Personally, I love the idea of allowing the network to be more
inclusive by allowing connections other than the current IPIP one. Rather than replace IPIP, I would suggest that we keep it and just allow people to act as hubs for those that are behind NAT/Limiting firewalls, etc.
This is what we already have working, and others have that too. A local VPN server that is connected to IPIP (and in our case BGP too). However, such a setup is a bit complicated because the IPIP mesh is not well supported on many router types, and having the two different network types integrated in the same router also is kind of tricky.
Not everyone gets that right: all routes have to be in the same routing table and evaluated from more-specific to less-specific. But you still need to handle cases where multiple routes to the same subnet (using different protocols) can exist. In some cases, people have resorted to having multiple routing tables and searching them in a specific sequence, but that does not work correctly in some cases. Also there is the issue of determining the correct source address. Sometimes such gateways send traffic with a non-44net source address through an IPIP tunnel, which of course is unwanted.
So my proposal is to drop the IPIP mesh to remove this additional complexity, and make the system easier to rollout and maintain.
While I think BGP would be great, it adds questions like: can people
announce their own non-44 space, can people use their own ASNs, how will we allocate ASNs, how do we confirm people are announcing space actually allocated to them. One thing we can do, is look at DN42 and how they work. Their network is similar to some of these suggestion with the exception that they use private space.
Some of those topics have already been addressed and resolved before. For example w.r.t. the AS numbers, we have agreed to use an allocation scheme for private AS numbers so this can be delegated to individual regions without chance of collisions. The scheme is to use "42"+iso country designator+5 digits, where these 5 digits can be subdivided in a region specific way. Large countries have several iso country designators so there should be ample space using this scheme. Here we use 42204+3digits+2digits where a router in our 44.137.aaa.bbb/16 subnet gets AS 44204aaann where nn=bbb/16.
Of course this network is only meant to distribute net44 addresses, our routefilters filter announcements outside that. But you can announce space for your friend inside net44. Actually the same as the current IPIP situation.
Indeed very similar to what DN42 does.
Rob