46 lines
2.1 KiB
Markdown
46 lines
2.1 KiB
Markdown
# 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.
|