The other extreme is that, what if you are singly-homed? Then it is useless again. My point is, I would guess that if you are not single-homed (in which case this is useless), you are multi-homed, and your traffic probably isn't symmetrical. Therefore, I think this feature is of limited usefullness.
It would prevent simple spoofing, yes, but that would not eliminate the Smurf attacks since to mount a Smurf attack you need to use the victim's address as your source address, and that one *is* typically "valid" according to the criteria you mention above (?).
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- Atheism is a non-prophet organization. I route, therefore I am. Alex Rubenstein, alex@nac.net, KC2BUO, ISP/C Charter Member Father of the Network and Head Bottle-Washer Net Access Corporation, 9 Mt. Pleasant Tpk., Denville, NJ 07834 Don't choose a spineless ISP! We have more backbone! http://www.nac.net -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --