feat(DASH): Add support for service descriptions

externs/shaka/manifest.js

@@ -20,7 +20,8 @@
  *   minBufferTime: number,
  *   sequenceMode: boolean,
  *   ignoreManifestTimestampsInSegmentsMode: boolean,
- *   type: string
+ *   type: string,
+ *   serviceDescription: ?shaka.extern.ServiceDescription
  * }}
  *
  * @description
@@ -89,6 +90,9 @@
  * @property {string} type
  *   Indicates the type of the manifest. It can be <code>'HLS'</code> or
  *   <code>'DASH'</code>.
+ * @property {?shaka.extern.ServiceDescription} serviceDescription
+ *   The service description for the manifest. Used to adapt playbackRate to
+ *   decrease latency.
  *
  * @exportDoc
  */
@@ -118,6 +122,26 @@ shaka.extern.Manifest;
  */
 shaka.extern.InitDataOverride;
 
+/**
+ * @typedef {{
+ *   maxLatency: ?number,
+ *   maxPlaybackRate: ?number
+ * }}
+ *
+ * @description
+ * Maximum latency and playback rate for a manifest. When max latency is reached
+ * playbackrate is updated to maxPlaybackRate to decrease latency.
+ * More information {@link https://dashif.org/docs/CR-Low-Latency-Live-r8.pdf here}.
+ *
+ * @property {?number} maxLatency
+ *  Maximum latency in seconds.
+ * @property {?number} maxPlaybackRate
+ *  Maximum playback rate.
+ *
+ * @exportDoc
+ */
+shaka.extern.ServiceDescription;
+
 
 /**
  * @typedef {{

lib/dash/dash_parser.js

@@ -509,6 +509,7 @@ shaka.dash.DashParser = class {
         sequenceMode: this.config_.dash.sequenceMode,
         ignoreManifestTimestampsInSegmentsMode: false,
         type: shaka.media.ManifestParser.DASH,
+        serviceDescription: this.parseServiceDescription_(mpd),
       };
 
       // We only need to do clock sync when we're using presentation start
@@ -547,6 +548,39 @@ shaka.dash.DashParser = class {
     this.playerInterface_.makeTextStreamsForClosedCaptions(this.manifest_);
   }
 
+  /**
+   * Reads maxLatency and maxPlaybackRate properties from service
+   * description element.
+   *
+   * @param {!Element} mpd
+   * @return {?shaka.extern.ServiceDescription}
+   * @private
+   */
+  parseServiceDescription_(mpd) {
+    const XmlUtils = shaka.util.XmlUtils;
+    const elem = XmlUtils.findChild(mpd, 'ServiceDescription');
+
+    if (!elem ) {
+      return null;
+    }
+
+    const latencyNode = XmlUtils.findChild(elem, 'Latency');
+    const playbackRateNode = XmlUtils.findChild(elem, 'PlaybackRate');
+
+    if ((latencyNode && latencyNode.getAttribute('max')) || playbackRateNode) {
+      const maxLatency = latencyNode && latencyNode.getAttribute('max') ?
+        parseInt(latencyNode.getAttribute('max'), 10) / 1000 :
+        null;
+      const maxPlaybackRate = playbackRateNode ?
+        parseFloat(playbackRateNode.getAttribute('max')) :
+        null;
+
+      return {maxLatency, maxPlaybackRate};
+    }
+
+    return null;
+  }
+
   /**
    * Reads and parses the periods from the manifest.  This first does some
    * partial parsing so the start and duration is available when parsing

lib/hls/hls_parser.js

@@ -801,6 +801,7 @@ shaka.hls.HlsParser = class {
       ignoreManifestTimestampsInSegmentsMode:
         this.config_.hls.ignoreManifestTimestampsInSegmentsMode,
       type: shaka.media.ManifestParser.HLS,
+      serviceDescription: null,
     };
     this.playerInterface_.makeTextStreamsForClosedCaptions(this.manifest_);
   }

lib/mss/mss_parser.js

@@ -376,6 +376,7 @@ shaka.mss.MssParser = class {
         sequenceMode: this.config_.mss.sequenceMode,
         ignoreManifestTimestampsInSegmentsMode: false,
         type: shaka.media.ManifestParser.MSS,
+        serviceDescription: null,
       };
 
       // This is the first point where we have a meaningful presentation start

lib/offline/manifest_converter.js

@@ -91,6 +91,7 @@ shaka.offline.ManifestConverter = class {
       sequenceMode: manifestDB.sequenceMode || false,
       ignoreManifestTimestampsInSegmentsMode: false,
       type: manifestDB.type || shaka.media.ManifestParser.UNKNOWN,
+      serviceDescription: null,
     };
   }
 

lib/player.js

@@ -2278,7 +2278,8 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
                         'arbitrary language initially');
     }
 
-    if (this.isLive() && this.config_.streaming.liveSync) {
+    if (this.isLive() && (this.config_.streaming.liveSync ||
+        this.manifest_.serviceDescription)) {
       const onTimeUpdate = () => this.onTimeUpdate_();
       this.loadEventManager_.listen(mediaElement, 'timeupdate', onTimeUpdate);
     }
@@ -5728,8 +5729,24 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
       // Bad stream?
       return;
     }
-    const liveSyncMaxLatency = this.config_.streaming.liveSyncMaxLatency;
-    const liveSyncPlaybackRate = this.config_.streaming.liveSyncPlaybackRate;
+
+    let liveSyncMaxLatency;
+    let liveSyncPlaybackRate;
+    if (this.config_.streaming.liveSync) {
+      liveSyncMaxLatency = this.config_.streaming.liveSyncMaxLatency;
+      liveSyncPlaybackRate = this.config_.streaming.liveSyncPlaybackRate;
+    } else {
+      // serviceDescription must override if it is defined in the MPD and
+      // liveSync configuration is not set.
+      if (this.manifest_ && this.manifest_.serviceDescription) {
+        liveSyncMaxLatency = this.manifest_.serviceDescription.maxLatency ||
+          this.config_.streaming.liveSyncMaxLatency;
+        liveSyncPlaybackRate =
+          this.manifest_.serviceDescription.maxPlaybackRate ||
+          this.config_.streaming.liveSyncPlaybackRate;
+      }
+    }
+
     const playbackRate = this.video_.playbackRate;
     const latency = seekRange.end - this.video_.currentTime;
     let offset = 0;
@@ -5745,8 +5762,12 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
       }
     }
 
-    if ((latency - offset) > liveSyncMaxLatency) {
+    if (liveSyncMaxLatency && liveSyncPlaybackRate &&
+        (latency - offset) > liveSyncMaxLatency) {
       if (playbackRate != liveSyncPlaybackRate) {
+        shaka.log.debug('Latency (' + latency + 's) ' +
+          'is greater than liveSyncMaxLatency (' + liveSyncMaxLatency + 's). ' +
+          'Updating playbackRate to ' + liveSyncPlaybackRate);
         this.trickPlay(liveSyncPlaybackRate);
       }
     } else if (playbackRate !== 1 && playbackRate !== 0) {

test/dash/dash_parser_manifest_unit.js

@@ -2556,4 +2556,26 @@ describe('DashParser Manifest', () => {
     expect(segments[0][1].startTime).toBe(15);
     expect(segments[1][1].startTime).toBe(15);
   });
+
+  describe('Parses ServiceDescription', () => {
+    it('with PlaybackRate and Latency', async () => {
+      const source = [
+        '<MPD minBufferTime="PT75S" type="dynamic"',
+        '     availabilityStartTime="1970-01-01T00:00:00Z">',
+        '  <ServiceDescription id="0">',
+        '    <Latency max="2000" min="2000" referenceId="0" target="4000" />',
+        '    <PlaybackRate max="1.10" min="0.96" />',
+        '  </ServiceDescription>',
+        '</MPD>',
+      ].join('\n');
+
+      fakeNetEngine.setResponseText('dummy://foo', source);
+
+      /** @type {shaka.extern.Manifest} */
+      const manifest = await parser.start('dummy://foo', playerInterface);
+
+      expect(manifest.serviceDescription.maxLatency).toBe(2);
+      expect(manifest.serviceDescription.maxPlaybackRate).toBe(1.1);
+    });
+  });
 });

test/media/playhead_unit.js

@@ -136,6 +136,7 @@ describe('Playhead', () => {
       sequenceMode: false,
       ignoreManifestTimestampsInSegmentsMode: false,
       type: 'UNKNOWN',
+      serviceDescription: null,
     };
 
     config = shaka.util.PlayerConfiguration.createDefault().streaming;

test/media/streaming_engine_integration.js

@@ -597,6 +597,7 @@ describe('StreamingEngine', () => {
         sequenceMode: false,
         ignoreManifestTimestampsInSegmentsMode: false,
         type: 'UNKNOWN',
+        serviceDescription: null,
         variants: [{
           id: 1,
           video: {

test/test/util/manifest_generator.js

@@ -101,6 +101,9 @@ shaka.test.ManifestGenerator.Manifest = class {
     this.ignoreManifestTimestampsInSegmentsMode = false;
     /** @type {string} */
     this.type = 'UNKNOWN';
+    /** @type {?shaka.extern.ServiceDescription} */
+    this.serviceDescription = null;
+
 
     /** @type {shaka.extern.Manifest} */
     const foo = this;

test/test/util/streaming_engine_util.js

@@ -286,6 +286,7 @@ shaka.test.StreamingEngineUtil = class {
       sequenceMode: false,
       ignoreManifestTimestampsInSegmentsMode: false,
       type: 'UNKNOWN',
+      serviceDescription: null,
     };
 
     /** @type {shaka.extern.Variant} */