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# TCP checksum in delphi

Posted on 2008-09-30

Can anyone tell me if there is a Delphi version of this routine out there to calculate the TCP checksum

/*

**************************************************************************

Function: tcp_sum_calc()

**************************************************************************

Description:

Calculate TCP checksum

***************************************************************************

*/

typedef unsigned short u16;

typedef unsigned long u32;

u16 tcp_sum_calc(u16 len_tcp, u16 src_addr[],u16 dest_addr[], BOOL padding, u16 buff[])

{

u16 prot_tcp=6;

u16 padd=0;

u16 word16;

u32 sum;

// Find out if the length of data is even or odd number. If odd,

// add a padding byte = 0 at the end of packet

if (padding&1==1){

padd=1;

buff[len_tcp]=0;

}

//initialize sum to zero

sum=0;

// make 16 bit words out of every two adjacent 8 bit words and

// calculate the sum of all 16 vit words

for (i=0;i<len_tcp+padd;i=i+2){

word16 =((buff[i]<<8)&0xFF00)+(buff[i+1]&0xFF);

sum = sum + (unsigned long)word16;

}

// add the TCP pseudo header which contains:

// the IP source and destinationn addresses,

for (i=0;i<4;i=i+2){

word16 =((src_addr[i]<<8)&0xFF00)+(src_addr[i+1]&0xFF);

sum=sum+word16;

}

for (i=0;i<4;i=i+2){

word16 =((dest_addr[i]<<8)&0xFF00)+(dest_addr[i+1]&0xFF);

sum=sum+word16;

}

// the protocol number and the length of the TCP packet

sum = sum + prot_tcp + len_tcp;

// keep only the last 16 bits of the 32 bit calculated sum and add the carries

while (sum>>16)

sum = (sum & 0xFFFF)+(sum >> 16);

// Take the one's complement of sum

sum = ~sum;

return ((unsigned short) sum);

}

Also how do I determine the len_tcp value and how do I get the buff[].