net_checksum.c

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/*
 * Copyright (c) 2009 Lukas Mejdrech
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <sys/types.h>

#include <net_checksum.h>

#define CRC_DIVIDER_BE  0x04c11db7

#define CRC_DIVIDER_LE  0xedb88320

uint16_t compact_checksum(uint32_t sum)
{
        /* Shorten to the 16 bits */
        while (sum >> 16)
                sum = (sum & 0xffff) + (sum >> 16);

        return (uint16_t) sum;
}

uint32_t compute_checksum(uint32_t seed, uint8_t *data, size_t length)
{
        size_t index;

        /* Sum all the 16 bit fields */
        for (index = 0; index + 1 < length; index += 2)
                seed += (data[index] << 8) + data[index + 1];

        /* Last odd byte with zero padding */
        if (index + 1 == length)
                seed += data[index] << 8;

        return seed;
}

uint32_t compute_crc32_be(uint32_t seed, uint8_t * data, size_t length)
{
        size_t index;

        /* Process full bytes */
        while (length >= 8) {
                /* Add the data */
                seed ^= (*data) << 24;
                
                /* For each added bit */
                for (index = 0; index < 8; ++index) {
                        /* If the first bit is set */
                        if (seed & 0x80000000) {
                                /* Shift and divide the checksum */
                                seed = (seed << 1) ^ ((uint32_t) CRC_DIVIDER_BE);
                        } else {
                                /* Shift otherwise */
                                seed <<= 1;
                        }
                }
                
                /* Move to the next byte */
                ++data;
                length -= 8;
        }

        /* Process the odd bits */
        if (length > 0) {
                /* Add the data with zero padding */
                seed ^= ((*data) & (0xff << (8 - length))) << 24;
                
                /* For each added bit */
                for (index = 0; index < length; ++index) {
                        /* If the first bit is set */
                        if (seed & 0x80000000) {
                                /* Shift and divide the checksum */
                                seed = (seed << 1) ^ ((uint32_t) CRC_DIVIDER_BE);
                        } else {
                                /* Shift otherwise */
                                seed <<= 1;
                        }
                }
        }

        return seed;
}

uint32_t compute_crc32_le(uint32_t seed, uint8_t * data, size_t length)
{
        size_t index;

        /* Process full bytes */
        while (length >= 8) {
                /* Add the data */
                seed ^= (*data);
                
                /* For each added bit */
                for (index = 0; index < 8; ++index) {
                        /* If the last bit is set */
                        if (seed & 1) {
                                /* Shift and divide the checksum */
                                seed = (seed >> 1) ^ ((uint32_t) CRC_DIVIDER_LE);
                        } else {
                                /* Shift otherwise */
                                seed >>= 1;
                        }
                }
                
                /* Move to the next byte */
                ++data;
                length -= 8;
        }

        /* Process the odd bits */
        if (length > 0) {
                /* Add the data with zero padding */
                seed ^= (*data) >> (8 - length);
                
                for (index = 0; index < length; ++index) {
                        /* If the last bit is set */
                        if (seed & 1) {
                                /* Shift and divide the checksum */
                                seed = (seed >> 1) ^ ((uint32_t) CRC_DIVIDER_LE);
                        } else {
                                /* Shift otherwise */
                                seed >>= 1;
                        }
                }
        }

        return seed;
}

uint16_t flip_checksum(uint16_t checksum)
{
        /* Flip, zero is returned as 0xFFFF (not flipped) */
        checksum = ~checksum;
        return checksum ? checksum : IP_CHECKSUM_ZERO;
}

uint16_t ip_checksum(uint8_t *data, size_t length)
{
        /* Compute, compact and flip the data checksum */
        return flip_checksum(compact_checksum(compute_checksum(0, data,
            length)));
}

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