FreeRTOS_TCP_Transmission.c

/*
 * FreeRTOS+TCP <DEVELOPMENT BRANCH>
 * Copyright (C) 2022 Amazon.com, Inc. or its affiliates.  All Rights Reserved.
 *
 * SPDX-License-Identifier: MIT
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of
 * this software and associated documentation files (the "Software"), to deal in
 * the Software without restriction, including without limitation the rights to
 * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
 * the Software, and to permit persons to whom the Software is furnished to do so,
 * subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
 * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
 * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
 * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 * http://aws.amazon.com/freertos
 * http://www.FreeRTOS.org
 */

/**
 * @file FreeRTOS_TCP_Transmission.c
 * @brief Module which prepares the packet to be sent through
 * a socket for FreeRTOS+TCP.
 * It depends on  FreeRTOS_TCP_WIN.c, which handles the TCP windowing
 * schemes.
 *
 * Endianness: in this module all ports and IP addresses are stored in
 * host byte-order, except fields in the IP-packets
 */

/* Standard includes. */
#include <stdint.h>
#include <stdio.h>

/* FreeRTOS includes. */
#include "FreeRTOS.h"
#include "task.h"
#include "queue.h"
#include "semphr.h"

/* FreeRTOS+TCP includes. */
#include "FreeRTOS_IP.h"
#include "FreeRTOS_Sockets.h"
#include "FreeRTOS_IP_Private.h"
#include "NetworkInterface.h"
#include "NetworkBufferManagement.h"
#include "FreeRTOSIPConfigDefaults.h"

#include "FreeRTOS_TCP_IP.h"
#include "FreeRTOS_TCP_Reception.h"
#include "FreeRTOS_TCP_Transmission.h"
#include "FreeRTOS_TCP_State_Handling.h"
#include "FreeRTOS_TCP_Utils.h"

/* Just make sure the contents doesn't get compiled if TCP is not enabled. */
#if ipconfigUSE_TCP == 1

    static BaseType_t prvTCPMakeSurePrepared( FreeRTOS_Socket_t * pxSocket );

/* Resolve the MAC-address of the peer and initialise the first SYN packet. */
    static BaseType_t prvTCPPrepareConnect( FreeRTOS_Socket_t * pxSocket );

    #if ipconfigIS_ENABLED( ipconfigUSE_TCP_WIN )
        static uint8_t prvWinScaleFactor( const FreeRTOS_Socket_t * pxSocket );
    #endif

/*------------------------------------------------------------------------*/

/**
 * @brief Check if the outgoing connection is already prepared, if not
 *         call prvTCPPrepareConnect() to continue the preparation.
 * @param[in] pxSocket The socket that wants to connect.
 * @return Returns pdTRUE if the connection is prepared, i.e. the MAC-
 *         address of the peer is already known. */
    static BaseType_t prvTCPMakeSurePrepared( FreeRTOS_Socket_t * pxSocket )
    {
        BaseType_t xReturn = pdTRUE;

        if( pxSocket->u.xTCP.bits.bConnPrepared == pdFALSE_UNSIGNED )
        {
            if( prvTCPPrepareConnect( pxSocket ) != pdTRUE )
            {
                /* The preparation of a connection ( resolution ) is not yet ready. */
                xReturn = pdFALSE;
            }
        }

        return xReturn;
    }
/*-----------------------------------------------------------*/

/**
 * @brief prvTCPSendPacket() will be called when the socket time-out has been reached.
 *
 * @param[in] pxSocket The socket owning the connection.
 *
 * @return Number of bytes to be sent.
 *
 * @note It is only called by xTCPSocketCheck().
 */
    int32_t prvTCPSendPacket( FreeRTOS_Socket_t * pxSocket )
    {
        int32_t lResult = 0;
        UBaseType_t uxOptionsLength, uxIntermediateResult = 0;
        NetworkBufferDescriptor_t * pxNetworkBuffer;

        if( pxSocket->u.xTCP.eTCPState != eCONNECT_SYN )
        {
            /* The connection is in a state other than SYN. */
            pxNetworkBuffer = NULL;

            /* prvTCPSendRepeated() will only create a network buffer if necessary,
             * i.e. when data must be sent to the peer. */
            lResult = prvTCPSendRepeated( pxSocket, &pxNetworkBuffer );

            if( pxNetworkBuffer != NULL )
            {
                vReleaseNetworkBufferAndDescriptor( pxNetworkBuffer );
            }
        }
        else
        {
            if( pxSocket->u.xTCP.ucRepCount >= 3U )
            {
                /* The connection is in the SYN status. The packet will be repeated
                 * to most 3 times.  When there is no response, the socket get the
                 * status 'eCLOSED'. */
                FreeRTOS_debug_printf( ( "Connect: giving up %xip:%u\n",
                                         ( unsigned ) pxSocket->u.xTCP.xRemoteIP.ulIP_IPv4, /* IP address of remote machine. */
                                         pxSocket->u.xTCP.usRemotePort ) );                 /* Port on remote machine. */
                vTCPStateChange( pxSocket, eCLOSED );
            }
            else if( prvTCPMakeSurePrepared( pxSocket ) == pdTRUE )
            {
                ProtocolHeaders_t * pxProtocolHeaders;

                /* Or else, if the connection has been prepared, or can be prepared
                 * now, proceed to send the packet with the SYN flag.
                 * prvTCPPrepareConnect() prepares 'xPacket' and returns pdTRUE if
                 * the Ethernet address of the peer or the gateway is found. */

                /* MISRA Ref 11.3.1 [Misaligned access] */
                /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
                /* coverity[misra_c_2012_rule_11_3_violation] */
                pxProtocolHeaders = ( ( ProtocolHeaders_t * ) &( pxSocket->u.xTCP.xPacket.u.ucLastPacket[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSizeSocket( pxSocket ) ] ) );

                /* About to send a SYN packet.  Call prvSetSynAckOptions() to set
                 * the proper options: The size of MSS and whether SACK's are
                 * allowed. */
                uxOptionsLength = prvSetSynAckOptions( pxSocket, &( pxProtocolHeaders->xTCPHeader ) );

                /* Return the number of bytes to be sent. */
                uxIntermediateResult = uxIPHeaderSizeSocket( pxSocket ) + ipSIZE_OF_TCP_HEADER + uxOptionsLength;
                lResult = ( int32_t ) uxIntermediateResult;

                /* Set the TCP offset field:  ipSIZE_OF_TCP_HEADER equals 20 and
                 * uxOptionsLength is always a multiple of 4.  The complete expression
                 * would be:
                 * ucTCPOffset = ( ( ipSIZE_OF_TCP_HEADER + uxOptionsLength ) / 4 ) << 4 */
                pxProtocolHeaders->xTCPHeader.ucTCPOffset = ( uint8_t ) ( ( ipSIZE_OF_TCP_HEADER + uxOptionsLength ) << 2 );

                /* Repeat Count is used for a connecting socket, to limit the number
                 * of tries. */
                pxSocket->u.xTCP.ucRepCount++;

                /* Send the SYN message to make a connection.  The messages is
                 * stored in the socket field 'xPacket'.  It will be wrapped in a
                 * pseudo network buffer descriptor before it will be sent. */
                prvTCPReturnPacket( pxSocket, NULL, ( uint32_t ) lResult, pdFALSE );
            }
            else
            {
                /* Nothing to do. */
            }
        }

        /* Return the total number of bytes sent. */
        return lResult;
    }
    /*-----------------------------------------------------------*/

/**
 * @brief prvTCPSendRepeated will try to send a series of messages, as
 *        long as there is data to be sent and as long as the transmit
 *        window isn't full.
 *
 * @param[in] pxSocket The socket owning the connection.
 * @param[in,out] ppxNetworkBuffer Pointer to pointer to the network buffer.
 *
 * @return Total number of bytes sent.
 */
    int32_t prvTCPSendRepeated( FreeRTOS_Socket_t * pxSocket,
                                NetworkBufferDescriptor_t ** ppxNetworkBuffer )
    {
        UBaseType_t uxIndex;
        int32_t lResult = 0;
        UBaseType_t uxOptionsLength = 0U;
        int32_t xSendLength;

        for( uxIndex = 0U; uxIndex < ( UBaseType_t ) SEND_REPEATED_COUNT; uxIndex++ )
        {
            /* prvTCPPrepareSend() might allocate a network buffer if there is data
             * to be sent. */
            xSendLength = prvTCPPrepareSend( pxSocket, ppxNetworkBuffer, uxOptionsLength );

            if( xSendLength <= 0 )
            {
                break;
            }

            /* And return the packet to the peer. */
            prvTCPReturnPacket( pxSocket, *ppxNetworkBuffer, ( uint32_t ) xSendLength, ipconfigZERO_COPY_TX_DRIVER );

            #if ( ipconfigZERO_COPY_TX_DRIVER != 0 )
            {
                *ppxNetworkBuffer = NULL;
            }
            #endif /* ipconfigZERO_COPY_TX_DRIVER */

            lResult += xSendLength;
        }

        /* Return the total number of bytes sent. */
        return lResult;
    }
    /*-----------------------------------------------------------*/

/**
 * @brief  Return (or send) a packet to the peer. The data is stored in pxBuffer,
 *         which may either point to a real network buffer or to a TCP socket field
 *         called 'xTCP.xPacket'. A temporary xNetworkBuffer will be used to pass
 *         the data to the NIC.
 *
 * @param[in] pxSocket The socket owning the connection.
 * @param[in] pxDescriptor The network buffer descriptor carrying the packet.
 * @param[in] ulLen Length of the packet being sent.
 * @param[in] xReleaseAfterSend pdTRUE if the ownership of the descriptor is
 *                               transferred to the network interface.
 */
    void prvTCPReturnPacket( FreeRTOS_Socket_t * pxSocket,
                             NetworkBufferDescriptor_t * pxDescriptor,
                             uint32_t ulLen,
                             BaseType_t xReleaseAfterSend )
    {
        const NetworkBufferDescriptor_t * pxNetworkBuffer = pxDescriptor;
        BaseType_t xIsIPv6 = pdFALSE;

        if( pxNetworkBuffer != NULL )
        {
            #if ( ipconfigUSE_IPv6 != 0 )
                if( uxIPHeaderSizePacket( pxNetworkBuffer ) == ipSIZE_OF_IPv6_HEADER )
                {
                    xIsIPv6 = pdTRUE;
                }
            #endif /* ( ipconfigUSE_IPv6 != 0 ) */
        }
        else if( pxSocket != NULL )
        {
            #if ( ipconfigUSE_IPv6 != 0 )
                if( uxIPHeaderSizeSocket( pxSocket ) == ipSIZE_OF_IPv6_HEADER )
                {
                    xIsIPv6 = pdTRUE;
                }
            #endif /* ( ipconfigUSE_IPv6 != 0 ) */
        }
        else
        {
            /* prvTCPReturnPacket_IPVx() needs either a network buffer, or a socket. */
            configASSERT( pdFALSE );
        }

        #if ( ipconfigUSE_IPv6 != 0 )
            if( xIsIPv6 == pdTRUE )
            {
                prvTCPReturnPacket_IPV6( pxSocket, pxDescriptor, ulLen, xReleaseAfterSend );
            }
        #endif /* ( ipconfigUSE_IPv6 != 0 ) */

        #if ( ipconfigUSE_IPv4 != 0 )
            if( xIsIPv6 == pdFALSE )
            {
                prvTCPReturnPacket_IPV4( pxSocket, pxDescriptor, ulLen, xReleaseAfterSend );
            }
        #endif /* ( ipconfigUSE_IPv4 != 0 ) */
    }
    /*-----------------------------------------------------------*/

/**
 * @brief Called by prvTCPReturnPacket(), this function will set the the window
 *        size on this side: 'xTCPHeader.usWindow'.
 * @param[in] pxSocket The socket on which the packet is being sent.
 * @param[in] pxNetworkBuffer The network buffer carrying the outgoing message.
 * @param[in] uxIPHeaderSize The size of the IP-header, which depends on the IP-type.
 */
    void prvTCPReturn_CheckTCPWindow( FreeRTOS_Socket_t * pxSocket,
                                      const NetworkBufferDescriptor_t * pxNetworkBuffer,
                                      size_t uxIPHeaderSize )
    {
        /* Calculate the space in the RX buffer in order to advertise the
         * size of this socket's reception window. */
        const TCPWindow_t * pxTCPWindow = &( pxSocket->u.xTCP.xTCPWindow );
        uint32_t ulFrontSpace, ulSpace, ulWinSize;
        ProtocolHeaders_t * pxProtocolHeaders;

        /* MISRA Ref 11.3.1 [Misaligned access] */
        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
        /* coverity[misra_c_2012_rule_11_3_violation] */
        pxProtocolHeaders = ( ( ProtocolHeaders_t * )
                              &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSize ] ) );

        if( pxSocket->u.xTCP.rxStream != NULL )
        {
            /* An RX stream was created already, see how much space is
             * available. */
            ulFrontSpace = ( uint32_t ) uxStreamBufferFrontSpace( pxSocket->u.xTCP.rxStream );
        }
        else
        {
            /* No RX stream has been created, the full stream size is
             * available. */
            ulFrontSpace = ( uint32_t ) pxSocket->u.xTCP.uxRxStreamSize;
        }

        /* Take the minimum of the RX buffer space and the RX window size. */
        ulSpace = FreeRTOS_min_uint32( pxTCPWindow->xSize.ulRxWindowLength, ulFrontSpace );

        if( ( pxSocket->u.xTCP.bits.bLowWater != pdFALSE_UNSIGNED ) || ( pxSocket->u.xTCP.bits.bRxStopped != pdFALSE_UNSIGNED ) )
        {
            /* The low-water mark was reached, meaning there was little
             * space left.  The socket will wait until the application has read
             * or flushed the incoming data, and 'zero-window' will be
             * advertised. */
            ulSpace = 0U;
        }

        /* If possible, advertise an RX window size of at least 1 MSS, otherwise
         * the peer might start 'zero window probing', i.e. sending small packets
         * (1, 2, 4, 8... bytes). */
        if( ( ulSpace < pxSocket->u.xTCP.usMSS ) && ( ulFrontSpace >= pxSocket->u.xTCP.usMSS ) )
        {
            ulSpace = pxSocket->u.xTCP.usMSS;
        }

        /* Avoid overflow of the 16-bit win field. */
        #if ( ipconfigUSE_TCP_WIN != 0 )
        {
            ulWinSize = ( ulSpace >> pxSocket->u.xTCP.ucMyWinScaleFactor );
        }
        #else
        {
            ulWinSize = ulSpace;
        }
        #endif

        if( ulWinSize > 0xfffcU )
        {
            ulWinSize = 0xfffcU;
        }

        pxProtocolHeaders->xTCPHeader.usWindow = FreeRTOS_htons( ( uint16_t ) ulWinSize );

        /* The new window size has been advertised, switch off the flag. */
        pxSocket->u.xTCP.bits.bWinChange = pdFALSE_UNSIGNED;

        /* Later on, when deciding to delay an ACK, a precise estimate is needed
         * of the free RX space.  At this moment, 'ulHighestRxAllowed' would be the
         * highest sequence number minus 1 that the socket will accept. */
        pxSocket->u.xTCP.ulHighestRxAllowed = pxTCPWindow->rx.ulCurrentSequenceNumber + ulSpace;
    }
/*-----------------------------------------------------------*/

/**
 * @brief Called by prvTCPReturnPacket(), this function sets the sequence and ack numbers
 *        in the TCP-header.
 * @param[in] pxSocket The socket on which the packet is being sent.
 * @param[in] pxNetworkBuffer The network buffer carrying the outgoing message.
 * @param[in] uxIPHeaderSize The size of the IP-header, which depends on the IP-type.
 * @param[in] ulLen The size of the packet minus the size of the Ethernet header.
 *
 */
    void prvTCPReturn_SetSequenceNumber( FreeRTOS_Socket_t * pxSocket,
                                         const NetworkBufferDescriptor_t * pxNetworkBuffer,
                                         size_t uxIPHeaderSize,
                                         uint32_t ulLen )
    {
        ProtocolHeaders_t * pxProtocolHeaders;
        const TCPWindow_t * pxTCPWindow = &( pxSocket->u.xTCP.xTCPWindow );

        /* MISRA Ref 11.3.1 [Misaligned access] */
        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
        /* coverity[misra_c_2012_rule_11_3_violation] */
        pxProtocolHeaders = ( ( ProtocolHeaders_t * )
                              &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSize ] ) );
        #if ( ipconfigTCP_KEEP_ALIVE == 1 )
            if( pxSocket->u.xTCP.bits.bSendKeepAlive != pdFALSE_UNSIGNED )
            {
                /* Sending a keep-alive packet, send the current sequence number
                 * minus 1, which will be recognised as a keep-alive packet and
                 * responded to by acknowledging the last byte. */
                pxSocket->u.xTCP.bits.bSendKeepAlive = pdFALSE_UNSIGNED;
                pxSocket->u.xTCP.bits.bWaitKeepAlive = pdTRUE_UNSIGNED;

                pxProtocolHeaders->xTCPHeader.ulSequenceNumber = pxSocket->u.xTCP.xTCPWindow.ulOurSequenceNumber - 1U;
                pxProtocolHeaders->xTCPHeader.ulSequenceNumber = FreeRTOS_htonl( pxProtocolHeaders->xTCPHeader.ulSequenceNumber );
            }
            else
        #endif /* if ( ipconfigTCP_KEEP_ALIVE == 1 ) */
        {
            pxProtocolHeaders->xTCPHeader.ulSequenceNumber = FreeRTOS_htonl( pxSocket->u.xTCP.xTCPWindow.ulOurSequenceNumber );

            if( ( pxProtocolHeaders->xTCPHeader.ucTCPFlags & ( uint8_t ) tcpTCP_FLAG_FIN ) != 0U )
            {
                /* Suppress FIN in case this packet carries earlier data to be
                 * retransmitted. */
                uint32_t ulDataLen = ( uint32_t ) ( ulLen - ( ipSIZE_OF_TCP_HEADER + uxIPHeaderSizeSocket( pxSocket ) ) );

                if( ( pxTCPWindow->ulOurSequenceNumber + ulDataLen ) != pxTCPWindow->tx.ulFINSequenceNumber )
                {
                    pxProtocolHeaders->xTCPHeader.ucTCPFlags &= ( ( uint8_t ) ~tcpTCP_FLAG_FIN );
                    FreeRTOS_debug_printf( ( "Suppress FIN for %u + %u < %u\n",
                                             ( unsigned int ) ( pxTCPWindow->ulOurSequenceNumber - pxTCPWindow->tx.ulFirstSequenceNumber ),
                                             ( unsigned int ) ulDataLen,
                                             ( unsigned int ) ( pxTCPWindow->tx.ulFINSequenceNumber - pxTCPWindow->tx.ulFirstSequenceNumber ) ) );
                }
            }
        }

        /* Tell which sequence number is expected next time */
        pxProtocolHeaders->xTCPHeader.ulAckNr = FreeRTOS_htonl( pxTCPWindow->rx.ulCurrentSequenceNumber );
    }
/*-----------------------------------------------------------*/

/**
 * @brief Create the TCP window for the given socket.
 *
 * @param[in] pxSocket The socket for which the window is being created.
 *
 * @note The SYN event is very important: the sequence numbers, which have a kind of
 *       random starting value, are being synchronized. The sliding window manager
 *       (in FreeRTOS_TCP_WIN.c) needs to know them, along with the Maximum Segment
 *       Size (MSS).
 */
    BaseType_t prvTCPCreateWindow( FreeRTOS_Socket_t * pxSocket )
    {
        BaseType_t xReturn;
        uint32_t ulRxWindowSize = ( uint32_t ) pxSocket->u.xTCP.uxRxWinSize;
        uint32_t ulTxWindowSize = ( uint32_t ) pxSocket->u.xTCP.uxTxWinSize;

        if( xTCPWindowLoggingLevel != 0 )
        {
            FreeRTOS_debug_printf( ( "Limits (using): TCP Win size %u Water %u <= %u <= %u\n",
                                     ( unsigned ) ( pxSocket->u.xTCP.uxRxWinSize * ipconfigTCP_MSS ),
                                     ( unsigned ) pxSocket->u.xTCP.uxLittleSpace,
                                     ( unsigned ) pxSocket->u.xTCP.uxEnoughSpace,
                                     ( unsigned ) pxSocket->u.xTCP.uxRxStreamSize ) );
        }

        xReturn = xTCPWindowCreate(
            &pxSocket->u.xTCP.xTCPWindow,
            ulRxWindowSize * ipconfigTCP_MSS,
            ulTxWindowSize * ipconfigTCP_MSS,
            pxSocket->u.xTCP.xTCPWindow.rx.ulCurrentSequenceNumber,
            pxSocket->u.xTCP.xTCPWindow.ulOurSequenceNumber,
            ( uint32_t ) pxSocket->u.xTCP.usMSS );

        return xReturn;
    }
    /*-----------------------------------------------------------*/

/**
 * @brief Resolve the MAC-address of the peer and initialise the first SYN packet.
 *
 * @param[in] pxSocket The socket owning the TCP connection. The first packet shall
 *               be created in this socket.
 *
 * @return pdTRUE: if the packet was successfully created and the first SYN can be sent.
 *         Else pdFALSE.
 *
 * @note Connecting sockets have a special state: eCONNECT_SYN. In this phase,
 *       the Ethernet address of the target will be found through address resolution.
 *       In case the target IP address is not within the netmask, the hardware address
 *       of the gateway will be used.
 */
    static BaseType_t prvTCPPrepareConnect( FreeRTOS_Socket_t * pxSocket )
    {
        BaseType_t xReturn = pdTRUE;

        switch( pxSocket->bits.bIsIPv6 ) /* LCOV_EXCL_BR_LINE */
        {
            #if ( ipconfigUSE_IPv4 != 0 )
                case pdFALSE_UNSIGNED:
                    xReturn = prvTCPPrepareConnect_IPV4( pxSocket );
                    break;
            #endif /* ( ipconfigUSE_IPv4 != 0 ) */

            #if ( ipconfigUSE_IPv6 != 0 )
                case pdTRUE_UNSIGNED:
                    xReturn = prvTCPPrepareConnect_IPV6( pxSocket );
                    break;
            #endif /* ( ipconfigUSE_IPv6 != 0 ) */

            default:   /* LCOV_EXCL_LINE */
                /* MISRA 16.4 Compliance */
                break; /* LCOV_EXCL_LINE */
        }

        return xReturn;
    }
    /*-----------------------------------------------------------*/

    #if ( ipconfigUSE_TCP_WIN != 0 )

/**
 * @brief Get the window scaling factor for the TCP connection.
 *
 * @param[in] pxSocket The socket owning the TCP connection.
 *
 * @return The scaling factor.
 */
        static uint8_t prvWinScaleFactor( const FreeRTOS_Socket_t * pxSocket )
        {
            size_t uxWinSize;
            uint8_t ucFactor;


            /* 'xTCP.uxRxWinSize' is the size of the reception window in units of MSS. */
            uxWinSize = pxSocket->u.xTCP.uxRxWinSize * ( size_t ) pxSocket->u.xTCP.usMSS;
            ucFactor = 0U;

            while( uxWinSize > 0xffffU )
            {
                /* Divide by two and increase the binary factor by 1. */
                uxWinSize >>= 1;
                ucFactor++;
            }

            FreeRTOS_debug_printf( ( "prvWinScaleFactor: uxRxWinSize %u MSS %u Factor %u\n",
                                     ( unsigned ) pxSocket->u.xTCP.uxRxWinSize,
                                     pxSocket->u.xTCP.usMSS,
                                     ucFactor ) );

            return ucFactor;
        }

    #endif /* if ( ipconfigUSE_TCP_WIN != 0 ) */
    /*-----------------------------------------------------------*/

/**
 * @brief When opening a TCP connection, while SYN's are being sent, the  parties may
 *        communicate what MSS (Maximum Segment Size) they intend to use, whether Selective
 *        ACK's ( SACK ) are supported, and the size of the reception window ( WSOPT ).
 *
 * @param[in] pxSocket The socket being used for communication. It is used to set
 *                      the MSS.
 * @param[in,out] pxTCPHeader The TCP packet header being used in the SYN transmission.
 *                             The MSS and corresponding options shall be set in this
 *                             header itself.
 *
 * @return The option length after the TCP header was updated.
 *
 * @note MSS is the net size of the payload, an is always smaller than MTU.
 */
    UBaseType_t prvSetSynAckOptions( FreeRTOS_Socket_t * pxSocket,
                                     TCPHeader_t * pxTCPHeader )
    {
        uint16_t usMSS = pxSocket->u.xTCP.usMSS;
        UBaseType_t uxOptionsLength;

        /* We send out the TCP Maximum Segment Size option with our SYN[+ACK]. */

        pxTCPHeader->ucOptdata[ 0 ] = ( uint8_t ) tcpTCP_OPT_MSS;
        pxTCPHeader->ucOptdata[ 1 ] = ( uint8_t ) tcpTCP_OPT_MSS_LEN;
        pxTCPHeader->ucOptdata[ 2 ] = ( uint8_t ) ( usMSS >> 8 );
        pxTCPHeader->ucOptdata[ 3 ] = ( uint8_t ) ( usMSS & 0xffU );

        #if ( ipconfigUSE_TCP_WIN != 0 )
        {
            pxSocket->u.xTCP.ucMyWinScaleFactor = prvWinScaleFactor( pxSocket );

            pxTCPHeader->ucOptdata[ 4 ] = tcpTCP_OPT_NOOP;
            pxTCPHeader->ucOptdata[ 5 ] = ( uint8_t ) ( tcpTCP_OPT_WSOPT );
            pxTCPHeader->ucOptdata[ 6 ] = ( uint8_t ) ( tcpTCP_OPT_WSOPT_LEN );
            pxTCPHeader->ucOptdata[ 7 ] = ( uint8_t ) pxSocket->u.xTCP.ucMyWinScaleFactor;
            uxOptionsLength = 8U;
        }
        #else
        {
            uxOptionsLength = 4U;
        }
        #endif /* if ( ipconfigUSE_TCP_WIN != 0 ) */

        #if ( ipconfigUSE_TCP_WIN != 0 )
        {
            pxTCPHeader->ucOptdata[ uxOptionsLength ] = tcpTCP_OPT_NOOP;
            pxTCPHeader->ucOptdata[ uxOptionsLength + 1U ] = tcpTCP_OPT_NOOP;
            pxTCPHeader->ucOptdata[ uxOptionsLength + 2U ] = tcpTCP_OPT_SACK_P; /* 4: Sack-Permitted Option. */
            pxTCPHeader->ucOptdata[ uxOptionsLength + 3U ] = 2U;                /* 2: length of this option. */
            uxOptionsLength += 4U;
        }
        #endif /* ipconfigUSE_TCP_WIN == 0 */
        return uxOptionsLength; /* bytes, not words. */
    }

/**
 * @brief Check if the size of a network buffer is big enough to hold the outgoing message.
 *        Allocate a new bigger network buffer when necessary.
 *
 * @param[in] pxSocket Socket whose buffer is being resized.
 * @param[in] pxNetworkBuffer The network buffer whose size is being increased.
 * @param[in] lDataLen Length of the data to be put in the buffer.
 * @param[in] uxOptionsLength Length of options.
 *
 * @return If the resizing is successful: The new buffer with the size being asked for
 *                with old data copied in it.
 *         Else, NULL.
 *
 * @note The old network buffer will be released if the resizing is successful and
 *       cannot be used any longer.
 */
    NetworkBufferDescriptor_t * prvTCPBufferResize( const FreeRTOS_Socket_t * pxSocket,
                                                    NetworkBufferDescriptor_t * pxNetworkBuffer,
                                                    int32_t lDataLen,
                                                    UBaseType_t uxOptionsLength )
    {
        NetworkBufferDescriptor_t * pxReturn;
        size_t uxNeeded;
        BaseType_t xResize;

        if( xBufferAllocFixedSize != pdFALSE )
        {
            /* Network buffers are created with a fixed size and can hold the largest
             * MTU. */
            uxNeeded = ( size_t ) ipTOTAL_ETHERNET_FRAME_SIZE;

            /* and therefore, the buffer won't be too small.
             * Only ask for a new network buffer in case none was supplied. */
            if( pxNetworkBuffer == NULL )
            {
                xResize = pdTRUE;
            }
            else
            {
                xResize = pdFALSE;
            }
        }
        else
        {
            /* Network buffers are created with a variable size. See if it must
             * grow. */
            uxNeeded = ipSIZE_OF_ETH_HEADER + uxIPHeaderSizeSocket( pxSocket ) + ipSIZE_OF_TCP_HEADER + uxOptionsLength;
            uxNeeded += ( size_t ) lDataLen;

            if( uxNeeded < sizeof( pxSocket->u.xTCP.xPacket.u.ucLastPacket ) )
            {
                uxNeeded = sizeof( pxSocket->u.xTCP.xPacket.u.ucLastPacket );
            }

            /* In case we were called from a TCP timer event, a buffer must be
             *  created.  Otherwise, test 'xDataLength' of the provided buffer. */
            if( ( pxNetworkBuffer == NULL ) || ( pxNetworkBuffer->xDataLength < uxNeeded ) )
            {
                xResize = pdTRUE;
            }
            else
            {
                xResize = pdFALSE;
            }
        }

        if( xResize != pdFALSE )
        {
            /* The caller didn't provide a network buffer or the provided buffer is
             * too small.  As we must send-out a data packet, a buffer will be created
             * here. */
            pxReturn = pxGetNetworkBufferWithDescriptor( uxNeeded, 0U );

            if( pxReturn != NULL )
            {
                /* Set the actual packet size, in case the returned buffer is larger. */
                pxReturn->xDataLength = uxNeeded;

                /* Copy the existing data to the new created buffer. */
                if( pxNetworkBuffer != NULL )
                {
                    /* Either from the previous buffer... */
                    ( void ) memcpy( pxReturn->pucEthernetBuffer, pxNetworkBuffer->pucEthernetBuffer, pxNetworkBuffer->xDataLength );

                    /* ...and release it. */
                    vReleaseNetworkBufferAndDescriptor( pxNetworkBuffer );
                }
                else
                {
                    /* Or from the socket field 'xTCP.xPacket'. */
                    ( void ) memcpy( pxReturn->pucEthernetBuffer, pxSocket->u.xTCP.xPacket.u.ucLastPacket, sizeof( pxSocket->u.xTCP.xPacket.u.ucLastPacket ) );
                }
            }
        }
        else
        {
            /* xResize is false, the network buffer provided was big enough. */
            configASSERT( pxNetworkBuffer != NULL ); /* LCOV_EXCL_BR_LINE this branch will not be covered, since it would never be NULL. to tell lint: when xResize is false, pxNetworkBuffer is not NULL. */
            pxReturn = pxNetworkBuffer;

            pxNetworkBuffer->xDataLength = ( size_t ) ( ipSIZE_OF_ETH_HEADER + uxIPHeaderSizeSocket( pxSocket ) + ipSIZE_OF_TCP_HEADER + uxOptionsLength ) + ( size_t ) lDataLen;
        }

        return pxReturn;
    }

/*-----------------------------------------------------------*/

/**
 * @brief Called by prvTCPReturnPacket(), this function makes sure that the network buffer
 *        has 'pxEndPoint' set properly.
 * @param[in] pxSocket The socket on which the packet is being sent.
 * @param[in] pxNetworkBuffer The network buffer carrying the outgoing message.
 * @param[in] uxIPHeaderSize The size of the IP-header, which depends on the IP-type.
 */
    void prvTCPReturn_SetEndPoint( const FreeRTOS_Socket_t * pxSocket,
                                   NetworkBufferDescriptor_t * pxNetworkBuffer,
                                   size_t uxIPHeaderSize )
    {
        #if ( ipconfigUSE_IPv4 != 0 )
            const IPHeader_t * pxIPHeader = NULL;
        #endif
        #if ( ipconfigUSE_IPv6 != 0 )
            const IPHeader_IPv6_t * pxIPHeader_IPv6 = NULL;
        #endif

        if( ( pxSocket != NULL ) && ( pxSocket->pxEndPoint != NULL ) )
        {
            pxNetworkBuffer->pxEndPoint = pxSocket->pxEndPoint;
        }
        else
        {
            FreeRTOS_printf( ( "prvTCPReturnPacket: No pxEndPoint yet?\n" ) );

            switch( uxIPHeaderSize )
            {
                #if ( ipconfigUSE_IPv4 != 0 )
                    case ipSIZE_OF_IPv4_HEADER:

                        /*_RB_ Was FreeRTOS_FindEndPointOnIP_IPv4() but changed to FreeRTOS_FindEndPointOnNetMask()
                         * as it is using the destination address.  I'm confused here as sometimes the addresses are swapped. */
                        /* MISRA Ref 11.3.1 [Misaligned access] */
                        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
                        /* coverity[misra_c_2012_rule_11_3_violation] */
                        pxIPHeader = ( ( IPHeader_t * ) &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER ] ) );
                        pxNetworkBuffer->pxEndPoint = FreeRTOS_FindEndPointOnNetMask( pxIPHeader->ulDestinationIPAddress );

                        if( pxNetworkBuffer->pxEndPoint == NULL )
                        {
                            FreeRTOS_printf( ( "prvTCPReturnPacket: no such end-point %xip => %xip\n",
                                               ( unsigned int ) FreeRTOS_ntohl( pxIPHeader->ulSourceIPAddress ),
                                               ( unsigned int ) FreeRTOS_ntohl( pxIPHeader->ulDestinationIPAddress ) ) );
                        }
                        break;
                #endif /* ( ipconfigUSE_IPv4 != 0 ) */

                #if ( ipconfigUSE_IPv6 != 0 )
                    case ipSIZE_OF_IPv6_HEADER:
                        /* MISRA Ref 11.3.1 [Misaligned access] */
                        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
                        /* coverity[misra_c_2012_rule_11_3_violation] */
                        pxIPHeader_IPv6 = ( ( IPHeader_IPv6_t * ) &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER ] ) );
                        pxNetworkBuffer->pxEndPoint = FreeRTOS_FindEndPointOnIP_IPv6( &( pxIPHeader_IPv6->xDestinationAddress ) );

                        if( pxNetworkBuffer->pxEndPoint == NULL )
                        {
                            FreeRTOS_printf( ( "prvTCPReturnPacket: no such end-point %pip => %pip\n",
                                               ( void * ) pxIPHeader_IPv6->xSourceAddress.ucBytes,
                                               ( void * ) pxIPHeader_IPv6->xDestinationAddress.ucBytes ) );
                        }
                        break;
                #endif /* ( ipconfigUSE_IPv6 != 0 ) */

                default:
                    /* Shouldn't reach here */
                    pxNetworkBuffer->pxEndPoint = NULL;
                    break;
            }

            if( pxNetworkBuffer->pxEndPoint != NULL )
            {
                FreeRTOS_printf( ( "prvTCPReturnPacket: packet's end-point %02x-%02x\n",
                                   pxNetworkBuffer->pxEndPoint->xMACAddress.ucBytes[ 4 ],
                                   pxNetworkBuffer->pxEndPoint->xMACAddress.ucBytes[ 5 ] ) );
            }
        }
    }
/*-----------------------------------------------------------*/

/**
 * @brief Prepare an outgoing message, in case anything has to be sent.
 *
 * @param[in] pxSocket The socket owning the connection.
 * @param[in,out] ppxNetworkBuffer Pointer to the pointer to the network buffer.
 * @param[in] uxOptionsLength The length of the TCP options.
 *
 * @return Length of the data to be sent if everything is correct. Else, -1
 *         is returned in case of any error.
 */
    int32_t prvTCPPrepareSend( FreeRTOS_Socket_t * pxSocket,
                               NetworkBufferDescriptor_t ** ppxNetworkBuffer,
                               UBaseType_t uxOptionsLength )
    {
        int32_t lDataLen;
        uint8_t * pucEthernetBuffer, * pucSendData;
        ProtocolHeaders_t * pxProtocolHeaders;
        size_t uxOffset;
        uint32_t ulDataGot, ulDistance;
        TCPWindow_t * pxTCPWindow;
        NetworkBufferDescriptor_t * pxNewBuffer;
        int32_t lStreamPos;
        UBaseType_t uxIntermediateResult = 0;

        if( ( *ppxNetworkBuffer ) != NULL )
        {
            /* A network buffer descriptor was already supplied */
            pucEthernetBuffer = ( *ppxNetworkBuffer )->pucEthernetBuffer;
        }
        else
        {
            /* For now let it point to the last packet header */
            pucEthernetBuffer = pxSocket->u.xTCP.xPacket.u.ucLastPacket;
        }

        /* Map the ethernet buffer onto the ProtocolHeader_t struct for easy access to the fields. */

        /* MISRA Ref 11.3.1 [Misaligned access] */
        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
        /* coverity[misra_c_2012_rule_11_3_violation] */
        pxProtocolHeaders = ( ( ProtocolHeaders_t * ) &( pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSizeSocket( pxSocket ) ] ) );
        pxTCPWindow = &( pxSocket->u.xTCP.xTCPWindow );
        lDataLen = 0;
        lStreamPos = 0;
        pxProtocolHeaders->xTCPHeader.ucTCPFlags |= tcpTCP_FLAG_ACK;

        if( pxSocket->u.xTCP.txStream != NULL )
        {
            /* ulTCPWindowTxGet will return the amount of data which may be sent
             * along with the position in the txStream.
             * Why check for MSS > 1 ?
             * Because some TCP-stacks (like uIP) use it for flow-control. */
            if( pxSocket->u.xTCP.usMSS > 1U )
            {
                lDataLen = ( int32_t ) ulTCPWindowTxGet( pxTCPWindow, pxSocket->u.xTCP.ulWindowSize, &lStreamPos );
            }

            if( lDataLen > 0 )
            {
                /* Check if the current network buffer is big enough, if not,
                 * resize it. */
                pxNewBuffer = prvTCPBufferResize( pxSocket, *ppxNetworkBuffer, lDataLen, uxOptionsLength );

                if( pxNewBuffer != NULL )
                {
                    *ppxNetworkBuffer = pxNewBuffer;
                    pucEthernetBuffer = pxNewBuffer->pucEthernetBuffer;

                    /* Map the byte stream onto ProtocolHeaders_t struct for easy
                     * access to the fields. */

                    /* MISRA Ref 11.3.1 [Misaligned access] */
/* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
                    /* coverity[misra_c_2012_rule_11_3_violation] */
                    pxProtocolHeaders = ( ( ProtocolHeaders_t * ) &( pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSizeSocket( pxSocket ) ] ) );

                    pucSendData = &( pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSizeSocket( pxSocket ) + ipSIZE_OF_TCP_HEADER + uxOptionsLength ] );

                    /* Translate the position in txStream to an offset from the tail
                     * marker. */
                    uxOffset = uxStreamBufferDistance( pxSocket->u.xTCP.txStream, pxSocket->u.xTCP.txStream->uxTail, ( size_t ) lStreamPos );

                    /* Here data is copied from the txStream in 'peek' mode.  Only
                     * when the packets are acked, the tail marker will be updated. */
                    ulDataGot = ( uint32_t ) uxStreamBufferGet( pxSocket->u.xTCP.txStream, uxOffset, pucSendData, ( size_t ) lDataLen, pdTRUE );

                    #if ( ipconfigHAS_DEBUG_PRINTF != 0 )
                    {
                        if( ulDataGot != ( uint32_t ) lDataLen )
                        {
                            FreeRTOS_debug_printf( ( "uxStreamBufferGet: pos %d offs %u only %u != %d\n",
                                                     ( int ) lStreamPos, ( unsigned ) uxOffset, ( unsigned ) ulDataGot, ( int ) lDataLen ) );
                        }
                    }
                    #endif

                    /* If the owner of the socket requests a closure, add the FIN
                     * flag to the last packet. */
                    if( pxSocket->u.xTCP.bits.bCloseRequested != pdFALSE_UNSIGNED )
                    {
                        ulDistance = ( uint32_t ) uxStreamBufferDistance( pxSocket->u.xTCP.txStream, ( size_t ) lStreamPos, pxSocket->u.xTCP.txStream->uxHead );

                        if( ulDistance == ulDataGot )
                        {
                            #if ( ipconfigHAS_DEBUG_PRINTF == 1 )
                            {
                                /* the order of volatile accesses is undefined
                                 *  so such workaround */
                                size_t uxHead = pxSocket->u.xTCP.txStream->uxHead;
                                size_t uxMid = pxSocket->u.xTCP.txStream->uxMid;
                                size_t uxTail = pxSocket->u.xTCP.txStream->uxTail;

                                FreeRTOS_debug_printf( ( "CheckClose %u <= %u (%u <= %u <= %u)\n",
                                                         ( unsigned ) ulDataGot, ( unsigned ) ulDistance,
                                                         ( unsigned ) uxTail, ( unsigned ) uxMid, ( unsigned ) uxHead ) );
                            }
                            #endif /* if ( ipconfigHAS_DEBUG_PRINTF == 1 ) */

                            /* Although the socket sends a FIN, it will stay in
                             * ESTABLISHED until all current data has been received or
                             * delivered. */
                            pxProtocolHeaders->xTCPHeader.ucTCPFlags |= tcpTCP_FLAG_FIN;
                            pxTCPWindow->tx.ulFINSequenceNumber = pxTCPWindow->ulOurSequenceNumber + ( uint32_t ) lDataLen;
                            pxSocket->u.xTCP.bits.bFinSent = pdTRUE_UNSIGNED;
                        }
                    }
                }
                else
                {
                    lDataLen = -1;
                }
            }
        }

        if( ( lDataLen >= 0 ) && ( pxSocket->u.xTCP.eTCPState == eESTABLISHED ) )
        {
            /* See if the socket owner wants to shutdown this connection. */
            if( ( pxSocket->u.xTCP.bits.bUserShutdown != pdFALSE_UNSIGNED ) &&
                ( xTCPWindowTxDone( pxTCPWindow ) != pdFALSE ) )
            {
                pxSocket->u.xTCP.bits.bUserShutdown = pdFALSE_UNSIGNED;
                pxProtocolHeaders->xTCPHeader.ucTCPFlags |= tcpTCP_FLAG_FIN;
                pxSocket->u.xTCP.bits.bFinSent = pdTRUE_UNSIGNED;
                pxSocket->u.xTCP.bits.bWinChange = pdTRUE_UNSIGNED;
                pxTCPWindow->tx.ulFINSequenceNumber = pxTCPWindow->tx.ulCurrentSequenceNumber;
                vTCPStateChange( pxSocket, eFIN_WAIT_1 );
            }

            #if ( ipconfigTCP_KEEP_ALIVE != 0 )
            {
                if( pxSocket->u.xTCP.ucKeepRepCount > 3U ) /*_RB_ Magic number. */
                {
                    FreeRTOS_debug_printf( ( "keep-alive: giving up %xip:%u\n",
                                             ( unsigned ) pxSocket->u.xTCP.xRemoteIP.ulIP_IPv4, /* IP address of remote machine. */
                                             pxSocket->u.xTCP.usRemotePort ) );                 /* Port on remote machine. */
                    vTCPStateChange( pxSocket, eCLOSE_WAIT );
                    lDataLen = -1;
                }

                if( ( lDataLen == 0 ) && ( pxSocket->u.xTCP.bits.bWinChange == pdFALSE_UNSIGNED ) )
                {
                    /* If there is no data to be sent, and no window-update message,
                     * we might want to send a keep-alive message. */
                    TickType_t xAge = xTaskGetTickCount() - pxSocket->u.xTCP.xLastAliveTime;
                    TickType_t xMax;
                    xMax = ( ( TickType_t ) ipconfigTCP_KEEP_ALIVE_INTERVAL * ( TickType_t ) configTICK_RATE_HZ );

                    if( pxSocket->u.xTCP.ucKeepRepCount != 0U )
                    {
                        xMax = 3U * configTICK_RATE_HZ;
                    }

                    if( xAge > xMax )
                    {
                        pxSocket->u.xTCP.xLastAliveTime = xTaskGetTickCount();

                        if( xTCPWindowLoggingLevel != 0 )
                        {
                            FreeRTOS_debug_printf( ( "keep-alive: %xip:%u count %u\n",
                                                     ( unsigned ) pxSocket->u.xTCP.xRemoteIP.ulIP_IPv4,
                                                     pxSocket->u.xTCP.usRemotePort,
                                                     pxSocket->u.xTCP.ucKeepRepCount ) );
                        }

                        pxSocket->u.xTCP.bits.bSendKeepAlive = pdTRUE_UNSIGNED;
                        pxSocket->u.xTCP.usTimeout = ( ( uint16_t ) pdMS_TO_TICKS( 2500U ) );
                        pxSocket->u.xTCP.ucKeepRepCount++;
                    }
                }
            }
            #endif /* ipconfigTCP_KEEP_ALIVE */
        }

        if( lDataLen >= 0 )
        {
            /* Anything to send, a change of the advertised window size, or maybe send a
             * keep-alive message? */
            if( ( lDataLen > 0 ) ||
                ( pxSocket->u.xTCP.bits.bWinChange != pdFALSE_UNSIGNED ) ||
                ( pxSocket->u.xTCP.bits.bSendKeepAlive != pdFALSE_UNSIGNED ) )
            {
                pxProtocolHeaders->xTCPHeader.ucTCPFlags &= ( ( uint8_t ) ~tcpTCP_FLAG_PSH );
                pxProtocolHeaders->xTCPHeader.ucTCPOffset = ( uint8_t ) ( ( ipSIZE_OF_TCP_HEADER + uxOptionsLength ) << 2 ); /*_RB_ "2" needs comment. */

                pxProtocolHeaders->xTCPHeader.ucTCPFlags |= ( uint8_t ) tcpTCP_FLAG_ACK;

                if( lDataLen != 0L )
                {
                    pxProtocolHeaders->xTCPHeader.ucTCPFlags |= ( uint8_t ) tcpTCP_FLAG_PSH;
                }

                uxIntermediateResult = uxIPHeaderSizeSocket( pxSocket ) + ipSIZE_OF_TCP_HEADER + uxOptionsLength;
                lDataLen += ( int32_t ) uxIntermediateResult;
            }
        }

        return lDataLen;
    }
    /*-----------------------------------------------------------*/


/**
 * @brief The API FreeRTOS_send() adds data to the TX stream. Add
 *        this data to the windowing system to it can be transmitted.
 *
 * @param[in] pxSocket The socket owning the connection.
 */
    void prvTCPAddTxData( FreeRTOS_Socket_t * pxSocket )
    {
        int32_t lCount, lLength;

        /* A txStream has been created already, see if the socket has new data for
         * the sliding window.
         *
         * uxStreamBufferMidSpace() returns the distance between rxHead and rxMid.  It
         * contains new Tx data which has not been passed to the sliding window yet.
         * The oldest data not-yet-confirmed can be found at rxTail. */
        lLength = ( int32_t ) uxStreamBufferMidSpace( pxSocket->u.xTCP.txStream );

        if( lLength > 0 )
        {
            /* All data between txMid and rxHead will now be passed to the sliding
             * window manager, so it can start transmitting them.
             *
             * Hand over the new data to the sliding window handler.  It will be
             * split-up in chunks of 1460 bytes each (or less, depending on
             * ipconfigTCP_MSS). */
            lCount = lTCPWindowTxAdd( &pxSocket->u.xTCP.xTCPWindow,
                                      ( uint32_t ) lLength,
                                      ( int32_t ) pxSocket->u.xTCP.txStream->uxMid,
                                      ( int32_t ) pxSocket->u.xTCP.txStream->LENGTH );

            /* Move the rxMid pointer forward up to rxHead. */
            if( lCount > 0 )
            {
                vStreamBufferMoveMid( pxSocket->u.xTCP.txStream, ( size_t ) lCount );
            }
        }
    }
    /*-----------------------------------------------------------*/


/**
 * @brief Set the TCP options (if any) for the outgoing packet.
 *
 * @param[in] pxSocket The socket owning the connection.
 * @param[in] pxNetworkBuffer The network buffer holding the packet.
 *
 * @return Length of the TCP options after they are set.
 */
    UBaseType_t prvSetOptions( FreeRTOS_Socket_t * pxSocket,
                               const NetworkBufferDescriptor_t * pxNetworkBuffer )
    {
        /* Map the ethernet buffer onto the ProtocolHeader_t struct for easy access to the fields. */

        /* MISRA Ref 11.3.1 [Misaligned access] */
/* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
        /* coverity[misra_c_2012_rule_11_3_violation] */
        ProtocolHeaders_t * pxProtocolHeaders = ( ( ProtocolHeaders_t * )
                                                  &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSizePacket( pxNetworkBuffer ) ] ) );
        TCPHeader_t * pxTCPHeader = &pxProtocolHeaders->xTCPHeader;
        const TCPWindow_t * pxTCPWindow = &pxSocket->u.xTCP.xTCPWindow;
        UBaseType_t uxOptionsLength = pxTCPWindow->ucOptionLength;

        #if ( ipconfigUSE_TCP_WIN == 1 )
            /* memcpy() helper variables for MISRA Rule 21.15 compliance*/
            const void * pvCopySource;
            void * pvCopyDest;

            if( uxOptionsLength != 0U )
            {
                /* TCP options must be sent because a packet which is out-of-order
                 * was received. */
                if( xTCPWindowLoggingLevel >= 0 )
                {
                    FreeRTOS_debug_printf( ( "SACK[%u,%u]: optlen %u sending %u - %u\n",
                                             pxSocket->usLocalPort,
                                             pxSocket->u.xTCP.usRemotePort,
                                             ( unsigned ) uxOptionsLength,
                                             ( unsigned ) ( FreeRTOS_ntohl( pxTCPWindow->ulOptionsData[ 1 ] ) - pxSocket->u.xTCP.xTCPWindow.rx.ulFirstSequenceNumber ),
                                             ( unsigned ) ( FreeRTOS_ntohl( pxTCPWindow->ulOptionsData[ 2 ] ) - pxSocket->u.xTCP.xTCPWindow.rx.ulFirstSequenceNumber ) ) );
                }

                /*
                 * Use helper variables for memcpy() source & dest to remain
                 * compliant with MISRA Rule 21.15.  These should be
                 * optimized away.
                 */
                pvCopySource = pxTCPWindow->ulOptionsData;
                pvCopyDest = pxTCPHeader->ucOptdata;
                ( void ) memcpy( pvCopyDest, pvCopySource, ( size_t ) uxOptionsLength );

                /* The header length divided by 4, goes into the higher nibble,
                 * effectively a shift-left 2. */
                pxTCPHeader->ucTCPOffset = ( uint8_t ) ( ( ipSIZE_OF_TCP_HEADER + uxOptionsLength ) << 2 );
            }
            else
        #endif /* ipconfigUSE_TCP_WIN */

        if( ( pxSocket->u.xTCP.eTCPState >= eESTABLISHED ) && ( pxSocket->u.xTCP.bits.bMssChange != pdFALSE_UNSIGNED ) )
        {
            /* TCP options must be sent because the MSS has changed. */
            pxSocket->u.xTCP.bits.bMssChange = pdFALSE_UNSIGNED;

            if( xTCPWindowLoggingLevel >= 0 )
            {
                FreeRTOS_debug_printf( ( "MSS: sending %u\n", pxSocket->u.xTCP.usMSS ) );
            }

            pxTCPHeader->ucOptdata[ 0 ] = tcpTCP_OPT_MSS;
            pxTCPHeader->ucOptdata[ 1 ] = tcpTCP_OPT_MSS_LEN;
            pxTCPHeader->ucOptdata[ 2 ] = ( uint8_t ) ( ( pxSocket->u.xTCP.usMSS ) >> 8 );
            pxTCPHeader->ucOptdata[ 3 ] = ( uint8_t ) ( ( pxSocket->u.xTCP.usMSS ) & 0xffU );
            uxOptionsLength = 4U;
            pxTCPHeader->ucTCPOffset = ( uint8_t ) ( ( ipSIZE_OF_TCP_HEADER + uxOptionsLength ) << 2 );
        }
        else
        {
            /* Nothing. */
        }

        return uxOptionsLength;
    }
    /*-----------------------------------------------------------*/


/**
 * @brief Called from prvTCPHandleState(). There is data to be sent. If
 *        ipconfigUSE_TCP_WIN is defined, and if only an ACK must be sent, it will be
 *        checked if it would better be postponed for efficiency.
 *
 * @param[in] pxSocket The socket owning the TCP connection.
 * @param[in] ppxNetworkBuffer Pointer to pointer to the network buffer.
 * @param[in] ulReceiveLength The length of the received buffer.
 * @param[in] xByteCount Length of the data to be sent.
 *
 * @return The number of bytes actually sent.
 */
    BaseType_t prvSendData( FreeRTOS_Socket_t * pxSocket,
                            NetworkBufferDescriptor_t ** ppxNetworkBuffer,
                            uint32_t ulReceiveLength,
                            BaseType_t xByteCount )
    {
        /* Map the buffer onto the ProtocolHeader_t struct for easy access to the fields. */

        /* MISRA Ref 11.3.1 [Misaligned access] */
/* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
        /* coverity[misra_c_2012_rule_11_3_violation] */
        const ProtocolHeaders_t * pxProtocolHeaders = ( ( ProtocolHeaders_t * )
                                                        &( ( *ppxNetworkBuffer )->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSizePacket( *ppxNetworkBuffer ) ] ) );
        const TCPHeader_t * pxTCPHeader = &pxProtocolHeaders->xTCPHeader;
        const TCPWindow_t * pxTCPWindow = &pxSocket->u.xTCP.xTCPWindow;
        /* Find out what window size we may advertised. */
        int32_t lRxSpace;
        BaseType_t xSendLength = xByteCount;
        uint32_t ulRxBufferSpace;

        #if ( ipconfigUSE_TCP_WIN == 1 )
            /* Two steps to please MISRA. */
            size_t uxSize = uxIPHeaderSizePacket( *ppxNetworkBuffer ) + ipSIZE_OF_TCP_HEADER;
            BaseType_t xSizeWithoutData = ( BaseType_t ) uxSize;

            int32_t lMinLength;
        #endif

        /* Set the time-out field, so that we'll be called by the IP-task in case no
         * next message will be received. */
        ulRxBufferSpace = pxSocket->u.xTCP.ulHighestRxAllowed - pxTCPWindow->rx.ulCurrentSequenceNumber;
        lRxSpace = ( int32_t ) ulRxBufferSpace;

        #if ipconfigUSE_TCP_WIN == 1
        {
            /* An ACK may be delayed if the peer has space for at least 2 x MSS. */
            lMinLength = ( ( int32_t ) 2 ) * ( ( int32_t ) pxSocket->u.xTCP.usMSS );

            /* In case we're receiving data continuously, we might postpone sending
             * an ACK to gain performance. */
            /* lint e9007 is OK because 'uxIPHeaderSizeSocket()' has no side-effects. */
            if( ( ulReceiveLength > 0U ) &&                               /* Data was sent to this socket. */
                ( lRxSpace >= lMinLength ) &&                             /* There is Rx space for more data. */
                ( pxSocket->u.xTCP.bits.bFinSent == pdFALSE_UNSIGNED ) && /* Not in a closure phase. */
                ( xSendLength == xSizeWithoutData ) &&                    /* No Tx data or options to be sent. */
                ( pxSocket->u.xTCP.eTCPState == eESTABLISHED ) &&         /* Connection established. */
                ( pxTCPHeader->ucTCPFlags == tcpTCP_FLAG_ACK ) )          /* There are no other flags than an ACK. */
            {
                uint32_t ulCurMSS = ( uint32_t ) pxSocket->u.xTCP.usMSS;

                if( pxSocket->u.xTCP.pxAckMessage != *ppxNetworkBuffer )
                {
                    /* There was still a delayed in queue, delete it. */
                    if( pxSocket->u.xTCP.pxAckMessage != NULL )
                    {
                        vReleaseNetworkBufferAndDescriptor( pxSocket->u.xTCP.pxAckMessage );
                    }

                    pxSocket->u.xTCP.pxAckMessage = *ppxNetworkBuffer;
                }

                if( ulReceiveLength < ulCurMSS ) /* Received a small message. */
                {
                    pxSocket->u.xTCP.usTimeout = ( uint16_t ) tcpDELAYED_ACK_SHORT_DELAY_MS;
                }
                else
                {
                    /* Normally a delayed ACK should wait 200 ms for a next incoming
                     * packet.  Only wait 20 ms here to gain performance.  A slow ACK
                     * for full-size message. */
                    pxSocket->u.xTCP.usTimeout = ( uint16_t ) pdMS_TO_TICKS( tcpDELAYED_ACK_LONGER_DELAY_MS );

                    if( pxSocket->u.xTCP.usTimeout < 1U ) /* LCOV_EXCL_BR_LINE, the second branch will never be hit */
                    {
                        pxSocket->u.xTCP.usTimeout = 1U;  /* LCOV_EXCL_LINE, this line will not be reached */
                    }
                }

                if( ( xTCPWindowLoggingLevel > 1 ) && ( ipconfigTCP_MAY_LOG_PORT( pxSocket->usLocalPort ) ) )
                {
                    FreeRTOS_debug_printf( ( "Send[%u->%u] del ACK %u SEQ %u (len %u) tmout %u d %d\n",
                                             pxSocket->usLocalPort,
                                             pxSocket->u.xTCP.usRemotePort,
                                             ( unsigned ) ( pxTCPWindow->rx.ulCurrentSequenceNumber - pxTCPWindow->rx.ulFirstSequenceNumber ),
                                             ( unsigned ) ( pxSocket->u.xTCP.xTCPWindow.ulOurSequenceNumber - pxTCPWindow->tx.ulFirstSequenceNumber ),
                                             ( unsigned ) xSendLength,
                                             pxSocket->u.xTCP.usTimeout,
                                             ( int ) lRxSpace ) );
                }

                *ppxNetworkBuffer = NULL;
                xSendLength = 0;
            }
            else if( pxSocket->u.xTCP.pxAckMessage != NULL )
            {
                /* As an ACK is not being delayed, remove any earlier delayed ACK
                 * message. */
                if( pxSocket->u.xTCP.pxAckMessage != *ppxNetworkBuffer )
                {
                    vReleaseNetworkBufferAndDescriptor( pxSocket->u.xTCP.pxAckMessage );
                }

                pxSocket->u.xTCP.pxAckMessage = NULL;
            }
            else
            {
                /* The ack will not be postponed, and there was no stored ack ( in 'pxAckMessage' ). */
            }
        }
        #else /* if ipconfigUSE_TCP_WIN == 1 */
        {
            /* Remove compiler warnings. */
            ( void ) ulReceiveLength;
            ( void ) pxTCPHeader;
            ( void ) lRxSpace;
        }
        #endif /* ipconfigUSE_TCP_WIN */

        if( xSendLength != 0 )
        {
            if( ( xTCPWindowLoggingLevel > 1 ) && ( ipconfigTCP_MAY_LOG_PORT( pxSocket->usLocalPort ) ) )
            {
                FreeRTOS_debug_printf( ( "Send[%u->%u] imm ACK %u SEQ %u (len %u)\n",
                                         pxSocket->usLocalPort,
                                         pxSocket->u.xTCP.usRemotePort,
                                         ( unsigned ) ( pxTCPWindow->rx.ulCurrentSequenceNumber - pxTCPWindow->rx.ulFirstSequenceNumber ),
                                         ( unsigned ) ( pxTCPWindow->ulOurSequenceNumber - pxTCPWindow->tx.ulFirstSequenceNumber ),
                                         ( unsigned ) xSendLength ) );
            }

            /* Set the parameter 'xReleaseAfterSend' to the value of
             * ipconfigZERO_COPY_TX_DRIVER. */
            prvTCPReturnPacket( pxSocket, *ppxNetworkBuffer, ( uint32_t ) xSendLength, ipconfigZERO_COPY_TX_DRIVER );
            #if ( ipconfigZERO_COPY_TX_DRIVER != 0 )
            {
                /* The driver has taken ownership of the Network Buffer. */
                *ppxNetworkBuffer = NULL;
            }
            #endif
        }

        return xSendLength;
    }
    /*-----------------------------------------------------------*/

/**
 * @brief Common code for sending a TCP protocol control packet (i.e. no options, no
 *        payload, just flags).
 *
 * @param[in] pxNetworkBuffer The network buffer received from the peer.
 * @param[in] ucTCPFlags The flags to determine what kind of packet this is.
 *
 * @return pdFAIL always indicating that the packet was not consumed.
 */
    BaseType_t prvTCPSendSpecialPacketHelper( NetworkBufferDescriptor_t * pxNetworkBuffer,
                                              uint8_t ucTCPFlags )
    {
        BaseType_t xReturn = pdTRUE;

        #if ( ipconfigIGNORE_UNKNOWN_PACKETS == 1 )
            /* Configured to ignore unknown packets just suppress a compiler warning. */
            ( void ) pxNetworkBuffer;
            ( void ) ucTCPFlags;
        #else
        {
            switch( uxIPHeaderSizePacket( pxNetworkBuffer ) )
            {
                #if ( ipconfigUSE_IPv4 != 0 )
                    case ipSIZE_OF_IPv4_HEADER:
                        xReturn = prvTCPSendSpecialPktHelper_IPV4( pxNetworkBuffer, ucTCPFlags );
                        break;
                #endif /* ( ipconfigUSE_IPv4 != 0 ) */

                #if ( ipconfigUSE_IPv6 != 0 )
                    case ipSIZE_OF_IPv6_HEADER:
                        xReturn = prvTCPSendSpecialPktHelper_IPV6( pxNetworkBuffer, ucTCPFlags );
                        break;
                #endif /* ( ipconfigUSE_IPv6 != 0 ) */

                default:
                    xReturn = pdFAIL;
                    break;
            }
        }
        #endif /* !ipconfigIGNORE_UNKNOWN_PACKETS */

        /* The packet was not consumed. */
        return xReturn;
    }
    /*-----------------------------------------------------------*/

/**
 * @brief A "challenge ACK" is as per https://tools.ietf.org/html/rfc5961#section-3.2,
 *        case #3. In summary, an RST was received with a sequence number that is
 *        unexpected but still within the window.
 *
 * @param[in] pxNetworkBuffer The network buffer descriptor with the packet.
 * @param[in] ulCurrentSequenceNumber The current expected sequence value by the connection.
 * @param[in] ulOurSequenceNumber The SEQ number to send out.
 *
 * @return Returns the value back from #prvTCPSendSpecialPacketHelper.
 */
    BaseType_t prvTCPSendChallengeAck( NetworkBufferDescriptor_t * pxNetworkBuffer,
                                       uint32_t ulCurrentSequenceNumber,
                                       uint32_t ulOurSequenceNumber )
    {
        /* MISRA Ref 11.3.1 [Misaligned access] */
        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-113 */
        /* coverity[misra_c_2012_rule_11_3_violation] */
        ProtocolHeaders_t * pxProtocolHeaders = ( ( ProtocolHeaders_t * )
                                                  &( pxNetworkBuffer->pucEthernetBuffer[ ipSIZE_OF_ETH_HEADER + uxIPHeaderSizePacket( pxNetworkBuffer ) ] ) );

        /* In https://tools.ietf.org/html/rfc5961#section-3.2 the field values are defined as follows:
         *  <SEQ=SND.NXT>
         *  <ACK=RCV.NXT>
         *
         * The prvTCPSendSpecialPacketHelper function uses the sequence number of the packet as the
         * ACK number and the ACK number as the sequence number, therefore the values are set swapped
         * here to match the RFC. */
        pxProtocolHeaders->xTCPHeader.ulSequenceNumber = FreeRTOS_htonl( ulCurrentSequenceNumber );
        pxProtocolHeaders->xTCPHeader.ulAckNr = FreeRTOS_htonl( ulOurSequenceNumber );

        return prvTCPSendSpecialPacketHelper( pxNetworkBuffer, tcpTCP_FLAG_ACK );
    }
    /*-----------------------------------------------------------*/

/**
 * @brief Send a RST (Reset) to peer in case the packet cannot be handled.
 *
 * @param[in] pxNetworkBuffer The network buffer descriptor with the packet.
 *
 * @return Returns the value back from #prvTCPSendSpecialPacketHelper.
 */
    BaseType_t prvTCPSendReset( NetworkBufferDescriptor_t * pxNetworkBuffer )
    {
        return prvTCPSendSpecialPacketHelper( pxNetworkBuffer,
                                              ( uint8_t ) tcpTCP_FLAG_ACK | ( uint8_t ) tcpTCP_FLAG_RST );
    }
    /*-----------------------------------------------------------*/

#endif /* ipconfigUSE_TCP == 1 */