[lwip-users] Duplicated ACK and retransmitted packets

[Lou Cypher]

I've already read of someone having problems with duplicated ACKs, and I've got
a similar issue.
Note that I'm debugging part of an application that could be bugged, though I
think that the part giving the dup ACKs is quite simple.

I'm attaching (part of) a Wireshark capture as an example, where the (FTP)
client is my embedded system running lwIP 1.3.2, and the (FTP) server is a PC
running FileZilla Server.

The problem arises in the FTP file transfer, on the data connection; I tried
separating the most of it, so now I have a couple of functions handling file
reception, after I send the RETR command to server:

 - the first one sets a buffer, with length, then calls tcp_recv() with the
second; an extract follows

        uint8_t         * pd, * pmax;

        pd   = (uint8_t *)p;
        pmax = p + size;

        pf->file_buf = pd;
        pf->file_put = pd;
        pf->file_len = size;

        tcp_recv( pcb, ftplw_file_recved );

        while( pf->file_put < pmax )
        {
             ... // lwIP TCP processing
        }

        tcp_recv( pcb, 0 );


 - the second one acts as the callback, and looks like the following:

static err_t    ftplw_file_recved( void *arg,
                                   struct tcp_pcb *pcb,
                                   struct pbuf *p,
                                   err_t err )
{
    struct sFTPc        * pf = (struct sFTPc *)arg;

    if( err == ERR_OK  &&  p == 0 )
    {
        tcp_close( (struct tcp_pcb *)pf->data_desc );
        pf->data_desc = 0;
        pf->data_conn = 0;
    }
    else
    if( err == ERR_OK  &&  p != 0 )
    {
        uint16_t        size;
        uint8_t         * pd, * pp, * pmax;
        struct pbuf     * pb;

        if( pf->data_desc )
        {
            //  Enqueues received data
            pp   = pf->file_put;
            pmax = pf->file_buf + pf->file_len;
            pb   = p;

            //  Processes all linked pbufs
            do {
                size = pb->len;
                pd   = pb->payload;

                while( size  &&  pp < pmax )
                {
                    *pp++ = *pd++ ;
                    size-- ;
                }

                pb = pb->next;

            } while( pb );

            pf->file_put = pp;
        }

        tcp_recved( pcb, p->tot_len );

        pbuf_free( p );
    }

    return  ERR_OK;
}

I think that the code above is quite straightforward, besides the fact that
*most* of the time it receives all the data correctly.
While sometimes I see data corruption happening, as well.

I'm using options as in the following, and checking stats I see no buffer
reaching any limit (resources usage is very low).
-----------------------------------------------------------------------
#define  NO_SYS                         1
#define  SYS_LIGHTWEIGHT_PROT           1

#define  MEM_ALIGNMENT                  4
#define  MEM_SIZE                       (128 * 1024)

#define  MEMP_NUM_RAW_PCB               16      // 8
#define  MEMP_NUM_UDP_PCB               8       // 4
#define  MEMP_NUM_PBUF                  32      // default 16
#define  MEMP_NUM_TCP_PCB               128     // 32   // default 5
#define  MEMP_NUM_TCP_PCB_LISTEN        8
#define  MEMP_NUM_TCP_SEG               256     // default 16
#define  PBUF_POOL_SIZE                 128     // default 16

#define  LWIP_DHCP                      1       // default 0
#define  DHCP_DOES_ARP_CHECK            1

#define  LWIP_AUTOIP                    1
#define  LWIP_DHCP_AUTOIP_COOP          1

#define  TCP_WND                        4096    // default 2048
#define  TCP_MSS                        1460
#define  TCP_SND_BUF                    (8 * TCP_MSS)

#define  PBUF_LINK_HLEN                 16      // default 14

#define  LWIP_NETCONN                   0       // default 1
#define  LWIP_SOCKET                    0       // default 1
#define  LWIP_COMPAT_SOCKETS            0

#define  LWIP_NETIF_HOSTNAME            1

#define  LWIP_DNS                       1


#define  LWIP_STATS                     1
#define  LWIP_STATS_DISPLAY             1

#define  LWIP_DEBUG                     1
#define  DBG_TYPES_ON           DBG_LEVEL_WARNING
#define  MEMP_DEBUG             (LWIP_DBG_LEVEL_WARNING | LWIP_DBG_ON)
-----------------------------------------------------------------------

I hope I enclosed everything here, since I'm writing this in a short time! :)
Any suggestion?

~ Lou

ftpup-part.pcap
Description: Binary data