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diff --git a/third_party/libwebrtc/pc/sdp_serializer.cc b/third_party/libwebrtc/pc/sdp_serializer.cc
new file mode 100644
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+++ b/third_party/libwebrtc/pc/sdp_serializer.cc
@@ -0,0 +1,393 @@
+/*
+ *  Copyright 2018 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_serializer.h"
+
+#include <map>
+#include <string>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "absl/algorithm/container.h"
+#include "absl/types/optional.h"
+#include "modules/rtp_rtcp/include/rtp_rtcp_defines.h"
+#include "rtc_base/checks.h"
+#include "rtc_base/string_encode.h"
+#include "rtc_base/string_to_number.h"
+#include "rtc_base/strings/string_builder.h"
+
+using cricket::RidDescription;
+using cricket::RidDirection;
+using cricket::SimulcastDescription;
+using cricket::SimulcastLayer;
+using cricket::SimulcastLayerList;
+
+namespace webrtc {
+
+namespace {
+
+// delimiters
+const char kDelimiterComma[] = ",";
+const char kDelimiterCommaChar = ',';
+const char kDelimiterEqual[] = "=";
+const char kDelimiterEqualChar = '=';
+const char kDelimiterSemicolon[] = ";";
+const char kDelimiterSemicolonChar = ';';
+const char kDelimiterSpace[] = " ";
+const char kDelimiterSpaceChar = ' ';
+
+// https://tools.ietf.org/html/draft-ietf-mmusic-sdp-simulcast-13#section-5.1
+// https://tools.ietf.org/html/draft-ietf-mmusic-rid-15#section-10
+const char kSimulcastPausedStream[] = "~";
+const char kSimulcastPausedStreamChar = '~';
+const char kSendDirection[] = "send";
+const char kReceiveDirection[] = "recv";
+const char kPayloadType[] = "pt";
+
+RTCError ParseError(const std::string& message) {
+  return RTCError(RTCErrorType::SYNTAX_ERROR, message);
+}
+
+// These methods serialize simulcast according to the specification:
+// https://tools.ietf.org/html/draft-ietf-mmusic-sdp-simulcast-13#section-5.1
+rtc::StringBuilder& operator<<(rtc::StringBuilder& builder,
+                               const SimulcastLayer& simulcast_layer) {
+  if (simulcast_layer.is_paused) {
+    builder << kSimulcastPausedStream;
+  }
+  builder << simulcast_layer.rid;
+  return builder;
+}
+
+rtc::StringBuilder& operator<<(
+    rtc::StringBuilder& builder,
+    const std::vector<SimulcastLayer>& layer_alternatives) {
+  bool first = true;
+  for (const SimulcastLayer& rid : layer_alternatives) {
+    if (!first) {
+      builder << kDelimiterComma;
+    }
+    builder << rid;
+    first = false;
+  }
+  return builder;
+}
+
+rtc::StringBuilder& operator<<(rtc::StringBuilder& builder,
+                               const SimulcastLayerList& simulcast_layers) {
+  bool first = true;
+  for (const auto& alternatives : simulcast_layers) {
+    if (!first) {
+      builder << kDelimiterSemicolon;
+    }
+    builder << alternatives;
+    first = false;
+  }
+  return builder;
+}
+// This method deserializes simulcast according to the specification:
+// https://tools.ietf.org/html/draft-ietf-mmusic-sdp-simulcast-13#section-5.1
+// sc-str-list  = sc-alt-list *( ";" sc-alt-list )
+// sc-alt-list  = sc-id *( "," sc-id )
+// sc-id-paused = "~"
+// sc-id        = [sc-id-paused] rid-id
+// rid-id       = 1*(alpha-numeric / "-" / "_") ; see: I-D.ietf-mmusic-rid
+RTCErrorOr<SimulcastLayerList> ParseSimulcastLayerList(const std::string& str) {
+  std::vector<absl::string_view> tokens =
+      rtc::split(str, kDelimiterSemicolonChar);
+  if (tokens.empty()) {
+    return ParseError("Layer list cannot be empty.");
+  }
+
+  SimulcastLayerList result;
+  for (const absl::string_view& token : tokens) {
+    if (token.empty()) {
+      return ParseError("Simulcast alternative layer list is empty.");
+    }
+
+    std::vector<absl::string_view> rid_tokens =
+        rtc::split(token, kDelimiterCommaChar);
+
+    if (rid_tokens.empty()) {
+      return ParseError("Simulcast alternative layer list is malformed.");
+    }
+
+    std::vector<SimulcastLayer> layers;
+    for (const absl::string_view& rid_token : rid_tokens) {
+      if (rid_token.empty() || rid_token == kSimulcastPausedStream) {
+        return ParseError("Rid must not be empty.");
+      }
+
+      bool paused = rid_token[0] == kSimulcastPausedStreamChar;
+      absl::string_view rid = paused ? rid_token.substr(1) : rid_token;
+      layers.push_back(SimulcastLayer(rid, paused));
+    }
+
+    result.AddLayerWithAlternatives(layers);
+  }
+
+  return std::move(result);
+}
+
+webrtc::RTCError ParseRidPayloadList(const std::string& payload_list,
+                                     RidDescription* rid_description) {
+  RTC_DCHECK(rid_description);
+  std::vector<int>& payload_types = rid_description->payload_types;
+  // Check that the description doesn't have any payload types or restrictions.
+  // If the pt= field is specified, it must be first and must not repeat.
+  if (!payload_types.empty()) {
+    return ParseError("Multiple pt= found in RID Description.");
+  }
+  if (!rid_description->restrictions.empty()) {
+    return ParseError("Payload list must appear first in the restrictions.");
+  }
+
+  // If the pt= field is specified, it must have a value.
+  if (payload_list.empty()) {
+    return ParseError("Payload list must have at least one value.");
+  }
+
+  // Tokenize the ',' delimited list
+  std::vector<std::string> string_payloads;
+  rtc::tokenize(payload_list, kDelimiterCommaChar, &string_payloads);
+  if (string_payloads.empty()) {
+    return ParseError("Payload list must have at least one value.");
+  }
+
+  for (const std::string& payload_type : string_payloads) {
+    absl::optional<int> value = rtc::StringToNumber<int>(payload_type);
+    if (!value.has_value()) {
+      return ParseError("Invalid payload type: " + payload_type);
+    }
+
+    // Check if the value already appears in the payload list.
+    if (absl::c_linear_search(payload_types, value.value())) {
+      return ParseError("Duplicate payload type in list: " + payload_type);
+    }
+    payload_types.push_back(value.value());
+  }
+
+  return RTCError::OK();
+}
+
+}  // namespace
+
+std::string SdpSerializer::SerializeSimulcastDescription(
+    const cricket::SimulcastDescription& simulcast) const {
+  rtc::StringBuilder sb;
+  std::string delimiter;
+
+  if (!simulcast.send_layers().empty()) {
+    sb << kSendDirection << kDelimiterSpace << simulcast.send_layers();
+    delimiter = kDelimiterSpace;
+  }
+
+  if (!simulcast.receive_layers().empty()) {
+    sb << delimiter << kReceiveDirection << kDelimiterSpace
+       << simulcast.receive_layers();
+  }
+
+  return sb.str();
+}
+
+// https://tools.ietf.org/html/draft-ietf-mmusic-sdp-simulcast-13#section-5.1
+// a:simulcast:<send> <streams> <recv> <streams>
+// Formal Grammar
+// sc-value     = ( sc-send [SP sc-recv] ) / ( sc-recv [SP sc-send] )
+// sc-send      = %s"send" SP sc-str-list
+// sc-recv      = %s"recv" SP sc-str-list
+// sc-str-list  = sc-alt-list *( ";" sc-alt-list )
+// sc-alt-list  = sc-id *( "," sc-id )
+// sc-id-paused = "~"
+// sc-id        = [sc-id-paused] rid-id
+// rid-id       = 1*(alpha-numeric / "-" / "_") ; see: I-D.ietf-mmusic-rid
+RTCErrorOr<SimulcastDescription> SdpSerializer::DeserializeSimulcastDescription(
+    absl::string_view string) const {
+  std::vector<std::string> tokens;
+  rtc::tokenize(std::string(string), kDelimiterSpaceChar, &tokens);
+
+  if (tokens.size() != 2 && tokens.size() != 4) {
+    return ParseError("Must have one or two <direction, streams> pairs.");
+  }
+
+  bool bidirectional = tokens.size() == 4;  // indicates both send and recv
+
+  // Tokens 0, 2 (if exists) should be send / recv
+  if ((tokens[0] != kSendDirection && tokens[0] != kReceiveDirection) ||
+      (bidirectional && tokens[2] != kSendDirection &&
+       tokens[2] != kReceiveDirection) ||
+      (bidirectional && tokens[0] == tokens[2])) {
+    return ParseError("Valid values: send / recv.");
+  }
+
+  // Tokens 1, 3 (if exists) should be alternative layer lists
+  RTCErrorOr<SimulcastLayerList> list1, list2;
+  list1 = ParseSimulcastLayerList(tokens[1]);
+  if (!list1.ok()) {
+    return list1.MoveError();
+  }
+
+  if (bidirectional) {
+    list2 = ParseSimulcastLayerList(tokens[3]);
+    if (!list2.ok()) {
+      return list2.MoveError();
+    }
+  }
+
+  // Set the layers so that list1 is for send and list2 is for recv
+  if (tokens[0] != kSendDirection) {
+    std::swap(list1, list2);
+  }
+
+  // Set the layers according to which pair is send and which is recv
+  // At this point if the simulcast is unidirectional then
+  // either `list1` or `list2` will be in 'error' state indicating that
+  // the value should not be used.
+  SimulcastDescription simulcast;
+  if (list1.ok()) {
+    simulcast.send_layers() = list1.MoveValue();
+  }
+
+  if (list2.ok()) {
+    simulcast.receive_layers() = list2.MoveValue();
+  }
+
+  return std::move(simulcast);
+}
+
+std::string SdpSerializer::SerializeRidDescription(
+    const RidDescription& rid_description) const {
+  RTC_DCHECK(!rid_description.rid.empty());
+  RTC_DCHECK(rid_description.direction == RidDirection::kSend ||
+             rid_description.direction == RidDirection::kReceive);
+
+  rtc::StringBuilder builder;
+  builder << rid_description.rid << kDelimiterSpace
+          << (rid_description.direction == RidDirection::kSend
+                  ? kSendDirection
+                  : kReceiveDirection);
+
+  const auto& payload_types = rid_description.payload_types;
+  const auto& restrictions = rid_description.restrictions;
+
+  // First property is separated by ' ', the next ones by ';'.
+  const char* propertyDelimiter = kDelimiterSpace;
+
+  // Serialize any codecs in the description.
+  if (!payload_types.empty()) {
+    builder << propertyDelimiter << kPayloadType << kDelimiterEqual;
+    propertyDelimiter = kDelimiterSemicolon;
+    const char* formatDelimiter = "";
+    for (int payload_type : payload_types) {
+      builder << formatDelimiter << payload_type;
+      formatDelimiter = kDelimiterComma;
+    }
+  }
+
+  // Serialize any restrictions in the description.
+  for (const auto& pair : restrictions) {
+    // Serialize key=val pairs. =val part is ommitted if val is empty.
+    builder << propertyDelimiter << pair.first;
+    if (!pair.second.empty()) {
+      builder << kDelimiterEqual << pair.second;
+    }
+
+    propertyDelimiter = kDelimiterSemicolon;
+  }
+
+  return builder.str();
+}
+
+// https://tools.ietf.org/html/draft-ietf-mmusic-rid-15#section-10
+// Formal Grammar
+// rid-syntax         = %s"a=rid:" rid-id SP rid-dir
+//                      [ rid-pt-param-list / rid-param-list ]
+// rid-id             = 1*(alpha-numeric / "-" / "_")
+// rid-dir            = %s"send" / %s"recv"
+// rid-pt-param-list  = SP rid-fmt-list *( ";" rid-param )
+// rid-param-list     = SP rid-param *( ";" rid-param )
+// rid-fmt-list       = %s"pt=" fmt *( "," fmt )
+// rid-param          = 1*(alpha-numeric / "-") [ "=" param-val ]
+// param-val          = *( %x20-58 / %x60-7E )
+//                      ; Any printable character except semicolon
+RTCErrorOr<RidDescription> SdpSerializer::DeserializeRidDescription(
+    absl::string_view string) const {
+  std::vector<std::string> tokens;
+  rtc::tokenize(std::string(string), kDelimiterSpaceChar, &tokens);
+
+  if (tokens.size() < 2) {
+    return ParseError("RID Description must contain <RID> <direction>.");
+  }
+
+  if (tokens.size() > 3) {
+    return ParseError("Invalid RID Description format. Too many arguments.");
+  }
+
+  if (!IsLegalRsidName(tokens[0])) {
+    return ParseError("Invalid RID value: " + tokens[0] + ".");
+  }
+
+  if (tokens[1] != kSendDirection && tokens[1] != kReceiveDirection) {
+    return ParseError("Invalid RID direction. Supported values: send / recv.");
+  }
+
+  RidDirection direction = tokens[1] == kSendDirection ? RidDirection::kSend
+                                                       : RidDirection::kReceive;
+
+  RidDescription rid_description(tokens[0], direction);
+
+  // If there is a third argument it is a payload list and/or restriction list.
+  if (tokens.size() == 3) {
+    std::vector<std::string> restrictions;
+    rtc::tokenize(tokens[2], kDelimiterSemicolonChar, &restrictions);
+
+    // Check for malformed restriction list, such as ';' or ';;;' etc.
+    if (restrictions.empty()) {
+      return ParseError("Invalid RID restriction list: " + tokens[2]);
+    }
+
+    // Parse the restrictions. The payload indicator (pt) can only appear first.
+    for (const std::string& restriction : restrictions) {
+      std::vector<std::string> parts;
+      rtc::tokenize(restriction, kDelimiterEqualChar, &parts);
+      if (parts.empty() || parts.size() > 2) {
+        return ParseError("Invalid format for restriction: " + restriction);
+      }
+
+      // `parts` contains at least one value and it does not contain a space.
+      // Note: `parts` and other values might still contain tab, newline,
+      // unprintable characters, etc. which will not generate errors here but
+      // will (most-likely) be ignored by components down stream.
+      if (parts[0] == kPayloadType) {
+        RTCError error = ParseRidPayloadList(
+            parts.size() > 1 ? parts[1] : std::string(), &rid_description);
+        if (!error.ok()) {
+          return std::move(error);
+        }
+
+        continue;
+      }
+
+      // Parse `parts` as a key=value pair which allows unspecified values.
+      if (rid_description.restrictions.find(parts[0]) !=
+          rid_description.restrictions.end()) {
+        return ParseError("Duplicate restriction specified: " + parts[0]);
+      }
+
+      rid_description.restrictions[parts[0]] =
+          parts.size() > 1 ? parts[1] : std::string();
+    }
+  }
+
+  return std::move(rid_description);
+}
+
+}  // namespace webrtc