To go forward, I'd like to say here that the new draft are prepared now
concerning the global communities.
While it's clear that a considerable amount of disagreement exists regarding
transitive communities dynamically doing things, it's extremely simple for
providers to just not pay attention to them.
Another potential application for global transitive communities, which is likely even more debatable than path selection issues, is using them in conjunction with MEDs and "more specifics" of provider aggregates (to fix some of the brokenness of aggregates and MEDs) in order to provide a safety net for potential route leaking.
This could be advantageous for several reasons. I think msot of us agree that a mechanism to re-introduce intelligence to "best-exit" type routing configurations is a good idea. It's a good idea not only because some providers want to perform "best-exit" as a value-add to their services, but also because it makes sense in order to provide the ability to compensate a peer (who's fussing about settlements and traffic asymmetries) by carrying the traffic longer on your network. It could also assist in more optimally regionalizing traffic exchange between networks, especially with the ever-growing geographically distributed inter-connectivty provided by direct interconnections.
The offshoots with providing more specifics to peers are obvious, I believe.
One problem is potential significant growth in routing and forwarding tables sizes, which was one of the primary drivers for aggregation techniques in the first place. If this is a problem, the provider can always opt to not accept the more specifics from the peer.
The other problem I can think of at the moment, which is likley more of a concern for most folks, is wrt providers leaking more specifics, either via BGP customers, or directly. This could be a concern because perceived "clue" of a peer, as well as simple errors in configurations, etc... This can be addressed to some extent by providing drafts that discuss these issues.
Then, there are the problems those accepting MEDs have regarding a networks ability to associate intelligent values with MEDs, or provide a *only* reasonable number of prefixes (versus "more specifics" expanding to thousands of /24s and longer). Of course, a large piece of this would be reliant upon a decent IP allocation plan that at worst provides router-based aggregates for more specifics, and preferrably PoP-based. This is difficult, of course, with all the older networks, acquisitions, etc...
Anyways, if a set of transitive communities were defined to provide a safety net that could catch the more specifcs, or some other mechanism were created to provide the same capabilities, I'd be interested.
I believe AboveNet and a few others actually have experience with accepting more specifics, and since I missed the BOF in Montreal (and no information is available on the web server as of yet?), I'd be interested in hearing what folks oinions are regarding this.