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Re: [lwip-users] TCP Retransmission when receiving consecutive packets

From: Sergio R. Caprile
Subject: Re: [lwip-users] TCP Retransmission when receiving consecutive packets
Date: Thu, 20 Mar 2014 11:10:01 -0300
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:24.0) Gecko/20100101 Thunderbird/24.3.0

Hi Julien.
Yes, there is a TCP retransmission from your host.
I bet that is because when the two packets come in such a close
proximity your Ethernet driver is losing the second one. This, in fact,
can be because there is a problem with the driver itself, or you have a
really low-memory condition and the driver can't find enough memory to
store the packet.
If I would have to hunt this witch, I would start by setting a
breakpoint on the Ethernet driver low_level_input() routine, somewhere
around here:

static struct pbuf *
low_level_input(struct netif *netif)
  /* We allocate a pbuf chain of pbufs from the pool. */
  p = pbuf_alloc(PBUF_RAW, len, PBUF_POOL);

  if (p != NULL) {
      read data into(q->payload, q->len);
    acknowledge that packet has been read();
  } else {
    drop packet();      <-------------

It actually depends on how the one who actually wrote the driver wanted
to make it difficult for you.
Let me know how it goes.


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