fix: correct checksum byte order and use monotonic clock

includes/icmp.h

@@ -4,6 +4,7 @@
 #include <stdint.h>
 #include <stddef.h>
 #include <netinet/in.h>
+#include <sys/types.h>
 #include <time.h>
 
 /* Opaque handle type */
@@ -45,4 +46,9 @@ int icmp_process(icmp_handle_t *h, icmp_callback_t cb, void *userdata);
 const char *icmp_strerror(const icmp_handle_t *h);
 int icmp_should_retry(const icmp_handle_t *h);
 
-#endif /* ICMP_H */
+/* Time helpers */
+int icmp_get_time(struct timespec *ts);
+int64_t icmp_time_diff_ns(const struct timespec *start,
+							const struct timespec *end);
+
+#endif

src/packet/build.c

@@ -3,6 +3,7 @@
 #include "internal/icmp_utils.h"
 #include <stddef.h>
 #include <string.h>
+#include <arpa/inet.h>
 
 /* Forward declarations */
 static void write_icmp_header(struct icmp_header *hdr, uint8_t type,
@@ -57,5 +58,5 @@ copy_payload(void *dest, const void *src, size_t len)
 static void
 compute_and_set_checksum(struct icmp_header *hdr, size_t total_len)
 {
-	hdr->checksum = icmp_checksum(hdr, total_len);
+	hdr->checksum = htons(icmp_checksum(hdr, total_len));
 }

src/utils/time.c

@@ -4,7 +4,7 @@
 int
 icmp_get_time(struct timespec *ts)
 {
-	if (0 != clock_gettime(CLOCK_REALTIME, ts))
+	if (0 != clock_gettime(CLOCK_MONOTONIC, ts))
 		return -1;
 	return 0;
 }