ReleaseUDPPayloadBuffer but for TCP?

Libraries
16

Hi Matt,

Some remarks about the progress we’ve made:

  1. There were unnecessary retransmissions because the IP-stack is impatient.

Recently I created a patch that should avoid this behaviour: “TCP minimum time for retransmissions (PR #387)”. See also this post.

For now you can try this change in FreeRTOS_TCP_WIN.c

-#define winSRTT_CAP_mS    50 /**< Cap in milliseconds. */
+#define winSRTT_CAP_mS  1000 /**< Cap in milliseconds. */

This avoids unnecessary retransmissions. But it doesn’t solve you problem.

  1. You found that the communication sometimes stalls for 5 seconds.

Could it be that FreeRTOS_send() can not deliver its data, and it will block for the maximum of 5 seconds?

Note that:

    static const TickType_t xSendTimeOut = pdMS_TO_TICKS( 5000 );

I think that FreeRTOS_send() blocks because of invalid settings, described in the next item 3).

  1. I noticed that the socket in your application has very little buffer space.
xWinProps.lTxBufSize = ipconfigTCP_TX_BUFFER_LENGTH;     // 1000 bytes
xWinProps.lTxWinSize = configECHO_SERVER_TX_WINDOW_SIZE; // 2 segments
xWinProps.lRxBufSize = ipconfigTCP_RX_BUFFER_LENGTH;     // 1000 bytes
xWinProps.lRxWinSize = configECHO_SERVER_RX_WINDOW_SIZE; // 2 segments;

The sizes of the buffers are far too small, less than a TCP segment.

These settings are passed to a socket with the socket option FREERTOS_SO_WIN_PROPERTIES.

Maybe we should adapt the handling in FreeRTOS_setsockopt() of the FREERTOS_SO_WIN_PROPERTIES option and refuse wrong settings.

It should be true that:

lTxBufSize >= lTxWinSize * ipconfigTCP_MSS
lRxBufSize >= lRxWinSize * ipconfigTCP_MSS

The help page shows this example:

    /* Fill in the required buffer and window sizes. */
    /* Unit: bytes */
    xWinProps.lTxBufSize = 4 * ipconfigTCP_MSS;
    /* Unit: MSS */
    xWinProps.lTxWinSize = 2;
    /* Unit: bytes */
    xWinProps.lRxBufSize = 4 * ipconfigTCP_MSS;
    /* Unit: MSS */
    xWinProps.lRxWinSize = 2;

It defines:

  • lTxBufSize: The size of the internal TX buffer (bytes)
  • lTxWinSize: The maximum TCP WIN size for transmissions ( unit MSS, e.g. 1460 bytes )
  • lRxBufSize: The size of the internal TX buffer (bytes)
  • lRxWinSize: The maximum TCP WIN size for reception ( unit MSS, e.g. 1460 bytes )

Transmission buffers are allocated as soon as they are needed: on reception of the first byte, or when sending the first packet. The configured buffer and WIN sizes are fixed for the life time of the socket.

Thanks,