ip4sum 0.1.0

Highly optimized IPv4 checksum calculation, no-std compatible
Documentation 
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/* SPDX-License-Identifier: MIT */
/* Copyright (c) 2026 Khashayar Fereidani */

#include <stdio.h>
#include <string.h>
#include <assert.h>
#include "checksum.h"

/* -- Reference checksum computed byte-by-byte in network order -- */

static uint16_t reference_checksum(const unsigned char *data, size_t len) {
    uint32_t sum = 0;
    size_t i = 0;
    while (i + 1 < len) {
        sum += ((uint32_t)data[i] << 8) | (uint32_t)data[i + 1];
        i += 2;
    }
    if (i < len) {
        sum += (uint32_t)data[i] << 8;
    }
    while (sum >> 16) {
        sum = (sum >> 16) + (sum & 0xffffu);
    }
    return ~(uint16_t)sum;
}

/* -- Helpers -- */

static void fill_deterministic(unsigned char *buf, size_t len,
                               unsigned char seed) {
    for (size_t i = 0; i < len; i++) {
        buf[i] = (unsigned char)((i * 31u + seed) & 0xFFu);
    }
}

/* ========================================================================= */
/* Basic correctness                                                         */
/* ========================================================================= */

static void test_empty(void) {
    assert(ip4sum_checksum_oneshot((const unsigned char *)"", 0)
           == reference_checksum((const unsigned char *)"", 0));
}

static void test_single_byte(void) {
    const unsigned char data[] = {0x45};
    assert(ip4sum_checksum_oneshot(data, sizeof(data))
           == reference_checksum(data, sizeof(data)));
}

static void test_two_bytes(void) {
    const unsigned char data[] = {0x45, 0x00};
    assert(ip4sum_checksum_oneshot(data, sizeof(data))
           == reference_checksum(data, sizeof(data)));
}

static void test_three_bytes(void) {
    const unsigned char data[] = {0x45, 0x00, 0xAB};
    assert(ip4sum_checksum_oneshot(data, sizeof(data))
           == reference_checksum(data, sizeof(data)));
}

static void test_four_bytes(void) {
    const unsigned char data[] = {0x45, 0x00, 0x00, 0x30};
    assert(ip4sum_checksum_oneshot(data, sizeof(data))
           == reference_checksum(data, sizeof(data)));
}

static void test_five_bytes(void) {
    const unsigned char data[] = {0x45, 0x00, 0x00, 0x30, 0xFF};
    assert(ip4sum_checksum_oneshot(data, sizeof(data))
           == reference_checksum(data, sizeof(data)));
}

/* ========================================================================= */
/* Known-good IPv4 headers                                                   */
/* ========================================================================= */

static void test_ipv4_header_1(void) {
    const unsigned char data[20] = {
        0x45, 0x00, 0x00, 0x30, 0x00, 0x00, 0x40, 0x00,
        0x40, 0x01, 0x00, 0x00, 0x0A, 0x00, 0x00, 0x01,
        0x0A, 0x00, 0x00, 0x02
    };
    unsigned char verified[20];
    memcpy(verified, data, 20);
    uint16_t csum = ip4sum_checksum_oneshot(data, 20);
    verified[10] = (unsigned char)(csum >> 8);
    verified[11] = (unsigned char)(csum & 0xFF);
    assert(ip4sum_checksum_oneshot(verified, 20) == 0);
}

static void test_ipv4_header_2(void) {
    const unsigned char data[20] = {
        0x45, 0x00, 0x00, 0x3C, 0x1C, 0x46, 0x40, 0x00,
        0x40, 0x06, 0x00, 0x00, 0xAC, 0x10, 0x0A, 0x63,
        0xAC, 0x10, 0x0A, 0x0C
    };
    unsigned char verified[20];
    memcpy(verified, data, 20);
    uint16_t csum = ip4sum_checksum_oneshot(data, 20);
    verified[10] = (unsigned char)(csum >> 8);
    verified[11] = (unsigned char)(csum & 0xFF);
    assert(ip4sum_checksum_oneshot(verified, 20) == 0);
}

/* ========================================================================= */
/* Incremental matches one-shot                                              */
/* ========================================================================= */

static void test_incremental_two_parts(void) {
    const unsigned char data[20] = {
        0x45, 0x00, 0x00, 0x30, 0x00, 0x00, 0x40, 0x00,
        0x40, 0x01, 0x00, 0x00, 0x0A, 0x00, 0x00, 0x01,
        0x0A, 0x00, 0x00, 0x02
    };
    uint16_t csum_oneshot = ip4sum_checksum_oneshot(data, 20);

    ip4sum_checksum c = ip4sum_checksum_new();
    ip4sum_checksum_update(&c, data, 10);
    ip4sum_checksum_update(&c, data + 10, 10);
    uint16_t csum_inc = ip4sum_checksum_finalize(c);

    assert(csum_oneshot == csum_inc);
}

static void test_incremental_three_parts(void) {
    const unsigned char data[20] = {
        0x45, 0x00, 0x00, 0x3C, 0x1C, 0x46, 0x40, 0x00,
        0x40, 0x06, 0x00, 0x00, 0xAC, 0x10, 0x0A, 0x63,
        0xAC, 0x10, 0x0A, 0x0C
    };
    uint16_t csum_oneshot = ip4sum_checksum_oneshot(data, 20);

    ip4sum_checksum c = ip4sum_checksum_new();
    ip4sum_checksum_update(&c, data, 4);
    ip4sum_checksum_update(&c, data + 4, 12);
    ip4sum_checksum_update(&c, data + 16, 4);
    uint16_t csum_inc = ip4sum_checksum_finalize(c);

    assert(csum_oneshot == csum_inc);
}

static void test_incremental_with_zeroed_checksum(void) {
    const unsigned char data[20] = {
        0x45, 0x00, 0x00, 0x30, 0x00, 0x00, 0x40, 0x00,
        0x40, 0x01, 0x00, 0x00, 0x0A, 0x00, 0x00, 0x01,
        0x0A, 0x00, 0x00, 0x02
    };
    uint16_t csum_oneshot = ip4sum_checksum_oneshot(data, 20);

    const unsigned char zero[2] = {0, 0};
    ip4sum_checksum c = ip4sum_checksum_new();
    ip4sum_checksum_update(&c, data, 10);
    ip4sum_checksum_update(&c, zero, 2);
    ip4sum_checksum_update(&c, data + 12, 8);
    uint16_t csum_inc = ip4sum_checksum_finalize(c);

    assert(csum_oneshot == csum_inc);
}

/* ========================================================================= */
/* Reset                                                                     */
/* ========================================================================= */

static void test_reset(void) {
    const unsigned char data[20] = {
        0x45, 0x00, 0x00, 0x30, 0x00, 0x00, 0x40, 0x00,
        0x40, 0x01, 0x00, 0x00, 0x0A, 0x00, 0x00, 0x01,
        0x0A, 0x00, 0x00, 0x02
    };
    uint16_t expected = ip4sum_checksum_oneshot(data, 20);

    ip4sum_checksum c = ip4sum_checksum_new();
    const unsigned char junk[] = {0xFF, 0xFF, 0xFF, 0xFF};
    ip4sum_checksum_update(&c, junk, sizeof(junk));
    ip4sum_checksum_reset(&c);
    ip4sum_checksum_update(&c, data, 20);
    assert(ip4sum_checksum_finalize(c) == expected);
}

/* ========================================================================= */
/* Boundary sizes                                                            */
/* ========================================================================= */

static void test_sizes_0_to_256(void) {
    unsigned char buf[256];
    fill_deterministic(buf, sizeof(buf), 0);
    for (size_t len = 0; len <= 256; len++) {
        uint16_t got = ip4sum_checksum_oneshot(buf, len);
        uint16_t want = reference_checksum(buf, len);
        assert(got == want);
    }
}

static void test_size_63_64_65(void) {
    unsigned char buf[256];
    memset(buf, 0xAB, sizeof(buf));
    const size_t lens[] = {63, 64, 65};
    for (size_t i = 0; i < 3; i++) {
        size_t len = lens[i];
        assert(ip4sum_checksum_oneshot(buf, len)
               == reference_checksum(buf, len));
    }
}

static void test_size_127_128_129(void) {
    unsigned char buf[256];
    memset(buf, 0xCD, sizeof(buf));
    const size_t lens[] = {127, 128, 129};
    for (size_t i = 0; i < 3; i++) {
        size_t len = lens[i];
        assert(ip4sum_checksum_oneshot(buf, len)
               == reference_checksum(buf, len));
    }
}

/* ========================================================================= */
/* Fold unit tests                                                           */
/* ========================================================================= */

static void test_fold_zero(void) {
    ip4sum_checksum c = ip4sum_checksum_new();
    assert(ip4sum_checksum_finalize(c) == 0xFFFF);
}

static void test_fold_known_value(void) {
    const unsigned char data[] = {0xFF, 0xFF};
    assert(ip4sum_checksum_oneshot(data, sizeof(data)) == 0);
}

/* ========================================================================= */
/* Verification property: inserting checksum yields zero                     */
/* ========================================================================= */

static void test_verification_property(void) {
    unsigned char buf[60];
    for (size_t i = 0; i < sizeof(buf); i++) {
        buf[i] = (unsigned char)((i * 7u + 0x5Au) & 0xFFu);
    }
    buf[10] = 0;
    buf[11] = 0;
    uint16_t csum = ip4sum_checksum_oneshot(buf, 60);
    buf[10] = (unsigned char)(csum >> 8);
    buf[11] = (unsigned char)(csum & 0xFF);
    assert(ip4sum_checksum_oneshot(buf, 60) == 0);
}

/* ========================================================================= */
/* Larger buffers                                                            */
/* ========================================================================= */

static void test_1500_bytes(void) {
    unsigned char buf[1500];
    for (size_t i = 0; i < sizeof(buf); i++) {
        buf[i] = (unsigned char)((i + 0x37u) & 0xFFu);
    }
    assert(ip4sum_checksum_oneshot(buf, 1500)
           == reference_checksum(buf, 1500));
}

static void test_odd_length_large(void) {
    unsigned char buf[1501];
    for (size_t i = 0; i < sizeof(buf); i++) {
        buf[i] = (unsigned char)((i * 13u) & 0xFFu);
    }
    assert(ip4sum_checksum_oneshot(buf, 1501)
           == reference_checksum(buf, 1501));
}

/* ========================================================================= */
/* TCP-style pseudo-header + header                                          */
/* ========================================================================= */

static void test_tcp_checksum_incremental(void) {
    const unsigned char pseudo[12] = {
        192, 168, 1, 1, 192, 168, 1, 2, 0, 0, 0, 6
    };
    const unsigned char tcp_length[2] = {0x00, 0x14};
    const unsigned char tcp_hdr[20] = {
        0x00, 0x50, 0x00, 0x50, 0x00, 0x00, 0x00, 0x01,
        0x00, 0x00, 0x00, 0x00, 0x50, 0x02, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00
    };

    unsigned char combined[34];
    memcpy(combined, pseudo, 12);
    memcpy(combined + 12, tcp_length, 2);
    memcpy(combined + 14, tcp_hdr, 20);
    uint16_t csum_oneshot = ip4sum_checksum_oneshot(combined, 34);

    ip4sum_checksum c = ip4sum_checksum_new();
    ip4sum_checksum_update(&c, pseudo, 12);
    ip4sum_checksum_update(&c, tcp_length, 2);
    ip4sum_checksum_update(&c, tcp_hdr, 20);
    uint16_t csum_inc = ip4sum_checksum_finalize(c);

    assert(csum_oneshot == csum_inc);
}

/* ========================================================================= */
/* main                                                                      */
/* ========================================================================= */

int main(void) {
    int pass = 0;

    /* basic correctness */
    test_empty();                          pass++;
    test_single_byte();                    pass++;
    test_two_bytes();                      pass++;
    test_three_bytes();                    pass++;
    test_four_bytes();                     pass++;
    test_five_bytes();                     pass++;

    /* known-good ipv4 headers */
    test_ipv4_header_1();                  pass++;
    test_ipv4_header_2();                  pass++;

    /* incremental */
    test_incremental_two_parts();          pass++;
    test_incremental_three_parts();        pass++;
    test_incremental_with_zeroed_checksum(); pass++;

    /* reset */
    test_reset();                          pass++;

    /* boundary sizes */
    test_sizes_0_to_256();                 pass++;
    test_size_63_64_65();                  pass++;
    test_size_127_128_129();               pass++;

    /* fold */
    test_fold_zero();                      pass++;
    test_fold_known_value();               pass++;

    /* verification property */
    test_verification_property();          pass++;

    /* larger buffers */
    test_1500_bytes();                     pass++;
    test_odd_length_large();               pass++;

    /* tcp pseudo-header */
    test_tcp_checksum_incremental();       pass++;

    printf("All %d tests passed.\n", pass);
    return 0;
}