forked from unom/punktfunk
enricobuehlerandClaude Fable 5
d0f68cbbcd
Phase 1 of the dual-stack plan (design/client-parity-and-network-resilience.md, plan 5):
the host sizes each session's shard_payload from the QUIC remote's address family instead
of assuming IPv4 — 1408 over v4 (unchanged), 1388 over v6 (40-byte header). Rides the
existing Welcome::shard_payload negotiation, so there is zero wire change and old clients
simply follow.
This has to land before any v6 data path exists: the v4-maximal 1408 makes every sealed
video datagram overshoot a 1500-MTU IPv6 hop, and v6 routers never fragment — that's a
blackhole (every datagram dropped), not the graceful-ish degradation of the b5c30df v4
fragmentation saga. IPv4-mapped v6 remotes (::ffff:a.b.c.d, what a dual-stack [::] socket
reports for a v4 client) correctly keep the v4 size — they ride IPv4 on the wire.
New mtu1500_shard_payload_v6()/mtu1500_shard_payload_for() in core config with the same
pinned never-fragments/maximality tests as the v4 constant, plus a family-selection test.
Verified: 82 core lib tests + loopback/c_abi green and host check/clippy clean on Linux
(home-worker-2); core tests green on macOS.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>