Check Packet Too Big feedback

A PTB can lower a relevant estimate; it cannot raise it or push it below 1280.

• ICMPv6 Packet Too Big is Type2, Code 0, with a 32-bit MTU field and as much of the invoking packet as fits the ICMPv6 error limit (RFC 4443§3.2). It has no TCP or UDP port of its own. The quote is context, not an authentication credential.
• Nodes should appropriately validate the PTB payload against transmitted traffic (§4). The first state assumes a matching packet on pathA. The unmatched-quote state applies this recommended validation as the teaching implementation’s rejection policy; it does not invent a new capitalized MUST.
• A PTB reporting an MTU less than 1280 must be discarded. A node must not reduce its estimate below the IPv6 minimum link MTU (§4). Discarding the message is distinct from accepting its number and merely clamping it to 1280.
• A node must not increase its PMTU estimate in response to a PTB (§4). An equal or larger reported number causes no estimate increase; the message may be stale, reflect a different path, or be forged. The sender of the original packet may still need drop notification (§5.2).
• A valid smaller report affects the relevant path representation (§5.2), not every unrelated interface or destination. A lower number that passes correlation is not thereby cryptographically authenticated (§6); false reductions can still harm throughput.
• The tentative-floor comparison described in §5.2 does not override §4’s explicit discard requirement for a reported MTU below 1280. The illustrated validation gate discards that message instead of changing the cache to a clamped value.