diff --git a/benchmark/src/main/java/org/capnproto/benchmark/TestCase.java b/benchmark/src/main/java/org/capnproto/benchmark/TestCase.java
index 0d9faf6..13a45e6 100644
--- a/benchmark/src/main/java/org/capnproto/benchmark/TestCase.java
+++ b/benchmark/src/main/java/org/capnproto/benchmark/TestCase.java
@@ -1,5 +1,7 @@
 package org.capnproto.benchmark;
 
+import java.nio.ByteBuffer;
+
 import org.capnproto.FromStructReader;
 import org.capnproto.FromStructBuilder;
 import org.capnproto.StructFactory;
@@ -32,14 +34,26 @@ public abstract class TestCase<RequestFactory extends StructFactory<RequestBuild
         }
     }
 
+    static final int SCRATCH_SIZE = 128 * 1024;
+
     public void passByBytes(RequestFactory requestFactory, ResponseFactory responseFactory,
                             long iters) {
 
+        ByteBuffer requestBytes = ByteBuffer.allocate(SCRATCH_SIZE * 8);
+        ByteBuffer responseBytes = ByteBuffer.allocate(SCRATCH_SIZE * 8);
         Common.FastRand rng = new Common.FastRand();
 
         for (int i = 0; i < iters; ++i) {
             MessageBuilder requestMessage = new MessageBuilder();
             MessageBuilder responseMessage = new MessageBuilder();
+            RequestBuilder request = requestMessage.initRoot(requestFactory);
+            Expectation expected = this.setupRequest(rng, request);
+            ResponseBuilder response = responseMessage.initRoot(responseFactory);
+
+            {
+                org.capnproto.ByteBufferWritableByteChannel writer = new org.capnproto.ByteBufferWritableByteChannel(requestBytes);
+                //org.capnproto.writeMessage
+            }
             // TODO
             throw new Error("unimplemented");
         }
diff --git a/runtime/src/main/java/org/capnproto/ByteBufferWritableByteChannel.java b/runtime/src/main/java/org/capnproto/ByteBufferWritableByteChannel.java
new file mode 100644
index 0000000..71d03b3
--- /dev/null
+++ b/runtime/src/main/java/org/capnproto/ByteBufferWritableByteChannel.java
@@ -0,0 +1,36 @@
+package org.capnproto;
+
+import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.channels.WritableByteChannel;
+
+public final class ByteBufferWritableByteChannel implements BufferedWritableByteChannel {
+
+    public final ByteBuffer buf;
+
+    public ByteBufferWritableByteChannel(ByteBuffer buf) {
+        this.buf = buf;
+    }
+
+    public final int write(ByteBuffer src) throws IOException {
+        int available = this.buf.remaining();
+        int size = src.remaining();
+        if (available < size) {
+            throw new IOException("backing buffer was not large enough");
+        }
+        this.buf.put(src);
+        return size;
+    }
+
+    public final ByteBuffer getWriteBuffer() {
+        return this.buf;
+    }
+
+    public final void close() throws IOException {
+        return;
+    }
+
+    public final boolean isOpen() {
+        return true;
+    }
+}