diff --git a/pw_transfer/java/main/dev/pigweed/pw_transfer/Transfer.java b/pw_transfer/java/main/dev/pigweed/pw_transfer/Transfer.java
index 7f1b702..6e75d19 100644
--- a/pw_transfer/java/main/dev/pigweed/pw_transfer/Transfer.java
+++ b/pw_transfer/java/main/dev/pigweed/pw_transfer/Transfer.java
@@ -309,7 +309,7 @@
 
     // Only call finish() if the sendChunk was successful. If it wasn't, the exception would have
     // already terminated the transfer.
-    if (sendChunk(newChunk(Chunk.Type.TRANSFER_COMPLETION).setStatus(status.code()))) {
+    if (sendChunk(newChunk(Chunk.Type.COMPLETION).setStatus(status.code()))) {
       cleanUp(status);
     }
   }
diff --git a/pw_transfer/java/main/dev/pigweed/pw_transfer/WriteTransfer.java b/pw_transfer/java/main/dev/pigweed/pw_transfer/WriteTransfer.java
index 40feed2..4daef2d 100644
--- a/pw_transfer/java/main/dev/pigweed/pw_transfer/WriteTransfer.java
+++ b/pw_transfer/java/main/dev/pigweed/pw_transfer/WriteTransfer.java
@@ -68,10 +68,7 @@
 
   @Override
   synchronized Chunk getInitialChunk() {
-    return newChunk(Chunk.Type.TRANSFER_START)
-        .setResourceId(getId())
-        .setRemainingBytes(data.length)
-        .build();
+    return newChunk(Chunk.Type.START).setResourceId(getId()).setRemainingBytes(data.length).build();
   }
 
   @Override
@@ -221,7 +218,7 @@
     // have a type field.
     return !chunk.hasType()
         || (chunk.getType().equals(Chunk.Type.PARAMETERS_RETRANSMIT)
-            || chunk.getType().equals(Chunk.Type.TRANSFER_START));
+            || chunk.getType().equals(Chunk.Type.START));
   }
 
   private static int getWindowEndOffset(Chunk chunk, int dataLength) {
@@ -234,8 +231,7 @@
   }
 
   private Chunk buildDataChunk(ByteString chunkData) {
-    Chunk.Builder chunk =
-        newChunk(Chunk.Type.TRANSFER_DATA).setOffset(sentOffset).setData(chunkData);
+    Chunk.Builder chunk = newChunk(Chunk.Type.DATA).setOffset(sentOffset).setData(chunkData);
 
     // If this is the last data chunk, setRemainingBytes to 0.
     if (sentOffset + chunkData.size() == data.length) {
diff --git a/pw_transfer/java/test/dev/pigweed/pw_transfer/TransferClientTest.java b/pw_transfer/java/test/dev/pigweed/pw_transfer/TransferClientTest.java
index a54d218..bca7c0b 100644
--- a/pw_transfer/java/test/dev/pigweed/pw_transfer/TransferClientTest.java
+++ b/pw_transfer/java/test/dev/pigweed/pw_transfer/TransferClientTest.java
@@ -73,10 +73,8 @@
     ListenableFuture<byte[]> future = manager.read(1);
     assertThat(future.isDone()).isFalse();
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, 1)
-                          .setOffset(0)
-                          .setData(TEST_DATA_SHORT)
-                          .setRemainingBytes(0));
+    receiveReadChunks(
+        newChunk(Chunk.Type.DATA, 1).setOffset(0).setData(TEST_DATA_SHORT).setRemainingBytes(0));
 
     assertThat(future.isDone()).isTrue();
     assertThat(future.get()).isEqualTo(TEST_DATA_SHORT.toByteArray());
@@ -102,10 +100,8 @@
                              .setMaxChunkSizeBytes(TRANSFER_PARAMETERS.maxChunkSizeBytes())
                              .build());
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, 1)
-                          .setOffset(0)
-                          .setData(TEST_DATA_SHORT)
-                          .setRemainingBytes(0));
+    receiveReadChunks(
+        newChunk(Chunk.Type.DATA, 1).setOffset(0).setData(TEST_DATA_SHORT).setRemainingBytes(0));
 
     assertThat(future.isDone()).isTrue();
     assertThat(future.get()).isEqualTo(TEST_DATA_SHORT.toByteArray());
@@ -142,30 +138,16 @@
     assertThat(future.isDone()).isFalse();
 
     receiveReadChunks(finalChunk(2, Status.OK),
-        newChunk(Chunk.Type.TRANSFER_DATA, 0)
-            .setOffset(0)
-            .setData(TEST_DATA_100B)
-            .setRemainingBytes(0),
-        newChunk(Chunk.Type.TRANSFER_DATA, 3)
-            .setOffset(0)
-            .setData(TEST_DATA_100B)
-            .setRemainingBytes(0));
+        newChunk(Chunk.Type.DATA, 0).setOffset(0).setData(TEST_DATA_100B).setRemainingBytes(0),
+        newChunk(Chunk.Type.DATA, 3).setOffset(0).setData(TEST_DATA_100B).setRemainingBytes(0));
     receiveWriteChunks(finalChunk(1, Status.OK),
-        newChunk(Chunk.Type.TRANSFER_DATA, 1)
-            .setOffset(0)
-            .setData(TEST_DATA_100B)
-            .setRemainingBytes(0),
-        newChunk(Chunk.Type.TRANSFER_DATA, 2)
-            .setOffset(0)
-            .setData(TEST_DATA_100B)
-            .setRemainingBytes(0));
+        newChunk(Chunk.Type.DATA, 1).setOffset(0).setData(TEST_DATA_100B).setRemainingBytes(0),
+        newChunk(Chunk.Type.DATA, 2).setOffset(0).setData(TEST_DATA_100B).setRemainingBytes(0));
 
     assertThat(future.isDone()).isFalse();
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, 1)
-                          .setOffset(0)
-                          .setData(TEST_DATA_SHORT)
-                          .setRemainingBytes(0));
+    receiveReadChunks(
+        newChunk(Chunk.Type.DATA, 1).setOffset(0).setData(TEST_DATA_SHORT).setRemainingBytes(0));
 
     assertThat(future.get()).isEqualTo(TEST_DATA_SHORT.toByteArray());
   }
@@ -175,7 +157,7 @@
     ListenableFuture<byte[]> future = manager.read(2);
     lastChunks(); // Discard initial chunk (tested elsewhere)
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, 2).setRemainingBytes(0));
+    receiveReadChunks(newChunk(Chunk.Type.DATA, 2).setRemainingBytes(0));
 
     assertThat(lastChunks()).containsExactly(finalChunk(2, Status.OK));
 
@@ -209,11 +191,9 @@
                              .setOffset(0)
                              .build());
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID)
-                          .setOffset(0)
-                          .setData(range(0, 20))
-                          .setRemainingBytes(70),
-        newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(20).setData(range(20, 40)));
+    receiveReadChunks(
+        newChunk(Chunk.Type.DATA, ID).setOffset(0).setData(range(0, 20)).setRemainingBytes(70),
+        newChunk(Chunk.Type.DATA, ID).setOffset(20).setData(range(20, 40)));
 
     assertThat(lastChunks())
         .containsExactly(newChunk(Chunk.Type.PARAMETERS_CONTINUE, ID)
@@ -223,7 +203,7 @@
                              .setWindowEndOffset(90)
                              .build());
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(40).setData(range(40, 70)));
+    receiveReadChunks(newChunk(Chunk.Type.DATA, ID).setOffset(40).setData(range(40, 70)));
 
     assertThat(lastChunks())
         .containsExactly(newChunk(Chunk.Type.PARAMETERS_CONTINUE, ID)
@@ -233,10 +213,8 @@
                              .setWindowEndOffset(120)
                              .build());
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID)
-                          .setOffset(70)
-                          .setData(range(70, 100))
-                          .setRemainingBytes(0));
+    receiveReadChunks(
+        newChunk(Chunk.Type.DATA, ID).setOffset(70).setData(range(70, 100)).setRemainingBytes(0));
 
     assertThat(lastChunks()).containsExactly(finalChunk(ID, Status.OK));
 
@@ -247,22 +225,13 @@
   public void read_progressCallbackIsCalled() {
     ListenableFuture<byte[]> future = manager.read(ID, TRANSFER_PARAMETERS, progressCallback);
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(0).setData(range(0, 30)),
-        newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(30).setData(range(30, 50)),
-        newChunk(Chunk.Type.TRANSFER_DATA, ID)
-            .setOffset(50)
-            .setData(range(50, 60))
-            .setRemainingBytes(5),
-        newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(60).setData(range(60, 70)),
-        newChunk(Chunk.Type.TRANSFER_DATA, ID)
-            .setOffset(70)
-            .setData(range(70, 80))
-            .setRemainingBytes(20),
-        newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(90).setData(range(90, 100)),
-        newChunk(Chunk.Type.TRANSFER_DATA, ID)
-            .setOffset(80)
-            .setData(range(80, 100))
-            .setRemainingBytes(0));
+    receiveReadChunks(newChunk(Chunk.Type.DATA, ID).setOffset(0).setData(range(0, 30)),
+        newChunk(Chunk.Type.DATA, ID).setOffset(30).setData(range(30, 50)),
+        newChunk(Chunk.Type.DATA, ID).setOffset(50).setData(range(50, 60)).setRemainingBytes(5),
+        newChunk(Chunk.Type.DATA, ID).setOffset(60).setData(range(60, 70)),
+        newChunk(Chunk.Type.DATA, ID).setOffset(70).setData(range(70, 80)).setRemainingBytes(20),
+        newChunk(Chunk.Type.DATA, ID).setOffset(90).setData(range(90, 100)),
+        newChunk(Chunk.Type.DATA, ID).setOffset(80).setData(range(80, 100)).setRemainingBytes(0));
 
     verify(progressCallback, times(6)).accept(progress.capture());
     assertThat(progress.getAllValues())
@@ -287,7 +256,7 @@
                              .setOffset(0)
                              .build());
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(50).setData(range(30, 50)));
+    receiveReadChunks(newChunk(Chunk.Type.DATA, ID).setOffset(50).setData(range(30, 50)));
 
     assertThat(lastChunks())
         .containsExactly(newChunk(Chunk.Type.PARAMETERS_RETRANSMIT, ID)
@@ -297,8 +266,8 @@
                              .setOffset(0)
                              .build());
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(0).setData(range(0, 30)),
-        newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(30).setData(range(30, 50)));
+    receiveReadChunks(newChunk(Chunk.Type.DATA, ID).setOffset(0).setData(range(0, 30)),
+        newChunk(Chunk.Type.DATA, ID).setOffset(30).setData(range(30, 50)));
 
     assertThat(lastChunks())
         .containsExactly(newChunk(Chunk.Type.PARAMETERS_CONTINUE, ID)
@@ -308,10 +277,8 @@
                              .setMaxChunkSizeBytes(30)
                              .build());
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID)
-                          .setOffset(80)
-                          .setData(range(80, 100))
-                          .setRemainingBytes(0));
+    receiveReadChunks(
+        newChunk(Chunk.Type.DATA, ID).setOffset(80).setData(range(80, 100)).setRemainingBytes(0));
 
     assertThat(lastChunks())
         .containsExactly(newChunk(Chunk.Type.PARAMETERS_RETRANSMIT, ID)
@@ -321,11 +288,8 @@
                              .setMaxChunkSizeBytes(30)
                              .build());
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(50).setData(range(50, 80)),
-        newChunk(Chunk.Type.TRANSFER_DATA, ID)
-            .setOffset(80)
-            .setData(range(80, 100))
-            .setRemainingBytes(0));
+    receiveReadChunks(newChunk(Chunk.Type.DATA, ID).setOffset(50).setData(range(50, 80)),
+        newChunk(Chunk.Type.DATA, ID).setOffset(80).setData(range(80, 100)).setRemainingBytes(0));
 
     assertThat(lastChunks())
         .containsExactly(newChunk(Chunk.Type.PARAMETERS_CONTINUE, ID)
@@ -344,10 +308,8 @@
     for (int i = 0; i < 3; ++i) {
       ListenableFuture<byte[]> future = manager.read(1);
 
-      receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, 1)
-                            .setOffset(0)
-                            .setData(TEST_DATA_SHORT)
-                            .setRemainingBytes(0));
+      receiveReadChunks(
+          newChunk(Chunk.Type.DATA, 1).setOffset(0).setData(TEST_DATA_SHORT).setRemainingBytes(0));
 
       assertThat(future.get()).isEqualTo(TEST_DATA_SHORT.toByteArray());
     }
@@ -381,12 +343,12 @@
   public void read_sendErrorOnLaterPacket_aborts() {
     ListenableFuture<byte[]> future = manager.read(ID, TRANSFER_PARAMETERS);
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(0).setData(range(0, 20)));
+    receiveReadChunks(newChunk(Chunk.Type.DATA, ID).setOffset(0).setData(range(0, 20)));
 
     ChannelOutputException exception = new ChannelOutputException("blah");
     client.setChannelOutputException(exception);
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(20).setData(range(20, 50)));
+    receiveReadChunks(newChunk(Chunk.Type.DATA, ID).setOffset(20).setData(range(20, 50)));
 
     ExecutionException thrown = assertThrows(ExecutionException.class, future::get);
     assertThat(thrown).hasCauseThat().isInstanceOf(TransferError.class);
@@ -399,7 +361,7 @@
 
     assertThat(future.cancel(true)).isTrue();
 
-    receiveReadChunks(newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(30).setData(range(30, 50)));
+    receiveReadChunks(newChunk(Chunk.Type.DATA, ID).setOffset(30).setData(range(30, 50)));
     assertThat(lastChunks()).contains(finalChunk(ID, Status.CANCELLED));
   }
 
@@ -554,9 +516,8 @@
                            .setMinDelayMicroseconds(1));
 
     assertThat(lastChunks())
-        .containsExactly(
-            newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(0).setData(range(0, 30)).build(),
-            newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(30).setData(range(30, 50)).build());
+        .containsExactly(newChunk(Chunk.Type.DATA, ID).setOffset(0).setData(range(0, 30)).build(),
+            newChunk(Chunk.Type.DATA, ID).setOffset(30).setData(range(30, 50)).build());
 
     receiveWriteChunks(newChunk(Chunk.Type.PARAMETERS_RETRANSMIT, ID)
                            .setOffset(50)
@@ -564,15 +525,14 @@
                            .setMaxChunkSizeBytes(25));
 
     assertThat(lastChunks())
-        .containsExactly(
-            newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(50).setData(range(50, 75)).build(),
-            newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(75).setData(range(75, 90)).build());
+        .containsExactly(newChunk(Chunk.Type.DATA, ID).setOffset(50).setData(range(50, 75)).build(),
+            newChunk(Chunk.Type.DATA, ID).setOffset(75).setData(range(75, 90)).build());
 
     receiveWriteChunks(
         newChunk(Chunk.Type.PARAMETERS_RETRANSMIT, ID).setOffset(90).setPendingBytes(50));
 
     assertThat(lastChunks())
-        .containsExactly(newChunk(Chunk.Type.TRANSFER_DATA, ID)
+        .containsExactly(newChunk(Chunk.Type.DATA, ID)
                              .setOffset(90)
                              .setData(range(90, 100))
                              .setRemainingBytes(0)
@@ -599,9 +559,8 @@
                            .setMinDelayMicroseconds(1));
 
     assertThat(lastChunks())
-        .containsExactly(
-            newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(0).setData(range(0, 30)).build(),
-            newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(30).setData(range(30, 50)).build());
+        .containsExactly(newChunk(Chunk.Type.DATA, ID).setOffset(0).setData(range(0, 30)).build(),
+            newChunk(Chunk.Type.DATA, ID).setOffset(30).setData(range(30, 50)).build());
 
     receiveWriteChunks(newChunk(Chunk.Type.PARAMETERS_CONTINUE, ID)
                            .setOffset(30)
@@ -611,7 +570,7 @@
     // Transfer doesn't roll back to offset 30 but instead continues sending up to 80.
     assertThat(lastChunks())
         .containsExactly(
-            newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(50).setData(range(50, 80)).build());
+            newChunk(Chunk.Type.DATA, ID).setOffset(50).setData(range(50, 80)).build());
 
     receiveWriteChunks(newChunk(Chunk.Type.PARAMETERS_CONTINUE, ID)
                            .setOffset(80)
@@ -619,7 +578,7 @@
                            .setWindowEndOffset(130));
 
     assertThat(lastChunks())
-        .containsExactly(newChunk(Chunk.Type.TRANSFER_DATA, ID)
+        .containsExactly(newChunk(Chunk.Type.DATA, ID)
                              .setOffset(80)
                              .setData(range(80, 100))
                              .setRemainingBytes(0)
@@ -646,8 +605,7 @@
                            .setMinDelayMicroseconds(1));
 
     assertThat(lastChunks())
-        .containsExactly(
-            newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(0).setData(range(0, 90)).build());
+        .containsExactly(newChunk(Chunk.Type.DATA, ID).setOffset(0).setData(range(0, 90)).build());
 
     receiveWriteChunks(
         // This stale packet with a window end before the offset should be ignored.
@@ -659,7 +617,7 @@
         newChunk(Chunk.Type.PARAMETERS_CONTINUE, ID).setOffset(80).setWindowEndOffset(100));
 
     assertThat(lastChunks())
-        .containsExactly(newChunk(Chunk.Type.TRANSFER_DATA, ID)
+        .containsExactly(newChunk(Chunk.Type.DATA, ID)
                              .setOffset(90)
                              .setData(range(90, 100))
                              .setRemainingBytes(0)
@@ -704,7 +662,7 @@
 
     assertThat(lastChunks())
         .containsExactly(initialWriteChunk(ID, ID, TEST_DATA_100B.size()),
-            newChunk(Chunk.Type.TRANSFER_DATA, ID).setOffset(100).setRemainingBytes(0).build());
+            newChunk(Chunk.Type.DATA, ID).setOffset(100).setRemainingBytes(0).build());
     assertThat(future.isDone()).isFalse();
   }
 
@@ -856,7 +814,7 @@
     ExecutionException exception = assertThrows(ExecutionException.class, future::get);
     assertThat(((TransferError) exception.getCause()).status()).isEqualTo(Status.DEADLINE_EXCEEDED);
 
-    Chunk data = newChunk(Chunk.Type.TRANSFER_DATA, ID)
+    Chunk data = newChunk(Chunk.Type.DATA, ID)
                      .setOffset(0)
                      .setData(TEST_DATA_SHORT)
                      .setRemainingBytes(0)
@@ -885,14 +843,14 @@
   }
 
   private static Chunk initialWriteChunk(int sessionId, int resourceId, int size) {
-    return newChunk(Chunk.Type.TRANSFER_START, sessionId)
+    return newChunk(Chunk.Type.START, sessionId)
         .setResourceId(resourceId)
         .setRemainingBytes(size)
         .build();
   }
 
   private static Chunk finalChunk(int sessionId, Status status) {
-    return newChunk(Chunk.Type.TRANSFER_COMPLETION, sessionId).setStatus(status.code()).build();
+    return newChunk(Chunk.Type.COMPLETION, sessionId).setStatus(status.code()).build();
   }
 
   private void receiveReadChunks(ChunkOrBuilder... chunks) {
diff --git a/pw_transfer/py/pw_transfer/transfer.py b/pw_transfer/py/pw_transfer/transfer.py
index 0004522..22e650e 100644
--- a/pw_transfer/py/pw_transfer/transfer.py
+++ b/pw_transfer/py/pw_transfer/transfer.py
@@ -201,10 +201,9 @@
     def _send_error(self, error: Status) -> None:
         """Sends an error chunk to the server and finishes the transfer."""
         self._send_chunk(
-            transfer_pb2.Chunk(
-                transfer_id=self.id,
-                status=error.value,
-                type=transfer_pb2.Chunk.Type.TRANSFER_COMPLETION))
+            transfer_pb2.Chunk(transfer_id=self.id,
+                               status=error.value,
+                               type=transfer_pb2.Chunk.Type.COMPLETION))
         self.finish(error)
 
 
@@ -248,7 +247,7 @@
         # server during an initial handshake.
         return transfer_pb2.Chunk(transfer_id=self.id,
                                   resource_id=self.id,
-                                  type=transfer_pb2.Chunk.Type.TRANSFER_START)
+                                  type=transfer_pb2.Chunk.Type.START)
 
     async def _handle_data_chunk(self, chunk: transfer_pb2.Chunk) -> None:
         """Processes an incoming chunk from the server.
@@ -298,9 +297,9 @@
 
         retransmit = True
         if chunk.HasField('type'):
-            retransmit = (
-                chunk.type == transfer_pb2.Chunk.Type.PARAMETERS_RETRANSMIT
-                or chunk.type == transfer_pb2.Chunk.Type.TRANSFER_START)
+            retransmit = (chunk.type
+                          == transfer_pb2.Chunk.Type.PARAMETERS_RETRANSMIT
+                          or chunk.type == transfer_pb2.Chunk.Type.START)
 
         if chunk.offset > len(self.data):
             # Bad offset; terminate the transfer.
@@ -355,7 +354,7 @@
         """Returns the next Chunk message to send in the data transfer."""
         chunk = transfer_pb2.Chunk(transfer_id=self.id,
                                    offset=self._offset,
-                                   type=transfer_pb2.Chunk.Type.TRANSFER_DATA)
+                                   type=transfer_pb2.Chunk.Type.DATA)
         max_bytes_in_chunk = min(self._max_chunk_size,
                                  self._window_end_offset - self._offset)
 
@@ -416,8 +415,7 @@
         return bytes(self._data)
 
     def _initial_chunk(self) -> transfer_pb2.Chunk:
-        return self._transfer_parameters(
-            transfer_pb2.Chunk.Type.TRANSFER_START)
+        return self._transfer_parameters(transfer_pb2.Chunk.Type.START)
 
     async def _handle_data_chunk(self, chunk: transfer_pb2.Chunk) -> None:
         """Processes an incoming chunk from the server.
@@ -446,7 +444,7 @@
                     transfer_pb2.Chunk(
                         transfer_id=self.id,
                         status=Status.OK.value,
-                        type=transfer_pb2.Chunk.Type.TRANSFER_COMPLETION))
+                        type=transfer_pb2.Chunk.Type.COMPLETION))
                 self.finish(Status.OK)
                 return
 
diff --git a/pw_transfer/py/tests/transfer_test.py b/pw_transfer/py/tests/transfer_test.py
index 4c6ea5a..63a73ee 100644
--- a/pw_transfer/py/tests/transfer_test.py
+++ b/pw_transfer/py/tests/transfer_test.py
@@ -473,18 +473,18 @@
         self.assertEqual(
             self._sent_chunks,
             [
-                transfer_pb2.Chunk(transfer_id=22,
-                                   resource_id=22,
-                                   type=transfer_pb2.Chunk.Type.TRANSFER_START
-                                   ),  # initial chunk
                 transfer_pb2.Chunk(
                     transfer_id=22,
                     resource_id=22,
-                    type=transfer_pb2.Chunk.Type.TRANSFER_START),  # retry 1
+                    type=transfer_pb2.Chunk.Type.START),  # initial chunk
                 transfer_pb2.Chunk(
                     transfer_id=22,
                     resource_id=22,
-                    type=transfer_pb2.Chunk.Type.TRANSFER_START),  # retry 2
+                    type=transfer_pb2.Chunk.Type.START),  # retry 1
+                transfer_pb2.Chunk(
+                    transfer_id=22,
+                    resource_id=22,
+                    type=transfer_pb2.Chunk.Type.START),  # retry 2
             ])
 
         exception = context.exception
@@ -506,23 +506,21 @@
         with self.assertRaises(pw_transfer.Error) as context:
             manager.write(22, b'0123456789')
 
-        last_data_chunk = transfer_pb2.Chunk(
-            transfer_id=22,
-            data=b'56789',
-            offset=5,
-            remaining_bytes=0,
-            type=transfer_pb2.Chunk.Type.TRANSFER_DATA)
+        last_data_chunk = transfer_pb2.Chunk(transfer_id=22,
+                                             data=b'56789',
+                                             offset=5,
+                                             remaining_bytes=0,
+                                             type=transfer_pb2.Chunk.Type.DATA)
 
         self.assertEqual(
             self._sent_chunks,
             [
-                transfer_pb2.Chunk(
-                    transfer_id=22,
-                    resource_id=22,
-                    type=transfer_pb2.Chunk.Type.TRANSFER_START),
+                transfer_pb2.Chunk(transfer_id=22,
+                                   resource_id=22,
+                                   type=transfer_pb2.Chunk.Type.START),
                 transfer_pb2.Chunk(transfer_id=22,
                                    data=b'01234',
-                                   type=transfer_pb2.Chunk.Type.TRANSFER_DATA),
+                                   type=transfer_pb2.Chunk.Type.DATA),
                 last_data_chunk,  # last chunk
                 last_data_chunk,  # retry 1
                 last_data_chunk,  # retry 2
diff --git a/pw_transfer/transfer.proto b/pw_transfer/transfer.proto
index e4bb5a8..b5eb3ab 100644
--- a/pw_transfer/transfer.proto
+++ b/pw_transfer/transfer.proto
@@ -143,10 +143,10 @@
 
   enum Type {
     // Chunk containing transfer data.
-    TRANSFER_DATA = 0;
+    DATA = 0;
 
     // First chunk of a transfer (only sent by the client).
-    TRANSFER_START = 1;
+    START = 1;
 
     // Transfer parameters indicating that the transmitter should retransmit
     // from the specified offset.
@@ -158,11 +158,21 @@
     PARAMETERS_CONTINUE = 3;
 
     // Sender of the chunk is terminating the transfer.
-    TRANSFER_COMPLETION = 4;
+    COMPLETION = 4;
 
     // Acknowledge the completion of a transfer. Currently unused.
     // TODO(konkers): Implement this behavior.
-    TRANSFER_COMPLETION_ACK = 5;
+    COMPLETION_ACK = 5;
+
+    // Acknowledges a transfer start request, assigning a session ID for the
+    // transfer and optionally negotiating the protocol version. Sent from
+    // server to client.
+    START_ACK = 6;
+
+    // Confirmation of a START_ACK's assigned session ID and negotiated
+    // parameters, sent by the client to the server. Initiates the data transfer
+    // proper.
+    START_ACK_CONFIRMATION = 7;
   };
 
   // The type of this chunk. This field should only be processed when present.
diff --git a/pw_transfer/ts/transfer.ts b/pw_transfer/ts/transfer.ts
index 8264f0a..e71f9bf 100644
--- a/pw_transfer/ts/transfer.ts
+++ b/pw_transfer/ts/transfer.ts
@@ -138,7 +138,7 @@
     const chunk = new Chunk();
     chunk.setStatus(error);
     chunk.setTransferId(this.id);
-    chunk.setType(Chunk.Type.TRANSFER_COMPLETION);
+    chunk.setType(Chunk.Type.COMPLETION);
     this.sendChunk(chunk);
     this.finish(error);
   }
@@ -253,7 +253,7 @@
   }
 
   protected get initialChunk(): Chunk {
-    return this.transferParameters(Chunk.Type.TRANSFER_START);
+    return this.transferParameters(Chunk.Type.START);
   }
 
   /** Builds an updated transfer parameters chunk to send the server. */
@@ -305,7 +305,7 @@
         const endChunk = new Chunk();
         endChunk.setTransferId(this.id);
         endChunk.setStatus(Status.OK);
-        endChunk.setType(Chunk.Type.TRANSFER_COMPLETION);
+        endChunk.setType(Chunk.Type.COMPLETION);
         this.sendChunk(endChunk);
         this.finish(Status.OK);
         return;
@@ -407,7 +407,7 @@
     const chunk = new Chunk();
     chunk.setTransferId(this.id);
     chunk.setResourceId(this.id);
-    chunk.setType(Chunk.Type.TRANSFER_START);
+    chunk.setType(Chunk.Type.START);
     return chunk;
   }
 
@@ -518,7 +518,7 @@
     const chunk = new Chunk();
     chunk.setTransferId(this.id);
     chunk.setOffset(this.offset);
-    chunk.setType(Chunk.Type.TRANSFER_DATA);
+    chunk.setType(Chunk.Type.DATA);
 
     const maxBytesInChunk = Math.min(
       this.maxChunkSize,
diff --git a/pw_transfer/ts/transfer_test.ts b/pw_transfer/ts/transfer_test.ts
index 584df42..a252dd9 100644
--- a/pw_transfer/ts/transfer_test.ts
+++ b/pw_transfer/ts/transfer_test.ts
@@ -614,17 +614,17 @@
         const expectedChunk1 = new Chunk();
         expectedChunk1.setTransferId(22);
         expectedChunk1.setResourceId(22);
-        expectedChunk1.setType(Chunk.Type.TRANSFER_START);
+        expectedChunk1.setType(Chunk.Type.START);
         const expectedChunk2 = new Chunk();
         expectedChunk2.setTransferId(22);
         expectedChunk2.setData(textEncoder.encode('01234'));
-        expectedChunk2.setType(Chunk.Type.TRANSFER_DATA);
+        expectedChunk2.setType(Chunk.Type.DATA);
         const lastChunk = new Chunk();
         lastChunk.setTransferId(22);
         lastChunk.setData(textEncoder.encode('56789'));
         lastChunk.setOffset(5);
         lastChunk.setRemainingBytes(0);
-        lastChunk.setType(Chunk.Type.TRANSFER_DATA);
+        lastChunk.setType(Chunk.Type.DATA);
 
         const expectedChunks = [
           expectedChunk1,