TCP校验和

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unsigned short checksum(unsigned short* buffer, int size)
{
	unsigned int sum = 0;
	for (sum = 0; size > 0;)
	{
		sum += *buffer++;
		size -= 2;
	}

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

	return (unsigned short)(~sum);
}

unsigned short TCPChecksum(IPV4_HDR* ipHeader, TCPHeader* tcpHeader, void* pData, int dataSize)
{
	PSD_HEADER psdHeader = { 0 };
	psdHeader.saddr = *(unsigned long*)ipHeader->ip_srcaddr;
	psdHeader.daddr = *(unsigned long*)ipHeader->ip_destaddr;
	psdHeader.mbz = 0;
	psdHeader.ptcl = ipHeader->ip_protocol;
	psdHeader.tcpl = htons(sizeof(TCPHeader) + dataSize);

	int bufSize = sizeof(PSD_HEADER) + sizeof(TCPHeader) + dataSize;
	if (0 != (bufSize % 2))
		bufSize += 1;

	char* pHeader = new char[bufSize];
	memset(pHeader, 0, bufSize);

	int offset = 0;
	memcpy(pHeader + offset, &psdHeader, sizeof(PSD_HEADER));
	offset += sizeof(PSD_HEADER);

	memcpy(pHeader + offset, tcpHeader, sizeof(TCPHeader));
	offset += sizeof(TCPHeader);

	if ((NULL != pData) && (dataSize > 0))
		memcpy(pHeader + offset, pData, dataSize);

	((TCPHeader*)(pHeader + sizeof(PSD_HEADER)))->th_sum = 0;
	((TCPHeader*)(pHeader + sizeof(PSD_HEADER)))->th_sum = checksum((unsigned short*)pHeader, bufSize);
	tcpHeader->th_sum = ((TCPHeader*)(pHeader + sizeof(PSD_HEADER)))->th_sum;

	delete[] pHeader;
	pHeader = NULL;

	return tcpHeader->th_sum;
}