RE: [lwip-users] raw api recv callback - procedure to (temporarily)refuse data?

[Karl Kobel]

All,

I'm stuck. I'm nearly done with a single threaded version of slip. I
will add it to contrib when I complete it.

The problem stems most likely from my lack of understanding of pbufs.

The follow2ing is the code that executes when a complete packet is
received:

        p = pbuf_alloc(PBUF_RAW, recved, PBUF_POOL);
        if (p != NULL)
        {
                memcpy(p->payload, SlipBuff, recved);
                p->len = recved;
                LWIP_DEBUGF(SLIP_DEBUG, ("slipif: Got packet\n"));
                recved = 0;
                return p;
        }

SlipBuff has the data,
Recved has the length.

Small packets are handled correctly. However larger packets (around 300
bytes) the TCP checksum fails. The following is the portion of tcp_input
that fails.

  /* Verify TCP checksum. */
  if (inet_chksum_pseudo(p, (struct ip_addr *)&(iphdr->src),
      (struct ip_addr *)&(iphdr->dest),
      IP_PROTO_TCP, p->tot_len) != 0) {
      LWIP_DEBUGF(TCP_INPUT_DEBUG, ("tcp_input: packet discarded due to
failing checksum 0x%04x\n",
        inet_chksum_pseudo(p, (struct ip_addr *)&(iphdr->src), (struct
ip_addr *)&(iphdr->dest),
      IP_PROTO_TCP, p->tot_len)));

The pbuf that fails has the next pointer pointing to another pbuf (four
total in the one I trapped), but the payload and len are correct for the
entire packet in the first pbuf. Also if I jump past the code after the
first pbuf checksum is totaled, the packet checksum is correct and the
packet is received correctly.

This has to be a result of me doing something wrong. Do I need to do
something different when I allocate the pbuf in the slip driver? I tried
PBUF_LINK as the first argument with the same results. Do I have to set
the next pointer to NULL?

Thanks,
Karl