Created
July 30, 2020 11:33
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Ipv4 header checksum calculation
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| /** | |
| * Compute Internet Checksum for "count" bytes | |
| * beginning at location "addr". | |
| * @param addr start of the ip header | |
| * @param count ip header size | |
| * @return Taking the ones' complement (flipping every bit) yields 0000, | |
| * which indicates that no error is detected. | |
| */ | |
| uint16_t checksum(void *addr, int count) { | |
| uint32_t sum = 0; | |
| auto *ptr_16 = static_cast<uint16_t *>(addr); | |
| for (; count > 1; count -= 2) { | |
| // This is the inner loop | |
| sum += *ptr_16++; | |
| } | |
| // Add left-over byte, if any | |
| if (count > 0) | |
| sum += *static_cast<uint8_t *>(addr); | |
| // Fold 32-bit sum to 16 bits | |
| while (sum >> 16u) | |
| sum = (sum & 0xffffu) + (sum >> 16u); | |
| return ~sum; | |
| } |
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