Re: Multiple routes to the same destination
- From: 'Claudio Jeker' <cjeker@xxxxxxxxxxxxxxxxx>
- Date: Fri, 23 Jun 2006 16:27:40 +0159
On Sat, Jun 24, 2006 at 12:04:25AM +1000, Christopher Martin wrote:
I doubt that. Doing a per packet round robin over different pathes will
kill your tcp performance because of out of order packets.
Noted. That's a very good reason. Maybe if there was a may to round robin on
a session basis to mitigate this. Not really going to be an easy fix,
however, so your point is very valid.
Most implementation do a per source/dst IP address hashing which should
result in a similar distribution.
internet
It would seem that you are assuming that I want to load balance two
connections which are NATed, in which case round robin might have issuesnegating
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
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.
I would have thought OSPF would have provided your dead hop issues, however
it does not resolve your point above, so we still seem out of luck.
OpenOSPFD will learn to cope with multipath routes in the next few weeks
but it will only work on OpenBSD.
To quote:
"...OSPF also supports multipath equal cost routing".
Yes it does but often you try to avoid that.
Because of your point above? Besides that, can you provide a couple of
examples of why we would try and avoid it?
Multipath setups are harder to debug as packets may flow differently.
Often it is easier to use a layer 2 trunk to aggregate links. It depends
on your network layout, etc.
It's more of a case where we would like to use BSD as a router/packetof
filtering firewall for sites with multiple WAN links between each site,
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).
Again, another good point. And it also answers the other query as to the
level of work involved in making it work.
I hope that we can get more routing stuff done in the next few weeks but
the way routing is implemented in BSD makes it harder then necessary.
I bet andre@ will start to port features to FreeBSD as soon as the
stabilised in OpenBSD.
--
:wq Claudio
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