Re: Multiple routes to the same destination

On Fri, Jun 23, 2006 at 10:19:06PM +1000, Christopher Martin wrote:


-----Original Message-----
From: Baldur Gislason [mailto:baldur@xxxxxx]
Sent: Friday, 23 June 2006 10:02 PM
To: Christopher Martin
Cc: FreeBSD Net Mailing list
Subject: Re: Multiple routes to the same destination

Well, round robin is really not what you want with IP packets.
And how are you going to detect that a route is good without a routing
protocol?


Actually, round robin is exactly what I want. And I am not saying I don't
use a routing protocol, in fact I do, but I want packets to be able to use
two or more diverse paths of equivalent cost.

I doubt that. Doing a per packet round robin over different pathes will
kill your tcp performance because of out of order packets.


It would seem that you are assuming that I want to load balance two internet
connections which are NATed, in which case round robin might have issues
with lost TCP sessions and weird reactions from servers as the apparent
source address changes from packet to packet, but in a routed internal
network the source address will not be changed by the router, thus negating
that issue.

It did seem at some stage someone was going to include it in OpenBSD:
http://undeadly.org/cgi?action=article&sid=20040425183024&mode=expanded


That's just part of the it. The rest was added in the last couple of days
because multipath routing and accepting more than one route per
destination is a scary thing. Additionally dead nexthop detection is not
available.

To quote:
"...OSPF also supports multipath equal cost routing".


Yes it does but often you try to avoid that.

It's more of a case where we would like to use BSD as a router/packet
filtering firewall for sites with multiple WAN links between each site, of
equal size, and not have one site idle until the other fails over. Round
robin is better than what we have: nothing.

OpenBSD is on the way to support this but it is still a long journey till
all issues are resolved. Btw. OpenBSD uses a hash-threshold mechanism to
select paths based on source/destination IP address pairs (round robin
will never be supported).

--
:wq Claudio
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