per connection routing

[Sergey V Kovalyov <sqk0316@scires.nyu.edu>]

I have a DLS connection to the Internet and a T1 connection to the other
office, which in turn is also connected to the Internet.
Both connections use IP masquerading with two completely different
addresses.

I want to put a firewall/router in the middle that would provide:

- load balancing for these two connections. Because of NAT, the choice of
which way to route has to be done on a per-connection basis.

- fault tolerance for the Internet connection: if oen link goes down
all new connections shold go through the other. The existing connections
will have to timeout.

- fault tolerance for the connection to the other office: it T1 goes down,
a VPN connection should be established through DSL.

Any ideas, siggestions, experinces or pointers ? What would be the most
appropriate OS for it ?

Sergey.