linux multipath per session route

Hope someone can tell me something about this.
I have a linux multipath routing system working. It's a 2 ISP
connection: one cable and one ADSL. I'm using iproute2 and some
scripts adapted from

file:///D:/Netscape.dat/linux/firewall/2-ISPs/dgd-usage.txt
http://www.skybuilders.com/Products/.../NanoSimon.txt

and my redundant, highly-available Internet connection works almost
fine.

The only problem is with connections that need to use the same path
throughout, like:

- large ftp/http downloads
- Windows Terminal Services client server comunication
- etc.

because at some moment the load balancing system exchanges the
interfaces through which the packets go.

My question is if I can write a set of ip rules (or anything similar)
to force the path to be the same while the session is the same; or
when the source port is the same (per session load balancing).

If it is not possible, is the only solution a per-destination load
balancing?

AP