diff --git a/src/relay/udprelay/mod.rs b/src/relay/udprelay/mod.rs
index fead9578..65a45cd2 100644
--- a/src/relay/udprelay/mod.rs
+++ b/src/relay/udprelay/mod.rs
@@ -71,4 +71,8 @@ mod utils;
 pub const MAXIMUM_UDP_PAYLOAD_SIZE: usize = 65536;
 
 /// Default association expire time
-pub const DEFAULT_TIMEOUT: Duration = Duration::from_secs(5 * 60);
+///
+/// FIXME: It is very hard to decide how long this value should be.
+/// For some usecases, clients may open a socket for sending just one packet,
+/// which will eventually cause too many *useless* associations are kept in the server.
+pub const DEFAULT_TIMEOUT: Duration = Duration::from_secs(15);