diff --git a/pc/sessiondescription_unittest.cc b/pc/sessiondescription_unittest.cc
new file mode 100644
index 0000000..b539524
--- /dev/null
+++ b/pc/sessiondescription_unittest.cc
@@ -0,0 +1,134 @@
+/*
+ *  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/sessiondescription.h"
+#include "rtc_base/gunit.h"
+
+namespace cricket {
+
+TEST(MediaContentDescriptionTest, ExtmapAllowMixedDefaultValue) {
+  VideoContentDescription video_desc;
+  EXPECT_EQ(MediaContentDescription::kNo,
+            video_desc.extmap_allow_mixed_headers());
+}
+
+TEST(MediaContentDescriptionTest, SetExtmapAllowMixed) {
+  VideoContentDescription video_desc;
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kNo);
+  EXPECT_EQ(MediaContentDescription::kNo,
+            video_desc.extmap_allow_mixed_headers());
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kMedia);
+  EXPECT_EQ(MediaContentDescription::kMedia,
+            video_desc.extmap_allow_mixed_headers());
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kSession);
+  EXPECT_EQ(MediaContentDescription::kSession,
+            video_desc.extmap_allow_mixed_headers());
+
+  // Not allowed to downgrade from kSession to kMedia.
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kMedia);
+  EXPECT_EQ(MediaContentDescription::kSession,
+            video_desc.extmap_allow_mixed_headers());
+
+  // Always okay to set not supported.
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kNo);
+  EXPECT_EQ(MediaContentDescription::kNo,
+            video_desc.extmap_allow_mixed_headers());
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kMedia);
+  EXPECT_EQ(MediaContentDescription::kMedia,
+            video_desc.extmap_allow_mixed_headers());
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kNo);
+  EXPECT_EQ(MediaContentDescription::kNo,
+            video_desc.extmap_allow_mixed_headers());
+}
+
+TEST(MediaContentDescriptionTest, MixedOneTwoByteHeaderSupported) {
+  VideoContentDescription video_desc;
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kNo);
+  EXPECT_FALSE(video_desc.mixed_one_two_byte_header_extensions_supported());
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kMedia);
+  EXPECT_TRUE(video_desc.mixed_one_two_byte_header_extensions_supported());
+  video_desc.set_extmap_allow_mixed_headers(MediaContentDescription::kSession);
+  EXPECT_TRUE(video_desc.mixed_one_two_byte_header_extensions_supported());
+}
+
+TEST(SessionDescriptionTest, SetExtmapAllowMixed) {
+  SessionDescription session_desc;
+  session_desc.set_extmap_allow_mixed_headers(true);
+  EXPECT_TRUE(session_desc.extmap_allow_mixed_headers());
+  session_desc.set_extmap_allow_mixed_headers(false);
+  EXPECT_FALSE(session_desc.extmap_allow_mixed_headers());
+}
+
+TEST(SessionDescriptionTest, SetExtmapAllowMixedPropagatesToMediaLevel) {
+  SessionDescription session_desc;
+  MediaContentDescription* video_desc = new VideoContentDescription();
+  session_desc.AddContent("video", MediaProtocolType::kRtp, video_desc);
+
+  // Setting true on session level propagates to media level.
+  session_desc.set_extmap_allow_mixed_headers(true);
+  EXPECT_EQ(MediaContentDescription::kSession,
+            video_desc->extmap_allow_mixed_headers());
+
+  // Don't downgrade from session level to media level
+  video_desc->set_extmap_allow_mixed_headers(MediaContentDescription::kMedia);
+  EXPECT_EQ(MediaContentDescription::kSession,
+            video_desc->extmap_allow_mixed_headers());
+
+  // Setting false on session level propagates to media level if the current
+  // state is kSession.
+  session_desc.set_extmap_allow_mixed_headers(false);
+  EXPECT_EQ(MediaContentDescription::kNo,
+            video_desc->extmap_allow_mixed_headers());
+
+  // Now possible to set at media level.
+  video_desc->set_extmap_allow_mixed_headers(MediaContentDescription::kMedia);
+  EXPECT_EQ(MediaContentDescription::kMedia,
+            video_desc->extmap_allow_mixed_headers());
+
+  // Setting false on session level does not override on media level if current
+  // state is kMedia.
+  session_desc.set_extmap_allow_mixed_headers(false);
+  EXPECT_EQ(MediaContentDescription::kMedia,
+            video_desc->extmap_allow_mixed_headers());
+
+  // Setting true on session level overrides setting on media level.
+  session_desc.set_extmap_allow_mixed_headers(true);
+  EXPECT_EQ(MediaContentDescription::kSession,
+            video_desc->extmap_allow_mixed_headers());
+}
+
+TEST(SessionDescriptionTest, AddContentTransfersExtmapAllowMixedSetting) {
+  SessionDescription session_desc;
+  session_desc.set_extmap_allow_mixed_headers(false);
+  MediaContentDescription* audio_desc = new AudioContentDescription();
+  audio_desc->set_extmap_allow_mixed_headers(MediaContentDescription::kMedia);
+
+  // If session setting is false, media level setting is preserved when new
+  // content is added.
+  session_desc.AddContent("audio", MediaProtocolType::kRtp, audio_desc);
+  EXPECT_EQ(MediaContentDescription::kMedia,
+            audio_desc->extmap_allow_mixed_headers());
+
+  // If session setting is true, it's transferred to media level when new
+  // content is added.
+  session_desc.set_extmap_allow_mixed_headers(true);
+  MediaContentDescription* video_desc = new VideoContentDescription();
+  session_desc.AddContent("video", MediaProtocolType::kRtp, video_desc);
+  EXPECT_EQ(MediaContentDescription::kSession,
+            video_desc->extmap_allow_mixed_headers());
+
+  // Session level setting overrides media level when new content is added.
+  MediaContentDescription* data_desc = new DataContentDescription;
+  data_desc->set_extmap_allow_mixed_headers(MediaContentDescription::kMedia);
+  session_desc.AddContent("data", MediaProtocolType::kRtp, data_desc);
+  EXPECT_EQ(MediaContentDescription::kSession,
+            data_desc->extmap_allow_mixed_headers());
+}
+
+}  // namespace cricket