IETF removed from the distribution list. On Fri, 11 Feb 2000, Paul Ferguson wrote:

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unicast RPF, but the best compromise is the built-in access filter. The solution must be general enough to work for multihomed, defaulting out customers with blocks from n providers,

No, that is a common misconception, or rather, an overstatement of a pretty easily described situation. It only breaks things in transit situations, and only in transit situations where you might not have the same forwarding path back to the source as you would via the same interface a packet came in on.

This is more common than you might believe. For Dialup and single homed, yes, this is not a problem in most cases. For a very large customer base, this problem does not scale all that well, especially for the large backbone carriers who are transiting a lot of traffic. As the internet grows more important to business, more and more people multihome.

This is a small percentage, I would thing, since the percentage of ISP's offering transit pales in comparison to all other "access" ISP's that do not. And in cases where ISP's _do_ offer transit, or have transit agreements, will they really do this on their transit interfaces? I think not.

I think you're solving something else. I submit that almost _all_ isp's offer transit for their customers. Thats where the I part of the SP comes in. For _peering_ links (peering being defined elsewhere), yes, this is a hard problem, but on the edges of the _peers_, this is not. If everyone filtered their T1/DSx/OCx/E1/E3/STMx customers at their edges, using Unicast RPF where appropriate and filters where appropriate, life would become better. /vijay