I said and meant "traditional type of
multihoming" and, more important in the previous sentence, "many
hosts and sites that are multihomed in the traditional sense of
advertising one set of endpoint addresses to the network and
letting the routing system sort things out". That is, very
specifically, one address per host, advertised to multiple ISPs/
networks/ paths and not "a chunk of PA IPv4 addresses from each
upstream".
But it's the same thing -- the customer numbers some of their network with
PA addresses from one provider and part of it with PA addresses from the
other. Then the customer sends out traffic from both address blocks to
both interfaces, and the upstreams can't tell whether an unknown address
is a legit one from another provider or a malicious spoof.
I'm pretty sure this is right, having talked to ISPs about it in the
context of why BCP38 is harder than you'd think.
R's,
John