asymmetric routing

[Dave Wilson]

I think the harm caused by sloppy aggregation of address space is clear. 
I think the harm caused by inadequate filtering of source addresses is 
also clear.

These are separate considerations, however. Forwarding traffic with a 
source address that is not your own, for good or ill, doesn't harm the 
routing table in the same way that unaggregated advertisements do, and 
so it seems to me it's entirely compatible with (or agnostic of) the 
principle of strict aggregation.

If the customer has legitimate reason for sending alien traffic (i.e. 
the space has been allocated to the customer by another ISP), I see no 
reason why the ISP can't choose to allow it. In fact, if an ISP has to 
advertise an address block just to be allowed source traffic from it, 
that is a big incentive to clutter the routing table.


> FYI: I just added the following text as a proposed requirement
> For IPv6 multihoming solutions.
> 
> "IPv6 multihoming solutions MUST be compatible with sites that
> implement RPF checks or filtering that prevents traffic to be sent
> back from a different interface it came in."


We get asymmetric traffic all the time in the default free zone, since 
we're all allowed to choose localprefs independently of each other. It's 
no big deal. Where a site chooses to allow a customer route traffic 
asymmetrically, and otherwise implements RFC2827, I don't understand the 
harm in allowing a multihoming solution based on this. My instinct says 
that we'd lose a whole swathe of potential solutions for arguably little 
benefit.

Dave