A cluster of open issues and competing proposals around how subscribers join a live stream mid-session. Tagged “Joining Fetch Dissent” on GitHub. The April five-way debate documented below was resolved at the June 11–12 London interim — see Resolution.
2026-06-22 — Resolution. The mid-April five-way contest collapsed at the London interim (June 11–12) into two consolidated workstreams: fill fetch (subscription filters delivering past groups over fetch-formatted streams — replaces Joining FETCH, absorbs REWIND / LargestGroup / CurrentGroupFill) and SWITCH_FROM (a parameter, not a standalone message, for ABR track switching — see switch-abr). The live wire-design item is Mo Zanaty’s Range Filters PR #1765 (June 16, OPEN). REWIND was parked at -02; the May 1 consensus call closed with no chair conclusion. As of June 22 the transport editorial pipeline is frozen for a 4th straight day, all these PRs OPEN, held for the June 22 virtual interim. See interim-meetings, discussions-2026-06.
The Problem
When joining a live stream, a subscriber needs historical data (e.g., the latest keyframe) to start rendering. The current joining-fetch mechanism has been contentious, generating multiple alternative proposals.
Open Issues (tagged “Joining Fetch Dissent”)
- #1313 - Joining FETCH as separate control message creates edge cases and feature gaps (ianswett, Past Deadline)
- #1391 - Unclear how to use Joining FETCH with New Group Request (ianswett)
- #1386 - Can a publisher ‘lie’ about what Largest Object is? (ianswett)
- #1358 - Subscribing to start of current Group could be optimized (ianswett)
- #1039 - Simplifying joining at the latest available join point (wilaw)
- #1023 - Subgroups + DELIVERY_TIMEOUT = pathological FETCH (afrind, Past Deadline)
Competing Proposals (and their fate)
| # | Proposal | Author | Fate (as of June 22) |
|---|---|---|---|
| 1 | Joining FETCH (separate FETCH alongside live SUBSCRIBE; moved-to-SUBSCRIBE-stream = PR #1604) | in draft-17/18 | CLOSED June 4 — replaced by fill fetch |
| 2 | PR #1362 Prior Group Subscription Filter | ianswett | folded into the filter line of work |
| 3 | Subscribe Rewind (draft-duke-moq-subscribe-rewind) | martin-duke | parked at -02; function folded into fill fetch |
| 4 | Join Subscription Filters (“more of an extension to Martin’s”) | alan-frindell | superseded by fill fetch |
| 5 | LargestGroup / CurrentGroup / CurrentGroupFill filters | Vasiliev / afrind / Luke | #1607 CLOSED June 4; CurrentGroup filter type absorbed into fill fetch |
| 6 | SWITCH (PR #1378, charter ABR argument) | gwendal-simon | superseded by SWITCH_FROM parameter (switch-abr) |
The Apr 17–18 detail on these:
- PR #1607 — Largest Available Group filter (victor-vasiliev, Apr 18): current group only, always complete group, no explicit relay backfill, “probably really easy to implement.” CLOSED June 4.
- afrind/moq-transport#15 CurrentGroupFill (alan-frindell): parallel draft targeting the same “current group” niche; absorbed into the official fill-fetch PRs (#1642 → #1673).
- Luke Curley’s LargestGroup proposal (mailing-list, informal) — same shape, framed as “the intended behaviour 99% of the time” so the first group stops being a special case.
Latest Developments
May–June 2026 consolidation (newest first)
- June 16 — Mo Zanaty opens PR #1765 “Add Range Filters” (follow-up to the London object-range-filter consensus + the action item to re-add the location filter separately; builds on his long-open PR #1518). “The active design thrust.” OPEN through the June editorial freeze.
- June 14 — alan-frindell opens PR #1673 “Replace Joining FETCH with fill fetch streams” (revises #1642, closes #1023) plus PR 1675 SWITCH_FROM hard/soft mode — all in a 33-minute window.
- June 11–12 — London interim consensus: fill fetch replaces Joining FETCH; SWITCH_FROM hard mode (soft deferred); keep the object-ID filter, split the location filter into a separate PR; draft-18 = Vienna interop target; Top-Tracks filter stays an extension.
- June 4 — REWIND/SWITCH consensus call closes; PR #1604 (Joining FETCH) and PR #1607 (LargestGroup) both CLOSED. DTS resolved June 10 as an extension (
draft-ietf-moq-dts4moq), not base-spec. - June 2 — afrind opens PR #1642 “Replace Joining Fetch with Subscription Fill, add Current Group.”
- May 1 — REWIND consensus call deadline reached with no chair conclusion (split outcome); REWIND parked at -02. The WG proceeded on the filter-based path.
April 2026 (historical)
- Interim moq-13 outcome (Apr 13) - WG declined to integrate REWIND into core v1. Editors will land minimal band-aids: FETCH timeouts (PR #1490, merged Apr 14) and subgroup filters; sophisticated joining deferred to future extensions.
- REWIND Consensus Call (Apr 16, Magnus Westerlund) - Three options open until 2026-05-01:
- No action until MOQT publishes
- Adopt REWIND as a MOQT extension draft
- Use REWIND as basis for a PR into MOQT when editors deem ready Targets issues #861, #1039, #1358, #1362, #1386.
- Alan Frindell’s reply (Apr 17) — backs Option 1. Argues FILL_TIMEOUT=0 (PR #1490, merged Apr 14) already removes the HOL-blocking scenarios REWIND was meant to solve (low-prio subgroup cache gaps, stream-per-object tracks, evicted groups). Residual case (cached low-prio data blocking) can be handled with filters. Recommends “stabilise around the core” and defer to future versions. Full message.
- Luke Curley’s reply (Apr 17, message) — against REWIND as-is. Sees FETCH’s VOD semantics as inherently HOL-prone and argues “smart” VOD clients are better served by per-group FETCHes (HLS/DASH-style). Proposes a LargestGroup filter for SUBSCRIBE to cover 99% of the join-live use case and eliminate the first-group special case. Follow-up Apr 18 (message): “Yeah, I just want to adopt CurrentGroup so we can make some progress.”
- Victor Vasiliev’s reply (Apr 18, message) — “Not against the LargestGroup idea.” Authored a concrete draft: PR #1607 — Largest Available Group filter. Only the current group is supported, always serves a complete group, no relay-side backfill, “probably really easy to implement.”
- Alan Frindell follow-up (Apr 18, message) — also does not object to a LargestGroup filter and has drafted a parallel “CurrentGroupFill” PR. Key caveat: the filter handles the common case (joining via one message, consistent response format) and the in-cache low-pri subgroup/datagram case of the current group, but does not address previous groups.
- Gwendal Simon’s dissent (Apr 18, message) — argues the emerging LargestGroup/CurrentGroup direction is insufficient for MoQ V1 because it does not cover ABR track switching, which is an explicit charter deliverable. Key points: (1) a subscriber is “almost always behind the live edge” during switches (congestion or buffering creates lag), so switching is not an edge case; (2) CurrentGroup helps with joining but only covers one group, whereas ABR switching requires “an arbitrary range of past groups”; (3) the real blocker is a semantic constraint — past objects are currently not allowed in a PUBLISH stream — not head-of-line blocking. He proposes reconsidering this constraint via Joining PUBLISH with live semantics, which he has prototyped in PR #1378 — SWITCH. See switch-abr.
- Emerging consensus vs. unresolved ABR case (as of Apr 19): Alan/Luke/Victor converging on a narrow LargestGroup/CurrentGroup filter for the join-live case; Gwendal contends this does not satisfy the charter’s ABR-switching requirement and keeps pushing the SWITCH / Joining-PUBLISH path.
- PR #1604 (Apr 10, updated Apr 16) - martin-duke implements #1602 to move Joining FETCH onto the SUBSCRIBE/PUBLISH stream. alan-frindell reviewed, noting subscriber priority cannot differ between fetch and subscription.
- PR #1490 (Merged Apr 14) - FILL_TIMEOUT parameter: subscriber’s max wait to fill a gap in a FETCH range before Unknown. Addresses part of #1023.
- draft-duke-moq-subscribe-rewind-02 (Apr 2) - Updated Rewind draft published with refined subscription filter semantics.
- #1603 (Apr 10-11) - martin-duke questions required-request-id. Ian Swett supports simplification, calling it unclear what functionality it provides. He also notes Joining FETCH’s dependency on another Request as a design concern.
- #1602 (Apr 9) - martin-duke proposes moving Joining Fetch to SUBSCRIBE/PUBLISH stream, eliminating race conditions
- #1601 (Closed) - Race condition in current design
- Restriction requiring “largest object” subscribes was removed
Debate at Interim 13
- will-law: Do OTT players actually benefit from REWIND if they require a complete buffer before playback?
- luke-curley + Victor Vasiliev: Best-effort rewind forces relays into complex upstream-fetching logic
- Cullen Jennings: Keep relays simple; push joining complexity into client libraries
- Will Law (counter): Only relays have cache visibility to make optimal decisions
Related
- joining-fetch - Current mechanism
- moq-transport - Protocol spec
- martin-duke - Rewind proposal author
- alan-frindell - Join Filters proposal author