Share the right path state

Notify all active users of a decreased path estimate, and separately identify the particular dropped packet.

• §5.2 leaves the local path representation to the implementation. Per-destination, per-flow or even a conservative global minimum are discussed possibilities, not one mandated cache key. For multiple interfaces, PMTU information should be maintained for each IPv6 link.
• The first two states choose a destination-plus-link cache key for illustration. PathA begins at first-hop 1500 and pathB at first-hop 9000. A correlated PTB 1400 on A changes only A; TCP and UDP users of A learn the decrease, while unrelated B stays 9000.
• All packetization-layer instances actively using the decreased path must be notified (§5.2), even TCP when the original quoted packet used UDP. This is separate from notifying the specific originating instance that its packet was dropped. Only that connection is eligible for an immediate retransmission response described in §5.4.
• If no estimate decrease occurs, the instance that sent the dropped packet should still receive a drop notification (§5.2). Notifications may use an asynchronous signal or suitable send-error/next-send design; no particular operating-system API is mandated.
• For a per-destination cache covering multiple paths, store the minimum learned PMTU across those paths (§5.2). A single source/destination pair can still have multiple paths, such as equal-cost multipath routing (ECMP). Per-flow storage can distinguish some of those choices when enough information exists.
• Multicast copies can follow different paths to several receivers. The minimum across the paths in use limits subsequent packets to the multicast destination (§3). Receiving one large-MTU report does not erase a smaller relevant path limit. A destination that appears on-link can still hide a proxy/router path and needs discovery (§3).