Packet reordering / VPN Bonding (Score 2, Interesting)

Obviously direct aggregation isn't possible, as each line will have a different source IP. What works is load balancing, but load balancing sucks. If you do per-TCP-connection load balancing on multiple lines, lots of sites will give you problems, as multiple requests for the same session are coming from different IPs. Online banking doesn't like this, ads-supported sites often don't like this (as the ad was loaded from a different IP). So this leaves you with per source-host load-balancing, and this only makes sense if there are lots of people who are two share the lines.

Doing real aggregation (bonding) requires a remote endpoint obviously, located in a datacenter somewhere for example. Problem: There is no standard protocol that works for a combination of different lines, Multilink-PPP will only work for several identical lines from the same ISP (ideally using the same clock source at the DSLAM etc). Why is that? That's because if you use multiple lines, they will have different latencies / round trip times. And if you bundle those, this means that TCP packets will overtake each other in-flight. So in the end whoever is receiving the re-assembled stream will get it out of order. And TCP can not differ between reordered and lost packet - if an unexpected (too high sequence number) packet is received, it is dropped. And this can not be solved by buffering at the router/PPP-device, because this buffering would interference with TCP windowing. In the end most of your aggregated bandwidth will therefore be eaten by retransmissions.

So, people may tell you to try this and that, but in the end everyone who has ever REALLY tried it himself will tell you: Forget about it, the performance will always be really bad (unless you have multiple identical lines).

There is a small german startup I work for which has solved the problem by creating a new bundled VPN protocol running on the way between the router in your office and the one in the datacenter, basically running a man-in-the-middle attack on TCP to get rid of the packet reordering in-flight. See http://www.viprinet.com/ for the available products and background info on how it works. Pricing starts at ~1000 USD, but obviously you'll need two boxes - probably not what you'd call "affordable". And sadly we do not yet have distributors inside the USA.