Add support for Content Steering and Variable Substitution #5074

Not a Contribution

Is your feature request related to a problem? Please describe.

Content Steering is defined in https://datatracker.ietf.org/doc/html/draft-pantos-hls-rfc8216bis-12#section-7

The main functions of Content Steering are to:

1. Group redundant Variant Streams into "Pathways"
2. Dynamically prioritize access to different Pathways

Describe the solution you'd like

Sub Tasks

• Load Steering Manifest if present (Parse #EXT-X-CONTENT-STEERING:SERVER-URI…)
  • Must support versions 1 (refuse other versions)
  • Include Steering Query Params (_HLS_pathway="”, _HLS_throughput=)
• Reload Steering Manifest (Manifest: TTL (seconds), RELOAD-URI)
  • Do not reload if HTTP status 410 Gone is received
  • Wait for “Retry-After” header time if HTTP 429 Too Many Requests is received
  • For other loading or parsing error, wait TTL or 300 seconds before attempting to reload again
• Define logical Pathways
  • Match Pathway IDs in PATHWAY-PRIORITY array to STREAM-INFs w/ PATHWAY-ID
  • Default Pathway ID (for variants without PATHWAY-ID) is “.”
• (optional) Support Pathway Cloning (Define logical Pathways pt.II)
  • BASE-ID > ID, URI-REPLACEMENT > HOST, PARAMS, PER-VARIANT-URIS (> STABLE-VARIANT-ID), PER-RENDITION-URIS (> STABLE-RENDITION-ID)
  • Clone Rendition Groups (PATHWAY-ID + GROUP-ID)
• Steering Client Responsibilities
  • Start playback using EXT-X-CONTENT-STEERING PATHWAY-ID Pathway when present, otherwise pick “first” level according to player config and current selection logic
    • preferred Pathway should “agree” with MVP (warn otherwise and/or delay playback by redirecting)
  • Content Steering evaluation
    • Apply Pathway when: none is applied, applied is not first in the list (accounting for removed pathways, and penalized pathways). Remain on current Pathway when no alternative is available.
  • Penalize Pathway when all variant streams in a Pathway fail (Look at player penalty box timeout, but consider re-evaluating after Content Steering Manifest refresh)
• (optional) Implement variable substitution
  • EXT-X-DEFINE
    • NAME, VALUE
    • IMPORT attribute from MVP to media playlist
  • Replace Variable References with defined values
    • a Variable Reference whose Variable Name has NOT been provided MUST trigger a parsing error