            if (lastByteSeq >= tailSeq)
            { // Incoming tail is overlapped
                tailSeq = i->first;
            }
        }
        ++i;
    }
    // We now know how much we are going to store, trim the packet
    if (headSeq >= tailSeq)
    {
        NS_LOG_LOGIC("Nothing to buffer");
        return false; // Nothing to buffer anyway
    }
    else
    {
        uint32_t start = static_cast<uint32_t>(headSeq - tcph.GetSequenceNumber());
        auto length = static_cast<uint32_t>(tailSeq - headSeq);
        p = p->CreateFragment(start, length);
        NS_ASSERT(length == p->GetSize());
    }
    // Insert packet into buffer
    NS_ASSERT(m_data.find(headSeq) == m_data.end()); // Shouldn't be there yet
    m_data[headSeq] = p;
 
    if (headSeq > m_nextRxSeq)
    {
        // Generate a new SACK block
        UpdateSackList(headSeq, tailSeq);
    }
 
    NS_LOG_LOGIC("Buffered packet of seqno=" << headSeq << " len=" << p->GetSize());
    // Update variables
    m_size += p->GetSize(); // Occupancy
    for (i = m_data.begin(); i != m_data.end(); ++i)
    {
        if (i->first < m_nextRxSeq)
        {
            continue;
        }
        else if (i->first > m_nextRxSeq)
        {
            break;
        };
        m_nextRxSeq = i->first + SequenceNumber32(i->second->GetSize());
        m_availBytes += i->second->GetSize();
        ClearSackList(m_nextRxSeq);
    }
    NS_LOG_LOGIC("Updated buffer occupancy=" << m_size << " nextRxSeq=" << m_nextRxSeq);
    if (m_gotFin && m_nextRxSeq == m_finSeq)
    { // Account for the FIN packet
        ++m_nextRxSeq;
    };
    return true;
}
 
uint32_t
TcpRxBuffer::GetSackListSize() const
{
    NS_LOG_FUNCTION(this);
 
    return static_cast<uint32_t>(m_sackList.size());
}
 
void
TcpRxBuffer::UpdateSackList(const SequenceNumber32& head, const SequenceNumber32& tail)
{
    NS_LOG_FUNCTION(this << head << tail);
    NS_ASSERT(head > m_nextRxSeq);
 
    TcpOptionSack::SackBlock current;
    current.first = head;
    current.second = tail;
 
    // The block "current" has been safely stored. Now we need to build the SACK
    // list, to be advertised. From RFC 2018:
    // (a) The first SACK block (i.e., the one immediately following the
    //     kind and length fields in the option) MUST specify the contiguous
    //     block of data containing the segment which triggered this ACK,
    //     unless that segment advanced the Acknowledgment Number field in
    //     the header.  This assures that the ACK with the SACK option
    //     reflects the most recent change in the data receiver's buffer
    //     queue.
    //
    // (b) The data receiver SHOULD include as many distinct SACK blocks as
    //     possible in the SACK option.  Note that the maximum available
    //     option space may not be sufficient to report all blocks present in
    //     the receiver's queue.
    //
    // (c) The SACK option SHOULD be filled out by repeating the most
    //     recently reported SACK blocks (based on first SACK blocks in
    //     previous SACK options) that are not subsets of a SACK block
    //     already included in the SACK option being constructed.  This
    //     assures that in normal operation, any segment remaining part of a
    //     non-contiguous block of data held by the data receiver is reported
    //     in at least three successive SACK options, even for large-window
    //     TCP implementations [RFC1323]).  After the first SACK block, the
    //     following SACK blocks in the SACK option may be listed in
    //     arbitrary order.
 
    m_sackList.push_front(current);
 
    // We have inserted the block at the beginning of the list. Now, we should
    // check if any existing blocks overlap with that.
    bool updated = false;
    auto it = m_sackList.begin();
    TcpOptionSack::SackBlock begin = *it;
    TcpOptionSack::SackBlock merged;
    ++it;
 
    // Iterates until we examined all blocks in the list (maximum 4)
    while (it != m_sackList.end())
    {
        current = *it;
 
        // This is a left merge:
        // [current_first; current_second] [beg_first; beg_second]
        if (begin.first == current.second)
        {
            NS_ASSERT(current.first < begin.second);
            merged = TcpOptionSack::SackBlock(current.first, begin.second);
            updated = true;
        }
        // while this is a right merge
        // [begin_first; begin_second] [current_first; current_second]
        else if (begin.second == current.first)
        {
            NS_ASSERT(begin.first < current.second);
            merged = TcpOptionSack::SackBlock(begin.first, current.second);
            updated = true;
        }
 
        // If we have merged the blocks (and the result is in merged) we should
        // delete the current block (it), the first block, and insert the merged
        // one at the beginning.
        if (updated)
        {
            m_sackList.erase(it);
            m_sackList.pop_front();
            m_sackList.push_front(merged);
            it = m_sackList.begin();
            begin = *it;
            updated = false;
        }
 
        ++it;
    }
 
    // Since the maximum blocks that fits into a TCP header are 4, there's no
    // point on maintaining the others.
    if (m_sackList.size() > 4)
    {
        m_sackList.pop_back();
    }
 
    // Please note that, if a block b is discarded and then a block contiguous
    // to b is received, only that new block (without the b part) is reported.
    // This is perfectly fine for the RFC point (a), given that we do not report any
    // overlapping blocks shortly after.
}
 
void
TcpRxBuffer::ClearSackList(const SequenceNumber32& seq)
{
    NS_LOG_FUNCTION(this << seq);
 
    for (auto it = m_sackList.begin(); it != m_sackList.end();)
    {
        TcpOptionSack::SackBlock block = *it;
        NS_ASSERT(block.first < block.second);
 
        if (block.second <= seq)
        {
            it = m_sackList.erase(it);
        }
        else
        {
            it++;
        }
    }
}
 
TcpOptionSack::SackList
TcpRxBuffer::GetSackList() const
{
    return m_sackList;
}
 
Ptr<Packet>
TcpRxBuffer::Extract(uint32_t maxSize)
{
    NS_LOG_FUNCTION(this << maxSize);
 
    uint32_t extractSize = std::min(maxSize, m_availBytes);
    NS_LOG_LOGIC("Requested to extract " << extractSize
                                         << " bytes from TcpRxBuffer of size=" << m_size);
    if (extractSize == 0)
    {
        return nullptr; // No contiguous block to return
    }
    NS_ASSERT(!m_data.empty());            // At least we have something to extract
    Ptr<Packet> outPkt = Create<Packet>(); // The packet that contains all the data to return
    BufIterator i;
    while (extractSize)
    { // Check the buffered data for delivery
        i = m_data.begin();
        NS_ASSERT(i->first <= m_nextRxSeq); // in-sequence data expected
        // Check if we send the whole pkt or just a partial
        uint32_t pktSize = i->second->GetSize();
        if (pktSize <= extractSize)
        { // Whole packet is extracted
            outPkt->AddAtEnd(i->second);
            m_data.erase(i);
            m_size -= pktSize;
            m_availBytes -= pktSize;
            extractSize -= pktSize;
        }
        else
        { // Partial is extracted and done
            outPkt->AddAtEnd(i->second->CreateFragment(0, extractSize));
            m_data[i->first + SequenceNumber32(extractSize)] =
                i->second->CreateFragment(extractSize, pktSize - extractSize);
            m_data.erase(i);
            m_size -= extractSize;
            m_availBytes -= extractSize;
            extractSize = 0;
        }
    }
    if (outPkt->GetSize() == 0)
    {
        NS_LOG_LOGIC("Nothing extracted.");
        return nullptr;
    }
    NS_LOG_LOGIC("Extracted " << outPkt->GetSize() << " bytes, bufsize=" << m_size
                              << ", num pkts in buffer=" << m_data.size());
    return outPkt;
}
 
} // namespace ns3