RE: [lwip-users] Duplicated ACK and retransmitted packets

[Bill Auerbach]

Lou,

When I fought a Dup Acks problem a long time ago and was sure it was caused
by lwIP, I was surprised to find that it was caused by a subtle problem in
my Ethernet driver.  Be sure your driver is rock solid.

Bill

>-----Original Message-----
>From: address@hidden
>[mailto:address@hidden On
>Behalf Of Lou Cypher
>Sent: Monday, February 08, 2010 1:47 PM
>To: Mailing list for lwIP users
>Subject: [lwip-users] Duplicated ACK and retransmitted packets
>
>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