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

Hi Dave,


El 13/07/2007, a las 0:40, David Meyer escribió:



	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.
basically you are assuming that the failure detection is performed by the routing system and that the information about the failures inside an aggregate are hidden by aggregation
But it is possible to perform end to end failure detection that is likely to be much faster than this (or at least it can be tuned by the endpoints to be faster than this and even to detect failure much faster than when using PI) 







        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).
exactly and if we want to restore such functionlity without burdening the routing system we will need additional mechanisms that will have a cost.
a similar considration can be applied to the TE discussion and using PA addresses. I mean, when we only use PA addresses and the only one that knows that 2 PA prefixes are assigned to a single site is the site itself, then the rest of the network cannot perform TE tricks. This is a similar trade off: the rest of the network is no longer overloaded with additional routing information, but as they no longer have the information about all the paths leading to a multihomed site, they can not longer play with different paths. If we want to restore such functionality, we will need to pay to publish that information, either by bigger routing tables, either having a parallel database that contains the multiple PA prefixes assigned to each multihomed site 

Regards, marcelo



	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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