diff --git a/common.go b/common.go
index 7e061f4..bdbedc6 100644
--- a/common.go
+++ b/common.go
@@ -206,7 +206,8 @@ type ClientSessionState struct {
 // ClientSessionCache is a cache of ClientSessionState objects that can be used
 // by a client to resume a TLS session with a given server. ClientSessionCache
 // implementations should expect to be called concurrently from different
-// goroutines.
+// goroutines. Only ticket-based resumption is supported, not SessionID-based
+// resumption.
 type ClientSessionCache interface {
 	// Get searches for a ClientSessionState associated with the given key.
 	// On return, ok is true if one was found.