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The host cursor stops riding the video and becomes a real OS cursor on the client (the Parsec/RDP model): pointer feel no longer pays the capture→encode→network→decode→present round trip. Wire (M2a): - Hello grows a client_caps trailing byte (CLIENT_CAP_CURSOR) after the fixed display_hdr block — presence disambiguated by remaining length, which caps the post-HDR tail at 27 bytes (documented); Welcome answers HOST_CAP_CURSOR (capable-and-asked, the 444/clipboard precedent). - CursorShape (0x50, control stream): serial + dims + hotspot + straight RGBA, ≤120px/side so the u16 frame always fits (128² would overshoot); client caches by serial — re-showing a known shape costs 14 bytes, not a bitmap (RDP pointer-cache for free). - CursorState (0xD0 datagram): serial + visible/relative_hint flags + position, sent once per encode-loop tick — latest-wins, self-healing under loss, no refresh timer. relative_hint is reserved for M3. - Client core: two new planes (control-task + datagram-task arms) → next_cursor_shape/next_cursor_state; connect() grows client_caps (C ABI passes 0 until the v11 cursor poll fns exist). Host (M2b, Linux portal only): - handshake::cursor_forward is THE predicate (client asked ∧ Linux ∧ compositor ≠ gamescope) — Welcome bit and session wiring both read it. - SessionPlan.cursor_blend goes false for a forwarding session; the encode loop ticks a CursorForwarder every iteration: shape-serial diff → control-task bridge (mirrors probe_result), state datagram → conn. - CursorOverlay/capture CursorState carry the hotspot through (nearest-neighbor downscale backstop for XL cursors, unit-tested). Presenter: - CursorChannel drains both planes per loop iteration; shapes become SDL color cursors (from_surface + hotspot), applied while the desktop mouse model is engaged; visibility follows the host; capture/released hands back the system cursor. Sessions advertise the cap when they START in desktop mode. Verified on .21: fmt + clippy -D warnings (7 crates) + tests green (core 218 incl. new wire roundtrips, host 245 incl. e2e + forwarder downscale tests). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
punktfunk-core
The shared protocol core — the one place where punktfunk's transport, forward error correction, and crypto live. It's linked into the host and every native client, so there's exactly one implementation of the wire format everywhere.
Written in Rust with no async on the per-frame path (native threads only). It exposes both a normal Rust API and a stable, versioned C ABI, so the Swift and Kotlin clients — and any C embedder — link the same code as the Rust ones.
What's in here
- Transport & session (
session.rs,transport/,packet.rs) — thepunktfunk/1data plane over raw UDP: packetization, reassembly (with attacker-bounded limits), pacing, and socket tuning. - FEC (
fec/) — the wall-breaker. Two codes:- GF(2⁸) classic Reed–Solomon with the Cauchy generator matrix — byte-identical to the
nanorslibrary Moonlight uses, so our parity is decodable by a stock Moonlight client. - GF(2¹⁶) Leopard-RS (SIMD, O(n log n)) — up to 65535 shards/block, which removes the ~1 Gbps
FEC ceiling.
punktfunk/1negotiates this one.
- GF(2⁸) classic Reed–Solomon with the Cauchy generator matrix — byte-identical to the
- Crypto (
crypto.rs) — AES-128-GCM session encryption with per-direction nonce salts and sequence-as-AAD; SPAKE2 PIN pairing lives behind thequicfeature. - QUIC control plane (
quic.rs,client.rs, featurequic) — the Hello/Welcome/Start handshake, cert pinning/TOFU, reverse audio, and the embeddableNativeClientconnector. This is the only placetokio/quinnare allowed; the feature is off by default so the core stays runtime-free. - C ABI (
abi.rs) — the versioned surface (punktfunk_abi_version(),PunktfunkConfigcarrying its ownstruct_size) that generatesinclude/punktfunk_core.hvia cbindgen at build time.
Build outputs
The crate builds three ways at once (crate-type = ["lib", "cdylib", "staticlib"]):
| Output | Used by |
|---|---|
lib (rlib) |
the host, probe, and tools link it as a normal Rust crate |
cdylib (.so/.dylib) |
the Swift / Kotlin clients via the C ABI |
staticlib (.a) |
the C test harness and static embedding |
Test
cargo test -p punktfunk-core # unit + proptest + loopback
cargo run -p loss-harness # FEC loss-resilience sweep (no network needed)
bash crates/punktfunk-core/tests/c/run.sh # standalone C-ABI link + round-trip proof
Design invariants (do not regress)
- One core, linked everywhere — protocol/FEC/crypto live only here, behind the stable C ABI.
- No async on the hot path — the per-frame pipeline is native threads only;
quic(tokio/quinn) is control-plane only, feature-gated, off by default. - Security hardening stays intact — the reassembler bounds attacker-controlled fields before
allocating; AES-GCM keeps per-direction nonce salts + seq-as-AAD; the ABI checks
struct_size. Regression tests exist — keep them green.
Related
punktfunk-host— the streaming host built on this core- Clients — the apps that link this core over the C ABI (or directly, in Rust)
- punktfunk-planning:
implementation-plan.md(internal planning repo) — why GF(2¹⁶) FEC, the latency budget, and the architecture thesis