xjsc16x xjsc16x - 2 months ago 20
C Question

fletcher checksum giving different values

I am trying to use a 16-bit Fletcher checksum here. Basically, my program simulates traffic over the physical layer by "sending" and "receiving" packets between two virtual entities. I am printing out the packets at both sides and they do match, but I am getting a different checksum calculated at the receiving end.

Packet structure:

#define MESSAGE_LENGTH 20
struct pkt {
int seqnum;
int acknum;
int checksum;
char payload[MESSAGE_LENGTH];
};


This is the code I'm using to compute the checksum of each packet:

/*
* Computes the fletcher checksum of the input packet
*/
uint16_t calcChecksum(struct pkt *packet) {
/* the data for the checksum needs to be continuous, so here I am making
a temporary block of memory to hold everything except the packet checksum */
size_t sizeint = sizeof(int);
size_t size = sizeof(struct pkt) - sizeint;
uint8_t *temp = malloc(size);
memcpy(temp, packet, sizeint * 2); // copy the seqnum and acknum
memcpy(temp + (2*sizeint), &packet->payload, MESSAGE_LENGTH); // copy data

// calculate checksum
uint16_t checksum = fletcher16((uint8_t const *) &temp, size);
free(temp);
return checksum;
}

/*
* This is a checksum algorithm that I shamelessly copied off a wikipedia page.
*/
uint16_t fletcher16( uint8_t const *data, size_t bytes ) {
uint16_t sum1 = 0xff, sum2 = 0xff;
size_t tlen;

while (bytes) {
tlen = bytes >= 20 ? 20 : bytes;
bytes -= tlen;
do {
sum2 += sum1 += *data++;
} while (--tlen);
sum1 = (sum1 & 0xff) + (sum1 >> 8);
sum2 = (sum2 & 0xff) + (sum2 >> 8);
}
/* Second reduction step to reduce sums to 8 bits */
sum1 = (sum1 & 0xff) + (sum1 >> 8);
sum2 = (sum2 & 0xff) + (sum2 >> 8);
return sum2 << 8 | sum1;
}


I don't know much about checksums and I copied that algorithm off a page I found, so if anyone can understand why the checksums for two otherwise identical packets are different I would greatly appreciate it. Thank you!

Answer

The problem occurs because you are not passing the address of temp data to the checksum function but rather the address to where the variable temp is stored on stack.

You should change

uint16_t checksum = fletcher16((uint8_t const *) &temp, size);

to

uint16_t checksum = fletcher16((uint8_t const *) temp, size);
                                                ^ no & operator