IETF MOQ WG: Top Tracks and SSTS (or ABR in general)

Message Snapshot

• List: moq@ietf.org
• Thread subject: Top Tracks and SSTS (or ABR in general)
• Message subject: [Moq] Re: Top Tracks and SSTS (or ABR in general)
• Author: Yu You (Nokia)
• Published: 2026-07-28T08:42:21.000Z
• Source tag: ietf-mailing-list
• Message URL: Open in IETF Mail Archive

Summary

Yu You (Nokia) posted to the MOQ working group mailing list on July 28, 2026. Here is my understanding and polished by AI when applying both Top Track filter and SSTS. #### When Sender-Side Track Switching (SSTS) is app…

Why It Matters

This message came from the public MOQ working group archive and captures active discussion, meeting coordination, or implementation feedback around Media over QUIC.

Message Excerpt

Here is my understanding and polished by AI when applying both Top Track filter and SSTS. #### When Sender-Side Track Switching (SSTS) is applied to tracks selected by a Top-N filter, the result is that the subscriber receives exactly one track at a time, effectively narrowing the "Top-N" pool down to a "Top-1" delive…

Plain-Text Body

Here is my understanding and polished by AI when applying both Top Track filter and SSTS.

When Sender-Side Track Switching (SSTS) is applied to tracks selected by a Top-N filter, the result is that the subscriber receives exactly one track at a time, effectively narrowing the "Top-N" pool down to a "Top-1" delivery.

However, rather than an "override," these two mechanisms work in a coordinated, two-step pipeline:

1. Step One: The Top-N Filter Defines the Pool

The Top Tracks Filter acts as a discovery and promotion mechanism. It evaluates all tracks in a namespace and promotes the (N) tracks with the highest property values (such as bitrate or popularity) to the selected state.

• The publisher sends a PUBLISH message for each of these (N) tracks to initiate subscriptions.
• At this stage, (N) tracks are authorized and available for delivery to the subscriber.

2. Step Two: SSTS Performs the Final Selection

Once the tracks are promoted by the Top-N filter, the subscriber can group them into a switching set by using the same Switching Set ID in the PUBLISH_OK messages.

• Relay Autonomy: The relay is responsible for maintaining an upstream "forward=1" state for all (N) tracks in that switching set to ensure it has all alternatives ready.
• Active Selection: The relay applies its SSTS algorithm (e.g., checking bandwidth thresholds) to select exactly one track from that set to forward downstream.
• Forwarding State: For the (N-1) tracks not chosen by the SSTS algorithm, the relay sets the downstream Forward state to 0, ensuring they are not sent over the congested link.

Summary of the Interaction

While the Top-N filter identifies which (N) tracks are currently the best candidates based on application logic (the "What"), SSTS decides which of those (N) tracks is the single best fit for the current network conditions (the "How").

Therefore, while the filter might promote a "Top-5," the SSTS mechanism ensures that the final delivery is effectively Top-1 to avoid over-delivery and response delays. If a track is later demoted out of the Top-N by the publisher, it is removed from the switching set entirely, and the relay continues to switch among the remaining promoted tracks.

Sent from Outlook for Mac From: Ian Swett ianswett=40google.com@dmarc.ietf.org Date: Monday, 27. July 2026 at 23.10 To: MOQ Mailing List moq@ietf.org Cc: Mo Zanaty mzanaty@cisco.com; Law, Will wilaw@akamai.com Subject: [Moq] Top Tracks and SSTS (or ABR in general)

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I reviewed the most recent Top Tracks PR, and I continue to wonder exactly how it will work with SSTS (or another ABR algorithm).

At a high level, I believe I know what one might want, but the mechanisms to achieve it are unclear to me.

I realize there are use cases for Top Tracks without ABR, but for the VC use…