DSL link aggregation?

[Charles Steinkuehler]

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Billy Crook wrote:
| For a number of technical reasons, it is not possible to use two
| residential internet connections to "accelerate" the path between the
| same two computers.  At least when using TCP, a persistent connection
| has to originate from an IP address.  Each of the dsl modems will be
| provisioning unique IP addresses.  Thus one connection can only
| originate from one of the two dsl modems at a time and only use one
| modem's worth of bandwidth.

Um...sort-of.

The above is true for the *RETURN* traffic, which will be routed based
on IP address.

The problem, however, is with *OUTBOUND* traffic, due to the
asymmetrical nature of the DSL connection.  It would be perfectly
acceptable to send half of the outbound packets via ISP #1, as usual,
and the other half of the packets via ISP #2.  The trick is, the source
IP needs to be the same for *ALL* of the packets.

As long as at least one of your ISPs isn't doing egress filtering for
spoofed source IPs, your traffic will get through, and you'll have twice
the upload bandwidth (assuming the system you're talking to on the other
end can easily handle the out-of-order packet arrival that will likely
result).

Setting this up will require some crafty playing with iptables (assuming
you're masquerading your internal machines) and the kernel routing
tables, but it should be quite possible.  Check into the 'ip' command
(iproute2) and the lartc HOWTO to get started:

http://lartc.org/howto/

You may also find a pre-canned solution like shorewall easier to
implement.  Even if you don't go this way, the documentation might be
helpful:

http://www.shorewall.net/MultiISP.html

NOTE:  If you're willing to forgo the download bandwidth of your
additional link, I believe you can use shorewall to combine the outbound
bandwidth of multiple links by properly specifying the masquerade
addresses used (ie: use the same public IP for all outbound traffic).

I assume there's some reason you can't just get a cable modem, or
alternate DSL plan with more upload traffic?  That would generally be
the easiest solution, and likely cheaper than paying for two separate links.

- --
Charles Steinkuehler
charles at steinkuehler.net
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