every.channel/evolution/proposals/ECP-0010-time-synchronized-chunking.md

ECP-0010: time-synchronized chunking

Status: Draft

Problem

To achieve byte-identical chunks across uncoordinated antennas, we need a shared timeline for chunk boundaries. If chunk boundaries are derived from local capture start time, identical broadcasts will diverge.

Decision

Define a time-synchronized chunking model based on broadcast clocks:

1. Primary clock: PCR
   • Use MPEG-TS PCR (27 MHz) as the canonical timeline for chunk boundaries.
2. UTC anchor (when available)
   • ATSC: use PSIP STT to compute UTC time.
   • DVB: use TDT/TOT to compute UTC time.
3. Chunk boundary rule
   • Compute chunk_index = floor((t_anchor) / D) where t_anchor is UTC time when available, otherwise PCR time.
   • D is fixed chunk duration (e.g., 2000 ms).
4. Alignment policy
   • On ingest, drop data until the next full boundary.
   • Each chunk contains frames/packets whose PTS fall within [chunk_start, chunk_end).
5. Discontinuity handling
   • If PCR/PTS discontinuity is detected, close the current group and start a new one at the next boundary.
6. Sync status
   • Streams with UTC anchor are marked synced and eligible for swarm convergence.
   • Streams without UTC anchor are unsynced and remain source-scoped.

Consequences

• Identical broadcasts on different antennas converge to the same chunk boundaries and can produce byte-identical object streams.
• Early ingestion can begin unsynced and promote to synced when broadcast time appears.
• Chunk IDs become deterministic functions of (UTC or PCR timeline, duration).

Alternatives considered

• Local wall clock (NTP/PTP): rejected as primary anchor because it is not broadcast-authoritative and varies between hosts.
• Capture-start-based chunking: rejected because it prevents convergence without coordination.

Rollout / teardown

• Add a time model to the ingest pipeline (PCR + optional UTC anchor).
• Require chunkers to align to the computed boundary.
• Promote StreamId from source scope to broadcast scope only when synced.
• Supersede this ECP if MoQ introduces a standardized global timeline.