diff --git a/src/emu/ovni/mark.c b/src/emu/ovni/mark.c
index 184a944..eceebd6 100644
--- a/src/emu/ovni/mark.c
+++ b/src/emu/ovni/mark.c
@@ -4,6 +4,7 @@
 #include "emu.h"
 #include "emu_ev.h"
 #include "emu_prv.h"
+#include "inttypes.h"
 #include "ovni.h"
 #include "ovni_priv.h"
 #include "parson.h"
@@ -12,28 +13,119 @@
 #include "uthash.h"
 #include <errno.h>
 
-struct mark_value {
-	int32_t type;
-	char title[MAX_PCF_LABEL];
-	struct chan ch;
-	struct pcf_type *pcftype;
-	UT_hash_handle hh; /* Indexed by type */
+struct mark_label {
+	int64_t value;
+	char label[MAX_PCF_LABEL];
+	UT_hash_handle hh; /* Indexed by value */
 };
 
 struct mark_chan {
 	long type;
-	char title[MAX_PCF_LABEL];
+	struct mark_label *labels; /* Hash table of labels */
 	struct chan ch;
+	char title[MAX_PCF_LABEL];
 	UT_hash_handle hh; /* Indexed by type */
 };
 
+static int
+parse_number(const char *str, int64_t *result)
+{
+	errno = 0;
+	char *endptr = NULL;
+	int64_t n = strtoll(str, &endptr, 10);
+
+	if (errno != 0 || endptr == str || endptr[0] != '\0') {
+		err("failed to parse number: %s", str);
+		return -1;
+	}
+
+	*result = n;
+	return 0;
+}
+
+static struct mark_label *
+find_label(struct mark_chan *c, int64_t value)
+{
+	struct mark_label *l;
+	HASH_FIND(hh, c->labels, &value, sizeof(value), l);
+	return l;
+}
+
+static int
+add_label(struct mark_chan *c, int64_t value, const char *label)
+{
+	struct mark_label *l = find_label(c, value);
+
+	if (l != NULL) {
+		if (strcmp(l->label, label) == 0) {
+			/* Already exists with the same label, all good */
+			return 0;
+		} else {
+			err("mark value %" PRIi64 " already defined with label %s", value, l->label);
+			return -1;
+		}
+	}
+
+	l = calloc(1, sizeof(*l));
+	if (l == NULL) {
+		err("calloc failed:");
+		return -1;
+	}
+
+	l->value = value;
+
+	int len = snprintf(l->label, MAX_PCF_LABEL, "%s", label);
+	if (len >= MAX_PCF_LABEL) {
+		err("mark label too long: %s", label);
+		return -1;
+	}
+
+	HASH_ADD(hh, c->labels, value, sizeof(value), l);
+
+	return 0;
+}
+
 static int
 parse_labels(struct mark_chan *c, JSON_Object *labels)
 {
 	UNUSED(c);
 	UNUSED(labels);
 
-	/* TODO: Implement */
+	/* It may happen that we call this function several times with
+	 * overlapping subsets of values. The only restriction is that we don't
+	 * define two values with different label. */
+
+	size_t n = json_object_get_count(labels);
+	for (size_t i = 0; i < n; i++) {
+		const char *valuestr = json_object_get_name(labels, i);
+		if (valuestr == NULL) {
+			err("json_object_get_name failed");
+			return -1;
+		}
+
+		int64_t value;
+		if (parse_number(valuestr, &value) != 0) {
+			err("parse_number failed");
+			return -1;
+		}
+
+		JSON_Value *labelval = json_object_get_value_at(labels, i);
+		if (labelval == NULL) {
+			err("json_object_get_value_at failed");
+			return -1;
+		}
+
+		const char *label = json_value_get_string(labelval);
+		if (label == NULL) {
+			err("json_value_get_string() for label failed");
+			return -1;
+		}
+
+		if (add_label(c, value, label) != 0) {
+			err("add_label() failed");
+			return -1;
+		}
+	}
 
 	return 0;
 }