Re: [LARTC] many ways to do load balancing (or not?)

[Christoph Simon <ciccio@kiosknet.com.br>]

On 21 Nov 2002 15:07:11 -0800
"William L. Thomson Jr." <support@obsidian-studios.com> wrote:

> > From the IP Command Reference:
> > 
> > 	equalize: allow packet by packet randomization on multipath
> > 	routes.
> > 
> > My understanding is, that for equalize to work, all lines must go to
> > the same point and that must not be the end point. Also, this same
> > point must implement the equalize very much the same way.
> 
> Not necessarily. I have two lines going in completely different
> directions, different private nets, and then via different ISP/Public
> IPs.

Two lines in different directions using equalize? What's the point of
this?

> > > Now if the request was generated from the inside it would still work
> > > some what the same. If I send two emails out at once, the first will
> > > use gw1 and the other will use gw2.
> > > 
> > > All packets for each will travel via the same route and use the same
> > > gateway from start to finish.
> > 
> > Not using equalize.
> 
> Yes using equalize, because the path will already be known and used from
> catch. Correct that if a further look up occurs the packets will use a
> different interface, but that is rare. Most of the time during a
> transfer the route is already known and kept in catch from the first
> initial lookup.

I didn't play too much with equalize, but using it for load balancing
and redundancy failed for reason by with equalize is defined. As far
as I understood how weighting works: There are several identic routes
so that the probability to pick one of them corresponds to the given
weights. Still on a packet base. With equalize, the balancing should
be perfect, which can never be the case with few connections, even if
they imply massive data transfer. But if there is no NAT later, it
just can't work.

> > unless there is a routing cache which will hold the path.
> 
> That's the problem. The route cache on the Linux load balancing router
> will be correct, but the route cache on the client will not be.

If "client" here is a computer reaching the internet through the linux
router (it can't be anything else, because you said that there are two
independent lines out of your control), and unless that client uses
source routing which might be ignored by the linux router, the
client's view of this situation really doesn't matter.

> The client will be caching the wrong path. So unless a redirect is sent
> the client will site there and expect further communication using the
> route in cache.

The client will cache the route which leads to the Linux router and
not bother with details happening later.

-- 
Christoph Simon
ciccio@kiosknet.com.br
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