diff --git a/api/test/fake_datagram_transport.h b/api/test/fake_datagram_transport.h
index 8cb399cd22..16cb8d5e7e 100644
--- a/api/test/fake_datagram_transport.h
+++ b/api/test/fake_datagram_transport.h
@@ -56,8 +56,6 @@ class FakeDatagramTransport : public DatagramTransportInterface {
 
   void SetDatagramSink(DatagramSinkInterface* sink) override {}
 
-  bool IsReadyToSend() const override { return false; }
-
   std::string GetTransportParameters() const override {
     if (settings_.remote_transport_parameters) {
       return *settings_.remote_transport_parameters;
@@ -65,6 +63,24 @@ class FakeDatagramTransport : public DatagramTransportInterface {
     return transport_parameters_;
   }
 
+  RTCError OpenChannel(int channel_id) override {
+    return RTCError(RTCErrorType::UNSUPPORTED_OPERATION);
+  }
+
+  RTCError SendData(int channel_id,
+                    const SendDataParams& params,
+                    const rtc::CopyOnWriteBuffer& buffer) override {
+    return RTCError(RTCErrorType::UNSUPPORTED_OPERATION);
+  }
+
+  RTCError CloseChannel(int channel_id) override {
+    return RTCError(RTCErrorType::UNSUPPORTED_OPERATION);
+  }
+
+  void SetDataSink(DataChannelSink* /*sink*/) override {}
+
+  bool IsReadyToSend() const override { return false; }
+
   rtc::PacketTransportInternal* packet_transport() { return packet_transport_; }
 
   void set_state(webrtc::MediaTransportState state) {
diff --git a/api/transport/BUILD.gn b/api/transport/BUILD.gn
index 12bcf099c1..972340713b 100644
--- a/api/transport/BUILD.gn
+++ b/api/transport/BUILD.gn
@@ -72,7 +72,6 @@ rtc_source_set("datagram_transport_interface") {
   visibility = [ "*" ]
   sources = [
     "congestion_control_interface.h",
-    "data_channel_transport_interface.cc",
     "data_channel_transport_interface.h",
     "datagram_transport_interface.h",
   ]
diff --git a/api/transport/data_channel_transport_interface.cc b/api/transport/data_channel_transport_interface.cc
deleted file mode 100644
index 122e282525..0000000000
--- a/api/transport/data_channel_transport_interface.cc
+++ /dev/null
@@ -1,40 +0,0 @@
-/* Copyright 2019 The WebRTC project authors. All Rights Reserved.
- *
- *  Use of this source code is governed by a BSD-style license
- *  that can be found in the LICENSE file in the root of the source
- *  tree. An additional intellectual property rights grant can be found
- *  in the file PATENTS.  All contributing project authors may
- *  be found in the AUTHORS file in the root of the source tree.
- */
-
-#include "api/transport/data_channel_transport_interface.h"
-
-namespace webrtc {
-
-// TODO(mellem):  Delete these default implementations and make these functions
-// pure virtual as soon as downstream implementations override them.
-
-RTCError DataChannelTransportInterface::OpenChannel(int channel_id) {
-  return RTCError(RTCErrorType::UNSUPPORTED_OPERATION);
-}
-
-RTCError DataChannelTransportInterface::SendData(
-    int channel_id,
-    const SendDataParams& params,
-    const rtc::CopyOnWriteBuffer& buffer) {
-  return RTCError(RTCErrorType::UNSUPPORTED_OPERATION);
-}
-
-RTCError DataChannelTransportInterface::CloseChannel(int channel_id) {
-  return RTCError(RTCErrorType::UNSUPPORTED_OPERATION);
-}
-
-void DataChannelTransportInterface::SetDataSink(DataChannelSink* /*sink*/) {}
-
-bool DataChannelTransportInterface::IsReadyToSend() const {
-  return false;
-}
-
-void DataChannelSink::OnReadyToSend() {}
-
-}  // namespace webrtc
diff --git a/api/transport/data_channel_transport_interface.h b/api/transport/data_channel_transport_interface.h
index 9b29323514..db53a5ed1d 100644
--- a/api/transport/data_channel_transport_interface.h
+++ b/api/transport/data_channel_transport_interface.h
@@ -83,8 +83,7 @@ class DataChannelSink {
   // registered if the transport is ready at that time.  This callback may be
   // invoked again following send errors (eg. due to the transport being
   // temporarily blocked or unavailable).
-  // TODO(mellem):  Make pure virtual when downstream sinks override this.
-  virtual void OnReadyToSend();
+  virtual void OnReadyToSend() = 0;
 };
 
 // Transport for data channels.
@@ -94,30 +93,29 @@ class DataChannelTransportInterface {
 
   // Opens a data |channel_id| for sending.  May return an error if the
   // specified |channel_id| is unusable.  Must be called before |SendData|.
-  virtual RTCError OpenChannel(int channel_id);
+  virtual RTCError OpenChannel(int channel_id) = 0;
 
   // Sends a data buffer to the remote endpoint using the given send parameters.
   // |buffer| may not be larger than 256 KiB. Returns an error if the send
   // fails.
   virtual RTCError SendData(int channel_id,
                             const SendDataParams& params,
-                            const rtc::CopyOnWriteBuffer& buffer);
+                            const rtc::CopyOnWriteBuffer& buffer) = 0;
 
   // Closes |channel_id| gracefully.  Returns an error if |channel_id| is not
   // open.  Data sent after the closing procedure begins will not be
   // transmitted. The channel becomes closed after pending data is transmitted.
-  virtual RTCError CloseChannel(int channel_id);
+  virtual RTCError CloseChannel(int channel_id) = 0;
 
   // Sets a sink for data messages and channel state callbacks. Before media
   // transport is destroyed, the sink must be unregistered by setting it to
   // nullptr.
-  virtual void SetDataSink(DataChannelSink* sink);
+  virtual void SetDataSink(DataChannelSink* sink) = 0;
 
   // Returns whether this data channel transport is ready to send.
   // Note: the default implementation always returns false (as it assumes no one
   // has implemented the interface).  This default implementation is temporary.
-  // TODO(mellem):  Change this to pure virtual.
-  virtual bool IsReadyToSend() const;
+  virtual bool IsReadyToSend() const = 0;
 };
 
 }  // namespace webrtc