[Hls-interest] Streaming Video Alliance Comments on HLS Content Steering Proposals

[Jason Thibeault <[e-mail]>]

Streaming Video Alliance (SVA) Comments on HLS Steering Proposal

The Streaming Video Alliance is a group of companies spanning the streaming
video value chain. These companies are dedicated to solving the critical
challenges facing online video to improve consumer adoption. Alliance
members have reviewed the HLS Content Steering Proposal 1.1b1 and the
subsequent Redundant Stream Steering Proposal (RSS) and provided comments
on both from the perspectives of our diverse membership (content providers,
CDNs, ISPs, open caching networks, multi-CDN management systems, encoding
systems, advertising insertion systems, video analytics systems, and video
player developers).

---- SEPARATION OF CONCERNS ----
* It is typical in multi-CDN deployments for different entities to provide
distinct parts of the solution. Manifests may be generated and manipulated
by multiple provider’s services, media segments may come from other
provider’s service, and CDN recommendation intelligence may come from yet
another service.

* The HLS Content Steering Proposal blurs the clean delineation of
responsibilities across these services, putting responsibility on the
Steering Service to generate CDN-specific path prefixes (and possibly
tokens).  A cleaner separation of concerns could be achieved by limiting
the steering service’s scope of responsibility strictly to recommending
pathways in a preferred order (as described in the Redundant Stream
Steering Proposal).
CDN-Specific Tokens

* Our most significant concern was initially about the inability to have
CDN-specific suffixes on the steering templates, precluding the real-world
situations where different tokenization schemes are used by different CDNs.

* While the addition of QUERY-PARAMS appears to address this problem when
tokens are expressed as query parameters, it still requires that the
steering server generates CDN tokens (see above: Separation of Concerns)
and it doesn’t address tokens that may be inserted anywhere in the path as
path parameters or segments.

---- IMPACT ON OPEN CACHING REQUEST ROUTING & EXISTING MULTI_CDN SOLUTIONS
----

* Open Caching Request Routing schemes (relying on several IETF’s CDNI
RFCs) can be configured to inspect and rewrite either main or media
manifests to direct traffic to downstream CDNs.

   - The HLS Content Steering Proposal may preclude the rewriting of media
manifest URLs in the main manifest, as the steering server also has
responsibility for generating media manifest URLs.
   - The HLS Content Steering Proposal adds complexity to request routing
based on HTTP redirect, as envisaged in IETF CDNI and SVA. It may, for
example, require steering server requests to be redirected to an open
caching controller for rewriting of URL prefixes.
   - To better support Open Caching redirect routing, the PREFIX attribute
in the steering manifest should be optional. For example, If the current
non-penalized pathway is the first in the list for which there is no PREFIX
attribute, the client would form the URL of variant streams attached to the
corresponding template in the conventional manner  (e.g. using the relative
URI).

* It should be noted that some multi-CDN solutions use DNS on media segment
hostnames to implement mid-stream CDN switching that is transparent to the
player. The HLS Content Steering Proposal’s focus on alternate manifests
will not break these types of solutions, but integration with such
solutions could be challenging.

* Compatibility can be maintained with client-side multi-CDN management
systems if HLS Video players allow client application override of the
steering server pathway recommendation (see Player Behavior section below).

---- SERVER-SIDE AD INSERTIONS ----

* SSAI systems typically generate ad loads at the time of the main manifest
request and persist session identifiers that need to be passed to media
manifests so that identical ad loads can be stitched into each rendition.
The addition of QUERY-PARAMS should be sufficient for allowing session IDs
to be passed on media manifest URLs.

---- PLAYER BEHAVIOR ----

In reviewing the HLS Content Steering Proposal, the SVA suggests the
following additions to the Player Behavior section:

* HLS Video Players should generate events whenever a pathway is selected,
exposing the PATHWAY-ID so that Video QoS systems can log CDN changes.

* HLS Video Players should generate events whenever requests are made to
the steering server, enabling Video QoS systems to log details from the
steering manifest such as TTL and Pathways on successful requests, and
error information on failed requests.

* For compatibility with existing client-side multi-CDN management systems,
HLS Video Players should expose an API that allows access to the existing
Pathway list and to force a pathway change (overriding the server-directed
steering).

--------------------------------------------------------------------

SVA SUPPORT FOR THE HLS REDUNDANT STREAM STEERING PROPOSAL (RSS)

The SVA supports the Redundant Stream Steering Proposal for the following
reasons:

* RSS embraces and extends redundant playlists (the existing HLS approach
for supporting multiple CDNs), and is backward compatible with any player
that supports redundant playlists.

* RSS provides a cleaner separation of concerns and simplifies the
responsibilities of the Steering Service, as it no longer participates in
generation of manifest URLs or tokens.

* RSS simplifies the HLS Stream Steering scheme by eliminating the steering
templates, prefixes, query parameters, and any code complexity associated
with supporting the template substitution.

* RSS avoids the token-generation challenges that have been identified, as
token generation for media manifest URLs remains in the main manifest.

-------------------------------------------------------------------

This document can also be found on the Streaming Video Alliance website at
https://www.streamingvideoalliance.org/document/comments-on-hls-steering-proposal/

----------

Jason Thibeault
Executive Director
Streaming Video Alliance
[phone]
https://www.streamingvideoalliance.org
Contributing Editor, Streaming Media Magazine
<https://www.streamingmedia.com/Search/?Query=jason%20thibeault>
https://www.linkedin.com/in/thejasonthibeault