Determine canSkip based on age of last playlist request

api-extractor/report/hls.js.api.md

@@ -267,7 +267,7 @@ export class BasePlaylistController extends Logger implements NetworkComponentAP
     // (undocumented)
     stopLoad(): void;
     // (undocumented)
-    protected switchParams(playlistUri: string, previous: LevelDetails | undefined): HlsUrlParameters | undefined;
+    protected switchParams(playlistUri: string, previous: LevelDetails | undefined, current: LevelDetails | undefined): HlsUrlParameters | undefined;
     // (undocumented)
     protected timer: number;
 }

src/controller/audio-track-controller.ts

@@ -339,7 +339,11 @@ class AudioTrackController extends BasePlaylistController {
     if (trackLoaded) {
       return;
     }
-    const hlsUrlParameters = this.switchParams(track.url, lastTrack?.details);
+    const hlsUrlParameters = this.switchParams(
+      track.url,
+      lastTrack?.details,
+      track.details,
+    );
     this.loadPlaylist(hlsUrlParameters);
   }
 

src/controller/base-playlist-controller.ts

@@ -1,6 +1,6 @@
 import type Hls from '../hls';
 import type { NetworkComponentAPI } from '../types/component-api';
-import { getSkipValue, HlsSkip, HlsUrlParameters, Level } from '../types/level';
+import { HlsSkip, HlsUrlParameters, Level, getSkipValue } from '../types/level';
 import { computeReloadInterval, mergeDetails } from '../utils/level-helper';
 import { ErrorData } from '../types/events';
 import { getRetryDelay, isTimeoutError } from '../utils/error-helper';
@@ -55,6 +55,7 @@ export default class BasePlaylistController
   protected switchParams(
     playlistUri: string,
     previous: LevelDetails | undefined,
+    current: LevelDetails | undefined,
   ): HlsUrlParameters | undefined {
     const renditionReports = previous?.renditionReports;
     if (renditionReports) {
@@ -92,11 +93,8 @@ export default class BasePlaylistController
             part += 1;
           }
         }
-        return new HlsUrlParameters(
-          msn,
-          part >= 0 ? part : undefined,
-          HlsSkip.No,
-        );
+        const skip = current && getSkipValue(current);
+        return new HlsUrlParameters(msn, part >= 0 ? part : undefined, skip);
       }
     }
   }
@@ -310,7 +308,7 @@ export default class BasePlaylistController
     msn?: number,
     part?: number,
   ): HlsUrlParameters {
-    let skip = getSkipValue(details, msn);
+    let skip = getSkipValue(details);
     if (previousDeliveryDirectives?.skip && details.deltaUpdateFailed) {
       msn = previousDeliveryDirectives.msn;
       part = previousDeliveryDirectives.part;

src/controller/level-controller.ts

@@ -461,7 +461,11 @@ export default class LevelController extends BasePlaylistController {
     const levelDetails = level.details;
     if (!levelDetails || levelDetails.live) {
       // level not retrieved yet, or live playlist we need to (re)load it
-      const hlsUrlParameters = this.switchParams(level.uri, lastLevel?.details);
+      const hlsUrlParameters = this.switchParams(
+        level.uri,
+        lastLevel?.details,
+        levelDetails,
+      );
       this.loadPlaylist(hlsUrlParameters);
     }
   }

src/controller/subtitle-track-controller.ts

@@ -537,7 +537,11 @@ class SubtitleTrackController extends BasePlaylistController {
       type,
       url,
     });
-    const hlsUrlParameters = this.switchParams(track.url, lastTrack?.details);
+    const hlsUrlParameters = this.switchParams(
+      track.url,
+      lastTrack?.details,
+      track.details,
+    );
     this.loadPlaylist(hlsUrlParameters);
   }
 

src/types/level.ts

@@ -59,10 +59,13 @@ export const enum HlsSkip {
   v2 = 'v2',
 }
 
-export function getSkipValue(details: LevelDetails, msn?: number): HlsSkip {
-  const { canSkipUntil, canSkipDateRanges, endSN } = details;
-  const snChangeGoal = msn !== undefined ? msn - endSN : 0;
-  if (canSkipUntil && snChangeGoal < canSkipUntil) {
+export function getSkipValue(details: LevelDetails): HlsSkip {
+  const { canSkipUntil, canSkipDateRanges, age } = details;
+  // A Client SHOULD NOT request a Playlist Delta Update unless it already
+  // has a version of the Playlist that is no older than one-half of the Skip Boundary.
+  // @see: https://datatracker.ietf.org/doc/html/draft-pantos-hls-rfc8216bis#section-6.3.7
+  const playlistRecentEnough = age < canSkipUntil / 2;
+  if (canSkipUntil && playlistRecentEnough) {
     if (canSkipDateRanges) {
       return HlsSkip.v2;
     }

tests/unit/controller/level-controller.ts

@@ -48,6 +48,7 @@ type LevelControllerTestable = Omit<LevelController, 'onManifestLoaded'> & {
   switchParams: (
     playlistUri: string,
     previous: LevelDetails | undefined,
+    current: LevelDetails | undefined,
   ) => void;
   redundantFailover: (levelIndex: number) => void;
 };
@@ -582,47 +583,51 @@ vfrag3.m4v
 #EXT-X-RENDITION-REPORT:URI="chunklist_vfrag100.m3u8",LAST-MSN=4,LAST-PART=1`;
 
     it('returns RENDITION-REPORT query values for the selected playlist URI', function () {
-      const levelDetails = M3U8Parser.parseLevelPlaylist(
+      const previousLevelDetails = M3U8Parser.parseLevelPlaylist(
         mediaPlaylist,
         'http://example.com/playlist.m3u8?abc=deg',
         0,
         PlaylistLevelType.MAIN,
         0,
         {},
       );
+      const mockCurrentDetails = undefined;
       const selectedUri = 'http://example.com/chunklist_vfrag1500.m3u8';
       const hlsUrlParameters = levelController.switchParams(
         selectedUri,
-        levelDetails,
+        previousLevelDetails,
+        mockCurrentDetails,
       );
       expect(hlsUrlParameters).to.have.property('msn').which.equals(4);
       expect(hlsUrlParameters).to.have.property('part').which.equals(1);
-      expect(hlsUrlParameters).to.have.property('skip').which.equals('');
+      expect(hlsUrlParameters).to.have.property('skip').to.be.undefined;
     });
 
     it('returns RENDITION-REPORT query values for the selected playlist URI with additional query params', function () {
-      const levelDetails = M3U8Parser.parseLevelPlaylist(
+      const previousDetails = M3U8Parser.parseLevelPlaylist(
         mediaPlaylist,
         'http://example.com/playlist.m3u8?abc=deg',
         0,
         PlaylistLevelType.MAIN,
         0,
         {},
       );
+      const mockCurrentDetails = undefined;
       const selectedUriWithQuery =
         'http://example.com/chunklist_vfrag1500.m3u8?abc=123';
       const hlsUrlParameters = levelController.switchParams(
         selectedUriWithQuery,
-        levelDetails,
+        previousDetails,
+        mockCurrentDetails,
       );
       expect(hlsUrlParameters).to.not.be.undefined;
       expect(hlsUrlParameters).to.have.property('msn').which.equals(4);
       expect(hlsUrlParameters).to.have.property('part').which.equals(1);
-      expect(hlsUrlParameters).to.have.property('skip').which.equals('');
+      expect(hlsUrlParameters).to.have.property('skip').to.be.undefined;
     });
 
     it('returns RENDITION-REPORT exact URI match over partial match for playlist URIs with additional query params', function () {
-      const levelDetails = M3U8Parser.parseLevelPlaylist(
+      const previousLevelDetails = M3U8Parser.parseLevelPlaylist(
         `#EXTM3U
 #EXT-X-VERSION:7
 #EXT-X-SERVER-CONTROL:CAN-BLOCK-RELOAD=YES
@@ -644,11 +649,13 @@ vfrag3.m4v
         0,
         {},
       );
+      const mockCurrentDetails = undefined;
       const selectedUriWithQuery =
         'http://example.com/chunklist.m3u8?token=123';
       const hlsUrlParameters = levelController.switchParams(
         selectedUriWithQuery,
-        levelDetails,
+        previousLevelDetails,
+        mockCurrentDetails,
       );
       expect(hlsUrlParameters).to.not.be.undefined;
       expect(hlsUrlParameters).to.have.property('msn').which.equals(6);