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Johnathan Corgan
037a965a93
When a transit forwarder drops an oversized data packet and reports the bottleneck back via MtuExceeded, the receive-side handler already updates per-session MmpSessionState::path_mtu (used by PTB synthesis to feed kernel TCP). It did not, however, update path_mtu_lookup — the per-destination map the TUN reader/writer consult at TCP MSS clamp time. So forward-path-asymmetry flows kept clamping at the discovery reverse-path value (too generous for the actual forward-path budget) on every subsequent SYN. Add the missing write at the same point apply_notification runs. Keep the tighter of existing-or-new — the clamp must never loosen. Same write-shape as seed_path_mtu_for_link_peer. Tests: - Three focused unit tests on handle_mtu_exceeded for the empty, tighten, and keep-tighter cases. - Extended test_multihop_pmtud_heterogeneous_mtu to assert the lookup tightens after the wire-level MtuExceeded propagation, alongside its existing PathMtuState assertion. Adds two #[cfg(test)] accessors on Node (path_mtu_lookup_get / path_mtu_lookup_insert) and bumps handle_mtu_exceeded to pub(in crate::node) for direct test invocation.