Re: [B.A.T.M.A.N.] Can b.a.t.m.a.n. be configured to ARP for unknown clients?

[Robert Bates <rbates@freewave.com>]

Hi Simon,

Thanks for the reply.  I'd already proposed changing the timer value to my team (e.g., to 1 hour), and we're likely going to get that done (again, we license this product from another developer, and don't even have direct access to the code under our current arrangement).  I agree that it seems to be the most straight-forward solution, but others in the team feel that the desired fix will involve either some type of "ARP if unknown", or timer-based ARP mechanism on the part of some other component/piece of software.  I like your idea of periodically reading tt_local and pinging the clients.  I'm going to bring that up when we have the next in-person discussion about a fix (a few people are still out on holiday vacation).  I like the fact that with that approach, the additional traffic introduced is only toward locally connected clients, and not over the mesh.  Each node is doing this with its locally connected clients, circumventing timeout and removal.  I like it.  You mentioned some potential pitfalls.  We will talk through what those might look like in our customer environments, but my sense is that there is no significant downside.  I could wrong.

Thanks again.

Robert



-----Original Message-----
From: Simon Wunderlich [mailto:sw@simonwunderlich.de]
Sent: Sunday, December 31, 2017 6:19 AM
To: b.a.t.m.a.n@lists.open-mesh.org
Cc: Robert Bates <rbates@freewave.com>
Subject: Re: [B.A.T.M.A.N.] Can b.a.t.m.a.n. be configured to ARP for unknown clients?

On Friday, December 29, 2017 5:01:23 PM CET Robert Bates wrote:
> Hello,
>
> Is it possible to have b.a.t.m.a.n. ARP if packets are received at
> bat0 for a client which has been removed/deleted due to timeout, and
> is therefore no longer in the translation tables?
>
> In one customer application of a product of ours (a mesh AP we've
> licensed from another vendor/developer, which is based on
> openWRT/b.a.t.m.a.n.), we are being adversely affected by the 10 minute inactivity timer on
> transtable_local.   The clients in this customer's network/application are
> stationary devices which basically do not speak unless spoken to (e.g.
> when they are polled for data).  They are periodically polled by a
> management platform, using an upper layer protocol running over TCP.
> The problem is that this customer's polling cycle time is variable,
> and occasionally it is taking longer than 10 minutes between
> successive polls of a given client/device.  When this happens, that
> client is of course removed from transtable_local, and transtable_global on the other nodes in the mesh.
> Meanwhile, the polling/management platform has a very long ARP cache
> life, so it never ARPs (and apparently it is not possible on this
> platform to have the customer implement dynamic, rather than static
> ARP table entries, in which it would ARP upon polling failure).  So
> once we get into this state, polls to this client device which has
> dropped out of the mesh are not possible, and their management
> platform throws alarms, etc.  To bring it back in service at that
> point requires an ARP, which the customer is manually triggering with a ping, whenever one of these "outages" occurs.
>
> We know that the transtable_local inactivity/removal timer value can
> be extended, and we will probably do that, but we would also like to
> know if it is possible to have b.a.t.m.a.n. ARP for the removed client
> in this case.  We prefer this approach, rather than arbitrarily
> changing the tt_local timer to some value which may not work well in
> some other customer's network/application.  I know that there is a
> statistically valid underlying assumption with this 10 minute
> inactivity timer on transtable_local, that clients will typically be
> "chatty".  But again, that is not the case in this application, which
> is a very common one in the industry in which we operate, where
> clients are very often fixed devices which only respond to explicit
> queries or commands.  This is a new product and protocol for us, and
> this could beg the question of whether or not b.a.t.man.-based meshing is the right solution in this type of application.
>  We believe it can be; it would just be helpful if we can configure it
> to ARP in this type of scenario.
>
> Can you please comment on how this might be possible (config or otherwise)?

Hi Robert,

batman-adv does not ARP on its own, so there is no way to configure this.

You should either increase the timer from 10 minutes to something (reasonably) high, or have another program sending some frames to refresh the ARP on behalf of the client - e.g. every 5 minutes, read the transtable local, send a packet for each MAC. However, in my opinion, this is just a more round-abot way with possibly more pitfalls on its own.

A general way to handle this would be to send those unicasts unknown to batman-adv  as broadcasts. However, this would be problematic for networks with big broadcasts domains which already suffer from too high broadcast load, but have a sane ARP mechanism in place otherwise.

So long story short, increasing the timeout seems to be the most easy and effective solution to me.

Cheers,
     Simon
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