/*
 *  Copyright (c) 2017 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.
 */
 /* eslint-env node */
'use strict';

var SDPUtils = require('sdp');

function fixStatsType(stat) {
  return {
    inboundrtp: 'inbound-rtp',
    outboundrtp: 'outbound-rtp',
    candidatepair: 'candidate-pair',
    localcandidate: 'local-candidate',
    remotecandidate: 'remote-candidate'
  }[stat.type] || stat.type;
}

function writeMediaSection(transceiver, caps, type, stream, dtlsRole) {
  var sdp = SDPUtils.writeRtpDescription(transceiver.kind, caps);

  // Map ICE parameters (ufrag, pwd) to SDP.
  sdp += SDPUtils.writeIceParameters(
      transceiver.iceGatherer.getLocalParameters());

  // Map DTLS parameters to SDP.
  sdp += SDPUtils.writeDtlsParameters(
      transceiver.dtlsTransport.getLocalParameters(),
      type === 'offer' ? 'actpass' : dtlsRole || 'active');

  sdp += 'a=mid:' + transceiver.mid + '\r\n';

  if (transceiver.rtpSender && transceiver.rtpReceiver) {
    sdp += 'a=sendrecv\r\n';
  } else if (transceiver.rtpSender) {
    sdp += 'a=sendonly\r\n';
  } else if (transceiver.rtpReceiver) {
    sdp += 'a=recvonly\r\n';
  } else {
    sdp += 'a=inactive\r\n';
  }

  if (transceiver.rtpSender) {
    var trackId = transceiver.rtpSender._initialTrackId ||
        transceiver.rtpSender.track.id;
    transceiver.rtpSender._initialTrackId = trackId;
    // spec.
    var msid = 'msid:' + (stream ? stream.id : '-') + ' ' +
        trackId + '\r\n';
    sdp += 'a=' + msid;
    // for Chrome. Legacy should no longer be required.
    sdp += 'a=ssrc:' + transceiver.sendEncodingParameters[0].ssrc +
        ' ' + msid;

    // RTX
    if (transceiver.sendEncodingParameters[0].rtx) {
      sdp += 'a=ssrc:' + transceiver.sendEncodingParameters[0].rtx.ssrc +
          ' ' + msid;
      sdp += 'a=ssrc-group:FID ' +
          transceiver.sendEncodingParameters[0].ssrc + ' ' +
          transceiver.sendEncodingParameters[0].rtx.ssrc +
          '\r\n';
    }
  }
  // FIXME: this should be written by writeRtpDescription.
  sdp += 'a=ssrc:' + transceiver.sendEncodingParameters[0].ssrc +
      ' cname:' + SDPUtils.localCName + '\r\n';
  if (transceiver.rtpSender && transceiver.sendEncodingParameters[0].rtx) {
    sdp += 'a=ssrc:' + transceiver.sendEncodingParameters[0].rtx.ssrc +
        ' cname:' + SDPUtils.localCName + '\r\n';
  }
  return sdp;
}

// Edge does not like
// 1) stun: filtered after 14393 unless ?transport=udp is present
// 2) turn: that does not have all of turn:host:port?transport=udp
// 3) turn: with ipv6 addresses
// 4) turn: occurring muliple times
function filterIceServers(iceServers, edgeVersion) {
  var hasTurn = false;
  iceServers = JSON.parse(JSON.stringify(iceServers));
  return iceServers.filter(function(server) {
    if (server && (server.urls || server.url)) {
      var urls = server.urls || server.url;
      if (server.url && !server.urls) {
        console.warn('RTCIceServer.url is deprecated! Use urls instead.');
      }
      var isString = typeof urls === 'string';
      if (isString) {
        urls = [urls];
      }
      urls = urls.filter(function(url) {
        var validTurn = url.indexOf('turn:') === 0 &&
            url.indexOf('transport=udp') !== -1 &&
            url.indexOf('turn:[') === -1 &&
            !hasTurn;

        if (validTurn) {
          hasTurn = true;
          return true;
        }
        return url.indexOf('stun:') === 0 && edgeVersion >= 14393 &&
            url.indexOf('?transport=udp') === -1;
      });

      delete server.url;
      server.urls = isString ? urls[0] : urls;
      return !!urls.length;
    }
  });
}

// Determines the intersection of local and remote capabilities.
function getCommonCapabilities(localCapabilities, remoteCapabilities) {
  var commonCapabilities = {
    codecs: [],
    headerExtensions: [],
    fecMechanisms: []
  };

  var findCodecByPayloadType = function(pt, codecs) {
    pt = parseInt(pt, 10);
    for (var i = 0; i < codecs.length; i++) {
      if (codecs[i].payloadType === pt ||
          codecs[i].preferredPayloadType === pt) {
        return codecs[i];
      }
    }
  };

  var rtxCapabilityMatches = function(lRtx, rRtx, lCodecs, rCodecs) {
    var lCodec = findCodecByPayloadType(lRtx.parameters.apt, lCodecs);
    var rCodec = findCodecByPayloadType(rRtx.parameters.apt, rCodecs);
    return lCodec && rCodec &&
        lCodec.name.toLowerCase() === rCodec.name.toLowerCase();
  };

  localCapabilities.codecs.forEach(function(lCodec) {
    for (var i = 0; i < remoteCapabilities.codecs.length; i++) {
      var rCodec = remoteCapabilities.codecs[i];
      if (lCodec.name.toLowerCase() === rCodec.name.toLowerCase() &&
          lCodec.clockRate === rCodec.clockRate) {
        if (lCodec.name.toLowerCase() === 'rtx' &&
            lCodec.parameters && rCodec.parameters.apt) {
          // for RTX we need to find the local rtx that has a apt
          // which points to the same local codec as the remote one.
          if (!rtxCapabilityMatches(lCodec, rCodec,
              localCapabilities.codecs, remoteCapabilities.codecs)) {
            continue;
          }
        }
        rCodec = JSON.parse(JSON.stringify(rCodec)); // deepcopy
        // number of channels is the highest common number of channels
        rCodec.numChannels = Math.min(lCodec.numChannels,
            rCodec.numChannels);
        // push rCodec so we reply with offerer payload type
        commonCapabilities.codecs.push(rCodec);

        // determine common feedback mechanisms
        rCodec.rtcpFeedback = rCodec.rtcpFeedback.filter(function(fb) {
          for (var j = 0; j < lCodec.rtcpFeedback.length; j++) {
            if (lCodec.rtcpFeedback[j].type === fb.type &&
                lCodec.rtcpFeedback[j].parameter === fb.parameter) {
              return true;
            }
          }
          return false;
        });
        // FIXME: also need to determine .parameters
        //  see https://github.com/openpeer/ortc/issues/569
        break;
      }
    }
  });

  localCapabilities.headerExtensions.forEach(function(lHeaderExtension) {
    for (var i = 0; i < remoteCapabilities.headerExtensions.length;
         i++) {
      var rHeaderExtension = remoteCapabilities.headerExtensions[i];
      if (lHeaderExtension.uri === rHeaderExtension.uri) {
        commonCapabilities.headerExtensions.push(rHeaderExtension);
        break;
      }
    }
  });

  // FIXME: fecMechanisms
  return commonCapabilities;
}

// is action=setLocalDescription with type allowed in signalingState
function isActionAllowedInSignalingState(action, type, signalingState) {
  return {
    offer: {
      setLocalDescription: ['stable', 'have-local-offer'],
      setRemoteDescription: ['stable', 'have-remote-offer']
    },
    answer: {
      setLocalDescription: ['have-remote-offer', 'have-local-pranswer'],
      setRemoteDescription: ['have-local-offer', 'have-remote-pranswer']
    }
  }[type][action].indexOf(signalingState) !== -1;
}

function maybeAddCandidate(iceTransport, candidate) {
  // Edge's internal representation adds some fields therefore
  // not all fieldѕ are taken into account.
  var alreadyAdded = iceTransport.getRemoteCandidates()
      .find(function(remoteCandidate) {
        return candidate.foundation === remoteCandidate.foundation &&
            candidate.ip === remoteCandidate.ip &&
            candidate.port === remoteCandidate.port &&
            candidate.priority === remoteCandidate.priority &&
            candidate.protocol === remoteCandidate.protocol &&
            candidate.type === remoteCandidate.type;
      });
  if (!alreadyAdded) {
    iceTransport.addRemoteCandidate(candidate);
  }
  return !alreadyAdded;
}


function makeError(name, description) {
  var e = new Error(description);
  e.name = name;
  // legacy error codes from https://heycam.github.io/webidl/#idl-DOMException-error-names
  e.code = {
    NotSupportedError: 9,
    InvalidStateError: 11,
    InvalidAccessError: 15,
    TypeError: undefined,
    OperationError: undefined
  }[name];
  return e;
}

module.exports = function(window, edgeVersion) {
  // https://w3c.github.io/mediacapture-main/#mediastream
  // Helper function to add the track to the stream and
  // dispatch the event ourselves.
  function addTrackToStreamAndFireEvent(track, stream) {
    stream.addTrack(track);
    stream.dispatchEvent(new window.MediaStreamTrackEvent('addtrack',
        {track: track}));
  }

  function removeTrackFromStreamAndFireEvent(track, stream) {
    stream.removeTrack(track);
    stream.dispatchEvent(new window.MediaStreamTrackEvent('removetrack',
        {track: track}));
  }

  function fireAddTrack(pc, track, receiver, streams) {
    var trackEvent = new Event('track');
    trackEvent.track = track;
    trackEvent.receiver = receiver;
    trackEvent.transceiver = {receiver: receiver};
    trackEvent.streams = streams;
    window.setTimeout(function() {
      pc._dispatchEvent('track', trackEvent);
    });
  }

  var RTCPeerConnection = function(config) {
    var pc = this;

    var _eventTarget = document.createDocumentFragment();
    ['addEventListener', 'removeEventListener', 'dispatchEvent']
        .forEach(function(method) {
          pc[method] = _eventTarget[method].bind(_eventTarget);
        });

    this.canTrickleIceCandidates = null;

    this.needNegotiation = false;

    this.localStreams = [];
    this.remoteStreams = [];

    this._localDescription = null;
    this._remoteDescription = null;

    this.signalingState = 'stable';
    this.iceConnectionState = 'new';
    this.connectionState = 'new';
    this.iceGatheringState = 'new';

    config = JSON.parse(JSON.stringify(config || {}));

    this.usingBundle = config.bundlePolicy === 'max-bundle';
    if (config.rtcpMuxPolicy === 'negotiate') {
      throw(makeError('NotSupportedError',
          'rtcpMuxPolicy \'negotiate\' is not supported'));
    } else if (!config.rtcpMuxPolicy) {
      config.rtcpMuxPolicy = 'require';
    }

    switch (config.iceTransportPolicy) {
      case 'all':
      case 'relay':
        break;
      default:
        config.iceTransportPolicy = 'all';
        break;
    }

    switch (config.bundlePolicy) {
      case 'balanced':
      case 'max-compat':
      case 'max-bundle':
        break;
      default:
        config.bundlePolicy = 'balanced';
        break;
    }

    config.iceServers = filterIceServers(config.iceServers || [], edgeVersion);

    this._iceGatherers = [];
    if (config.iceCandidatePoolSize) {
      for (var i = config.iceCandidatePoolSize; i > 0; i--) {
        this._iceGatherers.push(new window.RTCIceGatherer({
          iceServers: config.iceServers,
          gatherPolicy: config.iceTransportPolicy
        }));
      }
    } else {
      config.iceCandidatePoolSize = 0;
    }

    this._config = config;

    // per-track iceGathers, iceTransports, dtlsTransports, rtpSenders, ...
    // everything that is needed to describe a SDP m-line.
    this.transceivers = [];

    this._sdpSessionId = SDPUtils.generateSessionId();
    this._sdpSessionVersion = 0;

    this._dtlsRole = undefined; // role for a=setup to use in answers.

    this._isClosed = false;
  };

  Object.defineProperty(RTCPeerConnection.prototype, 'localDescription', {
    configurable: true,
    get: function() {
      return this._localDescription;
    }
  });
  Object.defineProperty(RTCPeerConnection.prototype, 'remoteDescription', {
    configurable: true,
    get: function() {
      return this._remoteDescription;
    }
  });

  // set up event handlers on prototype
  RTCPeerConnection.prototype.onicecandidate = null;
  RTCPeerConnection.prototype.onaddstream = null;
  RTCPeerConnection.prototype.ontrack = null;
  RTCPeerConnection.prototype.onremovestream = null;
  RTCPeerConnection.prototype.onsignalingstatechange = null;
  RTCPeerConnection.prototype.oniceconnectionstatechange = null;
  RTCPeerConnection.prototype.onconnectionstatechange = null;
  RTCPeerConnection.prototype.onicegatheringstatechange = null;
  RTCPeerConnection.prototype.onnegotiationneeded = null;
  RTCPeerConnection.prototype.ondatachannel = null;

  RTCPeerConnection.prototype._dispatchEvent = function(name, event) {
    if (this._isClosed) {
      return;
    }
    this.dispatchEvent(event);
    if (typeof this['on' + name] === 'function') {
      this['on' + name](event);
    }
  };

  RTCPeerConnection.prototype._emitGatheringStateChange = function() {
    var event = new Event('icegatheringstatechange');
    this._dispatchEvent('icegatheringstatechange', event);
  };

  RTCPeerConnection.prototype.getConfiguration = function() {
    return this._config;
  };

  RTCPeerConnection.prototype.getLocalStreams = function() {
    return this.localStreams;
  };

  RTCPeerConnection.prototype.getRemoteStreams = function() {
    return this.remoteStreams;
  };

  // internal helper to create a transceiver object.
  // (which is not yet the same as the WebRTC 1.0 transceiver)
  RTCPeerConnection.prototype._createTransceiver = function(kind, doNotAdd) {
    var hasBundleTransport = this.transceivers.length > 0;
    var transceiver = {
      track: null,
      iceGatherer: null,
      iceTransport: null,
      dtlsTransport: null,
      localCapabilities: null,
      remoteCapabilities: null,
      rtpSender: null,
      rtpReceiver: null,
      kind: kind,
      mid: null,
      sendEncodingParameters: null,
      recvEncodingParameters: null,
      stream: null,
      associatedRemoteMediaStreams: [],
      wantReceive: true
    };
    if (this.usingBundle && hasBundleTransport) {
      transceiver.iceTransport = this.transceivers[0].iceTransport;
      transceiver.dtlsTransport = this.transceivers[0].dtlsTransport;
    } else {
      var transports = this._createIceAndDtlsTransports();
      transceiver.iceTransport = transports.iceTransport;
      transceiver.dtlsTransport = transports.dtlsTransport;
    }
    if (!doNotAdd) {
      this.transceivers.push(transceiver);
    }
    return transceiver;
  };

  RTCPeerConnection.prototype.addTrack = function(track, stream) {
    if (this._isClosed) {
      throw makeError('InvalidStateError',
          'Attempted to call addTrack on a closed peerconnection.');
    }

    var alreadyExists = this.transceivers.find(function(s) {
      return s.track === track;
    });

    if (alreadyExists) {
      throw makeError('InvalidAccessError', 'Track already exists.');
    }

    var transceiver;
    for (var i = 0; i < this.transceivers.length; i++) {
      if (!this.transceivers[i].track &&
          this.transceivers[i].kind === track.kind) {
        transceiver = this.transceivers[i];
      }
    }
    if (!transceiver) {
      transceiver = this._createTransceiver(track.kind);
    }

    this._maybeFireNegotiationNeeded();

    if (this.localStreams.indexOf(stream) === -1) {
      this.localStreams.push(stream);
    }

    transceiver.track = track;
    transceiver.stream = stream;
    transceiver.rtpSender = new window.RTCRtpSender(track,
        transceiver.dtlsTransport);
    return transceiver.rtpSender;
  };

  RTCPeerConnection.prototype.addStream = function(stream) {
    var pc = this;
    if (edgeVersion >= 15025) {
      stream.getTracks().forEach(function(track) {
        pc.addTrack(track, stream);
      });
    } else {
      // Clone is necessary for local demos mostly, attaching directly
      // to two different senders does not work (build 10547).
      // Fixed in 15025 (or earlier)
      var clonedStream = stream.clone();
      stream.getTracks().forEach(function(track, idx) {
        var clonedTrack = clonedStream.getTracks()[idx];
        track.addEventListener('enabled', function(event) {
          clonedTrack.enabled = event.enabled;
        });
      });
      clonedStream.getTracks().forEach(function(track) {
        pc.addTrack(track, clonedStream);
      });
    }
  };

  RTCPeerConnection.prototype.removeTrack = function(sender) {
    if (this._isClosed) {
      throw makeError('InvalidStateError',
          'Attempted to call removeTrack on a closed peerconnection.');
    }

    if (!(sender instanceof window.RTCRtpSender)) {
      throw new TypeError('Argument 1 of RTCPeerConnection.removeTrack ' +
          'does not implement interface RTCRtpSender.');
    }

    var transceiver = this.transceivers.find(function(t) {
      return t.rtpSender === sender;
    });

    if (!transceiver) {
      throw makeError('InvalidAccessError',
          'Sender was not created by this connection.');
    }
    var stream = transceiver.stream;

    transceiver.rtpSender.stop();
    transceiver.rtpSender = null;
    transceiver.track = null;
    transceiver.stream = null;

    // remove the stream from the set of local streams
    var localStreams = this.transceivers.map(function(t) {
      return t.stream;
    });
    if (localStreams.indexOf(stream) === -1 &&
        this.localStreams.indexOf(stream) > -1) {
      this.localStreams.splice(this.localStreams.indexOf(stream), 1);
    }

    this._maybeFireNegotiationNeeded();
  };

  RTCPeerConnection.prototype.removeStream = function(stream) {
    var pc = this;
    stream.getTracks().forEach(function(track) {
      var sender = pc.getSenders().find(function(s) {
        return s.track === track;
      });
      if (sender) {
        pc.removeTrack(sender);
      }
    });
  };

  RTCPeerConnection.prototype.getSenders = function() {
    return this.transceivers.filter(function(transceiver) {
      return !!transceiver.rtpSender;
    })
    .map(function(transceiver) {
      return transceiver.rtpSender;
    });
  };

  RTCPeerConnection.prototype.getReceivers = function() {
    return this.transceivers.filter(function(transceiver) {
      return !!transceiver.rtpReceiver;
    })
    .map(function(transceiver) {
      return transceiver.rtpReceiver;
    });
  };


  RTCPeerConnection.prototype._createIceGatherer = function(sdpMLineIndex,
      usingBundle) {
    var pc = this;
    if (usingBundle && sdpMLineIndex > 0) {
      return this.transceivers[0].iceGatherer;
    } else if (this._iceGatherers.length) {
      return this._iceGatherers.shift();
    }
    var iceGatherer = new window.RTCIceGatherer({
      iceServers: this._config.iceServers,
      gatherPolicy: this._config.iceTransportPolicy
    });
    Object.defineProperty(iceGatherer, 'state',
        {value: 'new', writable: true}
    );

    this.transceivers[sdpMLineIndex].bufferedCandidateEvents = [];
    this.transceivers[sdpMLineIndex].bufferCandidates = function(event) {
      var end = !event.candidate || Object.keys(event.candidate).length === 0;
      // polyfill since RTCIceGatherer.state is not implemented in
      // Edge 10547 yet.
      iceGatherer.state = end ? 'completed' : 'gathering';
      if (pc.transceivers[sdpMLineIndex].bufferedCandidateEvents !== null) {
        pc.transceivers[sdpMLineIndex].bufferedCandidateEvents.push(event);
      }
    };
    iceGatherer.addEventListener('localcandidate',
      this.transceivers[sdpMLineIndex].bufferCandidates);
    return iceGatherer;
  };

  // start gathering from an RTCIceGatherer.
  RTCPeerConnection.prototype._gather = function(mid, sdpMLineIndex) {
    var pc = this;
    var iceGatherer = this.transceivers[sdpMLineIndex].iceGatherer;
    if (iceGatherer.onlocalcandidate) {
      return;
    }
    var bufferedCandidateEvents =
      this.transceivers[sdpMLineIndex].bufferedCandidateEvents;
    this.transceivers[sdpMLineIndex].bufferedCandidateEvents = null;
    iceGatherer.removeEventListener('localcandidate',
      this.transceivers[sdpMLineIndex].bufferCandidates);
    iceGatherer.onlocalcandidate = function(evt) {
      if (pc.usingBundle && sdpMLineIndex > 0) {
        // if we know that we use bundle we can drop candidates with
        // ѕdpMLineIndex > 0. If we don't do this then our state gets
        // confused since we dispose the extra ice gatherer.
        return;
      }
      var event = new Event('icecandidate');
      event.candidate = {sdpMid: mid, sdpMLineIndex: sdpMLineIndex};

      var cand = evt.candidate;
      // Edge emits an empty object for RTCIceCandidateComplete‥
      var end = !cand || Object.keys(cand).length === 0;
      if (end) {
        // polyfill since RTCIceGatherer.state is not implemented in
        // Edge 10547 yet.
        if (iceGatherer.state === 'new' || iceGatherer.state === 'gathering') {
          iceGatherer.state = 'completed';
        }
      } else {
        if (iceGatherer.state === 'new') {
          iceGatherer.state = 'gathering';
        }
        // RTCIceCandidate doesn't have a component, needs to be added
        cand.component = 1;
        // also the usernameFragment. TODO: update SDP to take both variants.
        cand.ufrag = iceGatherer.getLocalParameters().usernameFragment;

        var serializedCandidate = SDPUtils.writeCandidate(cand);
        event.candidate = Object.assign(event.candidate,
            SDPUtils.parseCandidate(serializedCandidate));

        event.candidate.candidate = serializedCandidate;
        event.candidate.toJSON = function() {
          return {
            candidate: event.candidate.candidate,
            sdpMid: event.candidate.sdpMid,
            sdpMLineIndex: event.candidate.sdpMLineIndex,
            usernameFragment: event.candidate.usernameFragment
          };
        };
      }

      // update local description.
      var sections = SDPUtils.getMediaSections(pc._localDescription.sdp);
      if (!end) {
        sections[event.candidate.sdpMLineIndex] +=
            'a=' + event.candidate.candidate + '\r\n';
      } else {
        sections[event.candidate.sdpMLineIndex] +=
            'a=end-of-candidates\r\n';
      }
      pc._localDescription.sdp =
          SDPUtils.getDescription(pc._localDescription.sdp) +
          sections.join('');
      var complete = pc.transceivers.every(function(transceiver) {
        return transceiver.iceGatherer &&
            transceiver.iceGatherer.state === 'completed';
      });

      if (pc.iceGatheringState !== 'gathering') {
        pc.iceGatheringState = 'gathering';
        pc._emitGatheringStateChange();
      }

      // Emit candidate. Also emit null candidate when all gatherers are
      // complete.
      if (!end) {
        pc._dispatchEvent('icecandidate', event);
      }
      if (complete) {
        pc._dispatchEvent('icecandidate', new Event('icecandidate'));
        pc.iceGatheringState = 'complete';
        pc._emitGatheringStateChange();
      }
    };

    // emit already gathered candidates.
    window.setTimeout(function() {
      bufferedCandidateEvents.forEach(function(e) {
        iceGatherer.onlocalcandidate(e);
      });
    }, 0);
  };

  // Create ICE transport and DTLS transport.
  RTCPeerConnection.prototype._createIceAndDtlsTransports = function() {
    var pc = this;
    var iceTransport = new window.RTCIceTransport(null);
    iceTransport.onicestatechange = function() {
      pc._updateIceConnectionState();
      pc._updateConnectionState();
    };

    var dtlsTransport = new window.RTCDtlsTransport(iceTransport);
    dtlsTransport.ondtlsstatechange = function() {
      pc._updateConnectionState();
    };
    dtlsTransport.onerror = function() {
      // onerror does not set state to failed by itself.
      Object.defineProperty(dtlsTransport, 'state',
          {value: 'failed', writable: true});
      pc._updateConnectionState();
    };

    return {
      iceTransport: iceTransport,
      dtlsTransport: dtlsTransport
    };
  };

  // Destroy ICE gatherer, ICE transport and DTLS transport.
  // Without triggering the callbacks.
  RTCPeerConnection.prototype._disposeIceAndDtlsTransports = function(
      sdpMLineIndex) {
    var iceGatherer = this.transceivers[sdpMLineIndex].iceGatherer;
    if (iceGatherer) {
      delete iceGatherer.onlocalcandidate;
      delete this.transceivers[sdpMLineIndex].iceGatherer;
    }
    var iceTransport = this.transceivers[sdpMLineIndex].iceTransport;
    if (iceTransport) {
      delete iceTransport.onicestatechange;
      delete this.transceivers[sdpMLineIndex].iceTransport;
    }
    var dtlsTransport = this.transceivers[sdpMLineIndex].dtlsTransport;
    if (dtlsTransport) {
      delete dtlsTransport.ondtlsstatechange;
      delete dtlsTransport.onerror;
      delete this.transceivers[sdpMLineIndex].dtlsTransport;
    }
  };

  // Start the RTP Sender and Receiver for a transceiver.
  RTCPeerConnection.prototype._transceive = function(transceiver,
      send, recv) {
    var params = getCommonCapabilities(transceiver.localCapabilities,
        transceiver.remoteCapabilities);
    if (send && transceiver.rtpSender) {
      params.encodings = transceiver.sendEncodingParameters;
      params.rtcp = {
        cname: SDPUtils.localCName,
        compound: transceiver.rtcpParameters.compound
      };
      if (transceiver.recvEncodingParameters.length) {
        params.rtcp.ssrc = transceiver.recvEncodingParameters[0].ssrc;
      }
      transceiver.rtpSender.send(params);
    }
    if (recv && transceiver.rtpReceiver && params.codecs.length > 0) {
      // remove RTX field in Edge 14942
      if (transceiver.kind === 'video'
          && transceiver.recvEncodingParameters
          && edgeVersion < 15019) {
        transceiver.recvEncodingParameters.forEach(function(p) {
          delete p.rtx;
        });
      }
      if (transceiver.recvEncodingParameters.length) {
        params.encodings = transceiver.recvEncodingParameters;
      } else {
        params.encodings = [{}];
      }
      params.rtcp = {
        compound: transceiver.rtcpParameters.compound
      };
      if (transceiver.rtcpParameters.cname) {
        params.rtcp.cname = transceiver.rtcpParameters.cname;
      }
      if (transceiver.sendEncodingParameters.length) {
        params.rtcp.ssrc = transceiver.sendEncodingParameters[0].ssrc;
      }
      transceiver.rtpReceiver.receive(params);
    }
  };

  RTCPeerConnection.prototype.setLocalDescription = function(description) {
    var pc = this;

    // Note: pranswer is not supported.
    if (['offer', 'answer'].indexOf(description.type) === -1) {
      return Promise.reject(makeError('TypeError',
          'Unsupported type "' + description.type + '"'));
    }

    if (!isActionAllowedInSignalingState('setLocalDescription',
        description.type, pc.signalingState) || pc._isClosed) {
      return Promise.reject(makeError('InvalidStateError',
          'Can not set local ' + description.type +
          ' in state ' + pc.signalingState));
    }

    var sections;
    var sessionpart;
    if (description.type === 'offer') {
      // VERY limited support for SDP munging. Limited to:
      // * changing the order of codecs
      sections = SDPUtils.splitSections(description.sdp);
      sessionpart = sections.shift();
      sections.forEach(function(mediaSection, sdpMLineIndex) {
        var caps = SDPUtils.parseRtpParameters(mediaSection);
        pc.transceivers[sdpMLineIndex].localCapabilities = caps;
      });

      pc.transceivers.forEach(function(transceiver, sdpMLineIndex) {
        pc._gather(transceiver.mid, sdpMLineIndex);
      });
    } else if (description.type === 'answer') {
      sections = SDPUtils.splitSections(pc._remoteDescription.sdp);
      sessionpart = sections.shift();
      var isIceLite = SDPUtils.matchPrefix(sessionpart,
          'a=ice-lite').length > 0;
      sections.forEach(function(mediaSection, sdpMLineIndex) {
        var transceiver = pc.transceivers[sdpMLineIndex];
        var iceGatherer = transceiver.iceGatherer;
        var iceTransport = transceiver.iceTransport;
        var dtlsTransport = transceiver.dtlsTransport;
        var localCapabilities = transceiver.localCapabilities;
        var remoteCapabilities = transceiver.remoteCapabilities;

        // treat bundle-only as not-rejected.
        var rejected = SDPUtils.isRejected(mediaSection) &&
            SDPUtils.matchPrefix(mediaSection, 'a=bundle-only').length === 0;

        if (!rejected && !transceiver.rejected) {
          var remoteIceParameters = SDPUtils.getIceParameters(
              mediaSection, sessionpart);
          var remoteDtlsParameters = SDPUtils.getDtlsParameters(
              mediaSection, sessionpart);
          if (isIceLite) {
            remoteDtlsParameters.role = 'server';
          }

          if (!pc.usingBundle || sdpMLineIndex === 0) {
            pc._gather(transceiver.mid, sdpMLineIndex);
            if (iceTransport.state === 'new') {
              iceTransport.start(iceGatherer, remoteIceParameters,
                  isIceLite ? 'controlling' : 'controlled');
            }
            if (dtlsTransport.state === 'new') {
              dtlsTransport.start(remoteDtlsParameters);
            }
          }

          // Calculate intersection of capabilities.
          var params = getCommonCapabilities(localCapabilities,
              remoteCapabilities);

          // Start the RTCRtpSender. The RTCRtpReceiver for this
          // transceiver has already been started in setRemoteDescription.
          pc._transceive(transceiver,
              params.codecs.length > 0,
              false);
        }
      });
    }

    pc._localDescription = {
      type: description.type,
      sdp: description.sdp
    };
    if (description.type === 'offer') {
      pc._updateSignalingState('have-local-offer');
    } else {
      pc._updateSignalingState('stable');
    }

    return Promise.resolve();
  };

  RTCPeerConnection.prototype.setRemoteDescription = function(description) {
    var pc = this;

    // Note: pranswer is not supported.
    if (['offer', 'answer'].indexOf(description.type) === -1) {
      return Promise.reject(makeError('TypeError',
          'Unsupported type "' + description.type + '"'));
    }

    if (!isActionAllowedInSignalingState('setRemoteDescription',
        description.type, pc.signalingState) || pc._isClosed) {
      return Promise.reject(makeError('InvalidStateError',
          'Can not set remote ' + description.type +
          ' in state ' + pc.signalingState));
    }

    var streams = {};
    pc.remoteStreams.forEach(function(stream) {
      streams[stream.id] = stream;
    });
    var receiverList = [];
    var sections = SDPUtils.splitSections(description.sdp);
    var sessionpart = sections.shift();
    var isIceLite = SDPUtils.matchPrefix(sessionpart,
        'a=ice-lite').length > 0;
    var usingBundle = SDPUtils.matchPrefix(sessionpart,
        'a=group:BUNDLE ').length > 0;
    pc.usingBundle = usingBundle;
    var iceOptions = SDPUtils.matchPrefix(sessionpart,
        'a=ice-options:')[0];
    if (iceOptions) {
      pc.canTrickleIceCandidates = iceOptions.substr(14).split(' ')
          .indexOf('trickle') >= 0;
    } else {
      pc.canTrickleIceCandidates = false;
    }

    sections.forEach(function(mediaSection, sdpMLineIndex) {
      var lines = SDPUtils.splitLines(mediaSection);
      var kind = SDPUtils.getKind(mediaSection);
      // treat bundle-only as not-rejected.
      var rejected = SDPUtils.isRejected(mediaSection) &&
          SDPUtils.matchPrefix(mediaSection, 'a=bundle-only').length === 0;
      var protocol = lines[0].substr(2).split(' ')[2];

      var direction = SDPUtils.getDirection(mediaSection, sessionpart);
      var remoteMsid = SDPUtils.parseMsid(mediaSection);

      var mid = SDPUtils.getMid(mediaSection) || SDPUtils.generateIdentifier();

      // Reject datachannels which are not implemented yet.
      if (rejected || (kind === 'application' && (protocol === 'DTLS/SCTP' ||
          protocol === 'UDP/DTLS/SCTP'))) {
        // TODO: this is dangerous in the case where a non-rejected m-line
        //     becomes rejected.
        pc.transceivers[sdpMLineIndex] = {
          mid: mid,
          kind: kind,
          protocol: protocol,
          rejected: true
        };
        return;
      }

      if (!rejected && pc.transceivers[sdpMLineIndex] &&
          pc.transceivers[sdpMLineIndex].rejected) {
        // recycle a rejected transceiver.
        pc.transceivers[sdpMLineIndex] = pc._createTransceiver(kind, true);
      }

      var transceiver;
      var iceGatherer;
      var iceTransport;
      var dtlsTransport;
      var rtpReceiver;
      var sendEncodingParameters;
      var recvEncodingParameters;
      var localCapabilities;

      var track;
      // FIXME: ensure the mediaSection has rtcp-mux set.
      var remoteCapabilities = SDPUtils.parseRtpParameters(mediaSection);
      var remoteIceParameters;
      var remoteDtlsParameters;
      if (!rejected) {
        remoteIceParameters = SDPUtils.getIceParameters(mediaSection,
            sessionpart);
        remoteDtlsParameters = SDPUtils.getDtlsParameters(mediaSection,
            sessionpart);
        remoteDtlsParameters.role = 'client';
      }
      recvEncodingParameters =
          SDPUtils.parseRtpEncodingParameters(mediaSection);

      var rtcpParameters = SDPUtils.parseRtcpParameters(mediaSection);

      var isComplete = SDPUtils.matchPrefix(mediaSection,
          'a=end-of-candidates', sessionpart).length > 0;
      var cands = SDPUtils.matchPrefix(mediaSection, 'a=candidate:')
          .map(function(cand) {
            return SDPUtils.parseCandidate(cand);
          })
          .filter(function(cand) {
            return cand.component === 1;
          });

      // Check if we can use BUNDLE and dispose transports.
      if ((description.type === 'offer' || description.type === 'answer') &&
          !rejected && usingBundle && sdpMLineIndex > 0 &&
          pc.transceivers[sdpMLineIndex]) {
        pc._disposeIceAndDtlsTransports(sdpMLineIndex);
        pc.transceivers[sdpMLineIndex].iceGatherer =
            pc.transceivers[0].iceGatherer;
        pc.transceivers[sdpMLineIndex].iceTransport =
            pc.transceivers[0].iceTransport;
        pc.transceivers[sdpMLineIndex].dtlsTransport =
            pc.transceivers[0].dtlsTransport;
        if (pc.transceivers[sdpMLineIndex].rtpSender) {
          pc.transceivers[sdpMLineIndex].rtpSender.setTransport(
              pc.transceivers[0].dtlsTransport);
        }
        if (pc.transceivers[sdpMLineIndex].rtpReceiver) {
          pc.transceivers[sdpMLineIndex].rtpReceiver.setTransport(
              pc.transceivers[0].dtlsTransport);
        }
      }
      if (description.type === 'offer' && !rejected) {
        transceiver = pc.transceivers[sdpMLineIndex] ||
            pc._createTransceiver(kind);
        transceiver.mid = mid;

        if (!transceiver.iceGatherer) {
          transceiver.iceGatherer = pc._createIceGatherer(sdpMLineIndex,
              usingBundle);
        }

        if (cands.length && transceiver.iceTransport.state === 'new') {
          if (isComplete && (!usingBundle || sdpMLineIndex === 0)) {
            transceiver.iceTransport.setRemoteCandidates(cands);
          } else {
            cands.forEach(function(candidate) {
              maybeAddCandidate(transceiver.iceTransport, candidate);
            });
          }
        }

        localCapabilities = window.RTCRtpReceiver.getCapabilities(kind);

        // filter RTX until additional stuff needed for RTX is implemented
        // in adapter.js
        if (edgeVersion < 15019) {
          localCapabilities.codecs = localCapabilities.codecs.filter(
              function(codec) {
                return codec.name !== 'rtx';
              });
        }

        sendEncodingParameters = transceiver.sendEncodingParameters || [{
          ssrc: (2 * sdpMLineIndex + 2) * 1001
        }];

        // TODO: rewrite to use http://w3c.github.io/webrtc-pc/#set-associated-remote-streams
        var isNewTrack = false;
        if (direction === 'sendrecv' || direction === 'sendonly') {
          isNewTrack = !transceiver.rtpReceiver;
          rtpReceiver = transceiver.rtpReceiver ||
              new window.RTCRtpReceiver(transceiver.dtlsTransport, kind);

          if (isNewTrack) {
            var stream;
            track = rtpReceiver.track;
            // FIXME: does not work with Plan B.
            if (remoteMsid && remoteMsid.stream === '-') {
              // no-op. a stream id of '-' means: no associated stream.
            } else if (remoteMsid) {
              if (!streams[remoteMsid.stream]) {
                streams[remoteMsid.stream] = new window.MediaStream();
                Object.defineProperty(streams[remoteMsid.stream], 'id', {
                  get: function() {
                    return remoteMsid.stream;
                  }
                });
              }
              Object.defineProperty(track, 'id', {
                get: function() {
                  return remoteMsid.track;
                }
              });
              stream = streams[remoteMsid.stream];
            } else {
              if (!streams.default) {
                streams.default = new window.MediaStream();
              }
              stream = streams.default;
            }
            if (stream) {
              addTrackToStreamAndFireEvent(track, stream);
              transceiver.associatedRemoteMediaStreams.push(stream);
            }
            receiverList.push([track, rtpReceiver, stream]);
          }
        } else if (transceiver.rtpReceiver && transceiver.rtpReceiver.track) {
          transceiver.associatedRemoteMediaStreams.forEach(function(s) {
            var nativeTrack = s.getTracks().find(function(t) {
              return t.id === transceiver.rtpReceiver.track.id;
            });
            if (nativeTrack) {
              removeTrackFromStreamAndFireEvent(nativeTrack, s);
            }
          });
          transceiver.associatedRemoteMediaStreams = [];
        }

        transceiver.localCapabilities = localCapabilities;
        transceiver.remoteCapabilities = remoteCapabilities;
        transceiver.rtpReceiver = rtpReceiver;
        transceiver.rtcpParameters = rtcpParameters;
        transceiver.sendEncodingParameters = sendEncodingParameters;
        transceiver.recvEncodingParameters = recvEncodingParameters;

        // Start the RTCRtpReceiver now. The RTPSender is started in
        // setLocalDescription.
        pc._transceive(pc.transceivers[sdpMLineIndex],
            false,
            isNewTrack);
      } else if (description.type === 'answer' && !rejected) {
        transceiver = pc.transceivers[sdpMLineIndex];
        iceGatherer = transceiver.iceGatherer;
        iceTransport = transceiver.iceTransport;
        dtlsTransport = transceiver.dtlsTransport;
        rtpReceiver = transceiver.rtpReceiver;
        sendEncodingParameters = transceiver.sendEncodingParameters;
        localCapabilities = transceiver.localCapabilities;

        pc.transceivers[sdpMLineIndex].recvEncodingParameters =
            recvEncodingParameters;
        pc.transceivers[sdpMLineIndex].remoteCapabilities =
            remoteCapabilities;
        pc.transceivers[sdpMLineIndex].rtcpParameters = rtcpParameters;

        if (cands.length && iceTransport.state === 'new') {
          if ((isIceLite || isComplete) &&
              (!usingBundle || sdpMLineIndex === 0)) {
            iceTransport.setRemoteCandidates(cands);
          } else {
            cands.forEach(function(candidate) {
              maybeAddCandidate(transceiver.iceTransport, candidate);
            });
          }
        }

        if (!usingBundle || sdpMLineIndex === 0) {
          if (iceTransport.state === 'new') {
            iceTransport.start(iceGatherer, remoteIceParameters,
                'controlling');
          }
          if (dtlsTransport.state === 'new') {
            dtlsTransport.start(remoteDtlsParameters);
          }
        }

        // If the offer contained RTX but the answer did not,
        // remove RTX from sendEncodingParameters.
        var commonCapabilities = getCommonCapabilities(
          transceiver.localCapabilities,
          transceiver.remoteCapabilities);

        var hasRtx = commonCapabilities.codecs.filter(function(c) {
          return c.name.toLowerCase() === 'rtx';
        }).length;
        if (!hasRtx && transceiver.sendEncodingParameters[0].rtx) {
          delete transceiver.sendEncodingParameters[0].rtx;
        }

        pc._transceive(transceiver,
            direction === 'sendrecv' || direction === 'recvonly',
            direction === 'sendrecv' || direction === 'sendonly');

        // TODO: rewrite to use http://w3c.github.io/webrtc-pc/#set-associated-remote-streams
        if (rtpReceiver &&
            (direction === 'sendrecv' || direction === 'sendonly')) {
          track = rtpReceiver.track;
          if (remoteMsid) {
            if (!streams[remoteMsid.stream]) {
              streams[remoteMsid.stream] = new window.MediaStream();
            }
            addTrackToStreamAndFireEvent(track, streams[remoteMsid.stream]);
            receiverList.push([track, rtpReceiver, streams[remoteMsid.stream]]);
          } else {
            if (!streams.default) {
              streams.default = new window.MediaStream();
            }
            addTrackToStreamAndFireEvent(track, streams.default);
            receiverList.push([track, rtpReceiver, streams.default]);
          }
        } else {
          // FIXME: actually the receiver should be created later.
          delete transceiver.rtpReceiver;
        }
      }
    });

    if (pc._dtlsRole === undefined) {
      pc._dtlsRole = description.type === 'offer' ? 'active' : 'passive';
    }

    pc._remoteDescription = {
      type: description.type,
      sdp: description.sdp
    };
    if (description.type === 'offer') {
      pc._updateSignalingState('have-remote-offer');
    } else {
      pc._updateSignalingState('stable');
    }
    Object.keys(streams).forEach(function(sid) {
      var stream = streams[sid];
      if (stream.getTracks().length) {
        if (pc.remoteStreams.indexOf(stream) === -1) {
          pc.remoteStreams.push(stream);
          var event = new Event('addstream');
          event.stream = stream;
          window.setTimeout(function() {
            pc._dispatchEvent('addstream', event);
          });
        }

        receiverList.forEach(function(item) {
          var track = item[0];
          var receiver = item[1];
          if (stream.id !== item[2].id) {
            return;
          }
          fireAddTrack(pc, track, receiver, [stream]);
        });
      }
    });
    receiverList.forEach(function(item) {
      if (item[2]) {
        return;
      }
      fireAddTrack(pc, item[0], item[1], []);
    });

    // check whether addIceCandidate({}) was called within four seconds after
    // setRemoteDescription.
    window.setTimeout(function() {
      if (!(pc && pc.transceivers)) {
        return;
      }
      pc.transceivers.forEach(function(transceiver) {
        if (transceiver.iceTransport &&
            transceiver.iceTransport.state === 'new' &&
            transceiver.iceTransport.getRemoteCandidates().length > 0) {
          console.warn('Timeout for addRemoteCandidate. Consider sending ' +
              'an end-of-candidates notification');
          transceiver.iceTransport.addRemoteCandidate({});
        }
      });
    }, 4000);

    return Promise.resolve();
  };

  RTCPeerConnection.prototype.close = function() {
    this.transceivers.forEach(function(transceiver) {
      /* not yet
      if (transceiver.iceGatherer) {
        transceiver.iceGatherer.close();
      }
      */
      if (transceiver.iceTransport) {
        transceiver.iceTransport.stop();
      }
      if (transceiver.dtlsTransport) {
        transceiver.dtlsTransport.stop();
      }
      if (transceiver.rtpSender) {
        transceiver.rtpSender.stop();
      }
      if (transceiver.rtpReceiver) {
        transceiver.rtpReceiver.stop();
      }
    });
    // FIXME: clean up tracks, local streams, remote streams, etc
    this._isClosed = true;
    this._updateSignalingState('closed');
  };

  // Update the signaling state.
  RTCPeerConnection.prototype._updateSignalingState = function(newState) {
    this.signalingState = newState;
    var event = new Event('signalingstatechange');
    this._dispatchEvent('signalingstatechange', event);
  };

  // Determine whether to fire the negotiationneeded event.
  RTCPeerConnection.prototype._maybeFireNegotiationNeeded = function() {
    var pc = this;
    if (this.signalingState !== 'stable' || this.needNegotiation === true) {
      return;
    }
    this.needNegotiation = true;
    window.setTimeout(function() {
      if (pc.needNegotiation) {
        pc.needNegotiation = false;
        var event = new Event('negotiationneeded');
        pc._dispatchEvent('negotiationneeded', event);
      }
    }, 0);
  };

  // Update the ice connection state.
  RTCPeerConnection.prototype._updateIceConnectionState = function() {
    var newState;
    var states = {
      'new': 0,
      closed: 0,
      checking: 0,
      connected: 0,
      completed: 0,
      disconnected: 0,
      failed: 0
    };
    this.transceivers.forEach(function(transceiver) {
      if (transceiver.iceTransport && !transceiver.rejected) {
        states[transceiver.iceTransport.state]++;
      }
    });

    newState = 'new';
    if (states.failed > 0) {
      newState = 'failed';
    } else if (states.checking > 0) {
      newState = 'checking';
    } else if (states.disconnected > 0) {
      newState = 'disconnected';
    } else if (states.new > 0) {
      newState = 'new';
    } else if (states.connected > 0) {
      newState = 'connected';
    } else if (states.completed > 0) {
      newState = 'completed';
    }

    if (newState !== this.iceConnectionState) {
      this.iceConnectionState = newState;
      var event = new Event('iceconnectionstatechange');
      this._dispatchEvent('iceconnectionstatechange', event);
    }
  };

  // Update the connection state.
  RTCPeerConnection.prototype._updateConnectionState = function() {
    var newState;
    var states = {
      'new': 0,
      closed: 0,
      connecting: 0,
      connected: 0,
      completed: 0,
      disconnected: 0,
      failed: 0
    };
    this.transceivers.forEach(function(transceiver) {
      if (transceiver.iceTransport && transceiver.dtlsTransport &&
          !transceiver.rejected) {
        states[transceiver.iceTransport.state]++;
        states[transceiver.dtlsTransport.state]++;
      }
    });
    // ICETransport.completed and connected are the same for this purpose.
    states.connected += states.completed;

    newState = 'new';
    if (states.failed > 0) {
      newState = 'failed';
    } else if (states.connecting > 0) {
      newState = 'connecting';
    } else if (states.disconnected > 0) {
      newState = 'disconnected';
    } else if (states.new > 0) {
      newState = 'new';
    } else if (states.connected > 0) {
      newState = 'connected';
    }

    if (newState !== this.connectionState) {
      this.connectionState = newState;
      var event = new Event('connectionstatechange');
      this._dispatchEvent('connectionstatechange', event);
    }
  };

  RTCPeerConnection.prototype.createOffer = function() {
    var pc = this;

    if (pc._isClosed) {
      return Promise.reject(makeError('InvalidStateError',
          'Can not call createOffer after close'));
    }

    var numAudioTracks = pc.transceivers.filter(function(t) {
      return t.kind === 'audio';
    }).length;
    var numVideoTracks = pc.transceivers.filter(function(t) {
      return t.kind === 'video';
    }).length;

    // Determine number of audio and video tracks we need to send/recv.
    var offerOptions = arguments[0];
    if (offerOptions) {
      // Reject Chrome legacy constraints.
      if (offerOptions.mandatory || offerOptions.optional) {
        throw new TypeError(
            'Legacy mandatory/optional constraints not supported.');
      }
      if (offerOptions.offerToReceiveAudio !== undefined) {
        if (offerOptions.offerToReceiveAudio === true) {
          numAudioTracks = 1;
        } else if (offerOptions.offerToReceiveAudio === false) {
          numAudioTracks = 0;
        } else {
          numAudioTracks = offerOptions.offerToReceiveAudio;
        }
      }
      if (offerOptions.offerToReceiveVideo !== undefined) {
        if (offerOptions.offerToReceiveVideo === true) {
          numVideoTracks = 1;
        } else if (offerOptions.offerToReceiveVideo === false) {
          numVideoTracks = 0;
        } else {
          numVideoTracks = offerOptions.offerToReceiveVideo;
        }
      }
    }

    pc.transceivers.forEach(function(transceiver) {
      if (transceiver.kind === 'audio') {
        numAudioTracks--;
        if (numAudioTracks < 0) {
          transceiver.wantReceive = false;
        }
      } else if (transceiver.kind === 'video') {
        numVideoTracks--;
        if (numVideoTracks < 0) {
          transceiver.wantReceive = false;
        }
      }
    });

    // Create M-lines for recvonly streams.
    while (numAudioTracks > 0 || numVideoTracks > 0) {
      if (numAudioTracks > 0) {
        pc._createTransceiver('audio');
        numAudioTracks--;
      }
      if (numVideoTracks > 0) {
        pc._createTransceiver('video');
        numVideoTracks--;
      }
    }

    var sdp = SDPUtils.writeSessionBoilerplate(pc._sdpSessionId,
        pc._sdpSessionVersion++);
    pc.transceivers.forEach(function(transceiver, sdpMLineIndex) {
      // For each track, create an ice gatherer, ice transport,
      // dtls transport, potentially rtpsender and rtpreceiver.
      var track = transceiver.track;
      var kind = transceiver.kind;
      var mid = transceiver.mid || SDPUtils.generateIdentifier();
      transceiver.mid = mid;

      if (!transceiver.iceGatherer) {
        transceiver.iceGatherer = pc._createIceGatherer(sdpMLineIndex,
            pc.usingBundle);
      }

      var localCapabilities = window.RTCRtpSender.getCapabilities(kind);
      // filter RTX until additional stuff needed for RTX is implemented
      // in adapter.js
      if (edgeVersion < 15019) {
        localCapabilities.codecs = localCapabilities.codecs.filter(
            function(codec) {
              return codec.name !== 'rtx';
            });
      }
      localCapabilities.codecs.forEach(function(codec) {
        // work around https://bugs.chromium.org/p/webrtc/issues/detail?id=6552
        // by adding level-asymmetry-allowed=1
        if (codec.name === 'H264' &&
            codec.parameters['level-asymmetry-allowed'] === undefined) {
          codec.parameters['level-asymmetry-allowed'] = '1';
        }

        // for subsequent offers, we might have to re-use the payload
        // type of the last offer.
        if (transceiver.remoteCapabilities &&
            transceiver.remoteCapabilities.codecs) {
          transceiver.remoteCapabilities.codecs.forEach(function(remoteCodec) {
            if (codec.name.toLowerCase() === remoteCodec.name.toLowerCase() &&
                codec.clockRate === remoteCodec.clockRate) {
              codec.preferredPayloadType = remoteCodec.payloadType;
            }
          });
        }
      });
      localCapabilities.headerExtensions.forEach(function(hdrExt) {
        var remoteExtensions = transceiver.remoteCapabilities &&
            transceiver.remoteCapabilities.headerExtensions || [];
        remoteExtensions.forEach(function(rHdrExt) {
          if (hdrExt.uri === rHdrExt.uri) {
            hdrExt.id = rHdrExt.id;
          }
        });
      });

      // generate an ssrc now, to be used later in rtpSender.send
      var sendEncodingParameters = transceiver.sendEncodingParameters || [{
        ssrc: (2 * sdpMLineIndex + 1) * 1001
      }];
      if (track) {
        // add RTX
        if (edgeVersion >= 15019 && kind === 'video' &&
            !sendEncodingParameters[0].rtx) {
          sendEncodingParameters[0].rtx = {
            ssrc: sendEncodingParameters[0].ssrc + 1
          };
        }
      }

      if (transceiver.wantReceive) {
        transceiver.rtpReceiver = new window.RTCRtpReceiver(
            transceiver.dtlsTransport, kind);
      }

      transceiver.localCapabilities = localCapabilities;
      transceiver.sendEncodingParameters = sendEncodingParameters;
    });

    // always offer BUNDLE and dispose on return if not supported.
    if (pc._config.bundlePolicy !== 'max-compat') {
      sdp += 'a=group:BUNDLE ' + pc.transceivers.map(function(t) {
        return t.mid;
      }).join(' ') + '\r\n';
    }
    sdp += 'a=ice-options:trickle\r\n';

    pc.transceivers.forEach(function(transceiver, sdpMLineIndex) {
      sdp += writeMediaSection(transceiver, transceiver.localCapabilities,
          'offer', transceiver.stream, pc._dtlsRole);
      sdp += 'a=rtcp-rsize\r\n';

      if (transceiver.iceGatherer && pc.iceGatheringState !== 'new' &&
          (sdpMLineIndex === 0 || !pc.usingBundle)) {
        transceiver.iceGatherer.getLocalCandidates().forEach(function(cand) {
          cand.component = 1;
          sdp += 'a=' + SDPUtils.writeCandidate(cand) + '\r\n';
        });

        if (transceiver.iceGatherer.state === 'completed') {
          sdp += 'a=end-of-candidates\r\n';
        }
      }
    });

    var desc = new window.RTCSessionDescription({
      type: 'offer',
      sdp: sdp
    });
    return Promise.resolve(desc);
  };

  RTCPeerConnection.prototype.createAnswer = function() {
    var pc = this;

    if (pc._isClosed) {
      return Promise.reject(makeError('InvalidStateError',
          'Can not call createAnswer after close'));
    }

    if (!(pc.signalingState === 'have-remote-offer' ||
        pc.signalingState === 'have-local-pranswer')) {
      return Promise.reject(makeError('InvalidStateError',
          'Can not call createAnswer in signalingState ' + pc.signalingState));
    }

    var sdp = SDPUtils.writeSessionBoilerplate(pc._sdpSessionId,
        pc._sdpSessionVersion++);
    if (pc.usingBundle) {
      sdp += 'a=group:BUNDLE ' + pc.transceivers.map(function(t) {
        return t.mid;
      }).join(' ') + '\r\n';
    }
    sdp += 'a=ice-options:trickle\r\n';

    var mediaSectionsInOffer = SDPUtils.getMediaSections(
        pc._remoteDescription.sdp).length;
    pc.transceivers.forEach(function(transceiver, sdpMLineIndex) {
      if (sdpMLineIndex + 1 > mediaSectionsInOffer) {
        return;
      }
      if (transceiver.rejected) {
        if (transceiver.kind === 'application') {
          if (transceiver.protocol === 'DTLS/SCTP') { // legacy fmt
            sdp += 'm=application 0 DTLS/SCTP 5000\r\n';
          } else {
            sdp += 'm=application 0 ' + transceiver.protocol +
                ' webrtc-datachannel\r\n';
          }
        } else if (transceiver.kind === 'audio') {
          sdp += 'm=audio 0 UDP/TLS/RTP/SAVPF 0\r\n' +
              'a=rtpmap:0 PCMU/8000\r\n';
        } else if (transceiver.kind === 'video') {
          sdp += 'm=video 0 UDP/TLS/RTP/SAVPF 120\r\n' +
              'a=rtpmap:120 VP8/90000\r\n';
        }
        sdp += 'c=IN IP4 0.0.0.0\r\n' +
            'a=inactive\r\n' +
            'a=mid:' + transceiver.mid + '\r\n';
        return;
      }

      // FIXME: look at direction.
      if (transceiver.stream) {
        var localTrack;
        if (transceiver.kind === 'audio') {
          localTrack = transceiver.stream.getAudioTracks()[0];
        } else if (transceiver.kind === 'video') {
          localTrack = transceiver.stream.getVideoTracks()[0];
        }
        if (localTrack) {
          // add RTX
          if (edgeVersion >= 15019 && transceiver.kind === 'video' &&
              !transceiver.sendEncodingParameters[0].rtx) {
            transceiver.sendEncodingParameters[0].rtx = {
              ssrc: transceiver.sendEncodingParameters[0].ssrc + 1
            };
          }
        }
      }

      // Calculate intersection of capabilities.
      var commonCapabilities = getCommonCapabilities(
          transceiver.localCapabilities,
          transceiver.remoteCapabilities);

      var hasRtx = commonCapabilities.codecs.filter(function(c) {
        return c.name.toLowerCase() === 'rtx';
      }).length;
      if (!hasRtx && transceiver.sendEncodingParameters[0].rtx) {
        delete transceiver.sendEncodingParameters[0].rtx;
      }

      sdp += writeMediaSection(transceiver, commonCapabilities,
          'answer', transceiver.stream, pc._dtlsRole);
      if (transceiver.rtcpParameters &&
          transceiver.rtcpParameters.reducedSize) {
        sdp += 'a=rtcp-rsize\r\n';
      }
    });

    var desc = new window.RTCSessionDescription({
      type: 'answer',
      sdp: sdp
    });
    return Promise.resolve(desc);
  };

  RTCPeerConnection.prototype.addIceCandidate = function(candidate) {
    var pc = this;
    var sections;
    if (candidate && !(candidate.sdpMLineIndex !== undefined ||
        candidate.sdpMid)) {
      return Promise.reject(new TypeError('sdpMLineIndex or sdpMid required'));
    }

    // TODO: needs to go into ops queue.
    return new Promise(function(resolve, reject) {
      if (!pc._remoteDescription) {
        return reject(makeError('InvalidStateError',
            'Can not add ICE candidate without a remote description'));
      } else if (!candidate || candidate.candidate === '') {
        for (var j = 0; j < pc.transceivers.length; j++) {
          if (pc.transceivers[j].rejected) {
            continue;
          }
          pc.transceivers[j].iceTransport.addRemoteCandidate({});
          sections = SDPUtils.getMediaSections(pc._remoteDescription.sdp);
          sections[j] += 'a=end-of-candidates\r\n';
          pc._remoteDescription.sdp =
              SDPUtils.getDescription(pc._remoteDescription.sdp) +
              sections.join('');
          if (pc.usingBundle) {
            break;
          }
        }
      } else {
        var sdpMLineIndex = candidate.sdpMLineIndex;
        if (candidate.sdpMid) {
          for (var i = 0; i < pc.transceivers.length; i++) {
            if (pc.transceivers[i].mid === candidate.sdpMid) {
              sdpMLineIndex = i;
              break;
            }
          }
        }
        var transceiver = pc.transceivers[sdpMLineIndex];
        if (transceiver) {
          if (transceiver.rejected) {
            return resolve();
          }
          var cand = Object.keys(candidate.candidate).length > 0 ?
              SDPUtils.parseCandidate(candidate.candidate) : {};
          // Ignore Chrome's invalid candidates since Edge does not like them.
          if (cand.protocol === 'tcp' && (cand.port === 0 || cand.port === 9)) {
            return resolve();
          }
          // Ignore RTCP candidates, we assume RTCP-MUX.
          if (cand.component && cand.component !== 1) {
            return resolve();
          }
          // when using bundle, avoid adding candidates to the wrong
          // ice transport. And avoid adding candidates added in the SDP.
          if (sdpMLineIndex === 0 || (sdpMLineIndex > 0 &&
              transceiver.iceTransport !== pc.transceivers[0].iceTransport)) {
            if (!maybeAddCandidate(transceiver.iceTransport, cand)) {
              return reject(makeError('OperationError',
                  'Can not add ICE candidate'));
            }
          }

          // update the remoteDescription.
          var candidateString = candidate.candidate.trim();
          if (candidateString.indexOf('a=') === 0) {
            candidateString = candidateString.substr(2);
          }
          sections = SDPUtils.getMediaSections(pc._remoteDescription.sdp);
          sections[sdpMLineIndex] += 'a=' +
              (cand.type ? candidateString : 'end-of-candidates')
              + '\r\n';
          pc._remoteDescription.sdp =
              SDPUtils.getDescription(pc._remoteDescription.sdp) +
              sections.join('');
        } else {
          return reject(makeError('OperationError',
              'Can not add ICE candidate'));
        }
      }
      resolve();
    });
  };

  RTCPeerConnection.prototype.getStats = function(selector) {
    if (selector && selector instanceof window.MediaStreamTrack) {
      var senderOrReceiver = null;
      this.transceivers.forEach(function(transceiver) {
        if (transceiver.rtpSender &&
            transceiver.rtpSender.track === selector) {
          senderOrReceiver = transceiver.rtpSender;
        } else if (transceiver.rtpReceiver &&
            transceiver.rtpReceiver.track === selector) {
          senderOrReceiver = transceiver.rtpReceiver;
        }
      });
      if (!senderOrReceiver) {
        throw makeError('InvalidAccessError', 'Invalid selector.');
      }
      return senderOrReceiver.getStats();
    }

    var promises = [];
    this.transceivers.forEach(function(transceiver) {
      ['rtpSender', 'rtpReceiver', 'iceGatherer', 'iceTransport',
          'dtlsTransport'].forEach(function(method) {
            if (transceiver[method]) {
              promises.push(transceiver[method].getStats());
            }
          });
    });
    return Promise.all(promises).then(function(allStats) {
      var results = new Map();
      allStats.forEach(function(stats) {
        stats.forEach(function(stat) {
          results.set(stat.id, stat);
        });
      });
      return results;
    });
  };

  // fix low-level stat names and return Map instead of object.
  var ortcObjects = ['RTCRtpSender', 'RTCRtpReceiver', 'RTCIceGatherer',
    'RTCIceTransport', 'RTCDtlsTransport'];
  ortcObjects.forEach(function(ortcObjectName) {
    var obj = window[ortcObjectName];
    if (obj && obj.prototype && obj.prototype.getStats) {
      var nativeGetstats = obj.prototype.getStats;
      obj.prototype.getStats = function() {
        return nativeGetstats.apply(this)
        .then(function(nativeStats) {
          var mapStats = new Map();
          Object.keys(nativeStats).forEach(function(id) {
            nativeStats[id].type = fixStatsType(nativeStats[id]);
            mapStats.set(id, nativeStats[id]);
          });
          return mapStats;
        });
      };
    }
  });

  // legacy callback shims. Should be moved to adapter.js some days.
  var methods = ['createOffer', 'createAnswer'];
  methods.forEach(function(method) {
    var nativeMethod = RTCPeerConnection.prototype[method];
    RTCPeerConnection.prototype[method] = function() {
      var args = arguments;
      if (typeof args[0] === 'function' ||
          typeof args[1] === 'function') { // legacy
        return nativeMethod.apply(this, [arguments[2]])
        .then(function(description) {
          if (typeof args[0] === 'function') {
            args[0].apply(null, [description]);
          }
        }, function(error) {
          if (typeof args[1] === 'function') {
            args[1].apply(null, [error]);
          }
        });
      }
      return nativeMethod.apply(this, arguments);
    };
  });

  methods = ['setLocalDescription', 'setRemoteDescription', 'addIceCandidate'];
  methods.forEach(function(method) {
    var nativeMethod = RTCPeerConnection.prototype[method];
    RTCPeerConnection.prototype[method] = function() {
      var args = arguments;
      if (typeof args[1] === 'function' ||
          typeof args[2] === 'function') { // legacy
        return nativeMethod.apply(this, arguments)
        .then(function() {
          if (typeof args[1] === 'function') {
            args[1].apply(null);
          }
        }, function(error) {
          if (typeof args[2] === 'function') {
            args[2].apply(null, [error]);
          }
        });
      }
      return nativeMethod.apply(this, arguments);
    };
  });

  // getStats is special. It doesn't have a spec legacy method yet we support
  // getStats(something, cb) without error callbacks.
  ['getStats'].forEach(function(method) {
    var nativeMethod = RTCPeerConnection.prototype[method];
    RTCPeerConnection.prototype[method] = function() {
      var args = arguments;
      if (typeof args[1] === 'function') {
        return nativeMethod.apply(this, arguments)
        .then(function() {
          if (typeof args[1] === 'function') {
            args[1].apply(null);
          }
        });
      }
      return nativeMethod.apply(this, arguments);
    };
  });

  return RTCPeerConnection;
};