diff --git a/p2p/base/dtls_transport.cc b/p2p/base/dtls_transport.cc
index 6f30c6dbdd..7547863bd6 100644
--- a/p2p/base/dtls_transport.cc
+++ b/p2p/base/dtls_transport.cc
@@ -727,7 +727,7 @@ void DtlsTransport::OnDtlsEvent(rtc::StreamInterface* dtls, int sig, int err) {
         NotifyPacketReceived(rtc::ReceivedPacket(
             rtc::MakeArrayView(buf, read), rtc::SocketAddress(),
             webrtc::Timestamp::Micros(rtc::TimeMicros()),
-            rtc::ReceivedPacket::kDtlsDecrypted));
+            rtc::EcnMarking::kNotEct, rtc::ReceivedPacket::kDtlsDecrypted));
       } else if (ret == rtc::SR_EOS) {
         // Remote peer shut down the association with no error.
         RTC_LOG(LS_INFO) << ToString() << ": DTLS transport closed by remote";
diff --git a/p2p/base/packet_transport_internal_unittest.cc b/p2p/base/packet_transport_internal_unittest.cc
index 1c9df852d1..b8e589a852 100644
--- a/p2p/base/packet_transport_internal_unittest.cc
+++ b/p2p/base/packet_transport_internal_unittest.cc
@@ -34,9 +34,9 @@ TEST(PacketTransportInternal,
             EXPECT_EQ(packet.decryption_info(),
                       rtc::ReceivedPacket::kDtlsDecrypted);
           });
-  packet_transport.NotifyPacketReceived(
-      rtc::ReceivedPacket({}, rtc::SocketAddress(), absl::nullopt,
-                          rtc::ReceivedPacket::kDtlsDecrypted));
+  packet_transport.NotifyPacketReceived(rtc::ReceivedPacket(
+      {}, rtc::SocketAddress(), absl::nullopt, rtc::EcnMarking::kNotEct,
+      rtc::ReceivedPacket::kDtlsDecrypted));
 
   packet_transport.DeregisterReceivedPacketCallback(&receiver);
 }
diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 775b9100e4..829f0970d1 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -1046,6 +1046,7 @@ rtc_library("threading") {
     "../api/task_queue:pending_task_safety_flag",
     "../api/units:time_delta",
     "../system_wrappers:field_trial",
+    "./network:ecn_marking",
     "synchronization:mutex",
     "system:no_unique_address",
     "system:rtc_export",
@@ -1092,6 +1093,7 @@ rtc_library("socket") {
     ":macromagic",
     ":socket_address",
     "../api/units:timestamp",
+    "./network:ecn_marking",
     "system:rtc_export",
     "third_party/sigslot",
   ]
diff --git a/rtc_base/async_udp_socket.cc b/rtc_base/async_udp_socket.cc
index 3d258bcb26..7f1301dea5 100644
--- a/rtc_base/async_udp_socket.cc
+++ b/rtc_base/async_udp_socket.cc
@@ -145,9 +145,9 @@ void AsyncUDPSocket::OnReadEvent(Socket* socket) {
     }
     *receive_buffer.arrival_time += *socket_time_offset_;
   }
-  NotifyPacketReceived(ReceivedPacket(receive_buffer.payload,
-                                      receive_buffer.source_address,
-                                      receive_buffer.arrival_time));
+  NotifyPacketReceived(
+      ReceivedPacket(receive_buffer.payload, receive_buffer.source_address,
+                     receive_buffer.arrival_time, receive_buffer.ecn));
 }
 
 void AsyncUDPSocket::OnWriteEvent(Socket* socket) {
diff --git a/rtc_base/network/BUILD.gn b/rtc_base/network/BUILD.gn
index 2be484e1e0..e2c6fc3c16 100644
--- a/rtc_base/network/BUILD.gn
+++ b/rtc_base/network/BUILD.gn
@@ -17,6 +17,11 @@ rtc_library("sent_packet") {
   absl_deps = [ "//third_party/abseil-cpp/absl/types:optional" ]
 }
 
+rtc_source_set("ecn_marking") {
+  visibility = [ "*" ]
+  sources = [ "ecn_marking.h" ]
+}
+
 rtc_library("received_packet") {
   visibility = [ "*" ]
   sources = [
@@ -24,6 +29,7 @@ rtc_library("received_packet") {
     "received_packet.h",
   ]
   deps = [
+    ":ecn_marking",
     "..:socket_address",
     "../../api:array_view",
     "../../api/units:timestamp",
diff --git a/rtc_base/network/ecn_marking.h b/rtc_base/network/ecn_marking.h
new file mode 100644
index 0000000000..e186080a2d
--- /dev/null
+++ b/rtc_base/network/ecn_marking.h
@@ -0,0 +1,42 @@
+/*
+ *  Copyright 2024 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 RTC_BASE_NETWORK_ECN_MARKING_H_
+#define RTC_BASE_NETWORK_ECN_MARKING_H_
+
+namespace rtc {
+
+// TODO(https://bugs.webrtc.org/15368): L4S support is slowly being developed.
+// Help is appreciated.
+
+// L4S Explicit Congestion Notification (ECN) .
+// https://www.rfc-editor.org/rfc/rfc9331.html ECT stands for ECN-Capable
+// Transport and CE stands for Congestion Experienced.
+
+// RFC-3168, Section 5
+// +-----+-----+
+// | ECN FIELD |
+// +-----+-----+
+//   ECT   CE         [Obsolete] RFC 2481 names for the ECN bits.
+//    0     0         Not-ECT
+//    0     1         ECT(1)
+//    1     0         ECT(0)
+//    1     1         CE
+
+enum class EcnMarking {
+  kNotEct = 0,  // Not ECN-Capable Transport
+  kEct1 = 1,    // ECN-Capable Transport
+  kEct0 = 2,    // Not used by L4s (or webrtc.)
+  kCe = 3,      // Congestion experienced
+};
+
+}  // namespace rtc
+
+#endif  // RTC_BASE_NETWORK_ECN_MARKING_H_
diff --git a/rtc_base/network/received_packet.cc b/rtc_base/network/received_packet.cc
index bf8a07ca89..9588e370d2 100644
--- a/rtc_base/network/received_packet.cc
+++ b/rtc_base/network/received_packet.cc
@@ -20,15 +20,17 @@ namespace rtc {
 ReceivedPacket::ReceivedPacket(rtc::ArrayView<const uint8_t> payload,
                                const SocketAddress& source_address,
                                absl::optional<webrtc::Timestamp> arrival_time,
+                               EcnMarking ecn,
                                DecryptionInfo decryption)
     : payload_(payload),
       arrival_time_(std::move(arrival_time)),
       source_address_(source_address),
+      ecn_(ecn),
       decryption_info_(decryption) {}
 
 ReceivedPacket ReceivedPacket::CopyAndSet(
     DecryptionInfo decryption_info) const {
-  return ReceivedPacket(payload_, source_address_, arrival_time_,
+  return ReceivedPacket(payload_, source_address_, arrival_time_, ecn_,
                         decryption_info);
 }
 
diff --git a/rtc_base/network/received_packet.h b/rtc_base/network/received_packet.h
index b5a6092d89..68dd33b1d1 100644
--- a/rtc_base/network/received_packet.h
+++ b/rtc_base/network/received_packet.h
@@ -15,6 +15,7 @@
 #include "absl/types/optional.h"
 #include "api/array_view.h"
 #include "api/units/timestamp.h"
+#include "rtc_base/network/ecn_marking.h"
 #include "rtc_base/socket_address.h"
 #include "rtc_base/system/rtc_export.h"
 
@@ -38,6 +39,7 @@ class RTC_EXPORT ReceivedPacket {
   ReceivedPacket(rtc::ArrayView<const uint8_t> payload,
                  const SocketAddress& source_address,
                  absl::optional<webrtc::Timestamp> arrival_time = absl::nullopt,
+                 EcnMarking ecn = EcnMarking::kNotEct,
                  DecryptionInfo decryption = kNotDecrypted);
 
   ReceivedPacket CopyAndSet(DecryptionInfo decryption_info) const;
@@ -52,6 +54,9 @@ class RTC_EXPORT ReceivedPacket {
     return arrival_time_;
   }
 
+  // L4S Explicit Congestion Notification.
+  EcnMarking ecn() const { return ecn_; }
+
   const DecryptionInfo& decryption_info() const { return decryption_info_; }
 
   static ReceivedPacket CreateFromLegacy(
@@ -73,6 +78,7 @@ class RTC_EXPORT ReceivedPacket {
   rtc::ArrayView<const uint8_t> payload_;
   absl::optional<webrtc::Timestamp> arrival_time_;
   const SocketAddress& source_address_;
+  EcnMarking ecn_;
   DecryptionInfo decryption_info_;
 };
 
diff --git a/rtc_base/physical_socket_server.cc b/rtc_base/physical_socket_server.cc
index c3bc1814a1..447e3da2ef 100644
--- a/rtc_base/physical_socket_server.cc
+++ b/rtc_base/physical_socket_server.cc
@@ -48,6 +48,7 @@
 #include "rtc_base/event.h"
 #include "rtc_base/ip_address.h"
 #include "rtc_base/logging.h"
+#include "rtc_base/network/ecn_marking.h"
 #include "rtc_base/network_monitor.h"
 #include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/time_utils.h"
@@ -108,6 +109,33 @@ typedef char* SockOptArg;
 #endif
 
 namespace {
+
+// RFC-3168, Section 5. ECN is the two least significant bits.
+static constexpr uint8_t kEcnMask = 0x03;
+
+#if defined(WEBRTC_POSIX)
+
+rtc::EcnMarking EcnFromDs(uint8_t ds) {
+  // RFC-3168, Section 5.
+  constexpr uint8_t ECN_ECT1 = 0x01;
+  constexpr uint8_t ECN_ECT0 = 0x02;
+  constexpr uint8_t ECN_CE = 0x03;
+  const uint8_t ecn = ds & kEcnMask;
+
+  if (ecn == ECN_ECT1) {
+    return rtc::EcnMarking::kEct1;
+  }
+  if (ecn == ECN_ECT0) {
+    return rtc::EcnMarking::kEct0;
+  }
+  if (ecn == ECN_CE) {
+    return rtc::EcnMarking::kCe;
+  }
+  return rtc::EcnMarking::kNotEct;
+}
+
+#endif
+
 class ScopedSetTrue {
  public:
   ScopedSetTrue(bool* value) : value_(value) {
@@ -125,6 +153,7 @@ class ScopedSetTrue {
 bool IsScmTimeStampExperimentDisabled() {
   return webrtc::field_trial::IsDisabled("WebRTC-SCM-Timestamp");
 }
+
 }  // namespace
 
 namespace rtc {
@@ -314,8 +343,19 @@ int PhysicalSocket::GetOption(Option opt, int* value) {
 #if defined(WEBRTC_POSIX)
     // unshift DSCP value to get six most significant bits of IP DiffServ field
     *value >>= 2;
+#endif
+  } else if (opt == OPT_SEND_ECN) {
+#if defined(WEBRTC_POSIX)
+    // Least 2 significant bits.
+    *value = *value & kEcnMask;
+#endif
+  } else if (opt == OPT_RECV_ECN) {
+#if defined(WEBRTC_POSIX)
+    // Least 2 significant bits.
+    *value = *value & kEcnMask;
 #endif
   }
+
   return ret;
 }
 
@@ -329,10 +369,13 @@ int PhysicalSocket::SetOption(Option opt, int value) {
     value = (value) ? IP_PMTUDISC_DO : IP_PMTUDISC_DONT;
 #endif
   } else if (opt == OPT_DSCP) {
-#if defined(WEBRTC_POSIX)
-    // shift DSCP value to fit six most significant bits of IP DiffServ field
-    value <<= 2;
-#endif
+    // IP DiffServ  consists of DSCP 6 most significant, ECN 2 least
+    // significant.
+    dscp_ = value << 2;
+    value = dscp_ + (ecn_ & kEcnMask);
+  } else if (opt == OPT_SEND_ECN) {
+    ecn_ = value;
+    value = dscp_ + (ecn_ & kEcnMask);
   }
 #if defined(WEBRTC_POSIX)
   if (sopt == IPV6_TCLASS) {
@@ -401,8 +444,8 @@ int PhysicalSocket::SendTo(const void* buffer,
 }
 
 int PhysicalSocket::Recv(void* buffer, size_t length, int64_t* timestamp) {
-  int received =
-      DoReadFromSocket(buffer, length, /*out_addr*/ nullptr, timestamp);
+  int received = DoReadFromSocket(buffer, length, /*out_addr*/ nullptr,
+                                  timestamp, /*ecn=*/nullptr);
   if ((received == 0) && (length != 0)) {
     // Note: on graceful shutdown, recv can return 0.  In this case, we
     // pretend it is blocking, and then signal close, so that simplifying
@@ -431,7 +474,7 @@ int PhysicalSocket::RecvFrom(void* buffer,
                              size_t length,
                              SocketAddress* out_addr,
                              int64_t* timestamp) {
-  int received = DoReadFromSocket(buffer, length, out_addr, timestamp);
+  int received = DoReadFromSocket(buffer, length, out_addr, timestamp, nullptr);
 
   UpdateLastError();
   int error = GetError();
@@ -450,9 +493,9 @@ int PhysicalSocket::RecvFrom(ReceiveBuffer& buffer) {
   static constexpr int BUF_SIZE = 64 * 1024;
   buffer.payload.EnsureCapacity(BUF_SIZE);
 
-  int received =
-      DoReadFromSocket(buffer.payload.data(), buffer.payload.capacity(),
-                       &buffer.source_address, &timestamp);
+  int received = DoReadFromSocket(
+      buffer.payload.data(), buffer.payload.capacity(), &buffer.source_address,
+      &timestamp, ecn_ ? &buffer.ecn : nullptr);
   buffer.payload.SetSize(received > 0 ? received : 0);
   if (received > 0 && timestamp != -1) {
     buffer.arrival_time = webrtc::Timestamp::Micros(timestamp);
@@ -472,7 +515,8 @@ int PhysicalSocket::RecvFrom(ReceiveBuffer& buffer) {
 int PhysicalSocket::DoReadFromSocket(void* buffer,
                                      size_t length,
                                      SocketAddress* out_addr,
-                                     int64_t* timestamp) {
+                                     int64_t* timestamp,
+                                     EcnMarking* ecn) {
   sockaddr_storage addr_storage;
   socklen_t addr_len = sizeof(addr_storage);
   sockaddr* addr = reinterpret_cast<sockaddr*>(&addr_storage);
@@ -487,8 +531,10 @@ int PhysicalSocket::DoReadFromSocket(void* buffer,
       msg.msg_name = addr;
       msg.msg_namelen = addr_len;
     }
-    char control[CMSG_SPACE(sizeof(struct timeval))] = {};
-    if (timestamp) {
+    // TODO(bugs.webrtc.org/15368): What size is needed? IPV6_TCLASS is supposed
+    // to be an int. Why is a larger size needed?
+    char control[CMSG_SPACE(sizeof(struct timeval) + 5 * sizeof(int))] = {};
+    if (timestamp || ecn) {
       *timestamp = -1;
       msg.msg_control = &control;
       msg.msg_controllen = sizeof(control);
@@ -498,17 +544,23 @@ int PhysicalSocket::DoReadFromSocket(void* buffer,
       // An error occured or shut down.
       return received;
     }
-    if (timestamp) {
+    if (timestamp || ecn) {
       struct cmsghdr* cmsg;
       for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
+        if (ecn) {
+          if ((cmsg->cmsg_type == IPV6_TCLASS &&
+               cmsg->cmsg_level == IPPROTO_IPV6) ||
+              (cmsg->cmsg_type == IP_TOS && cmsg->cmsg_level == IPPROTO_IP)) {
+            *ecn = EcnFromDs(CMSG_DATA(cmsg)[0]);
+          }
+        }
         if (cmsg->cmsg_level != SOL_SOCKET)
           continue;
-        if (cmsg->cmsg_type == SCM_TIMESTAMP) {
+        if (timestamp && cmsg->cmsg_type == SCM_TIMESTAMP) {
           timeval* ts = reinterpret_cast<timeval*>(CMSG_DATA(cmsg));
           *timestamp =
               rtc::kNumMicrosecsPerSec * static_cast<int64_t>(ts->tv_sec) +
               static_cast<int64_t>(ts->tv_usec);
-          break;
         }
       }
     }
@@ -699,6 +751,34 @@ int PhysicalSocket::TranslateOption(Option opt, int* slevel, int* sopt) {
 #else
       RTC_LOG(LS_WARNING) << "Socket::OPT_DSCP not supported.";
       return -1;
+#endif
+    case OPT_SEND_ECN:
+#if defined(WEBRTC_POSIX)
+      if (family_ == AF_INET6) {
+        *slevel = IPPROTO_IPV6;
+        *sopt = IPV6_TCLASS;
+      } else {
+        *slevel = IPPROTO_IP;
+        *sopt = IP_TOS;
+      }
+      break;
+#else
+      RTC_LOG(LS_WARNING) << "Socket::OPT_SEND_ESN not supported.";
+      return -1;
+#endif
+    case OPT_RECV_ECN:
+#if defined(WEBRTC_POSIX)
+      if (family_ == AF_INET6) {
+        *slevel = IPPROTO_IPV6;
+        *sopt = IPV6_RECVTCLASS;
+      } else {
+        *slevel = IPPROTO_IP;
+        *sopt = IP_RECVTOS;
+      }
+      break;
+#else
+      RTC_LOG(LS_WARNING) << "Socket::OPT_RECV_ECN not supported.";
+      return -1;
 #endif
     case OPT_RTP_SENDTIME_EXTN_ID:
       return -1;  // No logging is necessary as this not a OS socket option.
diff --git a/rtc_base/physical_socket_server.h b/rtc_base/physical_socket_server.h
index 2af563a3ca..ec30281e90 100644
--- a/rtc_base/physical_socket_server.h
+++ b/rtc_base/physical_socket_server.h
@@ -224,7 +224,8 @@ class PhysicalSocket : public Socket, public sigslot::has_slots<> {
   int DoReadFromSocket(void* buffer,
                        size_t length,
                        SocketAddress* out_addr,
-                       int64_t* timestamp);
+                       int64_t* timestamp,
+                       EcnMarking* ecn);
 
   void OnResolveResult(const webrtc::AsyncDnsResolverResult& resolver);
 
@@ -246,6 +247,8 @@ class PhysicalSocket : public Socket, public sigslot::has_slots<> {
   int error_ RTC_GUARDED_BY(mutex_);
   ConnState state_;
   std::unique_ptr<webrtc::AsyncDnsResolverInterface> resolver_;
+  uint8_t dscp_ = 0;  // 6bit.
+  uint8_t ecn_ = 0;   // 2bits.
 
 #if !defined(NDEBUG)
   std::string dbg_addr_;
diff --git a/rtc_base/physical_socket_server_unittest.cc b/rtc_base/physical_socket_server_unittest.cc
index de64a31812..ccb548d2f3 100644
--- a/rtc_base/physical_socket_server_unittest.cc
+++ b/rtc_base/physical_socket_server_unittest.cc
@@ -484,6 +484,19 @@ TEST_F(PhysicalSocketTest, TestSocketRecvTimestampIPv6ScmExperimentDisabled) {
 }
 #endif
 
+#if !defined(WEBRTC_MAC) && !defined(WEBRTC_IOS)
+// TODO(bugs.webrtc.org/15368): IpV4 fails on IOS and MAC. IPV6 works.
+TEST_F(PhysicalSocketTest, TestSocketSendRecvWithEcnIPv4) {
+  MAYBE_SKIP_IPV6;
+  SocketTest::TestSocketSendRecvWithEcnIPV4();
+}
+#endif
+
+TEST_F(PhysicalSocketTest, TestSocketSendRecvWithEcnIPv6) {
+  MAYBE_SKIP_IPV6;
+  SocketTest::TestSocketSendRecvWithEcnIPV6();
+}
+
 // Verify that if the socket was unable to be bound to a real network interface
 // (not loopback), Bind will return an error.
 TEST_F(PhysicalSocketTest,
diff --git a/rtc_base/socket.h b/rtc_base/socket.h
index 98e468e754..4e35406798 100644
--- a/rtc_base/socket.h
+++ b/rtc_base/socket.h
@@ -30,6 +30,7 @@
 
 #include "api/units/timestamp.h"
 #include "rtc_base/buffer.h"
+#include "rtc_base/network/ecn_marking.h"
 #include "rtc_base/socket_address.h"
 #include "rtc_base/system/rtc_export.h"
 #include "rtc_base/third_party/sigslot/sigslot.h"
@@ -91,6 +92,7 @@ class RTC_EXPORT Socket {
 
     absl::optional<webrtc::Timestamp> arrival_time;
     SocketAddress source_address;
+    EcnMarking ecn = EcnMarking::kNotEct;
     Buffer& payload;
   };
   virtual ~Socket() {}
@@ -144,6 +146,8 @@ class RTC_EXPORT Socket {
     OPT_RTP_SENDTIME_EXTN_ID,  // This is a non-traditional socket option param.
                                // This is specific to libjingle and will be used
                                // if SendTime option is needed at socket level.
+    OPT_SEND_ECN,              // 2-bit ECN
+    OPT_RECV_ECN
   };
   virtual int GetOption(Option opt, int* value) = 0;
   virtual int SetOption(Option opt, int value) = 0;
diff --git a/rtc_base/socket_unittest.cc b/rtc_base/socket_unittest.cc
index 5314128d0a..c5e791b293 100644
--- a/rtc_base/socket_unittest.cc
+++ b/rtc_base/socket_unittest.cc
@@ -234,6 +234,15 @@ void SocketTest::TestUdpSocketRecvTimestampUseRtcEpochIPv6() {
   UdpSocketRecvTimestampUseRtcEpoch(kIPv6Loopback);
 }
 
+void SocketTest::TestSocketSendRecvWithEcnIPV4() {
+  SocketSendRecvWithEcn(kIPv4Loopback);
+}
+
+void SocketTest::TestSocketSendRecvWithEcnIPV6() {
+  MAYBE_SKIP_IPV6;
+  SocketSendRecvWithEcn(kIPv6Loopback);
+}
+
 // For unbound sockets, GetLocalAddress / GetRemoteAddress return AF_UNSPEC
 // values on Windows, but an empty address of the same family on Linux/MacOS X.
 bool IsUnspecOrEmptyIP(const IPAddress& address) {
@@ -1079,6 +1088,18 @@ void SocketTest::GetSetOptionsInternal(const IPAddress& loopback) {
   ASSERT_NE(-1, socket->SetOption(Socket::OPT_DSCP, desired_dscp));
   ASSERT_NE(-1, socket->GetOption(Socket::OPT_DSCP, &current_dscp));
   ASSERT_EQ(desired_dscp, current_dscp);
+
+  int current_send_esn, desired_send_esn = 1;
+  ASSERT_NE(-1, socket->GetOption(Socket::OPT_SEND_ECN, &current_send_esn));
+  ASSERT_NE(-1, socket->SetOption(Socket::OPT_SEND_ECN, desired_send_esn));
+  ASSERT_NE(-1, socket->GetOption(Socket::OPT_SEND_ECN, &current_send_esn));
+  ASSERT_EQ(current_send_esn, desired_send_esn);
+
+  int current_recv_esn, desired_recv_esn = 1;
+  ASSERT_NE(-1, socket->GetOption(Socket::OPT_RECV_ECN, &current_recv_esn));
+  ASSERT_NE(-1, socket->SetOption(Socket::OPT_RECV_ECN, desired_recv_esn));
+  ASSERT_NE(-1, socket->GetOption(Socket::OPT_RECV_ECN, &current_recv_esn));
+  ASSERT_EQ(current_recv_esn, desired_recv_esn);
 #endif
 }
 
@@ -1143,4 +1164,41 @@ void SocketTest::UdpSocketRecvTimestampUseRtcEpoch(const IPAddress& loopback) {
   EXPECT_GT(packet_2->packet_time->us(), packet_1->packet_time->us());
   EXPECT_NEAR(packet_2->packet_time->us(), rtc::TimeMicros(), 1000'000);
 }
+
+void SocketTest::SocketSendRecvWithEcn(const IPAddress& loopback) {
+  StreamSink sink;
+  std::unique_ptr<Socket> socket(
+      socket_factory_->CreateSocket(loopback.family(), SOCK_DGRAM));
+  EXPECT_EQ(0, socket->Bind(SocketAddress(loopback, 0)));
+  SocketAddress address = socket->GetLocalAddress();
+  sink.Monitor(socket.get());
+  rtc::Buffer buffer;
+  Socket::ReceiveBuffer receive_buffer(buffer);
+
+  socket->SendTo("foo", 3, address);
+  EXPECT_TRUE_WAIT(sink.Check(socket.get(), SSE_READ), kTimeout);
+  ASSERT_GT(socket->RecvFrom(receive_buffer), 0);
+  EXPECT_EQ(receive_buffer.ecn, EcnMarking::kNotEct);
+
+  socket->SetOption(Socket::OPT_SEND_ECN, 1);  // Ect(1)
+  socket->SetOption(Socket::OPT_RECV_ECN, 1);
+
+  socket->SendTo("bar", 3, address);
+  EXPECT_TRUE_WAIT(sink.Check(socket.get(), SSE_READ), kTimeout);
+  ASSERT_GT(socket->RecvFrom(receive_buffer), 0);
+  EXPECT_EQ(receive_buffer.ecn, EcnMarking::kEct1);
+
+  socket->SetOption(Socket::OPT_SEND_ECN, 2);  // Ect(0)
+  socket->SendTo("bar", 3, address);
+  EXPECT_TRUE_WAIT(sink.Check(socket.get(), SSE_READ), kTimeout);
+  ASSERT_GT(socket->RecvFrom(receive_buffer), 0);
+  EXPECT_EQ(receive_buffer.ecn, EcnMarking::kEct0);
+
+  socket->SetOption(Socket::OPT_SEND_ECN, 3);  // Ce
+  socket->SendTo("bar", 3, address);
+  EXPECT_TRUE_WAIT(sink.Check(socket.get(), SSE_READ), kTimeout);
+  ASSERT_GT(socket->RecvFrom(receive_buffer), 0);
+  EXPECT_EQ(receive_buffer.ecn, EcnMarking::kCe);
+}
+
 }  // namespace rtc
diff --git a/rtc_base/socket_unittest.h b/rtc_base/socket_unittest.h
index db79be2eb8..4d24f3641d 100644
--- a/rtc_base/socket_unittest.h
+++ b/rtc_base/socket_unittest.h
@@ -64,6 +64,8 @@ class SocketTest : public ::testing::Test {
   void TestSocketRecvTimestampIPv6();
   void TestUdpSocketRecvTimestampUseRtcEpochIPv4();
   void TestUdpSocketRecvTimestampUseRtcEpochIPv6();
+  void TestSocketSendRecvWithEcnIPV4();
+  void TestSocketSendRecvWithEcnIPV6();
 
   static const int kTimeout = 5000;  // ms
   const IPAddress kIPv4Loopback;
@@ -95,6 +97,7 @@ class SocketTest : public ::testing::Test {
   void GetSetOptionsInternal(const IPAddress& loopback);
   void SocketRecvTimestamp(const IPAddress& loopback);
   void UdpSocketRecvTimestampUseRtcEpoch(const IPAddress& loopback);
+  void SocketSendRecvWithEcn(const IPAddress& loopback);
 
   SocketFactory* socket_factory_;
 };