Hey Marcelo, > > 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 Well, I don't know about "assuming". It definitely happens. > 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) Sure, but that's not what some people are relying on today. > > 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. Yep. > 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 Those are definitely possibilities. I wonder if there are others. Dave
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