I've been thinking about a benefit of PI addressing that I have not seen discussed on this list or others (at least recently). In particular, PI addressing enables a certain kind of "path selection" that might not be easy (or possibly desirable) to retain in any of the the LOC/ID split schemes we have been discussing. This is contrast with the standard PI stuff (e.g., I don't want to renumber, etc). Consider the following scenario: I'm a multihomed stub (I don't transit packets between my two upstreams). Further, I have PA delegations from each of up upstreams. Now, I'm corresponding with a remote site using addressing out of one of the PA blocks, call it X. Now, my link to the ISP aggregating X breaks. A packet destined for X will then travel very close to my site before learning that the link is down, possibly too far to be rerouted. And BTW, if I advertise X to my other upstream, then my advertisement of X has the same cost (to the routing system) as a PI advertisement. This problem is common to all (I think) of the schemes that seek to improve/optimize aggregatability in the core. For example: (a). In the 8+8/GSE case, the problem is that the packet will follow the RG in the src address all the way to the "end" of the path, that is, to the ISP that can forward it to the site. You don't learn that the site can't receive the packet until that point, and there is no way to reroute it. (b). The situation is similar with PA space, since the fact that the link at the "end" of the path might be down is hidden in the aggregate. You don't learn this until you are close to the "end" of the path, and there may be no way to reroute the packet. (c). The situation is similar with map/encap, since one of its goals to enable better PA-style aggregation. OTOH, if I announce PI space, "switching to the new path" is controlled by the announcement/withdrawal of the PI prefix, and can happen much closer to the source. So in this sense aggregation breaks a certain kind of "path selection". I think we all realize that there is no free lunch, and that this is a property (such as it is) of the fact that aggregation throws away information in the interest of computability (a standard technique). So folks are using PI for reasons other than the standard laundry list (i.e., avoiding renumbering, etc). In particular, advertising PI space can cause the "switch" to a different path during an outage to happen much closer to the source (i.e., much further back in the network). None of this to say that we shouldn't be moving forward with the various solutions we've speced out (quite the contrary). Rather, my question is really about revisiting assumptions, requirements, and tradeoffs. Dave
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