diff --git a/miniupnpd/asyncsendto.h b/miniupnpd/asyncsendto.h
index d46de69..6afb322 100644
--- a/miniupnpd/asyncsendto.h
+++ b/miniupnpd/asyncsendto.h
@@ -8,19 +8,28 @@
 #ifndef ASYNCSENDTO_H_INCLUDED
 #define ASYNCSENDTO_H_INCLUDED
 
+/* sendto_schedule() : see sendto(2)
+ * schedule sendto() call after delay (milliseconds) */
 ssize_t
 sendto_schedule(int sockfd, const void *buf, size_t len, int flags,
                 const struct sockaddr *dest_addr, socklen_t addrlen,
                 unsigned int delay);
 
+/* sendto_schedule() : see sendto(2)
+ * try sendto() at once and schedule if EINTR/EAGAIN/EWOULDBLOCK */
 ssize_t
 sendto_or_schedule(int sockfd, const void *buf, size_t len, int flags,
                    const struct sockaddr *dest_addr, socklen_t addrlen);
 
+/* get_next_scheduled_send()
+ * return number of scheduled sendto
+ * set next_send to timestamp to send next packet */
 int get_next_scheduled_send(struct timeval * next_send);
 
+/* execute sendto() for needed packets */
 int try_sendto(fd_set * writefds);
 
+/* set writefds before select() */
 int get_sendto_fds(fd_set * writefds, int * max_fd, const struct timeval * now);
 
 #endif