Refactor webrtc::PeerConnection to split out offer/answer

This reduces the size of peer_connection.cc by more than 2000 lines.

Design doc for refatoring (available on request):
https://docs.google.com/document/d/1ETeUhon9sJihEUpA9ZZHpOGhzDqlZGLQOk3cD_CjKDM/edit

Bug: webrtc:11995
Change-Id: I9ed8603807b45bb192a01df026755cb6b5365291
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/185801
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32212}
diff --git a/pc/BUILD.gn b/pc/BUILD.gn
index 1fad92a..7b7f863 100644
--- a/pc/BUILD.gn
+++ b/pc/BUILD.gn
@@ -206,6 +206,8 @@
     "rtp_transceiver.h",
     "sctp_data_channel.cc",
     "sctp_data_channel.h",
+    "sdp_offer_answer.cc",  # TODO: Make separate target when not circular
+    "sdp_offer_answer.h",  # dependent on peerconnection.h
     "sdp_serializer.cc",
     "sdp_serializer.h",
     "sdp_utils.cc",
diff --git a/pc/peer_connection.cc b/pc/peer_connection.cc
index 4c8a00b..75dc80b 100644
--- a/pc/peer_connection.cc
+++ b/pc/peer_connection.cc
@@ -47,6 +47,7 @@
 #include "pc/rtp_sender.h"
 #include "pc/sctp_transport.h"
 #include "pc/sctp_utils.h"
+#include "pc/sdp_offer_answer.h"
 #include "pc/sdp_utils.h"
 #include "pc/stream_collection.h"
 #include "pc/video_rtp_receiver.h"
@@ -82,35 +83,14 @@
 namespace webrtc {
 
 // Error messages
-const char kBundleWithoutRtcpMux[] =
-    "rtcp-mux must be enabled when BUNDLE "
-    "is enabled.";
-const char kInvalidCandidates[] = "Description contains invalid candidates.";
-const char kInvalidSdp[] = "Invalid session description.";
-const char kMlineMismatchInAnswer[] =
-    "The order of m-lines in answer doesn't match order in offer. Rejecting "
-    "answer.";
-const char kMlineMismatchInSubsequentOffer[] =
-    "The order of m-lines in subsequent offer doesn't match order from "
-    "previous offer/answer.";
-const char kSdpWithoutDtlsFingerprint[] =
-    "Called with SDP without DTLS fingerprint.";
-const char kSdpWithoutSdesCrypto[] = "Called with SDP without SDES crypto.";
-const char kSdpWithoutIceUfragPwd[] =
-    "Called with SDP without ice-ufrag and ice-pwd.";
 const char kSessionError[] = "Session error code: ";
 const char kSessionErrorDesc[] = "Session error description: ";
 
 namespace {
 
 // UMA metric names.
-const char kSimulcastVersionApplyLocalDescription[] =
-    "WebRTC.PeerConnection.Simulcast.ApplyLocalDescription";
-const char kSimulcastVersionApplyRemoteDescription[] =
-    "WebRTC.PeerConnection.Simulcast.ApplyRemoteDescription";
 const char kSimulcastNumberOfEncodings[] =
     "WebRTC.PeerConnection.Simulcast.NumberOfSendEncodings";
-const char kSimulcastDisabled[] = "WebRTC.PeerConnection.Simulcast.Disabled";
 
 static const char kDefaultStreamId[] = "default";
 static const char kDefaultAudioSenderId[] = "defaulta0";
@@ -169,22 +149,6 @@
   return true;
 }
 
-// If the direction is "recvonly" or "inactive", treat the description
-// as containing no streams.
-// See: https://code.google.com/p/webrtc/issues/detail?id=5054
-std::vector<cricket::StreamParams> GetActiveStreams(
-    const cricket::MediaContentDescription* desc) {
-  return RtpTransceiverDirectionHasSend(desc->direction())
-             ? desc->streams()
-             : std::vector<cricket::StreamParams>();
-}
-
-bool IsValidOfferToReceiveMedia(int value) {
-  typedef PeerConnectionInterface::RTCOfferAnswerOptions Options;
-  return (value >= Options::kUndefined) &&
-         (value <= Options::kMaxOfferToReceiveMedia);
-}
-
 // Add options to |[audio/video]_media_description_options| from |senders|.
 void AddPlanBRtpSenderOptions(
     const std::vector<rtc::scoped_refptr<
@@ -247,28 +211,6 @@
   return cricket::CF_NONE;
 }
 
-// Map internal signaling state name to spec name:
-//  https://w3c.github.io/webrtc-pc/#rtcsignalingstate-enum
-std::string GetSignalingStateString(
-    PeerConnectionInterface::SignalingState state) {
-  switch (state) {
-    case PeerConnectionInterface::kStable:
-      return "stable";
-    case PeerConnectionInterface::kHaveLocalOffer:
-      return "have-local-offer";
-    case PeerConnectionInterface::kHaveLocalPrAnswer:
-      return "have-local-pranswer";
-    case PeerConnectionInterface::kHaveRemoteOffer:
-      return "have-remote-offer";
-    case PeerConnectionInterface::kHaveRemotePrAnswer:
-      return "have-remote-pranswer";
-    case PeerConnectionInterface::kClosed:
-      return "closed";
-  }
-  RTC_NOTREACHED();
-  return "";
-}
-
 IceCandidatePairType GetIceCandidatePairCounter(
     const cricket::Candidate& local,
     const cricket::Candidate& remote) {
@@ -342,253 +284,6 @@
   return kIceCandidatePairMax;
 }
 
-// Logic to decide if an m= section can be recycled. This means that the new
-// m= section is not rejected, but the old local or remote m= section is
-// rejected. |old_content_one| and |old_content_two| refer to the m= section
-// of the old remote and old local descriptions in no particular order.
-// We need to check both the old local and remote because either
-// could be the most current from the latest negotation.
-bool IsMediaSectionBeingRecycled(SdpType type,
-                                 const ContentInfo& content,
-                                 const ContentInfo* old_content_one,
-                                 const ContentInfo* old_content_two) {
-  return type == SdpType::kOffer && !content.rejected &&
-         ((old_content_one && old_content_one->rejected) ||
-          (old_content_two && old_content_two->rejected));
-}
-
-// Verify that the order of media sections in |new_desc| matches
-// |current_desc|. The number of m= sections in |new_desc| should be no
-// less than |current_desc|. In the case of checking an answer's
-// |new_desc|, the |current_desc| is the last offer that was set as the
-// local or remote. In the case of checking an offer's |new_desc| we
-// check against the local and remote descriptions stored from the last
-// negotiation, because either of these could be the most up to date for
-// possible rejected m sections. These are the |current_desc| and
-// |secondary_current_desc|.
-bool MediaSectionsInSameOrder(const SessionDescription& current_desc,
-                              const SessionDescription* secondary_current_desc,
-                              const SessionDescription& new_desc,
-                              const SdpType type) {
-  if (current_desc.contents().size() > new_desc.contents().size()) {
-    return false;
-  }
-
-  for (size_t i = 0; i < current_desc.contents().size(); ++i) {
-    const cricket::ContentInfo* secondary_content_info = nullptr;
-    if (secondary_current_desc &&
-        i < secondary_current_desc->contents().size()) {
-      secondary_content_info = &secondary_current_desc->contents()[i];
-    }
-    if (IsMediaSectionBeingRecycled(type, new_desc.contents()[i],
-                                    &current_desc.contents()[i],
-                                    secondary_content_info)) {
-      // For new offer descriptions, if the media section can be recycled, it's
-      // valid for the MID and media type to change.
-      continue;
-    }
-    if (new_desc.contents()[i].name != current_desc.contents()[i].name) {
-      return false;
-    }
-    const MediaContentDescription* new_desc_mdesc =
-        new_desc.contents()[i].media_description();
-    const MediaContentDescription* current_desc_mdesc =
-        current_desc.contents()[i].media_description();
-    if (new_desc_mdesc->type() != current_desc_mdesc->type()) {
-      return false;
-    }
-  }
-  return true;
-}
-
-bool MediaSectionsHaveSameCount(const SessionDescription& desc1,
-                                const SessionDescription& desc2) {
-  return desc1.contents().size() == desc2.contents().size();
-}
-
-void NoteKeyProtocolAndMedia(KeyExchangeProtocolType protocol_type,
-                             cricket::MediaType media_type) {
-  // Array of structs needed to map {KeyExchangeProtocolType,
-  // cricket::MediaType} to KeyExchangeProtocolMedia without using std::map in
-  // order to avoid -Wglobal-constructors and -Wexit-time-destructors.
-  static constexpr struct {
-    KeyExchangeProtocolType protocol_type;
-    cricket::MediaType media_type;
-    KeyExchangeProtocolMedia protocol_media;
-  } kEnumCounterKeyProtocolMediaMap[] = {
-      {kEnumCounterKeyProtocolDtls, cricket::MEDIA_TYPE_AUDIO,
-       kEnumCounterKeyProtocolMediaTypeDtlsAudio},
-      {kEnumCounterKeyProtocolDtls, cricket::MEDIA_TYPE_VIDEO,
-       kEnumCounterKeyProtocolMediaTypeDtlsVideo},
-      {kEnumCounterKeyProtocolDtls, cricket::MEDIA_TYPE_DATA,
-       kEnumCounterKeyProtocolMediaTypeDtlsData},
-      {kEnumCounterKeyProtocolSdes, cricket::MEDIA_TYPE_AUDIO,
-       kEnumCounterKeyProtocolMediaTypeSdesAudio},
-      {kEnumCounterKeyProtocolSdes, cricket::MEDIA_TYPE_VIDEO,
-       kEnumCounterKeyProtocolMediaTypeSdesVideo},
-      {kEnumCounterKeyProtocolSdes, cricket::MEDIA_TYPE_DATA,
-       kEnumCounterKeyProtocolMediaTypeSdesData},
-  };
-
-  RTC_HISTOGRAM_ENUMERATION("WebRTC.PeerConnection.KeyProtocol", protocol_type,
-                            kEnumCounterKeyProtocolMax);
-
-  for (const auto& i : kEnumCounterKeyProtocolMediaMap) {
-    if (i.protocol_type == protocol_type && i.media_type == media_type) {
-      RTC_HISTOGRAM_ENUMERATION("WebRTC.PeerConnection.KeyProtocolByMedia",
-                                i.protocol_media,
-                                kEnumCounterKeyProtocolMediaTypeMax);
-    }
-  }
-}
-
-void NoteAddIceCandidateResult(int result) {
-  RTC_HISTOGRAM_ENUMERATION("WebRTC.PeerConnection.AddIceCandidate", result,
-                            kAddIceCandidateMax);
-}
-
-// Checks that each non-rejected content has SDES crypto keys or a DTLS
-// fingerprint, unless it's in a BUNDLE group, in which case only the
-// BUNDLE-tag section (first media section/description in the BUNDLE group)
-// needs a ufrag and pwd. Mismatches, such as replying with a DTLS fingerprint
-// to SDES keys, will be caught in JsepTransport negotiation, and backstopped
-// by Channel's |srtp_required| check.
-RTCError VerifyCrypto(const SessionDescription* desc, bool dtls_enabled) {
-  const cricket::ContentGroup* bundle =
-      desc->GetGroupByName(cricket::GROUP_TYPE_BUNDLE);
-  for (const cricket::ContentInfo& content_info : desc->contents()) {
-    if (content_info.rejected) {
-      continue;
-    }
-    // Note what media is used with each crypto protocol, for all sections.
-    NoteKeyProtocolAndMedia(dtls_enabled ? webrtc::kEnumCounterKeyProtocolDtls
-                                         : webrtc::kEnumCounterKeyProtocolSdes,
-                            content_info.media_description()->type());
-    const std::string& mid = content_info.name;
-    if (bundle && bundle->HasContentName(mid) &&
-        mid != *(bundle->FirstContentName())) {
-      // This isn't the first media section in the BUNDLE group, so it's not
-      // required to have crypto attributes, since only the crypto attributes
-      // from the first section actually get used.
-      continue;
-    }
-
-    // If the content isn't rejected or bundled into another m= section, crypto
-    // must be present.
-    const MediaContentDescription* media = content_info.media_description();
-    const TransportInfo* tinfo = desc->GetTransportInfoByName(mid);
-    if (!media || !tinfo) {
-      // Something is not right.
-      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, kInvalidSdp);
-    }
-    if (dtls_enabled) {
-      if (!tinfo->description.identity_fingerprint) {
-        RTC_LOG(LS_WARNING)
-            << "Session description must have DTLS fingerprint if "
-               "DTLS enabled.";
-        return RTCError(RTCErrorType::INVALID_PARAMETER,
-                        kSdpWithoutDtlsFingerprint);
-      }
-    } else {
-      if (media->cryptos().empty()) {
-        RTC_LOG(LS_WARNING)
-            << "Session description must have SDES when DTLS disabled.";
-        return RTCError(RTCErrorType::INVALID_PARAMETER, kSdpWithoutSdesCrypto);
-      }
-    }
-  }
-  return RTCError::OK();
-}
-
-// Checks that each non-rejected content has ice-ufrag and ice-pwd set, unless
-// it's in a BUNDLE group, in which case only the BUNDLE-tag section (first
-// media section/description in the BUNDLE group) needs a ufrag and pwd.
-bool VerifyIceUfragPwdPresent(const SessionDescription* desc) {
-  const cricket::ContentGroup* bundle =
-      desc->GetGroupByName(cricket::GROUP_TYPE_BUNDLE);
-  for (const cricket::ContentInfo& content_info : desc->contents()) {
-    if (content_info.rejected) {
-      continue;
-    }
-    const std::string& mid = content_info.name;
-    if (bundle && bundle->HasContentName(mid) &&
-        mid != *(bundle->FirstContentName())) {
-      // This isn't the first media section in the BUNDLE group, so it's not
-      // required to have ufrag/password, since only the ufrag/password from
-      // the first section actually get used.
-      continue;
-    }
-
-    // If the content isn't rejected or bundled into another m= section,
-    // ice-ufrag and ice-pwd must be present.
-    const TransportInfo* tinfo = desc->GetTransportInfoByName(mid);
-    if (!tinfo) {
-      // Something is not right.
-      RTC_LOG(LS_ERROR) << kInvalidSdp;
-      return false;
-    }
-    if (tinfo->description.ice_ufrag.empty() ||
-        tinfo->description.ice_pwd.empty()) {
-      RTC_LOG(LS_ERROR) << "Session description must have ice ufrag and pwd.";
-      return false;
-    }
-  }
-  return true;
-}
-
-// Returns true if |new_desc| requests an ICE restart (i.e., new ufrag/pwd).
-bool CheckForRemoteIceRestart(const SessionDescriptionInterface* old_desc,
-                              const SessionDescriptionInterface* new_desc,
-                              const std::string& content_name) {
-  if (!old_desc) {
-    return false;
-  }
-  const SessionDescription* new_sd = new_desc->description();
-  const SessionDescription* old_sd = old_desc->description();
-  const ContentInfo* cinfo = new_sd->GetContentByName(content_name);
-  if (!cinfo || cinfo->rejected) {
-    return false;
-  }
-  // If the content isn't rejected, check if ufrag and password has changed.
-  const cricket::TransportDescription* new_transport_desc =
-      new_sd->GetTransportDescriptionByName(content_name);
-  const cricket::TransportDescription* old_transport_desc =
-      old_sd->GetTransportDescriptionByName(content_name);
-  if (!new_transport_desc || !old_transport_desc) {
-    // No transport description exists. This is not an ICE restart.
-    return false;
-  }
-  if (cricket::IceCredentialsChanged(
-          old_transport_desc->ice_ufrag, old_transport_desc->ice_pwd,
-          new_transport_desc->ice_ufrag, new_transport_desc->ice_pwd)) {
-    RTC_LOG(LS_INFO) << "Remote peer requests ICE restart for " << content_name
-                     << ".";
-    return true;
-  }
-  return false;
-}
-
-// Generates a string error message for SetLocalDescription/SetRemoteDescription
-// from an RTCError.
-std::string GetSetDescriptionErrorMessage(cricket::ContentSource source,
-                                          SdpType type,
-                                          const RTCError& error) {
-  rtc::StringBuilder oss;
-  oss << "Failed to set " << (source == cricket::CS_LOCAL ? "local" : "remote")
-      << " " << SdpTypeToString(type) << " sdp: " << error.message();
-  return oss.Release();
-}
-
-std::string GetStreamIdsString(rtc::ArrayView<const std::string> stream_ids) {
-  std::string output = "streams=[";
-  const char* separator = "";
-  for (const auto& stream_id : stream_ids) {
-    output.append(separator).append(stream_id);
-    separator = ", ";
-  }
-  output.append("]");
-  return output;
-}
 
 absl::optional<int> RTCConfigurationToIceConfigOptionalInt(
     int rtc_configuration_parameter) {
@@ -599,88 +294,6 @@
   return rtc_configuration_parameter;
 }
 
-void ReportSimulcastApiVersion(const char* name,
-                               const SessionDescription& session) {
-  bool has_legacy = false;
-  bool has_spec_compliant = false;
-  for (const ContentInfo& content : session.contents()) {
-    if (!content.media_description()) {
-      continue;
-    }
-    has_spec_compliant |= content.media_description()->HasSimulcast();
-    for (const StreamParams& sp : content.media_description()->streams()) {
-      has_legacy |= sp.has_ssrc_group(cricket::kSimSsrcGroupSemantics);
-    }
-  }
-
-  if (has_legacy) {
-    RTC_HISTOGRAM_ENUMERATION(name, kSimulcastApiVersionLegacy,
-                              kSimulcastApiVersionMax);
-  }
-  if (has_spec_compliant) {
-    RTC_HISTOGRAM_ENUMERATION(name, kSimulcastApiVersionSpecCompliant,
-                              kSimulcastApiVersionMax);
-  }
-  if (!has_legacy && !has_spec_compliant) {
-    RTC_HISTOGRAM_ENUMERATION(name, kSimulcastApiVersionNone,
-                              kSimulcastApiVersionMax);
-  }
-}
-
-const ContentInfo* FindTransceiverMSection(
-    RtpTransceiverProxyWithInternal<RtpTransceiver>* transceiver,
-    const SessionDescriptionInterface* session_description) {
-  return transceiver->mid()
-             ? session_description->description()->GetContentByName(
-                   *transceiver->mid())
-             : nullptr;
-}
-
-// Wraps a CreateSessionDescriptionObserver and an OperationsChain operation
-// complete callback. When the observer is invoked, the wrapped observer is
-// invoked followed by invoking the completion callback.
-class CreateSessionDescriptionObserverOperationWrapper
-    : public CreateSessionDescriptionObserver {
- public:
-  CreateSessionDescriptionObserverOperationWrapper(
-      rtc::scoped_refptr<CreateSessionDescriptionObserver> observer,
-      std::function<void()> operation_complete_callback)
-      : observer_(std::move(observer)),
-        operation_complete_callback_(std::move(operation_complete_callback)) {
-    RTC_DCHECK(observer_);
-  }
-  ~CreateSessionDescriptionObserverOperationWrapper() override {
-    RTC_DCHECK(was_called_);
-  }
-
-  void OnSuccess(SessionDescriptionInterface* desc) override {
-    RTC_DCHECK(!was_called_);
-#ifdef RTC_DCHECK_IS_ON
-    was_called_ = true;
-#endif  // RTC_DCHECK_IS_ON
-    // Completing the operation before invoking the observer allows the observer
-    // to execute SetLocalDescription() without delay.
-    operation_complete_callback_();
-    observer_->OnSuccess(desc);
-  }
-
-  void OnFailure(RTCError error) override {
-    RTC_DCHECK(!was_called_);
-#ifdef RTC_DCHECK_IS_ON
-    was_called_ = true;
-#endif  // RTC_DCHECK_IS_ON
-    operation_complete_callback_();
-    observer_->OnFailure(std::move(error));
-  }
-
- private:
-#ifdef RTC_DCHECK_IS_ON
-  bool was_called_ = false;
-#endif  // RTC_DCHECK_IS_ON
-  rtc::scoped_refptr<CreateSessionDescriptionObserver> observer_;
-  std::function<void()> operation_complete_callback_;
-};
-
 // Check if the changes of IceTransportsType motives an ice restart.
 bool NeedIceRestart(bool surface_ice_candidates_on_ice_transport_type_changed,
                     PeerConnectionInterface::IceTransportsType current,
@@ -703,155 +316,6 @@
 
 }  // namespace
 
-// Used by parameterless SetLocalDescription() to create an offer or answer.
-// Upon completion of creating the session description, SetLocalDescription() is
-// invoked with the result.
-class PeerConnection::ImplicitCreateSessionDescriptionObserver
-    : public CreateSessionDescriptionObserver {
- public:
-  ImplicitCreateSessionDescriptionObserver(
-      rtc::WeakPtr<PeerConnection> pc,
-      rtc::scoped_refptr<SetLocalDescriptionObserverInterface>
-          set_local_description_observer)
-      : pc_(std::move(pc)),
-        set_local_description_observer_(
-            std::move(set_local_description_observer)) {}
-  ~ImplicitCreateSessionDescriptionObserver() override {
-    RTC_DCHECK(was_called_);
-  }
-
-  void SetOperationCompleteCallback(
-      std::function<void()> operation_complete_callback) {
-    operation_complete_callback_ = std::move(operation_complete_callback);
-  }
-
-  bool was_called() const { return was_called_; }
-
-  void OnSuccess(SessionDescriptionInterface* desc_ptr) override {
-    RTC_DCHECK(!was_called_);
-    std::unique_ptr<SessionDescriptionInterface> desc(desc_ptr);
-    was_called_ = true;
-
-    // Abort early if |pc_| is no longer valid.
-    if (!pc_) {
-      operation_complete_callback_();
-      return;
-    }
-    // DoSetLocalDescription() is a synchronous operation that invokes
-    // |set_local_description_observer_| with the result.
-    pc_->DoSetLocalDescription(std::move(desc),
-                               std::move(set_local_description_observer_));
-    operation_complete_callback_();
-  }
-
-  void OnFailure(RTCError error) override {
-    RTC_DCHECK(!was_called_);
-    was_called_ = true;
-    set_local_description_observer_->OnSetLocalDescriptionComplete(RTCError(
-        error.type(), std::string("SetLocalDescription failed to create "
-                                  "session description - ") +
-                          error.message()));
-    operation_complete_callback_();
-  }
-
- private:
-  bool was_called_ = false;
-  rtc::WeakPtr<PeerConnection> pc_;
-  rtc::scoped_refptr<SetLocalDescriptionObserverInterface>
-      set_local_description_observer_;
-  std::function<void()> operation_complete_callback_;
-};
-
-class PeerConnection::LocalIceCredentialsToReplace {
- public:
-  // Sets the ICE credentials that need restarting to the ICE credentials of
-  // the current and pending descriptions.
-  void SetIceCredentialsFromLocalDescriptions(
-      const SessionDescriptionInterface* current_local_description,
-      const SessionDescriptionInterface* pending_local_description) {
-    ice_credentials_.clear();
-    if (current_local_description) {
-      AppendIceCredentialsFromSessionDescription(*current_local_description);
-    }
-    if (pending_local_description) {
-      AppendIceCredentialsFromSessionDescription(*pending_local_description);
-    }
-  }
-
-  void ClearIceCredentials() { ice_credentials_.clear(); }
-
-  // Returns true if we have ICE credentials that need restarting.
-  bool HasIceCredentials() const { return !ice_credentials_.empty(); }
-
-  // Returns true if |local_description| shares no ICE credentials with the
-  // ICE credentials that need restarting.
-  bool SatisfiesIceRestart(
-      const SessionDescriptionInterface& local_description) const {
-    for (const auto& transport_info :
-         local_description.description()->transport_infos()) {
-      if (ice_credentials_.find(std::make_pair(
-              transport_info.description.ice_ufrag,
-              transport_info.description.ice_pwd)) != ice_credentials_.end()) {
-        return false;
-      }
-    }
-    return true;
-  }
-
- private:
-  void AppendIceCredentialsFromSessionDescription(
-      const SessionDescriptionInterface& desc) {
-    for (const auto& transport_info : desc.description()->transport_infos()) {
-      ice_credentials_.insert(
-          std::make_pair(transport_info.description.ice_ufrag,
-                         transport_info.description.ice_pwd));
-    }
-  }
-
-  std::set<std::pair<std::string, std::string>> ice_credentials_;
-};
-
-// Wrapper for SetSessionDescriptionObserver that invokes the success or failure
-// callback in a posted message handled by the peer connection. This introduces
-// a delay that prevents recursive API calls by the observer, but this also
-// means that the PeerConnection can be modified before the observer sees the
-// result of the operation. This is ill-advised for synchronizing states.
-//
-// Implements both the SetLocalDescriptionObserverInterface and the
-// SetRemoteDescriptionObserverInterface.
-class PeerConnection::SetSessionDescriptionObserverAdapter
-    : public SetLocalDescriptionObserverInterface,
-      public SetRemoteDescriptionObserverInterface {
- public:
-  SetSessionDescriptionObserverAdapter(
-      rtc::WeakPtr<PeerConnection> pc,
-      rtc::scoped_refptr<SetSessionDescriptionObserver> inner_observer)
-      : pc_(std::move(pc)), inner_observer_(std::move(inner_observer)) {}
-
-  // SetLocalDescriptionObserverInterface implementation.
-  void OnSetLocalDescriptionComplete(RTCError error) override {
-    OnSetDescriptionComplete(std::move(error));
-  }
-  // SetRemoteDescriptionObserverInterface implementation.
-  void OnSetRemoteDescriptionComplete(RTCError error) override {
-    OnSetDescriptionComplete(std::move(error));
-  }
-
- private:
-  void OnSetDescriptionComplete(RTCError error) {
-    if (!pc_)
-      return;
-    if (error.ok()) {
-      pc_->PostSetSessionDescriptionSuccess(inner_observer_);
-    } else {
-      pc_->PostSetSessionDescriptionFailure(inner_observer_, std::move(error));
-    }
-  }
-
-  rtc::WeakPtr<PeerConnection> pc_;
-  rtc::scoped_refptr<SetSessionDescriptionObserver> inner_observer_;
-};
-
 bool PeerConnectionInterface::RTCConfiguration::operator==(
     const PeerConnectionInterface::RTCConfiguration& o) const {
   // This static_assert prevents us from accidentally breaking operator==.
@@ -1003,14 +467,8 @@
   return cname;
 }
 
-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).
+// m= sectionss (in other words, nothing that involves a map/array).
 void ExtractSharedMediaSessionOptions(
     const PeerConnectionInterface::RTCOfferAnswerOptions& rtc_options,
     cricket::MediaSessionOptions* session_options) {
@@ -1026,30 +484,19 @@
     : factory_(factory),
       event_log_(std::move(event_log)),
       event_log_ptr_(event_log_.get()),
-      operations_chain_(rtc::OperationsChain::Create()),
       rtcp_cname_(GenerateRtcpCname()),
       local_streams_(StreamCollection::Create()),
       remote_streams_(StreamCollection::Create()),
       call_(std::move(call)),
       call_ptr_(call_.get()),
-      local_ice_credentials_to_replace_(new LocalIceCredentialsToReplace()),
-      data_channel_controller_(this),
-      weak_ptr_factory_(this) {
-  RTC_DCHECK(factory_);
-  // Note: call_ appears to be set to nullptr by some callers.
-  operations_chain_->SetOnChainEmptyCallback(
-      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr()]() {
-        if (!this_weak_ptr)
-          return;
-        this_weak_ptr->OnOperationsChainEmpty();
-      });
-}
+      sdp_handler_(this),
+      data_channel_controller_(this) {}
 
 PeerConnection::~PeerConnection() {
   TRACE_EVENT0("webrtc", "PeerConnection::~PeerConnection");
   RTC_DCHECK_RUN_ON(signaling_thread());
 
-  weak_ptr_factory_.InvalidateWeakPtrs();
+  sdp_handler_.PrepareForShutdown();
 
   // Need to stop transceivers before destroying the stats collector because
   // AudioRtpSender has a reference to the StatsCollector it will update when
@@ -1070,7 +517,7 @@
 
   RTC_LOG(LS_INFO) << "Session: " << session_id() << " is destroyed.";
 
-  webrtc_session_desc_factory_.reset();
+  sdp_handler_.ResetSessionDescFactory();
   transport_controller_.reset();
 
   // port_allocator_ lives on the network thread and should be destroyed there.
@@ -1314,19 +761,22 @@
     dependencies.cert_generator.reset();
   }
 
-  webrtc_session_desc_factory_.reset(new WebRtcSessionDescriptionFactory(
-      signaling_thread(), channel_manager(), this, session_id(),
-      std::move(dependencies.cert_generator), certificate, &ssrc_generator_));
-  webrtc_session_desc_factory_->SignalCertificateReady.connect(
+  auto webrtc_session_desc_factory =
+      std::make_unique<WebRtcSessionDescriptionFactory>(
+          signaling_thread(), channel_manager(), this, session_id(),
+          std::move(dependencies.cert_generator), certificate,
+          &ssrc_generator_);
+  webrtc_session_desc_factory->SignalCertificateReady.connect(
       this, &PeerConnection::OnCertificateReady);
 
   if (options.disable_encryption) {
-    webrtc_session_desc_factory_->SetSdesPolicy(cricket::SEC_DISABLED);
+    webrtc_session_desc_factory->SetSdesPolicy(cricket::SEC_DISABLED);
   }
 
-  webrtc_session_desc_factory_->set_enable_encrypted_rtp_header_extensions(
+  webrtc_session_desc_factory->set_enable_encrypted_rtp_header_extensions(
       GetCryptoOptions().srtp.enable_encrypted_rtp_header_extensions);
-  webrtc_session_desc_factory_->set_is_unified_plan(IsUnifiedPlan());
+  webrtc_session_desc_factory->set_is_unified_plan(IsUnifiedPlan());
+  sdp_handler_.SetSessionDescFactory(std::move(webrtc_session_desc_factory));
 
   // Add default audio/video transceivers for Plan B SDP.
   if (!IsUnifiedPlan()) {
@@ -1406,7 +856,7 @@
   }
 
   stats_->AddStream(local_stream);
-  UpdateNegotiationNeeded();
+  sdp_handler_.UpdateNegotiationNeeded();
   return true;
 }
 
@@ -1436,7 +886,7 @@
   if (IsClosed()) {
     return;
   }
-  UpdateNegotiationNeeded();
+  sdp_handler_.UpdateNegotiationNeeded();
 }
 
 RTCErrorOr<rtc::scoped_refptr<RtpSenderInterface>> PeerConnection::AddTrack(
@@ -1465,7 +915,7 @@
       (IsUnifiedPlan() ? AddTrackUnifiedPlan(track, stream_ids)
                        : AddTrackPlanB(track, stream_ids));
   if (sender_or_error.ok()) {
-    UpdateNegotiationNeeded();
+    sdp_handler_.UpdateNegotiationNeeded();
     stats_->AddTrack(track);
   }
   return sender_or_error;
@@ -1618,7 +1068,7 @@
           "Couldn't find sender " + sender->id() + " to remove.");
     }
   }
-  UpdateNegotiationNeeded();
+  sdp_handler_.UpdateNegotiationNeeded();
   return RTCError::OK();
 }
 
@@ -1775,7 +1225,7 @@
   transceiver->internal()->set_direction(init.direction);
 
   if (update_negotiation_needed) {
-    UpdateNegotiationNeeded();
+    sdp_handler_.UpdateNegotiationNeeded();
   }
 
   return rtc::scoped_refptr<RtpTransceiverInterface>(transceiver);
@@ -1837,6 +1287,7 @@
     rtc::scoped_refptr<RtpSenderProxyWithInternal<RtpSenderInternal>> sender,
     rtc::scoped_refptr<RtpReceiverProxyWithInternal<RtpReceiverInternal>>
         receiver) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   // Ensure that the new sender does not have an ID that is already in use by
   // another sender.
   // Allow receiver IDs to conflict since those come from remote SDP (which
@@ -1858,7 +1309,7 @@
 void PeerConnection::OnNegotiationNeeded() {
   RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(!IsClosed());
-  UpdateNegotiationNeeded();
+  sdp_handler_.UpdateNegotiationNeeded();
 }
 
 rtc::scoped_refptr<RtpSenderInterface> PeerConnection::CreateSender(
@@ -2064,7 +1515,7 @@
 
 PeerConnectionInterface::SignalingState PeerConnection::signaling_state() {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  return signaling_state_;
+  return sdp_handler_.signaling_state();
 }
 
 PeerConnectionInterface::IceConnectionState
@@ -2093,9 +1544,9 @@
 
 absl::optional<bool> PeerConnection::can_trickle_ice_candidates() {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  SessionDescriptionInterface* description = current_remote_description_.get();
+  const SessionDescriptionInterface* description = current_remote_description();
   if (!description) {
-    description = pending_remote_description_.get();
+    description = pending_remote_description();
   }
   if (!description) {
     return absl::nullopt;
@@ -2130,7 +1581,7 @@
   // Trigger the onRenegotiationNeeded event for every new RTP DataChannel, or
   // the first SCTP DataChannel.
   if (data_channel_type() == cricket::DCT_RTP || first_datachannel) {
-    UpdateNegotiationNeeded();
+    sdp_handler_.UpdateNegotiationNeeded();
   }
   NoteUsageEvent(UsageEvent::DATA_ADDED);
   return channel;
@@ -2138,95 +1589,24 @@
 
 void PeerConnection::RestartIce() {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  local_ice_credentials_to_replace_->SetIceCredentialsFromLocalDescriptions(
-      current_local_description_.get(), pending_local_description_.get());
-  UpdateNegotiationNeeded();
+  sdp_handler_.RestartIce();
 }
 
 void PeerConnection::CreateOffer(CreateSessionDescriptionObserver* observer,
                                  const RTCOfferAnswerOptions& options) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  // Chain this operation. If asynchronous operations are pending on the chain,
-  // this operation will be queued to be invoked, otherwise the contents of the
-  // lambda will execute immediately.
-  operations_chain_->ChainOperation(
-      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
-       observer_refptr =
-           rtc::scoped_refptr<CreateSessionDescriptionObserver>(observer),
-       options](std::function<void()> operations_chain_callback) {
-        // Abort early if |this_weak_ptr| is no longer valid.
-        if (!this_weak_ptr) {
-          observer_refptr->OnFailure(
-              RTCError(RTCErrorType::INTERNAL_ERROR,
-                       "CreateOffer failed because the session was shut down"));
-          operations_chain_callback();
-          return;
-        }
-        // The operation completes asynchronously when the wrapper is invoked.
-        rtc::scoped_refptr<CreateSessionDescriptionObserverOperationWrapper>
-            observer_wrapper(new rtc::RefCountedObject<
-                             CreateSessionDescriptionObserverOperationWrapper>(
-                std::move(observer_refptr),
-                std::move(operations_chain_callback)));
-        this_weak_ptr->DoCreateOffer(options, observer_wrapper);
-      });
+  sdp_handler_.CreateOffer(observer, options);
 }
 
-void PeerConnection::DoCreateOffer(
-    const RTCOfferAnswerOptions& options,
-    rtc::scoped_refptr<CreateSessionDescriptionObserver> observer) {
+void PeerConnection::CreateAnswer(CreateSessionDescriptionObserver* observer,
+                                  const RTCOfferAnswerOptions& options) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  TRACE_EVENT0("webrtc", "PeerConnection::DoCreateOffer");
-
-  if (!observer) {
-    RTC_LOG(LS_ERROR) << "CreateOffer - observer is NULL.";
-    return;
-  }
-
-  if (IsClosed()) {
-    std::string error = "CreateOffer called when PeerConnection is closed.";
-    RTC_LOG(LS_ERROR) << error;
-    PostCreateSessionDescriptionFailure(
-        observer, RTCError(RTCErrorType::INVALID_STATE, std::move(error)));
-    return;
-  }
-
-  // If a session error has occurred the PeerConnection is in a possibly
-  // inconsistent state so fail right away.
-  if (session_error() != SessionError::kNone) {
-    std::string error_message = GetSessionErrorMsg();
-    RTC_LOG(LS_ERROR) << "CreateOffer: " << error_message;
-    PostCreateSessionDescriptionFailure(
-        observer,
-        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
-    return;
-  }
-
-  if (!ValidateOfferAnswerOptions(options)) {
-    std::string error = "CreateOffer called with invalid options.";
-    RTC_LOG(LS_ERROR) << error;
-    PostCreateSessionDescriptionFailure(
-        observer, RTCError(RTCErrorType::INVALID_PARAMETER, std::move(error)));
-    return;
-  }
-
-  // Legacy handling for offer_to_receive_audio and offer_to_receive_video.
-  // Specified in WebRTC section 4.4.3.2 "Legacy configuration extensions".
-  if (IsUnifiedPlan()) {
-    RTCError error = HandleLegacyOfferOptions(options);
-    if (!error.ok()) {
-      PostCreateSessionDescriptionFailure(observer, std::move(error));
-      return;
-    }
-  }
-
-  cricket::MediaSessionOptions session_options;
-  GetOptionsForOffer(options, &session_options);
-  webrtc_session_desc_factory_->CreateOffer(observer, options, session_options);
+  sdp_handler_.CreateAnswer(observer, options);
 }
 
 RTCError PeerConnection::HandleLegacyOfferOptions(
     const RTCOfferAnswerOptions& options) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(IsUnifiedPlan());
 
   if (options.offer_to_receive_audio == 0) {
@@ -2299,216 +1679,34 @@
   return receiving_transceivers;
 }
 
-void PeerConnection::CreateAnswer(CreateSessionDescriptionObserver* observer,
-                                  const RTCOfferAnswerOptions& options) {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  // Chain this operation. If asynchronous operations are pending on the chain,
-  // this operation will be queued to be invoked, otherwise the contents of the
-  // lambda will execute immediately.
-  operations_chain_->ChainOperation(
-      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
-       observer_refptr =
-           rtc::scoped_refptr<CreateSessionDescriptionObserver>(observer),
-       options](std::function<void()> operations_chain_callback) {
-        // Abort early if |this_weak_ptr| is no longer valid.
-        if (!this_weak_ptr) {
-          observer_refptr->OnFailure(RTCError(
-              RTCErrorType::INTERNAL_ERROR,
-              "CreateAnswer failed because the session was shut down"));
-          operations_chain_callback();
-          return;
-        }
-        // The operation completes asynchronously when the wrapper is invoked.
-        rtc::scoped_refptr<CreateSessionDescriptionObserverOperationWrapper>
-            observer_wrapper(new rtc::RefCountedObject<
-                             CreateSessionDescriptionObserverOperationWrapper>(
-                std::move(observer_refptr),
-                std::move(operations_chain_callback)));
-        this_weak_ptr->DoCreateAnswer(options, observer_wrapper);
-      });
-}
-
-void PeerConnection::DoCreateAnswer(
-    const RTCOfferAnswerOptions& options,
-    rtc::scoped_refptr<CreateSessionDescriptionObserver> observer) {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  TRACE_EVENT0("webrtc", "PeerConnection::DoCreateAnswer");
-  if (!observer) {
-    RTC_LOG(LS_ERROR) << "CreateAnswer - observer is NULL.";
-    return;
-  }
-
-  // If a session error has occurred the PeerConnection is in a possibly
-  // inconsistent state so fail right away.
-  if (session_error() != SessionError::kNone) {
-    std::string error_message = GetSessionErrorMsg();
-    RTC_LOG(LS_ERROR) << "CreateAnswer: " << error_message;
-    PostCreateSessionDescriptionFailure(
-        observer,
-        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
-    return;
-  }
-
-  if (!(signaling_state_ == kHaveRemoteOffer ||
-        signaling_state_ == kHaveLocalPrAnswer)) {
-    std::string error =
-        "PeerConnection cannot create an answer in a state other than "
-        "have-remote-offer or have-local-pranswer.";
-    RTC_LOG(LS_ERROR) << error;
-    PostCreateSessionDescriptionFailure(
-        observer, RTCError(RTCErrorType::INVALID_STATE, std::move(error)));
-    return;
-  }
-
-  // The remote description should be set if we're in the right state.
-  RTC_DCHECK(remote_description());
-
-  if (IsUnifiedPlan()) {
-    if (options.offer_to_receive_audio != RTCOfferAnswerOptions::kUndefined) {
-      RTC_LOG(LS_WARNING) << "CreateAnswer: offer_to_receive_audio is not "
-                             "supported with Unified Plan semantics. Use the "
-                             "RtpTransceiver API instead.";
-    }
-    if (options.offer_to_receive_video != RTCOfferAnswerOptions::kUndefined) {
-      RTC_LOG(LS_WARNING) << "CreateAnswer: offer_to_receive_video is not "
-                             "supported with Unified Plan semantics. Use the "
-                             "RtpTransceiver API instead.";
-    }
-  }
-
-  cricket::MediaSessionOptions session_options;
-  GetOptionsForAnswer(options, &session_options);
-
-  webrtc_session_desc_factory_->CreateAnswer(observer, session_options);
-}
-
 void PeerConnection::SetLocalDescription(
     SetSessionDescriptionObserver* observer,
     SessionDescriptionInterface* desc_ptr) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  // Chain this operation. If asynchronous operations are pending on the chain,
-  // this operation will be queued to be invoked, otherwise the contents of the
-  // lambda will execute immediately.
-  operations_chain_->ChainOperation(
-      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
-       observer_refptr =
-           rtc::scoped_refptr<SetSessionDescriptionObserver>(observer),
-       desc = std::unique_ptr<SessionDescriptionInterface>(desc_ptr)](
-          std::function<void()> operations_chain_callback) mutable {
-        // Abort early if |this_weak_ptr| is no longer valid.
-        if (!this_weak_ptr) {
-          // For consistency with SetSessionDescriptionObserverAdapter whose
-          // posted messages doesn't get processed when the PC is destroyed, we
-          // do not inform |observer_refptr| that the operation failed.
-          operations_chain_callback();
-          return;
-        }
-        // SetSessionDescriptionObserverAdapter takes care of making sure the
-        // |observer_refptr| is invoked in a posted message.
-        this_weak_ptr->DoSetLocalDescription(
-            std::move(desc),
-            rtc::scoped_refptr<SetLocalDescriptionObserverInterface>(
-                new rtc::RefCountedObject<SetSessionDescriptionObserverAdapter>(
-                    this_weak_ptr, observer_refptr)));
-        // For backwards-compatability reasons, we declare the operation as
-        // completed here (rather than in a post), so that the operation chain
-        // is not blocked by this operation when the observer is invoked. This
-        // allows the observer to trigger subsequent offer/answer operations
-        // synchronously if the operation chain is now empty.
-        operations_chain_callback();
-      });
+  sdp_handler_.SetLocalDescription(observer, desc_ptr);
 }
 
 void PeerConnection::SetLocalDescription(
     std::unique_ptr<SessionDescriptionInterface> desc,
     rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  // Chain this operation. If asynchronous operations are pending on the chain,
-  // this operation will be queued to be invoked, otherwise the contents of the
-  // lambda will execute immediately.
-  operations_chain_->ChainOperation(
-      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(), observer,
-       desc = std::move(desc)](
-          std::function<void()> operations_chain_callback) mutable {
-        // Abort early if |this_weak_ptr| is no longer valid.
-        if (!this_weak_ptr) {
-          observer->OnSetLocalDescriptionComplete(RTCError(
-              RTCErrorType::INTERNAL_ERROR,
-              "SetLocalDescription failed because the session was shut down"));
-          operations_chain_callback();
-          return;
-        }
-        this_weak_ptr->DoSetLocalDescription(std::move(desc), observer);
-        // DoSetLocalDescription() is implemented as a synchronous operation.
-        // The |observer| will already have been informed that it completed, and
-        // we can mark this operation as complete without any loose ends.
-        operations_chain_callback();
-      });
+  sdp_handler_.SetLocalDescription(std::move(desc), observer);
 }
 
 void PeerConnection::SetLocalDescription(
     SetSessionDescriptionObserver* observer) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  SetLocalDescription(
-      new rtc::RefCountedObject<SetSessionDescriptionObserverAdapter>(
-          weak_ptr_factory_.GetWeakPtr(), observer));
+  sdp_handler_.SetLocalDescription(observer);
 }
 
 void PeerConnection::SetLocalDescription(
     rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  // The |create_sdp_observer| handles performing DoSetLocalDescription() with
-  // the resulting description as well as completing the operation.
-  rtc::scoped_refptr<ImplicitCreateSessionDescriptionObserver>
-      create_sdp_observer(
-          new rtc::RefCountedObject<ImplicitCreateSessionDescriptionObserver>(
-              weak_ptr_factory_.GetWeakPtr(), observer));
-  // Chain this operation. If asynchronous operations are pending on the chain,
-  // this operation will be queued to be invoked, otherwise the contents of the
-  // lambda will execute immediately.
-  operations_chain_->ChainOperation(
-      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
-       create_sdp_observer](std::function<void()> operations_chain_callback) {
-        // The |create_sdp_observer| is responsible for completing the
-        // operation.
-        create_sdp_observer->SetOperationCompleteCallback(
-            std::move(operations_chain_callback));
-        // Abort early if |this_weak_ptr| is no longer valid. This triggers the
-        // same code path as if DoCreateOffer() or DoCreateAnswer() failed.
-        if (!this_weak_ptr) {
-          create_sdp_observer->OnFailure(RTCError(
-              RTCErrorType::INTERNAL_ERROR,
-              "SetLocalDescription failed because the session was shut down"));
-          return;
-        }
-        switch (this_weak_ptr->signaling_state()) {
-          case PeerConnectionInterface::kStable:
-          case PeerConnectionInterface::kHaveLocalOffer:
-          case PeerConnectionInterface::kHaveRemotePrAnswer:
-            // TODO(hbos): If [LastCreatedOffer] exists and still represents the
-            // current state of the system, use that instead of creating another
-            // offer.
-            this_weak_ptr->DoCreateOffer(RTCOfferAnswerOptions(),
-                                         create_sdp_observer);
-            break;
-          case PeerConnectionInterface::kHaveLocalPrAnswer:
-          case PeerConnectionInterface::kHaveRemoteOffer:
-            // TODO(hbos): If [LastCreatedAnswer] exists and still represents
-            // the current state of the system, use that instead of creating
-            // another answer.
-            this_weak_ptr->DoCreateAnswer(RTCOfferAnswerOptions(),
-                                          create_sdp_observer);
-            break;
-          case PeerConnectionInterface::kClosed:
-            create_sdp_observer->OnFailure(RTCError(
-                RTCErrorType::INVALID_STATE,
-                "SetLocalDescription called when PeerConnection is closed."));
-            break;
-        }
-      });
+  sdp_handler_.SetLocalDescription(observer);
 }
 
 void PeerConnection::RemoveStoppedTransceivers() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   // 3.2.10.1: For each transceiver in the connection's set of transceivers
   //           run the following steps:
   if (!IsUnifiedPlan())
@@ -2549,338 +1747,6 @@
   }
 }
 
-void PeerConnection::DoSetLocalDescription(
-    std::unique_ptr<SessionDescriptionInterface> desc,
-    rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer) {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  TRACE_EVENT0("webrtc", "PeerConnection::DoSetLocalDescription");
-
-  if (!observer) {
-    RTC_LOG(LS_ERROR) << "SetLocalDescription - observer is NULL.";
-    return;
-  }
-
-  if (!desc) {
-    observer->OnSetLocalDescriptionComplete(
-        RTCError(RTCErrorType::INTERNAL_ERROR, "SessionDescription is NULL."));
-    return;
-  }
-
-  // If a session error has occurred the PeerConnection is in a possibly
-  // inconsistent state so fail right away.
-  if (session_error() != SessionError::kNone) {
-    std::string error_message = GetSessionErrorMsg();
-    RTC_LOG(LS_ERROR) << "SetLocalDescription: " << error_message;
-    observer->OnSetLocalDescriptionComplete(
-        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
-    return;
-  }
-
-  // For SLD we support only explicit rollback.
-  if (desc->GetType() == SdpType::kRollback) {
-    if (IsUnifiedPlan()) {
-      observer->OnSetLocalDescriptionComplete(Rollback(desc->GetType()));
-    } else {
-      observer->OnSetLocalDescriptionComplete(
-          RTCError(RTCErrorType::UNSUPPORTED_OPERATION,
-                   "Rollback not supported in Plan B"));
-    }
-    return;
-  }
-
-  RTCError error = ValidateSessionDescription(desc.get(), cricket::CS_LOCAL);
-  if (!error.ok()) {
-    std::string error_message = GetSetDescriptionErrorMessage(
-        cricket::CS_LOCAL, desc->GetType(), error);
-    RTC_LOG(LS_ERROR) << error_message;
-    observer->OnSetLocalDescriptionComplete(
-        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
-    return;
-  }
-
-  // Grab the description type before moving ownership to ApplyLocalDescription,
-  // which may destroy it before returning.
-  const SdpType type = desc->GetType();
-
-  error = ApplyLocalDescription(std::move(desc));
-  // |desc| may be destroyed at this point.
-
-  if (!error.ok()) {
-    // If ApplyLocalDescription fails, the PeerConnection could be in an
-    // inconsistent state, so act conservatively here and set the session error
-    // so that future calls to SetLocalDescription/SetRemoteDescription fail.
-    SetSessionError(SessionError::kContent, error.message());
-    std::string error_message =
-        GetSetDescriptionErrorMessage(cricket::CS_LOCAL, type, error);
-    RTC_LOG(LS_ERROR) << error_message;
-    observer->OnSetLocalDescriptionComplete(
-        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
-    return;
-  }
-  RTC_DCHECK(local_description());
-
-  if (local_description()->GetType() == SdpType::kAnswer) {
-    RemoveStoppedTransceivers();
-
-    // TODO(deadbeef): We already had to hop to the network thread for
-    // MaybeStartGathering...
-    network_thread()->Invoke<void>(
-        RTC_FROM_HERE, rtc::Bind(&cricket::PortAllocator::DiscardCandidatePool,
-                                 port_allocator_.get()));
-    // Make UMA notes about what was agreed to.
-    ReportNegotiatedSdpSemantics(*local_description());
-  }
-
-  observer->OnSetLocalDescriptionComplete(RTCError::OK());
-  NoteUsageEvent(UsageEvent::SET_LOCAL_DESCRIPTION_SUCCEEDED);
-
-  // Check if negotiation is needed. We must do this after informing the
-  // observer that SetLocalDescription() has completed to ensure negotiation is
-  // not needed prior to the promise resolving.
-  if (IsUnifiedPlan()) {
-    bool was_negotiation_needed = is_negotiation_needed_;
-    UpdateNegotiationNeeded();
-    if (signaling_state() == kStable && was_negotiation_needed &&
-        is_negotiation_needed_) {
-      // Legacy version.
-      Observer()->OnRenegotiationNeeded();
-      // Spec-compliant version; the event may get invalidated before firing.
-      GenerateNegotiationNeededEvent();
-    }
-  }
-
-  // MaybeStartGathering needs to be called after informing the observer so that
-  // we don't signal any candidates before signaling that SetLocalDescription
-  // completed.
-  transport_controller_->MaybeStartGathering();
-}
-
-RTCError PeerConnection::ApplyLocalDescription(
-    std::unique_ptr<SessionDescriptionInterface> desc) {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  RTC_DCHECK(desc);
-
-  // Update stats here so that we have the most recent stats for tracks and
-  // streams that might be removed by updating the session description.
-  stats_->UpdateStats(kStatsOutputLevelStandard);
-
-  // Take a reference to the old local description since it's used below to
-  // compare against the new local description. When setting the new local
-  // description, grab ownership of the replaced session description in case it
-  // is the same as |old_local_description|, to keep it alive for the duration
-  // of the method.
-  const SessionDescriptionInterface* old_local_description =
-      local_description();
-  std::unique_ptr<SessionDescriptionInterface> replaced_local_description;
-  SdpType type = desc->GetType();
-  if (type == SdpType::kAnswer) {
-    replaced_local_description = pending_local_description_
-                                     ? std::move(pending_local_description_)
-                                     : std::move(current_local_description_);
-    current_local_description_ = std::move(desc);
-    pending_local_description_ = nullptr;
-    current_remote_description_ = std::move(pending_remote_description_);
-  } else {
-    replaced_local_description = std::move(pending_local_description_);
-    pending_local_description_ = std::move(desc);
-  }
-  // The session description to apply now must be accessed by
-  // |local_description()|.
-  RTC_DCHECK(local_description());
-
-  // Report statistics about any use of simulcast.
-  ReportSimulcastApiVersion(kSimulcastVersionApplyLocalDescription,
-                            *local_description()->description());
-
-  if (!is_caller_) {
-    if (remote_description()) {
-      // Remote description was applied first, so this PC is the callee.
-      is_caller_ = false;
-    } else {
-      // Local description is applied first, so this PC is the caller.
-      is_caller_ = true;
-    }
-  }
-
-  RTCError error = PushdownTransportDescription(cricket::CS_LOCAL, type);
-  if (!error.ok()) {
-    return error;
-  }
-
-  if (IsUnifiedPlan()) {
-    RTCError error = UpdateTransceiversAndDataChannels(
-        cricket::CS_LOCAL, *local_description(), old_local_description,
-        remote_description());
-    if (!error.ok()) {
-      return error;
-    }
-    std::vector<rtc::scoped_refptr<RtpTransceiverInterface>> remove_list;
-    std::vector<rtc::scoped_refptr<MediaStreamInterface>> removed_streams;
-    for (const auto& transceiver : transceivers_) {
-      if (transceiver->stopped()) {
-        continue;
-      }
-
-      // 2.2.7.1.1.(6-9): Set sender and receiver's transport slots.
-      // Note that code paths that don't set MID won't be able to use
-      // information about DTLS transports.
-      if (transceiver->mid()) {
-        auto dtls_transport =
-            LookupDtlsTransportByMidInternal(*transceiver->mid());
-        transceiver->internal()->sender_internal()->set_transport(
-            dtls_transport);
-        transceiver->internal()->receiver_internal()->set_transport(
-            dtls_transport);
-      }
-
-      const ContentInfo* content =
-          FindMediaSectionForTransceiver(transceiver, local_description());
-      if (!content) {
-        continue;
-      }
-      const MediaContentDescription* media_desc = content->media_description();
-      // 2.2.7.1.6: If description is of type "answer" or "pranswer", then run
-      // the following steps:
-      if (type == SdpType::kPrAnswer || type == SdpType::kAnswer) {
-        // 2.2.7.1.6.1: If direction is "sendonly" or "inactive", and
-        // transceiver's [[FiredDirection]] slot is either "sendrecv" or
-        // "recvonly", process the removal of a remote track for the media
-        // description, given transceiver, removeList, and muteTracks.
-        if (!RtpTransceiverDirectionHasRecv(media_desc->direction()) &&
-            (transceiver->internal()->fired_direction() &&
-             RtpTransceiverDirectionHasRecv(
-                 *transceiver->internal()->fired_direction()))) {
-          ProcessRemovalOfRemoteTrack(transceiver, &remove_list,
-                                      &removed_streams);
-        }
-        // 2.2.7.1.6.2: Set transceiver's [[CurrentDirection]] and
-        // [[FiredDirection]] slots to direction.
-        transceiver->internal()->set_current_direction(media_desc->direction());
-        transceiver->internal()->set_fired_direction(media_desc->direction());
-      }
-    }
-    auto observer = Observer();
-    for (const auto& transceiver : remove_list) {
-      observer->OnRemoveTrack(transceiver->receiver());
-    }
-    for (const auto& stream : removed_streams) {
-      observer->OnRemoveStream(stream);
-    }
-  } else {
-    // Media channels will be created only when offer is set. These may use new
-    // transports just created by PushdownTransportDescription.
-    if (type == SdpType::kOffer) {
-      // TODO(bugs.webrtc.org/4676) - Handle CreateChannel failure, as new local
-      // description is applied. Restore back to old description.
-      RTCError error = CreateChannels(*local_description()->description());
-      if (!error.ok()) {
-        return error;
-      }
-    }
-    // Remove unused channels if MediaContentDescription is rejected.
-    RemoveUnusedChannels(local_description()->description());
-  }
-
-  error = UpdateSessionState(type, cricket::CS_LOCAL,
-                             local_description()->description());
-  if (!error.ok()) {
-    return error;
-  }
-
-  if (remote_description()) {
-    // Now that we have a local description, we can push down remote candidates.
-    UseCandidatesInSessionDescription(remote_description());
-  }
-
-  pending_ice_restarts_.clear();
-  if (session_error() != SessionError::kNone) {
-    LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR, GetSessionErrorMsg());
-  }
-
-  // If setting the description decided our SSL role, allocate any necessary
-  // SCTP sids.
-  rtc::SSLRole role;
-  if (IsSctpLike(data_channel_type()) && GetSctpSslRole(&role)) {
-    data_channel_controller_.AllocateSctpSids(role);
-  }
-
-  if (IsUnifiedPlan()) {
-    for (const auto& transceiver : transceivers_) {
-      if (transceiver->stopped()) {
-        continue;
-      }
-      const ContentInfo* content =
-          FindMediaSectionForTransceiver(transceiver, local_description());
-      if (!content) {
-        continue;
-      }
-      cricket::ChannelInterface* channel = transceiver->internal()->channel();
-      if (content->rejected || !channel || channel->local_streams().empty()) {
-        // 0 is a special value meaning "this sender has no associated send
-        // stream". Need to call this so the sender won't attempt to configure
-        // a no longer existing stream and run into DCHECKs in the lower
-        // layers.
-        transceiver->internal()->sender_internal()->SetSsrc(0);
-      } else {
-        // Get the StreamParams from the channel which could generate SSRCs.
-        const std::vector<StreamParams>& streams = channel->local_streams();
-        transceiver->internal()->sender_internal()->set_stream_ids(
-            streams[0].stream_ids());
-        transceiver->internal()->sender_internal()->SetSsrc(
-            streams[0].first_ssrc());
-      }
-    }
-  } else {
-    // Plan B semantics.
-
-    // Update state and SSRC of local MediaStreams and DataChannels based on the
-    // local session description.
-    const cricket::ContentInfo* audio_content =
-        GetFirstAudioContent(local_description()->description());
-    if (audio_content) {
-      if (audio_content->rejected) {
-        RemoveSenders(cricket::MEDIA_TYPE_AUDIO);
-      } else {
-        const cricket::AudioContentDescription* audio_desc =
-            audio_content->media_description()->as_audio();
-        UpdateLocalSenders(audio_desc->streams(), audio_desc->type());
-      }
-    }
-
-    const cricket::ContentInfo* video_content =
-        GetFirstVideoContent(local_description()->description());
-    if (video_content) {
-      if (video_content->rejected) {
-        RemoveSenders(cricket::MEDIA_TYPE_VIDEO);
-      } else {
-        const cricket::VideoContentDescription* video_desc =
-            video_content->media_description()->as_video();
-        UpdateLocalSenders(video_desc->streams(), video_desc->type());
-      }
-    }
-  }
-
-  const cricket::ContentInfo* data_content =
-      GetFirstDataContent(local_description()->description());
-  if (data_content) {
-    const cricket::RtpDataContentDescription* rtp_data_desc =
-        data_content->media_description()->as_rtp_data();
-    // rtp_data_desc will be null if this is an SCTP description.
-    if (rtp_data_desc) {
-      data_channel_controller_.UpdateLocalRtpDataChannels(
-          rtp_data_desc->streams());
-    }
-  }
-
-  if (type == SdpType::kAnswer &&
-      local_ice_credentials_to_replace_->SatisfiesIceRestart(
-          *current_local_description_)) {
-    local_ice_credentials_to_replace_->ClearIceCredentials();
-  }
-
-  return RTCError::OK();
-}
-
 // The SDP parser used to populate these values by default for the 'content
 // name' if an a=mid line was absent.
 static absl::string_view GetDefaultMidForPlanB(cricket::MediaType media_type) {
@@ -2898,6 +1764,7 @@
 
 void PeerConnection::FillInMissingRemoteMids(
     cricket::SessionDescription* new_remote_description) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(new_remote_description);
   const cricket::ContentInfos no_infos;
   const cricket::ContentInfos& local_contents =
@@ -2942,550 +1809,14 @@
     SetSessionDescriptionObserver* observer,
     SessionDescriptionInterface* desc_ptr) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  // Chain this operation. If asynchronous operations are pending on the chain,
-  // this operation will be queued to be invoked, otherwise the contents of the
-  // lambda will execute immediately.
-  operations_chain_->ChainOperation(
-      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
-       observer_refptr =
-           rtc::scoped_refptr<SetSessionDescriptionObserver>(observer),
-       desc = std::unique_ptr<SessionDescriptionInterface>(desc_ptr)](
-          std::function<void()> operations_chain_callback) mutable {
-        // Abort early if |this_weak_ptr| is no longer valid.
-        if (!this_weak_ptr) {
-          // For consistency with SetSessionDescriptionObserverAdapter whose
-          // posted messages doesn't get processed when the PC is destroyed, we
-          // do not inform |observer_refptr| that the operation failed.
-          operations_chain_callback();
-          return;
-        }
-        // SetSessionDescriptionObserverAdapter takes care of making sure the
-        // |observer_refptr| is invoked in a posted message.
-        this_weak_ptr->DoSetRemoteDescription(
-            std::move(desc),
-            rtc::scoped_refptr<SetRemoteDescriptionObserverInterface>(
-                new rtc::RefCountedObject<SetSessionDescriptionObserverAdapter>(
-                    this_weak_ptr, observer_refptr)));
-        // For backwards-compatability reasons, we declare the operation as
-        // completed here (rather than in a post), so that the operation chain
-        // is not blocked by this operation when the observer is invoked. This
-        // allows the observer to trigger subsequent offer/answer operations
-        // synchronously if the operation chain is now empty.
-        operations_chain_callback();
-      });
+  sdp_handler_.SetRemoteDescription(observer, desc_ptr);
 }
 
 void PeerConnection::SetRemoteDescription(
     std::unique_ptr<SessionDescriptionInterface> desc,
     rtc::scoped_refptr<SetRemoteDescriptionObserverInterface> observer) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  // Chain this operation. If asynchronous operations are pending on the chain,
-  // this operation will be queued to be invoked, otherwise the contents of the
-  // lambda will execute immediately.
-  operations_chain_->ChainOperation(
-      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(), observer,
-       desc = std::move(desc)](
-          std::function<void()> operations_chain_callback) mutable {
-        // Abort early if |this_weak_ptr| is no longer valid.
-        if (!this_weak_ptr) {
-          observer->OnSetRemoteDescriptionComplete(RTCError(
-              RTCErrorType::INTERNAL_ERROR,
-              "SetRemoteDescription failed because the session was shut down"));
-          operations_chain_callback();
-          return;
-        }
-        this_weak_ptr->DoSetRemoteDescription(std::move(desc),
-                                              std::move(observer));
-        // DoSetRemoteDescription() is implemented as a synchronous operation.
-        // The |observer| will already have been informed that it completed, and
-        // we can mark this operation as complete without any loose ends.
-        operations_chain_callback();
-      });
-}
-
-void PeerConnection::DoSetRemoteDescription(
-    std::unique_ptr<SessionDescriptionInterface> desc,
-    rtc::scoped_refptr<SetRemoteDescriptionObserverInterface> observer) {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  TRACE_EVENT0("webrtc", "PeerConnection::DoSetRemoteDescription");
-
-  if (!observer) {
-    RTC_LOG(LS_ERROR) << "SetRemoteDescription - observer is NULL.";
-    return;
-  }
-
-  if (!desc) {
-    observer->OnSetRemoteDescriptionComplete(RTCError(
-        RTCErrorType::INVALID_PARAMETER, "SessionDescription is NULL."));
-    return;
-  }
-
-  // If a session error has occurred the PeerConnection is in a possibly
-  // inconsistent state so fail right away.
-  if (session_error() != SessionError::kNone) {
-    std::string error_message = GetSessionErrorMsg();
-    RTC_LOG(LS_ERROR) << "SetRemoteDescription: " << error_message;
-    observer->OnSetRemoteDescriptionComplete(
-        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
-    return;
-  }
-  if (IsUnifiedPlan()) {
-    if (configuration_.enable_implicit_rollback) {
-      if (desc->GetType() == SdpType::kOffer &&
-          signaling_state() == kHaveLocalOffer) {
-        Rollback(desc->GetType());
-      }
-    }
-    // Explicit rollback.
-    if (desc->GetType() == SdpType::kRollback) {
-      observer->OnSetRemoteDescriptionComplete(Rollback(desc->GetType()));
-      return;
-    }
-  } else if (desc->GetType() == SdpType::kRollback) {
-    observer->OnSetRemoteDescriptionComplete(
-        RTCError(RTCErrorType::UNSUPPORTED_OPERATION,
-                 "Rollback not supported in Plan B"));
-    return;
-  }
-  if (desc->GetType() == SdpType::kOffer) {
-    // Report to UMA the format of the received offer.
-    ReportSdpFormatReceived(*desc);
-  }
-
-  // Handle remote descriptions missing a=mid lines for interop with legacy end
-  // points.
-  FillInMissingRemoteMids(desc->description());
-
-  RTCError error = ValidateSessionDescription(desc.get(), cricket::CS_REMOTE);
-  if (!error.ok()) {
-    std::string error_message = GetSetDescriptionErrorMessage(
-        cricket::CS_REMOTE, desc->GetType(), error);
-    RTC_LOG(LS_ERROR) << error_message;
-    observer->OnSetRemoteDescriptionComplete(
-        RTCError(error.type(), std::move(error_message)));
-    return;
-  }
-
-  // Grab the description type before moving ownership to
-  // ApplyRemoteDescription, which may destroy it before returning.
-  const SdpType type = desc->GetType();
-
-  error = ApplyRemoteDescription(std::move(desc));
-  // |desc| may be destroyed at this point.
-
-  if (!error.ok()) {
-    // If ApplyRemoteDescription fails, the PeerConnection could be in an
-    // inconsistent state, so act conservatively here and set the session error
-    // so that future calls to SetLocalDescription/SetRemoteDescription fail.
-    SetSessionError(SessionError::kContent, error.message());
-    std::string error_message =
-        GetSetDescriptionErrorMessage(cricket::CS_REMOTE, type, error);
-    RTC_LOG(LS_ERROR) << error_message;
-    observer->OnSetRemoteDescriptionComplete(
-        RTCError(error.type(), std::move(error_message)));
-    return;
-  }
-  RTC_DCHECK(remote_description());
-
-  if (type == SdpType::kAnswer) {
-    RemoveStoppedTransceivers();
-    // TODO(deadbeef): We already had to hop to the network thread for
-    // MaybeStartGathering...
-    network_thread()->Invoke<void>(
-        RTC_FROM_HERE, rtc::Bind(&cricket::PortAllocator::DiscardCandidatePool,
-                                 port_allocator_.get()));
-    // Make UMA notes about what was agreed to.
-    ReportNegotiatedSdpSemantics(*remote_description());
-  }
-
-  observer->OnSetRemoteDescriptionComplete(RTCError::OK());
-  NoteUsageEvent(UsageEvent::SET_REMOTE_DESCRIPTION_SUCCEEDED);
-
-  // Check if negotiation is needed. We must do this after informing the
-  // observer that SetRemoteDescription() has completed to ensure negotiation is
-  // not needed prior to the promise resolving.
-  if (IsUnifiedPlan()) {
-    bool was_negotiation_needed = is_negotiation_needed_;
-    UpdateNegotiationNeeded();
-    if (signaling_state() == kStable && was_negotiation_needed &&
-        is_negotiation_needed_) {
-      // Legacy version.
-      Observer()->OnRenegotiationNeeded();
-      // Spec-compliant version; the event may get invalidated before firing.
-      GenerateNegotiationNeededEvent();
-    }
-  }
-}
-
-RTCError PeerConnection::ApplyRemoteDescription(
-    std::unique_ptr<SessionDescriptionInterface> desc) {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  RTC_DCHECK(desc);
-
-  // Update stats here so that we have the most recent stats for tracks and
-  // streams that might be removed by updating the session description.
-  stats_->UpdateStats(kStatsOutputLevelStandard);
-
-  // Take a reference to the old remote description since it's used below to
-  // compare against the new remote description. When setting the new remote
-  // description, grab ownership of the replaced session description in case it
-  // is the same as |old_remote_description|, to keep it alive for the duration
-  // of the method.
-  const SessionDescriptionInterface* old_remote_description =
-      remote_description();
-  std::unique_ptr<SessionDescriptionInterface> replaced_remote_description;
-  SdpType type = desc->GetType();
-  if (type == SdpType::kAnswer) {
-    replaced_remote_description = pending_remote_description_
-                                      ? std::move(pending_remote_description_)
-                                      : std::move(current_remote_description_);
-    current_remote_description_ = std::move(desc);
-    pending_remote_description_ = nullptr;
-    current_local_description_ = std::move(pending_local_description_);
-  } else {
-    replaced_remote_description = std::move(pending_remote_description_);
-    pending_remote_description_ = std::move(desc);
-  }
-  // The session description to apply now must be accessed by
-  // |remote_description()|.
-  RTC_DCHECK(remote_description());
-
-  // Report statistics about any use of simulcast.
-  ReportSimulcastApiVersion(kSimulcastVersionApplyRemoteDescription,
-                            *remote_description()->description());
-
-  RTCError error = PushdownTransportDescription(cricket::CS_REMOTE, type);
-  if (!error.ok()) {
-    return error;
-  }
-  // Transport and Media channels will be created only when offer is set.
-  if (IsUnifiedPlan()) {
-    RTCError error = UpdateTransceiversAndDataChannels(
-        cricket::CS_REMOTE, *remote_description(), local_description(),
-        old_remote_description);
-    if (!error.ok()) {
-      return error;
-    }
-  } else {
-    // Media channels will be created only when offer is set. These may use new
-    // transports just created by PushdownTransportDescription.
-    if (type == SdpType::kOffer) {
-      // TODO(mallinath) - Handle CreateChannel failure, as new local
-      // description is applied. Restore back to old description.
-      RTCError error = CreateChannels(*remote_description()->description());
-      if (!error.ok()) {
-        return error;
-      }
-    }
-    // Remove unused channels if MediaContentDescription is rejected.
-    RemoveUnusedChannels(remote_description()->description());
-  }
-
-  // NOTE: Candidates allocation will be initiated only when
-  // SetLocalDescription is called.
-  error = UpdateSessionState(type, cricket::CS_REMOTE,
-                             remote_description()->description());
-  if (!error.ok()) {
-    return error;
-  }
-
-  if (local_description() &&
-      !UseCandidatesInSessionDescription(remote_description())) {
-    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, kInvalidCandidates);
-  }
-
-  if (old_remote_description) {
-    for (const cricket::ContentInfo& content :
-         old_remote_description->description()->contents()) {
-      // Check if this new SessionDescription contains new ICE ufrag and
-      // password that indicates the remote peer requests an ICE restart.
-      // TODO(deadbeef): When we start storing both the current and pending
-      // remote description, this should reset pending_ice_restarts and compare
-      // against the current description.
-      if (CheckForRemoteIceRestart(old_remote_description, remote_description(),
-                                   content.name)) {
-        if (type == SdpType::kOffer) {
-          pending_ice_restarts_.insert(content.name);
-        }
-      } else {
-        // We retain all received candidates only if ICE is not restarted.
-        // When ICE is restarted, all previous candidates belong to an old
-        // generation and should not be kept.
-        // TODO(deadbeef): This goes against the W3C spec which says the remote
-        // description should only contain candidates from the last set remote
-        // description plus any candidates added since then. We should remove
-        // this once we're sure it won't break anything.
-        WebRtcSessionDescriptionFactory::CopyCandidatesFromSessionDescription(
-            old_remote_description, content.name, mutable_remote_description());
-      }
-    }
-  }
-
-  if (session_error() != SessionError::kNone) {
-    LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR, GetSessionErrorMsg());
-  }
-
-  // Set the the ICE connection state to connecting since the connection may
-  // become writable with peer reflexive candidates before any remote candidate
-  // is signaled.
-  // TODO(pthatcher): This is a short-term solution for crbug/446908. A real fix
-  // is to have a new signal the indicates a change in checking state from the
-  // transport and expose a new checking() member from transport that can be
-  // read to determine the current checking state. The existing SignalConnecting
-  // actually means "gathering candidates", so cannot be be used here.
-  if (remote_description()->GetType() != SdpType::kOffer &&
-      remote_description()->number_of_mediasections() > 0u &&
-      ice_connection_state() == PeerConnectionInterface::kIceConnectionNew) {
-    SetIceConnectionState(PeerConnectionInterface::kIceConnectionChecking);
-  }
-
-  // If setting the description decided our SSL role, allocate any necessary
-  // SCTP sids.
-  rtc::SSLRole role;
-  if (IsSctpLike(data_channel_type()) && GetSctpSslRole(&role)) {
-    data_channel_controller_.AllocateSctpSids(role);
-  }
-
-  if (IsUnifiedPlan()) {
-    std::vector<rtc::scoped_refptr<RtpTransceiverInterface>>
-        now_receiving_transceivers;
-    std::vector<rtc::scoped_refptr<RtpTransceiverInterface>> remove_list;
-    std::vector<rtc::scoped_refptr<MediaStreamInterface>> added_streams;
-    std::vector<rtc::scoped_refptr<MediaStreamInterface>> removed_streams;
-    for (const auto& transceiver : transceivers_) {
-      const ContentInfo* content =
-          FindMediaSectionForTransceiver(transceiver, remote_description());
-      if (!content) {
-        continue;
-      }
-      const MediaContentDescription* media_desc = content->media_description();
-      RtpTransceiverDirection local_direction =
-          RtpTransceiverDirectionReversed(media_desc->direction());
-      // Roughly the same as steps 2.2.8.6 of section 4.4.1.6 "Set the
-      // RTCSessionDescription: Set the associated remote streams given
-      // transceiver.[[Receiver]], msids, addList, and removeList".
-      // https://w3c.github.io/webrtc-pc/#set-the-rtcsessiondescription
-      if (RtpTransceiverDirectionHasRecv(local_direction)) {
-        std::vector<std::string> stream_ids;
-        if (!media_desc->streams().empty()) {
-          // The remote description has signaled the stream IDs.
-          stream_ids = media_desc->streams()[0].stream_ids();
-        }
-        transceiver_stable_states_by_transceivers_[transceiver]
-            .SetRemoteStreamIdsIfUnset(transceiver->receiver()->stream_ids());
-
-        RTC_LOG(LS_INFO) << "Processing the MSIDs for MID=" << content->name
-                         << " (" << GetStreamIdsString(stream_ids) << ").";
-        SetAssociatedRemoteStreams(transceiver->internal()->receiver_internal(),
-                                   stream_ids, &added_streams,
-                                   &removed_streams);
-        // From the WebRTC specification, steps 2.2.8.5/6 of section 4.4.1.6
-        // "Set the RTCSessionDescription: If direction is sendrecv or recvonly,
-        // and transceiver's current direction is neither sendrecv nor recvonly,
-        // process the addition of a remote track for the media description.
-        if (!transceiver->fired_direction() ||
-            !RtpTransceiverDirectionHasRecv(*transceiver->fired_direction())) {
-          RTC_LOG(LS_INFO)
-              << "Processing the addition of a remote track for MID="
-              << content->name << ".";
-          now_receiving_transceivers.push_back(transceiver);
-        }
-      }
-      // 2.2.8.1.9: If direction is "sendonly" or "inactive", and transceiver's
-      // [[FiredDirection]] slot is either "sendrecv" or "recvonly", process the
-      // removal of a remote track for the media description, given transceiver,
-      // removeList, and muteTracks.
-      if (!RtpTransceiverDirectionHasRecv(local_direction) &&
-          (transceiver->fired_direction() &&
-           RtpTransceiverDirectionHasRecv(*transceiver->fired_direction()))) {
-        ProcessRemovalOfRemoteTrack(transceiver, &remove_list,
-                                    &removed_streams);
-      }
-      // 2.2.8.1.10: Set transceiver's [[FiredDirection]] slot to direction.
-      transceiver->internal()->set_fired_direction(local_direction);
-      // 2.2.8.1.11: If description is of type "answer" or "pranswer", then run
-      // the following steps:
-      if (type == SdpType::kPrAnswer || type == SdpType::kAnswer) {
-        // 2.2.8.1.11.1: Set transceiver's [[CurrentDirection]] slot to
-        // direction.
-        transceiver->internal()->set_current_direction(local_direction);
-        // 2.2.8.1.11.[3-6]: Set the transport internal slots.
-        if (transceiver->mid()) {
-          auto dtls_transport =
-              LookupDtlsTransportByMidInternal(*transceiver->mid());
-          transceiver->internal()->sender_internal()->set_transport(
-              dtls_transport);
-          transceiver->internal()->receiver_internal()->set_transport(
-              dtls_transport);
-        }
-      }
-      // 2.2.8.1.12: If the media description is rejected, and transceiver is
-      // not already stopped, stop the RTCRtpTransceiver transceiver.
-      if (content->rejected && !transceiver->stopped()) {
-        RTC_LOG(LS_INFO) << "Stopping transceiver for MID=" << content->name
-                         << " since the media section was rejected.";
-        transceiver->internal()->StopTransceiverProcedure();
-      }
-      if (!content->rejected &&
-          RtpTransceiverDirectionHasRecv(local_direction)) {
-        if (!media_desc->streams().empty() &&
-            media_desc->streams()[0].has_ssrcs()) {
-          uint32_t ssrc = media_desc->streams()[0].first_ssrc();
-          transceiver->internal()->receiver_internal()->SetupMediaChannel(ssrc);
-        } else {
-          transceiver->internal()
-              ->receiver_internal()
-              ->SetupUnsignaledMediaChannel();
-        }
-      }
-    }
-    // Once all processing has finished, fire off callbacks.
-    auto observer = Observer();
-    for (const auto& transceiver : now_receiving_transceivers) {
-      stats_->AddTrack(transceiver->receiver()->track());
-      observer->OnTrack(transceiver);
-      observer->OnAddTrack(transceiver->receiver(),
-                           transceiver->receiver()->streams());
-    }
-    for (const auto& stream : added_streams) {
-      observer->OnAddStream(stream);
-    }
-    for (const auto& transceiver : remove_list) {
-      observer->OnRemoveTrack(transceiver->receiver());
-    }
-    for (const auto& stream : removed_streams) {
-      observer->OnRemoveStream(stream);
-    }
-  }
-
-  const cricket::ContentInfo* audio_content =
-      GetFirstAudioContent(remote_description()->description());
-  const cricket::ContentInfo* video_content =
-      GetFirstVideoContent(remote_description()->description());
-  const cricket::AudioContentDescription* audio_desc =
-      GetFirstAudioContentDescription(remote_description()->description());
-  const cricket::VideoContentDescription* video_desc =
-      GetFirstVideoContentDescription(remote_description()->description());
-  const cricket::RtpDataContentDescription* rtp_data_desc =
-      GetFirstRtpDataContentDescription(remote_description()->description());
-
-  // Check if the descriptions include streams, just in case the peer supports
-  // MSID, but doesn't indicate so with "a=msid-semantic".
-  if (remote_description()->description()->msid_supported() ||
-      (audio_desc && !audio_desc->streams().empty()) ||
-      (video_desc && !video_desc->streams().empty())) {
-    remote_peer_supports_msid_ = true;
-  }
-
-  // We wait to signal new streams until we finish processing the description,
-  // since only at that point will new streams have all their tracks.
-  rtc::scoped_refptr<StreamCollection> new_streams(StreamCollection::Create());
-
-  if (!IsUnifiedPlan()) {
-    // TODO(steveanton): When removing RTP senders/receivers in response to a
-    // rejected media section, there is some cleanup logic that expects the
-    // voice/ video channel to still be set. But in this method the voice/video
-    // channel would have been destroyed by the SetRemoteDescription caller
-    // above so the cleanup that relies on them fails to run. The RemoveSenders
-    // calls should be moved to right before the DestroyChannel calls to fix
-    // this.
-
-    // Find all audio rtp streams and create corresponding remote AudioTracks
-    // and MediaStreams.
-    if (audio_content) {
-      if (audio_content->rejected) {
-        RemoveSenders(cricket::MEDIA_TYPE_AUDIO);
-      } else {
-        bool default_audio_track_needed =
-            !remote_peer_supports_msid_ &&
-            RtpTransceiverDirectionHasSend(audio_desc->direction());
-        UpdateRemoteSendersList(GetActiveStreams(audio_desc),
-                                default_audio_track_needed, audio_desc->type(),
-                                new_streams);
-      }
-    }
-
-    // Find all video rtp streams and create corresponding remote VideoTracks
-    // and MediaStreams.
-    if (video_content) {
-      if (video_content->rejected) {
-        RemoveSenders(cricket::MEDIA_TYPE_VIDEO);
-      } else {
-        bool default_video_track_needed =
-            !remote_peer_supports_msid_ &&
-            RtpTransceiverDirectionHasSend(video_desc->direction());
-        UpdateRemoteSendersList(GetActiveStreams(video_desc),
-                                default_video_track_needed, video_desc->type(),
-                                new_streams);
-      }
-    }
-
-    // If this is an RTP data transport, update the DataChannels with the
-    // information from the remote peer.
-    if (rtp_data_desc) {
-      data_channel_controller_.UpdateRemoteRtpDataChannels(
-          GetActiveStreams(rtp_data_desc));
-    }
-
-    // Iterate new_streams and notify the observer about new MediaStreams.
-    auto observer = Observer();
-    for (size_t i = 0; i < new_streams->count(); ++i) {
-      MediaStreamInterface* new_stream = new_streams->at(i);
-      stats_->AddStream(new_stream);
-      observer->OnAddStream(
-          rtc::scoped_refptr<MediaStreamInterface>(new_stream));
-    }
-
-    UpdateEndedRemoteMediaStreams();
-  }
-
-  if (type == SdpType::kAnswer &&
-      local_ice_credentials_to_replace_->SatisfiesIceRestart(
-          *current_local_description_)) {
-    local_ice_credentials_to_replace_->ClearIceCredentials();
-  }
-
-  return RTCError::OK();
-}
-
-void PeerConnection::SetAssociatedRemoteStreams(
-    rtc::scoped_refptr<RtpReceiverInternal> receiver,
-    const std::vector<std::string>& stream_ids,
-    std::vector<rtc::scoped_refptr<MediaStreamInterface>>* added_streams,
-    std::vector<rtc::scoped_refptr<MediaStreamInterface>>* removed_streams) {
-  std::vector<rtc::scoped_refptr<MediaStreamInterface>> media_streams;
-  for (const std::string& stream_id : stream_ids) {
-    rtc::scoped_refptr<MediaStreamInterface> stream =
-        remote_streams_->find(stream_id);
-    if (!stream) {
-      stream = MediaStreamProxy::Create(rtc::Thread::Current(),
-                                        MediaStream::Create(stream_id));
-      remote_streams_->AddStream(stream);
-      added_streams->push_back(stream);
-    }
-    media_streams.push_back(stream);
-  }
-  // Special case: "a=msid" missing, use random stream ID.
-  if (media_streams.empty() &&
-      !(remote_description()->description()->msid_signaling() &
-        cricket::kMsidSignalingMediaSection)) {
-    if (!missing_msid_default_stream_) {
-      missing_msid_default_stream_ = MediaStreamProxy::Create(
-          rtc::Thread::Current(), MediaStream::Create(rtc::CreateRandomUuid()));
-      added_streams->push_back(missing_msid_default_stream_);
-    }
-    media_streams.push_back(missing_msid_default_stream_);
-  }
-  std::vector<rtc::scoped_refptr<MediaStreamInterface>> previous_streams =
-      receiver->streams();
-  // SetStreams() will add/remove the receiver's track to/from the streams. This
-  // differs from the spec - the spec uses an "addList" and "removeList" to
-  // update the stream-track relationships in a later step. We do this earlier,
-  // changing the order of things, but the end-result is the same.
-  // TODO(hbos): When we remove remote_streams(), use set_stream_ids()
-  // instead. https://crbug.com/webrtc/9480
-  receiver->SetStreams(media_streams);
-  RemoveRemoteStreamsIfEmpty(previous_streams, removed_streams);
+  sdp_handler_.SetRemoteDescription(std::move(desc), observer);
 }
 
 void PeerConnection::ProcessRemovalOfRemoteTrack(
@@ -3507,6 +1838,7 @@
 void PeerConnection::RemoveRemoteStreamsIfEmpty(
     const std::vector<rtc::scoped_refptr<MediaStreamInterface>>& remote_streams,
     std::vector<rtc::scoped_refptr<MediaStreamInterface>>* removed_streams) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   // TODO(https://crbug.com/webrtc/9480): When we use stream IDs instead of
   // streams, see if the stream was removed by checking if this was the last
   // receiver with that stream ID.
@@ -3519,364 +1851,9 @@
   }
 }
 
-RTCError PeerConnection::UpdateTransceiversAndDataChannels(
-    cricket::ContentSource source,
-    const SessionDescriptionInterface& new_session,
-    const SessionDescriptionInterface* old_local_description,
-    const SessionDescriptionInterface* old_remote_description) {
-  RTC_DCHECK(IsUnifiedPlan());
-
-  const cricket::ContentGroup* bundle_group = nullptr;
-  if (new_session.GetType() == SdpType::kOffer) {
-    auto bundle_group_or_error =
-        GetEarlyBundleGroup(*new_session.description());
-    if (!bundle_group_or_error.ok()) {
-      return bundle_group_or_error.MoveError();
-    }
-    bundle_group = bundle_group_or_error.MoveValue();
-  }
-
-  const ContentInfos& new_contents = new_session.description()->contents();
-  for (size_t i = 0; i < new_contents.size(); ++i) {
-    const cricket::ContentInfo& new_content = new_contents[i];
-    cricket::MediaType media_type = new_content.media_description()->type();
-    mid_generator_.AddKnownId(new_content.name);
-    if (media_type == cricket::MEDIA_TYPE_AUDIO ||
-        media_type == cricket::MEDIA_TYPE_VIDEO) {
-      const cricket::ContentInfo* old_local_content = nullptr;
-      if (old_local_description &&
-          i < old_local_description->description()->contents().size()) {
-        old_local_content =
-            &old_local_description->description()->contents()[i];
-      }
-      const cricket::ContentInfo* old_remote_content = nullptr;
-      if (old_remote_description &&
-          i < old_remote_description->description()->contents().size()) {
-        old_remote_content =
-            &old_remote_description->description()->contents()[i];
-      }
-      // In the case where an m-section has completed its rejection,
-      // and is not being reused, we do not expect a transceiver.
-      if (old_local_content && old_local_content->rejected &&
-          old_remote_content && old_remote_content->rejected &&
-          new_content.rejected) {
-        continue;
-      }
-      auto transceiver_or_error =
-          AssociateTransceiver(source, new_session.GetType(), i, new_content,
-                               old_local_content, old_remote_content);
-      if (!transceiver_or_error.ok()) {
-        return transceiver_or_error.MoveError();
-      }
-      auto transceiver = transceiver_or_error.MoveValue();
-      RTCError error =
-          UpdateTransceiverChannel(transceiver, new_content, bundle_group);
-      if (!error.ok()) {
-        return error;
-      }
-    } else if (media_type == cricket::MEDIA_TYPE_DATA) {
-      if (GetDataMid() && new_content.name != *GetDataMid()) {
-        // Ignore all but the first data section.
-        RTC_LOG(LS_INFO) << "Ignoring data media section with MID="
-                         << new_content.name;
-        continue;
-      }
-      RTCError error = UpdateDataChannel(source, new_content, bundle_group);
-      if (!error.ok()) {
-        return error;
-      }
-    } else {
-      LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR,
-                           "Unknown section type.");
-    }
-  }
-
-  return RTCError::OK();
-}
-
-RTCError PeerConnection::UpdateTransceiverChannel(
-    rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
-        transceiver,
-    const cricket::ContentInfo& content,
-    const cricket::ContentGroup* bundle_group) {
-  RTC_DCHECK(IsUnifiedPlan());
-  RTC_DCHECK(transceiver);
-  // TODO(bugs.webrtc.org/11992): This function always returns RTCError::OK().
-  // Some of the below methods, specifically Create & Destroy, need to be called
-  // on the worker thread. Consider if there should be a split here where we do
-  // things asynchronously in two steps and change the return type of the
-  // function to be void. Note that in the case of 'create', that would/could
-  // mean that SetChannel might get called at a much later stage than it happens
-  // now.
-  cricket::ChannelInterface* channel = transceiver->internal()->channel();
-  if (content.rejected) {
-    if (channel) {
-      transceiver->internal()->SetChannel(nullptr);
-      DestroyChannelInterface(channel);
-    }
-  } else {
-    if (!channel) {
-      if (transceiver->media_type() == cricket::MEDIA_TYPE_AUDIO) {
-        channel = CreateVoiceChannel(content.name);
-      } else {
-        RTC_DCHECK_EQ(cricket::MEDIA_TYPE_VIDEO, transceiver->media_type());
-        channel = CreateVideoChannel(content.name);
-      }
-      if (!channel) {
-        LOG_AND_RETURN_ERROR(
-            RTCErrorType::INTERNAL_ERROR,
-            "Failed to create channel for mid=" + content.name);
-      }
-      transceiver->internal()->SetChannel(channel);
-    }
-  }
-  return RTCError::OK();
-}
-
-RTCError PeerConnection::UpdateDataChannel(
-    cricket::ContentSource source,
-    const cricket::ContentInfo& content,
-    const cricket::ContentGroup* bundle_group) {
-  if (data_channel_type() == cricket::DCT_NONE) {
-    // If data channels are disabled, ignore this media section. CreateAnswer
-    // will take care of rejecting it.
-    return RTCError::OK();
-  }
-  if (content.rejected) {
-    RTC_LOG(LS_INFO) << "Rejected data channel, mid=" << content.mid();
-    DestroyDataChannelTransport();
-  } else {
-    if (!data_channel_controller_.rtp_data_channel() &&
-        !data_channel_controller_.data_channel_transport()) {
-      RTC_LOG(LS_INFO) << "Creating data channel, mid=" << content.mid();
-      if (!CreateDataChannel(content.name)) {
-        LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR,
-                             "Failed to create data channel.");
-      }
-    }
-    if (source == cricket::CS_REMOTE) {
-      const MediaContentDescription* data_desc = content.media_description();
-      if (data_desc && cricket::IsRtpProtocol(data_desc->protocol())) {
-        data_channel_controller_.UpdateRemoteRtpDataChannels(
-            GetActiveStreams(data_desc));
-      }
-    }
-  }
-  return RTCError::OK();
-}
-
-// This method will extract any send encodings that were sent by the remote
-// connection. This is currently only relevant for Simulcast scenario (where
-// the number of layers may be communicated by the server).
-static std::vector<RtpEncodingParameters> GetSendEncodingsFromRemoteDescription(
-    const MediaContentDescription& desc) {
-  if (!desc.HasSimulcast()) {
-    return {};
-  }
-  std::vector<RtpEncodingParameters> result;
-  const SimulcastDescription& simulcast = desc.simulcast_description();
-
-  // This is a remote description, the parameters we are after should appear
-  // as receive streams.
-  for (const auto& alternatives : simulcast.receive_layers()) {
-    RTC_DCHECK(!alternatives.empty());
-    // There is currently no way to specify or choose from alternatives.
-    // We will always use the first alternative, which is the most preferred.
-    const SimulcastLayer& layer = alternatives[0];
-    RtpEncodingParameters parameters;
-    parameters.rid = layer.rid;
-    parameters.active = !layer.is_paused;
-    result.push_back(parameters);
-  }
-
-  return result;
-}
-
-static RTCError UpdateSimulcastLayerStatusInSender(
-    const std::vector<SimulcastLayer>& layers,
-    rtc::scoped_refptr<RtpSenderInternal> sender) {
-  RTC_DCHECK(sender);
-  RtpParameters parameters = sender->GetParametersInternal();
-  std::vector<std::string> disabled_layers;
-
-  // The simulcast envelope cannot be changed, only the status of the streams.
-  // So we will iterate over the send encodings rather than the layers.
-  for (RtpEncodingParameters& encoding : parameters.encodings) {
-    auto iter = std::find_if(layers.begin(), layers.end(),
-                             [&encoding](const SimulcastLayer& layer) {
-                               return layer.rid == encoding.rid;
-                             });
-    // A layer that cannot be found may have been removed by the remote party.
-    if (iter == layers.end()) {
-      disabled_layers.push_back(encoding.rid);
-      continue;
-    }
-
-    encoding.active = !iter->is_paused;
-  }
-
-  RTCError result = sender->SetParametersInternal(parameters);
-  if (result.ok()) {
-    result = sender->DisableEncodingLayers(disabled_layers);
-  }
-
-  return result;
-}
-
-static bool SimulcastIsRejected(
-    const ContentInfo* local_content,
-    const MediaContentDescription& answer_media_desc) {
-  bool simulcast_offered = local_content &&
-                           local_content->media_description() &&
-                           local_content->media_description()->HasSimulcast();
-  bool simulcast_answered = answer_media_desc.HasSimulcast();
-  bool rids_supported = RtpExtension::FindHeaderExtensionByUri(
-      answer_media_desc.rtp_header_extensions(), RtpExtension::kRidUri);
-  return simulcast_offered && (!simulcast_answered || !rids_supported);
-}
-
-static RTCError DisableSimulcastInSender(
-    rtc::scoped_refptr<RtpSenderInternal> sender) {
-  RTC_DCHECK(sender);
-  RtpParameters parameters = sender->GetParametersInternal();
-  if (parameters.encodings.size() <= 1) {
-    return RTCError::OK();
-  }
-
-  std::vector<std::string> disabled_layers;
-  std::transform(
-      parameters.encodings.begin() + 1, parameters.encodings.end(),
-      std::back_inserter(disabled_layers),
-      [](const RtpEncodingParameters& encoding) { return encoding.rid; });
-  return sender->DisableEncodingLayers(disabled_layers);
-}
-
-RTCErrorOr<rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>>
-PeerConnection::AssociateTransceiver(cricket::ContentSource source,
-                                     SdpType type,
-                                     size_t mline_index,
-                                     const ContentInfo& content,
-                                     const ContentInfo* old_local_content,
-                                     const ContentInfo* old_remote_content) {
-  RTC_DCHECK(IsUnifiedPlan());
-  // If this is an offer then the m= section might be recycled. If the m=
-  // section is being recycled (defined as: rejected in the current local or
-  // remote description and not rejected in new description), the transceiver
-  // should have been removed by RemoveStoppedTransceivers().
-  if (IsMediaSectionBeingRecycled(type, content, old_local_content,
-                                  old_remote_content)) {
-    const std::string& old_mid =
-        (old_local_content && old_local_content->rejected)
-            ? old_local_content->name
-            : old_remote_content->name;
-    auto old_transceiver = GetAssociatedTransceiver(old_mid);
-    // The transceiver should be disassociated in RemoveStoppedTransceivers()
-    RTC_DCHECK(!old_transceiver);
-  }
-  const MediaContentDescription* media_desc = content.media_description();
-  auto transceiver = GetAssociatedTransceiver(content.name);
-  if (source == cricket::CS_LOCAL) {
-    // Find the RtpTransceiver that corresponds to this m= section, using the
-    // mapping between transceivers and m= section indices established when
-    // creating the offer.
-    if (!transceiver) {
-      transceiver = GetTransceiverByMLineIndex(mline_index);
-    }
-    if (!transceiver) {
-      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                           "Unknown transceiver");
-    }
-  } else {
-    RTC_DCHECK_EQ(source, cricket::CS_REMOTE);
-    // If the m= section is sendrecv or recvonly, and there are RtpTransceivers
-    // of the same type...
-    // When simulcast is requested, a transceiver cannot be associated because
-    // AddTrack cannot be called to initialize it.
-    if (!transceiver &&
-        RtpTransceiverDirectionHasRecv(media_desc->direction()) &&
-        !media_desc->HasSimulcast()) {
-      transceiver = FindAvailableTransceiverToReceive(media_desc->type());
-    }
-    // If no RtpTransceiver was found in the previous step, create one with a
-    // recvonly direction.
-    if (!transceiver) {
-      RTC_LOG(LS_INFO) << "Adding "
-                       << cricket::MediaTypeToString(media_desc->type())
-                       << " transceiver for MID=" << content.name
-                       << " at i=" << mline_index
-                       << " in response to the remote description.";
-      std::string sender_id = rtc::CreateRandomUuid();
-      std::vector<RtpEncodingParameters> send_encodings =
-          GetSendEncodingsFromRemoteDescription(*media_desc);
-      auto sender = CreateSender(media_desc->type(), sender_id, nullptr, {},
-                                 send_encodings);
-      std::string receiver_id;
-      if (!media_desc->streams().empty()) {
-        receiver_id = media_desc->streams()[0].id;
-      } else {
-        receiver_id = rtc::CreateRandomUuid();
-      }
-      auto receiver = CreateReceiver(media_desc->type(), receiver_id);
-      transceiver = CreateAndAddTransceiver(sender, receiver);
-      transceiver->internal()->set_direction(
-          RtpTransceiverDirection::kRecvOnly);
-      if (type == SdpType::kOffer) {
-        transceiver_stable_states_by_transceivers_[transceiver]
-            .set_newly_created();
-      }
-    }
-    // Check if the offer indicated simulcast but the answer rejected it.
-    // This can happen when simulcast is not supported on the remote party.
-    if (SimulcastIsRejected(old_local_content, *media_desc)) {
-      RTC_HISTOGRAM_BOOLEAN(kSimulcastDisabled, true);
-      RTCError error =
-          DisableSimulcastInSender(transceiver->internal()->sender_internal());
-      if (!error.ok()) {
-        RTC_LOG(LS_ERROR) << "Failed to remove rejected simulcast.";
-        return std::move(error);
-      }
-    }
-  }
-  RTC_DCHECK(transceiver);
-  if (transceiver->media_type() != media_desc->type()) {
-    LOG_AND_RETURN_ERROR(
-        RTCErrorType::INVALID_PARAMETER,
-        "Transceiver type does not match media description type.");
-  }
-  if (media_desc->HasSimulcast()) {
-    std::vector<SimulcastLayer> layers =
-        source == cricket::CS_LOCAL
-            ? media_desc->simulcast_description().send_layers().GetAllLayers()
-            : media_desc->simulcast_description()
-                  .receive_layers()
-                  .GetAllLayers();
-    RTCError error = UpdateSimulcastLayerStatusInSender(
-        layers, transceiver->internal()->sender_internal());
-    if (!error.ok()) {
-      RTC_LOG(LS_ERROR) << "Failed updating status for simulcast layers.";
-      return std::move(error);
-    }
-  }
-  if (type == SdpType::kOffer) {
-    bool state_changes = transceiver->internal()->mid() != content.name ||
-                         transceiver->internal()->mline_index() != mline_index;
-    if (state_changes) {
-      transceiver_stable_states_by_transceivers_[transceiver]
-          .SetMSectionIfUnset(transceiver->internal()->mid(),
-                              transceiver->internal()->mline_index());
-    }
-  }
-  // Associate the found or created RtpTransceiver with the m= section by
-  // setting the value of the RtpTransceiver's mid property to the MID of the m=
-  // section, and establish a mapping between the transceiver and the index of
-  // the m= section.
-  transceiver->internal()->set_mid(content.name);
-  transceiver->internal()->set_mline_index(mline_index);
-  return std::move(transceiver);
-}
-
 rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
 PeerConnection::GetAssociatedTransceiver(const std::string& mid) const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(IsUnifiedPlan());
   for (auto transceiver : transceivers_) {
     if (transceiver->mid() == mid) {
@@ -3888,6 +1865,7 @@
 
 rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
 PeerConnection::GetTransceiverByMLineIndex(size_t mline_index) const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(IsUnifiedPlan());
   for (auto transceiver : transceivers_) {
     if (transceiver->internal()->mline_index() == mline_index) {
@@ -3900,6 +1878,7 @@
 rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
 PeerConnection::FindAvailableTransceiverToReceive(
     cricket::MediaType media_type) const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(IsUnifiedPlan());
   // From JSEP section 5.10 (Applying a Remote Description):
   // If the m= section is sendrecv or recvonly, and there are RtpTransceivers of
@@ -3920,6 +1899,7 @@
     rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
         transceiver,
     const SessionDescriptionInterface* sdesc) const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(transceiver);
   RTC_DCHECK(sdesc);
   if (IsUnifiedPlan()) {
@@ -4099,124 +2079,21 @@
 bool PeerConnection::AddIceCandidate(
     const IceCandidateInterface* ice_candidate) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  TRACE_EVENT0("webrtc", "PeerConnection::AddIceCandidate");
-  if (IsClosed()) {
-    RTC_LOG(LS_ERROR) << "AddIceCandidate: PeerConnection is closed.";
-    NoteAddIceCandidateResult(kAddIceCandidateFailClosed);
-    return false;
-  }
-
-  if (!remote_description()) {
-    RTC_LOG(LS_ERROR) << "AddIceCandidate: ICE candidates can't be added "
-                         "without any remote session description.";
-    NoteAddIceCandidateResult(kAddIceCandidateFailNoRemoteDescription);
-    return false;
-  }
-
-  if (!ice_candidate) {
-    RTC_LOG(LS_ERROR) << "AddIceCandidate: Candidate is null.";
-    NoteAddIceCandidateResult(kAddIceCandidateFailNullCandidate);
-    return false;
-  }
-
-  bool valid = false;
-  bool ready = ReadyToUseRemoteCandidate(ice_candidate, nullptr, &valid);
-  if (!valid) {
-    NoteAddIceCandidateResult(kAddIceCandidateFailNotValid);
-    return false;
-  }
-
-  // Add this candidate to the remote session description.
-  if (!mutable_remote_description()->AddCandidate(ice_candidate)) {
-    RTC_LOG(LS_ERROR) << "AddIceCandidate: Candidate cannot be used.";
-    NoteAddIceCandidateResult(kAddIceCandidateFailInAddition);
-    return false;
-  }
-
-  if (ready) {
-    bool result = UseCandidate(ice_candidate);
-    if (result) {
-      NoteUsageEvent(UsageEvent::ADD_ICE_CANDIDATE_SUCCEEDED);
-      NoteAddIceCandidateResult(kAddIceCandidateSuccess);
-    } else {
-      NoteAddIceCandidateResult(kAddIceCandidateFailNotUsable);
-    }
-    return result;
-  } else {
-    RTC_LOG(LS_INFO) << "AddIceCandidate: Not ready to use candidate.";
-    NoteAddIceCandidateResult(kAddIceCandidateFailNotReady);
-    return true;
-  }
+  return sdp_handler_.AddIceCandidate(ice_candidate);
 }
 
 void PeerConnection::AddIceCandidate(
     std::unique_ptr<IceCandidateInterface> candidate,
     std::function<void(RTCError)> callback) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  // Chain this operation. If asynchronous operations are pending on the chain,
-  // this operation will be queued to be invoked, otherwise the contents of the
-  // lambda will execute immediately.
-  operations_chain_->ChainOperation(
-      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
-       candidate = std::move(candidate), callback = std::move(callback)](
-          std::function<void()> operations_chain_callback) {
-        if (!this_weak_ptr) {
-          operations_chain_callback();
-          callback(RTCError(
-              RTCErrorType::INVALID_STATE,
-              "AddIceCandidate failed because the session was shut down"));
-          return;
-        }
-        if (!this_weak_ptr->AddIceCandidate(candidate.get())) {
-          operations_chain_callback();
-          // Fail with an error type and message consistent with Chromium.
-          // TODO(hbos): Fail with error types according to spec.
-          callback(RTCError(RTCErrorType::UNSUPPORTED_OPERATION,
-                            "Error processing ICE candidate"));
-          return;
-        }
-        operations_chain_callback();
-        callback(RTCError::OK());
-      });
+  sdp_handler_.AddIceCandidate(std::move(candidate), callback);
 }
 
 bool PeerConnection::RemoveIceCandidates(
     const std::vector<cricket::Candidate>& candidates) {
   TRACE_EVENT0("webrtc", "PeerConnection::RemoveIceCandidates");
   RTC_DCHECK_RUN_ON(signaling_thread());
-  if (IsClosed()) {
-    RTC_LOG(LS_ERROR) << "RemoveIceCandidates: PeerConnection is closed.";
-    return false;
-  }
-
-  if (!remote_description()) {
-    RTC_LOG(LS_ERROR) << "RemoveIceCandidates: ICE candidates can't be removed "
-                         "without any remote session description.";
-    return false;
-  }
-
-  if (candidates.empty()) {
-    RTC_LOG(LS_ERROR) << "RemoveIceCandidates: candidates are empty.";
-    return false;
-  }
-
-  size_t number_removed =
-      mutable_remote_description()->RemoveCandidates(candidates);
-  if (number_removed != candidates.size()) {
-    RTC_LOG(LS_ERROR)
-        << "RemoveIceCandidates: Failed to remove candidates. Requested "
-        << candidates.size() << " but only " << number_removed
-        << " are removed.";
-  }
-
-  // Remove the candidates from the transport controller.
-  RTCError error = transport_controller_->RemoveRemoteCandidates(candidates);
-  if (!error.ok()) {
-    RTC_LOG(LS_ERROR)
-        << "RemoveIceCandidates: Error when removing remote candidates: "
-        << error.message();
-  }
-  return true;
+  return sdp_handler_.RemoveIceCandidates(candidates);
 }
 
 RTCError PeerConnection::SetBitrate(const BitrateSettings& bitrate) {
@@ -4377,48 +2254,58 @@
 
 const SessionDescriptionInterface* PeerConnection::local_description() const {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  return pending_local_description_ ? pending_local_description_.get()
-                                    : current_local_description_.get();
+  return sdp_handler_.local_description();
 }
 
 const SessionDescriptionInterface* PeerConnection::remote_description() const {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  return pending_remote_description_ ? pending_remote_description_.get()
-                                     : current_remote_description_.get();
+  return sdp_handler_.remote_description();
 }
 
 const SessionDescriptionInterface* PeerConnection::current_local_description()
     const {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  return current_local_description_.get();
+  return sdp_handler_.current_local_description();
 }
 
 const SessionDescriptionInterface* PeerConnection::current_remote_description()
     const {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  return current_remote_description_.get();
+  return sdp_handler_.current_remote_description();
 }
 
 const SessionDescriptionInterface* PeerConnection::pending_local_description()
     const {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  return pending_local_description_.get();
+  return sdp_handler_.pending_local_description();
 }
 
 const SessionDescriptionInterface* PeerConnection::pending_remote_description()
     const {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  return pending_remote_description_.get();
+  return sdp_handler_.pending_remote_description();
 }
 
 void PeerConnection::Close() {
   RTC_DCHECK_RUN_ON(signaling_thread());
   TRACE_EVENT0("webrtc", "PeerConnection::Close");
+
+  if (IsClosed()) {
+    return;
+  }
   // Update stats here so that we have the most recent stats for tracks and
   // streams before the channels are closed.
   stats_->UpdateStats(kStatsOutputLevelStandard);
 
-  ChangeSignalingState(PeerConnectionInterface::kClosed);
+  ice_connection_state_ = PeerConnectionInterface::kIceConnectionClosed;
+  Observer()->OnIceConnectionChange(ice_connection_state_);
+  standardized_ice_connection_state_ =
+      PeerConnectionInterface::IceConnectionState::kIceConnectionClosed;
+  connection_state_ = PeerConnectionInterface::PeerConnectionState::kClosed;
+  Observer()->OnConnectionChange(connection_state_);
+
+  sdp_handler_.Close();
+
   NoteUsageEvent(UsageEvent::CLOSE_CALLED);
 
   for (const auto& transceiver : transceivers_) {
@@ -4442,7 +2329,7 @@
   // asynchronously and if the peer connection is closed without resetting the
   // WebRTC session description factory, the session description factory would
   // call the transport controller.
-  webrtc_session_desc_factory_.reset();
+  sdp_handler_.ResetSessionDescFactory();
   transport_controller_.reset();
 
   network_thread()->Invoke<void>(
@@ -4671,6 +2558,7 @@
 }
 
 void PeerConnection::SetIceConnectionState(IceConnectionState new_state) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   if (ice_connection_state_ == new_state) {
     return;
   }
@@ -4772,34 +2660,13 @@
   Observer()->OnIceSelectedCandidatePairChanged(event);
 }
 
-void PeerConnection::ChangeSignalingState(
-    PeerConnectionInterface::SignalingState signaling_state) {
-  if (signaling_state_ == signaling_state) {
-    return;
-  }
-  RTC_LOG(LS_INFO) << "Session: " << session_id() << " Old state: "
-                   << GetSignalingStateString(signaling_state_)
-                   << " New state: "
-                   << GetSignalingStateString(signaling_state);
-  signaling_state_ = signaling_state;
-  if (signaling_state == kClosed) {
-    ice_connection_state_ = kIceConnectionClosed;
-    Observer()->OnIceConnectionChange(ice_connection_state_);
-    standardized_ice_connection_state_ =
-        PeerConnectionInterface::IceConnectionState::kIceConnectionClosed;
-    connection_state_ = PeerConnectionInterface::PeerConnectionState::kClosed;
-    Observer()->OnConnectionChange(connection_state_);
-  }
-  Observer()->OnSignalingChange(signaling_state_);
-}
-
 void PeerConnection::OnAudioTrackAdded(AudioTrackInterface* track,
                                        MediaStreamInterface* stream) {
   if (IsClosed()) {
     return;
   }
   AddAudioTrack(track, stream);
-  UpdateNegotiationNeeded();
+  sdp_handler_.UpdateNegotiationNeeded();
 }
 
 void PeerConnection::OnAudioTrackRemoved(AudioTrackInterface* track,
@@ -4808,7 +2675,7 @@
     return;
   }
   RemoveAudioTrack(track, stream);
-  UpdateNegotiationNeeded();
+  sdp_handler_.UpdateNegotiationNeeded();
 }
 
 void PeerConnection::OnVideoTrackAdded(VideoTrackInterface* track,
@@ -4817,7 +2684,7 @@
     return;
   }
   AddVideoTrack(track, stream);
-  UpdateNegotiationNeeded();
+  sdp_handler_.UpdateNegotiationNeeded();
 }
 
 void PeerConnection::OnVideoTrackRemoved(VideoTrackInterface* track,
@@ -4826,7 +2693,7 @@
     return;
   }
   RemoveVideoTrack(track, stream);
-  UpdateNegotiationNeeded();
+  sdp_handler_.UpdateNegotiationNeeded();
 }
 
 void PeerConnection::PostSetSessionDescriptionSuccess(
@@ -4859,6 +2726,7 @@
 void PeerConnection::GetOptionsForOffer(
     const PeerConnectionInterface::RTCOfferAnswerOptions& offer_answer_options,
     cricket::MediaSessionOptions* session_options) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   ExtractSharedMediaSessionOptions(offer_answer_options, session_options);
 
   if (IsUnifiedPlan()) {
@@ -4878,8 +2746,8 @@
   }
 
   // Apply ICE restart flag and renomination flag.
-  bool ice_restart = offer_answer_options.ice_restart ||
-                     local_ice_credentials_to_replace_->HasIceCredentials();
+  bool ice_restart =
+      offer_answer_options.ice_restart || sdp_handler_.HasNewIceCredentials();
   for (auto& options : session_options->media_description_options) {
     options.transport_options.ice_restart = ice_restart;
     options.transport_options.enable_ice_renomination =
@@ -5191,6 +3059,7 @@
 void PeerConnection::GetOptionsForAnswer(
     const RTCOfferAnswerOptions& offer_answer_options,
     cricket::MediaSessionOptions* session_options) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   ExtractSharedMediaSessionOptions(offer_answer_options, session_options);
 
   if (IsUnifiedPlan()) {
@@ -5398,6 +3267,7 @@
 }
 
 absl::optional<std::string> PeerConnection::GetDataMid() const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   switch (data_channel_type()) {
     case cricket::DCT_RTP:
       if (!data_channel_controller_.rtp_data_channel()) {
@@ -5412,6 +3282,7 @@
 }
 
 void PeerConnection::RemoveSenders(cricket::MediaType media_type) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   UpdateLocalSenders(std::vector<cricket::StreamParams>(), media_type);
   UpdateRemoteSendersList(std::vector<cricket::StreamParams>(), false,
                           media_type, nullptr);
@@ -5422,6 +3293,7 @@
     bool default_sender_needed,
     cricket::MediaType media_type,
     StreamCollection* new_streams) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(!IsUnifiedPlan());
 
   std::vector<RtpSenderInfo>* current_senders =
@@ -5570,6 +3442,7 @@
 }
 
 void PeerConnection::UpdateEndedRemoteMediaStreams() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams_to_remove;
   for (size_t i = 0; i < remote_streams_->count(); ++i) {
     MediaStreamInterface* stream = remote_streams_->at(i);
@@ -5587,6 +3460,7 @@
 void PeerConnection::UpdateLocalSenders(
     const std::vector<cricket::StreamParams>& streams,
     cricket::MediaType media_type) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   std::vector<RtpSenderInfo>* current_senders = GetLocalSenderInfos(media_type);
 
   // Find removed tracks. I.e., tracks where the track id, stream id or ssrc
@@ -5920,8 +3794,8 @@
   absl::optional<rtc::SSLRole> dtls_role;
   if (sctp_mid_s_) {
     dtls_role = transport_controller_->GetDtlsRole(*sctp_mid_s_);
-    if (!dtls_role && is_caller_.has_value()) {
-      dtls_role = *is_caller_ ? rtc::SSL_SERVER : rtc::SSL_CLIENT;
+    if (!dtls_role && sdp_handler_.is_caller().has_value()) {
+      dtls_role = *sdp_handler_.is_caller() ? rtc::SSL_SERVER : rtc::SSL_CLIENT;
     }
     *role = *dtls_role;
     return true;
@@ -5956,48 +3830,6 @@
   }
 }
 
-RTCError PeerConnection::UpdateSessionState(
-    SdpType type,
-    cricket::ContentSource source,
-    const cricket::SessionDescription* description) {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-
-  // If there's already a pending error then no state transition should happen.
-  // But all call-sites should be verifying this before calling us!
-  RTC_DCHECK(session_error() == SessionError::kNone);
-
-  // If this is answer-ish we're ready to let media flow.
-  if (type == SdpType::kPrAnswer || type == SdpType::kAnswer) {
-    EnableSending();
-  }
-
-  // Update the signaling state according to the specified state machine (see
-  // https://w3c.github.io/webrtc-pc/#rtcsignalingstate-enum).
-  if (type == SdpType::kOffer) {
-    ChangeSignalingState(source == cricket::CS_LOCAL
-                             ? PeerConnectionInterface::kHaveLocalOffer
-                             : PeerConnectionInterface::kHaveRemoteOffer);
-  } else if (type == SdpType::kPrAnswer) {
-    ChangeSignalingState(source == cricket::CS_LOCAL
-                             ? PeerConnectionInterface::kHaveLocalPrAnswer
-                             : PeerConnectionInterface::kHaveRemotePrAnswer);
-  } else {
-    RTC_DCHECK(type == SdpType::kAnswer);
-    ChangeSignalingState(PeerConnectionInterface::kStable);
-    transceiver_stable_states_by_transceivers_.clear();
-    have_pending_rtp_data_channel_ = false;
-  }
-
-  // Update internal objects according to the session description's media
-  // descriptions.
-  RTCError error = PushdownMediaDescription(type, source);
-  if (!error.ok()) {
-    return error;
-  }
-
-  return RTCError::OK();
-}
-
 void PeerConnection::UpdatePayloadTypeDemuxingState(
     cricket::ContentSource source) {
   // We may need to delete any created default streams and disable creation of
@@ -6085,6 +3917,7 @@
   const SessionDescriptionInterface* sdesc =
       (source == cricket::CS_LOCAL ? local_description()
                                    : remote_description());
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RTC_DCHECK(sdesc);
 
   UpdatePayloadTypeDemuxingState(source);
@@ -6332,8 +4165,7 @@
 
 bool PeerConnection::IceRestartPending(const std::string& content_name) const {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  return pending_ice_restarts_.find(content_name) !=
-         pending_ice_restarts_.end();
+  return sdp_handler_.IceRestartPending(content_name);
 }
 
 bool PeerConnection::NeedsIceRestart(const std::string& content_name) const {
@@ -6406,9 +4238,7 @@
     // Use transport_name as the candidate media id.
     std::unique_ptr<JsepIceCandidate> candidate(
         new JsepIceCandidate(transport_name, sdp_mline_index, *citer));
-    if (local_description()) {
-      mutable_local_description()->AddCandidate(candidate.get());
-    }
+    sdp_handler_.AddLocalIceCandidate(candidate.get());
     OnIceCandidate(std::move(candidate));
   }
 }
@@ -6430,10 +4260,7 @@
       return;
     }
   }
-
-  if (local_description()) {
-    mutable_local_description()->RemoveCandidates(candidates);
-  }
+  sdp_handler_.RemoveLocalIceCandidates(candidates);
   OnIceCandidatesRemoved(candidates);
 }
 
@@ -6450,6 +4277,7 @@
 }
 
 void PeerConnection::EnableSending() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   for (const auto& transceiver : transceivers_) {
     cricket::ChannelInterface* channel = transceiver->internal()->channel();
     if (channel && !channel->enabled()) {
@@ -6485,6 +4313,7 @@
 
 bool PeerConnection::UseCandidatesInSessionDescription(
     const SessionDescriptionInterface* remote_desc) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   if (!remote_desc) {
     return true;
   }
@@ -6513,6 +4342,7 @@
 }
 
 bool PeerConnection::UseCandidate(const IceCandidateInterface* candidate) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RTCErrorOr<const cricket::ContentInfo*> result =
       FindContentInfo(remote_description(), candidate);
   if (!result.ok()) {
@@ -6585,6 +4415,7 @@
 }
 
 void PeerConnection::RemoveUnusedChannels(const SessionDescription* desc) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   // Destroy video channel first since it may have a pointer to the
   // voice channel.
   const cricket::ContentInfo* video_info = cricket::GetFirstVideoContent(desc);
@@ -6603,24 +4434,10 @@
   }
 }
 
-RTCErrorOr<const cricket::ContentGroup*> PeerConnection::GetEarlyBundleGroup(
-    const SessionDescription& desc) const {
-  const cricket::ContentGroup* bundle_group = nullptr;
-  if (configuration_.bundle_policy ==
-      PeerConnectionInterface::kBundlePolicyMaxBundle) {
-    bundle_group = desc.GetGroupByName(cricket::GROUP_TYPE_BUNDLE);
-    if (!bundle_group) {
-      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                           "max-bundle configured but session description "
-                           "has no BUNDLE group");
-    }
-  }
-  return bundle_group;
-}
-
 RTCError PeerConnection::CreateChannels(const SessionDescription& desc) {
   // Creating the media channels. Transports should already have been created
   // at this point.
+  RTC_DCHECK_RUN_ON(signaling_thread());
   const cricket::ContentInfo* voice = cricket::GetFirstAudioContent(&desc);
   if (voice && !voice->rejected &&
       !GetAudioTransceiver()->internal()->channel()) {
@@ -6659,6 +4476,7 @@
 // TODO(steveanton): Perhaps this should be managed by the RtpTransceiver.
 cricket::VoiceChannel* PeerConnection::CreateVoiceChannel(
     const std::string& mid) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RtpTransportInternal* rtp_transport = GetRtpTransport(mid);
 
   // TODO(bugs.webrtc.org/11992): CreateVoiceChannel internally switches to the
@@ -6681,6 +4499,7 @@
 // TODO(steveanton): Perhaps this should be managed by the RtpTransceiver.
 cricket::VideoChannel* PeerConnection::CreateVideoChannel(
     const std::string& mid) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   RtpTransportInternal* rtp_transport = GetRtpTransport(mid);
 
   // TODO(bugs.webrtc.org/11992): CreateVideoChannel internally switches to the
@@ -6701,6 +4520,7 @@
 }
 
 bool PeerConnection::CreateDataChannel(const std::string& mid) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   switch (data_channel_type()) {
     case cricket::DCT_SCTP:
       if (network_thread()->Invoke<bool>(
@@ -6813,130 +4633,6 @@
   return content->media_description()->rtcp_mux();
 }
 
-static RTCError ValidateMids(const cricket::SessionDescription& description) {
-  std::set<std::string> mids;
-  for (const cricket::ContentInfo& content : description.contents()) {
-    if (content.name.empty()) {
-      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                           "A media section is missing a MID attribute.");
-    }
-    if (!mids.insert(content.name).second) {
-      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                           "Duplicate a=mid value '" + content.name + "'.");
-    }
-  }
-  return RTCError::OK();
-}
-
-RTCError PeerConnection::ValidateSessionDescription(
-    const SessionDescriptionInterface* sdesc,
-    cricket::ContentSource source) {
-  if (session_error() != SessionError::kNone) {
-    LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR, GetSessionErrorMsg());
-  }
-
-  if (!sdesc || !sdesc->description()) {
-    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, kInvalidSdp);
-  }
-
-  SdpType type = sdesc->GetType();
-  if ((source == cricket::CS_LOCAL && !ExpectSetLocalDescription(type)) ||
-      (source == cricket::CS_REMOTE && !ExpectSetRemoteDescription(type))) {
-    LOG_AND_RETURN_ERROR(
-        RTCErrorType::INVALID_STATE,
-        "Called in wrong state: " + GetSignalingStateString(signaling_state()));
-  }
-
-  RTCError error = ValidateMids(*sdesc->description());
-  if (!error.ok()) {
-    return error;
-  }
-
-  // Verify crypto settings.
-  std::string crypto_error;
-  if (webrtc_session_desc_factory_->SdesPolicy() == cricket::SEC_REQUIRED ||
-      dtls_enabled_) {
-    RTCError crypto_error = VerifyCrypto(sdesc->description(), dtls_enabled_);
-    if (!crypto_error.ok()) {
-      return crypto_error;
-    }
-  }
-
-  // Verify ice-ufrag and ice-pwd.
-  if (!VerifyIceUfragPwdPresent(sdesc->description())) {
-    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                         kSdpWithoutIceUfragPwd);
-  }
-
-  if (!ValidateBundleSettings(sdesc->description())) {
-    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                         kBundleWithoutRtcpMux);
-  }
-
-  // TODO(skvlad): When the local rtcp-mux policy is Require, reject any
-  // m-lines that do not rtcp-mux enabled.
-
-  // Verify m-lines in Answer when compared against Offer.
-  if (type == SdpType::kPrAnswer || type == SdpType::kAnswer) {
-    // With an answer we want to compare the new answer session description with
-    // the offer's session description from the current negotiation.
-    const cricket::SessionDescription* offer_desc =
-        (source == cricket::CS_LOCAL) ? remote_description()->description()
-                                      : local_description()->description();
-    if (!MediaSectionsHaveSameCount(*offer_desc, *sdesc->description()) ||
-        !MediaSectionsInSameOrder(*offer_desc, nullptr, *sdesc->description(),
-                                  type)) {
-      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                           kMlineMismatchInAnswer);
-    }
-  } else {
-    // The re-offers should respect the order of m= sections in current
-    // description. See RFC3264 Section 8 paragraph 4 for more details.
-    // With a re-offer, either the current local or current remote descriptions
-    // could be the most up to date, so we would like to check against both of
-    // them if they exist. It could be the case that one of them has a 0 port
-    // for a media section, but the other does not. This is important to check
-    // against in the case that we are recycling an m= section.
-    const cricket::SessionDescription* current_desc = nullptr;
-    const cricket::SessionDescription* secondary_current_desc = nullptr;
-    if (local_description()) {
-      current_desc = local_description()->description();
-      if (remote_description()) {
-        secondary_current_desc = remote_description()->description();
-      }
-    } else if (remote_description()) {
-      current_desc = remote_description()->description();
-    }
-    if (current_desc &&
-        !MediaSectionsInSameOrder(*current_desc, secondary_current_desc,
-                                  *sdesc->description(), type)) {
-      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                           kMlineMismatchInSubsequentOffer);
-    }
-  }
-
-  if (IsUnifiedPlan()) {
-    // Ensure that each audio and video media section has at most one
-    // "StreamParams". This will return an error if receiving a session
-    // description from a "Plan B" endpoint which adds multiple tracks of the
-    // same type. With Unified Plan, there can only be at most one track per
-    // media section.
-    for (const ContentInfo& content : sdesc->description()->contents()) {
-      const MediaContentDescription& desc = *content.media_description();
-      if ((desc.type() == cricket::MEDIA_TYPE_AUDIO ||
-           desc.type() == cricket::MEDIA_TYPE_VIDEO) &&
-          desc.streams().size() > 1u) {
-        LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
-                             "Media section has more than one track specified "
-                             "with a=ssrc lines which is not supported with "
-                             "Unified Plan.");
-      }
-    }
-  }
-
-  return RTCError::OK();
-}
-
 bool PeerConnection::ExpectSetLocalDescription(SdpType type) {
   PeerConnectionInterface::SignalingState state = signaling_state();
   if (type == SdpType::kOffer) {
@@ -6975,6 +4671,7 @@
 }
 
 std::string PeerConnection::GetSessionErrorMsg() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   rtc::StringBuilder desc;
   desc << kSessionError << SessionErrorToString(session_error()) << ". ";
   desc << kSessionErrorDesc << session_error_desc() << ".";
@@ -7103,6 +4800,7 @@
     const IceCandidateInterface* candidate,
     const SessionDescriptionInterface* remote_desc,
     bool* valid) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   *valid = true;
 
   const SessionDescriptionInterface* current_remote_desc =
@@ -7128,7 +4826,8 @@
 
 bool PeerConnection::SrtpRequired() const {
   return (dtls_enabled_ ||
-          webrtc_session_desc_factory_->SdesPolicy() == cricket::SEC_REQUIRED);
+          sdp_handler_.webrtc_session_desc_factory()->SdesPolicy() ==
+              cricket::SEC_REQUIRED);
 }
 
 void PeerConnection::OnTransportControllerGatheringState(
@@ -7327,6 +5026,7 @@
 }
 
 void PeerConnection::DestroyDataChannelTransport() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
   if (data_channel_controller_.rtp_data_channel()) {
     data_channel_controller_.OnTransportChannelClosed();
     DestroyChannelInterface(data_channel_controller_.rtp_data_channel());
@@ -7395,7 +5095,7 @@
 void PeerConnection::OnSetStreams() {
   RTC_DCHECK_RUN_ON(signaling_thread());
   if (IsUnifiedPlan())
-    UpdateNegotiationNeeded();
+    sdp_handler_.UpdateNegotiationNeeded();
 }
 
 PeerConnectionObserver* PeerConnection::Observer() const {
@@ -7424,354 +5124,9 @@
                            nullptr);
 }
 
-void PeerConnection::UpdateNegotiationNeeded() {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  if (!IsUnifiedPlan()) {
-    Observer()->OnRenegotiationNeeded();
-    GenerateNegotiationNeededEvent();
-    return;
-  }
-
-  // In the spec, a task is queued here to run the following steps - this is
-  // meant to ensure we do not fire onnegotiationneeded prematurely if multiple
-  // changes are being made at once. In order to support Chromium's
-  // implementation where the JavaScript representation of the PeerConnection
-  // lives on a separate thread though, the queuing of a task is instead
-  // performed by the PeerConnectionObserver posting from the signaling thread
-  // to the JavaScript main thread that negotiation is needed. And because the
-  // Operations Chain lives on the WebRTC signaling thread,
-  // ShouldFireNegotiationNeededEvent() must be called before firing the event
-  // to ensure the Operations Chain is still empty and the event has not been
-  // invalidated.
-
-  // If connection's [[IsClosed]] slot is true, abort these steps.
-  if (IsClosed())
-    return;
-
-  // If connection's signaling state is not "stable", abort these steps.
-  if (signaling_state() != kStable)
-    return;
-
-  // NOTE
-  // The negotiation-needed flag will be updated once the state transitions to
-  // "stable", as part of the steps for setting an RTCSessionDescription.
-
-  // If the result of checking if negotiation is needed is false, clear the
-  // negotiation-needed flag by setting connection's [[NegotiationNeeded]] slot
-  // to false, and abort these steps.
-  bool is_negotiation_needed = CheckIfNegotiationIsNeeded();
-  if (!is_negotiation_needed) {
-    is_negotiation_needed_ = false;
-    // Invalidate any negotiation needed event that may previosuly have been
-    // generated.
-    ++negotiation_needed_event_id_;
-    return;
-  }
-
-  // If connection's [[NegotiationNeeded]] slot is already true, abort these
-  // steps.
-  if (is_negotiation_needed_)
-    return;
-
-  // Set connection's [[NegotiationNeeded]] slot to true.
-  is_negotiation_needed_ = true;
-
-  // Queue a task that runs the following steps:
-  // If connection's [[IsClosed]] slot is true, abort these steps.
-  // If connection's [[NegotiationNeeded]] slot is false, abort these steps.
-  // Fire an event named negotiationneeded at connection.
-  Observer()->OnRenegotiationNeeded();
-  // Fire the spec-compliant version; when ShouldFireNegotiationNeededEvent() is
-  // used in the task queued by the observer, this event will only fire when the
-  // chain is empty.
-  GenerateNegotiationNeededEvent();
-}
-
-bool PeerConnection::CheckIfNegotiationIsNeeded() {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  // 1. If any implementation-specific negotiation is required, as described at
-  // the start of this section, return true.
-
-  // 2. If connection.[[LocalIceCredentialsToReplace]] is not empty, return
-  // true.
-  if (local_ice_credentials_to_replace_->HasIceCredentials()) {
-    return true;
-  }
-
-  // 3. Let description be connection.[[CurrentLocalDescription]].
-  const SessionDescriptionInterface* description = current_local_description();
-  if (!description)
-    return true;
-
-  // 4. If connection has created any RTCDataChannels, and no m= section in
-  // description has been negotiated yet for data, return true.
-  if (data_channel_controller_.HasSctpDataChannels()) {
-    if (!cricket::GetFirstDataContent(description->description()->contents()))
-      return true;
-  }
-
-  // 5. For each transceiver in connection's set of transceivers, perform the
-  // following checks:
-  for (const auto& transceiver : transceivers_) {
-    const ContentInfo* current_local_msection =
-        FindTransceiverMSection(transceiver.get(), description);
-
-    const ContentInfo* current_remote_msection = FindTransceiverMSection(
-        transceiver.get(), current_remote_description());
-
-    // 5.4 If transceiver is stopped and is associated with an m= section,
-    // but the associated m= section is not yet rejected in
-    // connection.[[CurrentLocalDescription]] or
-    // connection.[[CurrentRemoteDescription]], return true.
-    if (transceiver->stopped()) {
-      RTC_DCHECK(transceiver->stopping());
-      if (current_local_msection && !current_local_msection->rejected &&
-          ((current_remote_msection && !current_remote_msection->rejected) ||
-           !current_remote_msection)) {
-        return true;
-      }
-      continue;
-    }
-
-    // 5.1 If transceiver.[[Stopping]] is true and transceiver.[[Stopped]] is
-    // false, return true.
-    if (transceiver->stopping() && !transceiver->stopped())
-      return true;
-
-    // 5.2 If transceiver isn't stopped and isn't yet associated with an m=
-    // section in description, return true.
-    if (!current_local_msection)
-      return true;
-
-    const MediaContentDescription* current_local_media_description =
-        current_local_msection->media_description();
-    // 5.3 If transceiver isn't stopped and is associated with an m= section
-    // in description then perform the following checks:
-
-    // 5.3.1 If transceiver.[[Direction]] is "sendrecv" or "sendonly", and the
-    // associated m= section in description either doesn't contain a single
-    // "a=msid" line, or the number of MSIDs from the "a=msid" lines in this
-    // m= section, or the MSID values themselves, differ from what is in
-    // transceiver.sender.[[AssociatedMediaStreamIds]], return true.
-    if (RtpTransceiverDirectionHasSend(transceiver->direction())) {
-      if (current_local_media_description->streams().size() == 0)
-        return true;
-
-      std::vector<std::string> msection_msids;
-      for (const auto& stream : current_local_media_description->streams()) {
-        for (const std::string& msid : stream.stream_ids())
-          msection_msids.push_back(msid);
-      }
-
-      std::vector<std::string> transceiver_msids =
-          transceiver->sender()->stream_ids();
-      if (msection_msids.size() != transceiver_msids.size())
-        return true;
-
-      absl::c_sort(transceiver_msids);
-      absl::c_sort(msection_msids);
-      if (transceiver_msids != msection_msids)
-        return true;
-    }
-
-    // 5.3.2 If description is of type "offer", and the direction of the
-    // associated m= section in neither connection.[[CurrentLocalDescription]]
-    // nor connection.[[CurrentRemoteDescription]] matches
-    // transceiver.[[Direction]], return true.
-    if (description->GetType() == SdpType::kOffer) {
-      if (!current_remote_description())
-        return true;
-
-      if (!current_remote_msection)
-        return true;
-
-      RtpTransceiverDirection current_local_direction =
-          current_local_media_description->direction();
-      RtpTransceiverDirection current_remote_direction =
-          current_remote_msection->media_description()->direction();
-      if (transceiver->direction() != current_local_direction &&
-          transceiver->direction() !=
-              RtpTransceiverDirectionReversed(current_remote_direction)) {
-        return true;
-      }
-    }
-
-    // 5.3.3 If description is of type "answer", and the direction of the
-    // associated m= section in the description does not match
-    // transceiver.[[Direction]] intersected with the offered direction (as
-    // described in [JSEP] (section 5.3.1.)), return true.
-    if (description->GetType() == SdpType::kAnswer) {
-      if (!remote_description())
-        return true;
-
-      const ContentInfo* offered_remote_msection =
-          FindTransceiverMSection(transceiver.get(), remote_description());
-
-      RtpTransceiverDirection offered_direction =
-          offered_remote_msection
-              ? offered_remote_msection->media_description()->direction()
-              : RtpTransceiverDirection::kInactive;
-
-      if (current_local_media_description->direction() !=
-          (RtpTransceiverDirectionIntersection(
-              transceiver->direction(),
-              RtpTransceiverDirectionReversed(offered_direction)))) {
-        return true;
-      }
-    }
-  }
-
-  // If all the preceding checks were performed and true was not returned,
-  // nothing remains to be negotiated; return false.
-  return false;
-}
-
-void PeerConnection::OnOperationsChainEmpty() {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  if (IsClosed() || !update_negotiation_needed_on_empty_chain_)
-    return;
-  update_negotiation_needed_on_empty_chain_ = false;
-  // Firing when chain is empty is only supported in Unified Plan to avoid Plan
-  // B regressions. (In Plan B, onnegotiationneeded is already broken anyway, so
-  // firing it even more might just be confusing.)
-  if (IsUnifiedPlan()) {
-    UpdateNegotiationNeeded();
-  }
-}
-
 bool PeerConnection::ShouldFireNegotiationNeededEvent(uint32_t event_id) {
   RTC_DCHECK_RUN_ON(signaling_thread());
-  // Plan B? Always fire to conform with useless legacy behavior.
-  if (!IsUnifiedPlan()) {
-    return true;
-  }
-  // The event ID has been invalidated. Either negotiation is no longer needed
-  // or a newer negotiation needed event has been generated.
-  if (event_id != negotiation_needed_event_id_) {
-    return false;
-  }
-  // The chain is no longer empty, update negotiation needed when it becomes
-  // empty. This should generate a newer negotiation needed event, making this
-  // one obsolete.
-  if (!operations_chain_->IsEmpty()) {
-    // Since we just suppressed an event that would have been fired, if
-    // negotiation is still needed by the time the chain becomes empty again, we
-    // must make sure to generate another event if negotiation is needed then.
-    // This happens when |is_negotiation_needed_| goes from false to true, so we
-    // set it to false until UpdateNegotiationNeeded() is called.
-    is_negotiation_needed_ = false;
-    update_negotiation_needed_on_empty_chain_ = true;
-    return false;
-  }
-  // We must not fire if the signaling state is no longer "stable". If
-  // negotiation is still needed when we return to "stable", a new negotiation
-  // needed event will be generated, so this one can safely be suppressed.
-  if (signaling_state_ != PeerConnectionInterface::kStable) {
-    return false;
-  }
-  // All checks have passed - please fire "negotiationneeded" now!
-  return true;
-}
-
-void PeerConnection::GenerateNegotiationNeededEvent() {
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  ++negotiation_needed_event_id_;
-  Observer()->OnNegotiationNeededEvent(negotiation_needed_event_id_);
-}
-
-RTCError PeerConnection::Rollback(SdpType desc_type) {
-  auto state = signaling_state();
-  if (state != PeerConnectionInterface::kHaveLocalOffer &&
-      state != PeerConnectionInterface::kHaveRemoteOffer) {
-    return RTCError(RTCErrorType::INVALID_STATE,
-                    "Called in wrong signalingState: " +
-                        GetSignalingStateString(signaling_state()));
-  }
-  RTC_DCHECK_RUN_ON(signaling_thread());
-  RTC_DCHECK(IsUnifiedPlan());
-  std::vector<rtc::scoped_refptr<MediaStreamInterface>> all_added_streams;
-  std::vector<rtc::scoped_refptr<MediaStreamInterface>> all_removed_streams;
-  std::vector<rtc::scoped_refptr<RtpReceiverInterface>> removed_receivers;
-
-  for (auto&& transceivers_stable_state_pair :
-       transceiver_stable_states_by_transceivers_) {
-    auto transceiver = transceivers_stable_state_pair.first;
-    auto state = transceivers_stable_state_pair.second;
-
-    if (state.remote_stream_ids()) {
-      std::vector<rtc::scoped_refptr<MediaStreamInterface>> added_streams;
-      std::vector<rtc::scoped_refptr<MediaStreamInterface>> removed_streams;
-      SetAssociatedRemoteStreams(transceiver->internal()->receiver_internal(),
-                                 state.remote_stream_ids().value(),
-                                 &added_streams, &removed_streams);
-      all_added_streams.insert(all_added_streams.end(), added_streams.begin(),
-                               added_streams.end());
-      all_removed_streams.insert(all_removed_streams.end(),
-                                 removed_streams.begin(),
-                                 removed_streams.end());
-      if (!state.has_m_section() && !state.newly_created()) {
-        continue;
-      }
-    }
-
-    RTC_DCHECK(transceiver->internal()->mid().has_value());
-    DestroyTransceiverChannel(transceiver);
-
-    if (signaling_state() == PeerConnectionInterface::kHaveRemoteOffer &&
-        transceiver->receiver()) {
-      removed_receivers.push_back(transceiver->receiver());
-    }
-    if (state.newly_created()) {
-      if (transceiver->internal()->reused_for_addtrack()) {
-        transceiver->internal()->set_created_by_addtrack(true);
-      } else {
-        int remaining_transceiver_count = 0;
-        for (auto&& t : transceivers_) {
-          if (t != transceiver) {
-            transceivers_[remaining_transceiver_count++] = t;
-          }
-        }
-        transceivers_.resize(remaining_transceiver_count);
-      }
-    }
-    transceiver->internal()->sender_internal()->set_transport(nullptr);
-    transceiver->internal()->receiver_internal()->set_transport(nullptr);
-    transceiver->internal()->set_mid(state.mid());
-    transceiver->internal()->set_mline_index(state.mline_index());
-  }
-  transport_controller_->RollbackTransports();
-  if (have_pending_rtp_data_channel_) {
-    DestroyDataChannelTransport();
-    have_pending_rtp_data_channel_ = false;
-  }
-  transceiver_stable_states_by_transceivers_.clear();
-  pending_local_description_.reset();
-  pending_remote_description_.reset();
-  ChangeSignalingState(PeerConnectionInterface::kStable);
-
-  // Once all processing has finished, fire off callbacks.
-  for (const auto& receiver : removed_receivers) {
-    Observer()->OnRemoveTrack(receiver);
-  }
-  for (const auto& stream : all_added_streams) {
-    Observer()->OnAddStream(stream);
-  }
-  for (const auto& stream : all_removed_streams) {
-    Observer()->OnRemoveStream(stream);
-  }
-
-  // The assumption is that in case of implicit rollback UpdateNegotiationNeeded
-  // gets called in SetRemoteDescription.
-  if (desc_type == SdpType::kRollback) {
-    UpdateNegotiationNeeded();
-    if (is_negotiation_needed_) {
-      // Legacy version.
-      Observer()->OnRenegotiationNeeded();
-      // Spec-compliant version; the event may get invalidated before firing.
-      GenerateNegotiationNeededEvent();
-    }
-  }
-  return RTCError::OK();
+  return sdp_handler_.ShouldFireNegotiationNeededEvent(event_id);
 }
 
 std::function<void(const rtc::CopyOnWriteBuffer& packet,
@@ -7807,4 +5162,5 @@
     }));
   };
 }
+
 }  // namespace webrtc
diff --git a/pc/peer_connection.h b/pc/peer_connection.h
index d92c734..d1fac2f 100644
--- a/pc/peer_connection.h
+++ b/pc/peer_connection.h
@@ -31,6 +31,7 @@
 #include "pc/rtp_sender.h"
 #include "pc/rtp_transceiver.h"
 #include "pc/sctp_transport.h"
+#include "pc/sdp_offer_answer.h"
 #include "pc/stats_collector.h"
 #include "pc/stream_collection.h"
 #include "pc/webrtc_session_description_factory.h"
@@ -46,6 +47,7 @@
 class MediaStreamObserver;
 class VideoRtpReceiver;
 class RtcEventLog;
+class SdpOfferAnswerHandler;
 
 // PeerConnection is the implementation of the PeerConnection object as defined
 // by the PeerConnectionInterface API surface.
@@ -326,7 +328,7 @@
   PeerConnectionObserver* Observer() const;
   bool IsClosed() const {
     RTC_DCHECK_RUN_ON(signaling_thread());
-    return signaling_state_ == PeerConnectionInterface::kClosed;
+    return sdp_handler_.signaling_state() == PeerConnectionInterface::kClosed;
   }
   // Get current SSL role used by SCTP's underlying transport.
   bool GetSctpSslRole(rtc::SSLRole* role);
@@ -335,6 +337,32 @@
 
   bool ShouldFireNegotiationNeededEvent(uint32_t event_id) override;
 
+  // Functions needed by SdpOfferAnswerHandler
+  StatsCollector* stats() {
+    RTC_DCHECK_RUN_ON(signaling_thread());
+    return stats_.get();
+  }
+  DataChannelController* data_channel_controller() {
+    RTC_DCHECK_RUN_ON(signaling_thread());
+    return &data_channel_controller_;
+  }
+  bool dtls_enabled() const {
+    RTC_DCHECK_RUN_ON(signaling_thread());
+    return dtls_enabled_;
+  }
+  const PeerConnectionInterface::RTCConfiguration* configuration() const {
+    RTC_DCHECK_RUN_ON(signaling_thread());
+    return &configuration_;
+  }
+  rtc::scoped_refptr<StreamCollection> remote_streams_internal() const {
+    RTC_DCHECK_RUN_ON(signaling_thread());
+    return remote_streams_;
+  }
+  rtc::UniqueStringGenerator* mid_generator() {
+    RTC_DCHECK_RUN_ON(signaling_thread());
+    return &mid_generator_;
+  }
+
   // Functions made public for testing.
   void ReturnHistogramVeryQuicklyForTesting() {
     RTC_DCHECK_RUN_ON(signaling_thread());
@@ -350,18 +378,9 @@
   ~PeerConnection() override;
 
  private:
-  class ImplicitCreateSessionDescriptionObserver;
-  friend class ImplicitCreateSessionDescriptionObserver;
-  class SetSessionDescriptionObserverAdapter;
-  friend class SetSessionDescriptionObserverAdapter;
-
-  // Represents the [[LocalIceCredentialsToReplace]] internal slot in the spec.
-  // It makes the next CreateOffer() produce new ICE credentials even if
-  // RTCOfferAnswerOptions::ice_restart is false.
-  // https://w3c.github.io/webrtc-pc/#dfn-localufragstoreplace
-  // TODO(hbos): When JsepTransportController/JsepTransport supports rollback,
-  // move this type of logic to JsepTransportController/JsepTransport.
-  class LocalIceCredentialsToReplace;
+  // While refactoring: Allow access from SDP negotiation
+  // TOOD(https://bugs.webrtc.org/11995): Remove friendship.
+  friend class SdpOfferAnswerHandler;
 
   struct RtpSenderInfo {
     RtpSenderInfo() : first_ssrc(0) {}
@@ -436,24 +455,9 @@
   rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
   GetFirstAudioTransceiver() const RTC_RUN_ON(signaling_thread());
 
-  // Implementation of the offer/answer exchange operations. These are chained
-  // onto the |operations_chain_| when the public CreateOffer(), CreateAnswer(),
-  // SetLocalDescription() and SetRemoteDescription() methods are invoked.
-  void DoCreateOffer(
-      const RTCOfferAnswerOptions& options,
-      rtc::scoped_refptr<CreateSessionDescriptionObserver> observer);
-  void DoCreateAnswer(
-      const RTCOfferAnswerOptions& options,
-      rtc::scoped_refptr<CreateSessionDescriptionObserver> observer);
-  void DoSetLocalDescription(
-      std::unique_ptr<SessionDescriptionInterface> desc,
-      rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer);
-  void DoSetRemoteDescription(
-      std::unique_ptr<SessionDescriptionInterface> desc,
-      rtc::scoped_refptr<SetRemoteDescriptionObserverInterface> observer);
 
   // Helper function to remove stopped transceivers.
-  void RemoveStoppedTransceivers() RTC_RUN_ON(signaling_thread());
+  void RemoveStoppedTransceivers();
 
   void CreateAudioReceiver(MediaStreamInterface* stream,
                            const RtpSenderInfo& remote_sender_info)
@@ -524,10 +528,9 @@
   CreateAndAddTransceiver(
       rtc::scoped_refptr<RtpSenderProxyWithInternal<RtpSenderInternal>> sender,
       rtc::scoped_refptr<RtpReceiverProxyWithInternal<RtpReceiverInternal>>
-          receiver) RTC_RUN_ON(signaling_thread());
+          receiver);
 
-  void SetIceConnectionState(IceConnectionState new_state)
-      RTC_RUN_ON(signaling_thread());
+  void SetIceConnectionState(IceConnectionState new_state);
   void SetStandardizedIceConnectionState(
       PeerConnectionInterface::IceConnectionState new_state)
       RTC_RUN_ON(signaling_thread());
@@ -556,10 +559,6 @@
       const cricket::CandidatePairChangeEvent& event)
       RTC_RUN_ON(signaling_thread());
 
-  // Update the state, signaling if necessary.
-  void ChangeSignalingState(SignalingState signaling_state)
-      RTC_RUN_ON(signaling_thread());
-
   // Signals from MediaStreamObserver.
   void OnAudioTrackAdded(AudioTrackInterface* track,
                          MediaStreamInterface* stream)
@@ -582,64 +581,19 @@
       CreateSessionDescriptionObserver* observer,
       RTCError error);
 
-  // Synchronous implementations of SetLocalDescription/SetRemoteDescription
-  // that return an RTCError instead of invoking a callback.
-  RTCError ApplyLocalDescription(
-      std::unique_ptr<SessionDescriptionInterface> desc);
-  RTCError ApplyRemoteDescription(
-      std::unique_ptr<SessionDescriptionInterface> desc);
-
-  // Updates the local RtpTransceivers according to the JSEP rules. Called as
-  // part of setting the local/remote description.
-  RTCError UpdateTransceiversAndDataChannels(
-      cricket::ContentSource source,
-      const SessionDescriptionInterface& new_session,
-      const SessionDescriptionInterface* old_local_description,
-      const SessionDescriptionInterface* old_remote_description)
-      RTC_RUN_ON(signaling_thread());
-
-  // Either creates or destroys the transceiver's BaseChannel according to the
-  // given media section.
-  RTCError UpdateTransceiverChannel(
-      rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
-          transceiver,
-      const cricket::ContentInfo& content,
-      const cricket::ContentGroup* bundle_group) RTC_RUN_ON(signaling_thread());
-
-  // Either creates or destroys the local data channel according to the given
-  // media section.
-  RTCError UpdateDataChannel(cricket::ContentSource source,
-                             const cricket::ContentInfo& content,
-                             const cricket::ContentGroup* bundle_group)
-      RTC_RUN_ON(signaling_thread());
-
-  // Associate the given transceiver according to the JSEP rules.
-  RTCErrorOr<
-      rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>>
-  AssociateTransceiver(cricket::ContentSource source,
-                       SdpType type,
-                       size_t mline_index,
-                       const cricket::ContentInfo& content,
-                       const cricket::ContentInfo* old_local_content,
-                       const cricket::ContentInfo* old_remote_content)
-      RTC_RUN_ON(signaling_thread());
-
   // Returns the RtpTransceiver, if found, that is associated to the given MID.
   rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
-  GetAssociatedTransceiver(const std::string& mid) const
-      RTC_RUN_ON(signaling_thread());
+  GetAssociatedTransceiver(const std::string& mid) const;
 
   // Returns the RtpTransceiver, if found, that was assigned to the given mline
   // index in CreateOffer.
   rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
-  GetTransceiverByMLineIndex(size_t mline_index) const
-      RTC_RUN_ON(signaling_thread());
+  GetTransceiverByMLineIndex(size_t mline_index) const;
 
   // Returns an RtpTransciever, if available, that can be used to receive the
   // given media type according to JSEP rules.
   rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
-  FindAvailableTransceiverToReceive(cricket::MediaType media_type) const
-      RTC_RUN_ON(signaling_thread());
+  FindAvailableTransceiverToReceive(cricket::MediaType media_type) const;
 
   // Returns the media section in the given session description that is
   // associated with the RtpTransceiver. Returns null if none found or this
@@ -648,17 +602,7 @@
   const cricket::ContentInfo* FindMediaSectionForTransceiver(
       rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
           transceiver,
-      const SessionDescriptionInterface* sdesc) const
-      RTC_RUN_ON(signaling_thread());
-
-  // Runs the algorithm **set the associated remote streams** specified in
-  // https://w3c.github.io/webrtc-pc/#set-associated-remote-streams.
-  void SetAssociatedRemoteStreams(
-      rtc::scoped_refptr<RtpReceiverInternal> receiver,
-      const std::vector<std::string>& stream_ids,
-      std::vector<rtc::scoped_refptr<MediaStreamInterface>>* added_streams,
-      std::vector<rtc::scoped_refptr<MediaStreamInterface>>* removed_streams)
-      RTC_RUN_ON(signaling_thread());
+      const SessionDescriptionInterface* sdesc) const;
 
   // Runs the algorithm **process the removal of a remote track** specified in
   // the WebRTC specification.
@@ -672,14 +616,12 @@
       rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
           transceiver,
       std::vector<rtc::scoped_refptr<RtpTransceiverInterface>>* remove_list,
-      std::vector<rtc::scoped_refptr<MediaStreamInterface>>* removed_streams)
-      RTC_RUN_ON(signaling_thread());
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>>* removed_streams);
 
   void RemoveRemoteStreamsIfEmpty(
       const std::vector<rtc::scoped_refptr<MediaStreamInterface>>&
           remote_streams,
-      std::vector<rtc::scoped_refptr<MediaStreamInterface>>* removed_streams)
-      RTC_RUN_ON(signaling_thread());
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>>* removed_streams);
 
   void OnNegotiationNeeded();
 
@@ -687,8 +629,7 @@
   // the local MediaStreams and DataChannels.
   void GetOptionsForOffer(const PeerConnectionInterface::RTCOfferAnswerOptions&
                               offer_answer_options,
-                          cricket::MediaSessionOptions* session_options)
-      RTC_RUN_ON(signaling_thread());
+                          cricket::MediaSessionOptions* session_options);
   void GetOptionsForPlanBOffer(
       const PeerConnectionInterface::RTCOfferAnswerOptions&
           offer_answer_options,
@@ -700,8 +641,7 @@
       cricket::MediaSessionOptions* session_options)
       RTC_RUN_ON(signaling_thread());
 
-  RTCError HandleLegacyOfferOptions(const RTCOfferAnswerOptions& options)
-      RTC_RUN_ON(signaling_thread());
+  RTCError HandleLegacyOfferOptions(const RTCOfferAnswerOptions& options);
   void RemoveRecvDirectionFromReceivingTransceiversOfType(
       cricket::MediaType media_type) RTC_RUN_ON(signaling_thread());
   void AddUpToOneReceivingTransceiverOfType(cricket::MediaType media_type);
@@ -713,8 +653,7 @@
   // Returns a MediaSessionOptions struct with options decided by
   // |constraints|, the local MediaStreams and DataChannels.
   void GetOptionsForAnswer(const RTCOfferAnswerOptions& offer_answer_options,
-                           cricket::MediaSessionOptions* session_options)
-      RTC_RUN_ON(signaling_thread());
+                           cricket::MediaSessionOptions* session_options);
   void GetOptionsForPlanBAnswer(
       const PeerConnectionInterface::RTCOfferAnswerOptions&
           offer_answer_options,
@@ -751,12 +690,11 @@
   // Returns the MID for the data section associated with either the
   // RtpDataChannel or SCTP data channel, if it has been set. If no data
   // channels are configured this will return nullopt.
-  absl::optional<std::string> GetDataMid() const RTC_RUN_ON(signaling_thread());
+  absl::optional<std::string> GetDataMid() const;
 
   // Remove all local and remote senders of type |media_type|.
   // Called when a media type is rejected (m-line set to port 0).
-  void RemoveSenders(cricket::MediaType media_type)
-      RTC_RUN_ON(signaling_thread());
+  void RemoveSenders(cricket::MediaType media_type);
 
   // Makes sure a MediaStreamTrack is created for each StreamParam in |streams|,
   // and existing MediaStreamTracks are removed if there is no corresponding
@@ -768,7 +706,7 @@
       const std::vector<cricket::StreamParams>& streams,
       bool default_track_needed,
       cricket::MediaType media_type,
-      StreamCollection* new_streams) RTC_RUN_ON(signaling_thread());
+      StreamCollection* new_streams);
 
   // Triggered when a remote sender has been seen for the first time in a remote
   // session description. It creates a remote MediaStreamTrackInterface
@@ -787,15 +725,14 @@
   // Finds remote MediaStreams without any tracks and removes them from
   // |remote_streams_| and notifies the observer that the MediaStreams no longer
   // exist.
-  void UpdateEndedRemoteMediaStreams() RTC_RUN_ON(signaling_thread());
+  void UpdateEndedRemoteMediaStreams();
 
   // Loops through the vector of |streams| and finds added and removed
   // StreamParams since last time this method was called.
   // For each new or removed StreamParam, OnLocalSenderSeen or
   // OnLocalSenderRemoved is invoked.
   void UpdateLocalSenders(const std::vector<cricket::StreamParams>& streams,
-                          cricket::MediaType media_type)
-      RTC_RUN_ON(signaling_thread());
+                          cricket::MediaType media_type);
 
   // Triggered when a local sender has been seen for the first time in a local
   // session description.
@@ -829,8 +766,7 @@
   // unique. To support legacy end points that do not supply a=mid lines, this
   // method will modify the session description to add MIDs generated according
   // to the SDP semantics.
-  void FillInMissingRemoteMids(cricket::SessionDescription* remote_description)
-      RTC_RUN_ON(signaling_thread());
+  void FillInMissingRemoteMids(cricket::SessionDescription* remote_description);
 
   // Return the RtpSender with the given track attached.
   rtc::scoped_refptr<RtpSenderProxyWithInternal<RtpSenderInternal>>
@@ -904,7 +840,8 @@
   };
 
   // Returns the last error in the session. See the enum above for details.
-  SessionError session_error() const RTC_RUN_ON(signaling_thread()) {
+  SessionError session_error() const {
+    RTC_DCHECK_RUN_ON(signaling_thread());
     return session_error_;
   }
   const std::string& session_error_desc() const { return session_error_desc_; }
@@ -921,33 +858,17 @@
   void OnCertificateReady(
       const rtc::scoped_refptr<rtc::RTCCertificate>& certificate);
 
-  // Non-const versions of local_description()/remote_description(), for use
-  // internally.
-  SessionDescriptionInterface* mutable_local_description()
-      RTC_RUN_ON(signaling_thread()) {
-    return pending_local_description_ ? pending_local_description_.get()
-                                      : current_local_description_.get();
-  }
-  SessionDescriptionInterface* mutable_remote_description()
-      RTC_RUN_ON(signaling_thread()) {
-    return pending_remote_description_ ? pending_remote_description_.get()
-                                       : current_remote_description_.get();
-  }
-
   // Updates the error state, signaling if necessary.
   void SetSessionError(SessionError error, const std::string& error_desc);
 
-  RTCError UpdateSessionState(SdpType type,
-                              cricket::ContentSource source,
-                              const cricket::SessionDescription* description);
   // Based on number of transceivers per media type, enabled or disable
   // payload type based demuxing in the affected channels.
   void UpdatePayloadTypeDemuxingState(cricket::ContentSource source)
       RTC_RUN_ON(signaling_thread());
   // Push the media parts of the local or remote session description
   // down to all of the channels.
-  RTCError PushdownMediaDescription(SdpType type, cricket::ContentSource source)
-      RTC_RUN_ON(signaling_thread());
+  RTCError PushdownMediaDescription(SdpType type,
+                                    cricket::ContentSource source);
 
   RTCError PushdownTransportDescription(cricket::ContentSource source,
                                         SdpType type);
@@ -962,7 +883,7 @@
   // Enables media channels to allow sending of media.
   // This enables media to flow on all configured audio/video channels and the
   // RtpDataChannel.
-  void EnableSending() RTC_RUN_ON(signaling_thread());
+  void EnableSending();
 
   // Destroys all BaseChannels and destroys the SCTP data channel, if present.
   void DestroyAllChannels() RTC_RUN_ON(signaling_thread());
@@ -975,40 +896,25 @@
       RTC_RUN_ON(signaling_thread());
   // Uses all remote candidates in |remote_desc| in this session.
   bool UseCandidatesInSessionDescription(
-      const SessionDescriptionInterface* remote_desc)
-      RTC_RUN_ON(signaling_thread());
+      const SessionDescriptionInterface* remote_desc);
   // Uses |candidate| in this session.
-  bool UseCandidate(const IceCandidateInterface* candidate)
-      RTC_RUN_ON(signaling_thread());
+  bool UseCandidate(const IceCandidateInterface* candidate);
   RTCErrorOr<const cricket::ContentInfo*> FindContentInfo(
       const SessionDescriptionInterface* description,
       const IceCandidateInterface* candidate) RTC_RUN_ON(signaling_thread());
   // Deletes the corresponding channel of contents that don't exist in |desc|.
   // |desc| can be null. This means that all channels are deleted.
-  void RemoveUnusedChannels(const cricket::SessionDescription* desc)
-      RTC_RUN_ON(signaling_thread());
+  void RemoveUnusedChannels(const cricket::SessionDescription* desc);
 
   // Allocates media channels based on the |desc|. If |desc| doesn't have
   // the BUNDLE option, this method will disable BUNDLE in PortAllocator.
   // This method will also delete any existing media channels before creating.
-  RTCError CreateChannels(const cricket::SessionDescription& desc)
-      RTC_RUN_ON(signaling_thread());
-
-  // If the BUNDLE policy is max-bundle, then we know for sure that all
-  // transports will be bundled from the start. This method returns the BUNDLE
-  // group if that's the case, or null if BUNDLE will be negotiated later. An
-  // error is returned if max-bundle is specified but the session description
-  // does not have a BUNDLE group.
-  RTCErrorOr<const cricket::ContentGroup*> GetEarlyBundleGroup(
-      const cricket::SessionDescription& desc) const
-      RTC_RUN_ON(signaling_thread());
+  RTCError CreateChannels(const cricket::SessionDescription& desc);
 
   // Helper methods to create media channels.
-  cricket::VoiceChannel* CreateVoiceChannel(const std::string& mid)
-      RTC_RUN_ON(signaling_thread());
-  cricket::VideoChannel* CreateVideoChannel(const std::string& mid)
-      RTC_RUN_ON(signaling_thread());
-  bool CreateDataChannel(const std::string& mid) RTC_RUN_ON(signaling_thread());
+  cricket::VoiceChannel* CreateVoiceChannel(const std::string& mid);
+  cricket::VideoChannel* CreateVideoChannel(const std::string& mid);
+  bool CreateDataChannel(const std::string& mid);
 
   bool SetupDataChannelTransport_n(const std::string& mid)
       RTC_RUN_ON(network_thread());
@@ -1016,10 +922,6 @@
 
   bool ValidateBundleSettings(const cricket::SessionDescription* desc);
   bool HasRtcpMuxEnabled(const cricket::ContentInfo* content);
-  // Below methods are helper methods which verifies SDP.
-  RTCError ValidateSessionDescription(const SessionDescriptionInterface* sdesc,
-                                      cricket::ContentSource source)
-      RTC_RUN_ON(signaling_thread());
 
   // Check if a call to SetLocalDescription is acceptable with a session
   // description of the given type.
@@ -1037,7 +939,7 @@
   // index is valid.
   bool ReadyToUseRemoteCandidate(const IceCandidateInterface* candidate,
                                  const SessionDescriptionInterface* remote_desc,
-                                 bool* valid) RTC_RUN_ON(signaling_thread());
+                                 bool* valid);
 
   // Returns true if SRTP (either using DTLS-SRTP or SDES) is required by
   // this session.
@@ -1064,7 +966,7 @@
   void OnTransportControllerDtlsHandshakeError(rtc::SSLHandshakeError error);
 
   const char* SessionErrorToString(SessionError error) const;
-  std::string GetSessionErrorMsg() RTC_RUN_ON(signaling_thread());
+  std::string GetSessionErrorMsg();
 
   // Report the UMA metric SdpFormatReceived for the given remote offer.
   void ReportSdpFormatReceived(const SessionDescriptionInterface& remote_offer);
@@ -1108,7 +1010,7 @@
 
   // Destroys the RTP data channel transport and/or the SCTP data channel
   // transport and clears it.
-  void DestroyDataChannelTransport() RTC_RUN_ON(signaling_thread());
+  void DestroyDataChannelTransport();
 
   // Destroys the given ChannelInterface.
   // The channel cannot be accessed after this method is called.
@@ -1142,14 +1044,6 @@
     return rtp_transport;
   }
 
-  void UpdateNegotiationNeeded();
-  bool CheckIfNegotiationIsNeeded();
-  void OnOperationsChainEmpty();
-  void GenerateNegotiationNeededEvent();
-
-  // | desc_type | is the type of the description that caused the rollback.
-  RTCError Rollback(SdpType desc_type);
-
   std::function<void(const rtc::CopyOnWriteBuffer& packet,
                      int64_t packet_time_us)>
   InitializeRtcpCallback();
@@ -1171,15 +1065,6 @@
   // pointer (but not touch the object) from any thread.
   RtcEventLog* const event_log_ptr_ RTC_PT_GUARDED_BY(worker_thread());
 
-  // The operations chain is used by the offer/answer exchange methods to ensure
-  // they are executed in the right order. For example, if
-  // SetRemoteDescription() is invoked while CreateOffer() is still pending, the
-  // SRD operation will not start until CreateOffer() has completed. See
-  // https://w3c.github.io/webrtc-pc/#dfn-operations-chain.
-  rtc::scoped_refptr<rtc::OperationsChain> operations_chain_
-      RTC_GUARDED_BY(signaling_thread());
-
-  SignalingState signaling_state_ RTC_GUARDED_BY(signaling_thread()) = kStable;
   IceConnectionState ice_connection_state_ RTC_GUARDED_BY(signaling_thread()) =
       kIceConnectionNew;
   PeerConnectionInterface::IceConnectionState standardized_ice_connection_state_
@@ -1234,8 +1119,6 @@
   std::vector<RtpSenderInfo> local_video_sender_infos_
       RTC_GUARDED_BY(signaling_thread());
 
-  bool remote_peer_supports_msid_ RTC_GUARDED_BY(signaling_thread()) = false;
-
   // The unique_ptr belongs to the worker thread, but the Call object manages
   // its own thread safety.
   std::unique_ptr<Call> call_ RTC_GUARDED_BY(worker_thread());
@@ -1269,12 +1152,6 @@
       transceivers_;  // TODO(bugs.webrtc.org/9987): Accessed on both signaling
                       // and network thread.
 
-  // In Unified Plan, if we encounter remote SDP that does not contain an a=msid
-  // line we create and use a stream with a random ID for our receivers. This is
-  // to support legacy endpoints that do not support the a=msid attribute (as
-  // opposed to streamless tracks with "a=msid:-").
-  rtc::scoped_refptr<MediaStreamInterface> missing_msid_default_stream_
-      RTC_GUARDED_BY(signaling_thread());
   // MIDs will be generated using this generator which will keep track of
   // all the MIDs that have been seen over the life of the PeerConnection.
   rtc::UniqueStringGenerator mid_generator_ RTC_GUARDED_BY(signaling_thread());
@@ -1299,28 +1176,11 @@
   absl::optional<std::string> sctp_mid_s_ RTC_GUARDED_BY(signaling_thread());
   absl::optional<std::string> sctp_mid_n_ RTC_GUARDED_BY(network_thread());
 
-  // Whether this peer is the caller. Set when the local description is applied.
-  absl::optional<bool> is_caller_ RTC_GUARDED_BY(signaling_thread());
+  // The machinery for handling offers and answers.
+  SdpOfferAnswerHandler sdp_handler_ RTC_GUARDED_BY(signaling_thread());
 
-
-
-  std::unique_ptr<SessionDescriptionInterface> current_local_description_
-      RTC_GUARDED_BY(signaling_thread());
-  std::unique_ptr<SessionDescriptionInterface> pending_local_description_
-      RTC_GUARDED_BY(signaling_thread());
-  std::unique_ptr<SessionDescriptionInterface> current_remote_description_
-      RTC_GUARDED_BY(signaling_thread());
-  std::unique_ptr<SessionDescriptionInterface> pending_remote_description_
-      RTC_GUARDED_BY(signaling_thread());
   bool dtls_enabled_ RTC_GUARDED_BY(signaling_thread()) = false;
 
-  // List of content names for which the remote side triggered an ICE restart.
-  std::set<std::string> pending_ice_restarts_
-      RTC_GUARDED_BY(signaling_thread());
-
-  std::unique_ptr<WebRtcSessionDescriptionFactory> webrtc_session_desc_factory_
-      RTC_GUARDED_BY(signaling_thread());
-
   // Member variables for caching global options.
   cricket::AudioOptions audio_options_ RTC_GUARDED_BY(signaling_thread());
   cricket::VideoOptions video_options_ RTC_GUARDED_BY(signaling_thread());
@@ -1344,16 +1204,7 @@
   std::unique_ptr<webrtc::VideoBitrateAllocatorFactory>
       video_bitrate_allocator_factory_;
 
-  std::unique_ptr<LocalIceCredentialsToReplace>
-      local_ice_credentials_to_replace_ RTC_GUARDED_BY(signaling_thread());
-  bool is_negotiation_needed_ RTC_GUARDED_BY(signaling_thread()) = false;
-  bool update_negotiation_needed_on_empty_chain_
-      RTC_GUARDED_BY(signaling_thread()) = false;
-  uint32_t negotiation_needed_event_id_ = 0;
-
   DataChannelController data_channel_controller_;
-  rtc::WeakPtrFactory<PeerConnection> weak_ptr_factory_
-      RTC_GUARDED_BY(signaling_thread());
 };
 
 }  // namespace webrtc
diff --git a/pc/sdp_offer_answer.cc b/pc/sdp_offer_answer.cc
new file mode 100644
index 0000000..51bb4ed
--- /dev/null
+++ b/pc/sdp_offer_answer.cc
@@ -0,0 +1,2928 @@
+/*
+ *  Copyright 2020 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "pc/sdp_offer_answer.h"
+
+#include "api/media_stream_proxy.h"
+#include "api/uma_metrics.h"
+#include "pc/media_stream.h"
+#include "pc/peer_connection.h"
+#include "pc/rtp_media_utils.h"
+#include "rtc_base/trace_event.h"
+#include "system_wrappers/include/metrics.h"
+
+using cricket::ContentInfo;
+using cricket::ContentInfos;
+using cricket::MediaContentDescription;
+using cricket::MediaProtocolType;
+using cricket::RidDescription;
+using cricket::RidDirection;
+using cricket::SessionDescription;
+using cricket::SimulcastDescription;
+using cricket::SimulcastLayer;
+using cricket::SimulcastLayerList;
+using cricket::StreamParams;
+using cricket::TransportInfo;
+
+using cricket::LOCAL_PORT_TYPE;
+using cricket::PRFLX_PORT_TYPE;
+using cricket::RELAY_PORT_TYPE;
+using cricket::STUN_PORT_TYPE;
+
+namespace webrtc {
+
+namespace {
+
+// Error messages
+const char kInvalidSdp[] = "Invalid session description.";
+const char kInvalidCandidates[] = "Description contains invalid candidates.";
+const char kBundleWithoutRtcpMux[] =
+    "rtcp-mux must be enabled when BUNDLE "
+    "is enabled.";
+const char kMlineMismatchInAnswer[] =
+    "The order of m-lines in answer doesn't match order in offer. Rejecting "
+    "answer.";
+const char kMlineMismatchInSubsequentOffer[] =
+    "The order of m-lines in subsequent offer doesn't match order from "
+    "previous offer/answer.";
+const char kSdpWithoutIceUfragPwd[] =
+    "Called with SDP without ice-ufrag and ice-pwd.";
+const char kSdpWithoutDtlsFingerprint[] =
+    "Called with SDP without DTLS fingerprint.";
+const char kSdpWithoutSdesCrypto[] = "Called with SDP without SDES crypto.";
+
+// UMA metric names.
+const char kSimulcastVersionApplyLocalDescription[] =
+    "WebRTC.PeerConnection.Simulcast.ApplyLocalDescription";
+const char kSimulcastVersionApplyRemoteDescription[] =
+    "WebRTC.PeerConnection.Simulcast.ApplyRemoteDescription";
+const char kSimulcastDisabled[] = "WebRTC.PeerConnection.Simulcast.Disabled";
+
+void NoteAddIceCandidateResult(int result) {
+  RTC_HISTOGRAM_ENUMERATION("WebRTC.PeerConnection.AddIceCandidate", result,
+                            kAddIceCandidateMax);
+}
+
+void NoteKeyProtocolAndMedia(KeyExchangeProtocolType protocol_type,
+                             cricket::MediaType media_type) {
+  // Array of structs needed to map {KeyExchangeProtocolType,
+  // cricket::MediaType} to KeyExchangeProtocolMedia without using std::map in
+  // order to avoid -Wglobal-constructors and -Wexit-time-destructors.
+  static constexpr struct {
+    KeyExchangeProtocolType protocol_type;
+    cricket::MediaType media_type;
+    KeyExchangeProtocolMedia protocol_media;
+  } kEnumCounterKeyProtocolMediaMap[] = {
+      {kEnumCounterKeyProtocolDtls, cricket::MEDIA_TYPE_AUDIO,
+       kEnumCounterKeyProtocolMediaTypeDtlsAudio},
+      {kEnumCounterKeyProtocolDtls, cricket::MEDIA_TYPE_VIDEO,
+       kEnumCounterKeyProtocolMediaTypeDtlsVideo},
+      {kEnumCounterKeyProtocolDtls, cricket::MEDIA_TYPE_DATA,
+       kEnumCounterKeyProtocolMediaTypeDtlsData},
+      {kEnumCounterKeyProtocolSdes, cricket::MEDIA_TYPE_AUDIO,
+       kEnumCounterKeyProtocolMediaTypeSdesAudio},
+      {kEnumCounterKeyProtocolSdes, cricket::MEDIA_TYPE_VIDEO,
+       kEnumCounterKeyProtocolMediaTypeSdesVideo},
+      {kEnumCounterKeyProtocolSdes, cricket::MEDIA_TYPE_DATA,
+       kEnumCounterKeyProtocolMediaTypeSdesData},
+  };
+
+  RTC_HISTOGRAM_ENUMERATION("WebRTC.PeerConnection.KeyProtocol", protocol_type,
+                            kEnumCounterKeyProtocolMax);
+
+  for (const auto& i : kEnumCounterKeyProtocolMediaMap) {
+    if (i.protocol_type == protocol_type && i.media_type == media_type) {
+      RTC_HISTOGRAM_ENUMERATION("WebRTC.PeerConnection.KeyProtocolByMedia",
+                                i.protocol_media,
+                                kEnumCounterKeyProtocolMediaTypeMax);
+    }
+  }
+}
+
+// Returns true if |new_desc| requests an ICE restart (i.e., new ufrag/pwd).
+bool CheckForRemoteIceRestart(const SessionDescriptionInterface* old_desc,
+                              const SessionDescriptionInterface* new_desc,
+                              const std::string& content_name) {
+  if (!old_desc) {
+    return false;
+  }
+  const SessionDescription* new_sd = new_desc->description();
+  const SessionDescription* old_sd = old_desc->description();
+  const ContentInfo* cinfo = new_sd->GetContentByName(content_name);
+  if (!cinfo || cinfo->rejected) {
+    return false;
+  }
+  // If the content isn't rejected, check if ufrag and password has changed.
+  const cricket::TransportDescription* new_transport_desc =
+      new_sd->GetTransportDescriptionByName(content_name);
+  const cricket::TransportDescription* old_transport_desc =
+      old_sd->GetTransportDescriptionByName(content_name);
+  if (!new_transport_desc || !old_transport_desc) {
+    // No transport description exists. This is not an ICE restart.
+    return false;
+  }
+  if (cricket::IceCredentialsChanged(
+          old_transport_desc->ice_ufrag, old_transport_desc->ice_pwd,
+          new_transport_desc->ice_ufrag, new_transport_desc->ice_pwd)) {
+    RTC_LOG(LS_INFO) << "Remote peer requests ICE restart for " << content_name
+                     << ".";
+    return true;
+  }
+  return false;
+}
+
+// Generates a string error message for SetLocalDescription/SetRemoteDescription
+// from an RTCError.
+std::string GetSetDescriptionErrorMessage(cricket::ContentSource source,
+                                          SdpType type,
+                                          const RTCError& error) {
+  rtc::StringBuilder oss;
+  oss << "Failed to set " << (source == cricket::CS_LOCAL ? "local" : "remote")
+      << " " << SdpTypeToString(type) << " sdp: " << error.message();
+  return oss.Release();
+}
+
+std::string GetStreamIdsString(rtc::ArrayView<const std::string> stream_ids) {
+  std::string output = "streams=[";
+  const char* separator = "";
+  for (const auto& stream_id : stream_ids) {
+    output.append(separator).append(stream_id);
+    separator = ", ";
+  }
+  output.append("]");
+  return output;
+}
+
+void ReportSimulcastApiVersion(const char* name,
+                               const SessionDescription& session) {
+  bool has_legacy = false;
+  bool has_spec_compliant = false;
+  for (const ContentInfo& content : session.contents()) {
+    if (!content.media_description()) {
+      continue;
+    }
+    has_spec_compliant |= content.media_description()->HasSimulcast();
+    for (const StreamParams& sp : content.media_description()->streams()) {
+      has_legacy |= sp.has_ssrc_group(cricket::kSimSsrcGroupSemantics);
+    }
+  }
+
+  if (has_legacy) {
+    RTC_HISTOGRAM_ENUMERATION(name, kSimulcastApiVersionLegacy,
+                              kSimulcastApiVersionMax);
+  }
+  if (has_spec_compliant) {
+    RTC_HISTOGRAM_ENUMERATION(name, kSimulcastApiVersionSpecCompliant,
+                              kSimulcastApiVersionMax);
+  }
+  if (!has_legacy && !has_spec_compliant) {
+    RTC_HISTOGRAM_ENUMERATION(name, kSimulcastApiVersionNone,
+                              kSimulcastApiVersionMax);
+  }
+}
+
+const ContentInfo* FindTransceiverMSection(
+    RtpTransceiverProxyWithInternal<RtpTransceiver>* transceiver,
+    const SessionDescriptionInterface* session_description) {
+  return transceiver->mid()
+             ? session_description->description()->GetContentByName(
+                   *transceiver->mid())
+             : nullptr;
+}
+
+// If the direction is "recvonly" or "inactive", treat the description
+// as containing no streams.
+// See: https://code.google.com/p/webrtc/issues/detail?id=5054
+std::vector<cricket::StreamParams> GetActiveStreams(
+    const cricket::MediaContentDescription* desc) {
+  return RtpTransceiverDirectionHasSend(desc->direction())
+             ? desc->streams()
+             : std::vector<cricket::StreamParams>();
+}
+
+// Logic to decide if an m= section can be recycled. This means that the new
+// m= section is not rejected, but the old local or remote m= section is
+// rejected. |old_content_one| and |old_content_two| refer to the m= section
+// of the old remote and old local descriptions in no particular order.
+// We need to check both the old local and remote because either
+// could be the most current from the latest negotation.
+bool IsMediaSectionBeingRecycled(SdpType type,
+                                 const ContentInfo& content,
+                                 const ContentInfo* old_content_one,
+                                 const ContentInfo* old_content_two) {
+  return type == SdpType::kOffer && !content.rejected &&
+         ((old_content_one && old_content_one->rejected) ||
+          (old_content_two && old_content_two->rejected));
+}
+
+// Verify that the order of media sections in |new_desc| matches
+// |current_desc|. The number of m= sections in |new_desc| should be no
+// less than |current_desc|. In the case of checking an answer's
+// |new_desc|, the |current_desc| is the last offer that was set as the
+// local or remote. In the case of checking an offer's |new_desc| we
+// check against the local and remote descriptions stored from the last
+// negotiation, because either of these could be the most up to date for
+// possible rejected m sections. These are the |current_desc| and
+// |secondary_current_desc|.
+bool MediaSectionsInSameOrder(const SessionDescription& current_desc,
+                              const SessionDescription* secondary_current_desc,
+                              const SessionDescription& new_desc,
+                              const SdpType type) {
+  if (current_desc.contents().size() > new_desc.contents().size()) {
+    return false;
+  }
+
+  for (size_t i = 0; i < current_desc.contents().size(); ++i) {
+    const cricket::ContentInfo* secondary_content_info = nullptr;
+    if (secondary_current_desc &&
+        i < secondary_current_desc->contents().size()) {
+      secondary_content_info = &secondary_current_desc->contents()[i];
+    }
+    if (IsMediaSectionBeingRecycled(type, new_desc.contents()[i],
+                                    &current_desc.contents()[i],
+                                    secondary_content_info)) {
+      // For new offer descriptions, if the media section can be recycled, it's
+      // valid for the MID and media type to change.
+      continue;
+    }
+    if (new_desc.contents()[i].name != current_desc.contents()[i].name) {
+      return false;
+    }
+    const MediaContentDescription* new_desc_mdesc =
+        new_desc.contents()[i].media_description();
+    const MediaContentDescription* current_desc_mdesc =
+        current_desc.contents()[i].media_description();
+    if (new_desc_mdesc->type() != current_desc_mdesc->type()) {
+      return false;
+    }
+  }
+  return true;
+}
+
+bool MediaSectionsHaveSameCount(const SessionDescription& desc1,
+                                const SessionDescription& desc2) {
+  return desc1.contents().size() == desc2.contents().size();
+}
+// Checks that each non-rejected content has SDES crypto keys or a DTLS
+// fingerprint, unless it's in a BUNDLE group, in which case only the
+// BUNDLE-tag section (first media section/description in the BUNDLE group)
+// needs a ufrag and pwd. Mismatches, such as replying with a DTLS fingerprint
+// to SDES keys, will be caught in JsepTransport negotiation, and backstopped
+// by Channel's |srtp_required| check.
+RTCError VerifyCrypto(const SessionDescription* desc, bool dtls_enabled) {
+  const cricket::ContentGroup* bundle =
+      desc->GetGroupByName(cricket::GROUP_TYPE_BUNDLE);
+  for (const cricket::ContentInfo& content_info : desc->contents()) {
+    if (content_info.rejected) {
+      continue;
+    }
+    // Note what media is used with each crypto protocol, for all sections.
+    NoteKeyProtocolAndMedia(dtls_enabled ? webrtc::kEnumCounterKeyProtocolDtls
+                                         : webrtc::kEnumCounterKeyProtocolSdes,
+                            content_info.media_description()->type());
+    const std::string& mid = content_info.name;
+    if (bundle && bundle->HasContentName(mid) &&
+        mid != *(bundle->FirstContentName())) {
+      // This isn't the first media section in the BUNDLE group, so it's not
+      // required to have crypto attributes, since only the crypto attributes
+      // from the first section actually get used.
+      continue;
+    }
+
+    // If the content isn't rejected or bundled into another m= section, crypto
+    // must be present.
+    const MediaContentDescription* media = content_info.media_description();
+    const TransportInfo* tinfo = desc->GetTransportInfoByName(mid);
+    if (!media || !tinfo) {
+      // Something is not right.
+      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, kInvalidSdp);
+    }
+    if (dtls_enabled) {
+      if (!tinfo->description.identity_fingerprint) {
+        RTC_LOG(LS_WARNING)
+            << "Session description must have DTLS fingerprint if "
+               "DTLS enabled.";
+        return RTCError(RTCErrorType::INVALID_PARAMETER,
+                        kSdpWithoutDtlsFingerprint);
+      }
+    } else {
+      if (media->cryptos().empty()) {
+        RTC_LOG(LS_WARNING)
+            << "Session description must have SDES when DTLS disabled.";
+        return RTCError(RTCErrorType::INVALID_PARAMETER, kSdpWithoutSdesCrypto);
+      }
+    }
+  }
+  return RTCError::OK();
+}
+
+// Checks that each non-rejected content has ice-ufrag and ice-pwd set, unless
+// it's in a BUNDLE group, in which case only the BUNDLE-tag section (first
+// media section/description in the BUNDLE group) needs a ufrag and pwd.
+bool VerifyIceUfragPwdPresent(const SessionDescription* desc) {
+  const cricket::ContentGroup* bundle =
+      desc->GetGroupByName(cricket::GROUP_TYPE_BUNDLE);
+  for (const cricket::ContentInfo& content_info : desc->contents()) {
+    if (content_info.rejected) {
+      continue;
+    }
+    const std::string& mid = content_info.name;
+    if (bundle && bundle->HasContentName(mid) &&
+        mid != *(bundle->FirstContentName())) {
+      // This isn't the first media section in the BUNDLE group, so it's not
+      // required to have ufrag/password, since only the ufrag/password from
+      // the first section actually get used.
+      continue;
+    }
+
+    // If the content isn't rejected or bundled into another m= section,
+    // ice-ufrag and ice-pwd must be present.
+    const TransportInfo* tinfo = desc->GetTransportInfoByName(mid);
+    if (!tinfo) {
+      // Something is not right.
+      RTC_LOG(LS_ERROR) << kInvalidSdp;
+      return false;
+    }
+    if (tinfo->description.ice_ufrag.empty() ||
+        tinfo->description.ice_pwd.empty()) {
+      RTC_LOG(LS_ERROR) << "Session description must have ice ufrag and pwd.";
+      return false;
+    }
+  }
+  return true;
+}
+
+static RTCError ValidateMids(const cricket::SessionDescription& description) {
+  std::set<std::string> mids;
+  for (const cricket::ContentInfo& content : description.contents()) {
+    if (content.name.empty()) {
+      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
+                           "A media section is missing a MID attribute.");
+    }
+    if (!mids.insert(content.name).second) {
+      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
+                           "Duplicate a=mid value '" + content.name + "'.");
+    }
+  }
+  return RTCError::OK();
+}
+
+bool IsValidOfferToReceiveMedia(int value) {
+  typedef PeerConnectionInterface::RTCOfferAnswerOptions Options;
+  return (value >= Options::kUndefined) &&
+         (value <= Options::kMaxOfferToReceiveMedia);
+}
+
+bool ValidateOfferAnswerOptions(
+    const PeerConnectionInterface::RTCOfferAnswerOptions& rtc_options) {
+  return IsValidOfferToReceiveMedia(rtc_options.offer_to_receive_audio) &&
+         IsValidOfferToReceiveMedia(rtc_options.offer_to_receive_video);
+}
+
+// Map internal signaling state name to spec name:
+//  https://w3c.github.io/webrtc-pc/#rtcsignalingstate-enum
+std::string GetSignalingStateString(
+    PeerConnectionInterface::SignalingState state) {
+  switch (state) {
+    case PeerConnectionInterface::kStable:
+      return "stable";
+    case PeerConnectionInterface::kHaveLocalOffer:
+      return "have-local-offer";
+    case PeerConnectionInterface::kHaveLocalPrAnswer:
+      return "have-local-pranswer";
+    case PeerConnectionInterface::kHaveRemoteOffer:
+      return "have-remote-offer";
+    case PeerConnectionInterface::kHaveRemotePrAnswer:
+      return "have-remote-pranswer";
+    case PeerConnectionInterface::kClosed:
+      return "closed";
+  }
+  RTC_NOTREACHED();
+  return "";
+}
+
+// This method will extract any send encodings that were sent by the remote
+// connection. This is currently only relevant for Simulcast scenario (where
+// the number of layers may be communicated by the server).
+static std::vector<RtpEncodingParameters> GetSendEncodingsFromRemoteDescription(
+    const MediaContentDescription& desc) {
+  if (!desc.HasSimulcast()) {
+    return {};
+  }
+  std::vector<RtpEncodingParameters> result;
+  const SimulcastDescription& simulcast = desc.simulcast_description();
+
+  // This is a remote description, the parameters we are after should appear
+  // as receive streams.
+  for (const auto& alternatives : simulcast.receive_layers()) {
+    RTC_DCHECK(!alternatives.empty());
+    // There is currently no way to specify or choose from alternatives.
+    // We will always use the first alternative, which is the most preferred.
+    const SimulcastLayer& layer = alternatives[0];
+    RtpEncodingParameters parameters;
+    parameters.rid = layer.rid;
+    parameters.active = !layer.is_paused;
+    result.push_back(parameters);
+  }
+
+  return result;
+}
+
+static RTCError UpdateSimulcastLayerStatusInSender(
+    const std::vector<SimulcastLayer>& layers,
+    rtc::scoped_refptr<RtpSenderInternal> sender) {
+  RTC_DCHECK(sender);
+  RtpParameters parameters = sender->GetParametersInternal();
+  std::vector<std::string> disabled_layers;
+
+  // The simulcast envelope cannot be changed, only the status of the streams.
+  // So we will iterate over the send encodings rather than the layers.
+  for (RtpEncodingParameters& encoding : parameters.encodings) {
+    auto iter = std::find_if(layers.begin(), layers.end(),
+                             [&encoding](const SimulcastLayer& layer) {
+                               return layer.rid == encoding.rid;
+                             });
+    // A layer that cannot be found may have been removed by the remote party.
+    if (iter == layers.end()) {
+      disabled_layers.push_back(encoding.rid);
+      continue;
+    }
+
+    encoding.active = !iter->is_paused;
+  }
+
+  RTCError result = sender->SetParametersInternal(parameters);
+  if (result.ok()) {
+    result = sender->DisableEncodingLayers(disabled_layers);
+  }
+
+  return result;
+}
+
+static bool SimulcastIsRejected(
+    const ContentInfo* local_content,
+    const MediaContentDescription& answer_media_desc) {
+  bool simulcast_offered = local_content &&
+                           local_content->media_description() &&
+                           local_content->media_description()->HasSimulcast();
+  bool simulcast_answered = answer_media_desc.HasSimulcast();
+  bool rids_supported = RtpExtension::FindHeaderExtensionByUri(
+      answer_media_desc.rtp_header_extensions(), RtpExtension::kRidUri);
+  return simulcast_offered && (!simulcast_answered || !rids_supported);
+}
+
+static RTCError DisableSimulcastInSender(
+    rtc::scoped_refptr<RtpSenderInternal> sender) {
+  RTC_DCHECK(sender);
+  RtpParameters parameters = sender->GetParametersInternal();
+  if (parameters.encodings.size() <= 1) {
+    return RTCError::OK();
+  }
+
+  std::vector<std::string> disabled_layers;
+  std::transform(
+      parameters.encodings.begin() + 1, parameters.encodings.end(),
+      std::back_inserter(disabled_layers),
+      [](const RtpEncodingParameters& encoding) { return encoding.rid; });
+  return sender->DisableEncodingLayers(disabled_layers);
+}
+
+}  // namespace
+
+// Used by parameterless SetLocalDescription() to create an offer or answer.
+// Upon completion of creating the session description, SetLocalDescription() is
+// invoked with the result.
+class SdpOfferAnswerHandler::ImplicitCreateSessionDescriptionObserver
+    : public CreateSessionDescriptionObserver {
+ public:
+  ImplicitCreateSessionDescriptionObserver(
+      rtc::WeakPtr<SdpOfferAnswerHandler> sdp_handler,
+      rtc::scoped_refptr<SetLocalDescriptionObserverInterface>
+          set_local_description_observer)
+      : sdp_handler_(std::move(sdp_handler)),
+        set_local_description_observer_(
+            std::move(set_local_description_observer)) {}
+  ~ImplicitCreateSessionDescriptionObserver() override {
+    RTC_DCHECK(was_called_);
+  }
+
+  void SetOperationCompleteCallback(
+      std::function<void()> operation_complete_callback) {
+    operation_complete_callback_ = std::move(operation_complete_callback);
+  }
+
+  bool was_called() const { return was_called_; }
+
+  void OnSuccess(SessionDescriptionInterface* desc_ptr) override {
+    RTC_DCHECK(!was_called_);
+    std::unique_ptr<SessionDescriptionInterface> desc(desc_ptr);
+    was_called_ = true;
+
+    // Abort early if |pc_| is no longer valid.
+    if (!sdp_handler_) {
+      operation_complete_callback_();
+      return;
+    }
+    // DoSetLocalDescription() is a synchronous operation that invokes
+    // |set_local_description_observer_| with the result.
+    sdp_handler_->DoSetLocalDescription(
+        std::move(desc), std::move(set_local_description_observer_));
+    operation_complete_callback_();
+  }
+
+  void OnFailure(RTCError error) override {
+    RTC_DCHECK(!was_called_);
+    was_called_ = true;
+    set_local_description_observer_->OnSetLocalDescriptionComplete(RTCError(
+        error.type(), std::string("SetLocalDescription failed to create "
+                                  "session description - ") +
+                          error.message()));
+    operation_complete_callback_();
+  }
+
+ private:
+  bool was_called_ = false;
+  rtc::WeakPtr<SdpOfferAnswerHandler> sdp_handler_;
+  rtc::scoped_refptr<SetLocalDescriptionObserverInterface>
+      set_local_description_observer_;
+  std::function<void()> operation_complete_callback_;
+};
+
+// Wraps a CreateSessionDescriptionObserver and an OperationsChain operation
+// complete callback. When the observer is invoked, the wrapped observer is
+// invoked followed by invoking the completion callback.
+class CreateSessionDescriptionObserverOperationWrapper
+    : public CreateSessionDescriptionObserver {
+ public:
+  CreateSessionDescriptionObserverOperationWrapper(
+      rtc::scoped_refptr<CreateSessionDescriptionObserver> observer,
+      std::function<void()> operation_complete_callback)
+      : observer_(std::move(observer)),
+        operation_complete_callback_(std::move(operation_complete_callback)) {
+    RTC_DCHECK(observer_);
+  }
+  ~CreateSessionDescriptionObserverOperationWrapper() override {
+    RTC_DCHECK(was_called_);
+  }
+
+  void OnSuccess(SessionDescriptionInterface* desc) override {
+    RTC_DCHECK(!was_called_);
+#ifdef RTC_DCHECK_IS_ON
+    was_called_ = true;
+#endif  // RTC_DCHECK_IS_ON
+    // Completing the operation before invoking the observer allows the observer
+    // to execute SetLocalDescription() without delay.
+    operation_complete_callback_();
+    observer_->OnSuccess(desc);
+  }
+
+  void OnFailure(RTCError error) override {
+    RTC_DCHECK(!was_called_);
+#ifdef RTC_DCHECK_IS_ON
+    was_called_ = true;
+#endif  // RTC_DCHECK_IS_ON
+    operation_complete_callback_();
+    observer_->OnFailure(std::move(error));
+  }
+
+ private:
+#ifdef RTC_DCHECK_IS_ON
+  bool was_called_ = false;
+#endif  // RTC_DCHECK_IS_ON
+  rtc::scoped_refptr<CreateSessionDescriptionObserver> observer_;
+  std::function<void()> operation_complete_callback_;
+};
+
+// Wrapper for SetSessionDescriptionObserver that invokes the success or failure
+// callback in a posted message handled by the peer connection. This introduces
+// a delay that prevents recursive API calls by the observer, but this also
+// means that the PeerConnection can be modified before the observer sees the
+// result of the operation. This is ill-advised for synchronizing states.
+//
+// Implements both the SetLocalDescriptionObserverInterface and the
+// SetRemoteDescriptionObserverInterface.
+class SdpOfferAnswerHandler::SetSessionDescriptionObserverAdapter
+    : public SetLocalDescriptionObserverInterface,
+      public SetRemoteDescriptionObserverInterface {
+ public:
+  SetSessionDescriptionObserverAdapter(
+      rtc::WeakPtr<SdpOfferAnswerHandler> handler,
+      rtc::scoped_refptr<SetSessionDescriptionObserver> inner_observer)
+      : handler_(std::move(handler)),
+        inner_observer_(std::move(inner_observer)) {}
+
+  // SetLocalDescriptionObserverInterface implementation.
+  void OnSetLocalDescriptionComplete(RTCError error) override {
+    OnSetDescriptionComplete(std::move(error));
+  }
+  // SetRemoteDescriptionObserverInterface implementation.
+  void OnSetRemoteDescriptionComplete(RTCError error) override {
+    OnSetDescriptionComplete(std::move(error));
+  }
+
+ private:
+  void OnSetDescriptionComplete(RTCError error) {
+    if (!handler_)
+      return;
+    if (error.ok()) {
+      handler_->pc_->PostSetSessionDescriptionSuccess(inner_observer_);
+    } else {
+      handler_->pc_->PostSetSessionDescriptionFailure(inner_observer_,
+                                                      std::move(error));
+    }
+  }
+
+  rtc::WeakPtr<SdpOfferAnswerHandler> handler_;
+  rtc::scoped_refptr<SetSessionDescriptionObserver> inner_observer_;
+};
+
+class SdpOfferAnswerHandler::LocalIceCredentialsToReplace {
+ public:
+  // Sets the ICE credentials that need restarting to the ICE credentials of
+  // the current and pending descriptions.
+  void SetIceCredentialsFromLocalDescriptions(
+      const SessionDescriptionInterface* current_local_description,
+      const SessionDescriptionInterface* pending_local_description) {
+    ice_credentials_.clear();
+    if (current_local_description) {
+      AppendIceCredentialsFromSessionDescription(*current_local_description);
+    }
+    if (pending_local_description) {
+      AppendIceCredentialsFromSessionDescription(*pending_local_description);
+    }
+  }
+
+  void ClearIceCredentials() { ice_credentials_.clear(); }
+
+  // Returns true if we have ICE credentials that need restarting.
+  bool HasIceCredentials() const { return !ice_credentials_.empty(); }
+
+  // Returns true if |local_description| shares no ICE credentials with the
+  // ICE credentials that need restarting.
+  bool SatisfiesIceRestart(
+      const SessionDescriptionInterface& local_description) const {
+    for (const auto& transport_info :
+         local_description.description()->transport_infos()) {
+      if (ice_credentials_.find(std::make_pair(
+              transport_info.description.ice_ufrag,
+              transport_info.description.ice_pwd)) != ice_credentials_.end()) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+ private:
+  void AppendIceCredentialsFromSessionDescription(
+      const SessionDescriptionInterface& desc) {
+    for (const auto& transport_info : desc.description()->transport_infos()) {
+      ice_credentials_.insert(
+          std::make_pair(transport_info.description.ice_ufrag,
+                         transport_info.description.ice_pwd));
+    }
+  }
+
+  std::set<std::pair<std::string, std::string>> ice_credentials_;
+};
+
+SdpOfferAnswerHandler::SdpOfferAnswerHandler(PeerConnection* pc)
+    : pc_(pc),
+      operations_chain_(rtc::OperationsChain::Create()),
+      local_ice_credentials_to_replace_(new LocalIceCredentialsToReplace()),
+      weak_ptr_factory_(this) {
+  operations_chain_->SetOnChainEmptyCallback(
+      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr()]() {
+        if (!this_weak_ptr)
+          return;
+        this_weak_ptr->OnOperationsChainEmpty();
+      });
+}
+
+SdpOfferAnswerHandler::~SdpOfferAnswerHandler() {}
+
+void SdpOfferAnswerHandler::PrepareForShutdown() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  weak_ptr_factory_.InvalidateWeakPtrs();
+}
+
+void SdpOfferAnswerHandler::Close() {
+  ChangeSignalingState(PeerConnectionInterface::kClosed);
+}
+
+void SdpOfferAnswerHandler::RestartIce() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  local_ice_credentials_to_replace_->SetIceCredentialsFromLocalDescriptions(
+      current_local_description(), pending_local_description());
+  UpdateNegotiationNeeded();
+}
+
+rtc::Thread* SdpOfferAnswerHandler::signaling_thread() const {
+  return pc_->signaling_thread();
+}
+
+void SdpOfferAnswerHandler::CreateOffer(
+    CreateSessionDescriptionObserver* observer,
+    const PeerConnectionInterface::RTCOfferAnswerOptions& options) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // Chain this operation. If asynchronous operations are pending on the chain,
+  // this operation will be queued to be invoked, otherwise the contents of the
+  // lambda will execute immediately.
+  operations_chain_->ChainOperation(
+      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
+       observer_refptr =
+           rtc::scoped_refptr<CreateSessionDescriptionObserver>(observer),
+       options](std::function<void()> operations_chain_callback) {
+        // Abort early if |this_weak_ptr| is no longer valid.
+        if (!this_weak_ptr) {
+          observer_refptr->OnFailure(
+              RTCError(RTCErrorType::INTERNAL_ERROR,
+                       "CreateOffer failed because the session was shut down"));
+          operations_chain_callback();
+          return;
+        }
+        // The operation completes asynchronously when the wrapper is invoked.
+        rtc::scoped_refptr<CreateSessionDescriptionObserverOperationWrapper>
+            observer_wrapper(new rtc::RefCountedObject<
+                             CreateSessionDescriptionObserverOperationWrapper>(
+                std::move(observer_refptr),
+                std::move(operations_chain_callback)));
+        this_weak_ptr->DoCreateOffer(options, observer_wrapper);
+      });
+}
+
+void SdpOfferAnswerHandler::SetLocalDescription(
+    SetSessionDescriptionObserver* observer,
+    SessionDescriptionInterface* desc_ptr) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // Chain this operation. If asynchronous operations are pending on the chain,
+  // this operation will be queued to be invoked, otherwise the contents of the
+  // lambda will execute immediately.
+  operations_chain_->ChainOperation(
+      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
+       observer_refptr =
+           rtc::scoped_refptr<SetSessionDescriptionObserver>(observer),
+       desc = std::unique_ptr<SessionDescriptionInterface>(desc_ptr)](
+          std::function<void()> operations_chain_callback) mutable {
+        // Abort early if |this_weak_ptr| is no longer valid.
+        if (!this_weak_ptr) {
+          // For consistency with SetSessionDescriptionObserverAdapter whose
+          // posted messages doesn't get processed when the PC is destroyed, we
+          // do not inform |observer_refptr| that the operation failed.
+          operations_chain_callback();
+          return;
+        }
+        // SetSessionDescriptionObserverAdapter takes care of making sure the
+        // |observer_refptr| is invoked in a posted message.
+        this_weak_ptr->DoSetLocalDescription(
+            std::move(desc),
+            rtc::scoped_refptr<SetLocalDescriptionObserverInterface>(
+                new rtc::RefCountedObject<SetSessionDescriptionObserverAdapter>(
+                    this_weak_ptr, observer_refptr)));
+        // For backwards-compatability reasons, we declare the operation as
+        // completed here (rather than in a post), so that the operation chain
+        // is not blocked by this operation when the observer is invoked. This
+        // allows the observer to trigger subsequent offer/answer operations
+        // synchronously if the operation chain is now empty.
+        operations_chain_callback();
+      });
+}
+
+void SdpOfferAnswerHandler::SetLocalDescription(
+    std::unique_ptr<SessionDescriptionInterface> desc,
+    rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // Chain this operation. If asynchronous operations are pending on the chain,
+  // this operation will be queued to be invoked, otherwise the contents of the
+  // lambda will execute immediately.
+  operations_chain_->ChainOperation(
+      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(), observer,
+       desc = std::move(desc)](
+          std::function<void()> operations_chain_callback) mutable {
+        // Abort early if |this_weak_ptr| is no longer valid.
+        if (!this_weak_ptr) {
+          observer->OnSetLocalDescriptionComplete(RTCError(
+              RTCErrorType::INTERNAL_ERROR,
+              "SetLocalDescription failed because the session was shut down"));
+          operations_chain_callback();
+          return;
+        }
+        this_weak_ptr->DoSetLocalDescription(std::move(desc), observer);
+        // DoSetLocalDescription() is implemented as a synchronous operation.
+        // The |observer| will already have been informed that it completed, and
+        // we can mark this operation as complete without any loose ends.
+        operations_chain_callback();
+      });
+}
+
+void SdpOfferAnswerHandler::SetLocalDescription(
+    SetSessionDescriptionObserver* observer) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  SetLocalDescription(
+      new rtc::RefCountedObject<SetSessionDescriptionObserverAdapter>(
+          weak_ptr_factory_.GetWeakPtr(), observer));
+}
+
+void SdpOfferAnswerHandler::SetLocalDescription(
+    rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // The |create_sdp_observer| handles performing DoSetLocalDescription() with
+  // the resulting description as well as completing the operation.
+  rtc::scoped_refptr<ImplicitCreateSessionDescriptionObserver>
+      create_sdp_observer(
+          new rtc::RefCountedObject<ImplicitCreateSessionDescriptionObserver>(
+              weak_ptr_factory_.GetWeakPtr(), observer));
+  // Chain this operation. If asynchronous operations are pending on the chain,
+  // this operation will be queued to be invoked, otherwise the contents of the
+  // lambda will execute immediately.
+  operations_chain_->ChainOperation(
+      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
+       create_sdp_observer](std::function<void()> operations_chain_callback) {
+        // The |create_sdp_observer| is responsible for completing the
+        // operation.
+        create_sdp_observer->SetOperationCompleteCallback(
+            std::move(operations_chain_callback));
+        // Abort early if |this_weak_ptr| is no longer valid. This triggers the
+        // same code path as if DoCreateOffer() or DoCreateAnswer() failed.
+        if (!this_weak_ptr) {
+          create_sdp_observer->OnFailure(RTCError(
+              RTCErrorType::INTERNAL_ERROR,
+              "SetLocalDescription failed because the session was shut down"));
+          return;
+        }
+        switch (this_weak_ptr->signaling_state()) {
+          case PeerConnectionInterface::kStable:
+          case PeerConnectionInterface::kHaveLocalOffer:
+          case PeerConnectionInterface::kHaveRemotePrAnswer:
+            // TODO(hbos): If [LastCreatedOffer] exists and still represents the
+            // current state of the system, use that instead of creating another
+            // offer.
+            this_weak_ptr->DoCreateOffer(
+                PeerConnectionInterface::RTCOfferAnswerOptions(),
+                create_sdp_observer);
+            break;
+          case PeerConnectionInterface::kHaveLocalPrAnswer:
+          case PeerConnectionInterface::kHaveRemoteOffer:
+            // TODO(hbos): If [LastCreatedAnswer] exists and still represents
+            // the current state of the system, use that instead of creating
+            // another answer.
+            this_weak_ptr->DoCreateAnswer(
+                PeerConnectionInterface::RTCOfferAnswerOptions(),
+                create_sdp_observer);
+            break;
+          case PeerConnectionInterface::kClosed:
+            create_sdp_observer->OnFailure(RTCError(
+                RTCErrorType::INVALID_STATE,
+                "SetLocalDescription called when PeerConnection is closed."));
+            break;
+        }
+      });
+}
+
+RTCError SdpOfferAnswerHandler::ApplyLocalDescription(
+    std::unique_ptr<SessionDescriptionInterface> desc) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  RTC_DCHECK(desc);
+
+  // Update stats here so that we have the most recent stats for tracks and
+  // streams that might be removed by updating the session description.
+  pc_->stats()->UpdateStats(PeerConnectionInterface::kStatsOutputLevelStandard);
+
+  // Take a reference to the old local description since it's used below to
+  // compare against the new local description. When setting the new local
+  // description, grab ownership of the replaced session description in case it
+  // is the same as |old_local_description|, to keep it alive for the duration
+  // of the method.
+  const SessionDescriptionInterface* old_local_description =
+      local_description();
+  std::unique_ptr<SessionDescriptionInterface> replaced_local_description;
+  SdpType type = desc->GetType();
+  if (type == SdpType::kAnswer) {
+    replaced_local_description = pending_local_description_
+                                     ? std::move(pending_local_description_)
+                                     : std::move(current_local_description_);
+    current_local_description_ = std::move(desc);
+    pending_local_description_ = nullptr;
+    current_remote_description_ = std::move(pending_remote_description_);
+  } else {
+    replaced_local_description = std::move(pending_local_description_);
+    pending_local_description_ = std::move(desc);
+  }
+  // The session description to apply now must be accessed by
+  // |local_description()|.
+  RTC_DCHECK(local_description());
+
+  // Report statistics about any use of simulcast.
+  ReportSimulcastApiVersion(kSimulcastVersionApplyLocalDescription,
+                            *local_description()->description());
+
+  if (!is_caller_) {
+    if (remote_description()) {
+      // Remote description was applied first, so this PC is the callee.
+      is_caller_ = false;
+    } else {
+      // Local description is applied first, so this PC is the caller.
+      is_caller_ = true;
+    }
+  }
+
+  RTCError error = pc_->PushdownTransportDescription(cricket::CS_LOCAL, type);
+  if (!error.ok()) {
+    return error;
+  }
+
+  if (IsUnifiedPlan()) {
+    RTCError error = UpdateTransceiversAndDataChannels(
+        cricket::CS_LOCAL, *local_description(), old_local_description,
+        remote_description());
+    if (!error.ok()) {
+      return error;
+    }
+    std::vector<rtc::scoped_refptr<RtpTransceiverInterface>> remove_list;
+    std::vector<rtc::scoped_refptr<MediaStreamInterface>> removed_streams;
+    for (const auto& transceiver : pc_->transceivers_) {
+      if (transceiver->stopped()) {
+        continue;
+      }
+
+      // 2.2.7.1.1.(6-9): Set sender and receiver's transport slots.
+      // Note that code paths that don't set MID won't be able to use
+      // information about DTLS transports.
+      if (transceiver->mid()) {
+        auto dtls_transport =
+            pc_->LookupDtlsTransportByMidInternal(*transceiver->mid());
+        transceiver->internal()->sender_internal()->set_transport(
+            dtls_transport);
+        transceiver->internal()->receiver_internal()->set_transport(
+            dtls_transport);
+      }
+
+      const ContentInfo* content =
+          pc_->FindMediaSectionForTransceiver(transceiver, local_description());
+      if (!content) {
+        continue;
+      }
+      const MediaContentDescription* media_desc = content->media_description();
+      // 2.2.7.1.6: If description is of type "answer" or "pranswer", then run
+      // the following steps:
+      if (type == SdpType::kPrAnswer || type == SdpType::kAnswer) {
+        // 2.2.7.1.6.1: If direction is "sendonly" or "inactive", and
+        // transceiver's [[FiredDirection]] slot is either "sendrecv" or
+        // "recvonly", process the removal of a remote track for the media
+        // description, given transceiver, removeList, and muteTracks.
+        if (!RtpTransceiverDirectionHasRecv(media_desc->direction()) &&
+            (transceiver->internal()->fired_direction() &&
+             RtpTransceiverDirectionHasRecv(
+                 *transceiver->internal()->fired_direction()))) {
+          pc_->ProcessRemovalOfRemoteTrack(transceiver, &remove_list,
+                                           &removed_streams);
+        }
+        // 2.2.7.1.6.2: Set transceiver's [[CurrentDirection]] and
+        // [[FiredDirection]] slots to direction.
+        transceiver->internal()->set_current_direction(media_desc->direction());
+        transceiver->internal()->set_fired_direction(media_desc->direction());
+      }
+    }
+    auto observer = pc_->Observer();
+    for (const auto& transceiver : remove_list) {
+      observer->OnRemoveTrack(transceiver->receiver());
+    }
+    for (const auto& stream : removed_streams) {
+      observer->OnRemoveStream(stream);
+    }
+  } else {
+    // Media channels will be created only when offer is set. These may use new
+    // transports just created by PushdownTransportDescription.
+    if (type == SdpType::kOffer) {
+      // TODO(bugs.webrtc.org/4676) - Handle CreateChannel failure, as new local
+      // description is applied. Restore back to old description.
+      RTCError error = pc_->CreateChannels(*local_description()->description());
+      if (!error.ok()) {
+        return error;
+      }
+    }
+    // Remove unused channels if MediaContentDescription is rejected.
+    pc_->RemoveUnusedChannels(local_description()->description());
+  }
+
+  error = UpdateSessionState(type, cricket::CS_LOCAL,
+                             local_description()->description());
+  if (!error.ok()) {
+    return error;
+  }
+
+  if (remote_description()) {
+    // Now that we have a local description, we can push down remote candidates.
+    pc_->UseCandidatesInSessionDescription(remote_description());
+  }
+
+  pending_ice_restarts_.clear();
+  if (pc_->session_error() != PeerConnection::SessionError::kNone) {
+    LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR,
+                         pc_->GetSessionErrorMsg());
+  }
+
+  // If setting the description decided our SSL role, allocate any necessary
+  // SCTP sids.
+  rtc::SSLRole role;
+  if (IsSctpLike(pc_->data_channel_type()) && pc_->GetSctpSslRole(&role)) {
+    pc_->data_channel_controller()->AllocateSctpSids(role);
+  }
+
+  if (IsUnifiedPlan()) {
+    for (const auto& transceiver : pc_->transceivers_) {
+      if (transceiver->stopped()) {
+        continue;
+      }
+      const ContentInfo* content =
+          pc_->FindMediaSectionForTransceiver(transceiver, local_description());
+      if (!content) {
+        continue;
+      }
+      cricket::ChannelInterface* channel = transceiver->internal()->channel();
+      if (content->rejected || !channel || channel->local_streams().empty()) {
+        // 0 is a special value meaning "this sender has no associated send
+        // stream". Need to call this so the sender won't attempt to configure
+        // a no longer existing stream and run into DCHECKs in the lower
+        // layers.
+        transceiver->internal()->sender_internal()->SetSsrc(0);
+      } else {
+        // Get the StreamParams from the channel which could generate SSRCs.
+        const std::vector<StreamParams>& streams = channel->local_streams();
+        transceiver->internal()->sender_internal()->set_stream_ids(
+            streams[0].stream_ids());
+        transceiver->internal()->sender_internal()->SetSsrc(
+            streams[0].first_ssrc());
+      }
+    }
+  } else {
+    // Plan B semantics.
+
+    // Update state and SSRC of local MediaStreams and DataChannels based on the
+    // local session description.
+    const cricket::ContentInfo* audio_content =
+        GetFirstAudioContent(local_description()->description());
+    if (audio_content) {
+      if (audio_content->rejected) {
+        pc_->RemoveSenders(cricket::MEDIA_TYPE_AUDIO);
+      } else {
+        const cricket::AudioContentDescription* audio_desc =
+            audio_content->media_description()->as_audio();
+        pc_->UpdateLocalSenders(audio_desc->streams(), audio_desc->type());
+      }
+    }
+
+    const cricket::ContentInfo* video_content =
+        GetFirstVideoContent(local_description()->description());
+    if (video_content) {
+      if (video_content->rejected) {
+        pc_->RemoveSenders(cricket::MEDIA_TYPE_VIDEO);
+      } else {
+        const cricket::VideoContentDescription* video_desc =
+            video_content->media_description()->as_video();
+        pc_->UpdateLocalSenders(video_desc->streams(), video_desc->type());
+      }
+    }
+  }
+
+  const cricket::ContentInfo* data_content =
+      GetFirstDataContent(local_description()->description());
+  if (data_content) {
+    const cricket::RtpDataContentDescription* rtp_data_desc =
+        data_content->media_description()->as_rtp_data();
+    // rtp_data_desc will be null if this is an SCTP description.
+    if (rtp_data_desc) {
+      pc_->data_channel_controller()->UpdateLocalRtpDataChannels(
+          rtp_data_desc->streams());
+    }
+  }
+
+  if (type == SdpType::kAnswer &&
+      local_ice_credentials_to_replace_->SatisfiesIceRestart(
+          *current_local_description_)) {
+    local_ice_credentials_to_replace_->ClearIceCredentials();
+  }
+
+  return RTCError::OK();
+}
+
+void SdpOfferAnswerHandler::SetRemoteDescription(
+    SetSessionDescriptionObserver* observer,
+    SessionDescriptionInterface* desc_ptr) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // Chain this operation. If asynchronous operations are pending on the chain,
+  // this operation will be queued to be invoked, otherwise the contents of the
+  // lambda will execute immediately.
+  operations_chain_->ChainOperation(
+      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
+       observer_refptr =
+           rtc::scoped_refptr<SetSessionDescriptionObserver>(observer),
+       desc = std::unique_ptr<SessionDescriptionInterface>(desc_ptr)](
+          std::function<void()> operations_chain_callback) mutable {
+        // Abort early if |this_weak_ptr| is no longer valid.
+        if (!this_weak_ptr) {
+          // For consistency with SetSessionDescriptionObserverAdapter whose
+          // posted messages doesn't get processed when the PC is destroyed, we
+          // do not inform |observer_refptr| that the operation failed.
+          operations_chain_callback();
+          return;
+        }
+        // SetSessionDescriptionObserverAdapter takes care of making sure the
+        // |observer_refptr| is invoked in a posted message.
+        this_weak_ptr->DoSetRemoteDescription(
+            std::move(desc),
+            rtc::scoped_refptr<SetRemoteDescriptionObserverInterface>(
+                new rtc::RefCountedObject<SetSessionDescriptionObserverAdapter>(
+                    this_weak_ptr, observer_refptr)));
+        // For backwards-compatability reasons, we declare the operation as
+        // completed here (rather than in a post), so that the operation chain
+        // is not blocked by this operation when the observer is invoked. This
+        // allows the observer to trigger subsequent offer/answer operations
+        // synchronously if the operation chain is now empty.
+        operations_chain_callback();
+      });
+}
+
+void SdpOfferAnswerHandler::SetRemoteDescription(
+    std::unique_ptr<SessionDescriptionInterface> desc,
+    rtc::scoped_refptr<SetRemoteDescriptionObserverInterface> observer) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // Chain this operation. If asynchronous operations are pending on the chain,
+  // this operation will be queued to be invoked, otherwise the contents of the
+  // lambda will execute immediately.
+  operations_chain_->ChainOperation(
+      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(), observer,
+       desc = std::move(desc)](
+          std::function<void()> operations_chain_callback) mutable {
+        // Abort early if |this_weak_ptr| is no longer valid.
+        if (!this_weak_ptr) {
+          observer->OnSetRemoteDescriptionComplete(RTCError(
+              RTCErrorType::INTERNAL_ERROR,
+              "SetRemoteDescription failed because the session was shut down"));
+          operations_chain_callback();
+          return;
+        }
+        this_weak_ptr->DoSetRemoteDescription(std::move(desc),
+                                              std::move(observer));
+        // DoSetRemoteDescription() is implemented as a synchronous operation.
+        // The |observer| will already have been informed that it completed, and
+        // we can mark this operation as complete without any loose ends.
+        operations_chain_callback();
+      });
+}
+
+RTCError SdpOfferAnswerHandler::ApplyRemoteDescription(
+    std::unique_ptr<SessionDescriptionInterface> desc) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  RTC_DCHECK(desc);
+
+  // Update stats here so that we have the most recent stats for tracks and
+  // streams that might be removed by updating the session description.
+  pc_->stats()->UpdateStats(PeerConnectionInterface::kStatsOutputLevelStandard);
+
+  // Take a reference to the old remote description since it's used below to
+  // compare against the new remote description. When setting the new remote
+  // description, grab ownership of the replaced session description in case it
+  // is the same as |old_remote_description|, to keep it alive for the duration
+  // of the method.
+  const SessionDescriptionInterface* old_remote_description =
+      remote_description();
+  std::unique_ptr<SessionDescriptionInterface> replaced_remote_description;
+  SdpType type = desc->GetType();
+  if (type == SdpType::kAnswer) {
+    replaced_remote_description = pending_remote_description_
+                                      ? std::move(pending_remote_description_)
+                                      : std::move(current_remote_description_);
+    current_remote_description_ = std::move(desc);
+    pending_remote_description_ = nullptr;
+    current_local_description_ = std::move(pending_local_description_);
+  } else {
+    replaced_remote_description = std::move(pending_remote_description_);
+    pending_remote_description_ = std::move(desc);
+  }
+  // The session description to apply now must be accessed by
+  // |remote_description()|.
+  RTC_DCHECK(remote_description());
+
+  // Report statistics about any use of simulcast.
+  ReportSimulcastApiVersion(kSimulcastVersionApplyRemoteDescription,
+                            *remote_description()->description());
+
+  RTCError error = pc_->PushdownTransportDescription(cricket::CS_REMOTE, type);
+  if (!error.ok()) {
+    return error;
+  }
+  // Transport and Media channels will be created only when offer is set.
+  if (IsUnifiedPlan()) {
+    RTCError error = UpdateTransceiversAndDataChannels(
+        cricket::CS_REMOTE, *remote_description(), local_description(),
+        old_remote_description);
+    if (!error.ok()) {
+      return error;
+    }
+  } else {
+    // Media channels will be created only when offer is set. These may use new
+    // transports just created by PushdownTransportDescription.
+    if (type == SdpType::kOffer) {
+      // TODO(mallinath) - Handle CreateChannel failure, as new local
+      // description is applied. Restore back to old description.
+      RTCError error =
+          pc_->CreateChannels(*remote_description()->description());
+      if (!error.ok()) {
+        return error;
+      }
+    }
+    // Remove unused channels if MediaContentDescription is rejected.
+    pc_->RemoveUnusedChannels(remote_description()->description());
+  }
+
+  // NOTE: Candidates allocation will be initiated only when
+  // SetLocalDescription is called.
+  error = UpdateSessionState(type, cricket::CS_REMOTE,
+                             remote_description()->description());
+  if (!error.ok()) {
+    return error;
+  }
+
+  if (local_description() &&
+      !pc_->UseCandidatesInSessionDescription(remote_description())) {
+    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, kInvalidCandidates);
+  }
+
+  if (old_remote_description) {
+    for (const cricket::ContentInfo& content :
+         old_remote_description->description()->contents()) {
+      // Check if this new SessionDescription contains new ICE ufrag and
+      // password that indicates the remote peer requests an ICE restart.
+      // TODO(deadbeef): When we start storing both the current and pending
+      // remote description, this should reset pending_ice_restarts and compare
+      // against the current description.
+      if (CheckForRemoteIceRestart(old_remote_description, remote_description(),
+                                   content.name)) {
+        if (type == SdpType::kOffer) {
+          pending_ice_restarts_.insert(content.name);
+        }
+      } else {
+        // We retain all received candidates only if ICE is not restarted.
+        // When ICE is restarted, all previous candidates belong to an old
+        // generation and should not be kept.
+        // TODO(deadbeef): This goes against the W3C spec which says the remote
+        // description should only contain candidates from the last set remote
+        // description plus any candidates added since then. We should remove
+        // this once we're sure it won't break anything.
+        WebRtcSessionDescriptionFactory::CopyCandidatesFromSessionDescription(
+            old_remote_description, content.name, mutable_remote_description());
+      }
+    }
+  }
+
+  if (pc_->session_error() != PeerConnection::SessionError::kNone) {
+    LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR,
+                         pc_->GetSessionErrorMsg());
+  }
+
+  // Set the the ICE connection state to connecting since the connection may
+  // become writable with peer reflexive candidates before any remote candidate
+  // is signaled.
+  // TODO(pthatcher): This is a short-term solution for crbug/446908. A real fix
+  // is to have a new signal the indicates a change in checking state from the
+  // transport and expose a new checking() member from transport that can be
+  // read to determine the current checking state. The existing SignalConnecting
+  // actually means "gathering candidates", so cannot be be used here.
+  if (remote_description()->GetType() != SdpType::kOffer &&
+      remote_description()->number_of_mediasections() > 0u &&
+      pc_->ice_connection_state() ==
+          PeerConnectionInterface::kIceConnectionNew) {
+    pc_->SetIceConnectionState(PeerConnectionInterface::kIceConnectionChecking);
+  }
+
+  // If setting the description decided our SSL role, allocate any necessary
+  // SCTP sids.
+  rtc::SSLRole role;
+  if (IsSctpLike(pc_->data_channel_type()) && pc_->GetSctpSslRole(&role)) {
+    pc_->data_channel_controller()->AllocateSctpSids(role);
+  }
+
+  if (IsUnifiedPlan()) {
+    std::vector<rtc::scoped_refptr<RtpTransceiverInterface>>
+        now_receiving_transceivers;
+    std::vector<rtc::scoped_refptr<RtpTransceiverInterface>> remove_list;
+    std::vector<rtc::scoped_refptr<MediaStreamInterface>> added_streams;
+    std::vector<rtc::scoped_refptr<MediaStreamInterface>> removed_streams;
+    for (const auto& transceiver : pc_->transceivers_) {
+      const ContentInfo* content = pc_->FindMediaSectionForTransceiver(
+          transceiver, remote_description());
+      if (!content) {
+        continue;
+      }
+      const MediaContentDescription* media_desc = content->media_description();
+      RtpTransceiverDirection local_direction =
+          RtpTransceiverDirectionReversed(media_desc->direction());
+      // Roughly the same as steps 2.2.8.6 of section 4.4.1.6 "Set the
+      // RTCSessionDescription: Set the associated remote streams given
+      // transceiver.[[Receiver]], msids, addList, and removeList".
+      // https://w3c.github.io/webrtc-pc/#set-the-rtcsessiondescription
+      if (RtpTransceiverDirectionHasRecv(local_direction)) {
+        std::vector<std::string> stream_ids;
+        if (!media_desc->streams().empty()) {
+          // The remote description has signaled the stream IDs.
+          stream_ids = media_desc->streams()[0].stream_ids();
+        }
+        pc_->transceiver_stable_states_by_transceivers_[transceiver]
+            .SetRemoteStreamIdsIfUnset(transceiver->receiver()->stream_ids());
+
+        RTC_LOG(LS_INFO) << "Processing the MSIDs for MID=" << content->name
+                         << " (" << GetStreamIdsString(stream_ids) << ").";
+        SetAssociatedRemoteStreams(transceiver->internal()->receiver_internal(),
+                                   stream_ids, &added_streams,
+                                   &removed_streams);
+        // From the WebRTC specification, steps 2.2.8.5/6 of section 4.4.1.6
+        // "Set the RTCSessionDescription: If direction is sendrecv or recvonly,
+        // and transceiver's current direction is neither sendrecv nor recvonly,
+        // process the addition of a remote track for the media description.
+        if (!transceiver->fired_direction() ||
+            !RtpTransceiverDirectionHasRecv(*transceiver->fired_direction())) {
+          RTC_LOG(LS_INFO)
+              << "Processing the addition of a remote track for MID="
+              << content->name << ".";
+          now_receiving_transceivers.push_back(transceiver);
+        }
+      }
+      // 2.2.8.1.9: If direction is "sendonly" or "inactive", and transceiver's
+      // [[FiredDirection]] slot is either "sendrecv" or "recvonly", process the
+      // removal of a remote track for the media description, given transceiver,
+      // removeList, and muteTracks.
+      if (!RtpTransceiverDirectionHasRecv(local_direction) &&
+          (transceiver->fired_direction() &&
+           RtpTransceiverDirectionHasRecv(*transceiver->fired_direction()))) {
+        pc_->ProcessRemovalOfRemoteTrack(transceiver, &remove_list,
+                                         &removed_streams);
+      }
+      // 2.2.8.1.10: Set transceiver's [[FiredDirection]] slot to direction.
+      transceiver->internal()->set_fired_direction(local_direction);
+      // 2.2.8.1.11: If description is of type "answer" or "pranswer", then run
+      // the following steps:
+      if (type == SdpType::kPrAnswer || type == SdpType::kAnswer) {
+        // 2.2.8.1.11.1: Set transceiver's [[CurrentDirection]] slot to
+        // direction.
+        transceiver->internal()->set_current_direction(local_direction);
+        // 2.2.8.1.11.[3-6]: Set the transport internal slots.
+        if (transceiver->mid()) {
+          auto dtls_transport =
+              pc_->LookupDtlsTransportByMidInternal(*transceiver->mid());
+          transceiver->internal()->sender_internal()->set_transport(
+              dtls_transport);
+          transceiver->internal()->receiver_internal()->set_transport(
+              dtls_transport);
+        }
+      }
+      // 2.2.8.1.12: If the media description is rejected, and transceiver is
+      // not already stopped, stop the RTCRtpTransceiver transceiver.
+      if (content->rejected && !transceiver->stopped()) {
+        RTC_LOG(LS_INFO) << "Stopping transceiver for MID=" << content->name
+                         << " since the media section was rejected.";
+        transceiver->internal()->StopTransceiverProcedure();
+      }
+      if (!content->rejected &&
+          RtpTransceiverDirectionHasRecv(local_direction)) {
+        if (!media_desc->streams().empty() &&
+            media_desc->streams()[0].has_ssrcs()) {
+          uint32_t ssrc = media_desc->streams()[0].first_ssrc();
+          transceiver->internal()->receiver_internal()->SetupMediaChannel(ssrc);
+        } else {
+          transceiver->internal()
+              ->receiver_internal()
+              ->SetupUnsignaledMediaChannel();
+        }
+      }
+    }
+    // Once all processing has finished, fire off callbacks.
+    auto observer = pc_->Observer();
+    for (const auto& transceiver : now_receiving_transceivers) {
+      pc_->stats()->AddTrack(transceiver->receiver()->track());
+      observer->OnTrack(transceiver);
+      observer->OnAddTrack(transceiver->receiver(),
+                           transceiver->receiver()->streams());
+    }
+    for (const auto& stream : added_streams) {
+      observer->OnAddStream(stream);
+    }
+    for (const auto& transceiver : remove_list) {
+      observer->OnRemoveTrack(transceiver->receiver());
+    }
+    for (const auto& stream : removed_streams) {
+      observer->OnRemoveStream(stream);
+    }
+  }
+
+  const cricket::ContentInfo* audio_content =
+      GetFirstAudioContent(remote_description()->description());
+  const cricket::ContentInfo* video_content =
+      GetFirstVideoContent(remote_description()->description());
+  const cricket::AudioContentDescription* audio_desc =
+      GetFirstAudioContentDescription(remote_description()->description());
+  const cricket::VideoContentDescription* video_desc =
+      GetFirstVideoContentDescription(remote_description()->description());
+  const cricket::RtpDataContentDescription* rtp_data_desc =
+      GetFirstRtpDataContentDescription(remote_description()->description());
+
+  // Check if the descriptions include streams, just in case the peer supports
+  // MSID, but doesn't indicate so with "a=msid-semantic".
+  if (remote_description()->description()->msid_supported() ||
+      (audio_desc && !audio_desc->streams().empty()) ||
+      (video_desc && !video_desc->streams().empty())) {
+    remote_peer_supports_msid_ = true;
+  }
+
+  // We wait to signal new streams until we finish processing the description,
+  // since only at that point will new streams have all their tracks.
+  rtc::scoped_refptr<StreamCollection> new_streams(StreamCollection::Create());
+
+  if (!IsUnifiedPlan()) {
+    // TODO(steveanton): When removing RTP senders/receivers in response to a
+    // rejected media section, there is some cleanup logic that expects the
+    // voice/ video channel to still be set. But in this method the voice/video
+    // channel would have been destroyed by the SetRemoteDescription caller
+    // above so the cleanup that relies on them fails to run. The RemoveSenders
+    // calls should be moved to right before the DestroyChannel calls to fix
+    // this.
+
+    // Find all audio rtp streams and create corresponding remote AudioTracks
+    // and MediaStreams.
+    if (audio_content) {
+      if (audio_content->rejected) {
+        pc_->RemoveSenders(cricket::MEDIA_TYPE_AUDIO);
+      } else {
+        bool default_audio_track_needed =
+            !remote_peer_supports_msid_ &&
+            RtpTransceiverDirectionHasSend(audio_desc->direction());
+        pc_->UpdateRemoteSendersList(GetActiveStreams(audio_desc),
+                                     default_audio_track_needed,
+                                     audio_desc->type(), new_streams);
+      }
+    }
+
+    // Find all video rtp streams and create corresponding remote VideoTracks
+    // and MediaStreams.
+    if (video_content) {
+      if (video_content->rejected) {
+        pc_->RemoveSenders(cricket::MEDIA_TYPE_VIDEO);
+      } else {
+        bool default_video_track_needed =
+            !remote_peer_supports_msid_ &&
+            RtpTransceiverDirectionHasSend(video_desc->direction());
+        pc_->UpdateRemoteSendersList(GetActiveStreams(video_desc),
+                                     default_video_track_needed,
+                                     video_desc->type(), new_streams);
+      }
+    }
+
+    // If this is an RTP data transport, update the DataChannels with the
+    // information from the remote peer.
+    if (rtp_data_desc) {
+      pc_->data_channel_controller()->UpdateRemoteRtpDataChannels(
+          GetActiveStreams(rtp_data_desc));
+    }
+
+    // Iterate new_streams and notify the observer about new MediaStreams.
+    auto observer = pc_->Observer();
+    for (size_t i = 0; i < new_streams->count(); ++i) {
+      MediaStreamInterface* new_stream = new_streams->at(i);
+      pc_->stats()->AddStream(new_stream);
+      observer->OnAddStream(
+          rtc::scoped_refptr<MediaStreamInterface>(new_stream));
+    }
+
+    pc_->UpdateEndedRemoteMediaStreams();
+  }
+
+  if (type == SdpType::kAnswer &&
+      local_ice_credentials_to_replace_->SatisfiesIceRestart(
+          *current_local_description_)) {
+    local_ice_credentials_to_replace_->ClearIceCredentials();
+  }
+
+  return RTCError::OK();
+}
+
+void SdpOfferAnswerHandler::DoSetLocalDescription(
+    std::unique_ptr<SessionDescriptionInterface> desc,
+    rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  TRACE_EVENT0("webrtc", "SdpOfferAnswerHandler::DoSetLocalDescription");
+
+  if (!observer) {
+    RTC_LOG(LS_ERROR) << "SetLocalDescription - observer is NULL.";
+    return;
+  }
+
+  if (!desc) {
+    observer->OnSetLocalDescriptionComplete(
+        RTCError(RTCErrorType::INTERNAL_ERROR, "SessionDescription is NULL."));
+    return;
+  }
+
+  // If a session error has occurred the PeerConnection is in a possibly
+  // inconsistent state so fail right away.
+  if (pc_->session_error() != PeerConnection::SessionError::kNone) {
+    std::string error_message = pc_->GetSessionErrorMsg();
+    RTC_LOG(LS_ERROR) << "SetLocalDescription: " << error_message;
+    observer->OnSetLocalDescriptionComplete(
+        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
+    return;
+  }
+
+  // For SLD we support only explicit rollback.
+  if (desc->GetType() == SdpType::kRollback) {
+    if (IsUnifiedPlan()) {
+      observer->OnSetLocalDescriptionComplete(Rollback(desc->GetType()));
+    } else {
+      observer->OnSetLocalDescriptionComplete(
+          RTCError(RTCErrorType::UNSUPPORTED_OPERATION,
+                   "Rollback not supported in Plan B"));
+    }
+    return;
+  }
+
+  RTCError error = ValidateSessionDescription(desc.get(), cricket::CS_LOCAL);
+  if (!error.ok()) {
+    std::string error_message = GetSetDescriptionErrorMessage(
+        cricket::CS_LOCAL, desc->GetType(), error);
+    RTC_LOG(LS_ERROR) << error_message;
+    observer->OnSetLocalDescriptionComplete(
+        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
+    return;
+  }
+
+  // Grab the description type before moving ownership to ApplyLocalDescription,
+  // which may destroy it before returning.
+  const SdpType type = desc->GetType();
+
+  error = ApplyLocalDescription(std::move(desc));
+  // |desc| may be destroyed at this point.
+
+  if (!error.ok()) {
+    // If ApplyLocalDescription fails, the PeerConnection could be in an
+    // inconsistent state, so act conservatively here and set the session error
+    // so that future calls to SetLocalDescription/SetRemoteDescription fail.
+    pc_->SetSessionError(PeerConnection::SessionError::kContent,
+                         error.message());
+    std::string error_message =
+        GetSetDescriptionErrorMessage(cricket::CS_LOCAL, type, error);
+    RTC_LOG(LS_ERROR) << error_message;
+    observer->OnSetLocalDescriptionComplete(
+        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
+    return;
+  }
+  RTC_DCHECK(local_description());
+
+  if (local_description()->GetType() == SdpType::kAnswer) {
+    pc_->RemoveStoppedTransceivers();
+
+    // TODO(deadbeef): We already had to hop to the network thread for
+    // MaybeStartGathering...
+    pc_->network_thread()->Invoke<void>(
+        RTC_FROM_HERE, rtc::Bind(&cricket::PortAllocator::DiscardCandidatePool,
+                                 pc_->port_allocator_.get()));
+    // Make UMA notes about what was agreed to.
+    pc_->ReportNegotiatedSdpSemantics(*local_description());
+  }
+
+  observer->OnSetLocalDescriptionComplete(RTCError::OK());
+  pc_->NoteUsageEvent(
+      PeerConnection::UsageEvent::SET_LOCAL_DESCRIPTION_SUCCEEDED);
+
+  // Check if negotiation is needed. We must do this after informing the
+  // observer that SetLocalDescription() has completed to ensure negotiation is
+  // not needed prior to the promise resolving.
+  if (IsUnifiedPlan()) {
+    bool was_negotiation_needed = is_negotiation_needed_;
+    UpdateNegotiationNeeded();
+    if (signaling_state() == PeerConnectionInterface::kStable &&
+        was_negotiation_needed && is_negotiation_needed_) {
+      // Legacy version.
+      pc_->Observer()->OnRenegotiationNeeded();
+      // Spec-compliant version; the event may get invalidated before firing.
+      GenerateNegotiationNeededEvent();
+    }
+  }
+
+  // MaybeStartGathering needs to be called after informing the observer so that
+  // we don't signal any candidates before signaling that SetLocalDescription
+  // completed.
+  pc_->transport_controller_->MaybeStartGathering();
+}
+
+void SdpOfferAnswerHandler::DoCreateOffer(
+    const PeerConnectionInterface::RTCOfferAnswerOptions& options,
+    rtc::scoped_refptr<CreateSessionDescriptionObserver> observer) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  TRACE_EVENT0("webrtc", "SdpOfferAnswerHandler::DoCreateOffer");
+
+  if (!observer) {
+    RTC_LOG(LS_ERROR) << "CreateOffer - observer is NULL.";
+    return;
+  }
+
+  if (pc_->IsClosed()) {
+    std::string error = "CreateOffer called when PeerConnection is closed.";
+    RTC_LOG(LS_ERROR) << error;
+    pc_->PostCreateSessionDescriptionFailure(
+        observer, RTCError(RTCErrorType::INVALID_STATE, std::move(error)));
+    return;
+  }
+
+  // If a session error has occurred the PeerConnection is in a possibly
+  // inconsistent state so fail right away.
+  if (pc_->session_error() != PeerConnection::SessionError::kNone) {
+    std::string error_message = pc_->GetSessionErrorMsg();
+    RTC_LOG(LS_ERROR) << "CreateOffer: " << error_message;
+    pc_->PostCreateSessionDescriptionFailure(
+        observer,
+        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
+    return;
+  }
+
+  if (!ValidateOfferAnswerOptions(options)) {
+    std::string error = "CreateOffer called with invalid options.";
+    RTC_LOG(LS_ERROR) << error;
+    pc_->PostCreateSessionDescriptionFailure(
+        observer, RTCError(RTCErrorType::INVALID_PARAMETER, std::move(error)));
+    return;
+  }
+
+  // Legacy handling for offer_to_receive_audio and offer_to_receive_video.
+  // Specified in WebRTC section 4.4.3.2 "Legacy configuration extensions".
+  if (IsUnifiedPlan()) {
+    RTCError error = pc_->HandleLegacyOfferOptions(options);
+    if (!error.ok()) {
+      pc_->PostCreateSessionDescriptionFailure(observer, std::move(error));
+      return;
+    }
+  }
+
+  cricket::MediaSessionOptions session_options;
+  pc_->GetOptionsForOffer(options, &session_options);
+  webrtc_session_desc_factory_->CreateOffer(observer, options, session_options);
+}
+
+void SdpOfferAnswerHandler::CreateAnswer(
+    CreateSessionDescriptionObserver* observer,
+    const PeerConnectionInterface::RTCOfferAnswerOptions& options) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // Chain this operation. If asynchronous operations are pending on the chain,
+  // this operation will be queued to be invoked, otherwise the contents of the
+  // lambda will execute immediately.
+  operations_chain_->ChainOperation(
+      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
+       observer_refptr =
+           rtc::scoped_refptr<CreateSessionDescriptionObserver>(observer),
+       options](std::function<void()> operations_chain_callback) {
+        // Abort early if |this_weak_ptr| is no longer valid.
+        if (!this_weak_ptr) {
+          observer_refptr->OnFailure(RTCError(
+              RTCErrorType::INTERNAL_ERROR,
+              "CreateAnswer failed because the session was shut down"));
+          operations_chain_callback();
+          return;
+        }
+        // The operation completes asynchronously when the wrapper is invoked.
+        rtc::scoped_refptr<CreateSessionDescriptionObserverOperationWrapper>
+            observer_wrapper(new rtc::RefCountedObject<
+                             CreateSessionDescriptionObserverOperationWrapper>(
+                std::move(observer_refptr),
+                std::move(operations_chain_callback)));
+        this_weak_ptr->DoCreateAnswer(options, observer_wrapper);
+      });
+}
+
+void SdpOfferAnswerHandler::DoCreateAnswer(
+    const PeerConnectionInterface::RTCOfferAnswerOptions& options,
+    rtc::scoped_refptr<CreateSessionDescriptionObserver> observer) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  TRACE_EVENT0("webrtc", "SdpOfferAnswerHandler::DoCreateAnswer");
+  if (!observer) {
+    RTC_LOG(LS_ERROR) << "CreateAnswer - observer is NULL.";
+    return;
+  }
+
+  // If a session error has occurred the PeerConnection is in a possibly
+  // inconsistent state so fail right away.
+  if (pc_->session_error() != PeerConnection::SessionError::kNone) {
+    std::string error_message = pc_->GetSessionErrorMsg();
+    RTC_LOG(LS_ERROR) << "CreateAnswer: " << error_message;
+    pc_->PostCreateSessionDescriptionFailure(
+        observer,
+        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
+    return;
+  }
+
+  if (!(signaling_state_ == PeerConnectionInterface::kHaveRemoteOffer ||
+        signaling_state_ == PeerConnectionInterface::kHaveLocalPrAnswer)) {
+    std::string error =
+        "PeerConnection cannot create an answer in a state other than "
+        "have-remote-offer or have-local-pranswer.";
+    RTC_LOG(LS_ERROR) << error;
+    pc_->PostCreateSessionDescriptionFailure(
+        observer, RTCError(RTCErrorType::INVALID_STATE, std::move(error)));
+    return;
+  }
+
+  // The remote description should be set if we're in the right state.
+  RTC_DCHECK(remote_description());
+
+  if (IsUnifiedPlan()) {
+    if (options.offer_to_receive_audio !=
+        PeerConnectionInterface::RTCOfferAnswerOptions::kUndefined) {
+      RTC_LOG(LS_WARNING) << "CreateAnswer: offer_to_receive_audio is not "
+                             "supported with Unified Plan semantics. Use the "
+                             "RtpTransceiver API instead.";
+    }
+    if (options.offer_to_receive_video !=
+        PeerConnectionInterface::RTCOfferAnswerOptions::kUndefined) {
+      RTC_LOG(LS_WARNING) << "CreateAnswer: offer_to_receive_video is not "
+                             "supported with Unified Plan semantics. Use the "
+                             "RtpTransceiver API instead.";
+    }
+  }
+
+  cricket::MediaSessionOptions session_options;
+  pc_->GetOptionsForAnswer(options, &session_options);
+  webrtc_session_desc_factory_->CreateAnswer(observer, session_options);
+}
+
+void SdpOfferAnswerHandler::DoSetRemoteDescription(
+    std::unique_ptr<SessionDescriptionInterface> desc,
+    rtc::scoped_refptr<SetRemoteDescriptionObserverInterface> observer) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  TRACE_EVENT0("webrtc", "SdpOfferAnswerHandler::DoSetRemoteDescription");
+
+  if (!observer) {
+    RTC_LOG(LS_ERROR) << "SetRemoteDescription - observer is NULL.";
+    return;
+  }
+
+  if (!desc) {
+    observer->OnSetRemoteDescriptionComplete(RTCError(
+        RTCErrorType::INVALID_PARAMETER, "SessionDescription is NULL."));
+    return;
+  }
+
+  // If a session error has occurred the PeerConnection is in a possibly
+  // inconsistent state so fail right away.
+  if (pc_->session_error() != PeerConnection::SessionError::kNone) {
+    std::string error_message = pc_->GetSessionErrorMsg();
+    RTC_LOG(LS_ERROR) << "SetRemoteDescription: " << error_message;
+    observer->OnSetRemoteDescriptionComplete(
+        RTCError(RTCErrorType::INTERNAL_ERROR, std::move(error_message)));
+    return;
+  }
+  if (IsUnifiedPlan()) {
+    if (pc_->configuration()->enable_implicit_rollback) {
+      if (desc->GetType() == SdpType::kOffer &&
+          signaling_state() == PeerConnectionInterface::kHaveLocalOffer) {
+        Rollback(desc->GetType());
+      }
+    }
+    // Explicit rollback.
+    if (desc->GetType() == SdpType::kRollback) {
+      observer->OnSetRemoteDescriptionComplete(Rollback(desc->GetType()));
+      return;
+    }
+  } else if (desc->GetType() == SdpType::kRollback) {
+    observer->OnSetRemoteDescriptionComplete(
+        RTCError(RTCErrorType::UNSUPPORTED_OPERATION,
+                 "Rollback not supported in Plan B"));
+    return;
+  }
+  if (desc->GetType() == SdpType::kOffer) {
+    // Report to UMA the format of the received offer.
+    pc_->ReportSdpFormatReceived(*desc);
+  }
+
+  // Handle remote descriptions missing a=mid lines for interop with legacy end
+  // points.
+  pc_->FillInMissingRemoteMids(desc->description());
+
+  RTCError error = ValidateSessionDescription(desc.get(), cricket::CS_REMOTE);
+  if (!error.ok()) {
+    std::string error_message = GetSetDescriptionErrorMessage(
+        cricket::CS_REMOTE, desc->GetType(), error);
+    RTC_LOG(LS_ERROR) << error_message;
+    observer->OnSetRemoteDescriptionComplete(
+        RTCError(error.type(), std::move(error_message)));
+    return;
+  }
+
+  // Grab the description type before moving ownership to
+  // ApplyRemoteDescription, which may destroy it before returning.
+  const SdpType type = desc->GetType();
+
+  error = ApplyRemoteDescription(std::move(desc));
+  // |desc| may be destroyed at this point.
+
+  if (!error.ok()) {
+    // If ApplyRemoteDescription fails, the PeerConnection could be in an
+    // inconsistent state, so act conservatively here and set the session error
+    // so that future calls to SetLocalDescription/SetRemoteDescription fail.
+    pc_->SetSessionError(PeerConnection::SessionError::kContent,
+                         error.message());
+    std::string error_message =
+        GetSetDescriptionErrorMessage(cricket::CS_REMOTE, type, error);
+    RTC_LOG(LS_ERROR) << error_message;
+    observer->OnSetRemoteDescriptionComplete(
+        RTCError(error.type(), std::move(error_message)));
+    return;
+  }
+  RTC_DCHECK(remote_description());
+
+  if (type == SdpType::kAnswer) {
+    pc_->RemoveStoppedTransceivers();
+    // TODO(deadbeef): We already had to hop to the network thread for
+    // MaybeStartGathering...
+    pc_->network_thread()->Invoke<void>(
+        RTC_FROM_HERE, rtc::Bind(&cricket::PortAllocator::DiscardCandidatePool,
+                                 pc_->port_allocator_.get()));
+    // Make UMA notes about what was agreed to.
+    pc_->ReportNegotiatedSdpSemantics(*remote_description());
+  }
+
+  observer->OnSetRemoteDescriptionComplete(RTCError::OK());
+  pc_->NoteUsageEvent(
+      PeerConnection::UsageEvent::SET_REMOTE_DESCRIPTION_SUCCEEDED);
+
+  // Check if negotiation is needed. We must do this after informing the
+  // observer that SetRemoteDescription() has completed to ensure negotiation is
+  // not needed prior to the promise resolving.
+  if (IsUnifiedPlan()) {
+    bool was_negotiation_needed = is_negotiation_needed_;
+    UpdateNegotiationNeeded();
+    if (signaling_state() == PeerConnectionInterface::kStable &&
+        was_negotiation_needed && is_negotiation_needed_) {
+      // Legacy version.
+      pc_->Observer()->OnRenegotiationNeeded();
+      // Spec-compliant version; the event may get invalidated before firing.
+      GenerateNegotiationNeededEvent();
+    }
+  }
+}
+
+void SdpOfferAnswerHandler::SetAssociatedRemoteStreams(
+    rtc::scoped_refptr<RtpReceiverInternal> receiver,
+    const std::vector<std::string>& stream_ids,
+    std::vector<rtc::scoped_refptr<MediaStreamInterface>>* added_streams,
+    std::vector<rtc::scoped_refptr<MediaStreamInterface>>* removed_streams) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  std::vector<rtc::scoped_refptr<MediaStreamInterface>> media_streams;
+  for (const std::string& stream_id : stream_ids) {
+    rtc::scoped_refptr<MediaStreamInterface> stream =
+        pc_->remote_streams_internal()->find(stream_id);
+    if (!stream) {
+      stream = MediaStreamProxy::Create(rtc::Thread::Current(),
+                                        MediaStream::Create(stream_id));
+      pc_->remote_streams_internal()->AddStream(stream);
+      added_streams->push_back(stream);
+    }
+    media_streams.push_back(stream);
+  }
+  // Special case: "a=msid" missing, use random stream ID.
+  if (media_streams.empty() &&
+      !(remote_description()->description()->msid_signaling() &
+        cricket::kMsidSignalingMediaSection)) {
+    if (!missing_msid_default_stream_) {
+      missing_msid_default_stream_ = MediaStreamProxy::Create(
+          rtc::Thread::Current(), MediaStream::Create(rtc::CreateRandomUuid()));
+      added_streams->push_back(missing_msid_default_stream_);
+    }
+    media_streams.push_back(missing_msid_default_stream_);
+  }
+  std::vector<rtc::scoped_refptr<MediaStreamInterface>> previous_streams =
+      receiver->streams();
+  // SetStreams() will add/remove the receiver's track to/from the streams. This
+  // differs from the spec - the spec uses an "addList" and "removeList" to
+  // update the stream-track relationships in a later step. We do this earlier,
+  // changing the order of things, but the end-result is the same.
+  // TODO(hbos): When we remove remote_streams(), use set_stream_ids()
+  // instead. https://crbug.com/webrtc/9480
+  receiver->SetStreams(media_streams);
+  pc_->RemoveRemoteStreamsIfEmpty(previous_streams, removed_streams);
+}
+
+bool SdpOfferAnswerHandler::AddIceCandidate(
+    const IceCandidateInterface* ice_candidate) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  TRACE_EVENT0("webrtc", "SdpOfferAnswerHandler::AddIceCandidate");
+  if (pc_->IsClosed()) {
+    RTC_LOG(LS_ERROR) << "AddIceCandidate: PeerConnection is closed.";
+    NoteAddIceCandidateResult(kAddIceCandidateFailClosed);
+    return false;
+  }
+
+  if (!remote_description()) {
+    RTC_LOG(LS_ERROR) << "AddIceCandidate: ICE candidates can't be added "
+                         "without any remote session description.";
+    NoteAddIceCandidateResult(kAddIceCandidateFailNoRemoteDescription);
+    return false;
+  }
+
+  if (!ice_candidate) {
+    RTC_LOG(LS_ERROR) << "AddIceCandidate: Candidate is null.";
+    NoteAddIceCandidateResult(kAddIceCandidateFailNullCandidate);
+    return false;
+  }
+
+  bool valid = false;
+  bool ready = pc_->ReadyToUseRemoteCandidate(ice_candidate, nullptr, &valid);
+  if (!valid) {
+    NoteAddIceCandidateResult(kAddIceCandidateFailNotValid);
+    return false;
+  }
+
+  // Add this candidate to the remote session description.
+  if (!mutable_remote_description()->AddCandidate(ice_candidate)) {
+    RTC_LOG(LS_ERROR) << "AddIceCandidate: Candidate cannot be used.";
+    NoteAddIceCandidateResult(kAddIceCandidateFailInAddition);
+    return false;
+  }
+
+  if (ready) {
+    bool result = pc_->UseCandidate(ice_candidate);
+    if (result) {
+      pc_->NoteUsageEvent(
+          PeerConnection::UsageEvent::ADD_ICE_CANDIDATE_SUCCEEDED);
+      NoteAddIceCandidateResult(kAddIceCandidateSuccess);
+    } else {
+      NoteAddIceCandidateResult(kAddIceCandidateFailNotUsable);
+    }
+    return result;
+  } else {
+    RTC_LOG(LS_INFO) << "AddIceCandidate: Not ready to use candidate.";
+    NoteAddIceCandidateResult(kAddIceCandidateFailNotReady);
+    return true;
+  }
+}
+
+void SdpOfferAnswerHandler::AddIceCandidate(
+    std::unique_ptr<IceCandidateInterface> candidate,
+    std::function<void(RTCError)> callback) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // Chain this operation. If asynchronous operations are pending on the chain,
+  // this operation will be queued to be invoked, otherwise the contents of the
+  // lambda will execute immediately.
+  operations_chain_->ChainOperation(
+      [this_weak_ptr = weak_ptr_factory_.GetWeakPtr(),
+       candidate = std::move(candidate), callback = std::move(callback)](
+          std::function<void()> operations_chain_callback) {
+        if (!this_weak_ptr) {
+          operations_chain_callback();
+          callback(RTCError(
+              RTCErrorType::INVALID_STATE,
+              "AddIceCandidate failed because the session was shut down"));
+          return;
+        }
+        if (!this_weak_ptr->AddIceCandidate(candidate.get())) {
+          operations_chain_callback();
+          // Fail with an error type and message consistent with Chromium.
+          // TODO(hbos): Fail with error types according to spec.
+          callback(RTCError(RTCErrorType::UNSUPPORTED_OPERATION,
+                            "Error processing ICE candidate"));
+          return;
+        }
+        operations_chain_callback();
+        callback(RTCError::OK());
+      });
+}
+
+bool SdpOfferAnswerHandler::RemoveIceCandidates(
+    const std::vector<cricket::Candidate>& candidates) {
+  TRACE_EVENT0("webrtc", "SdpOfferAnswerHandler::RemoveIceCandidates");
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  if (pc_->IsClosed()) {
+    RTC_LOG(LS_ERROR) << "RemoveIceCandidates: PeerConnection is closed.";
+    return false;
+  }
+
+  if (!remote_description()) {
+    RTC_LOG(LS_ERROR) << "RemoveIceCandidates: ICE candidates can't be removed "
+                         "without any remote session description.";
+    return false;
+  }
+
+  if (candidates.empty()) {
+    RTC_LOG(LS_ERROR) << "RemoveIceCandidates: candidates are empty.";
+    return false;
+  }
+
+  size_t number_removed =
+      mutable_remote_description()->RemoveCandidates(candidates);
+  if (number_removed != candidates.size()) {
+    RTC_LOG(LS_ERROR)
+        << "RemoveIceCandidates: Failed to remove candidates. Requested "
+        << candidates.size() << " but only " << number_removed
+        << " are removed.";
+  }
+
+  // Remove the candidates from the transport controller.
+  RTCError error =
+      pc_->transport_controller_->RemoveRemoteCandidates(candidates);
+  if (!error.ok()) {
+    RTC_LOG(LS_ERROR)
+        << "RemoveIceCandidates: Error when removing remote candidates: "
+        << error.message();
+  }
+  return true;
+}
+
+void SdpOfferAnswerHandler::AddLocalIceCandidate(
+    const JsepIceCandidate* candidate) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  if (local_description()) {
+    mutable_local_description()->AddCandidate(candidate);
+  }
+}
+
+void SdpOfferAnswerHandler::RemoveLocalIceCandidates(
+    const std::vector<cricket::Candidate>& candidates) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  if (local_description()) {
+    mutable_local_description()->RemoveCandidates(candidates);
+  }
+}
+
+const SessionDescriptionInterface* SdpOfferAnswerHandler::local_description()
+    const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return pending_local_description_ ? pending_local_description_.get()
+                                    : current_local_description_.get();
+}
+
+const SessionDescriptionInterface* SdpOfferAnswerHandler::remote_description()
+    const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return pending_remote_description_ ? pending_remote_description_.get()
+                                     : current_remote_description_.get();
+}
+
+const SessionDescriptionInterface*
+SdpOfferAnswerHandler::current_local_description() const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return current_local_description_.get();
+}
+
+const SessionDescriptionInterface*
+SdpOfferAnswerHandler::current_remote_description() const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return current_remote_description_.get();
+}
+
+const SessionDescriptionInterface*
+SdpOfferAnswerHandler::pending_local_description() const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return pending_local_description_.get();
+}
+
+const SessionDescriptionInterface*
+SdpOfferAnswerHandler::pending_remote_description() const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return pending_remote_description_.get();
+}
+
+PeerConnectionInterface::SignalingState SdpOfferAnswerHandler::signaling_state()
+    const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return signaling_state_;
+}
+
+void SdpOfferAnswerHandler::ChangeSignalingState(
+    PeerConnectionInterface::SignalingState signaling_state) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  if (signaling_state_ == signaling_state) {
+    return;
+  }
+  RTC_LOG(LS_INFO) << "Session: " << pc_->session_id() << " Old state: "
+                   << GetSignalingStateString(signaling_state_)
+                   << " New state: "
+                   << GetSignalingStateString(signaling_state);
+  signaling_state_ = signaling_state;
+  pc_->Observer()->OnSignalingChange(signaling_state_);
+}
+
+RTCError SdpOfferAnswerHandler::UpdateSessionState(
+    SdpType type,
+    cricket::ContentSource source,
+    const cricket::SessionDescription* description) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+
+  // If there's already a pending error then no state transition should happen.
+  // But all call-sites should be verifying this before calling us!
+  RTC_DCHECK(pc_->session_error() == PeerConnection::SessionError::kNone);
+
+  // If this is answer-ish we're ready to let media flow.
+  if (type == SdpType::kPrAnswer || type == SdpType::kAnswer) {
+    pc_->EnableSending();
+  }
+
+  // Update the signaling state according to the specified state machine (see
+  // https://w3c.github.io/webrtc-pc/#rtcsignalingstate-enum).
+  if (type == SdpType::kOffer) {
+    ChangeSignalingState(source == cricket::CS_LOCAL
+                             ? PeerConnectionInterface::kHaveLocalOffer
+                             : PeerConnectionInterface::kHaveRemoteOffer);
+  } else if (type == SdpType::kPrAnswer) {
+    ChangeSignalingState(source == cricket::CS_LOCAL
+                             ? PeerConnectionInterface::kHaveLocalPrAnswer
+                             : PeerConnectionInterface::kHaveRemotePrAnswer);
+  } else {
+    RTC_DCHECK_RUN_ON(pc_->signaling_thread());
+    RTC_DCHECK(type == SdpType::kAnswer);
+    ChangeSignalingState(PeerConnectionInterface::kStable);
+    pc_->transceiver_stable_states_by_transceivers_.clear();
+    pc_->have_pending_rtp_data_channel_ = false;
+  }
+
+  // Update internal objects according to the session description's media
+  // descriptions.
+  RTCError error = pc_->PushdownMediaDescription(type, source);
+  if (!error.ok()) {
+    return error;
+  }
+
+  return RTCError::OK();
+}
+
+bool SdpOfferAnswerHandler::ShouldFireNegotiationNeededEvent(
+    uint32_t event_id) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // Plan B? Always fire to conform with useless legacy behavior.
+  if (!IsUnifiedPlan()) {
+    return true;
+  }
+  // The event ID has been invalidated. Either negotiation is no longer needed
+  // or a newer negotiation needed event has been generated.
+  if (event_id != negotiation_needed_event_id_) {
+    return false;
+  }
+  // The chain is no longer empty, update negotiation needed when it becomes
+  // empty. This should generate a newer negotiation needed event, making this
+  // one obsolete.
+  if (!operations_chain_->IsEmpty()) {
+    // Since we just suppressed an event that would have been fired, if
+    // negotiation is still needed by the time the chain becomes empty again, we
+    // must make sure to generate another event if negotiation is needed then.
+    // This happens when |is_negotiation_needed_| goes from false to true, so we
+    // set it to false until UpdateNegotiationNeeded() is called.
+    is_negotiation_needed_ = false;
+    update_negotiation_needed_on_empty_chain_ = true;
+    return false;
+  }
+  // We must not fire if the signaling state is no longer "stable". If
+  // negotiation is still needed when we return to "stable", a new negotiation
+  // needed event will be generated, so this one can safely be suppressed.
+  if (signaling_state_ != PeerConnectionInterface::kStable) {
+    return false;
+  }
+  // All checks have passed - please fire "negotiationneeded" now!
+  return true;
+}
+
+RTCError SdpOfferAnswerHandler::Rollback(SdpType desc_type) {
+  auto state = signaling_state();
+  if (state != PeerConnectionInterface::kHaveLocalOffer &&
+      state != PeerConnectionInterface::kHaveRemoteOffer) {
+    return RTCError(RTCErrorType::INVALID_STATE,
+                    "Called in wrong signalingState: " +
+                        GetSignalingStateString(signaling_state()));
+  }
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  RTC_DCHECK(IsUnifiedPlan());
+  std::vector<rtc::scoped_refptr<MediaStreamInterface>> all_added_streams;
+  std::vector<rtc::scoped_refptr<MediaStreamInterface>> all_removed_streams;
+  std::vector<rtc::scoped_refptr<RtpReceiverInterface>> removed_receivers;
+
+  for (auto&& transceivers_stable_state_pair :
+       pc_->transceiver_stable_states_by_transceivers_) {
+    auto transceiver = transceivers_stable_state_pair.first;
+    auto state = transceivers_stable_state_pair.second;
+
+    if (state.remote_stream_ids()) {
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>> added_streams;
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>> removed_streams;
+      SetAssociatedRemoteStreams(transceiver->internal()->receiver_internal(),
+                                 state.remote_stream_ids().value(),
+                                 &added_streams, &removed_streams);
+      all_added_streams.insert(all_added_streams.end(), added_streams.begin(),
+                               added_streams.end());
+      all_removed_streams.insert(all_removed_streams.end(),
+                                 removed_streams.begin(),
+                                 removed_streams.end());
+      if (!state.has_m_section() && !state.newly_created()) {
+        continue;
+      }
+    }
+
+    RTC_DCHECK(transceiver->internal()->mid().has_value());
+    pc_->DestroyTransceiverChannel(transceiver);
+
+    if (signaling_state() == PeerConnectionInterface::kHaveRemoteOffer &&
+        transceiver->receiver()) {
+      removed_receivers.push_back(transceiver->receiver());
+    }
+    if (state.newly_created()) {
+      if (transceiver->internal()->reused_for_addtrack()) {
+        transceiver->internal()->set_created_by_addtrack(true);
+      } else {
+        int remaining_transceiver_count = 0;
+        for (auto&& t : pc_->transceivers_) {
+          if (t != transceiver) {
+            pc_->transceivers_[remaining_transceiver_count++] = t;
+          }
+        }
+        pc_->transceivers_.resize(remaining_transceiver_count);
+      }
+    }
+    transceiver->internal()->sender_internal()->set_transport(nullptr);
+    transceiver->internal()->receiver_internal()->set_transport(nullptr);
+    transceiver->internal()->set_mid(state.mid());
+    transceiver->internal()->set_mline_index(state.mline_index());
+  }
+  pc_->transport_controller_->RollbackTransports();
+  {
+    RTC_DCHECK_RUN_ON(pc_->signaling_thread());
+    if (pc_->have_pending_rtp_data_channel_) {
+      pc_->DestroyDataChannelTransport();
+      pc_->have_pending_rtp_data_channel_ = false;
+    }
+    pc_->transceiver_stable_states_by_transceivers_.clear();
+  }
+  pending_local_description_.reset();
+  pending_remote_description_.reset();
+  ChangeSignalingState(PeerConnectionInterface::kStable);
+
+  // Once all processing has finished, fire off callbacks.
+  for (const auto& receiver : removed_receivers) {
+    pc_->Observer()->OnRemoveTrack(receiver);
+  }
+  for (const auto& stream : all_added_streams) {
+    pc_->Observer()->OnAddStream(stream);
+  }
+  for (const auto& stream : all_removed_streams) {
+    pc_->Observer()->OnRemoveStream(stream);
+  }
+
+  // The assumption is that in case of implicit rollback UpdateNegotiationNeeded
+  // gets called in SetRemoteDescription.
+  if (desc_type == SdpType::kRollback) {
+    UpdateNegotiationNeeded();
+    if (is_negotiation_needed_) {
+      // Legacy version.
+      pc_->Observer()->OnRenegotiationNeeded();
+      // Spec-compliant version; the event may get invalidated before firing.
+      GenerateNegotiationNeededEvent();
+    }
+  }
+  return RTCError::OK();
+}
+
+bool SdpOfferAnswerHandler::IsUnifiedPlan() const {
+  RTC_DCHECK_RUN_ON(pc_->signaling_thread());
+  return pc_->IsUnifiedPlan();
+}
+
+void SdpOfferAnswerHandler::OnOperationsChainEmpty() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  if (pc_->IsClosed() || !update_negotiation_needed_on_empty_chain_)
+    return;
+  update_negotiation_needed_on_empty_chain_ = false;
+  // Firing when chain is empty is only supported in Unified Plan to avoid Plan
+  // B regressions. (In Plan B, onnegotiationneeded is already broken anyway, so
+  // firing it even more might just be confusing.)
+  if (IsUnifiedPlan()) {
+    UpdateNegotiationNeeded();
+  }
+}
+
+absl::optional<bool> SdpOfferAnswerHandler::is_caller() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return is_caller_;
+}
+
+bool SdpOfferAnswerHandler::HasNewIceCredentials() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return local_ice_credentials_to_replace_->HasIceCredentials();
+}
+
+bool SdpOfferAnswerHandler::IceRestartPending(
+    const std::string& content_name) const {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  return pending_ice_restarts_.find(content_name) !=
+         pending_ice_restarts_.end();
+}
+
+void SdpOfferAnswerHandler::UpdateNegotiationNeeded() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  if (!IsUnifiedPlan()) {
+    pc_->Observer()->OnRenegotiationNeeded();
+    GenerateNegotiationNeededEvent();
+    return;
+  }
+
+  // In the spec, a task is queued here to run the following steps - this is
+  // meant to ensure we do not fire onnegotiationneeded prematurely if multiple
+  // changes are being made at once. In order to support Chromium's
+  // implementation where the JavaScript representation of the PeerConnection
+  // lives on a separate thread though, the queuing of a task is instead
+  // performed by the PeerConnectionObserver posting from the signaling thread
+  // to the JavaScript main thread that negotiation is needed. And because the
+  // Operations Chain lives on the WebRTC signaling thread,
+  // ShouldFireNegotiationNeededEvent() must be called before firing the event
+  // to ensure the Operations Chain is still empty and the event has not been
+  // invalidated.
+
+  // If connection's [[IsClosed]] slot is true, abort these steps.
+  if (pc_->IsClosed())
+    return;
+
+  // If connection's signaling state is not "stable", abort these steps.
+  if (signaling_state() != PeerConnectionInterface::kStable)
+    return;
+
+  // NOTE
+  // The negotiation-needed flag will be updated once the state transitions to
+  // "stable", as part of the steps for setting an RTCSessionDescription.
+
+  // If the result of checking if negotiation is needed is false, clear the
+  // negotiation-needed flag by setting connection's [[NegotiationNeeded]] slot
+  // to false, and abort these steps.
+  bool is_negotiation_needed = CheckIfNegotiationIsNeeded();
+  if (!is_negotiation_needed) {
+    is_negotiation_needed_ = false;
+    // Invalidate any negotiation needed event that may previosuly have been
+    // generated.
+    ++negotiation_needed_event_id_;
+    return;
+  }
+
+  // If connection's [[NegotiationNeeded]] slot is already true, abort these
+  // steps.
+  if (is_negotiation_needed_)
+    return;
+
+  // Set connection's [[NegotiationNeeded]] slot to true.
+  is_negotiation_needed_ = true;
+
+  // Queue a task that runs the following steps:
+  // If connection's [[IsClosed]] slot is true, abort these steps.
+  // If connection's [[NegotiationNeeded]] slot is false, abort these steps.
+  // Fire an event named negotiationneeded at connection.
+  pc_->Observer()->OnRenegotiationNeeded();
+  // Fire the spec-compliant version; when ShouldFireNegotiationNeededEvent() is
+  // used in the task queued by the observer, this event will only fire when the
+  // chain is empty.
+  GenerateNegotiationNeededEvent();
+}
+
+bool SdpOfferAnswerHandler::CheckIfNegotiationIsNeeded() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  // 1. If any implementation-specific negotiation is required, as described at
+  // the start of this section, return true.
+
+  // 2. If connection.[[LocalIceCredentialsToReplace]] is not empty, return
+  // true.
+  if (local_ice_credentials_to_replace_->HasIceCredentials()) {
+    return true;
+  }
+
+  // 3. Let description be connection.[[CurrentLocalDescription]].
+  const SessionDescriptionInterface* description = current_local_description();
+  if (!description)
+    return true;
+
+  // 4. If connection has created any RTCDataChannels, and no m= section in
+  // description has been negotiated yet for data, return true.
+  if (pc_->data_channel_controller()->HasSctpDataChannels()) {
+    if (!cricket::GetFirstDataContent(description->description()->contents()))
+      return true;
+  }
+
+  // 5. For each transceiver in connection's set of transceivers, perform the
+  // following checks:
+  for (const auto& transceiver : pc_->transceivers_) {
+    const ContentInfo* current_local_msection =
+        FindTransceiverMSection(transceiver.get(), description);
+
+    const ContentInfo* current_remote_msection = FindTransceiverMSection(
+        transceiver.get(), current_remote_description());
+
+    // 5.4 If transceiver is stopped and is associated with an m= section,
+    // but the associated m= section is not yet rejected in
+    // connection.[[CurrentLocalDescription]] or
+    // connection.[[CurrentRemoteDescription]], return true.
+    if (transceiver->stopped()) {
+      RTC_DCHECK(transceiver->stopping());
+      if (current_local_msection && !current_local_msection->rejected &&
+          ((current_remote_msection && !current_remote_msection->rejected) ||
+           !current_remote_msection)) {
+        return true;
+      }
+      continue;
+    }
+
+    // 5.1 If transceiver.[[Stopping]] is true and transceiver.[[Stopped]] is
+    // false, return true.
+    if (transceiver->stopping() && !transceiver->stopped())
+      return true;
+
+    // 5.2 If transceiver isn't stopped and isn't yet associated with an m=
+    // section in description, return true.
+    if (!current_local_msection)
+      return true;
+
+    const MediaContentDescription* current_local_media_description =
+        current_local_msection->media_description();
+    // 5.3 If transceiver isn't stopped and is associated with an m= section
+    // in description then perform the following checks:
+
+    // 5.3.1 If transceiver.[[Direction]] is "sendrecv" or "sendonly", and the
+    // associated m= section in description either doesn't contain a single
+    // "a=msid" line, or the number of MSIDs from the "a=msid" lines in this
+    // m= section, or the MSID values themselves, differ from what is in
+    // transceiver.sender.[[AssociatedMediaStreamIds]], return true.
+    if (RtpTransceiverDirectionHasSend(transceiver->direction())) {
+      if (current_local_media_description->streams().size() == 0)
+        return true;
+
+      std::vector<std::string> msection_msids;
+      for (const auto& stream : current_local_media_description->streams()) {
+        for (const std::string& msid : stream.stream_ids())
+          msection_msids.push_back(msid);
+      }
+
+      std::vector<std::string> transceiver_msids =
+          transceiver->sender()->stream_ids();
+      if (msection_msids.size() != transceiver_msids.size())
+        return true;
+
+      absl::c_sort(transceiver_msids);
+      absl::c_sort(msection_msids);
+      if (transceiver_msids != msection_msids)
+        return true;
+    }
+
+    // 5.3.2 If description is of type "offer", and the direction of the
+    // associated m= section in neither connection.[[CurrentLocalDescription]]
+    // nor connection.[[CurrentRemoteDescription]] matches
+    // transceiver.[[Direction]], return true.
+    if (description->GetType() == SdpType::kOffer) {
+      if (!current_remote_description())
+        return true;
+
+      if (!current_remote_msection)
+        return true;
+
+      RtpTransceiverDirection current_local_direction =
+          current_local_media_description->direction();
+      RtpTransceiverDirection current_remote_direction =
+          current_remote_msection->media_description()->direction();
+      if (transceiver->direction() != current_local_direction &&
+          transceiver->direction() !=
+              RtpTransceiverDirectionReversed(current_remote_direction)) {
+        return true;
+      }
+    }
+
+    // 5.3.3 If description is of type "answer", and the direction of the
+    // associated m= section in the description does not match
+    // transceiver.[[Direction]] intersected with the offered direction (as
+    // described in [JSEP] (section 5.3.1.)), return true.
+    if (description->GetType() == SdpType::kAnswer) {
+      if (!remote_description())
+        return true;
+
+      const ContentInfo* offered_remote_msection =
+          FindTransceiverMSection(transceiver.get(), remote_description());
+
+      RtpTransceiverDirection offered_direction =
+          offered_remote_msection
+              ? offered_remote_msection->media_description()->direction()
+              : RtpTransceiverDirection::kInactive;
+
+      if (current_local_media_description->direction() !=
+          (RtpTransceiverDirectionIntersection(
+              transceiver->direction(),
+              RtpTransceiverDirectionReversed(offered_direction)))) {
+        return true;
+      }
+    }
+  }
+
+  // If all the preceding checks were performed and true was not returned,
+  // nothing remains to be negotiated; return false.
+  return false;
+}
+
+void SdpOfferAnswerHandler::GenerateNegotiationNeededEvent() {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  ++negotiation_needed_event_id_;
+  pc_->Observer()->OnNegotiationNeededEvent(negotiation_needed_event_id_);
+}
+
+RTCError SdpOfferAnswerHandler::ValidateSessionDescription(
+    const SessionDescriptionInterface* sdesc,
+    cricket::ContentSource source) {
+  if (pc_->session_error() != PeerConnection::SessionError::kNone) {
+    LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR,
+                         pc_->GetSessionErrorMsg());
+  }
+
+  if (!sdesc || !sdesc->description()) {
+    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER, kInvalidSdp);
+  }
+
+  SdpType type = sdesc->GetType();
+  if ((source == cricket::CS_LOCAL && !pc_->ExpectSetLocalDescription(type)) ||
+      (source == cricket::CS_REMOTE &&
+       !pc_->ExpectSetRemoteDescription(type))) {
+    LOG_AND_RETURN_ERROR(
+        RTCErrorType::INVALID_STATE,
+        "Called in wrong state: " + GetSignalingStateString(signaling_state()));
+  }
+
+  RTCError error = ValidateMids(*sdesc->description());
+  if (!error.ok()) {
+    return error;
+  }
+
+  // Verify crypto settings.
+  std::string crypto_error;
+  if (webrtc_session_desc_factory_->SdesPolicy() == cricket::SEC_REQUIRED ||
+      pc_->dtls_enabled()) {
+    RTCError crypto_error =
+        VerifyCrypto(sdesc->description(), pc_->dtls_enabled());
+    if (!crypto_error.ok()) {
+      return crypto_error;
+    }
+  }
+
+  // Verify ice-ufrag and ice-pwd.
+  if (!VerifyIceUfragPwdPresent(sdesc->description())) {
+    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
+                         kSdpWithoutIceUfragPwd);
+  }
+
+  if (!pc_->ValidateBundleSettings(sdesc->description())) {
+    LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
+                         kBundleWithoutRtcpMux);
+  }
+
+  // TODO(skvlad): When the local rtcp-mux policy is Require, reject any
+  // m-lines that do not rtcp-mux enabled.
+
+  // Verify m-lines in Answer when compared against Offer.
+  if (type == SdpType::kPrAnswer || type == SdpType::kAnswer) {
+    // With an answer we want to compare the new answer session description with
+    // the offer's session description from the current negotiation.
+    const cricket::SessionDescription* offer_desc =
+        (source == cricket::CS_LOCAL) ? remote_description()->description()
+                                      : local_description()->description();
+    if (!MediaSectionsHaveSameCount(*offer_desc, *sdesc->description()) ||
+        !MediaSectionsInSameOrder(*offer_desc, nullptr, *sdesc->description(),
+                                  type)) {
+      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
+                           kMlineMismatchInAnswer);
+    }
+  } else {
+    // The re-offers should respect the order of m= sections in current
+    // description. See RFC3264 Section 8 paragraph 4 for more details.
+    // With a re-offer, either the current local or current remote descriptions
+    // could be the most up to date, so we would like to check against both of
+    // them if they exist. It could be the case that one of them has a 0 port
+    // for a media section, but the other does not. This is important to check
+    // against in the case that we are recycling an m= section.
+    const cricket::SessionDescription* current_desc = nullptr;
+    const cricket::SessionDescription* secondary_current_desc = nullptr;
+    if (local_description()) {
+      current_desc = local_description()->description();
+      if (remote_description()) {
+        secondary_current_desc = remote_description()->description();
+      }
+    } else if (remote_description()) {
+      current_desc = remote_description()->description();
+    }
+    if (current_desc &&
+        !MediaSectionsInSameOrder(*current_desc, secondary_current_desc,
+                                  *sdesc->description(), type)) {
+      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
+                           kMlineMismatchInSubsequentOffer);
+    }
+  }
+
+  if (IsUnifiedPlan()) {
+    // Ensure that each audio and video media section has at most one
+    // "StreamParams". This will return an error if receiving a session
+    // description from a "Plan B" endpoint which adds multiple tracks of the
+    // same type. With Unified Plan, there can only be at most one track per
+    // media section.
+    for (const ContentInfo& content : sdesc->description()->contents()) {
+      const MediaContentDescription& desc = *content.media_description();
+      if ((desc.type() == cricket::MEDIA_TYPE_AUDIO ||
+           desc.type() == cricket::MEDIA_TYPE_VIDEO) &&
+          desc.streams().size() > 1u) {
+        LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
+                             "Media section has more than one track specified "
+                             "with a=ssrc lines which is not supported with "
+                             "Unified Plan.");
+      }
+    }
+  }
+
+  return RTCError::OK();
+}
+
+RTCError SdpOfferAnswerHandler::UpdateTransceiversAndDataChannels(
+    cricket::ContentSource source,
+    const SessionDescriptionInterface& new_session,
+    const SessionDescriptionInterface* old_local_description,
+    const SessionDescriptionInterface* old_remote_description) {
+  RTC_DCHECK_RUN_ON(signaling_thread());
+  RTC_DCHECK(IsUnifiedPlan());
+
+  const cricket::ContentGroup* bundle_group = nullptr;
+  if (new_session.GetType() == SdpType::kOffer) {
+    auto bundle_group_or_error =
+        GetEarlyBundleGroup(*new_session.description());
+    if (!bundle_group_or_error.ok()) {
+      return bundle_group_or_error.MoveError();
+    }
+    bundle_group = bundle_group_or_error.MoveValue();
+  }
+
+  const ContentInfos& new_contents = new_session.description()->contents();
+  for (size_t i = 0; i < new_contents.size(); ++i) {
+    const cricket::ContentInfo& new_content = new_contents[i];
+    cricket::MediaType media_type = new_content.media_description()->type();
+    pc_->mid_generator()->AddKnownId(new_content.name);
+    if (media_type == cricket::MEDIA_TYPE_AUDIO ||
+        media_type == cricket::MEDIA_TYPE_VIDEO) {
+      const cricket::ContentInfo* old_local_content = nullptr;
+      if (old_local_description &&
+          i < old_local_description->description()->contents().size()) {
+        old_local_content =
+            &old_local_description->description()->contents()[i];
+      }
+      const cricket::ContentInfo* old_remote_content = nullptr;
+      if (old_remote_description &&
+          i < old_remote_description->description()->contents().size()) {
+        old_remote_content =
+            &old_remote_description->description()->contents()[i];
+      }
+      // In the case where an m-section has completed its rejection,
+      // and is not being reused, we do not expect a transceiver.
+      if (old_local_content && old_local_content->rejected &&
+          old_remote_content && old_remote_content->rejected &&
+          new_content.rejected) {
+        continue;
+      }
+      auto transceiver_or_error =
+          AssociateTransceiver(source, new_session.GetType(), i, new_content,
+                               old_local_content, old_remote_content);
+      if (!transceiver_or_error.ok()) {
+        return transceiver_or_error.MoveError();
+      }
+      auto transceiver = transceiver_or_error.MoveValue();
+      RTCError error =
+          UpdateTransceiverChannel(transceiver, new_content, bundle_group);
+      if (!error.ok()) {
+        return error;
+      }
+    } else if (media_type == cricket::MEDIA_TYPE_DATA) {
+      if (pc_->GetDataMid() && new_content.name != *(pc_->GetDataMid())) {
+        // Ignore all but the first data section.
+        RTC_LOG(LS_INFO) << "Ignoring data media section with MID="
+                         << new_content.name;
+        continue;
+      }
+      RTCError error = UpdateDataChannel(source, new_content, bundle_group);
+      if (!error.ok()) {
+        return error;
+      }
+    } else {
+      LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR,
+                           "Unknown section type.");
+    }
+  }
+
+  return RTCError::OK();
+}
+
+RTCErrorOr<rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>>
+SdpOfferAnswerHandler::AssociateTransceiver(
+    cricket::ContentSource source,
+    SdpType type,
+    size_t mline_index,
+    const ContentInfo& content,
+    const ContentInfo* old_local_content,
+    const ContentInfo* old_remote_content) {
+  RTC_DCHECK(IsUnifiedPlan());
+  // If this is an offer then the m= section might be recycled. If the m=
+  // section is being recycled (defined as: rejected in the current local or
+  // remote description and not rejected in new description), the transceiver
+  // should have been removed by RemoveStoppedTransceivers().
+  if (IsMediaSectionBeingRecycled(type, content, old_local_content,
+                                  old_remote_content)) {
+    const std::string& old_mid =
+        (old_local_content && old_local_content->rejected)
+            ? old_local_content->name
+            : old_remote_content->name;
+    auto old_transceiver = pc_->GetAssociatedTransceiver(old_mid);
+    // The transceiver should be disassociated in RemoveStoppedTransceivers()
+    RTC_DCHECK(!old_transceiver);
+  }
+  const MediaContentDescription* media_desc = content.media_description();
+  auto transceiver = pc_->GetAssociatedTransceiver(content.name);
+  if (source == cricket::CS_LOCAL) {
+    // Find the RtpTransceiver that corresponds to this m= section, using the
+    // mapping between transceivers and m= section indices established when
+    // creating the offer.
+    if (!transceiver) {
+      transceiver = pc_->GetTransceiverByMLineIndex(mline_index);
+    }
+    if (!transceiver) {
+      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
+                           "Unknown transceiver");
+    }
+  } else {
+    RTC_DCHECK_EQ(source, cricket::CS_REMOTE);
+    // If the m= section is sendrecv or recvonly, and there are RtpTransceivers
+    // of the same type...
+    // When simulcast is requested, a transceiver cannot be associated because
+    // AddTrack cannot be called to initialize it.
+    if (!transceiver &&
+        RtpTransceiverDirectionHasRecv(media_desc->direction()) &&
+        !media_desc->HasSimulcast()) {
+      transceiver = pc_->FindAvailableTransceiverToReceive(media_desc->type());
+    }
+    // If no RtpTransceiver was found in the previous step, create one with a
+    // recvonly direction.
+    if (!transceiver) {
+      RTC_LOG(LS_INFO) << "Adding "
+                       << cricket::MediaTypeToString(media_desc->type())
+                       << " transceiver for MID=" << content.name
+                       << " at i=" << mline_index
+                       << " in response to the remote description.";
+      std::string sender_id = rtc::CreateRandomUuid();
+      std::vector<RtpEncodingParameters> send_encodings =
+          GetSendEncodingsFromRemoteDescription(*media_desc);
+      auto sender = pc_->CreateSender(media_desc->type(), sender_id, nullptr,
+                                      {}, send_encodings);
+      std::string receiver_id;
+      if (!media_desc->streams().empty()) {
+        receiver_id = media_desc->streams()[0].id;
+      } else {
+        receiver_id = rtc::CreateRandomUuid();
+      }
+      auto receiver = pc_->CreateReceiver(media_desc->type(), receiver_id);
+      transceiver = pc_->CreateAndAddTransceiver(sender, receiver);
+      transceiver->internal()->set_direction(
+          RtpTransceiverDirection::kRecvOnly);
+      if (type == SdpType::kOffer) {
+        pc_->transceiver_stable_states_by_transceivers_[transceiver]
+            .set_newly_created();
+      }
+    }
+    // Check if the offer indicated simulcast but the answer rejected it.
+    // This can happen when simulcast is not supported on the remote party.
+    if (SimulcastIsRejected(old_local_content, *media_desc)) {
+      RTC_HISTOGRAM_BOOLEAN(kSimulcastDisabled, true);
+      RTCError error =
+          DisableSimulcastInSender(transceiver->internal()->sender_internal());
+      if (!error.ok()) {
+        RTC_LOG(LS_ERROR) << "Failed to remove rejected simulcast.";
+        return std::move(error);
+      }
+    }
+  }
+  RTC_DCHECK(transceiver);
+  if (transceiver->media_type() != media_desc->type()) {
+    LOG_AND_RETURN_ERROR(
+        RTCErrorType::INVALID_PARAMETER,
+        "Transceiver type does not match media description type.");
+  }
+  if (media_desc->HasSimulcast()) {
+    std::vector<SimulcastLayer> layers =
+        source == cricket::CS_LOCAL
+            ? media_desc->simulcast_description().send_layers().GetAllLayers()
+            : media_desc->simulcast_description()
+                  .receive_layers()
+                  .GetAllLayers();
+    RTCError error = UpdateSimulcastLayerStatusInSender(
+        layers, transceiver->internal()->sender_internal());
+    if (!error.ok()) {
+      RTC_LOG(LS_ERROR) << "Failed updating status for simulcast layers.";
+      return std::move(error);
+    }
+  }
+  if (type == SdpType::kOffer) {
+    bool state_changes = transceiver->internal()->mid() != content.name ||
+                         transceiver->internal()->mline_index() != mline_index;
+    if (state_changes) {
+      pc_->transceiver_stable_states_by_transceivers_[transceiver]
+          .SetMSectionIfUnset(transceiver->internal()->mid(),
+                              transceiver->internal()->mline_index());
+    }
+  }
+  // Associate the found or created RtpTransceiver with the m= section by
+  // setting the value of the RtpTransceiver's mid property to the MID of the m=
+  // section, and establish a mapping between the transceiver and the index of
+  // the m= section.
+  transceiver->internal()->set_mid(content.name);
+  transceiver->internal()->set_mline_index(mline_index);
+  return std::move(transceiver);
+}
+
+RTCErrorOr<const cricket::ContentGroup*>
+SdpOfferAnswerHandler::GetEarlyBundleGroup(
+    const SessionDescription& desc) const {
+  const cricket::ContentGroup* bundle_group = nullptr;
+  if (pc_->configuration()->bundle_policy ==
+      PeerConnectionInterface::kBundlePolicyMaxBundle) {
+    bundle_group = desc.GetGroupByName(cricket::GROUP_TYPE_BUNDLE);
+    if (!bundle_group) {
+      LOG_AND_RETURN_ERROR(RTCErrorType::INVALID_PARAMETER,
+                           "max-bundle configured but session description "
+                           "has no BUNDLE group");
+    }
+  }
+  return bundle_group;
+}
+
+RTCError SdpOfferAnswerHandler::UpdateTransceiverChannel(
+    rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
+        transceiver,
+    const cricket::ContentInfo& content,
+    const cricket::ContentGroup* bundle_group) {
+  RTC_DCHECK(IsUnifiedPlan());
+  RTC_DCHECK(transceiver);
+  cricket::ChannelInterface* channel = transceiver->internal()->channel();
+  if (content.rejected) {
+    if (channel) {
+      transceiver->internal()->SetChannel(nullptr);
+      pc_->DestroyChannelInterface(channel);
+    }
+  } else {
+    if (!channel) {
+      if (transceiver->media_type() == cricket::MEDIA_TYPE_AUDIO) {
+        channel = pc_->CreateVoiceChannel(content.name);
+      } else {
+        RTC_DCHECK_EQ(cricket::MEDIA_TYPE_VIDEO, transceiver->media_type());
+        channel = pc_->CreateVideoChannel(content.name);
+      }
+      if (!channel) {
+        LOG_AND_RETURN_ERROR(
+            RTCErrorType::INTERNAL_ERROR,
+            "Failed to create channel for mid=" + content.name);
+      }
+      transceiver->internal()->SetChannel(channel);
+    }
+  }
+  return RTCError::OK();
+}
+
+RTCError SdpOfferAnswerHandler::UpdateDataChannel(
+    cricket::ContentSource source,
+    const cricket::ContentInfo& content,
+    const cricket::ContentGroup* bundle_group) {
+  if (pc_->data_channel_type() == cricket::DCT_NONE) {
+    // If data channels are disabled, ignore this media section. CreateAnswer
+    // will take care of rejecting it.
+    return RTCError::OK();
+  }
+  if (content.rejected) {
+    RTC_LOG(LS_INFO) << "Rejected data channel, mid=" << content.mid();
+    pc_->DestroyDataChannelTransport();
+  } else {
+    if (!pc_->data_channel_controller_.rtp_data_channel() &&
+        !pc_->data_channel_controller_.data_channel_transport()) {
+      RTC_LOG(LS_INFO) << "Creating data channel, mid=" << content.mid();
+      if (!pc_->CreateDataChannel(content.name)) {
+        LOG_AND_RETURN_ERROR(RTCErrorType::INTERNAL_ERROR,
+                             "Failed to create data channel.");
+      }
+    }
+    if (source == cricket::CS_REMOTE) {
+      const MediaContentDescription* data_desc = content.media_description();
+      if (data_desc && cricket::IsRtpProtocol(data_desc->protocol())) {
+        pc_->data_channel_controller_.UpdateRemoteRtpDataChannels(
+            GetActiveStreams(data_desc));
+      }
+    }
+  }
+  return RTCError::OK();
+}
+
+}  // namespace webrtc
diff --git a/pc/sdp_offer_answer.h b/pc/sdp_offer_answer.h
new file mode 100644
index 0000000..780dd93
--- /dev/null
+++ b/pc/sdp_offer_answer.h
@@ -0,0 +1,313 @@
+/*
+ *  Copyright 2020 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef PC_SDP_OFFER_ANSWER_H_
+#define PC_SDP_OFFER_ANSWER_H_
+
+#include <map>
+#include <memory>
+#include <set>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "api/jsep_ice_candidate.h"
+#include "api/peer_connection_interface.h"
+#include "api/transport/data_channel_transport_interface.h"
+#include "api/turn_customizer.h"
+#include "pc/data_channel_controller.h"
+#include "pc/ice_server_parsing.h"
+#include "pc/jsep_transport_controller.h"
+#include "pc/peer_connection_factory.h"
+#include "pc/peer_connection_internal.h"
+#include "pc/rtc_stats_collector.h"
+#include "pc/rtp_sender.h"
+#include "pc/rtp_transceiver.h"
+#include "pc/sctp_transport.h"
+#include "pc/stats_collector.h"
+#include "pc/stream_collection.h"
+#include "pc/webrtc_session_description_factory.h"
+#include "rtc_base/experiments/field_trial_parser.h"
+#include "rtc_base/operations_chain.h"
+#include "rtc_base/race_checker.h"
+#include "rtc_base/unique_id_generator.h"
+#include "rtc_base/weak_ptr.h"
+
+namespace webrtc {
+
+class MediaStreamObserver;
+class PeerConnection;
+class VideoRtpReceiver;
+class RtcEventLog;
+
+// SdpOfferAnswerHandler is a component
+// of the PeerConnection object as defined
+// by the PeerConnectionInterface API surface.
+// The class is responsible for the following:
+// - Parsing and interpreting SDP.
+// - Generating offers and answers based on the current state.
+// This class lives on the signaling thread.
+class SdpOfferAnswerHandler {
+ public:
+  explicit SdpOfferAnswerHandler(PeerConnection* pc);
+  ~SdpOfferAnswerHandler();
+
+  void SetSessionDescFactory(
+      std::unique_ptr<WebRtcSessionDescriptionFactory> factory) {
+    RTC_DCHECK_RUN_ON(signaling_thread());
+    webrtc_session_desc_factory_ = std::move(factory);
+  }
+  void ResetSessionDescFactory() {
+    RTC_DCHECK_RUN_ON(signaling_thread());
+    webrtc_session_desc_factory_.reset();
+  }
+  const WebRtcSessionDescriptionFactory* webrtc_session_desc_factory() const {
+    RTC_DCHECK_RUN_ON(signaling_thread());
+    return webrtc_session_desc_factory_.get();
+  }
+
+  // Change signaling state to Closed, and perform appropriate actions.
+  void Close();
+
+  // Called as part of destroying the owning PeerConnection.
+  void PrepareForShutdown();
+
+  PeerConnectionInterface::SignalingState signaling_state() const;
+
+  const SessionDescriptionInterface* local_description() const;
+  const SessionDescriptionInterface* remote_description() const;
+  const SessionDescriptionInterface* current_local_description() const;
+  const SessionDescriptionInterface* current_remote_description() const;
+  const SessionDescriptionInterface* pending_local_description() const;
+  const SessionDescriptionInterface* pending_remote_description() const;
+
+  void RestartIce();
+
+  // JSEP01
+  void CreateOffer(
+      CreateSessionDescriptionObserver* observer,
+      const PeerConnectionInterface::RTCOfferAnswerOptions& options);
+  void CreateAnswer(
+      CreateSessionDescriptionObserver* observer,
+      const PeerConnectionInterface::RTCOfferAnswerOptions& options);
+
+  void SetLocalDescription(
+      std::unique_ptr<SessionDescriptionInterface> desc,
+      rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer);
+  void SetLocalDescription(
+      rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer);
+  void SetLocalDescription(SetSessionDescriptionObserver* observer,
+                           SessionDescriptionInterface* desc);
+  void SetLocalDescription(SetSessionDescriptionObserver* observer);
+
+  void SetRemoteDescription(
+      std::unique_ptr<SessionDescriptionInterface> desc,
+      rtc::scoped_refptr<SetRemoteDescriptionObserverInterface> observer);
+  void SetRemoteDescription(SetSessionDescriptionObserver* observer,
+                            SessionDescriptionInterface* desc);
+
+  PeerConnectionInterface::RTCConfiguration GetConfiguration();
+  RTCError SetConfiguration(
+      const PeerConnectionInterface::RTCConfiguration& configuration);
+  bool AddIceCandidate(const IceCandidateInterface* candidate);
+  void AddIceCandidate(std::unique_ptr<IceCandidateInterface> candidate,
+                       std::function<void(RTCError)> callback);
+  bool RemoveIceCandidates(const std::vector<cricket::Candidate>& candidates);
+  // Adds a locally generated candidate to the local description.
+  void AddLocalIceCandidate(const JsepIceCandidate* candidate);
+  void RemoveLocalIceCandidates(
+      const std::vector<cricket::Candidate>& candidates);
+  bool ShouldFireNegotiationNeededEvent(uint32_t event_id);
+
+  absl::optional<bool> is_caller();
+  bool HasNewIceCredentials();
+  bool IceRestartPending(const std::string& content_name) const;
+  void UpdateNegotiationNeeded();
+
+ private:
+  class ImplicitCreateSessionDescriptionObserver;
+  friend class ImplicitCreateSessionDescriptionObserver;
+  class SetSessionDescriptionObserverAdapter;
+  friend class SetSessionDescriptionObserverAdapter;
+
+  // Represents the [[LocalIceCredentialsToReplace]] internal slot in the spec.
+  // It makes the next CreateOffer() produce new ICE credentials even if
+  // RTCOfferAnswerOptions::ice_restart is false.
+  // https://w3c.github.io/webrtc-pc/#dfn-localufragstoreplace
+  // TODO(hbos): When JsepTransportController/JsepTransport supports rollback,
+  // move this type of logic to JsepTransportController/JsepTransport.
+  class LocalIceCredentialsToReplace;
+
+  rtc::Thread* signaling_thread() const;
+  // Non-const versions of local_description()/remote_description(), for use
+  // internally.
+  SessionDescriptionInterface* mutable_local_description()
+      RTC_RUN_ON(signaling_thread()) {
+    return pending_local_description_ ? pending_local_description_.get()
+                                      : current_local_description_.get();
+  }
+  SessionDescriptionInterface* mutable_remote_description()
+      RTC_RUN_ON(signaling_thread()) {
+    return pending_remote_description_ ? pending_remote_description_.get()
+                                       : current_remote_description_.get();
+  }
+
+  // Synchronous implementations of SetLocalDescription/SetRemoteDescription
+  // that return an RTCError instead of invoking a callback.
+  RTCError ApplyLocalDescription(
+      std::unique_ptr<SessionDescriptionInterface> desc);
+  RTCError ApplyRemoteDescription(
+      std::unique_ptr<SessionDescriptionInterface> desc);
+
+  // Implementation of the offer/answer exchange operations. These are chained
+  // onto the |operations_chain_| when the public CreateOffer(), CreateAnswer(),
+  // SetLocalDescription() and SetRemoteDescription() methods are invoked.
+  void DoCreateOffer(
+      const PeerConnectionInterface::RTCOfferAnswerOptions& options,
+      rtc::scoped_refptr<CreateSessionDescriptionObserver> observer);
+  void DoCreateAnswer(
+      const PeerConnectionInterface::RTCOfferAnswerOptions& options,
+      rtc::scoped_refptr<CreateSessionDescriptionObserver> observer);
+  void DoSetLocalDescription(
+      std::unique_ptr<SessionDescriptionInterface> desc,
+      rtc::scoped_refptr<SetLocalDescriptionObserverInterface> observer);
+  void DoSetRemoteDescription(
+      std::unique_ptr<SessionDescriptionInterface> desc,
+      rtc::scoped_refptr<SetRemoteDescriptionObserverInterface> observer);
+
+  // Update the state, signaling if necessary.
+  void ChangeSignalingState(
+      PeerConnectionInterface::SignalingState signaling_state);
+
+  RTCError UpdateSessionState(SdpType type,
+                              cricket::ContentSource source,
+                              const cricket::SessionDescription* description);
+
+  bool IsUnifiedPlan() const RTC_RUN_ON(signaling_thread());
+
+  // | desc_type | is the type of the description that caused the rollback.
+  RTCError Rollback(SdpType desc_type);
+  void OnOperationsChainEmpty();
+
+  // Runs the algorithm **set the associated remote streams** specified in
+  // https://w3c.github.io/webrtc-pc/#set-associated-remote-streams.
+  void SetAssociatedRemoteStreams(
+      rtc::scoped_refptr<RtpReceiverInternal> receiver,
+      const std::vector<std::string>& stream_ids,
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>>* added_streams,
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>>* removed_streams);
+
+  bool CheckIfNegotiationIsNeeded();
+  void GenerateNegotiationNeededEvent();
+  // Helper method which verifies SDP.
+  RTCError ValidateSessionDescription(const SessionDescriptionInterface* sdesc,
+                                      cricket::ContentSource source)
+      RTC_RUN_ON(signaling_thread());
+
+  // Updates the local RtpTransceivers according to the JSEP rules. Called as
+  // part of setting the local/remote description.
+  RTCError UpdateTransceiversAndDataChannels(
+      cricket::ContentSource source,
+      const SessionDescriptionInterface& new_session,
+      const SessionDescriptionInterface* old_local_description,
+      const SessionDescriptionInterface* old_remote_description);
+
+  // Associate the given transceiver according to the JSEP rules.
+  RTCErrorOr<
+      rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>>
+  AssociateTransceiver(cricket::ContentSource source,
+                       SdpType type,
+                       size_t mline_index,
+                       const cricket::ContentInfo& content,
+                       const cricket::ContentInfo* old_local_content,
+                       const cricket::ContentInfo* old_remote_content)
+      RTC_RUN_ON(signaling_thread());
+
+  // If the BUNDLE policy is max-bundle, then we know for sure that all
+  // transports will be bundled from the start. This method returns the BUNDLE
+  // group if that's the case, or null if BUNDLE will be negotiated later. An
+  // error is returned if max-bundle is specified but the session description
+  // does not have a BUNDLE group.
+  RTCErrorOr<const cricket::ContentGroup*> GetEarlyBundleGroup(
+      const cricket::SessionDescription& desc) const
+      RTC_RUN_ON(signaling_thread());
+
+  // Either creates or destroys the transceiver's BaseChannel according to the
+  // given media section.
+  RTCError UpdateTransceiverChannel(
+      rtc::scoped_refptr<RtpTransceiverProxyWithInternal<RtpTransceiver>>
+          transceiver,
+      const cricket::ContentInfo& content,
+      const cricket::ContentGroup* bundle_group) RTC_RUN_ON(signaling_thread());
+
+  // Either creates or destroys the local data channel according to the given
+  // media section.
+  RTCError UpdateDataChannel(cricket::ContentSource source,
+                             const cricket::ContentInfo& content,
+                             const cricket::ContentGroup* bundle_group)
+      RTC_RUN_ON(signaling_thread());
+
+  // ===================================================================
+
+  PeerConnection* const pc_;
+
+  std::unique_ptr<WebRtcSessionDescriptionFactory> webrtc_session_desc_factory_
+      RTC_GUARDED_BY(signaling_thread());
+
+  std::unique_ptr<SessionDescriptionInterface> current_local_description_
+      RTC_GUARDED_BY(signaling_thread());
+  std::unique_ptr<SessionDescriptionInterface> pending_local_description_
+      RTC_GUARDED_BY(signaling_thread());
+  std::unique_ptr<SessionDescriptionInterface> current_remote_description_
+      RTC_GUARDED_BY(signaling_thread());
+  std::unique_ptr<SessionDescriptionInterface> pending_remote_description_
+      RTC_GUARDED_BY(signaling_thread());
+
+  PeerConnectionInterface::SignalingState signaling_state_
+      RTC_GUARDED_BY(signaling_thread()) = PeerConnectionInterface::kStable;
+
+  // Whether this peer is the caller. Set when the local description is applied.
+  absl::optional<bool> is_caller_ RTC_GUARDED_BY(signaling_thread());
+
+  // The operations chain is used by the offer/answer exchange methods to ensure
+  // they are executed in the right order. For example, if
+  // SetRemoteDescription() is invoked while CreateOffer() is still pending, the
+  // SRD operation will not start until CreateOffer() has completed. See
+  // https://w3c.github.io/webrtc-pc/#dfn-operations-chain.
+  rtc::scoped_refptr<rtc::OperationsChain> operations_chain_
+      RTC_GUARDED_BY(signaling_thread());
+
+  // List of content names for which the remote side triggered an ICE restart.
+  std::set<std::string> pending_ice_restarts_
+      RTC_GUARDED_BY(signaling_thread());
+
+  std::unique_ptr<LocalIceCredentialsToReplace>
+      local_ice_credentials_to_replace_ RTC_GUARDED_BY(signaling_thread());
+
+  bool remote_peer_supports_msid_ RTC_GUARDED_BY(signaling_thread()) = false;
+  bool is_negotiation_needed_ RTC_GUARDED_BY(signaling_thread()) = false;
+  uint32_t negotiation_needed_event_id_ = 0;
+  bool update_negotiation_needed_on_empty_chain_
+      RTC_GUARDED_BY(signaling_thread()) = false;
+
+  // In Unified Plan, if we encounter remote SDP that does not contain an a=msid
+  // line we create and use a stream with a random ID for our receivers. This is
+  // to support legacy endpoints that do not support the a=msid attribute (as
+  // opposed to streamless tracks with "a=msid:-").
+  rtc::scoped_refptr<MediaStreamInterface> missing_msid_default_stream_
+      RTC_GUARDED_BY(signaling_thread());
+
+  rtc::WeakPtrFactory<SdpOfferAnswerHandler> weak_ptr_factory_
+      RTC_GUARDED_BY(signaling_thread());
+};
+
+}  // namespace webrtc
+
+#endif  // PC_SDP_OFFER_ANSWER_H_