feat(host/wire): MTU resilience for the video data plane

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.

This commit is contained in:

enricobuehler

2026-08-04 18:30:33 +02:00

parent 2c03290a5e

commit 4bc7eecf05

6 changed files with 338 additions and 4 deletions

									
										include/punktfunk_core.h
									
		+6
		
												View File
												
				@@ -333,6 +333,12 @@

				#define INBOUND_REQ_FLAG 2147483648

				#endif

				// Floor for a negotiated `shard_payload` (even, well under every real path). A path whose UDP

				// budget lands below this can't carry the QUIC control plane either (QUIC's own minimum is a

				// 1200-byte UDP payload), so shrinking video shards further buys nothing — the clamp helpers

				// bottom out here instead of producing degenerate confetti-sized shards.

				#define MIN_SHARD_PAYLOAD 512

				// 16-byte AEAD authentication tag appended by either session cipher.

				#define TAG_LEN 16