That's once again a matter of defaults -- routers should _by
default_ discard all packets from interfaces which they won't
use for forwarding those packets back.
This is a sweeping statement. I'm not sure we should buy into it
without thinking about it pretty hard. However, I am no routing
theorist, so maybe this has already been thoroughly explored by
the high-level networking gurus.
I see three obvious advantages:
1. Prevention of IP spoofing.
2. Prevention of asymmetric routing.
3. Better TCP windowing and better performance when all
packets follow the same path.
However, are the benefits worth the overhead? What are the
effects on network redundancy? What other issues am I