diff --git a/draft-ietf-quic-transport.md b/draft-ietf-quic-transport.md
index 5ae1cfb396..586f1cd6ce 100644
--- a/draft-ietf-quic-transport.md
+++ b/draft-ietf-quic-transport.md
@@ -650,7 +650,7 @@ QUIC encodes the transport parameters and options as tag-value pairs, all as
 
 * MSPC: Maximum number of incoming streams per connection.
 
-* ICSL: ?
+* ICSL: Idle timeout in seconds.  The maximum value is 600 seconds (10 minutes).
 
 #### Optional Transport Parameters