diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/ProtobufWrapperLegacy.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/ProtobufWrapperLegacy.java
new file mode 100644
index 00000000000..e00b1a80055
--- /dev/null
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/ProtobufWrapperLegacy.java
@@ -0,0 +1,122 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.hadoop.ipc;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.util.concurrent.atomic.AtomicBoolean;
+
+import org.apache.hadoop.classification.InterfaceAudience;
+import org.apache.hadoop.util.Preconditions;
+
+/**
+ * A RpcWritable wrapper for unshaded protobuf messages.
+ * This class isolates unshaded protobuf classes from
+ * the rest of the RPC codebase, so it can operate without
+ * needing that on the classpath <i>at runtime</i>.
+ * The classes are needed at compile time; and if
+ * unshaded protobuf messages are to be marshalled, they
+ * will need to be on the classpath then.
+ * That is implicit: it is impossible to pass in a class
+ * which is a protobuf message unless that condition is met.
+ */
+@InterfaceAudience.Private
+class ProtobufWrapperLegacy extends RpcWritable {
+
+  private com.google.protobuf.Message message;
+
+  /**
+   * Construct.
+   * The type of the parameter is Object so as to keep the casting internal
+   * to this class.
+   * @param message message to wrap.
+   * @throws IllegalArgumentException if the class is not a protobuf message.
+   */
+  ProtobufWrapperLegacy(Object message) {
+    Preconditions.checkArgument(isUnshadedProtobufMessage(message),
+        "message class is not an unshaded protobuf message %s",
+        message.getClass());
+    this.message = (com.google.protobuf.Message) message;
+  }
+
+  com.google.protobuf.Message getMessage() {
+    return message;
+  }
+
+  @Override
+  void writeTo(ResponseBuffer out) throws IOException {
+    int length = message.getSerializedSize();
+    length += com.google.protobuf.CodedOutputStream.
+        computeUInt32SizeNoTag(length);
+    out.ensureCapacity(length);
+    message.writeDelimitedTo(out);
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  <T> T readFrom(ByteBuffer bb) throws IOException {
+    // using the parser with a byte[]-backed coded input stream is the
+    // most efficient way to deserialize a protobuf.  it has a direct
+    // path to the PB ctor that doesn't create multi-layered streams
+    // that internally buffer.
+    com.google.protobuf.CodedInputStream cis =
+        com.google.protobuf.CodedInputStream.newInstance(
+            bb.array(), bb.position() + bb.arrayOffset(), bb.remaining());
+    try {
+      cis.pushLimit(cis.readRawVarint32());
+      message = message.getParserForType().parseFrom(cis);
+      cis.checkLastTagWas(0);
+    } finally {
+      // advance over the bytes read.
+      bb.position(bb.position() + cis.getTotalBytesRead());
+    }
+    return (T) message;
+  }
+
+  /**
+   * Has protobuf been looked for and is known as absent?
+   * Saves a check on every message.
+   */
+  private static final AtomicBoolean PROTOBUF_KNOWN_NOT_FOUND = new AtomicBoolean(false);
+
+  /**
+   * Is a message an unshaded protobuf message?
+   * @param payload payload
+   * @return true if protobuf.jar is on the classpath and the payload is a Message
+   */
+  static boolean isUnshadedProtobufMessage(Object payload) {
+    if (PROTOBUF_KNOWN_NOT_FOUND.get()) {
+      // protobuf is known to be absent. fail fast without examining
+      // jars or generating exceptions.
+      return false;
+    }
+    // load the protobuf message class.
+    // if it does not load, then the payload is guaranteed not to be
+    // an unshaded protobuf message
+    // this relies on classloader caching for performance
+    try {
+      Class<?> protobufMessageClazz = Class.forName("com.google.protobuf.Message");
+      return protobufMessageClazz.isAssignableFrom(payload.getClass());
+    } catch (ClassNotFoundException e) {
+      PROTOBUF_KNOWN_NOT_FOUND.set(true);
+      return false;
+    }
+
+  }
+}
diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/RpcWritable.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/RpcWritable.java
index f5f0d071f39..c2dc57573f2 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/RpcWritable.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/RpcWritable.java
@@ -41,9 +41,11 @@ public abstract class RpcWritable implements Writable {
     if (o instanceof RpcWritable) {
       return (RpcWritable)o;
     } else if (o instanceof Message) {
+      // hadoop shaded protobuf
       return new ProtobufWrapper((Message)o);
-    } else if (o instanceof com.google.protobuf.Message) {
-      return new ProtobufWrapperLegacy((com.google.protobuf.Message) o);
+    } else if (ProtobufWrapperLegacy.isUnshadedProtobufMessage(o)) {
+      // unshaded protobuf
+      return new ProtobufWrapperLegacy( o);
     } else if (o instanceof Writable) {
       return new WritableWrapper((Writable)o);
     }
@@ -134,49 +136,6 @@ public abstract class RpcWritable implements Writable {
     }
   }
 
-  // adapter for Protobufs.
-  static class ProtobufWrapperLegacy extends RpcWritable {
-    private com.google.protobuf.Message message;
-
-    ProtobufWrapperLegacy(com.google.protobuf.Message message) {
-      this.message = message;
-    }
-
-    com.google.protobuf.Message getMessage() {
-      return message;
-    }
-
-    @Override
-    void writeTo(ResponseBuffer out) throws IOException {
-      int length = message.getSerializedSize();
-      length += com.google.protobuf.CodedOutputStream.
-          computeUInt32SizeNoTag(length);
-      out.ensureCapacity(length);
-      message.writeDelimitedTo(out);
-    }
-
-    @SuppressWarnings("unchecked")
-    @Override
-    <T> T readFrom(ByteBuffer bb) throws IOException {
-      // using the parser with a byte[]-backed coded input stream is the
-      // most efficient way to deserialize a protobuf.  it has a direct
-      // path to the PB ctor that doesn't create multi-layered streams
-      // that internally buffer.
-      com.google.protobuf.CodedInputStream cis =
-          com.google.protobuf.CodedInputStream.newInstance(
-              bb.array(), bb.position() + bb.arrayOffset(), bb.remaining());
-      try {
-        cis.pushLimit(cis.readRawVarint32());
-        message = message.getParserForType().parseFrom(cis);
-        cis.checkLastTagWas(0);
-      } finally {
-        // advance over the bytes read.
-        bb.position(bb.position() + cis.getTotalBytesRead());
-      }
-      return (T)message;
-    }
-  }
-
   /**
    * adapter to allow decoding of writables and protobufs from a byte buffer.
    */
diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/package-info.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/package-info.java
index cb35e938483..f91fa23194c 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/package-info.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/package-info.java
@@ -18,8 +18,12 @@
 
 /**
  * Tools to help define network clients and servers.
+ * Other ASF projects use this package, often with their own shaded/unshaded
+ * versions of protobuf messages.
+ * Changes to the API signatures will break things, especially changes to
+ * {@link org.apache.hadoop.ipc.RPC} and {@link org.apache.hadoop.ipc.RpcEngine}.
  */
-@InterfaceAudience.LimitedPrivate({"HBase", "HDFS", "MapReduce"})
+@InterfaceAudience.LimitedPrivate({"HBase", "HDFS", "MapReduce", "YARN", "Hive", "Ozone"})
 @InterfaceStability.Evolving
 package org.apache.hadoop.ipc;
 import org.apache.hadoop.classification.InterfaceAudience;