docs: add docs for content steering (#1442)

docs/content-steering.md

@@ -0,0 +1,47 @@
+# Content Steering
+
+Content Steering provides content creators a method of runtime control over
+the location from which segments are fetched via a content steering server and
+pathways defined in the content manifest. For a working example visit
+https://www.content-steering.com/.
+
+HLS and DASH each define their own specific Content Steering tags and properties
+that prescribe how the client should fetch the content steering manifest as well
+as make steering decisions. `#EXT-X-CONTENT-STEERING` and `<ContentSteering>` respectively.
+
+For reference, HLS spec section 4.4.6.6:
+https://datatracker.ietf.org/doc/html/draft-pantos-hls-rfc8216bis#section-4.4.6.6
+
+DASH-IF:
+https://dashif.org/docs/DASH-IF-CTS-00XX-Content-Steering-Community-Review.pdf
+
+Both protocols rely on a content steering server to provide steering guidance.
+VHS will request the content steering manifest from the location defined in the
+content steering tag in the `.m3u8` or `.mpd` and refresh the steering manifest
+at an interval defined in that manifest.
+
+A content steering manifest response will look something like this:
+```
+{
+ "VERSION": 1,
+ "TTL": 300,
+ "RELOAD-URI": "https://steeringservice.com/app/instance12345?session=abc",
+ "CDN-PRIORITY": ["beta","alpha"]
+}
+```
+`CDN-PRIORITY` represents either `PATHWAY-PRIORITY` for HLS or `SERVICE-LOCATION-PRIORITY` for DASH. This list of keys in priority order will match with either a `PATHWAY-ID` or `serviceLocation` (HLS and DASH respectively) associated with a location where VHS can fetch segments.
+
+VHS will attempt to fetch segments from the locations defined in the steering manifest response in the order. Then, during playback, VHS will provide quality of experience metrics back to the steering server which can adjust the steering guidance accordingly.
+
+## Notable Support
+
+### HLS
+ * Pathway Cloning
+### DASH
+ * queryBeforeStart
+ * proxyServerURL
+
+## Currently Missing Support
+
+### DASH
+ * Extended HTTP GET request parametrization, see: ISO/IEC 23009-1 [2], clause I.3

docs/supported-features.md

@@ -65,6 +65,7 @@ not meant serve as an exhaustive list.
   tracks
 * [AES-128] segment encryption
 * DASH In-manifest EventStream and Event tags are automatically translated into HTML5 metadata cues
+* [Content Steering](content-steering.md) for both HLS and DASH.
 
 ## Notable Missing Features