Re: [RRG] some musings on PI v. PA, and assumptions, requirements, and tradeoffs

[Dino Farinacci <dino@cisco.com>]

> Haven't we been round this particular mulberry bush a few times  
> already?   Yes, the routing system offers the path of least  
> resistance when attempting to solve a number of issues, and thats  
> because routing solutions are invariable simple and (today) cheap.  
> And the Loc/ID split seems to be a pretty massive solution for  
> something that does not appear to be a commonly accepted problem in  
> the first place. So yes, we continue to use the routing system for  
> multi-homing, traffic engineering and similar. It just works.

To connect to stationary hosts yes. But if they are moving around and  
keep their addresses regardless if those addresses are from PI or PA  
space, then location needs to be topological. Furthermore, moving  
around a lot and injecting and withdrawing more specifics is just  
going to add churn in a major way.

> The only concern I've heard voiced that seems to me to be a real  
> issue in all this is the question "what if we all did that?" Are  
> there numbers for "we all" that appear to be beyond the capability  
> of the technology curve? Are there numbers for "we all" that might  
> even be beyond the capability of the protocol? If so, then we  
> probably need some

All valid questions with no magic answers. But if we are concerned  
with churn and convergence of control-plane tables into forward-plane  
tables and looking at 10^6 sites plus the mobility aspect, the  
numbers are getting large. But this is not scientific but just gut feel.

> kind of plan B, and it does appear to me that if you want to get  
> out of the tyranny of incrementalism then you need to consider  
> something a little more fundamental in terms of network  
> architectures when you start to look at what would make a Plan B  
> effective. For me this path is where the split loc/ID approaches  
> start to gain traction. But to be clear, the split loc/ID approach  
> does not appear to me to be a viable tactical response to routing  
> inflation in the near term future.

As I have said before Geoff, solving the routing table size problem  
is just a side benefit if we first solve the "non-search-site"  
multihoming problem. That is to allow them to use both of their links  
(I believe you said that 80% of all multi-homed sites have exactly 2  
connections) and to be able to tell hosts returning packets to them  
which link to use and how to failover.

A level of indirection can solve this but the big question is it too  
much of a solution. That is, do we have the guts to just brute-force  
and continue to send in PI prefixes.

Dino

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