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.
enricobuehler