Date: Mon, 11 Mar 2002 17:29:31 +0200 From: "alpha" <alpha_byte@centras.lt> To: "Ryan Thompson" <ryan@sasknow.com> Cc: "FreeBSD" <freebsd-net@freebsd.org> Subject: Re: help with tcp checksum Message-ID: <003901c1c911$b5f90f30$a61b3bd4@alpha> References: <20020311090940.B99093-100000@catalyst.sasknow.net>
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hi,
it the first place thanx for answering. i'm affraid i can't show my work
becouse it's confidential. I'm working on one comercial project.
> Well, honestly, I haven't had my coffee yet, so I'd probably screw it
> up anyway. I haven't hand-calculated one of these this century, but
> perhaps you are forgetting to take the checksum of the IP fields? In
?? you mean IP fields must be calculated in TCP header checksum too?
> Maybe if you post some of your work, outline the steps (as opposed to
> the wrong answer :-), someone can spot how you got that wrong answer.
you know, i downloaded free example from internet, but text portion's
checksum of TCP header was writed by hand. i relalculated it and i got wrong
answer. and i don't know why. here a part from this example:
load_TCP_header( 0x5000 | TCP_ACK | TCP_FIN, 0x07AC, 148);
^^^^^^
this
is text portion's checksum, but
i
get 0798h checksum somehow
// Number of bytes include in load_IP_header call
printf(outdata,"HTTP/1.0 200 OK\r\n");
printf(outdata,"Content-type: text/html\r\n\r\n");
printf(outdata,"<HTML><body>SimmStick® Web Server<br>");
printf(outdata,"using a PIC 16F874<br>");
printf(outdata,"by David C. Witt (12/24/00)</body></HTML>");
............................................................................
...................................
void load_TCP_header(long int data_flags, long int tcp_chksum, long int
tcp_length) {
long int prev_chksum;
chksum = tcp_chksum;
// calculate checksum of pseudo header
prev_chksum = chksum;
chksum += ((long)my_ip[0]<<8) + my_ip[1];
if (chksum < prev_chksum) chksum++;
prev_chksum = chksum;
chksum += ((long)my_ip[2]<<8) + my_ip[3];
if (chksum < prev_chksum) chksum++;
prev_chksum = chksum;
chksum += ((long)source_ip[0]<<8) + source_ip[1];
if (chksum < prev_chksum) chksum++;
prev_chksum = chksum;
chksum += ((long)source_ip[2]<<8) + source_ip[3];
if (chksum < prev_chksum) chksum++;
prev_chksum = chksum;
chksum += TCP;
if (chksum < prev_chksum) chksum++;
prev_chksum = chksum;
chksum += tcp_length;
if (chksum < prev_chksum) chksum++;
// source port
outportw(NIC_DATA, tcp_dest_port);
// destination port
outportw(NIC_DATA, tcp_source_port);
// sequence number
outportw(NIC_DATA, xm_seq[0]);
outportw(NIC_DATA, xm_seq[1]);
// acknowledment number
outportw(NIC_DATA, xm_ack[0]);
outportw(NIC_DATA, xm_ack[1]);
// data offset and flags
outportw(NIC_DATA, data_flags);
// window
outportw(NIC_DATA, 1460);
// checksum
outportw(NIC_DATA, ~chksum);
// urgent pointer
outportw(NIC_DATA, 0);
............................................................................
.............
be so kind to help me please.
Darius
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