Adaptive Bitrate (ABR) in MOQ

How MOQ supports switching between quality levels during streaming.

Overview

moq-msf defines support for adaptive bitrate switching between time-synchronized tracks. Different quality representations are published as separate tracks that share a common timeline.

Mechanism

1. The catalog-format describes all available tracks and their quality parameters (in MSF -01, codec/width/height are required for video, sampleRate/channels for audio, so a subscriber can pick a rendition without parsing the init segment; targetBuffer gives per-track buffer guidance)
2. Tracks within a renderGroup share a timeline, but strict per-group time alignment has been loosened — MSF #128 (“relax time alignment on RenderGroup”) and #155 (“sequence-aligned groups are too restrictive”, luke-curley) both closed, settling on shared PTS without mandatory group-boundary alignment
3. Subscribers can switch between tracks by changing their subscription
4. The publish-subscribe model means switching is a client-side decision

Latency Modes

MSF defines three latency regimes that affect ABR behavior:

• Real-time (<500ms) - Minimal buffering, aggressive quality drops
• Interactive (500-2500ms) - Some buffer for smoothing
• Standard (>2500ms) - Traditional ABR with deep buffers

Zapping

PR #122 in the MSF repo adds initial text on “zapping” - quickly switching between different content streams (like changing TV channels). This is related to but distinct from ABR switching.

Transport-Level Switching (June 2026)

Whether the transport needs dedicated machinery for seamless ABR switching was debated through spring 2026. The June London interim settled it: track switching uses a SWITCH_FROM parameter (afrind PR #1674, hard mode) rather than a standalone SWITCH message, and the catch-up half is handled by fill fetch + Range Filters. Decode-timestamp signaling proceeds as an extension (draft-ietf-moq-dts4moq). See switch-abr and joining-fetch-dissent for the full resolution.

Related

• moq-msf - Streaming format with ABR support
• catalog-format - Describes available quality tracks
• joining-fetch - ABR implications when joining mid-stream
• media-packaging - Container format affects ABR granularity