diff --git a/hadoop-common-project/hadoop-common/CHANGES.txt b/hadoop-common-project/hadoop-common/CHANGES.txt
index 339bc7baeb0..bea151a7c51 100644
--- a/hadoop-common-project/hadoop-common/CHANGES.txt
+++ b/hadoop-common-project/hadoop-common/CHANGES.txt
@@ -148,6 +148,8 @@ Trunk (Unreleased)
 
     HADOOP-9277. Improve javadoc for FileContext. (Andrew Wang via suresh)
 
+    HADOOP-9218 Document the Rpc-wrappers used internally (sanjay Radia)
+
   BUG FIXES
 
     HADOOP-8419. Fixed GzipCode NPE reset for IBM JDK. (Yu Li via eyang)
diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/ProtobufRpcEngine.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/ProtobufRpcEngine.java
index 198a618947e..0483c7c4457 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/ProtobufRpcEngine.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/ProtobufRpcEngine.java
@@ -62,7 +62,7 @@ public class ProtobufRpcEngine implements RpcEngine {
   
   static { // Register the rpcRequest deserializer for WritableRpcEngine 
     org.apache.hadoop.ipc.Server.registerProtocolEngine(
-        RPC.RpcKind.RPC_PROTOCOL_BUFFER, RpcRequestWritable.class,
+        RPC.RpcKind.RPC_PROTOCOL_BUFFER, RpcRequestWrapper.class,
         new Server.ProtoBufRpcInvoker());
   }
 
@@ -122,7 +122,7 @@ public class ProtobufRpcEngine implements RpcEngine {
     public Invoker(Class<?> protocol, Client.ConnectionId connId,
         Configuration conf, SocketFactory factory) {
       this.remoteId = connId;
-      this.client = CLIENTS.getClient(conf, factory, RpcResponseWritable.class);
+      this.client = CLIENTS.getClient(conf, factory, RpcResponseWrapper.class);
       this.protocolName = RPC.getProtocolName(protocol);
       this.clientProtocolVersion = RPC
           .getProtocolVersion(protocol);
@@ -191,7 +191,7 @@ public class ProtobufRpcEngine implements RpcEngine {
       }
 
       RequestProto rpcRequest = constructRpcRequest(method, args);
-      RpcResponseWritable val = null;
+      RpcResponseWrapper val = null;
       
       if (LOG.isTraceEnabled()) {
         LOG.trace(Thread.currentThread().getId() + ": Call -> " +
@@ -199,8 +199,8 @@ public class ProtobufRpcEngine implements RpcEngine {
             " {" + TextFormat.shortDebugString((Message) args[1]) + "}");
       }
       try {
-        val = (RpcResponseWritable) client.call(RPC.RpcKind.RPC_PROTOCOL_BUFFER,
-            new RpcRequestWritable(rpcRequest), remoteId);
+        val = (RpcResponseWrapper) client.call(RPC.RpcKind.RPC_PROTOCOL_BUFFER,
+            new RpcRequestWrapper(rpcRequest), remoteId);
 
       } catch (Throwable e) {
         if (LOG.isTraceEnabled()) {
@@ -268,16 +268,20 @@ public class ProtobufRpcEngine implements RpcEngine {
   }
 
   /**
-   * Writable Wrapper for Protocol Buffer Requests
+   * Wrapper for Protocol Buffer Requests
+   * 
+   * Note while this wrapper is writable, the request on the wire is in
+   * Protobuf. Several methods on {@link org.apache.hadoop.ipc.Server and RPC} 
+   * use type Writable as a wrapper to work across multiple RpcEngine kinds.
    */
-  private static class RpcRequestWritable implements Writable {
+  private static class RpcRequestWrapper implements Writable {
     RequestProto message;
 
     @SuppressWarnings("unused")
-    public RpcRequestWritable() {
+    public RpcRequestWrapper() {
     }
 
-    RpcRequestWritable(RequestProto message) {
+    RpcRequestWrapper(RequestProto message) {
       this.message = message;
     }
 
@@ -303,16 +307,20 @@ public class ProtobufRpcEngine implements RpcEngine {
   }
 
   /**
-   * Writable Wrapper for Protocol Buffer Responses
+   *  Wrapper for Protocol Buffer Responses
+   * 
+   * Note while this wrapper is writable, the request on the wire is in
+   * Protobuf. Several methods on {@link org.apache.hadoop.ipc.Server and RPC} 
+   * use type Writable as a wrapper to work across multiple RpcEngine kinds.
    */
-  private static class RpcResponseWritable implements Writable {
+  private static class RpcResponseWrapper implements Writable {
     byte[] responseMessage;
 
     @SuppressWarnings("unused")
-    public RpcResponseWritable() {
+    public RpcResponseWrapper() {
     }
 
-    public RpcResponseWritable(Message message) {
+    public RpcResponseWrapper(Message message) {
       this.responseMessage = message.toByteArray();
     }
 
@@ -336,7 +344,7 @@ public class ProtobufRpcEngine implements RpcEngine {
   @InterfaceStability.Unstable
   static Client getClient(Configuration conf) {
     return CLIENTS.getClient(conf, SocketFactory.getDefault(),
-        RpcResponseWritable.class);
+        RpcResponseWrapper.class);
   }
   
  
@@ -425,7 +433,7 @@ public class ProtobufRpcEngine implements RpcEngine {
        */
       public Writable call(RPC.Server server, String connectionProtocolName,
           Writable writableRequest, long receiveTime) throws Exception {
-        RpcRequestWritable request = (RpcRequestWritable) writableRequest;
+        RpcRequestWrapper request = (RpcRequestWrapper) writableRequest;
         RequestProto rpcRequest = request.message;
         String methodName = rpcRequest.getMethodName();
         
@@ -487,7 +495,7 @@ public class ProtobufRpcEngine implements RpcEngine {
         } catch (Exception e) {
           throw e;
         }
-        return new RpcResponseWritable(result);
+        return new RpcResponseWrapper(result);
       }
     }
   }