[ILUG] ip route equalize

[Kieran Murphy]

(oops... and to the list too)

What you need to undestand about Equal Cost Multi-Path, is that its a
per-flow load-balancing.
Not per-packet, which I think is what you're expecting to see.

A flow is defined by the Src IP, Src Port, Dst IP and Dst Port.

So, in theory, if you changed one of these, it should re-balance.
If you have enough diverse flows, they should even out the balance over your
links... if that makes sense.

Hope this helps.

Regards,
Kieran.

On Mon, Sep 1, 2008 at 1:21 PM, Ronan Mullally <ronan at iol.ie> wrote:

> A back to school question for everybody ;-)
>
> I've got a scenario as shown below:
>
>   +-----------+                            +-----------+
>   | linux src |=========== Internet -------| linux dst |
>   +-----------+                            +-----------+
>
> There are two (or more) paths from the src box to the Internet.  It is
> sending traffic to the dst box, and wants to balance UDP traffic across
> these multiple links.  Traffic in the other direction is not an issue.
>
> Sane options like Multilink PPP are not avaialble.  Less sane options like
> running IPSEC/VPN tunnels are not available.  The option I'm left with is
> to load balance outbound traffic across the links.  The insane twist is
> that these packets will need to be source NATed on egress, but let's
> ignore that for the moment.
>
> I'm trying to use the equalize flag to the ip route command.  I can set up
> a multipath route okay:
>
>  # ip route add 172.16.0.0/24 equalize nexthop dev dummy1 nexthop dev
> dummy2
>
>  # ip route show
>  172.16.0.0/24 equalize
>        nexthop dev dummy1 weight 1
>        nexthop dev dummy2 weight 1
>
> And would expect to see a ping to a host in the above range to send
> packets via each link in turn.  It doesn't.  It sends all traffic via the
> first path, so the routing decision is getting cached rather than
> re-calculated per packet.
>
> The man page for 'ip' indicates that the kernel needs to be patched to
> support equalisation.  I was hoping that CONFIG_IP_ROUTE_MULTIPATH would
> be sufficient, but it's not.  I've found the patch I think the man page
> refers to at http://trash.net/~kaber/equalize/equalize_2.4.18.patch<http://trash.net/%7Ekaber/equalize/equalize_2.4.18.patch>but
> it doesn't apply cleanly against a recent 2.6 kernel.
>
> Am I missing something obvious, or can anybody think of a more elegant
> solution?  The only other alternative I can think of (apart from trying
> to re-write the patch for 2.6) is the eql driver in the kernel, but that
> looks well and truly dead (circa 1995) and I don't think it'll work in a
> reverse-path-filtering environment.
>
>
> -Ronan
>
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