jajabin

func chksum(data []byte, srcip, dstip []byte) uint16 {
	data[16],data[17] = 0,0 //zero on original check sum
	psheader := []byte{
		srcip[0], srcip[1], srcip[2], srcip[3],
		dstip[0], dstip[1], dstip[2], dstip[3],
		0,
		6,                  //  TCP
		0, byte(len(data)), //  length (16 bits)
	}

	csum := make([]byte, 0, len(psheader)+len(data))
	csum = append(csum, psheader...)
	csum = append(csum, data...)


	lenSumThis := len(csum)
	var word uint16
	var sum uint32
	for i := 0; i+1 < lenSumThis; i += 2 {
		word = uint16(csum[i])<<8 | uint16(csum[i+1])
		sum += uint32(word)
	}
	if lenSumThis%2 != 0 {

		sum += uint32(csum[len(csum)-1])
	}

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

	return uint16(^sum)
}