#40 the ip check sum field compute method have a bug

[Anonymous]

1. in IPPacket:

method protected int onesCompSum(byte[] bytes, int
start, int len)

The method mising the one-compilent on the checksum

sum = ~sum & 0xffff ;

currenlty returning the wrong check sum.

c code from stevens:

unsigned short ip_cksum(unsigned short *addr,int len)
{
int nleft = len;
int sum = 0;
unsigned short *w = addr;
unsigned short answer = 0;

while (nleft>1)
{
sum+=*w++;
nleft-=2;
}
if (nleft == 1 )
{
*(unsigned char *) (&answer) = *(unsigned
char *) w;
sum += answer;
}

sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
answer = ~sum;

return(answer);
}

2.the checksum can be any 16 bit number and not just
0xffff as in method

/**
* Check if the IP packet is valid, checksum-wise.
*/
public boolean isValidChecksum()

[Nobody/Anonymous]

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my email ronybary@yahoo.com

[EJP]

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The problem is twofold.

1. The ipChecksum() method only computes the ones-complement sum, whereas the method in Stevens computes the ones-complement of the ones-complement sum.

2. The result in either case is 16 bits with no sign extension. The Stevens result should be compared to 0; the ipChecksum() result should be complemented and compared to zero, but in fact it is compared to -1 in 32 bits, i.e. 0xffffffff, which can never succeed, as the upper 16 bits can never be set.

NB the Stevens method has a bug too! The final odd byte if any should be left-shifted 8 before addition.

[EJP]

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This has been addressed in the forthcoming release 2.0.