diff --git a/balancer/balancer.go b/balancer/balancer.go
index b7c685ad..8bdfe069 100644
--- a/balancer/balancer.go
+++ b/balancer/balancer.go
@@ -63,6 +63,11 @@ func Get(name string) Builder {
 // When the connection encounters an error, it will reconnect immediately.
 // When the connection becomes IDLE, it will not reconnect unless Connect is
 // called.
+//
+// This interface is to be implemented by gRPC. Users should not need a
+// brand new implementation of this interface. For the situations like
+// testing, the new implementation should embed this interface. This allows
+// gRPC to add new methods to this interface.
 type SubConn interface {
 	// UpdateAddresses updates the addresses used in this SubConn.
 	// gRPC checks if currently-connected address is still in the new list.
@@ -80,6 +85,11 @@ type SubConn interface {
 type NewSubConnOptions struct{}
 
 // ClientConn represents a gRPC ClientConn.
+//
+// This interface is to be implemented by gRPC. Users should not need a
+// brand new implementation of this interface. For the situations like
+// testing, the new implementation should embed this interface. This allows
+// gRPC to add new methods to this interface.
 type ClientConn interface {
 	// NewSubConn is called by balancer to create a new SubConn.
 	// It doesn't block and wait for the connections to be established.
diff --git a/resolver/resolver.go b/resolver/resolver.go
index 15be62dc..9efcffb3 100644
--- a/resolver/resolver.go
+++ b/resolver/resolver.go
@@ -94,6 +94,11 @@ type BuildOption struct {
 
 // ClientConn contains the callbacks for resolver to notify any updates
 // to the gRPC ClientConn.
+//
+// This interface is to be implemented by gRPC. Users should not need a
+// brand new implementation of this interface. For the situations like
+// testing, the new implementation should embed this interface. This allows
+// gRPC to add new methods to this interface.
 type ClientConn interface {
 	// NewAddress is called by resolver to notify ClientConn a new list
 	// of resolved addresses.