Re: [ECOS] Can't Connect,TCP CHECKSUM INCORRECT

["ariga masahiro" <ariga@link-lab.co.jp>]

Hello,

Please read this mail with next consent.
I never doubt anything of eCos source code.
I just like to find out suffering points because of hardware oddity and
my bad concoction.I beseech you to help me find that suffering points.
And I mail this if my discovery have any merit for you to pinpoint that
suffering points.I am not good at TCP/IP.My opinion is greatly loaded
with conjectures.

I traced tcp_output function.
It looks to me, at tcp_output's next points, append option part.
line 688-691
 if (optlen) {
  bcopy(opt, th + 1, optlen);
  th->th_off = (sizeof (struct tcphdr) + optlen) >> 2;
 }
Although peer sent 8 bytes option,optlen was 4.

Also it looks to me, in generating SYN-ACK packet, at next switch sentence,
case (TH_SYN|TH_ACK) context should be executed.
But it all passed out these block, never entered if sentence(line 445) 
block.
At that time tp->t_flags was 0xA1,flags was 0x12.

line 445-510
  /*
  * Send `CC-family' options if our side wants to use them (TF_REQ_CC),
  * options are allowed (!TF_NOOPT) and it's not a RST.
   */
  if ((tp->t_flags & (TF_REQ_CC|TF_NOOPT)) == TF_REQ_CC &&
       (flags & TH_RST) == 0) {
  switch (flags & (TH_SYN|TH_ACK)) {
  /*
   * This is a normal ACK, send CC if we received CC before
   * from our peer.
   */
  case TH_ACK:
   if (!(tp->t_flags & TF_RCVD_CC))
    break;
   /*FALLTHROUGH*/

  /*
   * We can only get here in T/TCP's SYN_SENT* state, when
   * we're a sending a non-SYN segment without waiting for
   * the ACK of our SYN.  A check above assures that we only
   * do this if our peer understands T/TCP.
   */
  case 0:
   opt[optlen++] = TCPOPT_NOP;
   opt[optlen++] = TCPOPT_NOP;
   opt[optlen++] = TCPOPT_CC;
   opt[optlen++] = TCPOLEN_CC;
   *(u_int32_t *)&opt[optlen] = htonl(tp->cc_send);

   optlen += 4;
   break;

  /*
   * This is our initial SYN, check whether we have to use
   * CC or CC.new.
   */
  case TH_SYN:
   opt[optlen++] = TCPOPT_NOP;
   opt[optlen++] = TCPOPT_NOP;
   opt[optlen++] = tp->t_flags & TF_SENDCCNEW ?
      TCPOPT_CCNEW : TCPOPT_CC;
   opt[optlen++] = TCPOLEN_CC;
   *(u_int32_t *)&opt[optlen] = htonl(tp->cc_send);
    optlen += 4;
   break;

  /*
   * This is a SYN,ACK; send CC and CC.echo if we received
   * CC from our peer.
   */
  case (TH_SYN|TH_ACK):
   if (tp->t_flags & TF_RCVD_CC) {
    opt[optlen++] = TCPOPT_NOP;
    opt[optlen++] = TCPOPT_NOP;
    opt[optlen++] = TCPOPT_CC;
    opt[optlen++] = TCPOLEN_CC;
    *(u_int32_t *)&opt[optlen] =
     htonl(tp->cc_send);
    optlen += 4;
    opt[optlen++] = TCPOPT_NOP;
    opt[optlen++] = TCPOPT_NOP;
    opt[optlen++] = TCPOPT_CCECHO;
    opt[optlen++] = TCPOLEN_CC;
    *(u_int32_t *)&opt[optlen] =
     htonl(tp->cc_recv);
    optlen += 4;
   }
   break;
  }
  }

Possibly I am wrong,anyway I inform you on the risk of showing ignarance.

I sincerely beseech your help.

Masahiro Ariga



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