diff --git a/RELEASENOTES.md b/RELEASENOTES.md
index d95288bb4d..f56a511f6a 100644
--- a/RELEASENOTES.md
+++ b/RELEASENOTES.md
@@ -27,6 +27,8 @@
   * Added callbacks to `MediaSourceEventListener` to get notified when media
     periods are created, released and being read from.
   * Support live stream clipping with `ClippingMediaSource`.
+  * Allow setting custom tags for all media sources in their factories. The tag
+    of the current window can be retrieved with `ExoPlayer.getCurrentTag`.
 * Audio:
   * Factor out `AudioTrack` position tracking from `DefaultAudioSink`.
   * Fix an issue where the playback position would pause just after playback
diff --git a/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastPlayer.java b/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastPlayer.java
index 50c883c3f6..7e0753ba10 100644
--- a/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastPlayer.java
+++ b/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastPlayer.java
@@ -481,6 +481,14 @@ public final class CastPlayer implements Player {
         : currentTimeline.getPreviousWindowIndex(getCurrentWindowIndex(), repeatMode, false);
   }
 
+  @Override
+  public @Nullable Object getCurrentTag() {
+    int windowIndex = getCurrentWindowIndex();
+    return windowIndex > currentTimeline.getWindowCount()
+        ? null
+        : currentTimeline.getWindow(windowIndex, window, /* setTag= */ true).tag;
+  }
+
   // TODO: Fill the cast timeline information with ProgressListener's duration updates.
   // See [Internal: b/65152553].
   @Override
diff --git a/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastTimeline.java b/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastTimeline.java
index a0be844439..24d815bae2 100644
--- a/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastTimeline.java
+++ b/extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastTimeline.java
@@ -73,12 +73,22 @@ import java.util.Map;
   }
 
   @Override
-  public Window getWindow(int windowIndex, Window window, boolean setIds,
-      long defaultPositionProjectionUs) {
+  public Window getWindow(
+      int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
     long durationUs = durationsUs[windowIndex];
     boolean isDynamic = durationUs == C.TIME_UNSET;
-    return window.set(ids[windowIndex], C.TIME_UNSET, C.TIME_UNSET, !isDynamic, isDynamic,
-        defaultPositionsUs[windowIndex], durationUs, windowIndex, windowIndex, 0);
+    Object tag = setTag ? ids[windowIndex] : null;
+    return window.set(
+        tag,
+        /* presentationStartTimeMs= */ C.TIME_UNSET,
+        /* windowStartTimeMs= */ C.TIME_UNSET,
+        /* isSeekable= */ !isDynamic,
+        isDynamic,
+        defaultPositionsUs[windowIndex],
+        durationUs,
+        /* firstPeriodIndex= */ windowIndex,
+        /* lastPeriodIndex= */ windowIndex,
+        /* positionInFirstPeriodUs= */ 0);
   }
 
   @Override
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java b/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java
index ae55870baf..e33b7358a4 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java
@@ -311,6 +311,14 @@ import java.util.concurrent.CopyOnWriteArraySet;
     internalPlayer.setSeekParameters(seekParameters);
   }
 
+  @Override
+  public @Nullable Object getCurrentTag() {
+    int windowIndex = getCurrentWindowIndex();
+    return windowIndex > playbackInfo.timeline.getWindowCount()
+        ? null
+        : playbackInfo.timeline.getWindow(windowIndex, window, /* setTag= */ true).tag;
+  }
+
   @Override
   public void stop() {
     stop(/* reset= */ false);
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/Player.java b/library/core/src/main/java/com/google/android/exoplayer2/Player.java
index 443ff8a2ea..85872339a3 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/Player.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/Player.java
@@ -655,6 +655,12 @@ public interface Player {
    */
   int getPreviousWindowIndex();
 
+  /**
+   * Returns the tag of the currently playing window in the timeline. May be null if no tag is set
+   * or the timeline is not yet available.
+   */
+  @Nullable Object getCurrentTag();
+
   /**
    * Returns the duration of the current window in milliseconds, or {@link C#TIME_UNSET} if the
    * duration is not known.
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java b/library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java
index 419e082dea..e6979b4a60 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java
@@ -664,6 +664,11 @@ public class SimpleExoPlayer implements ExoPlayer, Player.VideoComponent, Player
     player.setSeekParameters(seekParameters);
   }
 
+  @Override
+  public @Nullable Object getCurrentTag() {
+    return player.getCurrentTag();
+  }
+
   @Override
   public void stop() {
     player.stop();
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/Timeline.java b/library/core/src/main/java/com/google/android/exoplayer2/Timeline.java
index 50a3e66880..884bdc0f4d 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/Timeline.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/Timeline.java
@@ -118,10 +118,8 @@ public abstract class Timeline {
    */
   public static final class Window {
 
-    /**
-     * An identifier for the window. Not necessarily unique.
-     */
-    public Object id;
+    /** A custom tag for the window. Not necessarily unique. */
+    public Object tag;
 
     /**
      * The start time of the presentation to which this window belongs in milliseconds since the
@@ -174,13 +172,19 @@ public abstract class Timeline {
      */
     public long positionInFirstPeriodUs;
 
-    /**
-     * Sets the data held by this window.
-     */
-    public Window set(Object id, long presentationStartTimeMs, long windowStartTimeMs,
-        boolean isSeekable, boolean isDynamic, long defaultPositionUs, long durationUs,
-        int firstPeriodIndex, int lastPeriodIndex, long positionInFirstPeriodUs) {
-      this.id = id;
+    /** Sets the data held by this window. */
+    public Window set(
+        Object tag,
+        long presentationStartTimeMs,
+        long windowStartTimeMs,
+        boolean isSeekable,
+        boolean isDynamic,
+        long defaultPositionUs,
+        long durationUs,
+        int firstPeriodIndex,
+        int lastPeriodIndex,
+        long positionInFirstPeriodUs) {
+      this.tag = tag;
       this.presentationStartTimeMs = presentationStartTimeMs;
       this.windowStartTimeMs = windowStartTimeMs;
       this.isSeekable = isSeekable;
@@ -486,38 +490,36 @@ public abstract class Timeline {
 
   }
 
-  /**
-   * An empty timeline.
-   */
-  public static final Timeline EMPTY = new Timeline() {
+  /** An empty timeline. */
+  public static final Timeline EMPTY =
+      new Timeline() {
 
-    @Override
-    public int getWindowCount() {
-      return 0;
-    }
+        @Override
+        public int getWindowCount() {
+          return 0;
+        }
 
-    @Override
-    public Window getWindow(int windowIndex, Window window, boolean setIds,
-        long defaultPositionProjectionUs) {
-      throw new IndexOutOfBoundsException();
-    }
+        @Override
+        public Window getWindow(
+            int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
+          throw new IndexOutOfBoundsException();
+        }
 
-    @Override
-    public int getPeriodCount() {
-      return 0;
-    }
+        @Override
+        public int getPeriodCount() {
+          return 0;
+        }
 
-    @Override
-    public Period getPeriod(int periodIndex, Period period, boolean setIds) {
-      throw new IndexOutOfBoundsException();
-    }
+        @Override
+        public Period getPeriod(int periodIndex, Period period, boolean setIds) {
+          throw new IndexOutOfBoundsException();
+        }
 
-    @Override
-    public int getIndexOfPeriod(Object uid) {
-      return C.INDEX_UNSET;
-    }
-
-  };
+        @Override
+        public int getIndexOfPeriod(Object uid) {
+          return C.INDEX_UNSET;
+        }
+      };
 
   /**
    * Returns whether the timeline is empty.
@@ -607,7 +609,7 @@ public abstract class Timeline {
 
   /**
    * Populates a {@link Window} with data for the window at the specified index. Does not populate
-   * {@link Window#id}.
+   * {@link Window#tag}.
    *
    * @param windowIndex The index of the window.
    * @param window The {@link Window} to populate. Must not be null.
@@ -622,12 +624,12 @@ public abstract class Timeline {
    *
    * @param windowIndex The index of the window.
    * @param window The {@link Window} to populate. Must not be null.
-   * @param setIds Whether {@link Window#id} should be populated. If false, the field will be set to
-   *     null. The caller should pass false for efficiency reasons unless the field is required.
+   * @param setTag Whether {@link Window#tag} should be populated. If false, the field will be set
+   *     to null. The caller should pass false for efficiency reasons unless the field is required.
    * @return The populated {@link Window}, for convenience.
    */
-  public final Window getWindow(int windowIndex, Window window, boolean setIds) {
-    return getWindow(windowIndex, window, setIds, 0);
+  public final Window getWindow(int windowIndex, Window window, boolean setTag) {
+    return getWindow(windowIndex, window, setTag, 0);
   }
 
   /**
@@ -635,14 +637,14 @@ public abstract class Timeline {
    *
    * @param windowIndex The index of the window.
    * @param window The {@link Window} to populate. Must not be null.
-   * @param setIds Whether {@link Window#id} should be populated. If false, the field will be set to
-   *     null. The caller should pass false for efficiency reasons unless the field is required.
+   * @param setTag Whether {@link Window#tag} should be populated. If false, the field will be set
+   *     to null. The caller should pass false for efficiency reasons unless the field is required.
    * @param defaultPositionProjectionUs A duration into the future that the populated window's
    *     default start position should be projected.
    * @return The populated {@link Window}, for convenience.
    */
-  public abstract Window getWindow(int windowIndex, Window window, boolean setIds,
-      long defaultPositionProjectionUs);
+  public abstract Window getWindow(
+      int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs);
 
   /**
    * Returns the number of periods in the timeline.
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/AbstractConcatenatedTimeline.java b/library/core/src/main/java/com/google/android/exoplayer2/source/AbstractConcatenatedTimeline.java
index 696a6f6fad..8663b4c05c 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/AbstractConcatenatedTimeline.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/AbstractConcatenatedTimeline.java
@@ -155,13 +155,14 @@ import com.google.android.exoplayer2.Timeline;
   }
 
   @Override
-  public final Window getWindow(int windowIndex, Window window, boolean setIds,
-      long defaultPositionProjectionUs) {
+  public final Window getWindow(
+      int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
     int childIndex = getChildIndexByWindowIndex(windowIndex);
     int firstWindowIndexInChild = getFirstWindowIndexByChildIndex(childIndex);
     int firstPeriodIndexInChild = getFirstPeriodIndexByChildIndex(childIndex);
-    getTimelineByChildIndex(childIndex).getWindow(windowIndex - firstWindowIndexInChild, window,
-        setIds, defaultPositionProjectionUs);
+    getTimelineByChildIndex(childIndex)
+        .getWindow(
+            windowIndex - firstWindowIndexInChild, window, setTag, defaultPositionProjectionUs);
     window.firstPeriodIndex += firstPeriodIndexInChild;
     window.lastPeriodIndex += firstPeriodIndexInChild;
     return window;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/ClippingMediaSource.java b/library/core/src/main/java/com/google/android/exoplayer2/source/ClippingMediaSource.java
index a3f1b10d30..f633dd8f15 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/ClippingMediaSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/ClippingMediaSource.java
@@ -363,10 +363,10 @@ public final class ClippingMediaSource extends CompositeMediaSource<Void> {
     }
 
     @Override
-    public Window getWindow(int windowIndex, Window window, boolean setIds,
-        long defaultPositionProjectionUs) {
+    public Window getWindow(
+        int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
       timeline.getWindow(
-          /* windowIndex= */ 0, window, setIds, /* defaultPositionProjectionUs= */ 0);
+          /* windowIndex= */ 0, window, setTag, /* defaultPositionProjectionUs= */ 0);
       window.positionInFirstPeriodUs += startUs;
       window.durationUs = durationUs;
       window.isDynamic = isDynamic;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java b/library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java
index 7e44895ac7..3e39139918 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java
@@ -865,9 +865,9 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
 
     @Override
     public Window getWindow(
-        int windowIndex, Window window, boolean setIds, long defaultPositionProjectionUs) {
+        int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
       return window.set(
-          /* id= */ null,
+          /* tag= */ null,
           /* presentationStartTimeMs= */ C.TIME_UNSET,
           /* windowStartTimeMs= */ C.TIME_UNSET,
           /* isSeekable= */ false,
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/ExtractorMediaSource.java b/library/core/src/main/java/com/google/android/exoplayer2/source/ExtractorMediaSource.java
index ed467af830..c4a0487bd9 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/ExtractorMediaSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/ExtractorMediaSource.java
@@ -96,6 +96,7 @@ public final class ExtractorMediaSource extends BaseMediaSource
   private final int minLoadableRetryCount;
   private final String customCacheKey;
   private final int continueLoadingCheckIntervalBytes;
+  private final @Nullable Object tag;
 
   private long timelineDurationUs;
   private boolean timelineIsSeekable;
@@ -107,6 +108,7 @@ public final class ExtractorMediaSource extends BaseMediaSource
 
     private @Nullable ExtractorsFactory extractorsFactory;
     private @Nullable String customCacheKey;
+    private @Nullable Object tag;
     private int minLoadableRetryCount;
     private int continueLoadingCheckIntervalBytes;
     private boolean isCreateCalled;
@@ -153,6 +155,21 @@ public final class ExtractorMediaSource extends BaseMediaSource
       return this;
     }
 
+    /**
+     * Sets a tag for the media source which will be published in the {@link
+     * com.google.android.exoplayer2.Timeline} of the source as {@link
+     * com.google.android.exoplayer2.Timeline.Window#tag}.
+     *
+     * @param tag A tag for the media source.
+     * @return This factory, for convenience.
+     * @throws IllegalStateException If one of the {@code create} methods has already been called.
+     */
+    public Factory setTag(Object tag) {
+      Assertions.checkState(!isCreateCalled);
+      this.tag = tag;
+      return this;
+    }
+
     /**
      * Sets the minimum number of times to retry if a loading error occurs. The default value is
      * {@link #MIN_RETRY_COUNT_DEFAULT_FOR_MEDIA}.
@@ -202,7 +219,8 @@ public final class ExtractorMediaSource extends BaseMediaSource
           extractorsFactory,
           minLoadableRetryCount,
           customCacheKey,
-          continueLoadingCheckIntervalBytes);
+          continueLoadingCheckIntervalBytes,
+          tag);
     }
 
     /**
@@ -300,7 +318,8 @@ public final class ExtractorMediaSource extends BaseMediaSource
         extractorsFactory,
         minLoadableRetryCount,
         customCacheKey,
-        continueLoadingCheckIntervalBytes);
+        continueLoadingCheckIntervalBytes,
+        /* tag= */ null);
     if (eventListener != null && eventHandler != null) {
       addEventListener(eventHandler, new EventListenerWrapper(eventListener));
     }
@@ -312,7 +331,8 @@ public final class ExtractorMediaSource extends BaseMediaSource
       ExtractorsFactory extractorsFactory,
       int minLoadableRetryCount,
       @Nullable String customCacheKey,
-      int continueLoadingCheckIntervalBytes) {
+      int continueLoadingCheckIntervalBytes,
+      @Nullable Object tag) {
     this.uri = uri;
     this.dataSourceFactory = dataSourceFactory;
     this.extractorsFactory = extractorsFactory;
@@ -320,6 +340,7 @@ public final class ExtractorMediaSource extends BaseMediaSource
     this.customCacheKey = customCacheKey;
     this.continueLoadingCheckIntervalBytes = continueLoadingCheckIntervalBytes;
     this.timelineDurationUs = C.TIME_UNSET;
+    this.tag = tag;
   }
 
   @Override
@@ -377,7 +398,8 @@ public final class ExtractorMediaSource extends BaseMediaSource
     timelineIsSeekable = isSeekable;
     // TODO: Make timeline dynamic until its duration is known. This is non-trivial. See b/69703223.
     refreshSourceInfo(
-        new SinglePeriodTimeline(timelineDurationUs, timelineIsSeekable, /* isDynamic= */ false),
+        new SinglePeriodTimeline(
+            timelineDurationUs, timelineIsSeekable, /* isDynamic= */ false, tag),
         /* manifest= */ null);
   }
 
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/ForwardingTimeline.java b/library/core/src/main/java/com/google/android/exoplayer2/source/ForwardingTimeline.java
index cfa5cec387..c7ab7615d8 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/ForwardingTimeline.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/ForwardingTimeline.java
@@ -57,9 +57,9 @@ public abstract class ForwardingTimeline extends Timeline {
   }
 
   @Override
-  public Window getWindow(int windowIndex, Window window, boolean setIds,
-      long defaultPositionProjectionUs) {
-    return timeline.getWindow(windowIndex, window, setIds, defaultPositionProjectionUs);
+  public Window getWindow(
+      int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
+    return timeline.getWindow(windowIndex, window, setTag, defaultPositionProjectionUs);
   }
 
   @Override
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/SinglePeriodTimeline.java b/library/core/src/main/java/com/google/android/exoplayer2/source/SinglePeriodTimeline.java
index 9cce67f68c..27636d41cb 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/SinglePeriodTimeline.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/SinglePeriodTimeline.java
@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.source;
 
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.Timeline;
 import com.google.android.exoplayer2.util.Assertions;
@@ -24,7 +25,7 @@ import com.google.android.exoplayer2.util.Assertions;
  */
 public final class SinglePeriodTimeline extends Timeline {
 
-  private static final Object ID = new Object();
+  private static final Object UID = new Object();
 
   private final long presentationStartTimeMs;
   private final long windowStartTimeMs;
@@ -34,6 +35,7 @@ public final class SinglePeriodTimeline extends Timeline {
   private final long windowDefaultStartPositionUs;
   private final boolean isSeekable;
   private final boolean isDynamic;
+  private final @Nullable Object tag;
 
   /**
    * Creates a timeline containing a single period and a window that spans it.
@@ -43,7 +45,27 @@ public final class SinglePeriodTimeline extends Timeline {
    * @param isDynamic Whether the window may change when the timeline is updated.
    */
   public SinglePeriodTimeline(long durationUs, boolean isSeekable, boolean isDynamic) {
-    this(durationUs, durationUs, 0, 0, isSeekable, isDynamic);
+    this(durationUs, isSeekable, isDynamic, /* tag= */ null);
+  }
+
+  /**
+   * Creates a timeline containing a single period and a window that spans it.
+   *
+   * @param durationUs The duration of the period, in microseconds.
+   * @param isSeekable Whether seeking is supported within the period.
+   * @param isDynamic Whether the window may change when the timeline is updated.
+   * @param tag A custom tag used for {@link Timeline.Window#tag}.
+   */
+  public SinglePeriodTimeline(
+      long durationUs, boolean isSeekable, boolean isDynamic, @Nullable Object tag) {
+    this(
+        durationUs,
+        durationUs,
+        /* windowPositionInPeriodUs= */ 0,
+        /* windowDefaultStartPositionUs= */ 0,
+        isSeekable,
+        isDynamic,
+        tag);
   }
 
   /**
@@ -58,12 +80,26 @@ public final class SinglePeriodTimeline extends Timeline {
    *     which to begin playback, in microseconds.
    * @param isSeekable Whether seeking is supported within the window.
    * @param isDynamic Whether the window may change when the timeline is updated.
+   * @param tag A custom tag used for {@link Timeline.Window#tag}.
    */
-  public SinglePeriodTimeline(long periodDurationUs, long windowDurationUs,
-      long windowPositionInPeriodUs, long windowDefaultStartPositionUs, boolean isSeekable,
-      boolean isDynamic) {
-    this(C.TIME_UNSET, C.TIME_UNSET, periodDurationUs, windowDurationUs, windowPositionInPeriodUs,
-        windowDefaultStartPositionUs, isSeekable, isDynamic);
+  public SinglePeriodTimeline(
+      long periodDurationUs,
+      long windowDurationUs,
+      long windowPositionInPeriodUs,
+      long windowDefaultStartPositionUs,
+      boolean isSeekable,
+      boolean isDynamic,
+      @Nullable Object tag) {
+    this(
+        /* presentationStartTimeMs= */ C.TIME_UNSET,
+        /* windowStartTimeMs= */ C.TIME_UNSET,
+        periodDurationUs,
+        windowDurationUs,
+        windowPositionInPeriodUs,
+        windowDefaultStartPositionUs,
+        isSeekable,
+        isDynamic,
+        tag);
   }
 
   /**
@@ -81,10 +117,19 @@ public final class SinglePeriodTimeline extends Timeline {
    *     which to begin playback, in microseconds.
    * @param isSeekable Whether seeking is supported within the window.
    * @param isDynamic Whether the window may change when the timeline is updated.
+   * @param tag A custom tag used for {@link Timeline.Window#tag}. If null, an arbitrary default tag
+   *     is used.
    */
-  public SinglePeriodTimeline(long presentationStartTimeMs, long windowStartTimeMs,
-      long periodDurationUs, long windowDurationUs, long windowPositionInPeriodUs,
-      long windowDefaultStartPositionUs, boolean isSeekable, boolean isDynamic) {
+  public SinglePeriodTimeline(
+      long presentationStartTimeMs,
+      long windowStartTimeMs,
+      long periodDurationUs,
+      long windowDurationUs,
+      long windowPositionInPeriodUs,
+      long windowDefaultStartPositionUs,
+      boolean isSeekable,
+      boolean isDynamic,
+      @Nullable Object tag) {
     this.presentationStartTimeMs = presentationStartTimeMs;
     this.windowStartTimeMs = windowStartTimeMs;
     this.periodDurationUs = periodDurationUs;
@@ -93,6 +138,7 @@ public final class SinglePeriodTimeline extends Timeline {
     this.windowDefaultStartPositionUs = windowDefaultStartPositionUs;
     this.isSeekable = isSeekable;
     this.isDynamic = isDynamic;
+    this.tag = tag;
   }
 
   @Override
@@ -101,10 +147,10 @@ public final class SinglePeriodTimeline extends Timeline {
   }
 
   @Override
-  public Window getWindow(int windowIndex, Window window, boolean setIds,
-      long defaultPositionProjectionUs) {
+  public Window getWindow(
+      int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
     Assertions.checkIndex(windowIndex, 0, 1);
-    Object id = setIds ? ID : null;
+    Object tag = setTag ? this.tag : null;
     long windowDefaultStartPositionUs = this.windowDefaultStartPositionUs;
     if (isDynamic && defaultPositionProjectionUs != 0) {
       if (windowDurationUs == C.TIME_UNSET) {
@@ -118,8 +164,17 @@ public final class SinglePeriodTimeline extends Timeline {
         }
       }
     }
-    return window.set(id, presentationStartTimeMs, windowStartTimeMs, isSeekable, isDynamic,
-        windowDefaultStartPositionUs, windowDurationUs, 0, 0, windowPositionInPeriodUs);
+    return window.set(
+        tag,
+        presentationStartTimeMs,
+        windowStartTimeMs,
+        isSeekable,
+        isDynamic,
+        windowDefaultStartPositionUs,
+        windowDurationUs,
+        0,
+        0,
+        windowPositionInPeriodUs);
   }
 
   @Override
@@ -130,13 +185,13 @@ public final class SinglePeriodTimeline extends Timeline {
   @Override
   public Period getPeriod(int periodIndex, Period period, boolean setIds) {
     Assertions.checkIndex(periodIndex, 0, 1);
-    Object id = setIds ? ID : null;
-    return period.set(id, id, 0, periodDurationUs, -windowPositionInPeriodUs);
+    Object uid = setIds ? UID : null;
+    return period.set(/* id= */ null, uid, 0, periodDurationUs, -windowPositionInPeriodUs);
   }
 
   @Override
   public int getIndexOfPeriod(Object uid) {
-    return ID.equals(uid) ? 0 : C.INDEX_UNSET;
+    return UID.equals(uid) ? 0 : C.INDEX_UNSET;
   }
 
 }
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java b/library/core/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java
index f8d29a6408..2c651bef59 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java
@@ -58,6 +58,7 @@ public final class SingleSampleMediaSource extends BaseMediaSource {
     private int minLoadableRetryCount;
     private boolean treatLoadErrorsAsEndOfStream;
     private boolean isCreateCalled;
+    private @Nullable Object tag;
 
     /**
      * Creates a factory for {@link SingleSampleMediaSource}s.
@@ -70,6 +71,20 @@ public final class SingleSampleMediaSource extends BaseMediaSource {
       this.minLoadableRetryCount = DEFAULT_MIN_LOADABLE_RETRY_COUNT;
     }
 
+    /**
+     * Sets a tag for the media source which will be published in the {@link Timeline} of the source
+     * as {@link Timeline.Window#tag}.
+     *
+     * @param tag A tag for the media source.
+     * @return This factory, for convenience.
+     * @throws IllegalStateException If one of the {@code create} methods has already been called.
+     */
+    public Factory setTag(Object tag) {
+      Assertions.checkState(!isCreateCalled);
+      this.tag = tag;
+      return this;
+    }
+
     /**
      * Sets the minimum number of times to retry if a loading error occurs. The default value is
      * {@link #DEFAULT_MIN_LOADABLE_RETRY_COUNT}.
@@ -116,7 +131,8 @@ public final class SingleSampleMediaSource extends BaseMediaSource {
           format,
           durationUs,
           minLoadableRetryCount,
-          treatLoadErrorsAsEndOfStream);
+          treatLoadErrorsAsEndOfStream,
+          tag);
     }
 
     /**
@@ -188,7 +204,8 @@ public final class SingleSampleMediaSource extends BaseMediaSource {
         format,
         durationUs,
         minLoadableRetryCount,
-        /* treatLoadErrorsAsEndOfStream= */ false);
+        /* treatLoadErrorsAsEndOfStream= */ false,
+        /* tag= */ null);
   }
 
   /**
@@ -223,7 +240,8 @@ public final class SingleSampleMediaSource extends BaseMediaSource {
         format,
         durationUs,
         minLoadableRetryCount,
-        treatLoadErrorsAsEndOfStream);
+        treatLoadErrorsAsEndOfStream,
+        /* tag= */ null);
     if (eventHandler != null && eventListener != null) {
       addEventListener(eventHandler, new EventListenerWrapper(eventListener, eventSourceId));
     }
@@ -235,14 +253,16 @@ public final class SingleSampleMediaSource extends BaseMediaSource {
       Format format,
       long durationUs,
       int minLoadableRetryCount,
-      boolean treatLoadErrorsAsEndOfStream) {
+      boolean treatLoadErrorsAsEndOfStream,
+      @Nullable Object tag) {
     this.dataSourceFactory = dataSourceFactory;
     this.format = format;
     this.durationUs = durationUs;
     this.minLoadableRetryCount = minLoadableRetryCount;
     this.treatLoadErrorsAsEndOfStream = treatLoadErrorsAsEndOfStream;
     dataSpec = new DataSpec(uri);
-    timeline = new SinglePeriodTimeline(durationUs, true, false);
+    timeline =
+        new SinglePeriodTimeline(durationUs, /* isSeekable= */ true, /* isDynamic= */ false, tag);
   }
 
   // MediaSource implementation.
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/source/ads/SinglePeriodAdTimeline.java b/library/core/src/main/java/com/google/android/exoplayer2/source/ads/SinglePeriodAdTimeline.java
index ec0d6cb2fe..b0c245b706 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/source/ads/SinglePeriodAdTimeline.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/source/ads/SinglePeriodAdTimeline.java
@@ -55,9 +55,9 @@ import com.google.android.exoplayer2.util.Assertions;
   }
 
   @Override
-  public Window getWindow(int windowIndex, Window window, boolean setIds,
-      long defaultPositionProjectionUs) {
-    window = super.getWindow(windowIndex, window, setIds, defaultPositionProjectionUs);
+  public Window getWindow(
+      int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
+    window = super.getWindow(windowIndex, window, setTag, defaultPositionProjectionUs);
     if (window.durationUs == C.TIME_UNSET) {
       window.durationUs = adPlaybackState.contentDurationUs;
     }
diff --git a/library/core/src/test/java/com/google/android/exoplayer2/TimelineTest.java b/library/core/src/test/java/com/google/android/exoplayer2/TimelineTest.java
index b5457555ab..88617ce8ec 100644
--- a/library/core/src/test/java/com/google/android/exoplayer2/TimelineTest.java
+++ b/library/core/src/test/java/com/google/android/exoplayer2/TimelineTest.java
@@ -34,7 +34,7 @@ public class TimelineTest {
   @Test
   public void testSinglePeriodTimeline() {
     Timeline timeline = new FakeTimeline(new TimelineWindowDefinition(1, 111));
-    TimelineAsserts.assertWindowIds(timeline, 111);
+    TimelineAsserts.assertWindowTags(timeline, 111);
     TimelineAsserts.assertPeriodCounts(timeline, 1);
     TimelineAsserts.assertPreviousWindowIndices(
         timeline, Player.REPEAT_MODE_OFF, false, C.INDEX_UNSET);
@@ -48,7 +48,7 @@ public class TimelineTest {
   @Test
   public void testMultiPeriodTimeline() {
     Timeline timeline = new FakeTimeline(new TimelineWindowDefinition(5, 111));
-    TimelineAsserts.assertWindowIds(timeline, 111);
+    TimelineAsserts.assertWindowTags(timeline, 111);
     TimelineAsserts.assertPeriodCounts(timeline, 5);
     TimelineAsserts.assertPreviousWindowIndices(
         timeline, Player.REPEAT_MODE_OFF, false, C.INDEX_UNSET);
diff --git a/library/core/src/test/java/com/google/android/exoplayer2/source/ClippingMediaSourceTest.java b/library/core/src/test/java/com/google/android/exoplayer2/source/ClippingMediaSourceTest.java
index f0389fb6c2..e853529ae6 100644
--- a/library/core/src/test/java/com/google/android/exoplayer2/source/ClippingMediaSourceTest.java
+++ b/library/core/src/test/java/com/google/android/exoplayer2/source/ClippingMediaSourceTest.java
@@ -183,7 +183,8 @@ public final class ClippingMediaSourceTest {
             /* windowPositionInPeriodUs= */ TEST_PERIOD_DURATION_US,
             /* windowDefaultStartPositionUs= */ TEST_CLIP_AMOUNT_US,
             /* isSeekable= */ true,
-            /* isDynamic= */ true);
+            /* isDynamic= */ true,
+            /* tag= */ null);
 
     Timeline clippedTimeline = getClippedTimeline(timeline, /* durationUs= */ TEST_CLIP_AMOUNT_US);
     assertThat(clippedTimeline.getWindow(0, window).getDurationUs()).isEqualTo(TEST_CLIP_AMOUNT_US);
@@ -203,7 +204,8 @@ public final class ClippingMediaSourceTest {
             /* windowPositionInPeriodUs= */ TEST_PERIOD_DURATION_US,
             /* windowDefaultStartPositionUs= */ TEST_CLIP_AMOUNT_US,
             /* isSeekable= */ true,
-            /* isDynamic= */ true);
+            /* isDynamic= */ true,
+            /* tag= */ null);
     Timeline timeline2 =
         new SinglePeriodTimeline(
             /* periodDurationUs= */ 3 * TEST_PERIOD_DURATION_US,
@@ -211,7 +213,8 @@ public final class ClippingMediaSourceTest {
             /* windowPositionInPeriodUs= */ 2 * TEST_PERIOD_DURATION_US,
             /* windowDefaultStartPositionUs= */ TEST_CLIP_AMOUNT_US,
             /* isSeekable= */ true,
-            /* isDynamic= */ true);
+            /* isDynamic= */ true,
+            /* tag= */ null);
 
     Timeline[] clippedTimelines =
         getClippedTimelines(
@@ -248,7 +251,8 @@ public final class ClippingMediaSourceTest {
             /* windowPositionInPeriodUs= */ TEST_PERIOD_DURATION_US,
             /* windowDefaultStartPositionUs= */ TEST_CLIP_AMOUNT_US,
             /* isSeekable= */ true,
-            /* isDynamic= */ true);
+            /* isDynamic= */ true,
+            /* tag= */ null);
     Timeline timeline2 =
         new SinglePeriodTimeline(
             /* periodDurationUs= */ 4 * TEST_PERIOD_DURATION_US,
@@ -256,7 +260,8 @@ public final class ClippingMediaSourceTest {
             /* windowPositionInPeriodUs= */ 3 * TEST_PERIOD_DURATION_US,
             /* windowDefaultStartPositionUs= */ TEST_CLIP_AMOUNT_US,
             /* isSeekable= */ true,
-            /* isDynamic= */ true);
+            /* isDynamic= */ true,
+            /* tag= */ null);
 
     Timeline[] clippedTimelines =
         getClippedTimelines(
@@ -293,7 +298,8 @@ public final class ClippingMediaSourceTest {
             /* windowPositionInPeriodUs= */ TEST_PERIOD_DURATION_US,
             /* windowDefaultStartPositionUs= */ TEST_CLIP_AMOUNT_US,
             /* isSeekable= */ true,
-            /* isDynamic= */ true);
+            /* isDynamic= */ true,
+            /* tag= */ null);
     Timeline timeline2 =
         new SinglePeriodTimeline(
             /* periodDurationUs= */ 3 * TEST_PERIOD_DURATION_US,
@@ -301,7 +307,8 @@ public final class ClippingMediaSourceTest {
             /* windowPositionInPeriodUs= */ 2 * TEST_PERIOD_DURATION_US,
             /* windowDefaultStartPositionUs= */ TEST_CLIP_AMOUNT_US,
             /* isSeekable= */ true,
-            /* isDynamic= */ true);
+            /* isDynamic= */ true,
+            /* tag= */ null);
 
     Timeline[] clippedTimelines =
         getClippedTimelines(
@@ -339,7 +346,8 @@ public final class ClippingMediaSourceTest {
             /* windowPositionInPeriodUs= */ TEST_PERIOD_DURATION_US,
             /* windowDefaultStartPositionUs= */ TEST_CLIP_AMOUNT_US,
             /* isSeekable= */ true,
-            /* isDynamic= */ true);
+            /* isDynamic= */ true,
+            /* tag= */ null);
     Timeline timeline2 =
         new SinglePeriodTimeline(
             /* periodDurationUs= */ 4 * TEST_PERIOD_DURATION_US,
@@ -347,7 +355,8 @@ public final class ClippingMediaSourceTest {
             /* windowPositionInPeriodUs= */ 3 * TEST_PERIOD_DURATION_US,
             /* windowDefaultStartPositionUs= */ TEST_CLIP_AMOUNT_US,
             /* isSeekable= */ true,
-            /* isDynamic= */ true);
+            /* isDynamic= */ true,
+            /* tag= */ null);
 
     Timeline[] clippedTimelines =
         getClippedTimelines(
@@ -382,7 +391,7 @@ public final class ClippingMediaSourceTest {
     Timeline clippedTimeline =
         getClippedTimeline(
             timeline, TEST_CLIP_AMOUNT_US, TEST_PERIOD_DURATION_US - TEST_CLIP_AMOUNT_US);
-    TimelineAsserts.assertWindowIds(clippedTimeline, 111);
+    TimelineAsserts.assertWindowTags(clippedTimeline, 111);
     TimelineAsserts.assertPeriodCounts(clippedTimeline, 1);
     TimelineAsserts.assertPreviousWindowIndices(
         clippedTimeline, Player.REPEAT_MODE_OFF, false, C.INDEX_UNSET);
diff --git a/library/core/src/test/java/com/google/android/exoplayer2/source/ConcatenatingMediaSourceTest.java b/library/core/src/test/java/com/google/android/exoplayer2/source/ConcatenatingMediaSourceTest.java
index 89ebaac071..5231fc22ed 100644
--- a/library/core/src/test/java/com/google/android/exoplayer2/source/ConcatenatingMediaSourceTest.java
+++ b/library/core/src/test/java/com/google/android/exoplayer2/source/ConcatenatingMediaSourceTest.java
@@ -75,50 +75,50 @@ public final class ConcatenatingMediaSourceTest {
     mediaSource.addMediaSource(childSources[0]);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 1);
-    TimelineAsserts.assertWindowIds(timeline, 111);
+    TimelineAsserts.assertWindowTags(timeline, 111);
 
     // Add at front of queue.
     mediaSource.addMediaSource(0, childSources[1]);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 2, 1);
-    TimelineAsserts.assertWindowIds(timeline, 222, 111);
+    TimelineAsserts.assertWindowTags(timeline, 222, 111);
 
     // Add at back of queue.
     mediaSource.addMediaSource(childSources[2]);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 2, 1, 3);
-    TimelineAsserts.assertWindowIds(timeline, 222, 111, 333);
+    TimelineAsserts.assertWindowTags(timeline, 222, 111, 333);
 
     // Add in the middle.
     mediaSource.addMediaSource(1, childSources[3]);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 2, 4, 1, 3);
-    TimelineAsserts.assertWindowIds(timeline, 222, 444, 111, 333);
+    TimelineAsserts.assertWindowTags(timeline, 222, 444, 111, 333);
 
     // Add bulk.
     mediaSource.addMediaSources(
         3, Arrays.<MediaSource>asList(childSources[4], childSources[5], childSources[6]));
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 2, 4, 1, 5, 6, 7, 3);
-    TimelineAsserts.assertWindowIds(timeline, 222, 444, 111, 555, 666, 777, 333);
+    TimelineAsserts.assertWindowTags(timeline, 222, 444, 111, 555, 666, 777, 333);
 
     // Move sources.
     mediaSource.moveMediaSource(2, 3);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 2, 4, 5, 1, 6, 7, 3);
-    TimelineAsserts.assertWindowIds(timeline, 222, 444, 555, 111, 666, 777, 333);
+    TimelineAsserts.assertWindowTags(timeline, 222, 444, 555, 111, 666, 777, 333);
     mediaSource.moveMediaSource(3, 2);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 2, 4, 1, 5, 6, 7, 3);
-    TimelineAsserts.assertWindowIds(timeline, 222, 444, 111, 555, 666, 777, 333);
+    TimelineAsserts.assertWindowTags(timeline, 222, 444, 111, 555, 666, 777, 333);
     mediaSource.moveMediaSource(0, 6);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 4, 1, 5, 6, 7, 3, 2);
-    TimelineAsserts.assertWindowIds(timeline, 444, 111, 555, 666, 777, 333, 222);
+    TimelineAsserts.assertWindowTags(timeline, 444, 111, 555, 666, 777, 333, 222);
     mediaSource.moveMediaSource(6, 0);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 2, 4, 1, 5, 6, 7, 3);
-    TimelineAsserts.assertWindowIds(timeline, 222, 444, 111, 555, 666, 777, 333);
+    TimelineAsserts.assertWindowTags(timeline, 222, 444, 111, 555, 666, 777, 333);
 
     // Remove in the middle.
     mediaSource.removeMediaSource(3);
@@ -130,7 +130,7 @@ public final class ConcatenatingMediaSourceTest {
     mediaSource.removeMediaSource(1);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 2, 1, 3);
-    TimelineAsserts.assertWindowIds(timeline, 222, 111, 333);
+    TimelineAsserts.assertWindowTags(timeline, 222, 111, 333);
     for (int i = 3; i <= 6; i++) {
       childSources[i].assertReleased();
     }
@@ -169,14 +169,14 @@ public final class ConcatenatingMediaSourceTest {
     mediaSource.removeMediaSource(0);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 1, 3);
-    TimelineAsserts.assertWindowIds(timeline, 111, 333);
+    TimelineAsserts.assertWindowTags(timeline, 111, 333);
     childSources[1].assertReleased();
 
     // Remove at back of queue.
     mediaSource.removeMediaSource(1);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 1);
-    TimelineAsserts.assertWindowIds(timeline, 111);
+    TimelineAsserts.assertWindowTags(timeline, 111);
     childSources[2].assertReleased();
 
     // Remove last source.
@@ -200,7 +200,7 @@ public final class ConcatenatingMediaSourceTest {
 
     Timeline timeline = testRunner.prepareSource();
     TimelineAsserts.assertPeriodCounts(timeline, 3, 4, 2);
-    TimelineAsserts.assertWindowIds(timeline, 333, 444, 222);
+    TimelineAsserts.assertWindowTags(timeline, 333, 444, 222);
     TimelineAsserts.assertNextWindowIndices(
         timeline, Player.REPEAT_MODE_OFF, false, 1, 2, C.INDEX_UNSET);
     TimelineAsserts.assertPreviousWindowIndices(
@@ -241,7 +241,7 @@ public final class ConcatenatingMediaSourceTest {
     // placeholder information for lazy sources.
     Timeline timeline = testRunner.prepareSource();
     TimelineAsserts.assertPeriodCounts(timeline, 1, 1);
-    TimelineAsserts.assertWindowIds(timeline, 111, null);
+    TimelineAsserts.assertWindowTags(timeline, 111, null);
     TimelineAsserts.assertWindowIsDynamic(timeline, false, true);
 
     // Trigger source info refresh for lazy source and check that the timeline now contains all
@@ -255,7 +255,7 @@ public final class ConcatenatingMediaSourceTest {
         });
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 1, 9);
-    TimelineAsserts.assertWindowIds(timeline, 111, 999);
+    TimelineAsserts.assertWindowTags(timeline, 111, 999);
     TimelineAsserts.assertWindowIsDynamic(timeline, false, false);
     testRunner.assertPrepareAndReleaseAllPeriods();
     testRunner.assertCompletedManifestLoads(0, 1);
@@ -272,7 +272,7 @@ public final class ConcatenatingMediaSourceTest {
     mediaSource.removeMediaSource(2);
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 1, 1, 2, 9);
-    TimelineAsserts.assertWindowIds(timeline, null, 111, 222, 999);
+    TimelineAsserts.assertWindowTags(timeline, null, 111, 222, 999);
     TimelineAsserts.assertWindowIsDynamic(timeline, true, false, false, false);
 
     // Create a period from an unprepared lazy media source and assert Callback.onPrepared is not
@@ -300,7 +300,7 @@ public final class ConcatenatingMediaSourceTest {
         });
     timeline = testRunner.assertTimelineChangeBlocking();
     TimelineAsserts.assertPeriodCounts(timeline, 8, 1, 2, 9);
-    TimelineAsserts.assertWindowIds(timeline, 888, 111, 222, 999);
+    TimelineAsserts.assertWindowTags(timeline, 888, 111, 222, 999);
     TimelineAsserts.assertWindowIsDynamic(timeline, false, false, false, false);
     assertThat(preparedCondition.getCount()).isEqualTo(0);
 
@@ -346,7 +346,7 @@ public final class ConcatenatingMediaSourceTest {
     testRunner.assertTimelineChangeBlocking();
     mediaSource.addMediaSource(6, mediaSources[2]);
     timeline = testRunner.assertTimelineChangeBlocking();
-    TimelineAsserts.assertWindowIds(timeline, 111, 222, 333);
+    TimelineAsserts.assertWindowTags(timeline, 111, 222, 333);
     TimelineAsserts.assertPeriodCounts(timeline, 1, 2, 3);
     TimelineAsserts.assertPreviousWindowIndices(
         timeline, Player.REPEAT_MODE_OFF, false, C.INDEX_UNSET, 0, 1);
@@ -685,7 +685,7 @@ public final class ConcatenatingMediaSourceTest {
     testRunner = new MediaSourceTestRunner(mediaSource, null);
     mediaSource.addMediaSources(Arrays.<MediaSource>asList(createMediaSources(3)));
     Timeline timeline = testRunner.prepareSource();
-    TimelineAsserts.assertWindowIds(timeline, 111, 222, 333);
+    TimelineAsserts.assertWindowTags(timeline, 111, 222, 333);
     TimelineAsserts.assertPeriodCounts(timeline, 1, 2, 3);
     TimelineAsserts.assertPreviousWindowIndices(
         timeline, Player.REPEAT_MODE_OFF, /* shuffleModeEnabled= */ false, C.INDEX_UNSET, 0, 1);
@@ -736,7 +736,7 @@ public final class ConcatenatingMediaSourceTest {
     nestedSource2.addMediaSource(childSources[3]);
     Timeline timeline = testRunner.assertTimelineChangeBlocking();
 
-    TimelineAsserts.assertWindowIds(timeline, 111, 222, 333, 444);
+    TimelineAsserts.assertWindowTags(timeline, 111, 222, 333, 444);
     TimelineAsserts.assertPeriodCounts(timeline, 1, 2, 3, 4);
     TimelineAsserts.assertPreviousWindowIndices(
         timeline, Player.REPEAT_MODE_OFF, /* shuffleModeEnabled= */ false, C.INDEX_UNSET, 0, 1, 2);
diff --git a/library/core/src/test/java/com/google/android/exoplayer2/source/LoopingMediaSourceTest.java b/library/core/src/test/java/com/google/android/exoplayer2/source/LoopingMediaSourceTest.java
index 6aa710aff4..d639bc168a 100644
--- a/library/core/src/test/java/com/google/android/exoplayer2/source/LoopingMediaSourceTest.java
+++ b/library/core/src/test/java/com/google/android/exoplayer2/source/LoopingMediaSourceTest.java
@@ -50,7 +50,7 @@ public class LoopingMediaSourceTest {
   @Test
   public void testSingleLoop() throws IOException {
     Timeline timeline = getLoopingTimeline(multiWindowTimeline, 1);
-    TimelineAsserts.assertWindowIds(timeline, 111, 222, 333);
+    TimelineAsserts.assertWindowTags(timeline, 111, 222, 333);
     TimelineAsserts.assertPeriodCounts(timeline, 1, 1, 1);
     for (boolean shuffled : new boolean[] {false, true}) {
       TimelineAsserts.assertPreviousWindowIndices(
@@ -69,7 +69,7 @@ public class LoopingMediaSourceTest {
   @Test
   public void testMultiLoop() throws IOException {
     Timeline timeline = getLoopingTimeline(multiWindowTimeline, 3);
-    TimelineAsserts.assertWindowIds(timeline, 111, 222, 333, 111, 222, 333, 111, 222, 333);
+    TimelineAsserts.assertWindowTags(timeline, 111, 222, 333, 111, 222, 333, 111, 222, 333);
     TimelineAsserts.assertPeriodCounts(timeline, 1, 1, 1, 1, 1, 1, 1, 1, 1);
     for (boolean shuffled : new boolean[] {false, true}) {
       TimelineAsserts.assertPreviousWindowIndices(
@@ -90,7 +90,7 @@ public class LoopingMediaSourceTest {
   @Test
   public void testInfiniteLoop() throws IOException {
     Timeline timeline = getLoopingTimeline(multiWindowTimeline, Integer.MAX_VALUE);
-    TimelineAsserts.assertWindowIds(timeline, 111, 222, 333);
+    TimelineAsserts.assertWindowTags(timeline, 111, 222, 333);
     TimelineAsserts.assertPeriodCounts(timeline, 1, 1, 1);
     for (boolean shuffled : new boolean[] {false, true}) {
       TimelineAsserts.assertPreviousWindowIndices(
diff --git a/library/core/src/test/java/com/google/android/exoplayer2/source/SinglePeriodTimelineTest.java b/library/core/src/test/java/com/google/android/exoplayer2/source/SinglePeriodTimelineTest.java
index 2627052cc5..2587b78d99 100644
--- a/library/core/src/test/java/com/google/android/exoplayer2/source/SinglePeriodTimelineTest.java
+++ b/library/core/src/test/java/com/google/android/exoplayer2/source/SinglePeriodTimelineTest.java
@@ -56,8 +56,15 @@ public final class SinglePeriodTimelineTest {
   @Test
   public void testGetPeriodPositionDynamicWindowKnownDuration() {
     long windowDurationUs = 1000;
-    SinglePeriodTimeline timeline = new SinglePeriodTimeline(windowDurationUs, windowDurationUs, 0,
-        0, false, true);
+    SinglePeriodTimeline timeline =
+        new SinglePeriodTimeline(
+            windowDurationUs,
+            windowDurationUs,
+            /* windowPositionInPeriodUs= */ 0,
+            /* windowDefaultStartPositionUs= */ 0,
+            /* isSeekable= */ false,
+            /* isDynamic= */ true,
+            /* tag= */ null);
     // Should return null with a positive position projection beyond window duration.
     Pair<Integer, Long> position = timeline.getPeriodPosition(window, period, 0, C.TIME_UNSET,
         windowDurationUs + 1);
@@ -72,4 +79,48 @@ public final class SinglePeriodTimelineTest {
     assertThat(position.second).isEqualTo(0);
   }
 
+  @Test
+  public void setNullTag_returnsNullTag_butUsesDefaultUid() {
+    SinglePeriodTimeline timeline =
+        new SinglePeriodTimeline(
+            /* durationUs= */ C.TIME_UNSET,
+            /* isSeekable= */ false,
+            /* isDynamic= */ false,
+            /* tag= */ null);
+
+    assertThat(timeline.getWindow(/* windowIndex= */ 0, window, /* setTag= */ false).tag).isNull();
+    assertThat(timeline.getWindow(/* windowIndex= */ 0, window, /* setTag= */ true).tag).isNull();
+    assertThat(timeline.getPeriod(/* periodIndex= */ 0, period, /* setIds= */ false).id).isNull();
+    assertThat(timeline.getPeriod(/* periodIndex= */ 0, period, /* setIds= */ true).id).isNull();
+    assertThat(timeline.getPeriod(/* periodIndex= */ 0, period, /* setIds= */ false).uid).isNull();
+    assertThat(timeline.getPeriod(/* periodIndex= */ 0, period, /* setIds= */ true).uid)
+        .isNotNull();
+  }
+
+  @Test
+  public void setTag_isUsedForWindowTag() {
+    Object tag = new Object();
+    SinglePeriodTimeline timeline =
+        new SinglePeriodTimeline(
+            /* durationUs= */ C.TIME_UNSET, /* isSeekable= */ false, /* isDynamic= */ false, tag);
+
+    assertThat(timeline.getWindow(/* windowIndex= */ 0, window, /* setTag= */ false).tag).isNull();
+    assertThat(timeline.getWindow(/* windowIndex= */ 0, window, /* setTag= */ true).tag)
+        .isEqualTo(tag);
+  }
+
+  @Test
+  public void getIndexOfPeriod_returnsPeriod() {
+    SinglePeriodTimeline timeline =
+        new SinglePeriodTimeline(
+            /* durationUs= */ C.TIME_UNSET,
+            /* isSeekable= */ false,
+            /* isDynamic= */ false,
+            /* tag= */ null);
+    Object uid = timeline.getPeriod(/* periodIndex= */ 0, period, /* setIds= */ true).uid;
+
+    assertThat(timeline.getIndexOfPeriod(uid)).isEqualTo(0);
+    assertThat(timeline.getIndexOfPeriod(/* uid= */ null)).isEqualTo(C.INDEX_UNSET);
+    assertThat(timeline.getIndexOfPeriod(/* uid= */ new Object())).isEqualTo(C.INDEX_UNSET);
+  }
 }
diff --git a/library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java b/library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java
index b6371c2219..7b854e9d29 100644
--- a/library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java
+++ b/library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java
@@ -77,6 +77,7 @@ public final class DashMediaSource extends BaseMediaSource {
     private int minLoadableRetryCount;
     private long livePresentationDelayMs;
     private boolean isCreateCalled;
+    private @Nullable Object tag;
 
     /**
      * Creates a new factory for {@link DashMediaSource}s.
@@ -97,6 +98,21 @@ public final class DashMediaSource extends BaseMediaSource {
       compositeSequenceableLoaderFactory = new DefaultCompositeSequenceableLoaderFactory();
     }
 
+    /**
+     * Sets a tag for the media source which will be published in the {@link
+     * com.google.android.exoplayer2.Timeline} of the source as {@link
+     * com.google.android.exoplayer2.Timeline.Window#tag}.
+     *
+     * @param tag A tag for the media source.
+     * @return This factory, for convenience.
+     * @throws IllegalStateException If one of the {@code create} methods has already been called.
+     */
+    public Factory setTag(Object tag) {
+      Assertions.checkState(!isCreateCalled);
+      this.tag = tag;
+      return this;
+    }
+
     /**
      * Sets the minimum number of times to retry if a loading error occurs. The default value is
      * {@link #DEFAULT_MIN_LOADABLE_RETRY_COUNT}.
@@ -175,13 +191,14 @@ public final class DashMediaSource extends BaseMediaSource {
       isCreateCalled = true;
       return new DashMediaSource(
           manifest,
-          null,
-          null,
-          null,
+          /* manifestUri= */ null,
+          /* manifestDataSourceFactory= */ null,
+          /* manifestParser= */ null,
           chunkSourceFactory,
           compositeSequenceableLoaderFactory,
           minLoadableRetryCount,
-          livePresentationDelayMs);
+          livePresentationDelayMs,
+          tag);
     }
 
     /**
@@ -213,14 +230,15 @@ public final class DashMediaSource extends BaseMediaSource {
         manifestParser = new DashManifestParser();
       }
       return new DashMediaSource(
-          null,
+          /* manifest= */ null,
           Assertions.checkNotNull(manifestUri),
           manifestDataSourceFactory,
           manifestParser,
           chunkSourceFactory,
           compositeSequenceableLoaderFactory,
           minLoadableRetryCount,
-          livePresentationDelayMs);
+          livePresentationDelayMs,
+          tag);
     }
 
     /**
@@ -290,6 +308,7 @@ public final class DashMediaSource extends BaseMediaSource {
   private final Runnable simulateManifestRefreshRunnable;
   private final PlayerEmsgCallback playerEmsgCallback;
   private final LoaderErrorThrower manifestLoadErrorThrower;
+  private final @Nullable Object tag;
 
   private DataSource dataSource;
   private Loader loader;
@@ -349,13 +368,14 @@ public final class DashMediaSource extends BaseMediaSource {
       MediaSourceEventListener eventListener) {
     this(
         manifest,
-        null,
-        null,
-        null,
+        /* manifestUri= */ null,
+        /* manifestDataSourceFactory= */ null,
+        /* manifestParser= */ null,
         chunkSourceFactory,
         new DefaultCompositeSequenceableLoaderFactory(),
         minLoadableRetryCount,
-        DEFAULT_LIVE_PRESENTATION_DELAY_PREFER_MANIFEST_MS);
+        DEFAULT_LIVE_PRESENTATION_DELAY_PREFER_MANIFEST_MS,
+        /* tag= */ null);
     if (eventHandler != null && eventListener != null) {
       addEventListener(eventHandler, eventListener);
     }
@@ -444,14 +464,15 @@ public final class DashMediaSource extends BaseMediaSource {
       Handler eventHandler,
       MediaSourceEventListener eventListener) {
     this(
-        null,
+        /* manifest= */ null,
         manifestUri,
         manifestDataSourceFactory,
         manifestParser,
         chunkSourceFactory,
         new DefaultCompositeSequenceableLoaderFactory(),
         minLoadableRetryCount,
-        livePresentationDelayMs);
+        livePresentationDelayMs,
+        /* tag= */ null);
     if (eventHandler != null && eventListener != null) {
       addEventListener(eventHandler, eventListener);
     }
@@ -465,7 +486,8 @@ public final class DashMediaSource extends BaseMediaSource {
       DashChunkSource.Factory chunkSourceFactory,
       CompositeSequenceableLoaderFactory compositeSequenceableLoaderFactory,
       int minLoadableRetryCount,
-      long livePresentationDelayMs) {
+      long livePresentationDelayMs,
+      @Nullable Object tag) {
     this.initialManifestUri = manifestUri;
     this.manifest = manifest;
     this.manifestUri = manifestUri;
@@ -475,6 +497,7 @@ public final class DashMediaSource extends BaseMediaSource {
     this.minLoadableRetryCount = minLoadableRetryCount;
     this.livePresentationDelayMs = livePresentationDelayMs;
     this.compositeSequenceableLoaderFactory = compositeSequenceableLoaderFactory;
+    this.tag = tag;
     sideloadedManifest = manifest != null;
     manifestEventDispatcher = createEventDispatcher(/* mediaPeriodId= */ null);
     manifestUriLock = new Object();
@@ -862,9 +885,16 @@ public final class DashMediaSource extends BaseMediaSource {
     }
     long windowStartTimeMs = manifest.availabilityStartTimeMs
         + manifest.getPeriod(0).startMs + C.usToMs(currentStartTimeUs);
-    DashTimeline timeline = new DashTimeline(manifest.availabilityStartTimeMs, windowStartTimeMs,
-        firstPeriodId, currentStartTimeUs, windowDurationUs, windowDefaultStartPositionUs,
-        manifest);
+    DashTimeline timeline =
+        new DashTimeline(
+            manifest.availabilityStartTimeMs,
+            windowStartTimeMs,
+            firstPeriodId,
+            currentStartTimeUs,
+            windowDurationUs,
+            windowDefaultStartPositionUs,
+            manifest,
+            tag);
     refreshSourceInfo(timeline, manifest);
 
     if (!sideloadedManifest) {
@@ -993,10 +1023,17 @@ public final class DashMediaSource extends BaseMediaSource {
     private final long windowDurationUs;
     private final long windowDefaultStartPositionUs;
     private final DashManifest manifest;
+    private final @Nullable Object windowTag;
 
-    public DashTimeline(long presentationStartTimeMs, long windowStartTimeMs, int firstPeriodId,
-        long offsetInFirstPeriodUs, long windowDurationUs, long windowDefaultStartPositionUs,
-        DashManifest manifest) {
+    public DashTimeline(
+        long presentationStartTimeMs,
+        long windowStartTimeMs,
+        int firstPeriodId,
+        long offsetInFirstPeriodUs,
+        long windowDurationUs,
+        long windowDefaultStartPositionUs,
+        DashManifest manifest,
+        @Nullable Object windowTag) {
       this.presentationStartTimeMs = presentationStartTimeMs;
       this.windowStartTimeMs = windowStartTimeMs;
       this.firstPeriodId = firstPeriodId;
@@ -1004,6 +1041,7 @@ public final class DashMediaSource extends BaseMediaSource {
       this.windowDurationUs = windowDurationUs;
       this.windowDefaultStartPositionUs = windowDefaultStartPositionUs;
       this.manifest = manifest;
+      this.windowTag = windowTag;
     }
 
     @Override
@@ -1028,14 +1066,23 @@ public final class DashMediaSource extends BaseMediaSource {
     }
 
     @Override
-    public Window getWindow(int windowIndex, Window window, boolean setIdentifier,
-        long defaultPositionProjectionUs) {
+    public Window getWindow(
+        int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
       Assertions.checkIndex(windowIndex, 0, 1);
       long windowDefaultStartPositionUs = getAdjustedWindowDefaultStartPositionUs(
           defaultPositionProjectionUs);
-      return window.set(null, presentationStartTimeMs, windowStartTimeMs, true /* isSeekable */,
-          manifest.dynamic, windowDefaultStartPositionUs, windowDurationUs, 0,
-          manifest.getPeriodCount() - 1, offsetInFirstPeriodUs);
+      Object tag = setTag ? windowTag : null;
+      return window.set(
+          tag,
+          presentationStartTimeMs,
+          windowStartTimeMs,
+          /* isSeekable= */ true,
+          manifest.dynamic,
+          windowDefaultStartPositionUs,
+          windowDurationUs,
+          /* firstPeriodIndex= */ 0,
+          manifest.getPeriodCount() - 1,
+          offsetInFirstPeriodUs);
     }
 
     @Override
diff --git a/library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaSource.java b/library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaSource.java
index 37d37575c2..01bb36f6ce 100644
--- a/library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaSource.java
+++ b/library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsMediaSource.java
@@ -62,6 +62,7 @@ public final class HlsMediaSource extends BaseMediaSource
     private int minLoadableRetryCount;
     private boolean allowChunklessPreparation;
     private boolean isCreateCalled;
+    private @Nullable Object tag;
 
     /**
      * Creates a new factory for {@link HlsMediaSource}s.
@@ -87,6 +88,21 @@ public final class HlsMediaSource extends BaseMediaSource
       compositeSequenceableLoaderFactory = new DefaultCompositeSequenceableLoaderFactory();
     }
 
+    /**
+     * Sets a tag for the media source which will be published in the {@link
+     * com.google.android.exoplayer2.Timeline} of the source as {@link
+     * com.google.android.exoplayer2.Timeline.Window#tag}.
+     *
+     * @param tag A tag for the media source.
+     * @return This factory, for convenience.
+     * @throws IllegalStateException If one of the {@code create} methods has already been called.
+     */
+    public Factory setTag(Object tag) {
+      Assertions.checkState(!isCreateCalled);
+      this.tag = tag;
+      return this;
+    }
+
     /**
      * Sets the factory for {@link Extractor}s for the segments. The default value is {@link
      * HlsExtractorFactory#DEFAULT}.
@@ -181,7 +197,8 @@ public final class HlsMediaSource extends BaseMediaSource
           compositeSequenceableLoaderFactory,
           minLoadableRetryCount,
           playlistParser,
-          allowChunklessPreparation);
+          allowChunklessPreparation,
+          tag);
     }
 
     /**
@@ -218,6 +235,7 @@ public final class HlsMediaSource extends BaseMediaSource
   private final int minLoadableRetryCount;
   private final ParsingLoadable.Parser<HlsPlaylist> playlistParser;
   private final boolean allowChunklessPreparation;
+  private final @Nullable Object tag;
 
   private HlsPlaylistTracker playlistTracker;
 
@@ -292,7 +310,8 @@ public final class HlsMediaSource extends BaseMediaSource
         new DefaultCompositeSequenceableLoaderFactory(),
         minLoadableRetryCount,
         playlistParser,
-        false);
+        /* allowChunklessPreparation= */ false,
+        /* tag= */ null);
     if (eventHandler != null && eventListener != null) {
       addEventListener(eventHandler, eventListener);
     }
@@ -305,7 +324,8 @@ public final class HlsMediaSource extends BaseMediaSource
       CompositeSequenceableLoaderFactory compositeSequenceableLoaderFactory,
       int minLoadableRetryCount,
       ParsingLoadable.Parser<HlsPlaylist> playlistParser,
-      boolean allowChunklessPreparation) {
+      boolean allowChunklessPreparation,
+      @Nullable Object tag) {
     this.manifestUri = manifestUri;
     this.dataSourceFactory = dataSourceFactory;
     this.extractorFactory = extractorFactory;
@@ -313,6 +333,7 @@ public final class HlsMediaSource extends BaseMediaSource
     this.minLoadableRetryCount = minLoadableRetryCount;
     this.playlistParser = playlistParser;
     this.allowChunklessPreparation = allowChunklessPreparation;
+    this.tag = tag;
   }
 
   @Override
@@ -388,7 +409,8 @@ public final class HlsMediaSource extends BaseMediaSource
               /* windowPositionInPeriodUs= */ offsetFromInitialStartTimeUs,
               windowDefaultStartPositionUs,
               /* isSeekable= */ true,
-              /* isDynamic= */ !playlist.hasEndTag);
+              /* isDynamic= */ !playlist.hasEndTag,
+              tag);
     } else /* not live */ {
       if (windowDefaultStartPositionUs == C.TIME_UNSET) {
         windowDefaultStartPositionUs = 0;
@@ -402,7 +424,8 @@ public final class HlsMediaSource extends BaseMediaSource
               /* windowPositionInPeriodUs= */ 0,
               windowDefaultStartPositionUs,
               /* isSeekable= */ true,
-              /* isDynamic= */ false);
+              /* isDynamic= */ false,
+              tag);
     }
     refreshSourceInfo(timeline, new HlsManifest(playlistTracker.getMasterPlaylist(), playlist));
   }
diff --git a/library/smoothstreaming/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/SsMediaSource.java b/library/smoothstreaming/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/SsMediaSource.java
index d07cfd116b..72d1ba1efd 100644
--- a/library/smoothstreaming/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/SsMediaSource.java
+++ b/library/smoothstreaming/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/SsMediaSource.java
@@ -66,6 +66,7 @@ public final class SsMediaSource extends BaseMediaSource
     private int minLoadableRetryCount;
     private long livePresentationDelayMs;
     private boolean isCreateCalled;
+    private @Nullable Object tag;
 
     /**
      * Creates a new factory for {@link SsMediaSource}s.
@@ -86,6 +87,20 @@ public final class SsMediaSource extends BaseMediaSource
       compositeSequenceableLoaderFactory = new DefaultCompositeSequenceableLoaderFactory();
     }
 
+    /**
+     * Sets a tag for the media source which will be published in the {@link Timeline} of the source
+     * as {@link Timeline.Window#tag}.
+     *
+     * @param tag A tag for the media source.
+     * @return This factory, for convenience.
+     * @throws IllegalStateException If one of the {@code create} methods has already been called.
+     */
+    public Factory setTag(Object tag) {
+      Assertions.checkState(!isCreateCalled);
+      this.tag = tag;
+      return this;
+    }
+
     /**
      * Sets the minimum number of times to retry if a loading error occurs. The default value is
      * {@link #DEFAULT_MIN_LOADABLE_RETRY_COUNT}.
@@ -161,13 +176,14 @@ public final class SsMediaSource extends BaseMediaSource
       isCreateCalled = true;
       return new SsMediaSource(
           manifest,
-          null,
-          null,
-          null,
+          /* manifestUri= */ null,
+          /* manifestDataSourceFactory= */ null,
+          /* manifestParser= */ null,
           chunkSourceFactory,
           compositeSequenceableLoaderFactory,
           minLoadableRetryCount,
-          livePresentationDelayMs);
+          livePresentationDelayMs,
+          tag);
     }
 
     /**
@@ -199,14 +215,15 @@ public final class SsMediaSource extends BaseMediaSource
         manifestParser = new SsManifestParser();
       }
       return new SsMediaSource(
-          null,
+          /* manifest= */ null,
           Assertions.checkNotNull(manifestUri),
           manifestDataSourceFactory,
           manifestParser,
           chunkSourceFactory,
           compositeSequenceableLoaderFactory,
           minLoadableRetryCount,
-          livePresentationDelayMs);
+          livePresentationDelayMs,
+          tag);
     }
 
     /**
@@ -261,6 +278,7 @@ public final class SsMediaSource extends BaseMediaSource
   private final EventDispatcher manifestEventDispatcher;
   private final ParsingLoadable.Parser<? extends SsManifest> manifestParser;
   private final ArrayList<SsMediaPeriod> mediaPeriods;
+  private final @Nullable Object tag;
 
   private DataSource manifestDataSource;
   private Loader manifestLoader;
@@ -309,13 +327,14 @@ public final class SsMediaSource extends BaseMediaSource
       MediaSourceEventListener eventListener) {
     this(
         manifest,
-        null,
-        null,
-        null,
+        /* manifestUri= */ null,
+        /* manifestDataSourceFactory= */ null,
+        /* manifestParser= */ null,
         chunkSourceFactory,
         new DefaultCompositeSequenceableLoaderFactory(),
         minLoadableRetryCount,
-        DEFAULT_LIVE_PRESENTATION_DELAY_MS);
+        DEFAULT_LIVE_PRESENTATION_DELAY_MS,
+        /* tag= */ null);
     if (eventHandler != null && eventListener != null) {
       addEventListener(eventHandler, eventListener);
     }
@@ -400,14 +419,15 @@ public final class SsMediaSource extends BaseMediaSource
       Handler eventHandler,
       MediaSourceEventListener eventListener) {
     this(
-        null,
+        /* manifest= */ null,
         manifestUri,
         manifestDataSourceFactory,
         manifestParser,
         chunkSourceFactory,
         new DefaultCompositeSequenceableLoaderFactory(),
         minLoadableRetryCount,
-        livePresentationDelayMs);
+        livePresentationDelayMs,
+        /* tag= */ null);
     if (eventHandler != null && eventListener != null) {
       addEventListener(eventHandler, eventListener);
     }
@@ -421,7 +441,8 @@ public final class SsMediaSource extends BaseMediaSource
       SsChunkSource.Factory chunkSourceFactory,
       CompositeSequenceableLoaderFactory compositeSequenceableLoaderFactory,
       int minLoadableRetryCount,
-      long livePresentationDelayMs) {
+      long livePresentationDelayMs,
+      @Nullable Object tag) {
     Assertions.checkState(manifest == null || !manifest.isLive);
     this.manifest = manifest;
     this.manifestUri = manifestUri == null ? null : SsUtil.fixManifestUri(manifestUri);
@@ -432,6 +453,7 @@ public final class SsMediaSource extends BaseMediaSource
     this.minLoadableRetryCount = minLoadableRetryCount;
     this.livePresentationDelayMs = livePresentationDelayMs;
     this.manifestEventDispatcher = createEventDispatcher(/* mediaPeriodId= */ null);
+    this.tag = tag;
     sideloadedManifest = manifest != null;
     mediaPeriods = new ArrayList<>();
   }
@@ -555,8 +577,15 @@ public final class SsMediaSource extends BaseMediaSource
     Timeline timeline;
     if (startTimeUs == Long.MAX_VALUE) {
       long periodDurationUs = manifest.isLive ? C.TIME_UNSET : 0;
-      timeline = new SinglePeriodTimeline(periodDurationUs, 0, 0, 0, true /* isSeekable */,
-          manifest.isLive /* isDynamic */);
+      timeline =
+          new SinglePeriodTimeline(
+              periodDurationUs,
+              /* windowDurationUs= */ 0,
+              /* windowPositionInPeriodUs= */ 0,
+              /* windowDefaultStartPositionUs= */ 0,
+              /* isSeekable= */ true,
+              manifest.isLive,
+              tag);
     } else if (manifest.isLive) {
       if (manifest.dvrWindowLengthUs != C.TIME_UNSET && manifest.dvrWindowLengthUs > 0) {
         startTimeUs = Math.max(startTimeUs, endTimeUs - manifest.dvrWindowLengthUs);
@@ -569,13 +598,27 @@ public final class SsMediaSource extends BaseMediaSource
         // it to the middle of the window.
         defaultStartPositionUs = Math.min(MIN_LIVE_DEFAULT_START_POSITION_US, durationUs / 2);
       }
-      timeline = new SinglePeriodTimeline(C.TIME_UNSET, durationUs, startTimeUs,
-          defaultStartPositionUs, true /* isSeekable */, true /* isDynamic */);
+      timeline =
+          new SinglePeriodTimeline(
+              /* periodDurationUs= */ C.TIME_UNSET,
+              durationUs,
+              startTimeUs,
+              defaultStartPositionUs,
+              /* isSeekable= */ true,
+              /* isDynamic= */ true,
+              tag);
     } else {
       long durationUs = manifest.durationUs != C.TIME_UNSET ? manifest.durationUs
           : endTimeUs - startTimeUs;
-      timeline = new SinglePeriodTimeline(startTimeUs + durationUs, durationUs, startTimeUs, 0,
-          true /* isSeekable */, false /* isDynamic */);
+      timeline =
+          new SinglePeriodTimeline(
+              startTimeUs + durationUs,
+              durationUs,
+              startTimeUs,
+              /* windowDefaultStartPositionUs= */ 0,
+              /* isSeekable= */ true,
+              /* isDynamic= */ false,
+              tag);
     }
     refreshSourceInfo(timeline, manifest);
   }
diff --git a/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeTimeline.java b/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeTimeline.java
index 521c8ee52a..4e118366d7 100644
--- a/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeTimeline.java
+++ b/testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeTimeline.java
@@ -164,13 +164,21 @@ public final class FakeTimeline extends Timeline {
   }
 
   @Override
-  public Window getWindow(int windowIndex, Window window, boolean setIds,
-      long defaultPositionProjectionUs) {
+  public Window getWindow(
+      int windowIndex, Window window, boolean setTag, long defaultPositionProjectionUs) {
     TimelineWindowDefinition windowDefinition = windowDefinitions[windowIndex];
-    Object id = setIds ? windowDefinition.id : null;
-    return window.set(id, C.TIME_UNSET, C.TIME_UNSET, windowDefinition.isSeekable,
-        windowDefinition.isDynamic, 0, windowDefinition.durationUs, periodOffsets[windowIndex],
-        periodOffsets[windowIndex + 1] - 1, 0);
+    Object tag = setTag ? windowDefinition.id : null;
+    return window.set(
+        tag,
+        /* presentationStartTimeMs= */ C.TIME_UNSET,
+        /* windowStartTimeMs= */ C.TIME_UNSET,
+        windowDefinition.isSeekable,
+        windowDefinition.isDynamic,
+        /* defaultPositionUs= */ 0,
+        windowDefinition.durationUs,
+        periodOffsets[windowIndex],
+        periodOffsets[windowIndex + 1] - 1,
+        /* positionInFirstPeriodUs= */ 0);
   }
 
   @Override
diff --git a/testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/StubExoPlayer.java b/testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/StubExoPlayer.java
index af8b10e6d3..8de92ddfa9 100644
--- a/testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/StubExoPlayer.java
+++ b/testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/StubExoPlayer.java
@@ -16,6 +16,7 @@
 package com.google.android.exoplayer2.testutil;
 
 import android.os.Looper;
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.PlaybackParameters;
 import com.google.android.exoplayer2.Player;
@@ -142,6 +143,11 @@ public abstract class StubExoPlayer implements ExoPlayer {
     throw new UnsupportedOperationException();
   }
 
+  @Override
+  public @Nullable Object getCurrentTag() {
+    throw new UnsupportedOperationException();
+  }
+
   @Override
   public void stop() {
     throw new UnsupportedOperationException();
diff --git a/testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/TimelineAsserts.java b/testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/TimelineAsserts.java
index 17045f749a..a0ca6af8a9 100644
--- a/testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/TimelineAsserts.java
+++ b/testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/TimelineAsserts.java
@@ -34,7 +34,7 @@ public final class TimelineAsserts {
 
   /** Assert that timeline is empty (i.e. has no windows or periods). */
   public static void assertEmpty(Timeline timeline) {
-    assertWindowIds(timeline);
+    assertWindowTags(timeline);
     assertPeriodCounts(timeline);
     for (boolean shuffled : new boolean[] {false, true}) {
       assertThat(timeline.getFirstWindowIndex(shuffled)).isEqualTo(C.INDEX_UNSET);
@@ -43,18 +43,18 @@ public final class TimelineAsserts {
   }
 
   /**
-   * Asserts that window IDs are set correctly.
+   * Asserts that window tags are set correctly.
    *
-   * @param expectedWindowIds A list of expected window IDs. If an ID is unknown or not important
+   * @param expectedWindowTags A list of expected window tags. If a tag is unknown or not important
    *     {@code null} can be passed to skip this window.
    */
-  public static void assertWindowIds(Timeline timeline, Object... expectedWindowIds) {
+  public static void assertWindowTags(Timeline timeline, Object... expectedWindowTags) {
     Window window = new Window();
-    assertThat(timeline.getWindowCount()).isEqualTo(expectedWindowIds.length);
+    assertThat(timeline.getWindowCount()).isEqualTo(expectedWindowTags.length);
     for (int i = 0; i < timeline.getWindowCount(); i++) {
       timeline.getWindow(i, window, true);
-      if (expectedWindowIds[i] != null) {
-        assertThat(window.id).isEqualTo(expectedWindowIds[i]);
+      if (expectedWindowTags[i] != null) {
+        assertThat(window.tag).isEqualTo(expectedWindowTags[i]);
       }
     }
   }