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: [Select all] [Expand/Collapse]

1. static void convert_hex_to_uint8(const char *data_hex)
2. {
3.     unsigned int temp;
4.     for (size_t i = 0; i < sizeof(mensajeUDP); i++) {
5.         sscanf(&data_hex[i*3], "%02x", &temp);
6.         mensajeUDP[i] = temp;
7.     }
8. }
9.  
10. static void enviarConfiguracionABalanza(void)
11. {
12. uint8_t src_mac[] = {0x00, 0x08, 0xef, 0x00, 0x01, 0x37};
13. uint8_t dest_mac[] = {0xff, 0xff, 0xff, 0xff, 0xff, 0xff};
14. uint32_t src_ip = 0xc0a80a98;
15. uint32_t dest_ip = 0xc0a80aff;
16. uint16_t src_port = 6000;
17. uint16_t dest_port = 6000;
18. int msg_len;
19.    
20.     if (enviarRespuestaUDP == false)
21.         return;
22.     else
23.         enviarRespuestaUDP = false;
24.  
25.     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";
26.     msg_len = strlen(data_hex);
27.     printf ("%d",msg_len);
28.     convert_hex_to_uint8(data_hex);
29.  
30.     ethernet_header_t *eth_header = (ethernet_header_t *)bufferUDP;
31.     ip_header_t *ip_header = (ip_header_t *)(bufferUDP + sizeof(ethernet_header_t));
32.     udp_header_t *udp_header = (udp_header_t *)(bufferUDP + sizeof(ethernet_header_t) + sizeof(ip_header_t));
33.  
34.     memcpy(eth_header->src_mac, src_mac, 6);
35.     memcpy(eth_header->dest_mac, dest_mac, 6);
36.     eth_header->ethertype = htons(0x0800); // IP
37.  
38.     ip_header->version_ihl = 0x45;
39.     ip_header->type_of_service = 0;
40.     ip_header->total_length = 0x2001;
41.     ip_header->identification = 0x2900;
42.     ip_header->flags_fragment_offset = 0;
43.     ip_header->ttl = 128;
44.     ip_header->header_checksum = 0;
45.     ip_header->protocol = 17; // UDP
46.     ip_header->src_ip = htonl(src_ip);
47.     ip_header->dest_ip = htonl(dest_ip);
48.  
49.     ip_header->header_checksum = calculate_ip_checksum(ip_header);
50.  
51.     udp_header->src_port = htons(src_port);
52.     udp_header->dest_port = htons(dest_port);
53.     udp_header->checksum = 0;
54.     udp_header->length = 0x0c01;
55.  
56.     udp_header->checksum = calculate_udp_checksum(ip_header, udp_header, mensajeUDP, 260);
57.  
58.     memcpy(bufferUDP + sizeof(ethernet_header_t) + sizeof(ip_header_t) + sizeof(udp_header_t), mensajeUDP, 260);
59.  
60.     if (esp_eth_transmit(s_eth_handle[0], bufferUDP, 302) != ESP_OK)
61.         ESP_LOGE(TAG, "Error");
62. }
63.  
64. static uint16_t calculate_checksum(uint16_t *buffer, int length)
65. {
66.     uint32_t sum = 0;
67.  
68.     while (length > 1) {
69.         sum += *buffer++;
70.         length -= 2;
71.     }
72.  
73.     if (length > 0) {
74.         sum += *(uint8_t *)buffer;
75.     }
76.  
77.     while (sum >> 16) {
78.         sum = (sum & 0xffff) + (sum >> 16);
79.     }
80.  
81.     return (uint16_t)~sum;
82. }
83.  
84. static uint16_t calculate_ip_checksum(ip_header_t *ip_header)
85. {
86.     ip_header->header_checksum = 0;
87.     return (calculate_checksum((uint16_t *)ip_header, sizeof(ip_header_t)));
88. }
89.  
90. static uint16_t calculate_udp_checksum(ip_header_t *ip_header, udp_header_t *udp_header, uint8_t *data, uint16_t data_len)
91. {
92.     uint16_t total_length = sizeof(udp_header_t) + data_len;
93.  
94.     uint8_t pseudo_header_buffer[sizeof(pseudo_header_t) + sizeof(udp_header_t)];
95.     pseudo_header_t *pseudo_header = (pseudo_header_t *)pseudo_header_buffer;
96.  
97.     pseudo_header->src_ip = ip_header->src_ip;
98.     pseudo_header->dest_ip = ip_header->dest_ip;
99.     pseudo_header->zero = 0;
100.     pseudo_header->protocol = ip_header->protocol;
101.     pseudo_header->udp_length = htons(total_length);
102.  
103.     uint32_t sum = 0;
104.  
105.     sum += calculate_checksum((uint16_t *)pseudo_header, sizeof(pseudo_header_t));
106.     sum += calculate_checksum((uint16_t *)udp_header, sizeof(udp_header_t));
107.     sum += calculate_checksum((uint16_t *)data, data_len);
108.  
109.     while (sum >> 16) {
110.         sum = (sum & 0xffff) + (sum >> 16);
111.     }
112.     return (uint16_t)~sum;
113. }
114.  

GeSHi © Codebox Plus Extension

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)