Adding support for Unified Plan offer/answer negotiation.

webrtc/pc/peerconnection.cc

Index: webrtc/pc/peerconnection.cc
diff --git a/webrtc/pc/peerconnection.cc b/webrtc/pc/peerconnection.cc
index 3af56671af451927f6b1ecc4a687695a0c14cd41..454a1893b1934b9e9becd23446fca44b6a0ffde0 100644
--- a/webrtc/pc/peerconnection.cc
+++ b/webrtc/pc/peerconnection.cc
@@ -133,32 +133,45 @@ bool IsValidOfferToReceiveMedia(int value) {
          (value <= Options::kMaxOfferToReceiveMedia);
 }
 
-// Add the stream and RTP data channel info to |session_options|.
-void AddSendStreams(
-    cricket::MediaSessionOptions* session_options,
+// Add options to |[audio/video]_media_description_options| from |senders|.
+void AddRtpSenderOptions(
     const std::vector<rtc::scoped_refptr<
         RtpSenderProxyWithInternal<RtpSenderInternal>>>& senders,
-    const std::map<std::string, rtc::scoped_refptr<DataChannel>>&
-        rtp_data_channels) {
-  session_options->streams.clear();
+    cricket::MediaDescriptionOptions* audio_media_description_options,
+    cricket::MediaDescriptionOptions* video_media_description_options) {
   for (const auto& sender : senders) {
-    session_options->AddSendStream(sender->media_type(), sender->id(),
-                                   sender->internal()->stream_id());
+    if (sender->media_type() == cricket::MEDIA_TYPE_AUDIO) {
+      if (audio_media_description_options) {
+        audio_media_description_options->AddAudioSender(
+            sender->id(), sender->internal()->stream_id());
+      }
+    } 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);
+      }
+    }
   }
+}
 
+// Add options to |session_options| from |rtp_data_channels|.
+void AddRtpDataChannelOptions(
+    const std::map<std::string, rtc::scoped_refptr<DataChannel>>&
+        rtp_data_channels,
+    cricket::MediaDescriptionOptions* data_media_description_options) {
+  if (!data_media_description_options) {
+    return;
+  }
   // Check for data channels.
   for (const auto& kv : rtp_data_channels) {
     const DataChannel* channel = kv.second;
     if (channel->state() == DataChannel::kConnecting ||
         channel->state() == DataChannel::kOpen) {
-      // |streamid| and |sync_label| are both set to the DataChannel label
-      // here so they can be signaled the same way as MediaStreams and Tracks.
-      // For MediaStreams, the sync_label is the MediaStream label and the
-      // track label is the same as |streamid|.
-      const std::string& streamid = channel->label();
-      const std::string& sync_label = channel->label();
-      session_options->AddSendStream(cricket::MEDIA_TYPE_DATA, streamid,
-                                     sync_label);
+      // Legacy RTP data channels are signaled with the track/stream ID set to
+      // the data channel's label.
+      data_media_description_options->AddRtpDataChannel(channel->label(),
+                                                        channel->label());
     }
   }
 }
@@ -314,92 +327,62 @@ std::string GenerateRtcpCname() {
   return cname;
 }
 
-bool ExtractMediaSessionOptions(
+bool ValidateOfferAnswerOptions(
+    const PeerConnectionInterface::RTCOfferAnswerOptions& rtc_options) {
+  return IsValidOfferToReceiveMedia(rtc_options.offer_to_receive_audio) &&
+         IsValidOfferToReceiveMedia(rtc_options.offer_to_receive_video);
+}
+
+// From |rtc_options|, fill parts of |session_options| shared by all generated
+// m= sections (in other words, nothing that involves a map/array).
+void ExtractSharedMediaSessionOptions(
     const PeerConnectionInterface::RTCOfferAnswerOptions& rtc_options,
-    bool is_offer,
     cricket::MediaSessionOptions* session_options) {
-  typedef PeerConnectionInterface::RTCOfferAnswerOptions RTCOfferAnswerOptions;
-  if (!IsValidOfferToReceiveMedia(rtc_options.offer_to_receive_audio) ||
-      !IsValidOfferToReceiveMedia(rtc_options.offer_to_receive_video)) {
-    return false;
-  }
-
-  // If constraints don't prevent us, we always accept video.
-  if (rtc_options.offer_to_receive_audio != RTCOfferAnswerOptions::kUndefined) {
-    session_options->recv_audio = (rtc_options.offer_to_receive_audio > 0);
-  } else {
-    session_options->recv_audio = true;
-  }
-  // For offers, we only offer video if we have it or it's forced by options.
-  // For answers, we will always accept video (if offered).
-  if (rtc_options.offer_to_receive_video != RTCOfferAnswerOptions::kUndefined) {
-    session_options->recv_video = (rtc_options.offer_to_receive_video > 0);
-  } else if (is_offer) {
-    session_options->recv_video = false;
-  } else {
-    session_options->recv_video = true;
-  }
-
   session_options->vad_enabled = rtc_options.voice_activity_detection;
   session_options->bundle_enabled = rtc_options.use_rtp_mux;
-  for (auto& kv : session_options->transport_options) {
-    kv.second.ice_restart = rtc_options.ice_restart;
-  }
-
-  return true;
 }
 
-bool ParseConstraintsForAnswer(const MediaConstraintsInterface* constraints,
-                               cricket::MediaSessionOptions* session_options) {
+bool ConvertConstraintsToOfferAnswerOptions(
+    const MediaConstraintsInterface* constraints,
+    PeerConnectionInterface::RTCOfferAnswerOptions* offer_answer_options) {
+  if (!constraints) {
+    return true;
+  }
+
   bool value = false;
   size_t mandatory_constraints_satisfied = 0;
 
-  // kOfferToReceiveAudio defaults to true according to spec.
-  if (!FindConstraint(constraints,
-                      MediaConstraintsInterface::kOfferToReceiveAudio, &value,
-                      &mandatory_constraints_satisfied) ||
-      value) {
-    session_options->recv_audio = true;
+  if (FindConstraint(constraints,
+                     MediaConstraintsInterface::kOfferToReceiveAudio, &value,
+                     &mandatory_constraints_satisfied)) {
+    offer_answer_options->offer_to_receive_audio =
+        value ? PeerConnectionInterface::RTCOfferAnswerOptions::
+                    kOfferToReceiveMediaTrue
+              : 0;
   }
 
-  // kOfferToReceiveVideo defaults to false according to spec. But
-  // if it is an answer and video is offered, we should still accept video
-  // per default.
-  value = false;
-  if (!FindConstraint(constraints,
-                      MediaConstraintsInterface::kOfferToReceiveVideo, &value,
-                      &mandatory_constraints_satisfied) ||
-      value) {
-    session_options->recv_video = true;
+  if (FindConstraint(constraints,
+                     MediaConstraintsInterface::kOfferToReceiveVideo, &value,
+                     &mandatory_constraints_satisfied)) {
+    offer_answer_options->offer_to_receive_video =
+        value ? PeerConnectionInterface::RTCOfferAnswerOptions::
+                    kOfferToReceiveMediaTrue
+              : 0;
   }
-
   if (FindConstraint(constraints,
                      MediaConstraintsInterface::kVoiceActivityDetection, &value,
                      &mandatory_constraints_satisfied)) {
-    session_options->vad_enabled = value;
+    offer_answer_options->voice_activity_detection = value;
   }
-
   if (FindConstraint(constraints, MediaConstraintsInterface::kUseRtpMux, &value,
                      &mandatory_constraints_satisfied)) {
-    session_options->bundle_enabled = value;
-  } else {
-    // kUseRtpMux defaults to true according to spec.
-    session_options->bundle_enabled = true;
+    offer_answer_options->use_rtp_mux = value;
   }
-
-  bool ice_restart = false;
   if (FindConstraint(constraints, MediaConstraintsInterface::kIceRestart,
                      &value, &mandatory_constraints_satisfied)) {
-    // kIceRestart defaults to false according to spec.
-    ice_restart = true;
-  }
-  for (auto& kv : session_options->transport_options) {
-    kv.second.ice_restart = ice_restart;
+    offer_answer_options->ice_restart = value;
   }
 
-  if (!constraints) {
-    return true;
-  }
   return mandatory_constraints_satisfied == constraints->GetMandatory().size();
 }
 
@@ -842,49 +825,16 @@ void PeerConnection::CreateOffer(CreateSessionDescriptionObserver* observer,
     LOG(LS_ERROR) << "CreateOffer - observer is NULL.";
     return;
   }
-  RTCOfferAnswerOptions options;
-
-  bool value;
-  size_t mandatory_constraints = 0;
-
-  if (FindConstraint(constraints,
-                     MediaConstraintsInterface::kOfferToReceiveAudio,
-                     &value,
-                     &mandatory_constraints)) {
-    options.offer_to_receive_audio =
-        value ? RTCOfferAnswerOptions::kOfferToReceiveMediaTrue : 0;
-  }
-
-  if (FindConstraint(constraints,
-                     MediaConstraintsInterface::kOfferToReceiveVideo,
-                     &value,
-                     &mandatory_constraints)) {
-    options.offer_to_receive_video =
-        value ? RTCOfferAnswerOptions::kOfferToReceiveMediaTrue : 0;
-  }
-
-  if (FindConstraint(constraints,
-                     MediaConstraintsInterface::kVoiceActivityDetection,
-                     &value,
-                     &mandatory_constraints)) {
-    options.voice_activity_detection = value;
-  }
-
-  if (FindConstraint(constraints,
-                     MediaConstraintsInterface::kIceRestart,
-                     &value,
-                     &mandatory_constraints)) {
-    options.ice_restart = value;
-  }
-
-  if (FindConstraint(constraints,
-                     MediaConstraintsInterface::kUseRtpMux,
-                     &value,
-                     &mandatory_constraints)) {
-    options.use_rtp_mux = value;
+  PeerConnectionInterface::RTCOfferAnswerOptions offer_answer_options;
+  if (!ConvertConstraintsToOfferAnswerOptions(constraints,
+                                              &offer_answer_options)) {
+    std::string error = "CreateOffer called with invalid constraints.";
+    LOG(LS_ERROR) << error;
+    PostCreateSessionDescriptionFailure(observer, error);
+    return;
   }
 
-  CreateOffer(observer, options);
+  CreateOffer(observer, offer_answer_options);
 }
 
 void PeerConnection::CreateOffer(CreateSessionDescriptionObserver* observer,
@@ -895,14 +845,15 @@ void PeerConnection::CreateOffer(CreateSessionDescriptionObserver* observer,
     return;
   }
 
-  cricket::MediaSessionOptions session_options;
-  if (!GetOptionsForOffer(options, &session_options)) {
+  if (!ValidateOfferAnswerOptions(options)) {
     std::string error = "CreateOffer called with invalid options.";
     LOG(LS_ERROR) << error;
     PostCreateSessionDescriptionFailure(observer, error);
     return;
   }
 
+  cricket::MediaSessionOptions session_options;
+  GetOptionsForOffer(options, &session_options);
   session_->CreateOffer(observer, options, session_options);
 }
 
@@ -915,14 +866,26 @@ void PeerConnection::CreateAnswer(
     return;
   }
 
-  cricket::MediaSessionOptions session_options;
-  if (!GetOptionsForAnswer(constraints, &session_options)) {
+  if (!session_->remote_description() ||
+      session_->remote_description()->type() !=
+          SessionDescriptionInterface::kOffer) {
+    std::string error = "CreateAnswer called without remote offer.";
+    LOG(LS_ERROR) << error;
+    PostCreateSessionDescriptionFailure(observer, error);
+    return;
+  }
+
+  PeerConnectionInterface::RTCOfferAnswerOptions offer_answer_options;
+  if (!ConvertConstraintsToOfferAnswerOptions(constraints,
+                                              &offer_answer_options)) {
     std::string error = "CreateAnswer called with invalid constraints.";
     LOG(LS_ERROR) << error;
     PostCreateSessionDescriptionFailure(observer, error);
     return;
   }
 
+  cricket::MediaSessionOptions session_options;
+  GetOptionsForAnswer(offer_answer_options, &session_options);
   session_->CreateAnswer(observer, session_options);
 }
 
@@ -935,12 +898,7 @@ void PeerConnection::CreateAnswer(CreateSessionDescriptionObserver* observer,
   }
 
   cricket::MediaSessionOptions session_options;
-  if (!GetOptionsForAnswer(options, &session_options)) {
-    std::string error = "CreateAnswer called with invalid options.";
-    LOG(LS_ERROR) << error;
-    PostCreateSessionDescriptionFailure(observer, error);
-    return;
-  }
+  GetOptionsForAnswer(options, &session_options);
 
   session_->CreateAnswer(observer, session_options);
 }
@@ -1698,82 +1656,169 @@ void PeerConnection::PostCreateSessionDescriptionFailure(
                            MSG_CREATE_SESSIONDESCRIPTION_FAILED, msg);
 }
 
-bool PeerConnection::GetOptionsForOffer(
+void PeerConnection::GetOptionsForOffer(
     const PeerConnectionInterface::RTCOfferAnswerOptions& rtc_options,
     cricket::MediaSessionOptions* session_options) {
-  // TODO(deadbeef): Once we have transceivers, enumerate them here instead of
-  // ContentInfos.
-  if (session_->local_description()) {
-    for (const cricket::ContentInfo& content :
-         session_->local_description()->description()->contents()) {
-      session_options->transport_options[content.name] =
-          cricket::TransportOptions();
-    }
-  }
-  session_options->enable_ice_renomination =
-      configuration_.enable_ice_renomination;
+  ExtractSharedMediaSessionOptions(rtc_options, session_options);
 
-  if (!ExtractMediaSessionOptions(rtc_options, true, session_options)) {
-    return false;
-  }
+  // Figure out transceiver directional preferences.
+  bool send_audio = HasRtpSender(cricket::MEDIA_TYPE_AUDIO);
+  bool send_video = HasRtpSender(cricket::MEDIA_TYPE_VIDEO);
+
+  // By default, generate sendrecv/recvonly m= sections.
+  bool recv_audio = true;
+  bool recv_video = true;
 
-  AddSendStreams(session_options, senders_, rtp_data_channels_);
-  // Offer to receive audio/video if the constraint is not set and there are
-  // send streams, or we're currently receiving.
-  if (rtc_options.offer_to_receive_audio == RTCOfferAnswerOptions::kUndefined) {
-    session_options->recv_audio =
-        session_options->HasSendMediaStream(cricket::MEDIA_TYPE_AUDIO) ||
-        !remote_audio_tracks_.empty();
+  // By default, only offer a new m= section if we have media to send with it.
+  bool offer_new_audio_description = send_audio;
+  bool offer_new_video_description = send_video;
+  bool offer_new_data_description = HasDataChannels();
+
+  // The "offer_to_receive_X" options allow those defaults to be overridden.
+  if (rtc_options.offer_to_receive_audio != RTCOfferAnswerOptions::kUndefined) {
+    recv_audio = (rtc_options.offer_to_receive_audio > 0);
+    offer_new_audio_description = (rtc_options.offer_to_receive_audio > 0);
   }
-  if (rtc_options.offer_to_receive_video == RTCOfferAnswerOptions::kUndefined) {
-    session_options->recv_video =
-        session_options->HasSendMediaStream(cricket::MEDIA_TYPE_VIDEO) ||
-        !remote_video_tracks_.empty();
+  if (rtc_options.offer_to_receive_video != RTCOfferAnswerOptions::kUndefined) {
+    recv_video = (rtc_options.offer_to_receive_video > 0);
+    offer_new_video_description = (rtc_options.offer_to_receive_video > 0);
   }
 
+  int audio_index = -1;
+  int video_index = -1;
+  int data_index = -1;
+  // If a current description exists, generate m= sections in the same order,
+  // using the first audio/video/data section that appears and rejecting
+  // extraneous ones.
+  if (session_->local_description()) {
+    GenerateMediaDescrptionOptions(

[Taylor Brandstetter, 2017/08/03 00:24:48]

nit: "Description" is missing an "i".

[Zhi Huang, 2017/08/04 18:38:25]

Done.

+        session_->local_description(),
+        cricket::RtpTransceiverDirection(send_audio, recv_audio),
+        cricket::RtpTransceiverDirection(send_video, recv_video), &audio_index,
+        &video_index, &data_index, session_options);
+  }
+
+  // Add audio/video/data m= sections to the end if needed.
+  if (audio_index == -1 && offer_new_audio_description) {
+    session_options->media_description_options.push_back(
+        cricket::MediaDescriptionOptions(
+            cricket::MEDIA_TYPE_AUDIO, cricket::CN_AUDIO,
+            cricket::RtpTransceiverDirection(send_audio, recv_audio), false));
+    audio_index = session_options->media_description_options.size() - 1;
+  }
+  if (video_index == -1 && offer_new_video_description) {
+    session_options->media_description_options.push_back(
+        cricket::MediaDescriptionOptions(
+            cricket::MEDIA_TYPE_VIDEO, cricket::CN_VIDEO,
+            cricket::RtpTransceiverDirection(send_video, recv_video), false));
+    video_index = session_options->media_description_options.size() - 1;
+  }
+  if (data_index == -1 && offer_new_data_description) {
+    session_options->media_description_options.push_back(
+        cricket::MediaDescriptionOptions(
+            cricket::MEDIA_TYPE_DATA, cricket::CN_DATA,
+            cricket::RtpTransceiverDirection(true, true), false));
+    data_index = session_options->media_description_options.size() - 1;
+  }
+
+  cricket::MediaDescriptionOptions* audio_media_description_options =
+      audio_index == -1
+          ? nullptr
+          : &session_options->media_description_options[audio_index];
+  cricket::MediaDescriptionOptions* video_media_description_options =
+      video_index == -1
+          ? nullptr
+          : &session_options->media_description_options[video_index];
+  cricket::MediaDescriptionOptions* data_media_description_options =
+      data_index == -1
+          ? nullptr
+          : &session_options->media_description_options[data_index];
+
+  // Apply ICE restart flag and renomination flag.
+  for (auto& options : session_options->media_description_options) {
+    options.transport_options.ice_restart = rtc_options.ice_restart;
+    options.transport_options.enable_ice_renomination =
+        configuration_.enable_ice_renomination;
+  }
+
+  AddRtpSenderOptions(senders_, audio_media_description_options,
+                      video_media_description_options);
+  AddRtpDataChannelOptions(rtp_data_channels_, data_media_description_options);
+
   // Intentionally unset the data channel type for RTP data channel with the
   // second condition. Otherwise the RTP data channels would be successfully
   // negotiated by default and the unit tests in WebRtcDataBrowserTest will fail
   // when building with chromium. We want to leave RTP data channels broken, so
   // people won't try to use them.
-  if (HasDataChannels() && session_->data_channel_type() != cricket::DCT_RTP) {
+  if (session_->data_channel_type() != cricket::DCT_RTP) {
     session_options->data_channel_type = session_->data_channel_type();
   }
 
-  session_options->bundle_enabled =
-      session_options->bundle_enabled &&
-      (session_options->has_audio() || session_options->has_video() ||
-       session_options->has_data());
-
   session_options->rtcp_cname = rtcp_cname_;
   session_options->crypto_options = factory_->options().crypto_options;
-  return true;
 }
 
-void PeerConnection::InitializeOptionsForAnswer(
+void PeerConnection::GetOptionsForAnswer(
+    const RTCOfferAnswerOptions& rtc_options,
     cricket::MediaSessionOptions* session_options) {
-  session_options->recv_audio = false;
-  session_options->recv_video = false;
-  session_options->enable_ice_renomination =
-      configuration_.enable_ice_renomination;
-}
+  ExtractSharedMediaSessionOptions(rtc_options, session_options);
 
-void PeerConnection::FinishOptionsForAnswer(
-    cricket::MediaSessionOptions* session_options) {
-  // TODO(deadbeef): Once we have transceivers, enumerate them here instead of
-  // ContentInfos.
-  if (session_->remote_description()) {
-    // Initialize the transport_options map.
-    for (const cricket::ContentInfo& content :
-         session_->remote_description()->description()->contents()) {
-      session_options->transport_options[content.name] =
-          cricket::TransportOptions();
-    }
+  // Figure out transceiver directional preferences.
+  bool send_audio = HasRtpSender(cricket::MEDIA_TYPE_AUDIO);
+  bool send_video = HasRtpSender(cricket::MEDIA_TYPE_VIDEO);
+
+  // By default, generate sendrecv/recvonly m= sections. The direction is also
+  // restricted by the direction in the offer.
+  bool recv_audio = true;
+  bool recv_video = true;
+
+  // The "offer_to_receive_X" options allow those defaults to be overridden.
+  if (rtc_options.offer_to_receive_audio != RTCOfferAnswerOptions::kUndefined) {
+    recv_audio = (rtc_options.offer_to_receive_audio > 0);
   }
-  AddSendStreams(session_options, senders_, rtp_data_channels_);
-  // RTP data channel is handled in MediaSessionOptions::AddStream. SCTP streams
-  // are not signaled in the SDP so does not go through that path and must be
-  // handled here.
+  if (rtc_options.offer_to_receive_video != RTCOfferAnswerOptions::kUndefined) {
+    recv_video = (rtc_options.offer_to_receive_video > 0);
+  }
+
+  int audio_index = -1;
+  int video_index = -1;
+  int data_index = -1;
+  // There should be a pending remote description that's an offer...
+  RTC_DCHECK(session_->remote_description());
+  RTC_DCHECK(session_->remote_description()->type() ==
+             SessionDescriptionInterface::kOffer);
+  // Generate m= sections that match those in the offer.
+  // Note that mediasession.cc will handle intersection our preferred direction
+  // with the offered direction.
+  GenerateMediaDescrptionOptions(
+      session_->remote_description(),
+      cricket::RtpTransceiverDirection(send_audio, recv_audio),
+      cricket::RtpTransceiverDirection(send_video, recv_video), &audio_index,
+      &video_index, &data_index, session_options);
+
+  cricket::MediaDescriptionOptions* audio_media_description_options =
+      audio_index == -1
+          ? nullptr
+          : &session_options->media_description_options[audio_index];
+  cricket::MediaDescriptionOptions* video_media_description_options =
+      video_index == -1
+          ? nullptr
+          : &session_options->media_description_options[video_index];
+  cricket::MediaDescriptionOptions* data_media_description_options =
+      data_index == -1
+          ? nullptr
+          : &session_options->media_description_options[data_index];
+
+  // Apply ICE renomination flag.
+  for (auto& options : session_options->media_description_options) {
+    options.transport_options.enable_ice_renomination =
+        configuration_.enable_ice_renomination;
+  }
+
+  AddRtpSenderOptions(senders_, audio_media_description_options,
+                      video_media_description_options);
+  AddRtpDataChannelOptions(rtp_data_channels_, data_media_description_options);
+
   // Intentionally unset the data channel type for RTP data channel. Otherwise
   // the RTP data channels would be successfully negotiated by default and the
   // unit tests in WebRtcDataBrowserTest will fail when building with chromium.
@@ -1781,38 +1826,67 @@ void PeerConnection::FinishOptionsForAnswer(
   if (session_->data_channel_type() != cricket::DCT_RTP) {
     session_options->data_channel_type = session_->data_channel_type();
   }
-  session_options->bundle_enabled =
-      session_options->bundle_enabled &&
-      (session_options->has_audio() || session_options->has_video() ||
-       session_options->has_data());
-
-  session_options->crypto_options = factory_->options().crypto_options;
-}
-
-bool PeerConnection::GetOptionsForAnswer(
-    const MediaConstraintsInterface* constraints,
-    cricket::MediaSessionOptions* session_options) {
-  InitializeOptionsForAnswer(session_options);
-  if (!ParseConstraintsForAnswer(constraints, session_options)) {
-    return false;
-  }
   session_options->rtcp_cname = rtcp_cname_;
-
-  FinishOptionsForAnswer(session_options);
-  return true;
+  session_options->crypto_options = factory_->options().crypto_options;
 }
 
-bool PeerConnection::GetOptionsForAnswer(
-    const RTCOfferAnswerOptions& options,
+void PeerConnection::GenerateMediaDescrptionOptions(
+    const SessionDescriptionInterface* session_desc,
+    cricket::RtpTransceiverDirection audio_direction,
+    cricket::RtpTransceiverDirection video_direction,
+    int* audio_index,
+    int* video_index,
+    int* data_index,
     cricket::MediaSessionOptions* session_options) {
-  InitializeOptionsForAnswer(session_options);
-  if (!ExtractMediaSessionOptions(options, false, session_options)) {
-    return false;
+  for (const cricket::ContentInfo& content :
+       session_desc->description()->contents()) {
+    if (IsAudioContent(&content)) {
+      // If we already have an audio m= section, reject this extra one.
+      if (*audio_index != -1) {
+        session_options->media_description_options.push_back(
+            cricket::MediaDescriptionOptions(
+                cricket::MEDIA_TYPE_AUDIO, content.name,
+                cricket::RtpTransceiverDirection(false, false), true));
+      } else {
+        session_options->media_description_options.push_back(
+            cricket::MediaDescriptionOptions(
+                cricket::MEDIA_TYPE_AUDIO, content.name, audio_direction,
+                !audio_direction.send && !audio_direction.recv));
+        *audio_index = session_options->media_description_options.size() - 1;
+      }
+    } else if (IsVideoContent(&content)) {
+      // If we already have an video m= section, reject this extra one.
+      if (*video_index != -1) {
+        session_options->media_description_options.push_back(
+            cricket::MediaDescriptionOptions(
+                cricket::MEDIA_TYPE_VIDEO, content.name,
+                cricket::RtpTransceiverDirection(false, false), true));
+      } else {
+        session_options->media_description_options.push_back(
+            cricket::MediaDescriptionOptions(
+                cricket::MEDIA_TYPE_VIDEO, content.name, video_direction,
+                !video_direction.send && !video_direction.recv));
+        *video_index = session_options->media_description_options.size() - 1;
+      }
+    } else {
+      RTC_DCHECK(IsDataContent(&content));
+      // If we already have an data m= section, reject this extra one.
+      if (*data_index != -1) {
+        session_options->media_description_options.push_back(
+            cricket::MediaDescriptionOptions(
+                cricket::MEDIA_TYPE_DATA, content.name,
+                cricket::RtpTransceiverDirection(false, false), true));
+      } else {
+        session_options->media_description_options.push_back(
+            cricket::MediaDescriptionOptions(
+                cricket::MEDIA_TYPE_DATA, content.name,
+                // Direction for data sections is meaningless, but legacy
+                // endpoints might expect sendrecv.
+                cricket::RtpTransceiverDirection(true, true), false));
+        *data_index = session_options->media_description_options.size() - 1;
+      }
+    }
   }
-  session_options->rtcp_cname = rtcp_cname_;
-
-  FinishOptionsForAnswer(session_options);
-  return true;
 }
 
 void PeerConnection::RemoveTracks(cricket::MediaType media_type) {
@@ -2285,6 +2359,15 @@ void PeerConnection::OnDataChannelOpenMessage(
   observer_->OnDataChannel(std::move(proxy_channel));
 }
 
+bool PeerConnection::HasRtpSender(cricket::MediaType type) const {
+  return std::find_if(
+             senders_.begin(), senders_.end(),
+             [type](const rtc::scoped_refptr<
+                    RtpSenderProxyWithInternal<RtpSenderInternal>>& sender) {
+               return sender->media_type() == type;
+             }) != senders_.end();
+}
+
 RtpSenderInternal* PeerConnection::FindSenderById(const std::string& id) {
   auto it = std::find_if(
       senders_.begin(), senders_.end(),