diff --git a/api/mediastreaminterface.h b/api/mediastreaminterface.h
index 5d65447..c3966dc 100644
--- a/api/mediastreaminterface.h
+++ b/api/mediastreaminterface.h
@@ -300,6 +300,7 @@
 class MediaStreamInterface : public rtc::RefCountInterface,
                              public NotifierInterface {
  public:
+  // TODO(steveanton): This could be renamed to id() to match the spec.
   virtual std::string label() const = 0;
 
   virtual AudioTrackVector GetAudioTracks() = 0;
diff --git a/pc/mediasession.cc b/pc/mediasession.cc
index 2d9fd94..e1c3dd4 100644
--- a/pc/mediasession.cc
+++ b/pc/mediasession.cc
@@ -498,7 +498,9 @@
         }
       }
       stream_param.cname = rtcp_cname;
-      stream_param.sync_label = sender.stream_id;
+      // TODO(steveanton): Support any number of stream ids.
+      RTC_CHECK(sender.stream_ids.size() == 1U);
+      stream_param.sync_label = sender.stream_ids[0];
       content_description->AddStream(stream_param);
 
       // Store the new StreamParams in current_streams.
@@ -508,7 +510,9 @@
       // Use existing generated SSRCs/groups, but update the sync_label if
       // necessary. This may be needed if a MediaStreamTrack was moved from one
       // MediaStream to another.
-      param->sync_label = sender.stream_id;
+      // TODO(steveanton): Support any number of stream ids.
+      RTC_CHECK(sender.stream_ids.size() == 1U);
+      param->sync_label = sender.stream_ids[0];
       content_description->AddStream(*param);
     }
   }
@@ -1250,29 +1254,36 @@
   return dir_str;
 }
 
-void MediaDescriptionOptions::AddAudioSender(const std::string& track_id,
-                                             const std::string& stream_id) {
+void MediaDescriptionOptions::AddAudioSender(
+    const std::string& track_id,
+    const std::vector<std::string>& stream_ids) {
   RTC_DCHECK(type == MEDIA_TYPE_AUDIO);
-  AddSenderInternal(track_id, stream_id, 1);
+  AddSenderInternal(track_id, stream_ids, 1);
 }
 
-void MediaDescriptionOptions::AddVideoSender(const std::string& track_id,
-                                             const std::string& stream_id,
-                                             int num_sim_layers) {
+void MediaDescriptionOptions::AddVideoSender(
+    const std::string& track_id,
+    const std::vector<std::string>& stream_ids,
+    int num_sim_layers) {
   RTC_DCHECK(type == MEDIA_TYPE_VIDEO);
-  AddSenderInternal(track_id, stream_id, num_sim_layers);
+  AddSenderInternal(track_id, stream_ids, num_sim_layers);
 }
 
 void MediaDescriptionOptions::AddRtpDataChannel(const std::string& track_id,
                                                 const std::string& stream_id) {
   RTC_DCHECK(type == MEDIA_TYPE_DATA);
-  AddSenderInternal(track_id, stream_id, 1);
+  // TODO(steveanton): Is it the case that RtpDataChannel will never have more
+  // than one stream?
+  AddSenderInternal(track_id, {stream_id}, 1);
 }
 
-void MediaDescriptionOptions::AddSenderInternal(const std::string& track_id,
-                                                const std::string& stream_id,
-                                                int num_sim_layers) {
-  sender_options.push_back(SenderOptions{track_id, stream_id, num_sim_layers});
+void MediaDescriptionOptions::AddSenderInternal(
+    const std::string& track_id,
+    const std::vector<std::string>& stream_ids,
+    int num_sim_layers) {
+  // TODO(steveanton): Support any number of stream ids.
+  RTC_CHECK(stream_ids.size() == 1U);
+  sender_options.push_back(SenderOptions{track_id, stream_ids, num_sim_layers});
 }
 
 bool MediaSessionOptions::HasMediaDescription(MediaType type) const {
diff --git a/pc/mediasession.h b/pc/mediasession.h
index 35bd444..ee7183f 100644
--- a/pc/mediasession.h
+++ b/pc/mediasession.h
@@ -105,7 +105,9 @@
 // Options for an RtpSender contained with an media description/"m=" section.
 struct SenderOptions {
   std::string track_id;
-  std::string stream_id;
+  // TODO(steveanton): As part of work towards Unified Plan, this has been
+  // changed to be a vector. But for now this can only have exactly one.
+  std::vector<std::string> stream_ids;
   int num_sim_layers;
 };
 
@@ -120,9 +122,9 @@
   // TODO(deadbeef): When we don't support Plan B, there will only be one
   // sender per media description and this can be simplified.
   void AddAudioSender(const std::string& track_id,
-                      const std::string& stream_id);
+                      const std::vector<std::string>& stream_ids);
   void AddVideoSender(const std::string& track_id,
-                      const std::string& stream_id,
+                      const std::vector<std::string>& stream_ids,
                       int num_sim_layers);
 
   // Internally just uses sender_options.
@@ -141,7 +143,7 @@
  private:
   // Doesn't DCHECK on |type|.
   void AddSenderInternal(const std::string& track_id,
-                         const std::string& stream_id,
+                         const std::vector<std::string>& stream_ids,
                          int num_sim_layers);
 };
 
diff --git a/pc/mediasession_unittest.cc b/pc/mediasession_unittest.cc
index f87dda1..28883f2 100644
--- a/pc/mediasession_unittest.cc
+++ b/pc/mediasession_unittest.cc
@@ -284,22 +284,24 @@
   AddMediaSection(MEDIA_TYPE_DATA, "data", direction, kActive, opts);
 }
 
-static void AttachSenderToMediaSection(const std::string& mid,
-                                       MediaType type,
-                                       const std::string& track_id,
-                                       const std::string& stream_id,
-                                       int num_sim_layer,
-                                       MediaSessionOptions* session_options) {
+static void AttachSenderToMediaSection(
+    const std::string& mid,
+    MediaType type,
+    const std::string& track_id,
+    const std::vector<std::string>& stream_ids,
+    int num_sim_layer,
+    MediaSessionOptions* session_options) {
   auto it = FindFirstMediaDescriptionByMid(mid, session_options);
   switch (type) {
     case MEDIA_TYPE_AUDIO:
-      it->AddAudioSender(track_id, stream_id);
+      it->AddAudioSender(track_id, stream_ids);
       break;
     case MEDIA_TYPE_VIDEO:
-      it->AddVideoSender(track_id, stream_id, num_sim_layer);
+      it->AddVideoSender(track_id, stream_ids, num_sim_layer);
       break;
     case MEDIA_TYPE_DATA:
-      it->AddRtpDataChannel(track_id, stream_id);
+      RTC_CHECK(stream_ids.size() == 1U);
+      it->AddRtpDataChannel(track_id, stream_ids[0]);
       break;
     default:
       RTC_NOTREACHED();
@@ -882,9 +884,9 @@
   MediaSessionOptions opts;
   AddAudioVideoSections(cricket::MD_SENDONLY, &opts);
   AttachSenderToMediaSection("video", MEDIA_TYPE_VIDEO, kVideoTrack1,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
   AttachSenderToMediaSection("audio", MEDIA_TYPE_AUDIO, kAudioTrack1,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
 
   std::unique_ptr<SessionDescription> offer(f1_.CreateOffer(opts, NULL));
   ASSERT_TRUE(offer.get() != NULL);
@@ -1652,17 +1654,17 @@
   MediaSessionOptions opts;
   AddAudioVideoSections(cricket::MD_SENDRECV, &opts);
   AttachSenderToMediaSection("video", MEDIA_TYPE_VIDEO, kVideoTrack1,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
   AttachSenderToMediaSection("audio", MEDIA_TYPE_AUDIO, kAudioTrack1,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
   AttachSenderToMediaSection("audio", MEDIA_TYPE_AUDIO, kAudioTrack2,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
 
   AddDataSection(cricket::DCT_RTP, cricket::MD_SENDRECV, &opts);
   AttachSenderToMediaSection("data", MEDIA_TYPE_DATA, kDataTrack1,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
   AttachSenderToMediaSection("data", MEDIA_TYPE_DATA, kDataTrack2,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
 
   f1_.set_secure(SEC_ENABLED);
   std::unique_ptr<SessionDescription> offer(f1_.CreateOffer(opts, NULL));
@@ -1730,13 +1732,13 @@
   // Update the offer. Add a new video track that is not synched to the
   // other tracks and replace audio track 2 with audio track 3.
   AttachSenderToMediaSection("video", MEDIA_TYPE_VIDEO, kVideoTrack2,
-                             kMediaStream2, 1, &opts);
+                             {kMediaStream2}, 1, &opts);
   DetachSenderFromMediaSection("audio", kAudioTrack2, &opts);
   AttachSenderToMediaSection("audio", MEDIA_TYPE_AUDIO, kAudioTrack3,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
   DetachSenderFromMediaSection("data", kDataTrack2, &opts);
   AttachSenderToMediaSection("data", MEDIA_TYPE_DATA, kDataTrack3,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
   std::unique_ptr<SessionDescription> updated_offer(
       f1_.CreateOffer(opts, offer.get()));
 
@@ -1804,7 +1806,7 @@
                   &opts);
   const int num_sim_layers = 3;
   AttachSenderToMediaSection("video", MEDIA_TYPE_VIDEO, kVideoTrack1,
-                             kMediaStream1, num_sim_layers, &opts);
+                             {kMediaStream1}, num_sim_layers, &opts);
   std::unique_ptr<SessionDescription> offer(f1_.CreateOffer(opts, NULL));
 
   ASSERT_TRUE(offer.get() != NULL);
@@ -1847,18 +1849,18 @@
   AddMediaSection(MEDIA_TYPE_VIDEO, "video", cricket::MD_SENDRECV, kActive,
                   &answer_opts);
   AttachSenderToMediaSection("video", MEDIA_TYPE_VIDEO, kVideoTrack1,
-                             kMediaStream1, 1, &answer_opts);
+                             {kMediaStream1}, 1, &answer_opts);
   AttachSenderToMediaSection("audio", MEDIA_TYPE_AUDIO, kAudioTrack1,
-                             kMediaStream1, 1, &answer_opts);
+                             {kMediaStream1}, 1, &answer_opts);
   AttachSenderToMediaSection("audio", MEDIA_TYPE_AUDIO, kAudioTrack2,
-                             kMediaStream1, 1, &answer_opts);
+                             {kMediaStream1}, 1, &answer_opts);
 
   AddMediaSection(MEDIA_TYPE_DATA, "data", cricket::MD_SENDRECV, kActive,
                   &answer_opts);
   AttachSenderToMediaSection("data", MEDIA_TYPE_DATA, kDataTrack1,
-                             kMediaStream1, 1, &answer_opts);
+                             {kMediaStream1}, 1, &answer_opts);
   AttachSenderToMediaSection("data", MEDIA_TYPE_DATA, kDataTrack2,
-                             kMediaStream1, 1, &answer_opts);
+                             {kMediaStream1}, 1, &answer_opts);
   answer_opts.data_channel_type = cricket::DCT_RTP;
 
   std::unique_ptr<SessionDescription> answer(
@@ -1927,7 +1929,7 @@
   // Update the answer. Add a new video track that is not synched to the
   // other tracks and remove 1 audio track.
   AttachSenderToMediaSection("video", MEDIA_TYPE_VIDEO, kVideoTrack2,
-                             kMediaStream2, 1, &answer_opts);
+                             {kMediaStream2}, 1, &answer_opts);
   DetachSenderFromMediaSection("audio", kAudioTrack2, &answer_opts);
   DetachSenderFromMediaSection("data", kDataTrack2, &answer_opts);
   std::unique_ptr<SessionDescription> updated_answer(
@@ -2325,7 +2327,7 @@
                   &opts);
   // Add simulcast streams.
   AttachSenderToMediaSection("video", MEDIA_TYPE_VIDEO, "stream1",
-                             "stream1label", 3, &opts);
+                             {"stream1label"}, 3, &opts);
 
   // Use a single real codec, and then add RTX for it.
   std::vector<VideoCodec> f1_codecs;
@@ -2366,7 +2368,7 @@
                   &opts);
   // Add single stream.
   AttachSenderToMediaSection("video", MEDIA_TYPE_VIDEO, "stream1",
-                             "stream1label", 1, &opts);
+                             {"stream1label"}, 1, &opts);
 
   // Use a single real codec, and then add FlexFEC for it.
   std::vector<VideoCodec> f1_codecs;
@@ -2406,7 +2408,7 @@
                   &opts);
   // Add simulcast streams.
   AttachSenderToMediaSection("video", MEDIA_TYPE_VIDEO, "stream1",
-                             "stream1label", 3, &opts);
+                             {"stream1label"}, 3, &opts);
 
   // Use a single real codec, and then add FlexFEC for it.
   std::vector<VideoCodec> f1_codecs;
@@ -3018,22 +3020,22 @@
   AddMediaSection(MEDIA_TYPE_AUDIO, "audio_1", cricket::MD_SENDRECV, kActive,
                   &opts);
   AttachSenderToMediaSection("audio_1", MEDIA_TYPE_AUDIO, kAudioTrack1,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
 
   AddMediaSection(MEDIA_TYPE_VIDEO, "video_1", cricket::MD_SENDRECV, kActive,
                   &opts);
   AttachSenderToMediaSection("video_1", MEDIA_TYPE_VIDEO, kVideoTrack1,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
 
   AddMediaSection(MEDIA_TYPE_AUDIO, "audio_2", cricket::MD_SENDRECV, kActive,
                   &opts);
   AttachSenderToMediaSection("audio_2", MEDIA_TYPE_AUDIO, kAudioTrack2,
-                             kMediaStream2, 1, &opts);
+                             {kMediaStream2}, 1, &opts);
 
   AddMediaSection(MEDIA_TYPE_VIDEO, "video_2", cricket::MD_SENDRECV, kActive,
                   &opts);
   AttachSenderToMediaSection("video_2", MEDIA_TYPE_VIDEO, kVideoTrack2,
-                             kMediaStream2, 1, &opts);
+                             {kMediaStream2}, 1, &opts);
   std::unique_ptr<SessionDescription> offer(f1_.CreateOffer(opts, nullptr));
   ASSERT_TRUE(offer);
 
@@ -3077,22 +3079,22 @@
   AddMediaSection(MEDIA_TYPE_AUDIO, "audio_1", cricket::MD_SENDRECV, kActive,
                   &opts);
   AttachSenderToMediaSection("audio_1", MEDIA_TYPE_AUDIO, kAudioTrack1,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
 
   AddMediaSection(MEDIA_TYPE_VIDEO, "video_1", cricket::MD_SENDRECV, kActive,
                   &opts);
   AttachSenderToMediaSection("video_1", MEDIA_TYPE_VIDEO, kVideoTrack1,
-                             kMediaStream1, 1, &opts);
+                             {kMediaStream1}, 1, &opts);
 
   AddMediaSection(MEDIA_TYPE_AUDIO, "audio_2", cricket::MD_SENDRECV, kActive,
                   &opts);
   AttachSenderToMediaSection("audio_2", MEDIA_TYPE_AUDIO, kAudioTrack2,
-                             kMediaStream2, 1, &opts);
+                             {kMediaStream2}, 1, &opts);
 
   AddMediaSection(MEDIA_TYPE_VIDEO, "video_2", cricket::MD_SENDRECV, kActive,
                   &opts);
   AttachSenderToMediaSection("video_2", MEDIA_TYPE_VIDEO, kVideoTrack2,
-                             kMediaStream2, 1, &opts);
+                             {kMediaStream2}, 1, &opts);
 
   std::unique_ptr<SessionDescription> offer(f1_.CreateOffer(opts, nullptr));
   ASSERT_TRUE(offer);
@@ -3487,7 +3489,7 @@
 
   if (RtpTransceiverDirection::FromMediaContentDirection(direction).send) {
     AttachSenderToMediaSection("audio", MEDIA_TYPE_AUDIO, kAudioTrack1,
-                               kMediaStream1, 1, &opts);
+                               {kMediaStream1}, 1, &opts);
   }
 
   std::unique_ptr<SessionDescription> offer(sf.CreateOffer(opts, NULL));
@@ -3587,7 +3589,7 @@
   if (RtpTransceiverDirection::FromMediaContentDirection(offer_direction)
           .send) {
     AttachSenderToMediaSection("audio", MEDIA_TYPE_AUDIO, kAudioTrack1,
-                               kMediaStream1, 1, &offer_opts);
+                               {kMediaStream1}, 1, &offer_opts);
   }
 
   std::unique_ptr<SessionDescription> offer(
@@ -3601,7 +3603,7 @@
   if (RtpTransceiverDirection::FromMediaContentDirection(answer_direction)
           .send) {
     AttachSenderToMediaSection("audio", MEDIA_TYPE_AUDIO, kAudioTrack1,
-                               kMediaStream1, 1, &answer_opts);
+                               {kMediaStream1}, 1, &answer_opts);
   }
   std::unique_ptr<SessionDescription> answer(
       answer_factory.CreateAnswer(offer.get(), answer_opts, NULL));
diff --git a/pc/peerconnection.cc b/pc/peerconnection.cc
index b258c1f..64b842b 100644
--- a/pc/peerconnection.cc
+++ b/pc/peerconnection.cc
@@ -143,13 +143,13 @@
     if (sender->media_type() == cricket::MEDIA_TYPE_AUDIO) {
       if (audio_media_description_options) {
         audio_media_description_options->AddAudioSender(
-            sender->id(), sender->internal()->stream_id());
+            sender->id(), sender->internal()->stream_ids());
       }
     } else {
       RTC_DCHECK(sender->media_type() == cricket::MEDIA_TYPE_VIDEO);
       if (video_media_description_options) {
         video_media_description_options->AddVideoSender(
-            sender->id(), sender->internal()->stream_id(), 1);
+            sender->id(), sender->internal()->stream_ids(), 1);
       }
     }
   }
@@ -1475,8 +1475,8 @@
   rtc::scoped_refptr<RtpSenderProxyWithInternal<RtpSenderInternal>> new_sender =
       RtpSenderProxyWithInternal<RtpSenderInternal>::Create(
           signaling_thread(),
-          new AudioRtpSender(track, stream->label(), session_->voice_channel(),
-                             stats_.get()));
+          new AudioRtpSender(track, {stream->label()},
+                             session_->voice_channel(), stats_.get()));
   senders_.push_back(new_sender);
   // If the sender has already been configured in SDP, we call SetSsrc,
   // which will connect the sender to the underlying transport. This can
@@ -1520,7 +1520,7 @@
   // Normal case; we've never seen this track before.
   rtc::scoped_refptr<RtpSenderProxyWithInternal<RtpSenderInternal>> new_sender =
       RtpSenderProxyWithInternal<RtpSenderInternal>::Create(
-          signaling_thread(), new VideoRtpSender(track, stream->label(),
+          signaling_thread(), new VideoRtpSender(track, {stream->label()},
                                                  session_->video_channel()));
   senders_.push_back(new_sender);
   const TrackInfo* track_info =
diff --git a/pc/rtpsender.cc b/pc/rtpsender.cc
index 9d920e6..c95befd 100644
--- a/pc/rtpsender.cc
+++ b/pc/rtpsender.cc
@@ -45,16 +45,19 @@
 }
 
 AudioRtpSender::AudioRtpSender(AudioTrackInterface* track,
-                               const std::string& stream_id,
+                               const std::vector<std::string>& stream_ids,
                                cricket::VoiceChannel* channel,
                                StatsCollector* stats)
     : id_(track->id()),
-      stream_id_(stream_id),
+      stream_ids_(stream_ids),
       channel_(channel),
       stats_(stats),
       track_(track),
       cached_track_enabled_(track->enabled()),
       sink_adapter_(new LocalAudioSinkAdapter()) {
+  // TODO(steveanton): Relax this constraint once more Unified Plan work is
+  // done.
+  RTC_CHECK(stream_ids_.size() == 1U);
   track_->RegisterObserver(this);
   track_->AddSink(sink_adapter_.get());
   CreateDtmfSender();
@@ -64,7 +67,8 @@
                                cricket::VoiceChannel* channel,
                                StatsCollector* stats)
     : id_(track->id()),
-      stream_id_(rtc::CreateRandomUuid()),
+      // TODO(steveanton): With Unified Plan this should be empty.
+      stream_ids_({rtc::CreateRandomUuid()}),
       channel_(channel),
       stats_(stats),
       track_(track),
@@ -78,7 +82,8 @@
 AudioRtpSender::AudioRtpSender(cricket::VoiceChannel* channel,
                                StatsCollector* stats)
     : id_(rtc::CreateRandomUuid()),
-      stream_id_(rtc::CreateRandomUuid()),
+      // TODO(steveanton): With Unified Plan this should be empty.
+      stream_ids_({rtc::CreateRandomUuid()}),
       channel_(channel),
       stats_(stats),
       sink_adapter_(new LocalAudioSinkAdapter()) {
@@ -297,21 +302,25 @@
 }
 
 VideoRtpSender::VideoRtpSender(VideoTrackInterface* track,
-                               const std::string& stream_id,
+                               const std::vector<std::string>& stream_ids,
                                cricket::VideoChannel* channel)
     : id_(track->id()),
-      stream_id_(stream_id),
+      stream_ids_({stream_ids}),
       channel_(channel),
       track_(track),
       cached_track_enabled_(track->enabled()),
       cached_track_content_hint_(track->content_hint()) {
+  // TODO(steveanton): Relax this constraint once more Unified Plan work is
+  // done.
+  RTC_CHECK(stream_ids_.size() == 1U);
   track_->RegisterObserver(this);
 }
 
 VideoRtpSender::VideoRtpSender(VideoTrackInterface* track,
                                cricket::VideoChannel* channel)
     : id_(track->id()),
-      stream_id_(rtc::CreateRandomUuid()),
+      // TODO(steveanton): With Unified Plan this should be empty.
+      stream_ids_({rtc::CreateRandomUuid()}),
       channel_(channel),
       track_(track),
       cached_track_enabled_(track->enabled()),
@@ -321,7 +330,8 @@
 
 VideoRtpSender::VideoRtpSender(cricket::VideoChannel* channel)
     : id_(rtc::CreateRandomUuid()),
-      stream_id_(rtc::CreateRandomUuid()),
+      // TODO(steveanton): With Unified Plan this should be empty.
+      stream_ids_({rtc::CreateRandomUuid()}),
       channel_(channel) {}
 
 VideoRtpSender::~VideoRtpSender() {
diff --git a/pc/rtpsender.h b/pc/rtpsender.h
index 24edb4b..de679cf 100644
--- a/pc/rtpsender.h
+++ b/pc/rtpsender.h
@@ -40,9 +40,13 @@
   // description).
   virtual void SetSsrc(uint32_t ssrc) = 0;
 
-  // TODO(deadbeef): Support one sender having multiple stream ids.
+  // TODO(steveanton): With Unified Plan, a track/RTCRTPSender can be part of
+  // multiple streams (or no stream at all). Replace these singular methods with
+  // their corresponding plural methods.
+  // Until these are removed, RtpSenders must have exactly one stream.
   virtual void set_stream_id(const std::string& stream_id) = 0;
   virtual std::string stream_id() const = 0;
+  virtual void set_stream_ids(const std::vector<std::string>& stream_ids) = 0;
 
   virtual void Stop() = 0;
 };
@@ -79,7 +83,7 @@
   // at the appropriate times.
   // |channel| can be null if one does not exist yet.
   AudioRtpSender(AudioTrackInterface* track,
-                 const std::string& stream_id,
+                 const std::vector<std::string>& stream_id,
                  cricket::VoiceChannel* channel,
                  StatsCollector* stats);
 
@@ -117,10 +121,7 @@
 
   std::string id() const override { return id_; }
 
-  std::vector<std::string> stream_ids() const override {
-    std::vector<std::string> ret = {stream_id_};
-    return ret;
-  }
+  std::vector<std::string> stream_ids() const override { return stream_ids_; }
 
   RtpParameters GetParameters() const override;
   bool SetParameters(const RtpParameters& parameters) override;
@@ -131,9 +132,12 @@
   void SetSsrc(uint32_t ssrc) override;
 
   void set_stream_id(const std::string& stream_id) override {
-    stream_id_ = stream_id;
+    stream_ids_ = {stream_id};
   }
-  std::string stream_id() const override { return stream_id_; }
+  std::string stream_id() const override { return stream_ids_[0]; }
+  void set_stream_ids(const std::vector<std::string>& stream_ids) override {
+    stream_ids_ = stream_ids;
+  }
 
   void Stop() override;
 
@@ -156,7 +160,9 @@
   sigslot::signal0<> SignalDestroyed;
 
   std::string id_;
-  std::string stream_id_;
+  // TODO(steveanton): Until more Unified Plan work is done, this can only have
+  // exactly one element.
+  std::vector<std::string> stream_ids_;
   cricket::VoiceChannel* channel_ = nullptr;
   StatsCollector* stats_;
   rtc::scoped_refptr<AudioTrackInterface> track_;
@@ -175,7 +181,7 @@
  public:
   // |channel| can be null if one does not exist yet.
   VideoRtpSender(VideoTrackInterface* track,
-                 const std::string& stream_id,
+                 const std::vector<std::string>& stream_id,
                  cricket::VideoChannel* channel);
 
   // Randomly generates stream_id.
@@ -205,10 +211,7 @@
 
   std::string id() const override { return id_; }
 
-  std::vector<std::string> stream_ids() const override {
-    std::vector<std::string> ret = {stream_id_};
-    return ret;
-  }
+  std::vector<std::string> stream_ids() const override { return stream_ids_; }
 
   RtpParameters GetParameters() const override;
   bool SetParameters(const RtpParameters& parameters) override;
@@ -219,9 +222,12 @@
   void SetSsrc(uint32_t ssrc) override;
 
   void set_stream_id(const std::string& stream_id) override {
-    stream_id_ = stream_id;
+    stream_ids_ = {stream_id};
   }
-  std::string stream_id() const override { return stream_id_; }
+  std::string stream_id() const override { return stream_ids_[0]; }
+  void set_stream_ids(const std::vector<std::string>& stream_ids) override {
+    stream_ids_ = stream_ids;
+  }
 
   void Stop() override;
 
@@ -238,7 +244,9 @@
   void ClearVideoSend();
 
   std::string id_;
-  std::string stream_id_;
+  // TODO(steveanton): Until more Unified Plan work is done, this can only have
+  // exactly one element.
+  std::vector<std::string> stream_ids_;
   cricket::VideoChannel* channel_ = nullptr;
   rtc::scoped_refptr<VideoTrackInterface> track_;
   uint32_t ssrc_ = 0;
diff --git a/pc/rtpsenderreceiver_unittest.cc b/pc/rtpsenderreceiver_unittest.cc
index 9779eb6..a00b980 100644
--- a/pc/rtpsenderreceiver_unittest.cc
+++ b/pc/rtpsenderreceiver_unittest.cc
@@ -136,7 +136,7 @@
     EXPECT_TRUE(local_stream_->AddTrack(audio_track_));
     audio_rtp_sender_ =
         new AudioRtpSender(local_stream_->GetAudioTracks()[0],
-                           local_stream_->label(), voice_channel_, nullptr);
+                           {local_stream_->label()}, voice_channel_, nullptr);
     audio_rtp_sender_->SetSsrc(kAudioSsrc);
     audio_rtp_sender_->GetOnDestroyedSignal()->connect(
         this, &RtpSenderReceiverTest::OnAudioSenderDestroyed);
@@ -151,7 +151,7 @@
     AddVideoTrack(is_screencast);
     video_rtp_sender_ =
         new VideoRtpSender(local_stream_->GetVideoTracks()[0],
-                           local_stream_->label(), video_channel_);
+                           {local_stream_->label()}, video_channel_);
     video_rtp_sender_->SetSsrc(kVideoSsrc);
     VerifyVideoChannelInput();
   }
@@ -715,7 +715,7 @@
   video_track_->set_content_hint(VideoTrackInterface::ContentHint::kDetailed);
   video_rtp_sender_ =
       new VideoRtpSender(local_stream_->GetVideoTracks()[0],
-                         local_stream_->label(), video_channel_);
+                         {local_stream_->label()}, video_channel_);
   video_track_->set_enabled(true);
 
   // Sender is not ready to send (no SSRC) so no option should have been set.
diff --git a/pc/webrtcsession_unittest.cc b/pc/webrtcsession_unittest.cc
index 6c9dcec..2088327 100644
--- a/pc/webrtcsession_unittest.cc
+++ b/pc/webrtcsession_unittest.cc
@@ -596,10 +596,10 @@
             cricket::MEDIA_TYPE_AUDIO, media_description_options.mid,
             cricket::RtpTransceiverDirection(send_audio, recv_audio), stopped);
         if (send_stream_1_ && send_audio) {
-          media_desc_options.AddAudioSender(kAudioTrack1, kStream1);
+          media_desc_options.AddAudioSender(kAudioTrack1, {kStream1});
         }
         if (send_stream_2_ && send_audio) {
-          media_desc_options.AddAudioSender(kAudioTrack2, kStream2);
+          media_desc_options.AddAudioSender(kAudioTrack2, {kStream2});
         }
         session_options->media_description_options.push_back(
             media_desc_options);
@@ -609,11 +609,11 @@
             cricket::MEDIA_TYPE_VIDEO, media_description_options.mid,
             cricket::RtpTransceiverDirection(send_video, recv_video), stopped);
         if (send_stream_1_ && send_video) {
-          media_desc_options.AddVideoSender(kVideoTrack1, kStream1,
+          media_desc_options.AddVideoSender(kVideoTrack1, {kStream1},
                                             num_sim_layer);
         }
         if (send_stream_2_ && send_video) {
-          media_desc_options.AddVideoSender(kVideoTrack2, kStream2,
+          media_desc_options.AddVideoSender(kVideoTrack2, {kStream2},
                                             num_sim_layer);
         }
         session_options->media_description_options.push_back(
@@ -649,10 +649,10 @@
               cricket::RtpTransceiverDirection(send_audio, recv_audio),
               kActive);
       if (send_stream_1_ && send_audio) {
-        media_desc_options.AddAudioSender(kAudioTrack1, kStream1);
+        media_desc_options.AddAudioSender(kAudioTrack1, {kStream1});
       }
       if (send_stream_2_ && send_audio) {
-        media_desc_options.AddAudioSender(kAudioTrack2, kStream2);
+        media_desc_options.AddAudioSender(kAudioTrack2, {kStream2});
       }
       session_options->media_description_options.push_back(media_desc_options);
       offered_media_sections_.push_back(media_desc_options);
@@ -666,11 +666,11 @@
               kActive);
       int num_sim_layer = 1;
       if (send_stream_1_ && send_video) {
-        media_desc_options.AddVideoSender(kVideoTrack1, kStream1,
+        media_desc_options.AddVideoSender(kVideoTrack1, {kStream1},
                                           num_sim_layer);
       }
       if (send_stream_2_ && send_video) {
-        media_desc_options.AddVideoSender(kVideoTrack2, kStream2,
+        media_desc_options.AddVideoSender(kVideoTrack2, {kStream2},
                                           num_sim_layer);
       }
       session_options->media_description_options.push_back(media_desc_options);