![]() ![]() This is a simple to compute checksum and experimental evidence For purposes ofĬomputing the checksum, the value of the checksum field is zero. The checksum algorithm is: The checksum field is the 16 bit one's complement of the one'sĬomplement sum of all 16 bit words in the header. (e.g., time to live), this is recomputed and verified at each point Internet Header Format Header Checksum: 16 bitsĪ checksum on the header only. If the header checksum fails, the internetÄatagram is discarded at once by the entity which detects the error.Ä£.1. In processing internet datagram has been transmitted correctly. Operation The Header Checksum provides a verification that the information used For further reading, refer to RFC 791 INTERNET PROTOCOL:Ä¡.4. To be clear, the IP Header Checksum helps protect against errors in the IP header only, and not with errors in the payload. Memcpy((char*)(pData + 40), &un16TempChecksum, sizeof(uint16_t)) UnChecksum += *reinterpret_cast(pD16)If (len) // if total lenght the data is odd PD16 = reinterpret_cast(pData + CommonHelper::DATAGRAME_DATA_OFFSET)Size_t len = unDataLen - CommonHelper::DATAGRAME_DATA_OFFSET // 42 is the Ethernet header PD16 = reinterpret_cast(pData + 36) PD16 = reinterpret_cast(pData + 34) PD16 = reinterpret_cast(pData + 38)handle data lenght, from IP layer to udp data layer PD8 = reinterpret_cast(pData + 23) PD16 = reinterpret_cast(pData + 30)UnChecksum += _byteswap_ushort(*pD16++) // since wireshark file is big-endian, but c++ in PC is little-endian PD16 = reinterpret_cast(pData + 26)Here is what I do for the UDP checksum: void ReCalculateCheckSum_UDP_Pkt(u_char* pData, unsigned int unDataLen) Memcpy((char*)(pData + 24), &un16TempChecksum, sizeof(uint16_t)) Un16TempChecksum = _byteswap_ushort(un16TempChecksum) Uint16_t un16TempChecksum = static_cast(~unChecksum)UnChecksum = (unChecksum & 0xffff) + (unChecksum > 16) PD16 = reinterpret_cast(pData + CommonHelper::IP_HEADER_OFFSET)Here is what I do for IP Header checksum: void ReCalculateCheckSum_IPHeader(u_char* pData) I have tried to search on the web but what I found is the check sum calculation for the whole UDP packet which seems not the same checksum that I am asking for on the above. I have a udp packet captured through multicast channel and I found that there is a Header checksum under Internet Protocol Version 4. ![]()
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