forked from unom/punktfunk
enricobuehler
4bc7eecf05
Video datagrams are sealed at a shard payload sized for a clean 1500-byte MTU (1472-byte UDP payloads). A host whose route to the client crosses a smaller-MTU hop (a VPN/overlay adapter claiming the LAN route, a lowered NIC MTU) delivers every small flow — QUIC control, hole punch, input, audio — while 100% of video datagrams die: the client sits on a black screen reporting zero loss and the host streams into the void with every gauge green. Field-reported as 'connects fine, black screen forever'. Three legs, none of which changes a session on a healthy path: - PUNKTFUNK_WIRE_MTU operator override: shard payload derived from a given on-wire IP MTU. Wire-compatible — Welcome::shard_payload is already negotiated per session (the v4/v6 split ships two values today) and every client follows the negotiated value. - Detection: the QUIC MTU-discovery probe ceiling moves from quinn's stock 1452 to exactly the sealed video-datagram size (1472), so a control connection's settled MTU becomes a verdict on the path: settled at the ceiling proves it carries video, settled below proves it cannot. A per-session watcher samples after the search has settled (live-connection guard against mid-search false learns) and logs an actionable WARN naming the failure shape and the diagnosis commands. - Healing: the measured budget is recorded per peer IP; the next handshake clamps shard_payload to fit, so a reconnect self-heals. A later session that reaches the ceiling erases the record. Verified: core 286/286 --features quic + clippy -D warnings (macOS); host clippy -D warnings + native:: tests 44/44 (pf-lxcheck container). The regenerated C header picks up the new MIN_SHARD_PAYLOAD constant.