(2) There is no way that these decisions can be made solely at the
transport or application layer, because source (and to a lesser degree
destination) address selection is tightly tied to the first-hop
forwarding decision. The outbound interface, source address and default
router all have to be selected in a coordinate process, to avoid sending
traffic that will be discarded on the outbound path, due to router
filters.
Actually, applications can to do that today, using the socket API, if the stack
implements the "strong host" model. The application just needs to bind the
socket to a specific IP address. Doing that ensures that packets sourced by the
application will use the specified address, will go out through the interface
corresponding to that address, and will use the default gateway associated with
that interface.
-- Christian Huitema
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