Routing over multiple interfaces

[David Woodhouse <dwmw2@infradead.org>]

I have two ADSL lines, and the ISP routes the same set of addresses over
both of them -- so I can saturate my incoming bandwidth over both lines
with a single TCP connection. This is not MLPPP; just two separate PPP
connections.

TCP can cope with a little bit of packet re-ordering, which obviously
happens when packets come over both lines.

Everything works really well for downstream traffic -- but not so well
for upstream, because Linux doesn't use the full available upstream
bandwidth. I have the default route set up thus:

default  src 90.155.92.214 
	nexthop dev ppp0 weight 1
	nexthop dev ppp1 weight 1

But when I do a large upload, I find that the kernel is only ever using
a *single* link at a time, rather than both. How can I make it use
*both* links? It's fine to confine each flow to a single link if it
doesn't saturate that link... but once the queue is full, it should
overflow onto the other device.

I worked around this earlier today by splitting the large file I needed
to upload into two parts, shifting one of them to a *different* machine,
connecting that to the VPN too and then uploading the two parts
separately. But that isn't really a viable option in the general case.

-- 
dwmw2