Re: [lwip-users] Checksum Incorrect caused by Concatenation

[Simon Goldschmidt]

Did you read Kieran's response to your mail yesterday?

I also doubt the changes are correct and find it quite strange that it makes 
things better, not worse.

Simon


"Hervé GARAT : Audemat" <address@hidden> wrote:
> 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

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