weird checksum function

sagitt13 · 10-30-2006, 11:47 PM

Hi,

I'm taking a networking class and my current project is to implement a "reliable data transfer protocol". So we're given a checksum function to both create a checksum for our packets and to check if the packets are error free. My problem is everytime I tried to check, I get a result that says packet has errors.

Here's the function:

Code:

uint16_t checksum(uint16_t *buf, int nwords)
{
	uint32_t sum;

	for (sum = 0; nwords > 0; nwords--)
		sum += *buf++;
	sum = (sum >> 16) + (sum & 0xffff);
	sum += (sum >> 16); 
	return ~sum;
}

What I did is: I create a packet (a structure), memset it to 0, add in data/headers, feed the struct into this function and get back a checksum to put in the same packet. I sent the packet to the receiving host. At that end I first save the packet checksum into a temp variable, set the checksum in the packet struct to 0, feed it back in the function and what it returns should be the same as the saved checksum I got from the packet itself. But everytime I do that the results don't match. (btw the checksum field is 16 bits)

So roughly:
on sender side:

Code:

Packet p;
memset(&p sizeof(Packet), 0);      .
//set contents;
p.checksum = checksum((uint16_t *)&p, sizeof(Packet)/2);

on receiver side:

Code:

uint16_t check = p.checksum;
p.checksum = 0;
 if (checksum((uint16_t *)&p, sizeof(Packet)/2) != check)
            //corrupted

From what I see, this checksum function adds up 16 bit words of the packet, add the sum to its upper 16 bit value and the lower 16 bit value, then add that to the upper 16 bit again and returns the complement. I'm still confused as to how this will give me a unique checksum. But my priority right now is to get this to work / use it correctly.

If anyone can help/comment I'll greatly appreciate it, Thanks!

**Dave_Sinkula** · 10-30-2006, 11:56 PM

Sorry, didn't look to closely. I'd take a closer look if you had a single, simple bit of code that demonstrated the problem.

Code:

p.checksum = checksum((uint16_t *)&p, sizeof(Packet)/2);

Why are you doing sizeof(Packet)/2?

Many checksums in which the checksum itself is included add to zero.

vart · 10-31-2006, 12:02 AM

Have you checked your structure? is it filled exactly as you sent it?

vart · 10-31-2006, 12:04 AM

Quote:

Originally Posted by Dave_Sinkula

Why are you doing sizeof(Packet)/2?

He is scanning it in WORDs

so actually it should be

Code:

sizeof(Packet)/sizeof(uint16_t)

But I'm not sure what about aligning and bytes order

sagitt13 · 10-31-2006, 12:44 AM

Thanks for the input, I tried dividing by sizeof(uint16_t) but it still doesn't work... Here's a little test program I used to test the function:

Code:

#include "saw.h"

uint16_t checksum(uint16_t *buf, int nwords)
{
	uint32_t sum;

	for (sum = 0; nwords > 0; nwords--)
		sum += *buf++;
	sum = (sum >> 16) + (sum & 0xffff);
	sum += (sum >> 16); 
	return ~sum;
}

int main(int argc, char *argv[]) {

	sawhdr packet;
	uint16_t check;
	memset(&packet, sizeof(sawhdr), 0);	
	packet.type = SYNACK;
	packet.checksum = checksum((uint16_t *)&packet, sizeof(sawhdr)/sizeof(uint16_t));
	check = packet.checksum;
	packet.checksum = 0;
	if (checksum((uint16_t *)&packet, sizeof(sawhdr)/sizeof(uint16_t)) != check)
			fprintf(stderr, "checksum bad!!\n");
	else
		printf("checksum ok\n");
}

And here's the stuff from saw.h that're relevant:

Code:

#define SYNACK   1

typedef struct {
	uint8_t  type;            /* packet type (e.g. SYN, DATA, ACK, FIN)     */
	uint8_t  seqnum;          /* sequence number of the packet              */
    uint16_t checksum;        /* header and payload checksum                */
    uint8_t data[DATALEN];    /* payload                     */
	size_t numlen;
} __attribute__((packed)) sawhdr;

So when I ran the test program I get "bad checksum..."

vart · 10-31-2006, 12:53 AM

Code:

void *memset( void *dest, int c, size_t count );

It means

Code:

memset(&packet, sizeof(sawhdr), 0);

should be

Code:

memset(&packet,0,sizeof(sawhdr));

Laserve · 10-31-2006, 12:58 AM

Your problem is here:

Code:

memset(&packet, sizeof(sawhdr), 0);

you're setting 0 bytes to sizeof(sawhdr)
change to

Code:

memset(&packet, 0, sizeof(sawhdr));

sagitt13 · 10-31-2006, 01:25 AM

Oh DUH....
I feel stupid now...

It works now! Thanks...haha..

10-30-2006, 11:47 PM	#1
sagitt13 Registered User Join Date: Aug 2004 Posts: 7	weird checksum function Hi, I'm taking a networking class and my current project is to implement a "reliable data transfer protocol". So we're given a checksum function to both create a checksum for our packets and to check if the packets are error free. My problem is everytime I tried to check, I get a result that says packet has errors. Here's the function: Code: uint16_t checksum(uint16_t buf, int nwords) { uint32_t sum; for (sum = 0; nwords > 0; nwords--) sum += buf++; sum = (sum >> 16) + (sum & 0xffff); sum += (sum >> 16); return ~sum; } What I did is: I create a packet (a structure), memset it to 0, add in data/headers, feed the struct into this function and get back a checksum to put in the same packet. I sent the packet to the receiving host. At that end I first save the packet checksum into a temp variable, set the checksum in the packet struct to 0, feed it back in the function and what it returns should be the same as the saved checksum I got from the packet itself. But everytime I do that the results don't match. (btw the checksum field is 16 bits) So roughly: on sender side: Code: Packet p; memset(&p sizeof(Packet), 0); . //set contents; p.checksum = checksum((uint16_t )&p, sizeof(Packet)/2); on receiver side: Code: uint16_t check = p.checksum; p.checksum = 0; if (checksum((uint16_t )&p, sizeof(Packet)/2) != check) //corrupted From what I see, this checksum function adds up 16 bit words of the packet, add the sum to its upper 16 bit value and the lower 16 bit value, then add that to the upper 16 bit again and returns the complement. I'm still confused as to how this will give me a unique checksum. But my priority right now is to get this to work / use it correctly. If anyone can help/comment I'll greatly appreciate it, Thanks!
sagitt13 is offline

10-30-2006, 11:56 PM	#2
Dave_Sinkula Just Lurking Join Date: Oct 2002 Posts: 4,990	Sorry, didn't look to closely. I'd take a closer look if you had a single, simple bit of code that demonstrated the problem. Code: p.checksum = checksum((uint16_t )&p, sizeof(Packet)/2); Why are you doing sizeof(Packet)/2? Many checksums in which the checksum itself is included add to zero. __________________ 7. It is easier to write an incorrect program than understand a correct one. 40. There are two ways to write error-free programs; only the third one works.
Dave_Sinkula is offline

10-31-2006, 12:02 AM	#3
vart CSharpener Join Date: Oct 2006 Posts: 5,242	Have you checked your structure? is it filled exactly as you sent it? __________________ If I have eight hours for cutting wood, I spend six sharpening my axe.
vart is offline

10-31-2006, 12:53 AM	#6
vart CSharpener Join Date: Oct 2006 Posts: 5,242	Code: void memset( void dest, int c, size_t count ); It means Code: memset(&packet, sizeof(sawhdr), 0); should be Code: memset(&packet,0,sizeof(sawhdr)); __________________ If I have eight hours for cutting wood, I spend six sharpening my axe. Last edited by vart; 10-31-2006 at 01:02 AM.
vart is offline

10-31-2006, 12:58 AM	#7
Laserve Registered User Join Date: Jun 2004 Posts: 201	Your problem is here: Code: memset(&packet, sizeof(sawhdr), 0); you're setting 0 bytes to sizeof(sawhdr) change to Code: memset(&packet, 0, sizeof(sawhdr));
Laserve is offline

10-31-2006, 01:25 AM	#8
sagitt13 Registered User Join Date: Aug 2004 Posts: 7	Oh DUH.... I feel stupid now... It works now! Thanks...haha..
sagitt13 is offline

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