[Babel-users] Babel and multipath [was: atomic updates ...]

[Leen Besselink]

On Sun, May 05, 2013 at 05:27:10PM +0200, Juliusz Chroboczek wrote:
> > Now that Linux 3.8 has support for IPv6 ECMP is there any plan to
> > use that again to make updates more attomic
> 
> The current code works on a variety of kernels, and is good enough for
> me.  Everytime I've tried to change it, somebody hit a bug on some
> exotic kernel variant.
> 
> I can change my mind, of course.
> 
> > or even support multipath directly.
> 
> [...]
> 
> > Do I understand it correctly that in the case of babeld it might
> > 'just' be make babeld recognize equal routes, have a way to keep
> > track of them inside babeld and add multiple routes to the kernel.
> 
> It's a little more complicated than that.  First of all, Babel has
> 16-bit metrics, so if two routes have similar but not quite equal
> metrics, you might want to install both of them -- call that "almost
> equal cost multipath".  I'm not sure it's standard, but I believe that
> Cisco do that in EIGRP.
> 

I had a feeling that might be true.

> Second, on a wireless network multipath might be unproductive if the
> two routes interfere.  So you might want to use the interference
> information provided by Babel-Z to only select non-interfering routes.
> 

Ahh, yes that is true.

> Third, Babel has a fairly sophisticated loop avoidance mechanism, and
> it's not quite trivial to generalise it to multipath.  A Babel route
> announcement consists of four pieces of data:
> 
>   (prefix, metric, router-id, seqno)
> 
> rather than just (prefix, metric) as in ordinary Bellman-Ford.  A FIB
> route is readvertised as
> 
>   (prefix, cost + metric, router-id, seqno)
> 
> Now suppose you're doing AECMP on two routes
> 
>   (prefix, metric, router-id, seqno)
>   (prefix, metric', router-id', seqno')
> 
> If router-id and router-id' are the same, then you should advertise
> 
>   (prefix, max(cost + metric, cost' + metric'), router-id, min(seqno, seqno'))
> 
> The max of the metrics is absolutely essential -- if you don't do
> that, then you'll get persistent routing loops.  The min of the seqnos
> is what makes loop avoidance work in this case.
> 
> If the two router-ids are different, then there is no good solution --
> you won't have any persistent loops, but you might get transient loops
> in some cases.  A fairly good solution is to advertise the router-id
> and seqno that realise the max of the metrics, but apply the
> update_source procedure to both routes.  This can cause transient
> loops in some pathological cases, but I'm pretty confident they will
> disappear speedily.
> 
> If anyone wants to work on that, make sure you understand Babel's
> loop-avoidance properties.  I'll be glad to look at your code anytime.
> 

My use case was actually a lot simpeler where so I totally forgot about
these things. I should have mentioned what it was from the start.

What I actually want to do is create a routed L2-multipath solution for
wired.

Which means a machine is connected to multiple different L2-domains
and automatically finds other machines connected to those L2-domains.

The L3-routing comes in to play because the IP-address of the machine
is connected to the local loopback. And dynamic routing is used to find
the other machines.

I'm not actually interrested in routing traffic on the machine.

Just have multiple routes to reach other machines on the same L2-domains.

My plan was to define filters to prevent it from announcing other routes.

Protocols like OSPF use a centralized/distributed protocol, but I actually
want each machine to make it own routing decisions so I thought a mesh
protocol would be a better fit.

I hope that description makes some sense to you folks.

Babel seems to fit that profile better than any other solution I've seen.

> -- Juliusz