Checksum of TCP over IPv6

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Hello Guys,
I am trying to find out checksum of TCP over IPV6. Please see the below cod

What I have tried:

char src_addr[] = {
  0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0xa1, 0xe4, 0x22, 0x2c, 0x0c, 0x9b, 0x57, 0x22
};

char dest_addr[] = {
  0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x18, 0xd7, 0xc9, 0x57, 0x52, 0xd7, 0x0f, 0xcd
};

_inline unsigned short TOWORD(unsigned int a)
{
    return (unsigned short)( ((a>>8)&0x00FFL) + ((a<<8)&0xFF00L) );
}


int _tmain(int argc, _TCHAR* argv[])
{
	unsigned int sum=0;

	for(int i = 0; i < 8; i++)
	{
		sum += *((unsigned short*)(src_addr+i));
	}

	for(int i = 0; i < 8; i++)
	{
		sum += *(((unsigned short*)dest_addr+i));
	}
	
	unsigned short type = 0x06; // TCP protocol
	sum += TOWORD(type);

	unsigned int tcplen = 0x1c; // 28 bytes length

	sum += ((tcplen <<8) & 0xFFFF) + (tcplen >> 8);

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

	sum = (~sum & 0xFFFF);
	unsigned short sum1 =  sum;

	printf("Checksum = %x\n", sum1);
	return 0;
}

gives output of 3a96.

However WireShak shows checksum of 0x3c3b.

Can you please help?

Posted 6-Feb-17 3:49am

vikramlinux

Updated 24-Jan-18 8:07am

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3 solutions

Solution 3

Tried below code as per above suggestion.

char src_addr[] = {
  0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0xa1, 0xe4, 0x22, 0x2c, 0x0c, 0x9b, 0x57, 0x22
};

char dest_addr[] = {
  0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x18, 0xd7, 0xc9, 0x57, 0x52, 0xd7, 0x0f, 0xcd
};


_inline unsigned short TOWORD(unsigned int a)
{
    return (unsigned short)( ((a>>8)&0x00FFL) + ((a<<8)&0xFF00L) );
}


int _tmain(int argc, _TCHAR* argv[])
{
	unsigned int sum=0;

	for(int i = 0; i < 16; i += 2)
	{
		sum += src_addr[i] + (src_addr[i+1] << 8U);
	}

	for(int i = 0; i < 16; i += 2)
	{
		sum += dest_addr[i] + (dest_addr[i+1] << 8U);
	}

	
	unsigned short type = 0x06; // TCP protocol
	sum += TOWORD(type);

	unsigned int tcplen = 0x1c; // 28 bytes length
	sum += ((tcplen <<8) & 0xFFFF) + (tcplen >> 8);

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

	sum = (~sum & 0xFFFF);
	unsigned short sum1 =  sum;

	printf("Checksum = %x\n", sum1);
	return 0;
}

results into checksum of 3e9c

Posted 6-Feb-17 4:21am

vikramlinux

Solution 4

The TCP header + data was missing. Thanks Jochen.

Posted 10-Feb-17 3:47am

vikramlinux

Comments

Member 13641570 24-Jan-18 14:08pm

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Jochen Arndt · Accepted Answer · 2017-02-06T04:05:00

This is obviously wrong and a fine example why casting can be unsafe:

for(int i = 0; i < 8; i++)
{
    sum += *((unsigned short*)(src_addr+i));
}

You are casting a char pointer to unsigned short*. While this might be used, you forgot at least that you have to increment the pointer by two then.

I don't know about the byte order in this case, so a possible solution might be one of these:

for(int i = 0; i < 16; i += 2)
{
    sum += src_addr[i] + (src_addr[i+1] << 8U);
    // Or this (EDIT: I guess this is it)
    sum += (src_addr[i] << 8U) + src_addr[i+1];
}

[EDIT]
There may be also other errors and/or a wrong implementation. The IPv6 TCP checksum is generated on the TCP header, the pseudo header, and the data.
Your code calculates the checksum for the pseudo header. I don't know which checksum is reported by WireShark but would expect that it is the complete one.
[/EDIT]

Checksum of TCP over IPv6

3 solutions

Solution 1

Solution 3

Solution 4

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