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VerseVDI-Data-Plane/openspec/changes/truthful-fixed-frame-qualification/design.md
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Context

The current harness sends one fixed 1,179-byte payload per logical sample. It reaches the production path but does not represent encoded frames at 60/120 FPS or exercise realistic fragmentation, reassembly, queue bytes, and keyframe pressure.

Goals / Non-Goals

Goals:

  • Deterministically generate complete variable-size frame units at exact profile frame rates and target bitrates.
  • Include bounded periodic keyframes while preserving exact aggregate bytes.
  • Measure the existing production path and independent reassembly with frame-level accounting.

Non-Goals:

  • A real encoder, codec parsing, a second simulator, or a normative run before immutable Protocol publication.

Decisions

  • Derive bytes per fixed interval from bitrate and FPS, distribute integer remainder deterministically, and shift bounded bytes into periodic keyframes while keeping the interval total exact.
  • Carry a deterministic frame index/pattern only in the generated payload bytes; no codec semantics are claimed.
  • Keep the existing path/impairment/resource driver and change its unit from datagram payload to complete frame.

Risks / Trade-offs

  • [Keyframes can exceed queue budget] → use the reviewed 1 MiB frame ceiling and production byte-bound queue.
  • [Short smoke windows have rounding effects] → assert exact generated totals and report measured duration separately from normative ten-minute gates.