D008 - Frame grab protocol

Status: decided Date: 2026-09-26

The frame grabber is a resident service on its own port (9022) that answers GRAB\n with a self-describing header, the pixels and an FNV-1a checksum. Format and stride pass through as the target reports them; a non-zero status means no pixels follow; a short read is an error.

Why: diffing needs repeated grabs, and a resident listener is what every other service in the chain already is. A frame whose stride was guessed fails as an emulator defect. The threat is a truncated transfer on a local network, which a non-cryptographic checksum covers in a few lines of freestanding C.

Rejected:

• A one-shot payload writing back over the loader's socket: that socket exists only when the loader started it, and each grab would need a re-send.
• A cryptographic digest: guards against substitution, which is not the threat here.