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FreeRTOS+TCP Configuration Examples
Configuring the RTOS's TCP/IP stack to either minimise RAM consumption or maximise throughput

The FreeRTOS+TCP configuration file page documents each TCP/IP stack configuration option. This page provides suggestions on how to set key TCP parameters to tailor the TCP/IP stack to minimise its RAM consumption, and then to maximise its throughput. Note that minimising RAM consumption and maximising throughput are somewhat mutually exclusive objectives - up to a point the more RAM that is allocated to the TCP/IP stack the higher the throughput will be.

In all cases RAM consumption and CPU load can be minimised by using network drivers that make full use of any hardware features available, such as checksum offloading and MAC address filtering. If the hardware does not offer these facilities then the network driver can still improve RAM consumption and CPU load by performing any filtering it can before passing packets to the TCP/IP stack.


TCP/IP Stack Configuration To Minimise RAM Consumption

If you have a tiny CPU with less than 64KB of RAM, do not use sliding Windows: 
#define ipconfigUSE_TCP_WIN       0
The window size will be fixed to 1 MSS. The buffer size can be declared as 1 or 2 MSS: 
#define ipconfigTCP_TX_BUFFER_LENGTH   ( 2 * ipconfigTCP_MSS )
#define ipconfigTCP_RX_BUFFER_LENGTH   ( 2 * ipconfigTCP_MSS )
If RAM is really constrained then use smaller segments: 
#define ipconfigNETWORK_MTU   576
#define ipconfigTCP_MSS       522
All peers will understand this and only send small packets.

Only allocate the minimum number of network buffer descriptors you can get away with. This also has the effect of preventing high network traffic resulting in memory exhaustion as network buffers will not be allocated if no descriptors are available: 

#define ipconfigNUM_NETWORK_BUFFER_DESCRIPTORS  [a small number]
Finally, ensure only the amount of RAM that is actually required is allocated at any given time by using the BufferAllocation_2.c.


TCP/IP Stack Configuration to Maximise Throughput

First - note that network driver implementation is crucial in the throughput that can be obtained. Network drivers should copy as little data as possible (none!), use DMAs, calculate checksums in hardware, make full use of hardware filtering, and make use of software filtering where hardware filtering is not possible (to ensure only packets that actually require processing are passed to the TCP/IP stack).

Second the advanced features of the TCP/IP stack, including the functionality mentioned in the description of the ipconfigUSE_LINKED_RX_MESSAGES parameter and the callback API are provided with the aim of maximising throughput - however these features are considered to be for advanced users only.

If you have enough RAM then the following declarations will help performance: 

#define ipconfigNETWORK_MTU     1526
#define ipconfigTCP_MSS         1460
#define ipconfigTCP_TX_BUFFER_LENGTH  ( 16 * ipconfigTCP_MSS )
#define ipconfigTCP_RX_BUFFER_LENGTH  ( 16 * ipconfigTCP_MSS )
On a LAN, the sliding windows will get a size of ( 8 * ipconfigTCP_MSS ), meaning that only one out of 8 packets will receive an ACK.

For more flexibility FreeRTOS_setsockopt() can be used to set sizes between a socket being created and the same socket getting used. 


    /* Declare the variable used to set the configuration parameters. */
    WinProperties_t xWinProps;

    /* Start with everything set to 0. */
    memset( &xWinProps, '\0', sizeof( xWinProps ) );

    /* Configure as required. */
    xWinProps.ulTxBufSize = 24 * ipconfigTCP_MSS;
    xWinProps.ulTxWinSize = 8;
    xWinProps.ulRxBufSize = 24 * ipconfigTCP_MSS;
    xWinProps.ulRxWinSize = 8;

    /* Set the socket options. */
    FreeRTOS_setsockopt( sock,
                         0,
                         FREERTOS_SO_WIN_PROPERTIES,
                         ( void * ) &xWinProps,
                         sizeof( xWinProps ) );
Using FreeRTOS_setsockopt() to set TCP/IP options

Usually nothing is to be gained by setting the windows larger than ( 8 * MSS ), unless the CPU and MAC are very fast and connected to a 1 Gbit LAN. Using larger buffers for reception does make sense in case the end-point is slow, for instance if all the received data must be written to an SD-card.

Finally, ensure fast and deterministic buffer allocation that can also be used directly from within the MAC interrupt by using BufferAllocation_1.c.







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