[lwip-users] Why does enabling Debug fix this tcp_sndbuf()

[Osborne, David]

Hi All,

 

I’m using LwIP + FreeRTOS + Sockets on STM32H7 with external SDRAM and FLASH (Ethernet buffers are internal).  It works well until the server has to send a very large message (> 18Kbytes).  One in five attempts ends in failure.  During the transmission tcp_sndbuf() (in api_msg.c) returns zero and the connection falls over.  From what I’ve read, a common cause is that it’s waiting for an outstanding ACK.  I’m no expert, but it looks like the ACK has been sent (see Wireshark attachment).

 

When I enable certain debug settings it works every time (don’t you hate that).

 

#define TCP_DEBUG LWIP_DBG_ON

#define TCP_OUTPUT_DEBUG LWIP_DBG_ON

 

It has to be both debugs settings (just one doesn’t fix it).

 

Is there any TCP experts out there who can tell me why tcp_sndbuf() is returning 0 and the client is retransmitting (when debug is off)?

 

 

Regards

David

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failed_exchange.pcapng
Description: failed_exchange.pcapng