multi-hop routing breaks pmtu discovery

[rade]

I haven't actually tested this, but...

When a weave router forwards a packet to another peer, it uses the same logic for dealing with too big frames as if the packet had been captured from the local interface. In particular, an ICMP 3.4 packet will be injected on the interface. Now, if weave subsequently actually captured that same packet then things might sort of work, but a) it probably won't since we tell pcap to capture inbound packets only, and b) this is a rather round-about and inefficient way of handling pmtu discovery in the forwarding scenario.

Instead we should directly sent the generated ICMP 3.4 packet to the appropriate peer(s).

[msackman]

It did work for me but that was possibly before the pcap "inbound-only" changes.
With the conn-limit stuff, it's pretty easy to establish a linear chain of routers - thus not too hard to test this.
I do agree it's fairly round-about though. Routing it properly would be good.