One of the most debated topics in MOQ - whether the transport layer needs a dedicated SWITCH message for adaptive bitrate track switching.
Background
In traditional ABR streaming (HLS/DASH), the client decides which quality to fetch next. In MOQ’s publish-subscribe model, switching quality means changing which track you subscribe to. The question is whether SUBSCRIBE/UNSUBSCRIBE is sufficient or a dedicated SWITCH message is needed.
PR #1378 - SWITCH for Client-Side ABR
Author: Gwendal Simon (Nov 2025) Labels: Needs Discussion, ABR, Design
Proposes a SWITCH message at the transport level to enable seamless client-side ABR. The PR adds a new control message that atomically transitions a subscription from one track to another.
Issue #1354 - Why do we need a dedicated SWITCH message?
Author: Ali C. Begen Comments: 39 (most discussed open issue)
The issue consolidates the history of ABR discussions in MOQ:
- #259 - Sender-side ABR: publisher decides what to send based on congestion
- #370 - Probing track approach for bandwidth estimation
- #471 - Client-side upswitch causing excessive bandwidth
The core tension: should the publisher or subscriber make quality decisions?
Related Open Issues
- #1507 - Mechanism to get sender’s bitrate (Luke Curley). Equivalent to CMSD for MOQ. Essential for client-side ABR to make informed decisions.
- #1453 - Send Rate parameter (Will Law). Publisher-reported send rate.
- #1365 - If you can’t deliver an entire Group, should you send any Objects? Affects ABR drop behavior.
- #1352 - SUBSCRIBE doesn’t need a forward parameter if we have filters (Parked)
Current Status (April 2026)
Unresolved. The PR is labeled “Needs Discussion” and hasn’t been merged or closed. The community is split between:
- Transport-level SWITCH - Atomic transition, relay-aware, can optimize delivery
- Application-level switching - Just UNSUBSCRIBE old track + SUBSCRIBE new track, keep transport simple
- Sender-side ABR - Publisher decides quality, subscriber specifies constraints
The April 13 interim meeting may address this.
Related
- adaptive-bitrate - ABR overview in MOQ
- moq-transport - Protocol spec
- publish-subscribe - Subscription model
- moq-msf - Streaming format with ABR support