Calculating the checksum of a UDP packet

[endi83]

In an ESP32C6 project I am sending UDP packets over ethernet to another device. I am trying to calculate the checksum of the UDP packet I have to send.
To do this I have:

Code: Untitled.c Select all

static void convert_hex_to_uint8(const char *data_hex)

{

    unsigned int temp;

    for (size_t i = 0; i < sizeof(mensajeUDP); i++) {

        sscanf(&data_hex[i*3], "%02x", &temp);

        mensajeUDP[i] = temp;

    }

}



static void enviarConfiguracionABalanza(void)

{

uint8_t src_mac[] = {0x00, 0x08, 0xef, 0x00, 0x01, 0x37};

uint8_t dest_mac[] = {0xff, 0xff, 0xff, 0xff, 0xff, 0xff};

uint32_t src_ip = 0xc0a80a98;

uint32_t dest_ip = 0xc0a80aff;

uint16_t src_port = 6000;

uint16_t dest_port = 6000;

int msg_len;

    

    if (enviarRespuestaUDP == false)

        return;

    else

        enviarRespuestaUDP = false;



    const char *data_hex = "44 50 48 00 04 01 00 ff 00 00 00 00 02 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 01 4d 41 53 00 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 2b 2b 2b 2b 2b 2b 2b 2b 2b 00 2a 2a 2a 2a 2a 2a 2a 2a 2a 2a 98 0a a8 c0 0e 00 00 00 31 31 37 00 20 20 45 00 00 00 00 00 01 00 00 00 46 00 00 00 30 2e 31 37 00 00 00 00 00 00 00 05 30 00 00 03 00 00 00 00 00 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ff ff ff 00 00 01 00 00 00 00 00";

    msg_len = strlen(data_hex);

    printf ("%d",msg_len);

    convert_hex_to_uint8(data_hex);



    ethernet_header_t *eth_header = (ethernet_header_t *)bufferUDP;

    ip_header_t *ip_header = (ip_header_t *)(bufferUDP + sizeof(ethernet_header_t));

    udp_header_t *udp_header = (udp_header_t *)(bufferUDP + sizeof(ethernet_header_t) + sizeof(ip_header_t));



    memcpy(eth_header->src_mac, src_mac, 6);

    memcpy(eth_header->dest_mac, dest_mac, 6);

    eth_header->ethertype = htons(0x0800); // IP



    ip_header->version_ihl = 0x45; 

    ip_header->type_of_service = 0;

    ip_header->total_length = 0x2001;

    ip_header->identification = 0x2900;

    ip_header->flags_fragment_offset = 0;

    ip_header->ttl = 128;

    ip_header->header_checksum = 0;

    ip_header->protocol = 17; // UDP

    ip_header->src_ip = htonl(src_ip);

    ip_header->dest_ip = htonl(dest_ip);



    ip_header->header_checksum = calculate_ip_checksum(ip_header);



    udp_header->src_port = htons(src_port);

    udp_header->dest_port = htons(dest_port);

    udp_header->checksum = 0;

    udp_header->length = 0x0c01;



    udp_header->checksum = calculate_udp_checksum(ip_header, udp_header, mensajeUDP, 260);



    memcpy(bufferUDP + sizeof(ethernet_header_t) + sizeof(ip_header_t) + sizeof(udp_header_t), mensajeUDP, 260);



    if (esp_eth_transmit(s_eth_handle[0], bufferUDP, 302) != ESP_OK)

        ESP_LOGE(TAG, "Error");

}



static uint16_t calculate_checksum(uint16_t *buffer, int length)

{

    uint32_t sum = 0;



    while (length > 1) {

        sum += *buffer++;

        length -= 2;

    }



    if (length > 0) {

        sum += *(uint8_t *)buffer;

    }



    while (sum >> 16) {

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

    }



    return (uint16_t)~sum;

}



static uint16_t calculate_ip_checksum(ip_header_t *ip_header)

{

    ip_header->header_checksum = 0;

    return (calculate_checksum((uint16_t *)ip_header, sizeof(ip_header_t)));

}



static uint16_t calculate_udp_checksum(ip_header_t *ip_header, udp_header_t *udp_header, uint8_t *data, uint16_t data_len)

{

    uint16_t total_length = sizeof(udp_header_t) + data_len;



    uint8_t pseudo_header_buffer[sizeof(pseudo_header_t) + sizeof(udp_header_t)];

    pseudo_header_t *pseudo_header = (pseudo_header_t *)pseudo_header_buffer;



    pseudo_header->src_ip = ip_header->src_ip;

    pseudo_header->dest_ip = ip_header->dest_ip;

    pseudo_header->zero = 0;

    pseudo_header->protocol = ip_header->protocol;

    pseudo_header->udp_length = htons(total_length);



    uint32_t sum = 0;



    sum += calculate_checksum((uint16_t *)pseudo_header, sizeof(pseudo_header_t));

    sum += calculate_checksum((uint16_t *)udp_header, sizeof(udp_header_t));

    sum += calculate_checksum((uint16_t *)data, data_len);



    while (sum >> 16) {

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

    }

    return (uint16_t)~sum;

}

I know that the result of the checksum of the UDP header is "ec 16" but the result I get is "13 e9".

Can someone tell me what I have wrong?

[MicroController]

I know that the result of the checksum of the UDP header is "ec 16" but the result I get is "13 e9".

Can someone tell me what I have wrong?

0xec16 happens to be (0x13e9 ^ 0xffff), which hints at one inversion too much.
See e.g. https://gist.github.com/fxlv/81209bbd15 ... 5ff076c9f3
You'd need to sum up all values first, then invert only once at the end. So you may want to change "return (uint16_t)~sum;" in calculate_checksum(...) to "return (uint16_t)sum;" but keep the single inversion at the end of calculate_udp_checksum(...).

[endi83]

Thanks to your comment I managed to solve the problem!

[chegewara]

Maybe late and stupid question, but since when you are passing uint16_t* to calculate CRC?

Code: Select all

uint16_t calculate_checksum(uint16_t *buffer, int length)