[lwip-users] Checksum Incorrect caused by Concatenation

[Hervé GARAT : Audemat]

Hi All,

 

I use the last CVS Head LWIP on LPC2468 with FreeRTOS. My embedded system communicates by IP with a PC. When the PC IP interfaces restart, I look for a checksum incorrect packet on Wireshark.

 

This error seems to be a concatenate error. 172.17.2.173:my embedded system with LWIP; 172.17.2.172 : PC

 

    111 7.819682    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [PSH, ACK] Seq=55493 Ack=0 Win=5840 Len=209

    112 7.825543    172.17.2.172          172.17.2.173          TCP      48955 > 5678 [ACK] Seq=0 Ack=55702 Win=65535 Len=0

    113 7.993714    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [PSH, ACK] Seq=55702 Ack=0 Win=5840 Len=531

    114 7.997536    172.17.2.172          172.17.2.173          TCP      48955 > 5678 [ACK] Seq=0 Ack=56233 Win=65535 Len=0

    115 8.211446    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [ACK] Seq=56233 Ack=0 Win=5840 Len=1460

    116 8.301504    172.17.2.172          172.17.2.173          TCP      48955 > 5678 [ACK] Seq=0 Ack=57693 Win=65535 Len=0

    117 8.326219    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [PSH, ACK] Seq=57693 Ack=0 Win=5840 Len=197

    118 8.333736    172.17.2.172          172.17.2.173          TCP      48955 > 5678 [ACK] Seq=0 Ack=57890 Win=65535 Len=0

    119 8.422016    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [PSH, ACK] Seq=57890 Ack=0 Win=5840 Len=1126

    120 8.429543    172.17.2.172          172.17.2.173          TCP      48955 > 5678 [ACK] Seq=0 Ack=59016 Win=65535 Len=0

    121 8.632683    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [PSH, ACK] Seq=59016 Ack=0 Win=5840 Len=934

    122 8.634010    172.17.2.172          172.17.2.173          TCP      48955 > 5678 [ACK] Seq=0 Ack=59950 Win=65535 Len=0

    123 8.869443    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [ACK] Seq=59950 Ack=0 Win=5840 Len=1460       <- Restart of the PC IP interfaces

    125 8.871878    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [ACK] Seq=61410 Ack=0 Win=5840 Len=1460

    129 8.874047    172.17.2.172          172.17.2.173          TCP      48955 > 5678 [ACK] Seq=0 Ack=62870 Win=65535 Len=0

    130 9.307578    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [PSH, ACK] Seq=62870 Ack=0 Win=5840 [TCP CHECKSUM INCORRECT] Len=740    <- this packet is a concatenation of the same packets than 111 and 113

    131 9.309162    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [ACK] Seq=63610 Ack=0 Win=5840 Len=1460

    132 9.309293    172.17.2.172          172.17.2.173          TCP      [TCP Dup ACK 129#1] 48955 > 5678 [ACK] Seq=0 Ack=62870 Win=65535 Len=0

    142 9.789090    172.17.2.173          172.17.2.172          TCP      5678 > 48955 [PSH, ACK] Seq=65070 Ack=0 Win=5840 [TCP CHECKSUM INCORRECT] Len=1323   <- this packet is a concatenation of the same packets than 117 and 119ss

    153 10.003902   172.17.2.173          172.17.2.172          TCP      5678 > 48955 [PSH, ACK] Seq=66393 Ack=0 Win=5840 Len=934

    156 10.019539   172.17.2.172          172.17.2.173          TCP      [TCP Dup ACK 129#2] 48955 > 5678 [ACK] Seq=0 Ack=62870 Win=65535 Len=0

    157 10.905015   172.17.2.173          172.17.2.172          TCP      [TCP Retransmission] 5678 > 48955 [PSH, ACK] Seq=62870 Ack=0 Win=5840 [TCP CHECKSUM INCORRECT] Len=740

    158 13.209979   172.17.2.172          172.17.2.173          TCP      48955 > 5678 [PSH, ACK] Seq=0 Ack=62870 Win=65535 Len=1

    159 13.210040   172.17.2.172          172.17.2.173          TCP      48955 > 5678 [FIN, ACK] Seq=1 Ack=62870 Win=65535 Len=0

    160 13.210332   172.17.2.172          172.17.2.173          TCP      48956 > 5678 [SYN] Seq=0 Len=0 MSS=1460 TSV=1711037 TSER=0 WS=6

    161 13.241498   172.17.2.173          172.17.2.172          TCP      5678 > 48955 [ACK] Seq=67327 Ack=2 Win=5838 Len=0

    162 13.279770   172.17.2.173          172.17.2.172          TCP      5678 > 48956 [SYN, ACK] Seq=0 Ack=1 Win=5840 Len=0 MSS=1460

    163 13.279930   172.17.2.172          172.17.2.173          TCP      48956 > 5678 [ACK] Seq=1 Ack=1 Win=5840 Len=0

    164 14.205872   172.17.2.173          172.17.2.172          TCP      [TCP Retransmission] 5678 > 48955 [PSH, ACK] Seq=62870 Ack=2 Win=5838 [TCP CHECKSUM INCORRECT] Len=740

 

 

In the pbuf_cat function “p->next” must not be equal to “t”, but must be equal to “NULL”. And we must write “p->len+=t->len;”. And next after the call of “pbuf_cat(useg->p, queue->p);” in “tcp_enqueue fuction” tcp_out.c  , we have Add two line:

 

    pbuf_cat(useg->p, queue->p);

    useg->len += queue->len;

    useg->next = queue->next;

    LWIP_DEBUGF(TCP_OUTPUT_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_STATE, ("tcp_enqueue: chaining segments, new len %"U16_F"\n", useg->len));

    if (seg == queue)

    {

      seg = useg;

      seglen = useg->len;

      queuelen--;               //decrementation of the SND_QUEUELEN due to concatenation

    }

    memp_free(MEMP_TCP_SEG, queue);

    pbuf_free(queue->p);   /dereference a pbuf chain due to concatenation

 

 

Thanks to this modification I haven’t a TCP CHECKSUM INCORRECT in wireshark and the communication works fine.

 

Can you tell me  if this modification are the best  and if they will not be cause of a problem.

 

Thanks In advance,

 

Hervé GARAT