Adding support for Unified Plan offer/answer negotiation.

webrtc/pc/mediasession.cc

Index: webrtc/pc/mediasession.cc
diff --git a/webrtc/pc/mediasession.cc b/webrtc/pc/mediasession.cc
index 2f973deb7f6476a223d2b8713fa66906d18ba777..709e5c15debf25b164127c955b4dcd7cb6921075 100644
--- a/webrtc/pc/mediasession.cc
+++ b/webrtc/pc/mediasession.cc
@@ -159,11 +159,12 @@ bool CreateMediaCryptos(const std::vector<std::string>& crypto_suites,
 }
 #endif
 
-const CryptoParamsVec* GetCryptos(const MediaContentDescription* media) {
-  if (!media) {
-    return NULL;
+const CryptoParamsVec* GetCryptos(const ContentInfo* content) {
+  if (!content) {
+    return nullptr;
   }
-  return &media->cryptos();
+  return &(static_cast<const MediaContentDescription*>(content->description)
+               ->cryptos());
 }
 
 bool FindMatchingCrypto(const CryptoParamsVec& cryptos,
@@ -332,28 +333,34 @@ class UsedIds {
   void FindAndSetIdUsed(std::vector<Id>* ids) {
     for (typename std::vector<Id>::iterator it = ids->begin();
          it != ids->end(); ++it) {
-      FindAndSetIdUsed(&*it);
+      FindAndSetIdUsed(&*it, true);
     }
   }
 
   // Finds and sets an unused id if the |idstruct| id is already in use.
-  void FindAndSetIdUsed(IdStruct* idstruct) {
+  // If |can_reassign_id| is false, the ID of |idstruct| won't be reassigned,
+  // and ID conflicts will simply result in false being returned.
+  bool FindAndSetIdUsed(IdStruct* idstruct, bool can_reassign_id) {
     const int original_id = idstruct->id;
     int new_id = idstruct->id;
 
     if (original_id > max_allowed_id_ || original_id < min_allowed_id_) {
       // If the original id is not in range - this is an id that can't be
       // dynamically changed.
-      return;
+      return true;
     }
 
     if (IsIdUsed(original_id)) {
+      if (!can_reassign_id) {
+        return false;
+      }
       new_id = FindUnusedId();
       LOG(LS_WARNING) << "Duplicate id found. Reassigning from " << original_id
           << " to " << new_id;
       idstruct->id = new_id;
     }
     SetIdUsed(new_id);
+    return true;
   }
 
  private:
@@ -390,7 +397,6 @@ class UsedPayloadTypes : public UsedIds<Codec> {
       : UsedIds<Codec>(kDynamicPayloadTypeMin, kDynamicPayloadTypeMax) {
   }
 
-
  private:
   static const int kDynamicPayloadTypeMin = 96;
   static const int kDynamicPayloadTypeMax = 127;
@@ -414,15 +420,15 @@ static bool IsSctp(const MediaContentDescription* desc) {
           (desc->protocol() == kMediaProtocolDtlsSctp));
 }
 
-// Adds a StreamParams for each Stream in Streams with media type
-// media_type to content_description.
-// |current_params| - All currently known StreamParams of any media type.
+// Adds a StreamParams for each SenderOptions in |sender_options| to
+// content_description.
+// |current_params| - All currently known StreamParams.
 template <class C>
-static bool AddStreamParams(MediaType media_type,
-                            const MediaSessionOptions& options,
-                            StreamParamsVec* current_streams,
-                            MediaContentDescriptionImpl<C>* content_description,
-                            const bool add_legacy_stream) {
+static bool AddStreamParams(
+    const std::vector<SenderOptions>& sender_options,
+    const std::string& rtcp_cname,
+    StreamParamsVec* current_streams,
+    MediaContentDescriptionImpl<C>* content_description) {
   // SCTP streams are not negotiated using SDP/ContentDescriptions.
   if (IsSctp(content_description)) {
     return true;
@@ -431,45 +437,25 @@ static bool AddStreamParams(MediaType media_type,
   const bool include_rtx_streams =
       ContainsRtxCodec(content_description->codecs());
 
-  const MediaSessionOptions::Streams& streams = options.streams;
-  if (streams.empty() && add_legacy_stream) {
-    // TODO(perkj): Remove this legacy stream when all apps use StreamParams.
-    std::vector<uint32_t> ssrcs;
-    int num_ssrcs = include_rtx_streams ? 2 : 1;
-    GenerateSsrcs(*current_streams, num_ssrcs, &ssrcs);
-    if (include_rtx_streams) {
-      content_description->AddLegacyStream(ssrcs[0], ssrcs[1]);
-      content_description->set_multistream(true);
-    } else {
-      content_description->AddLegacyStream(ssrcs[0]);
-    }
-    return true;
-  }
-
   const bool include_flexfec_stream =
       ContainsFlexfecCodec(content_description->codecs());
 
-  MediaSessionOptions::Streams::const_iterator stream_it;
-  for (stream_it = streams.begin();
-       stream_it != streams.end(); ++stream_it) {
-    if (stream_it->type != media_type)
-      continue;  // Wrong media type.
-
-    const StreamParams* param =
-        GetStreamByIds(*current_streams, "", stream_it->id);
+  for (const SenderOptions& sender : sender_options) {
     // groupid is empty for StreamParams generated using
     // MediaSessionDescriptionFactory.
+    const StreamParams* param =
+        GetStreamByIds(*current_streams, "", sender.track_id);
     if (!param) {
-      // This is a new stream.
+      // This is a new sender.
       std::vector<uint32_t> ssrcs;
-      GenerateSsrcs(*current_streams, stream_it->num_sim_layers, &ssrcs);
+      GenerateSsrcs(*current_streams, sender.num_sim_layers, &ssrcs);
       StreamParams stream_param;
-      stream_param.id = stream_it->id;
+      stream_param.id = sender.track_id;
       // Add the generated ssrc.
       for (size_t i = 0; i < ssrcs.size(); ++i) {
         stream_param.ssrcs.push_back(ssrcs[i]);
       }
-      if (stream_it->num_sim_layers > 1) {
+      if (sender.num_sim_layers > 1) {
         SsrcGroup group(kSimSsrcGroupSemantics, stream_param.ssrcs);
         stream_param.ssrc_groups.push_back(group);
       }
@@ -499,8 +485,8 @@ static bool AddStreamParams(MediaType media_type,
               << "media streams however, so no FlexFEC SSRC will be generated.";
         }
       }
-      stream_param.cname = options.rtcp_cname;
-      stream_param.sync_label = stream_it->sync_label;
+      stream_param.cname = rtcp_cname;
+      stream_param.sync_label = sender.stream_id;
       content_description->AddStream(stream_param);
 
       // Store the new StreamParams in current_streams.
@@ -718,46 +704,35 @@ static bool IsFlexfecCodec(const C& codec) {
   return stricmp(codec.name.c_str(), kFlexfecCodecName) == 0;
 }
 
-static TransportOptions GetTransportOptions(const MediaSessionOptions& options,
-                                            const std::string& content_name) {
-  TransportOptions transport_options;
-  auto it = options.transport_options.find(content_name);
-  if (it != options.transport_options.end()) {
-    transport_options = it->second;
-  }
-  transport_options.enable_ice_renomination = options.enable_ice_renomination;
-  return transport_options;
-}
-
-// Create a media content to be offered in a session-initiate,
-// according to the given options.rtcp_mux, options.is_muc,
-// options.streams, codecs, secure_transport, crypto, and streams.  If we don't
+// Create a media content to be offered in a session-initiate for the
+// given |sender_options|, according to the given options.rtcp_mux,
+// session_options.is_muc,
+// codecs, secure_transport, crypto, and current_streams. If we don't
 // currently have crypto (in current_cryptos) and it is enabled (in
-// secure_policy), crypto is created (according to crypto_suites).  If
-// add_legacy_stream is true, and current_streams is empty, a legacy
-// stream is created.  The created content is added to the offer.
+// secure_policy), crypto is created (according to crypto_suites). The created
+// content is added to the offer.
 template <class C>
 static bool CreateMediaContentOffer(
-    const MediaSessionOptions& options,
+    const std::vector<SenderOptions>& sender_options,
+    const MediaSessionOptions& session_options,
     const std::vector<C>& codecs,
     const SecurePolicy& secure_policy,
     const CryptoParamsVec* current_cryptos,
     const std::vector<std::string>& crypto_suites,
     const RtpHeaderExtensions& rtp_extensions,
-    bool add_legacy_stream,
     StreamParamsVec* current_streams,
     MediaContentDescriptionImpl<C>* offer) {
   offer->AddCodecs(codecs);
 
-  offer->set_rtcp_mux(options.rtcp_mux_enabled);
+  offer->set_rtcp_mux(session_options.rtcp_mux_enabled);
   if (offer->type() == cricket::MEDIA_TYPE_VIDEO) {
     offer->set_rtcp_reduced_size(true);
   }
-  offer->set_multistream(options.is_muc);
+  offer->set_multistream(session_options.is_muc);
   offer->set_rtp_header_extensions(rtp_extensions);
 
-  if (!AddStreamParams(offer->type(), options, current_streams, offer,
-                       add_legacy_stream)) {
+  if (!AddStreamParams(sender_options, session_options.rtcp_cname,
+                       current_streams, offer)) {
     return false;
   }
 
@@ -867,23 +842,54 @@ static bool FindMatchingCodec(const std::vector<C>& codecs1,
   return false;
 }
 
-// Adds all codecs from |reference_codecs| to |offered_codecs| that dont'
-// already exist in |offered_codecs| and ensure the payload types don't
-// collide.
+// Find the codec in |codec_list| that |rtx_codec| is associated with.
 template <class C>
-static void FindCodecsToOffer(
-    const std::vector<C>& reference_codecs,
-    std::vector<C>* offered_codecs,
-    UsedPayloadTypes* used_pltypes) {
+static const C* GetAssociatedCodec(const std::vector<C>& codec_list,
+                                   const C& rtx_codec) {
+  std::string associated_pt_str;
+  if (!rtx_codec.GetParam(kCodecParamAssociatedPayloadType,
+                          &associated_pt_str)) {
+    LOG(LS_WARNING) << "RTX codec " << rtx_codec.name
+                    << " is missing an associated payload type.";
+    return nullptr;
+  }
+
+  int associated_pt;
+  if (!rtc::FromString(associated_pt_str, &associated_pt)) {
+    LOG(LS_WARNING) << "Couldn't convert payload type " << associated_pt_str
+                    << " of RTX codec " << rtx_codec.name << " to an integer.";
+    return nullptr;
+  }
 
+  // Find the associated reference codec for the reference RTX codec.
+  const C* associated_codec = FindCodecById(codec_list, associated_pt);
+  if (!associated_codec) {
+    LOG(LS_WARNING) << "Couldn't find associated codec with payload type "
+                    << associated_pt << " for RTX codec " << rtx_codec.name
+                    << ".";
+  }
+  return associated_codec;
+}
+
+// Adds all codecs from |reference_codecs| to |offered_codecs| that don't
+// already exist in |offered_codecs| and ensure the payload types don't
+// collide. If |can_modify_codecs| is false, a codec is only added if it
+// can be added without changing any parameters (namely payload type,
+// associated payload type).
+template <class C>
+static void MergeCodecs(const std::vector<C>& reference_codecs,
+                        bool can_modify_codecs,
+                        std::vector<C>* offered_codecs,
+                        UsedPayloadTypes* used_pltypes) {
   // Add all new codecs that are not RTX codecs.
   for (const C& reference_codec : reference_codecs) {
     if (!IsRtxCodec(reference_codec) &&
         !FindMatchingCodec<C>(reference_codecs, *offered_codecs,
                               reference_codec, nullptr)) {
       C codec = reference_codec;
-      used_pltypes->FindAndSetIdUsed(&codec);
-      offered_codecs->push_back(codec);
+      if (used_pltypes->FindAndSetIdUsed(&codec, can_modify_codecs)) {
+        offered_codecs->push_back(codec);
+      }
     }
   }
 
@@ -893,30 +899,9 @@ static void FindCodecsToOffer(
         !FindMatchingCodec<C>(reference_codecs, *offered_codecs,
                               reference_codec, nullptr)) {
       C rtx_codec = reference_codec;
-
-      std::string associated_pt_str;
-      if (!rtx_codec.GetParam(kCodecParamAssociatedPayloadType,
-                              &associated_pt_str)) {
-        LOG(LS_WARNING) << "RTX codec " << rtx_codec.name
-                        << " is missing an associated payload type.";
-        continue;
-      }
-
-      int associated_pt;
-      if (!rtc::FromString(associated_pt_str, &associated_pt)) {
-        LOG(LS_WARNING) << "Couldn't convert payload type " << associated_pt_str
-                        << " of RTX codec " << rtx_codec.name
-                        << " to an integer.";
-        continue;
-      }
-
-      // Find the associated reference codec for the reference RTX codec.
       const C* associated_codec =
-          FindCodecById(reference_codecs, associated_pt);
+          GetAssociatedCodec(reference_codecs, rtx_codec);
       if (!associated_codec) {
-        LOG(LS_WARNING) << "Couldn't find associated codec with payload type "
-                        << associated_pt << " for RTX codec " << rtx_codec.name
-                        << ".";
         continue;
       }
 
@@ -930,10 +915,15 @@ static void FindCodecsToOffer(
         continue;
       }
 
+      if (!can_modify_codecs && matching_codec.id != associated_codec->id) {
+        continue;
+      }
+
       rtx_codec.params[kCodecParamAssociatedPayloadType] =
           rtc::ToString(matching_codec.id);
-      used_pltypes->FindAndSetIdUsed(&rtx_codec);
-      offered_codecs->push_back(rtx_codec);
+      if (used_pltypes->FindAndSetIdUsed(&rtx_codec, can_modify_codecs)) {
+        offered_codecs->push_back(rtx_codec);
+      }
     }
   }
 }
@@ -954,40 +944,28 @@ static bool FindByUri(const RtpHeaderExtensions& extensions,
   return false;
 }
 
-// Iterates through |offered_extensions|, adding each one to |all_extensions|
-// and |used_ids|, and resolving ID conflicts. If an offered extension has the
-// same URI as one in |all_extensions|, it will re-use the same ID and won't be
-// treated as a conflict.
-static void FindAndSetRtpHdrExtUsed(RtpHeaderExtensions* offered_extensions,
-                                    RtpHeaderExtensions* all_extensions,
-                                    UsedRtpHeaderExtensionIds* used_ids) {
-  for (auto& extension : *offered_extensions) {
-    webrtc::RtpExtension existing;
-    if (FindByUri(*all_extensions, extension, &existing)) {
-      extension.id = existing.id;
-    } else {
-      used_ids->FindAndSetIdUsed(&extension);
-      all_extensions->push_back(extension);
-    }
-  }
-}
-
-// Adds |reference_extensions| to |offered_extensions|, while updating
-// |all_extensions| and |used_ids|.
-static void FindRtpHdrExtsToOffer(
-    const RtpHeaderExtensions& reference_extensions,
-    RtpHeaderExtensions* offered_extensions,
-    RtpHeaderExtensions* all_extensions,
-    UsedRtpHeaderExtensionIds* used_ids) {
+// Adds all extensions from |reference_extensions| to |offered_extensions| that
+// don't already exist in |offered_extensions| and ensure the IDs don't
+// collide. If an extension is added, it's also added to |all_extensions|, and
+// if the extension is in |all_extensions| its ID is used. If
+// |can_modify_extensions| is false, a extension is only added if it can be
+// added without changing the ID.
+static void MergeRtpHdrExts(const RtpHeaderExtensions& reference_extensions,
+                            bool can_modify_extension,
+                            RtpHeaderExtensions* offered_extensions,
+                            RtpHeaderExtensions* all_extensions,
+                            UsedRtpHeaderExtensionIds* used_ids) {
   for (auto reference_extension : reference_extensions) {
     if (!FindByUri(*offered_extensions, reference_extension, NULL)) {
       webrtc::RtpExtension existing;
       if (FindByUri(*all_extensions, reference_extension, &existing)) {
         offered_extensions->push_back(existing);
       } else {
-        used_ids->FindAndSetIdUsed(&reference_extension);
-        all_extensions->push_back(reference_extension);
-        offered_extensions->push_back(reference_extension);
+        webrtc::RtpExtension extension = reference_extension;
+        if (used_ids->FindAndSetIdUsed(&extension, can_modify_extension)) {
+          all_extensions->push_back(extension);
+          offered_extensions->push_back(extension);
+        }
       }
     }
   }
@@ -1019,25 +997,27 @@ static void StripCNCodecs(AudioCodecs* audio_codecs) {
   }
 }
 
-// Create a media content to be answered in a session-accept,
-// according to the given options.rtcp_mux, options.streams, codecs,
-// crypto, and streams.  If we don't currently have crypto (in
-// current_cryptos) and it is enabled (in secure_policy), crypto is
-// created (according to crypto_suites).  If add_legacy_stream is
-// true, and current_streams is empty, a legacy stream is created.
-// The codecs, rtcp_mux, and crypto are all negotiated with the offer
-// from the incoming session-initiate.  If the negotiation fails, this
-// method returns false.  The created content is added to the offer.
+// Create a media content to be answered in a session-accept for
+// the given |sender_options|, according to the given session_options.rtcp_mux,
+// session_options.streams, codecs, crypto, and current_streams. If we don't
+// currently
+// have crypto (in current_cryptos) and it is enabled (in secure_policy),
+// crypto is created (according to crypto_suites). The codecs, rtcp_mux, and
+// crypto are all negotiated with the offer from the incoming session-initiate.
+// If the negotiation fails, this method returns false. The created content is
+// added to the offer.
 template <class C>
 static bool CreateMediaContentAnswer(
     const MediaContentDescriptionImpl<C>* offer,
-    const MediaSessionOptions& options,
+    const std::vector<SenderOptions>& sender_options,
+    // TODO: combine these two params into mediadescriptionoptions
+    RtpTransceiverDirection desired_direction,
+    const MediaSessionOptions& session_options,
     const std::vector<C>& local_codecs,
     const SecurePolicy& sdes_policy,
     const CryptoParamsVec* current_cryptos,
     const RtpHeaderExtensions& local_rtp_extenstions,
     StreamParamsVec* current_streams,
-    bool add_legacy_stream,
     bool bundle_enabled,
     MediaContentDescriptionImpl<C>* answer) {
   std::vector<C> negotiated_codecs;
@@ -1050,14 +1030,15 @@ static bool CreateMediaContentAnswer(
                                &negotiated_rtp_extensions);
   answer->set_rtp_header_extensions(negotiated_rtp_extensions);
 
-  answer->set_rtcp_mux(options.rtcp_mux_enabled && offer->rtcp_mux());
+  answer->set_rtcp_mux(session_options.rtcp_mux_enabled && offer->rtcp_mux());
   if (answer->type() == cricket::MEDIA_TYPE_VIDEO) {
     answer->set_rtcp_reduced_size(offer->rtcp_reduced_size());
   }
 
   if (sdes_policy != SEC_DISABLED) {
     CryptoParams crypto;
-    if (SelectCrypto(offer, bundle_enabled, options.crypto_options, &crypto)) {
+    if (SelectCrypto(offer, bundle_enabled, session_options.crypto_options,
+                     &crypto)) {
       if (current_cryptos) {
         FindMatchingCrypto(*current_cryptos, crypto, &crypto);
       }
@@ -1069,29 +1050,16 @@ static bool CreateMediaContentAnswer(
     return false;
   }
 
-  if (!AddStreamParams(answer->type(), options, current_streams, answer,
-                       add_legacy_stream)) {
+  if (!AddStreamParams(sender_options, session_options.rtcp_cname,
+                       current_streams, answer)) {
     return false;  // Something went seriously wrong.
   }
 
-  // Make sure the answer media content direction is per default set as
-  // described in RFC3264 section 6.1.
-  const bool is_data = !IsRtpProtocol(answer->protocol());
-  const bool has_send_streams = !answer->streams().empty();
-  const bool wants_send = has_send_streams || is_data;
-  const bool recv_audio =
-      answer->type() == cricket::MEDIA_TYPE_AUDIO && options.recv_audio;
-  const bool recv_video =
-      answer->type() == cricket::MEDIA_TYPE_VIDEO && options.recv_video;
-  const bool recv_data =
-      answer->type() == cricket::MEDIA_TYPE_DATA;
-  const bool wants_receive = recv_audio || recv_video || recv_data;
-
   auto offer_rtd =
       RtpTransceiverDirection::FromMediaContentDirection(offer->direction());
-  auto wants_rtd = RtpTransceiverDirection(wants_send, wants_receive);
-  answer->set_direction(NegotiateRtpTransceiverDirection(offer_rtd, wants_rtd)
-                        .ToMediaContentDirection());
+  answer->set_direction(
+      NegotiateRtpTransceiverDirection(offer_rtd, desired_direction)
+          .ToMediaContentDirection());
   return true;
 }
 
@@ -1173,21 +1141,17 @@ static const TransportDescription* GetTransportDescription(
 }
 
 // Gets the current DTLS state from the transport description.
-static bool IsDtlsActive(
-    const std::string& content_name,
-    const SessionDescription* current_description) {
-  if (!current_description)
-    return false;
-
-  const ContentInfo* content =
-      current_description->GetContentByName(content_name);
-  if (!content)
+static bool IsDtlsActive(const ContentInfo* content,
+                         const SessionDescription* current_description) {
+  if (!content) {
     return false;
+  }
 
   const TransportDescription* current_tdesc =
-      GetTransportDescription(content_name, current_description);
-  if (!current_tdesc)
+      GetTransportDescription(content->name, current_description);
+  if (!current_tdesc) {
     return false;
+  }
 
   return current_tdesc->secure();
 }
@@ -1234,67 +1198,54 @@ std::string MediaContentDirectionToString(MediaContentDirection direction) {
   return dir_str;
 }
 
-void MediaSessionOptions::AddSendStream(MediaType type,
-                                    const std::string& id,
-                                    const std::string& sync_label) {
-  AddSendStreamInternal(type, id, sync_label, 1);
+void MediaDescriptionOptions::AddAudioSender(const std::string& track_id,
+                                             const std::string& stream_id) {
+  RTC_DCHECK(type == MEDIA_TYPE_AUDIO);
+  AddSenderInternal(track_id, stream_id, 1);
 }
 
-void MediaSessionOptions::AddSendVideoStream(
-    const std::string& id,
-    const std::string& sync_label,
-    int num_sim_layers) {
-  AddSendStreamInternal(MEDIA_TYPE_VIDEO, id, sync_label, num_sim_layers);
+void MediaDescriptionOptions::AddVideoSender(const std::string& track_id,
+                                             const std::string& stream_id,
+                                             int num_sim_layers) {
+  RTC_DCHECK(type == MEDIA_TYPE_VIDEO);
+  AddSenderInternal(track_id, stream_id, num_sim_layers);
 }
 
-void MediaSessionOptions::AddSendStreamInternal(
-    MediaType type,
-    const std::string& id,
-    const std::string& sync_label,
-    int num_sim_layers) {
-  streams.push_back(Stream(type, id, sync_label, 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);
+}
 
-  // If we haven't already set the data_channel_type, and we add a
-  // stream, we assume it's an RTP data stream.
-  if (type == MEDIA_TYPE_DATA && data_channel_type == DCT_NONE)
-    data_channel_type = DCT_RTP;
+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 MediaSessionOptions::RemoveSendStream(MediaType type,
-                                       const std::string& id) {
-  Streams::iterator stream_it = streams.begin();
-  for (; stream_it != streams.end(); ++stream_it) {
-    if (stream_it->type == type && stream_it->id == id) {
-      streams.erase(stream_it);
-      return;
-    }
-  }
-  ASSERT(false);
+bool MediaSessionOptions::HasMediaDescription(MediaType type) const {
+  return std::find_if(media_description_options.begin(),
+                      media_description_options.end(),
+                      [type](const MediaDescriptionOptions& t) {
+                        return t.type == type;
+                      }) != media_description_options.end();
 }
 
-bool MediaSessionOptions::HasSendMediaStream(MediaType type) const {
-  Streams::const_iterator stream_it = streams.begin();
-  for (; stream_it != streams.end(); ++stream_it) {
-    if (stream_it->type == type) {
-      return true;
-    }
-  }
-  return false;
+std::vector<MediaDescriptionOptions>::iterator
+MediaSessionOptions::FindMediaDescription(const std::string& mid) {
+  return std::find_if(
+      media_description_options.begin(), media_description_options.end(),
+      [mid](const MediaDescriptionOptions& t) { return t.mid == mid; });
 }
 
 MediaSessionDescriptionFactory::MediaSessionDescriptionFactory(
     const TransportDescriptionFactory* transport_desc_factory)
-    : secure_(SEC_DISABLED),
-      add_legacy_(true),
-      transport_desc_factory_(transport_desc_factory) {
-}
+    : transport_desc_factory_(transport_desc_factory) {}
 
 MediaSessionDescriptionFactory::MediaSessionDescriptionFactory(
     ChannelManager* channel_manager,
     const TransportDescriptionFactory* transport_desc_factory)
-    : secure_(SEC_DISABLED),
-      add_legacy_(true),
-      transport_desc_factory_(transport_desc_factory) {
+    : transport_desc_factory_(transport_desc_factory) {
   channel_manager->GetSupportedAudioSendCodecs(&audio_send_codecs_);
   channel_manager->GetSupportedAudioReceiveCodecs(&audio_recv_codecs_);
   channel_manager->GetSupportedAudioSendCodecs(&audio_send_codecs_);
@@ -1302,8 +1253,23 @@ MediaSessionDescriptionFactory::MediaSessionDescriptionFactory(
   channel_manager->GetSupportedVideoCodecs(&video_codecs_);
   channel_manager->GetSupportedVideoRtpHeaderExtensions(&video_rtp_extensions_);
   channel_manager->GetSupportedDataCodecs(&data_codecs_);
-  NegotiateCodecs(audio_recv_codecs_, audio_send_codecs_,
-                  &audio_sendrecv_codecs_);
+  // Precompute the intersection and union of audio send/recv codecs.
+  for (const AudioCodec& recv : audio_recv_codecs_) {
+    all_audio_codecs_.push_back(recv);
+    if (FindMatchingCodec<AudioCodec>(audio_recv_codecs_, audio_send_codecs_,
+                                      recv, nullptr)) {
+      audio_sendrecv_codecs_.push_back(recv);
+    }
+  }
+  for (const AudioCodec& send : audio_send_codecs_) {
+    if (!FindMatchingCodec<AudioCodec>(audio_send_codecs_, audio_recv_codecs_,
+                                       send, nullptr)) {
+      // It doesn't make sense to have an RTX codec we support sending but not
+      // receiving.
+      RTC_DCHECK(!IsRtxCodec(send));
+      all_audio_codecs_.push_back(send);
+    }
+  }
 }
 
 const AudioCodecs& MediaSessionDescriptionFactory::audio_sendrecv_codecs()
@@ -1333,102 +1299,93 @@ void MediaSessionDescriptionFactory::set_audio_codecs(
 }
 
 SessionDescription* MediaSessionDescriptionFactory::CreateOffer(
-    const MediaSessionOptions& options,
+    const MediaSessionOptions& session_options,
     const SessionDescription* current_description) const {
   std::unique_ptr<SessionDescription> offer(new SessionDescription());
 
   StreamParamsVec current_streams;
   GetCurrentStreamParams(current_description, &current_streams);
 
-  const bool wants_send =
-      options.HasSendMediaStream(MEDIA_TYPE_AUDIO) || add_legacy_;
-  const AudioCodecs& supported_audio_codecs =
-      GetAudioCodecsForOffer({wants_send, options.recv_audio});
-
-  AudioCodecs audio_codecs;
-  VideoCodecs video_codecs;
-  DataCodecs data_codecs;
-  GetCodecsToOffer(current_description, supported_audio_codecs,
-                   video_codecs_, data_codecs_,
-                   &audio_codecs, &video_codecs, &data_codecs);
+  AudioCodecs offer_audio_codecs;
+  VideoCodecs offer_video_codecs;
+  DataCodecs offer_data_codecs;
+  GetCodecsForOffer(current_description, &offer_audio_codecs,
+                    &offer_video_codecs, &offer_data_codecs);
 
-  if (!options.vad_enabled) {
+  if (!session_options.vad_enabled) {
     // If application doesn't want CN codecs in offer.
-    StripCNCodecs(&audio_codecs);
+    StripCNCodecs(&offer_audio_codecs);
   }
+  FilterDataCodecs(&offer_data_codecs,
+                   session_options.data_channel_type == DCT_SCTP);
 
   RtpHeaderExtensions audio_rtp_extensions;
   RtpHeaderExtensions video_rtp_extensions;
   GetRtpHdrExtsToOffer(current_description, &audio_rtp_extensions,
                        &video_rtp_extensions);
 
-  bool audio_added = false;
-  bool video_added = false;
-  bool data_added = false;
-
-  // Iterate through the contents of |current_description| to maintain the order
-  // of the m-lines in the new offer.
+  // Must have options for each existing section.
   if (current_description) {
-    ContentInfos::const_iterator it = current_description->contents().begin();
-    for (; it != current_description->contents().end(); ++it) {
-      if (IsMediaContentOfType(&*it, MEDIA_TYPE_AUDIO)) {
-        if (!AddAudioContentForOffer(options, current_description,
-                                     audio_rtp_extensions, audio_codecs,
+    RTC_DCHECK(current_description->contents().size() <=
+               session_options.media_description_options.size());
+  }
+  // Iterate through the media description options, matching with existing
+  // media descriptions in |current_description|.
+  int msection_index = 0;
+  for (const MediaDescriptionOptions& media_description_options :
+       session_options.media_description_options) {
+    const ContentInfo* current_content = nullptr;
+    if (current_description &&
+        msection_index <
+            static_cast<int>(current_description->contents().size())) {
+      current_content = &current_description->contents()[msection_index];
+      // Media types and mids must match.
+      RTC_DCHECK(IsMediaContentOfType(current_content,
+                                      media_description_options.type));
+      RTC_DCHECK(media_description_options.mid == current_content->name);
+    }
+    switch (media_description_options.type) {
+      case MEDIA_TYPE_AUDIO:
+        if (!AddAudioContentForOffer(media_description_options, session_options,
+                                     current_content, current_description,
+                                     audio_rtp_extensions, offer_audio_codecs,
                                      &current_streams, offer.get())) {
-          return NULL;
+          return nullptr;
         }
-        audio_added = true;
-      } else if (IsMediaContentOfType(&*it, MEDIA_TYPE_VIDEO)) {
-        if (!AddVideoContentForOffer(options, current_description,
-                                     video_rtp_extensions, video_codecs,
+        break;
+      case MEDIA_TYPE_VIDEO:
+        if (!AddVideoContentForOffer(media_description_options, session_options,
+                                     current_content, current_description,
+                                     video_rtp_extensions, offer_video_codecs,
                                      &current_streams, offer.get())) {
-          return NULL;
-        }
-        video_added = true;
-      } else if (IsMediaContentOfType(&*it, MEDIA_TYPE_DATA)) {
-        MediaSessionOptions options_copy(options);
-        if (IsSctp(static_cast<const MediaContentDescription*>(
-                it->description))) {
-          options_copy.data_channel_type = DCT_SCTP;
+          return nullptr;
         }
-        if (!AddDataContentForOffer(options_copy, current_description,
-                                    &data_codecs, &current_streams,
+        break;
+      case MEDIA_TYPE_DATA:
+        if (!AddDataContentForOffer(media_description_options, session_options,
+                                    current_content, current_description,
+                                    offer_data_codecs, &current_streams,
                                     offer.get())) {
-          return NULL;
+          return nullptr;
         }
-        data_added = true;
-      } else {
-        ASSERT(false);
-      }
+        break;
+      default:
+        RTC_NOTREACHED();
     }
-  }
-
-  // Append contents that are not in |current_description|.
-  if (!audio_added && options.has_audio() &&
-      !AddAudioContentForOffer(options, current_description,
-                               audio_rtp_extensions, audio_codecs,
-                               &current_streams, offer.get())) {
-    return NULL;
-  }
-  if (!video_added && options.has_video() &&
-      !AddVideoContentForOffer(options, current_description,
-                               video_rtp_extensions, video_codecs,
-                               &current_streams, offer.get())) {
-    return NULL;
-  }
-  if (!data_added && options.has_data() &&
-      !AddDataContentForOffer(options, current_description, &data_codecs,
-                              &current_streams, offer.get())) {
-    return NULL;
+    ++msection_index;
   }
 
   // Bundle the contents together, if we've been asked to do so, and update any
   // parameters that need to be tweaked for BUNDLE.
-  if (options.bundle_enabled) {
+  if (session_options.bundle_enabled && offer->contents().size() > 0u) {
     ContentGroup offer_bundle(GROUP_TYPE_BUNDLE);
-    for (ContentInfos::const_iterator content = offer->contents().begin();
-       content != offer->contents().end(); ++content) {
-      offer_bundle.AddContentName(content->name);
+    for (const ContentInfo& content : offer->contents()) {
+      // TODO(deadbeef): There are conditions that make bundling two media
+      // descriptions together illegal. For example, they use the same payload
+      // type to represent different codecs, or same IDs for different header
+      // extensions. We need to detect this and not try to bundle those media
+      // descriptions together.
+      offer_bundle.AddContentName(content.name);
     }
     offer->AddGroup(offer_bundle);
     if (!UpdateTransportInfoForBundle(offer_bundle, offer.get())) {
@@ -1445,8 +1402,10 @@ SessionDescription* MediaSessionDescriptionFactory::CreateOffer(
 }
 
 SessionDescription* MediaSessionDescriptionFactory::CreateAnswer(
-    const SessionDescription* offer, const MediaSessionOptions& options,
+    const SessionDescription* offer,
+    const MediaSessionOptions& session_options,
     const SessionDescription* current_description) const {
+  RTC_DCHECK(offer);
   // The answer contains the intersection of the codecs in the offer with the
   // codecs we support. As indicated by XEP-0167, we retain the same payload ids
   // from the offer in the answer.
@@ -1455,32 +1414,82 @@ SessionDescription* MediaSessionDescriptionFactory::CreateAnswer(
   StreamParamsVec current_streams;
   GetCurrentStreamParams(current_description, &current_streams);
 
-  if (offer) {
-    ContentInfos::const_iterator it = offer->contents().begin();
-    for (; it != offer->contents().end(); ++it) {
-      if (IsMediaContentOfType(&*it, MEDIA_TYPE_AUDIO)) {
-        if (!AddAudioContentForAnswer(offer, options, current_description,
-                                  &current_streams, answer.get())) {
-          return NULL;
+  // Get list of all possible codecs that respects existing payload type
+  // mappings and uses a single payload type space.
+  //
+  // Note that these lists may be further filtered for each m= section; this
+  // step is done just to establish the payload type mappings shared by all
+  // sections.
+  AudioCodecs answer_audio_codecs;
+  VideoCodecs answer_video_codecs;
+  DataCodecs answer_data_codecs;
+  GetCodecsForAnswer(current_description, offer, &answer_audio_codecs,
+                     &answer_video_codecs, &answer_data_codecs);
+
+  if (!session_options.vad_enabled) {
+    // If application doesn't want CN codecs in answer.
+    StripCNCodecs(&answer_audio_codecs);
+  }
+  FilterDataCodecs(&answer_data_codecs,
+                   session_options.data_channel_type == DCT_SCTP);
+
+  // Must have options for exactly as many sections as in the offer.
+  RTC_DCHECK(offer->contents().size() ==
+             session_options.media_description_options.size());
+  // Iterate through the media description options, matching with existing
+  // media descriptions in |current_description|.
+  int msection_index = 0;
+  for (const MediaDescriptionOptions& media_description_options :
+       session_options.media_description_options) {
+    const ContentInfo* offer_content = &offer->contents()[msection_index];
+    // Media types and mids must match.
+    RTC_DCHECK(
+        IsMediaContentOfType(offer_content, media_description_options.type));
+    RTC_DCHECK(media_description_options.mid == offer_content->name);
+    const ContentInfo* current_content = nullptr;
+    if (current_description &&
+        msection_index <
+            static_cast<int>(current_description->contents().size())) {
+      current_content = &current_description->contents()[msection_index];
+      // Media types and mids must match.
+      RTC_DCHECK(IsMediaContentOfType(current_content,
+                                      media_description_options.type));
+      RTC_DCHECK(media_description_options.mid == current_content->name);
+    }
+    switch (media_description_options.type) {
+      case MEDIA_TYPE_AUDIO:
+        if (!AddAudioContentForAnswer(
+                media_description_options, session_options, offer_content,
+                offer, current_content, current_description,
+                answer_audio_codecs, &current_streams, answer.get())) {
+          return nullptr;
         }
-      } else if (IsMediaContentOfType(&*it, MEDIA_TYPE_VIDEO)) {
-        if (!AddVideoContentForAnswer(offer, options, current_description,
-                                      &current_streams, answer.get())) {
-          return NULL;
+        break;
+      case MEDIA_TYPE_VIDEO:
+        if (!AddVideoContentForAnswer(
+                media_description_options, session_options, offer_content,
+                offer, current_content, current_description,
+                answer_video_codecs, &current_streams, answer.get())) {
+          return nullptr;
         }
-      } else {
-        ASSERT(IsMediaContentOfType(&*it, MEDIA_TYPE_DATA));
-        if (!AddDataContentForAnswer(offer, options, current_description,
+        break;
+      case MEDIA_TYPE_DATA:
+        if (!AddDataContentForAnswer(media_description_options, session_options,
+                                     offer_content, offer, current_content,
+                                     current_description, answer_data_codecs,
                                      &current_streams, answer.get())) {
-          return NULL;
+          return nullptr;
         }
-      }
+        break;
+      default:
+        RTC_NOTREACHED();
     }
+    ++msection_index;
   }
 
   // If the offer supports BUNDLE, and we want to use it too, create a BUNDLE
   // group in the answer with the appropriate content names.
-  if (offer->HasGroup(GROUP_TYPE_BUNDLE) && options.bundle_enabled) {
+  if (offer->HasGroup(GROUP_TYPE_BUNDLE) && session_options.bundle_enabled) {
     const ContentGroup* offer_bundle = offer->GetGroupByName(GROUP_TYPE_BUNDLE);
     ContentGroup answer_bundle(GROUP_TYPE_BUNDLE);
     for (ContentInfos::const_iterator content = answer->contents().begin();
@@ -1541,11 +1550,40 @@ const AudioCodecs& MediaSessionDescriptionFactory::GetAudioCodecsForAnswer(
   }
 }
 
-void MediaSessionDescriptionFactory::GetCodecsToOffer(
+void MergeCodecsFromDescription(const SessionDescription* description,
+                                bool can_modify_codecs,
+                                AudioCodecs* audio_codecs,
+                                VideoCodecs* video_codecs,
+                                DataCodecs* data_codecs,
+                                UsedPayloadTypes* used_pltypes) {
+  for (const ContentInfo& content : description->contents()) {
+    if (IsMediaContentOfType(&content, MEDIA_TYPE_AUDIO)) {
+      const AudioContentDescription* audio =
+          static_cast<AudioContentDescription*>(content.description);
+      MergeCodecs<AudioCodec>(audio->codecs(), can_modify_codecs, audio_codecs,
+                              used_pltypes);
+    } else if (IsMediaContentOfType(&content, MEDIA_TYPE_VIDEO)) {
+      const VideoContentDescription* video =
+          static_cast<VideoContentDescription*>(content.description);
+      MergeCodecs<VideoCodec>(video->codecs(), can_modify_codecs, video_codecs,
+                              used_pltypes);
+    } else if (IsMediaContentOfType(&content, MEDIA_TYPE_DATA)) {
+      const DataContentDescription* data =
+          static_cast<DataContentDescription*>(content.description);
+      MergeCodecs<DataCodec>(data->codecs(), can_modify_codecs, data_codecs,
+                             used_pltypes);
+    }
+  }
+}
+
+// Getting codecs for an offer involves these steps:
+//
+// 1. Construct payload type -> codec mappings for current description.
+// 2. Add any reference codecs that weren't already present
+// 3. For each individual media description (m= section), filter codecs based
+//    on the directional attribute (happens in another method).
+void MediaSessionDescriptionFactory::GetCodecsForOffer(
     const SessionDescription* current_description,
-    const AudioCodecs& supported_audio_codecs,
-    const VideoCodecs& supported_video_codecs,
-    const DataCodecs& supported_data_codecs,
     AudioCodecs* audio_codecs,
     VideoCodecs* video_codecs,
     DataCodecs* data_codecs) const {
@@ -1554,77 +1592,167 @@ void MediaSessionDescriptionFactory::GetCodecsToOffer(
   video_codecs->clear();
   data_codecs->clear();
 
-
   // First - get all codecs from the current description if the media type
-  // is used.
-  // Add them to |used_pltypes| so the payloadtype is not reused if a new media
-  // type is added.
+  // is used. Add them to |used_pltypes| so the payload type is not reused if a
+  // new media type is added.
+  bool can_modify_codecs = false;
   if (current_description) {
-    const AudioContentDescription* audio =
-        GetFirstAudioContentDescription(current_description);
-    if (audio) {
-      *audio_codecs = audio->codecs();
-      used_pltypes.FindAndSetIdUsed<AudioCodec>(audio_codecs);
-    }
-    const VideoContentDescription* video =
-        GetFirstVideoContentDescription(current_description);
-    if (video) {
-      *video_codecs = video->codecs();
-      used_pltypes.FindAndSetIdUsed<VideoCodec>(video_codecs);
-    }
-    const DataContentDescription* data =
-        GetFirstDataContentDescription(current_description);
-    if (data) {
-      *data_codecs = data->codecs();
-      used_pltypes.FindAndSetIdUsed<DataCodec>(data_codecs);
-    }
+    MergeCodecsFromDescription(current_description, can_modify_codecs,
+                               audio_codecs, video_codecs, data_codecs,
+                               &used_pltypes);
   }
 
   // Add our codecs that are not in |current_description|.
-  FindCodecsToOffer<AudioCodec>(supported_audio_codecs, audio_codecs,
-                                &used_pltypes);
-  FindCodecsToOffer<VideoCodec>(supported_video_codecs, video_codecs,
-                                &used_pltypes);
-  FindCodecsToOffer<DataCodec>(supported_data_codecs, data_codecs,
+  // We can modify our reference codecs (payload type, associated payload type,
+  // etc.) since they're not part of a negotiated session description.
+  can_modify_codecs = true;
+  MergeCodecs<AudioCodec>(all_audio_codecs_, can_modify_codecs, audio_codecs,
+                          &used_pltypes);
+  MergeCodecs<VideoCodec>(video_codecs_, can_modify_codecs, video_codecs,
+                          &used_pltypes);
+  MergeCodecs<DataCodec>(data_codecs_, can_modify_codecs, data_codecs,
+                         &used_pltypes);
+}
+
+// Getting codecs for an answer involves these steps:
+//
+// 1. Construct payload type -> codec mappings for current description.
+// 2. Add any codecs from the offer that weren't already present.
+// 3. Add any remaining codecs that weren't already present.
+// 4. For each individual media description (m= section), filter codecs based
+//    on the directional attribute (happens in another method).
+void MediaSessionDescriptionFactory::GetCodecsForAnswer(
+    const SessionDescription* current_description,
+    const SessionDescription* remote_offer,
+    AudioCodecs* audio_codecs,
+    VideoCodecs* video_codecs,
+    DataCodecs* data_codecs) const {
+  UsedPayloadTypes used_pltypes;
+  audio_codecs->clear();
+  video_codecs->clear();
+  data_codecs->clear();
+
+  // First - get all codecs from the current description if the media type
+  // is used. Add them to |used_pltypes| so the payload type is not reused if a
+  // new media type is added.
+  bool can_modify_codecs = false;
+  if (current_description) {
+    MergeCodecsFromDescription(current_description, can_modify_codecs,
+                               audio_codecs, video_codecs, data_codecs,
                                &used_pltypes);
+  }
+
+  // First, filter out codecs that we don't support at all and should ignore.
+  AudioCodecs filtered_offered_audio_codecs;
+  VideoCodecs filtered_offered_video_codecs;
+  DataCodecs filtered_offered_data_codecs;
+  for (const ContentInfo& content : remote_offer->contents()) {
+    if (IsMediaContentOfType(&content, MEDIA_TYPE_AUDIO)) {
+      const AudioContentDescription* audio =
+          static_cast<AudioContentDescription*>(content.description);
+      for (const AudioCodec& offered_audio_codec : audio->codecs()) {
+        if (!FindMatchingCodec<AudioCodec>(audio->codecs(),
+                                           filtered_offered_audio_codecs,
+                                           offered_audio_codec, nullptr) &&
+            FindMatchingCodec<AudioCodec>(audio->codecs(), all_audio_codecs_,
+                                          offered_audio_codec, nullptr)) {
+          filtered_offered_audio_codecs.push_back(offered_audio_codec);
+        }
+      }
+    } else if (IsMediaContentOfType(&content, MEDIA_TYPE_VIDEO)) {
+      const VideoContentDescription* video =
+          static_cast<VideoContentDescription*>(content.description);
+      for (const VideoCodec& offered_video_codec : video->codecs()) {
+        if (!FindMatchingCodec<VideoCodec>(video->codecs(),
+                                           filtered_offered_video_codecs,
+                                           offered_video_codec, nullptr) &&
+            FindMatchingCodec<VideoCodec>(video->codecs(), video_codecs_,
+                                          offered_video_codec, nullptr)) {
+          filtered_offered_video_codecs.push_back(offered_video_codec);
+        }
+      }
+    } else if (IsMediaContentOfType(&content, MEDIA_TYPE_DATA)) {
+      const DataContentDescription* data =
+          static_cast<DataContentDescription*>(content.description);
+      for (const DataCodec& offered_data_codec : data->codecs()) {
+        if (!FindMatchingCodec<DataCodec>(data->codecs(),
+                                          filtered_offered_data_codecs,
+                                          offered_data_codec, nullptr) &&
+            FindMatchingCodec<DataCodec>(data->codecs(), data_codecs_,
+                                         offered_data_codec, nullptr)) {
+          filtered_offered_data_codecs.push_back(offered_data_codec);
+        }
+      }
+    }
+  }
+
+  // Add codecs that are not in |current_description| but were in
+  // |remote_offer|. Do two passes, first adding codecs with payload types that
+  // match the offer, then adding remaining codecs with remapped payload types.
+  //
+  // The two passes are needed so that a payload type isn't remapped to
+  // conflict with another codec. For example, PT "96" is in current
+  // description, and a remote offer has "96" and "97". If "96" were remapped
+  // in the first pass (to "97", since that's the next number), that's now not
+  // available for "97", causing "97" to be remapped to "98" and a payload type
+  // to be wasted.
+  MergeCodecs<AudioCodec>(filtered_offered_audio_codecs, can_modify_codecs,
+                          audio_codecs, &used_pltypes);
+  MergeCodecs<VideoCodec>(filtered_offered_video_codecs, can_modify_codecs,
+                          video_codecs, &used_pltypes);
+  MergeCodecs<DataCodec>(filtered_offered_data_codecs, can_modify_codecs,
+                         data_codecs, &used_pltypes);
+  can_modify_codecs = true;
+  MergeCodecs<AudioCodec>(filtered_offered_audio_codecs, can_modify_codecs,
+                          audio_codecs, &used_pltypes);
+  MergeCodecs<VideoCodec>(filtered_offered_video_codecs, can_modify_codecs,
+                          video_codecs, &used_pltypes);
+  MergeCodecs<DataCodec>(filtered_offered_data_codecs, can_modify_codecs,
+                         data_codecs, &used_pltypes);
 }
 
 void MediaSessionDescriptionFactory::GetRtpHdrExtsToOffer(
     const SessionDescription* current_description,
-    RtpHeaderExtensions* audio_extensions,
-    RtpHeaderExtensions* video_extensions) const {
+    RtpHeaderExtensions* offer_audio_extensions,
+    RtpHeaderExtensions* offer_video_extensions) const {
   // All header extensions allocated from the same range to avoid potential
   // issues when using BUNDLE.
   UsedRtpHeaderExtensionIds used_ids;
   RtpHeaderExtensions all_extensions;
-  audio_extensions->clear();
-  video_extensions->clear();
+  offer_audio_extensions->clear();
+  offer_video_extensions->clear();
 
   // First - get all extensions from the current description if the media type
-  // is used.
-  // Add them to |used_ids| so the local ids are not reused if a new media
-  // type is added.
+  // is used. Add them to |used_ids| so the local ids are not reused if a new
+  // media type is added.
+  bool can_modify_extensions = false;
   if (current_description) {
-    const AudioContentDescription* audio =
-        GetFirstAudioContentDescription(current_description);
-    if (audio) {
-      *audio_extensions = audio->rtp_header_extensions();
-      FindAndSetRtpHdrExtUsed(audio_extensions, &all_extensions, &used_ids);
-    }
-    const VideoContentDescription* video =
-        GetFirstVideoContentDescription(current_description);
-    if (video) {
-      *video_extensions = video->rtp_header_extensions();
-      FindAndSetRtpHdrExtUsed(video_extensions, &all_extensions, &used_ids);
+    for (const ContentInfo& content : current_description->contents()) {
+      if (IsMediaContentOfType(&content, MEDIA_TYPE_DATA)) {
+        continue;
+      }
+      if (IsMediaContentOfType(&content, MEDIA_TYPE_AUDIO)) {
+        const AudioContentDescription* audio =
+            static_cast<const AudioContentDescription*>(content.description);
+        MergeRtpHdrExts(audio->rtp_header_extensions(), can_modify_extensions,
+                        offer_audio_extensions, &all_extensions, &used_ids);
+      } else if (IsMediaContentOfType(&content, MEDIA_TYPE_VIDEO)) {
+        const VideoContentDescription* video =
+            static_cast<const VideoContentDescription*>(content.description);
+        MergeRtpHdrExts(video->rtp_header_extensions(), can_modify_extensions,
+                        offer_video_extensions, &all_extensions, &used_ids);
+      }
     }
   }
+  // TODO: Do this for answer too?
 
   // Add our default RTP header extensions that are not in
   // |current_description|.
-  FindRtpHdrExtsToOffer(audio_rtp_header_extensions(), audio_extensions,
-                        &all_extensions, &used_ids);
-  FindRtpHdrExtsToOffer(video_rtp_header_extensions(), video_extensions,
-                        &all_extensions, &used_ids);
+  can_modify_extensions = true;
+  MergeRtpHdrExts(audio_rtp_header_extensions(), can_modify_extensions,
+                  offer_audio_extensions, &all_extensions, &used_ids);
+  MergeRtpHdrExts(video_rtp_header_extensions(), can_modify_extensions,
+                  offer_video_extensions, &all_extensions, &used_ids);
 }
 
 bool MediaSessionDescriptionFactory::AddTransportOffer(
@@ -1677,34 +1805,38 @@ bool MediaSessionDescriptionFactory::AddTransportAnswer(
 }
 
 bool MediaSessionDescriptionFactory::AddAudioContentForOffer(
-    const MediaSessionOptions& options,
+    const MediaDescriptionOptions& media_description_options,
+    const MediaSessionOptions& session_options,
+    const ContentInfo* current_content,
     const SessionDescription* current_description,
     const RtpHeaderExtensions& audio_rtp_extensions,
     const AudioCodecs& audio_codecs,
     StreamParamsVec* current_streams,
     SessionDescription* desc) const {
-  const ContentInfo* current_audio_content =
-      GetFirstAudioContent(current_description);
-  std::string content_name =
-      current_audio_content ? current_audio_content->name : CN_AUDIO;
+  // Filter audio_codecs (which includes all codecs, with correctly remapped
+  // payload types) based on transceiver direction.
+  const AudioCodecs& supported_audio_codecs =
+      GetAudioCodecsForOffer(media_description_options.direction);
+  AudioCodecs filtered_codecs;
+  for (const AudioCodec& codec : audio_codecs) {
+    if (FindMatchingCodec<AudioCodec>(audio_codecs, supported_audio_codecs,
+                                      codec, nullptr)) {
+      filtered_codecs.push_back(codec);
+    }
+  }
 
   cricket::SecurePolicy sdes_policy =
-      IsDtlsActive(content_name, current_description) ? cricket::SEC_DISABLED
-                                                      : secure();
+      IsDtlsActive(current_content, current_description) ? cricket::SEC_DISABLED
+                                                         : secure();
 
   std::unique_ptr<AudioContentDescription> audio(new AudioContentDescription());
   std::vector<std::string> crypto_suites;
-  GetSupportedAudioCryptoSuiteNames(options.crypto_options, &crypto_suites);
+  GetSupportedAudioCryptoSuiteNames(session_options.crypto_options,
+                                    &crypto_suites);
   if (!CreateMediaContentOffer(
-          options,
-          audio_codecs,
-          sdes_policy,
-          GetCryptos(GetFirstAudioContentDescription(current_description)),
-          crypto_suites,
-          audio_rtp_extensions,
-          add_legacy_,
-          current_streams,
-          audio.get())) {
+          media_description_options.sender_options, session_options,
+          filtered_codecs, sdes_policy, GetCryptos(current_content),
+          crypto_suites, audio_rtp_extensions, current_streams, audio.get())) {
     return false;
   }
   audio->set_lang(lang_);
@@ -1712,13 +1844,13 @@ bool MediaSessionDescriptionFactory::AddAudioContentForOffer(
   bool secure_transport = (transport_desc_factory_->secure() != SEC_DISABLED);
   SetMediaProtocol(secure_transport, audio.get());
 
-  auto offer_rtd =
-      RtpTransceiverDirection(!audio->streams().empty(), options.recv_audio);
-  audio->set_direction(offer_rtd.ToMediaContentDirection());
+  audio->set_direction(
+      media_description_options.direction.ToMediaContentDirection());
 
-  desc->AddContent(content_name, NS_JINGLE_RTP, audio.release());
-  if (!AddTransportOffer(content_name,
-                         GetTransportOptions(options, content_name),
+  desc->AddContent(media_description_options.mid, NS_JINGLE_RTP,
+                   audio.release());
+  if (!AddTransportOffer(media_description_options.mid,
+                         media_description_options.transport_options,
                          current_description, desc)) {
     return false;
   }
@@ -1727,87 +1859,72 @@ bool MediaSessionDescriptionFactory::AddAudioContentForOffer(
 }
 
 bool MediaSessionDescriptionFactory::AddVideoContentForOffer(
-    const MediaSessionOptions& options,
+    const MediaDescriptionOptions& media_description_options,
+    const MediaSessionOptions& session_options,
+    const ContentInfo* current_content,
     const SessionDescription* current_description,
     const RtpHeaderExtensions& video_rtp_extensions,
     const VideoCodecs& video_codecs,
     StreamParamsVec* current_streams,
     SessionDescription* desc) const {
-  const ContentInfo* current_video_content =
-      GetFirstVideoContent(current_description);
-  std::string content_name =
-      current_video_content ? current_video_content->name : CN_VIDEO;
+  // Filter video_codecs (which includes all codecs, with correctly remapped
+  // payload types).
+  VideoCodecs filtered_codecs;
+  for (const VideoCodec& codec : video_codecs) {
+    if (FindMatchingCodec<VideoCodec>(video_codecs, video_codecs_, codec,
+                                      nullptr)) {
+      filtered_codecs.push_back(codec);
+    }
+  }
 
   cricket::SecurePolicy sdes_policy =
-      IsDtlsActive(content_name, current_description) ? cricket::SEC_DISABLED
-                                                      : secure();
+      IsDtlsActive(current_content, current_description) ? cricket::SEC_DISABLED
+                                                         : secure();
 
   std::unique_ptr<VideoContentDescription> video(new VideoContentDescription());
   std::vector<std::string> crypto_suites;
-  GetSupportedVideoCryptoSuiteNames(options.crypto_options, &crypto_suites);
+  GetSupportedVideoCryptoSuiteNames(session_options.crypto_options,
+                                    &crypto_suites);
   if (!CreateMediaContentOffer(
-          options,
-          video_codecs,
-          sdes_policy,
-          GetCryptos(GetFirstVideoContentDescription(current_description)),
-          crypto_suites,
-          video_rtp_extensions,
-          add_legacy_,
-          current_streams,
-          video.get())) {
+          media_description_options.sender_options, session_options,
+          filtered_codecs, sdes_policy, GetCryptos(current_content),
+          crypto_suites, video_rtp_extensions, current_streams, video.get())) {
     return false;
   }
-
-  video->set_bandwidth(options.video_bandwidth);
+  video->set_bandwidth(kAutoBandwidth);
 
   bool secure_transport = (transport_desc_factory_->secure() != SEC_DISABLED);
   SetMediaProtocol(secure_transport, video.get());
 
-  if (!video->streams().empty()) {
-    if (options.recv_video) {
-      video->set_direction(MD_SENDRECV);
-    } else {
-      video->set_direction(MD_SENDONLY);
-    }
-  } else {
-    if (options.recv_video) {
-      video->set_direction(MD_RECVONLY);
-    } else {
-      video->set_direction(MD_INACTIVE);
-    }
-  }
+  video->set_direction(
+      media_description_options.direction.ToMediaContentDirection());
 
-  desc->AddContent(content_name, NS_JINGLE_RTP, video.release());
-  if (!AddTransportOffer(content_name,
-                         GetTransportOptions(options, content_name),
+  desc->AddContent(media_description_options.mid, NS_JINGLE_RTP,
+                   video.release());
+  if (!AddTransportOffer(media_description_options.mid,
+                         media_description_options.transport_options,
                          current_description, desc)) {
     return false;
   }
-
   return true;
 }
 
 bool MediaSessionDescriptionFactory::AddDataContentForOffer(
-    const MediaSessionOptions& options,
+    const MediaDescriptionOptions& media_description_options,
+    const MediaSessionOptions& session_options,
+    const ContentInfo* current_content,
     const SessionDescription* current_description,
-    DataCodecs* data_codecs,
+    const DataCodecs& data_codecs,
     StreamParamsVec* current_streams,
     SessionDescription* desc) const {
   bool secure_transport = (transport_desc_factory_->secure() != SEC_DISABLED);
 
   std::unique_ptr<DataContentDescription> data(new DataContentDescription());
-  bool is_sctp = (options.data_channel_type == DCT_SCTP);
-
-  FilterDataCodecs(data_codecs, is_sctp);
-
-  const ContentInfo* current_data_content =
-      GetFirstDataContent(current_description);
-  std::string content_name =
-      current_data_content ? current_data_content->name : CN_DATA;
+  bool is_sctp = (session_options.data_channel_type == DCT_SCTP);
 
   cricket::SecurePolicy sdes_policy =
-      IsDtlsActive(content_name, current_description) ? cricket::SEC_DISABLED
-                                                      : secure();
+      IsDtlsActive(current_content, current_description) ? cricket::SEC_DISABLED
+                                                         : secure();
   std::vector<std::string> crypto_suites;
   if (is_sctp) {
     // SDES doesn't make sense for SCTP, so we disable it, and we only
@@ -1820,31 +1937,29 @@ bool MediaSessionDescriptionFactory::AddDataContentForOffer(
     data->set_protocol(
         secure_transport ? kMediaProtocolDtlsSctp : kMediaProtocolSctp);
   } else {
-    GetSupportedDataCryptoSuiteNames(options.crypto_options, &crypto_suites);
+    GetSupportedDataCryptoSuiteNames(session_options.crypto_options,
+                                     &crypto_suites);
   }
 
+  // Even SCTP uses a "codec".
   if (!CreateMediaContentOffer(
-          options,
-          *data_codecs,
-          sdes_policy,
-          GetCryptos(GetFirstDataContentDescription(current_description)),
-          crypto_suites,
-          RtpHeaderExtensions(),
-          add_legacy_,
-          current_streams,
-          data.get())) {
+          media_description_options.sender_options, session_options,
+          data_codecs, sdes_policy, GetCryptos(current_content), crypto_suites,
+          RtpHeaderExtensions(), current_streams, data.get())) {
     return false;
   }
 
   if (is_sctp) {
-    desc->AddContent(content_name, NS_JINGLE_DRAFT_SCTP, data.release());
+    desc->AddContent(media_description_options.mid, NS_JINGLE_DRAFT_SCTP,
+                     data.release());
   } else {
-    data->set_bandwidth(options.data_bandwidth);
+    data->set_bandwidth(kDataMaxBandwidth);
     SetMediaProtocol(secure_transport, data.get());
-    desc->AddContent(content_name, NS_JINGLE_RTP, data.release());
+    desc->AddContent(media_description_options.mid, NS_JINGLE_RTP,
+                     data.release());
   }
-  if (!AddTransportOffer(content_name,
-                         GetTransportOptions(options, content_name),
+  if (!AddTransportOffer(media_description_options.mid,
+                         media_description_options.transport_options,
                          current_description, desc)) {
     return false;
   }
@@ -1852,175 +1967,173 @@ bool MediaSessionDescriptionFactory::AddDataContentForOffer(
 }
 
 bool MediaSessionDescriptionFactory::AddAudioContentForAnswer(
-    const SessionDescription* offer,
-    const MediaSessionOptions& options,
+    const MediaDescriptionOptions& media_description_options,
+    const MediaSessionOptions& session_options,
+    const ContentInfo* offer_content,
+    const SessionDescription* offer_description,
+    const ContentInfo* current_content,
     const SessionDescription* current_description,
+    const AudioCodecs& audio_codecs,
     StreamParamsVec* current_streams,
     SessionDescription* answer) const {
-  const ContentInfo* audio_content = GetFirstAudioContent(offer);
-  const AudioContentDescription* offer_audio =
-      static_cast<const AudioContentDescription*>(audio_content->description);
+  const AudioContentDescription* offer_audio_description =
+      static_cast<const AudioContentDescription*>(offer_content->description);
 
   std::unique_ptr<TransportDescription> audio_transport(CreateTransportAnswer(
-      audio_content->name, offer,
-      GetTransportOptions(options, audio_content->name), current_description));
+      media_description_options.mid, offer_description,
+      media_description_options.transport_options, current_description));
   if (!audio_transport) {
     return false;
   }
 
-  // Pick codecs based on the requested communications direction in the offer.
-  const bool wants_send =
-      options.HasSendMediaStream(MEDIA_TYPE_AUDIO) || add_legacy_;
-  auto wants_rtd = RtpTransceiverDirection(wants_send, options.recv_audio);
-  auto offer_rtd =
-      RtpTransceiverDirection::FromMediaContentDirection(
-          offer_audio->direction());
+  // Pick codecs based on the requested communications direction in the offer
+  // and the selected direction in the answer.
+  // Note these will be filtered one final time in CreateMediaContentAnswer.
+  auto wants_rtd = media_description_options.direction;
+  auto offer_rtd = RtpTransceiverDirection::FromMediaContentDirection(
+      offer_audio_description->direction());
   auto answer_rtd = NegotiateRtpTransceiverDirection(offer_rtd, wants_rtd);
-  AudioCodecs audio_codecs = GetAudioCodecsForAnswer(offer_rtd, answer_rtd);
-  if (!options.vad_enabled) {
-    StripCNCodecs(&audio_codecs);
+  const AudioCodecs& supported_audio_codecs =
+      GetAudioCodecsForAnswer(offer_rtd, answer_rtd);
+  AudioCodecs filtered_codecs;
+  for (const AudioCodec& codec : audio_codecs) {
+    if (FindMatchingCodec<AudioCodec>(audio_codecs, supported_audio_codecs,
+                                      codec, nullptr) &&
+        FindMatchingCodec<AudioCodec>(audio_codecs, supported_audio_codecs,
+                                      codec, nullptr)) {
+      filtered_codecs.push_back(codec);
+    }
   }
 
-  bool bundle_enabled =
-      offer->HasGroup(GROUP_TYPE_BUNDLE) && options.bundle_enabled;
+  bool bundle_enabled = offer_description->HasGroup(GROUP_TYPE_BUNDLE) &&
+                        session_options.bundle_enabled;
   std::unique_ptr<AudioContentDescription> audio_answer(
       new AudioContentDescription());
   // Do not require or create SDES cryptos if DTLS is used.
   cricket::SecurePolicy sdes_policy =
       audio_transport->secure() ? cricket::SEC_DISABLED : secure();
   if (!CreateMediaContentAnswer(
-          offer_audio,
-          options,
-          audio_codecs,
-          sdes_policy,
-          GetCryptos(GetFirstAudioContentDescription(current_description)),
-          audio_rtp_extensions_,
-          current_streams,
-          add_legacy_,
-          bundle_enabled,
-          audio_answer.get())) {
+          offer_audio_description, media_description_options.sender_options,
+          media_description_options.direction, session_options, filtered_codecs,
+          sdes_policy, GetCryptos(current_content), audio_rtp_extensions_,
+          current_streams, bundle_enabled, audio_answer.get())) {
     return false;  // Fails the session setup.
   }
 
-  bool rejected = !options.has_audio() || audio_content->rejected ||
-      !IsMediaProtocolSupported(MEDIA_TYPE_AUDIO,
-                                audio_answer->protocol(),
+  bool rejected =
+      media_description_options.stopped || offer_content->rejected ||
+      !IsMediaProtocolSupported(MEDIA_TYPE_AUDIO, audio_answer->protocol(),
                                 audio_transport->secure());
   if (!rejected) {
-    AddTransportAnswer(audio_content->name, *(audio_transport.get()), answer);
+    AddTransportAnswer(media_description_options.mid, *(audio_transport.get()),
+                       answer);
   } else {
-    // RFC 3264
-    // The answer MUST contain the same number of m-lines as the offer.
-    LOG(LS_INFO) << "Audio is not supported in the answer.";
+    LOG(LS_INFO) << "Audio m= section '" << media_description_options.mid
+                 << "' being rejected in answer.";
   }
 
-  answer->AddContent(audio_content->name, audio_content->type, rejected,
-                     audio_answer.release());
+  answer->AddContent(media_description_options.mid, offer_content->type,
+                     rejected, audio_answer.release());
   return true;
 }
 
 bool MediaSessionDescriptionFactory::AddVideoContentForAnswer(
-    const SessionDescription* offer,
-    const MediaSessionOptions& options,
+    const MediaDescriptionOptions& media_description_options,
+    const MediaSessionOptions& session_options,
+    const ContentInfo* offer_content,
+    const SessionDescription* offer_description,
+    const ContentInfo* current_content,
     const SessionDescription* current_description,
+    const VideoCodecs& video_codecs,
     StreamParamsVec* current_streams,
     SessionDescription* answer) const {
-  const ContentInfo* video_content = GetFirstVideoContent(offer);
+  const VideoContentDescription* offer_video_description =
+      static_cast<const VideoContentDescription*>(offer_content->description);
+
   std::unique_ptr<TransportDescription> video_transport(CreateTransportAnswer(
-      video_content->name, offer,
-      GetTransportOptions(options, video_content->name), current_description));
+      media_description_options.mid, offer_description,
+      media_description_options.transport_options, current_description));
   if (!video_transport) {
     return false;
   }
 
+  bool bundle_enabled = offer_description->HasGroup(GROUP_TYPE_BUNDLE) &&
+                        session_options.bundle_enabled;
   std::unique_ptr<VideoContentDescription> video_answer(
       new VideoContentDescription());
   // Do not require or create SDES cryptos if DTLS is used.
   cricket::SecurePolicy sdes_policy =
       video_transport->secure() ? cricket::SEC_DISABLED : secure();
-  bool bundle_enabled =
-      offer->HasGroup(GROUP_TYPE_BUNDLE) && options.bundle_enabled;
   if (!CreateMediaContentAnswer(
-          static_cast<const VideoContentDescription*>(
-              video_content->description),
-          options,
-          video_codecs_,
-          sdes_policy,
-          GetCryptos(GetFirstVideoContentDescription(current_description)),
-          video_rtp_extensions_,
-          current_streams,
-          add_legacy_,
-          bundle_enabled,
-          video_answer.get())) {
-    return false;
+          offer_video_description, media_description_options.sender_options,
+          media_description_options.direction, session_options, video_codecs,
+          sdes_policy, GetCryptos(current_content), video_rtp_extensions_,
+          current_streams, bundle_enabled, video_answer.get())) {
+    return false;  // Fails the session setup.
   }
-  bool rejected = !options.has_video() || video_content->rejected ||
-      !IsMediaProtocolSupported(MEDIA_TYPE_VIDEO,
-                                video_answer->protocol(),
+
+  bool rejected =
+      media_description_options.stopped || offer_content->rejected ||
+      !IsMediaProtocolSupported(MEDIA_TYPE_VIDEO, video_answer->protocol(),
                                 video_transport->secure());
   if (!rejected) {
-    if (!AddTransportAnswer(video_content->name, *(video_transport.get()),
-                            answer)) {
+    if (!AddTransportAnswer(media_description_options.mid,
+                            *(video_transport.get()), answer)) {
       return false;
     }
-    video_answer->set_bandwidth(options.video_bandwidth);
+    video_answer->set_bandwidth(kAutoBandwidth);
   } else {
-    // RFC 3264
-    // The answer MUST contain the same number of m-lines as the offer.
-    LOG(LS_INFO) << "Video is not supported in the answer.";
+    LOG(LS_INFO) << "Video m= section '" << media_description_options.mid
+                 << "' being rejected in answer.";
   }
-  answer->AddContent(video_content->name, video_content->type, rejected,
-                     video_answer.release());
+
+  answer->AddContent(media_description_options.mid, offer_content->type,
+                     rejected, video_answer.release());
   return true;
 }
 
 bool MediaSessionDescriptionFactory::AddDataContentForAnswer(
-    const SessionDescription* offer,
-    const MediaSessionOptions& options,
+    const MediaDescriptionOptions& media_description_options,
+    const MediaSessionOptions& session_options,
+    const ContentInfo* offer_content,
+    const SessionDescription* offer_description,
+    const ContentInfo* current_content,
     const SessionDescription* current_description,
+    const DataCodecs& data_codecs,
     StreamParamsVec* current_streams,
     SessionDescription* answer) const {
-  const ContentInfo* data_content = GetFirstDataContent(offer);
   std::unique_ptr<TransportDescription> data_transport(CreateTransportAnswer(
-      data_content->name, offer,
-      GetTransportOptions(options, data_content->name), current_description));
+      media_description_options.mid, offer_description,
+      media_description_options.transport_options, current_description));
   if (!data_transport) {
     return false;
   }
-  bool is_sctp = (options.data_channel_type == DCT_SCTP);
-  std::vector<DataCodec> data_codecs(data_codecs_);
-  FilterDataCodecs(&data_codecs, is_sctp);
 
   std::unique_ptr<DataContentDescription> data_answer(
       new DataContentDescription());
   // Do not require or create SDES cryptos if DTLS is used.
   cricket::SecurePolicy sdes_policy =
       data_transport->secure() ? cricket::SEC_DISABLED : secure();
-  bool bundle_enabled =
-      offer->HasGroup(GROUP_TYPE_BUNDLE) && options.bundle_enabled;
+  bool bundle_enabled = offer_description->HasGroup(GROUP_TYPE_BUNDLE) &&
+                        session_options.bundle_enabled;
   if (!CreateMediaContentAnswer(
           static_cast<const DataContentDescription*>(
-              data_content->description),
-          options,
-          data_codecs_,
-          sdes_policy,
-          GetCryptos(GetFirstDataContentDescription(current_description)),
-          RtpHeaderExtensions(),
-          current_streams,
-          add_legacy_,
-          bundle_enabled,
-          data_answer.get())) {
+              offer_content->description),
+          media_description_options.sender_options,
+          media_description_options.direction, session_options, data_codecs,
+          sdes_policy, GetCryptos(current_content), RtpHeaderExtensions(),
+          current_streams, bundle_enabled, data_answer.get())) {
     return false;  // Fails the session setup.
   }
 
-  bool rejected = !options.has_data() || data_content->rejected ||
-      !IsMediaProtocolSupported(MEDIA_TYPE_DATA,
-                                data_answer->protocol(),
+  bool rejected =
+      media_description_options.stopped || offer_content->rejected ||
+      !IsMediaProtocolSupported(MEDIA_TYPE_DATA, data_answer->protocol(),
                                 data_transport->secure());
   if (!rejected) {
-    data_answer->set_bandwidth(options.data_bandwidth);
-    if (!AddTransportAnswer(data_content->name, *(data_transport.get()),
-                            answer)) {
+    data_answer->set_bandwidth(kDataMaxBandwidth);
+    if (!AddTransportAnswer(media_description_options.mid,
+                            *(data_transport.get()), answer)) {
       return false;
     }
   } else {
@@ -2028,8 +2141,8 @@ bool MediaSessionDescriptionFactory::AddDataContentForAnswer(
     // The answer MUST contain the same number of m-lines as the offer.
     LOG(LS_INFO) << "Data is not supported in the answer.";
   }
-  answer->AddContent(data_content->name, data_content->type, rejected,
-                     data_answer.release());
+  answer->AddContent(media_description_options.mid, offer_content->type,
+                     rejected, data_answer.release());
   return true;
 }