Shulou(Shulou.com)06/01 Report--

Calculation of ipv4 checksum

Principle:

Calculation method 1: remove the two bits of the checksum and add the other bits: 45+00+00+3c+55+81+00+00+40+01+ac+1c

0fend0dfolk acidity 1cparts 0frank0e =

Calculation method 2:

The checksum (checksum) algorithm is simply a 16-bit cumulative inverse operation:

The calculation function is as follows:

When we calculate, it is the host byte order, and when the calculation result is encapsulated into the IP packet, it is the network byte order. Pay attention to the difference between the two. When we read from the IP package, we need to convert it to the host byte order, and when we save it into the IP packet, we need to convert it to the network byte order.

UINT32 Checksum (UINT32 cksum, VOID*pBuffer, UINT32 size)

{

INT8 num = 0

UINT8 * p = (UINT8 *) pBuffer

If ((NULL = = pBuffer) | | (0 = size))

{

Return cksum

}

While (size > 1)

{

Cksum + = ((UINT16) p [num] 0)

/ / if the length is odd

{

Cksum + = (UINT16) p [num] > 16)

{

Cksum = (cksum & 0xFFFF) + (cksum > > 16)

/ / add the high 16 bits to the low 16 bits after accumulation, and repeat the process until the high 16 bits are all 0

}

Return cksum

}

Note: the type of UINT32 cksum, which is 4 bytes here, prevents data overflow during accumulation (for example, memory overflow occurs when 0xFF is accumulated)

The detailed calculation process and principle are as follows

1: calculation of ip headers:

Accumulate the header data directly (excluding the original checksum value):

1. Ipv4 header

IpHeadLen = (pIpHeader- > ver_ihl & 0x0F) chksum = 0

Because the original checksum value is not included, set the value of checksum to 0 before each calculation, and then calculate

Sum = Checksum (0, (VOID *) pIpHeader, ipHeadLen)

The accumulation of the entire ip header

PIpHeader- > chksum = HTONS ((UINT16) (~ sum))

The result is the inverse of the calculated value, (don't forget to convert it to network byte order)

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