diff --git a/common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/protocol/BlockTransferMessage.java b/common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/protocol/BlockTransferMessage.java
index 9af6759f5d5f..de54a81b0f2f 100644
--- a/common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/protocol/BlockTransferMessage.java
+++ b/common/network-shuffle/src/main/java/org/apache/spark/network/shuffle/protocol/BlockTransferMessage.java
@@ -35,6 +35,9 @@
  *     shuffle service. It returns a StreamHandle.
  *   - UploadBlock is only handled by the NettyBlockTransferService.
  *   - RegisterExecutor is only handled by the external shuffle service.
+ *   - StreamHandle is only handled by the OneForOneBlockFetcher.
+ *   - RegisterDriver is only handled by the MesosExternalShuffleService.
+ *   - HearBeat is only handled by the MesosExternalShuffleService.
  */
 public abstract class BlockTransferMessage implements Encodable {
   protected abstract Type type();