TCP checksums are evaluated by attaching a pseudo
header in the start of the TCP segment which u are
going to be sent. Actually that pseudo header is like
this...
struct pseudohdr
{
struct in_addr source_address; // 32 bit
netid/hostid
struct in_addr dest_address; // 32 bit
netid/hostid
unsigned char place_holder;
unsigned char protocol;
unsigned short length;
struct tcphdr tcp;
} phdr;
where the protocol is IPPROTO_TCP , place holder is
zero and length is htons(sizeof(struct tcphdr));
After this u can directly compute this checksum by
of this function which does exactly as what has been
explained in comer or anyother book (e.g W.R.Stevens)
int in_chksum(u_short *ptr, int nbytes)
// where ptr is the pointer to the tcp segment
prefixed
// with pseudo header and nbytes is
// sizeof(tcpsegment+pseudoheader); by tcpsegment I
// mean tcpdata + tcpheader;
{
long sum;
u_short oddbyte; // 16 bit
u_short answer;
sum=0;
while(nbytes > 1)
{
sum += *ptr++;
nbytes -= 2;
}
if (nbytes == 1)
{
oddbyte = 0;
*((u_char *)&oddbyte) = *(u_char
*)ptr;
sum+= oddbyte;
}
sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
answer =~sum;
return(answer);
}
--- Bill Geissler <billgeissler@yahoo.com> wrote:
>
> How is the checksum for the data field in a TCP
> packet
> computed, and how is that checksum incorporated into
> the checksum field in the TCP header?
>
> I see a number of checksum routines, and, frankly,
> I'm
> getting confused.
>
> Any help would be appreciated.
>
> Bill
>
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