[nf-failover] Re: [RFC] ct_sync 0.15 (corrected)

Tobias DiPasquale codeslinger at gmail.com
Wed Sep 29 05:00:23 CEST 2004


On Wed, 29 Sep 2004 01:36:19 +0200 (CEST), Henrik Nordstrom
<hno at marasystems.com> wrote:
> There is many ways to get traffic on an node, each having their
> complications and limitations.
> 
> For unicast you need to find some method to divide the address space using
> existing metrics supported by your network. If we assume the trivial setup
> of an HA active-active server then this can for example be done by routing
> the traffic based on the source IP or by client based balancing by
> publishing the service using multiple IPs. In a forwarding NAT firewall it
> gets a little more complex as you have two sides with slightly different
> metrics to care about.  And N node setups gets even more complex as you
> then need to be able to divide and merge contexts across several nodes
> when adding/removing nodes unless you accept a imbalance in the load if a
> node member has failed (fixed contexts, where one node at a time is master
> per context).

The idea of what I was saying is still the same: active-active implies
making multiple machines answer for a single address, whether its a
firewall, load-balancer, DNS server, HTTP server or anything else.
Advertising the service on multiple IPs just means you have x * y
machines, where x == number of advertised addresses and y == number of
machines that actively service each individual advertised address.

> If you do not have any reasonable support for balancing in your network
> layer then you can either introduce a load balancer layer or use multicast
> (broadcast MAC, or by MAC cloning if supported by the network equipment..
> not all switches like MAC cloning)

I thought that having all of the active nodes respond to ARP requests
for the virtual IP would handle that, as the switch could no longer
bind an IP address to a particular switch port? Is that not the case
with some hardware?
 
> The multicast approach provides full flexibility in how connections are
> migrated among the nodes, but does not scale very well as all nodes sees
> all traffic.

Yeah, that's for sure.
 
> A simple unicast load balancing which does work with most equipment is
> policy routing based on IP's. Ports is also doable in most equipment but
> limits the protocols which can be supported.
> 
> Statically divide the "clients" in N groups, each group assigned to one
> virtual firewall with a virtual MAC address.
> 
> failover this virtual firewall among the nodes in the cluster as needed.
> there may only be one master at a time per virtual firewall but each node
> can be master for several virtual firewalls.

Right, but same situation as above. There are still multiple machines
acting as one by way of a virtual address.
 
> connections need to be ct_sync:ed from the current master to all potential
> backups, multiplied by each virtual firewall.

Is that desired? I'm now thinking that perhaps the goals of ct_sync
are not in line with in-cluster active-active load-balancing.

Here's what I mean: if you have a 4-node cluster of firewalls in
active-active configuration, then the states of all connections
flowing through ALL the boxes are replicated on all of the boxes. This
defeats the purpose of active-active load-balancing as each of the
boxes would then handle almost all of the load of the whole cluster. I
don't see why I'd want to synchronize the connection states between
_all_ of the machines in an active-active cluster.

> For virtual MAC address support to Linux see the mac_vlan patch. Allows
> you to create any number of virtual ethernet interfaces per real
> interface, each with their own MAC.

Nice, that sounds better than the hidden patch I was looking at. Thanks :)

-- 
[ Tobias DiPasquale ]
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