[c-nsp] Private IP in point to point link on internet

[Pierre Emeriaud]

>> I'm not 100 percent sure how it breaks the path discovery, I would love to
>> test this too, as we have a few of these setups in place.
>
>
> The issue is that many routers, when the need arises to fragment packets,
> will send back an icmp 'fragmentation needed' message, *from the source ip
> address of the interface that was traversed*. So, if you have a p2p link
> with your end being 192.168.1.1, your router may very well send the packet
> with that ip. And, many providers filter packets with rfc1918 addresses
> inbound and outbound,

That, and urpf. We use unannounced netblocs (but publics) for our
backbone links and loopbacks, and while this has served us well (go
on, try to send a packet -or a billion- to it), our network is pretty
much opaque to anyone doing urpf, because ttl expired will be dropped.
This makes traceroutes less useful...

when two best practices collides...