* Path MTU discovery: that's part of the IP contract. IP provides an unreliable datagram service with an MTU that varies based on destination endpoint but will

* Path MTU discovery: that's part of the IP contract. IP provides an unreliable datagram service with an MTU that varies based on destination endpoint but will never be below 1280b (in IPv6 - IPv4 was 576b). IPv6 also wisely doesn't do fragmentation; sizing your packets correctly is the job of layer 4. * TCP over wireless links: TCP's congestion control mechanism is a heuristic based on ever-evolving understanding of the characteristics of links in the wild. There are things that layer 3 can do that unambiguously get in layer 4's way (bufferbloat makes low-latency response unfeasible), but it's layer 4's job to deal with reliability and congestion control. (By the way - unlike LFNs, WiFi is actually not a pathological case for TCP congestion control and buffering. A good mental model for those periodic WiFi drops is of an Ethernet cable being disconnected and reconnected with a different one picked at random from a supply closet. In a lot of very common cases, when traffic gets passed again it will not be at the same throughput as before and so the endpoints need to rediscover the available throughput.) To your more general suggestions about alternative designs: generally, schemes that have the link layer communicate with the endpoints using them scale BADLY to large internetworks, and the global internet is the largest.