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



	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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