diff --git a/lib/includes/ngtcp2/ngtcp2.h b/lib/includes/ngtcp2/ngtcp2.h
index 9d01615fc199c6acb1bdf72a2751d9e2fe9531cb..f65519222327dc23525863d54a706a2564d27db2 100644
--- a/lib/includes/ngtcp2/ngtcp2.h
+++ b/lib/includes/ngtcp2/ngtcp2.h
@@ -508,6 +508,7 @@ typedef struct {
   uint64_t max_data;
   uint64_t max_streams_bidi;
   uint64_t max_streams_uni;
+  /* idle_timeout is specified in millisecond resolution */
   uint64_t idle_timeout;
   uint64_t max_packet_size;
   uint64_t active_connection_id_limit;