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[lwip-users] IP fragmentation

From: Vic
Subject: [lwip-users] IP fragmentation
Date: Mon, 22 Oct 2012 06:09:17 -0700 (PDT)

Hi all,

I have trouble getting the IP fragmentation to work. I try to transfer 4000
byte of data, therefor I call the function "tcp_write(tpcb, test_data,
sizeof(test_data), 0);". test_data is defined as "uint8_t test_data[4000];"
and filled with "DEADBEEF" test pattern. In ip.c at the point

  /* don't fragment if interface has mtu set to 0 [loopif] */
  if (netif->mtu && (p->tot_len > netif->mtu)) {
    return ip_frag(p, netif, dest);
#endif /* IP_FRAG */

This point is passed 3 times but ip_frag is never executed because
p->tot_len is smaller than the MTU. For the first and second p it is 1486
and for the last p 1148.
I defined the TCP_MSS as 1446 and the MTU is 1500.

Is there anything I have to consider when transferring data larger than MSS?
Have anybody a clue for me what could be wrong?

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