[Freifunk-Bonn] On compat version 15 in the Freifunk-KBU network

[Jan Lühr]

Hello folks,

I'd like to give some thought's for the upcoming batman-adv release, especially on compat version 15. This somewhat summarizes my discussion with T_X on the wireless community weekend (WCW -- 2013-05-10 - 2013-05-12). As some of you may know already, we're running a small freifunk network in western Germany (Köln-Bonn-area) - about ~100 nodes in total; ~40 are online at the moment.
It's not my intention to bash on batman-adv in general or to start flame-wars (like: batman-adv vs. olsr or something else) - I'd like to provide some statement on the impact of the upcoming protocol changes.

1. The upcoming protocol change appears to be painful - we have no suitable migration strategy.
Compat version 14 and 15 will be incompatible. Nodes will loose mesh connectivity (to older nodes). Since we cannot upgrade all nodes at once, we'll have to run different networks in parallel.
In fact, we're running two networks at the moment (compat 13 and 14). We aren't able to upgrade the existing compat 13 nodes in the next months - by that, we'll probably have some compat 13 ones for at least 1/2 year. There is no way of telling newer nodes to use compat version 13. Since some "supernodes" (dedicated servers) are part of the batman-adv cloud, we need twice the servers as well.
Upgrading to compat version 15 will require a huge amount of work: New infrastructure (servers) must be deployed and nodes meshing with each other must be upgraded in parallel.

2. Backwards-compatilibilty doesn't seem to be a design target.
Looking at other routing protocols (BGP-4 - for example) - they provide decent ways for protocol extension while still providing backwards compatibility for the version specified 7 years ago. Looking at batman-adv a lot of protocol changes have been introduced in the past years - but no backwards compatibility is there. Eg.
- There will be no flag for using compat 14 version in newer version of batman-adv - if 15 is out.
- Nodes using 14 and 15 cannot mesh.
Since batman-adv depends on the kernel (old batman-adv versions won't build with recent kernels) using 14 will not be an option in a few months: If some router hardware requires recent Kernel / OpenWRT releases version 14 might no longer be used.

3. Conclusion
-> Freifunk KBU will not use compat version 15 in the foreseeable future
-> We're aware, that we cannot use version 14 in some months
-> At the point, when 14 becomes unusable (introduced into OpenWRT Kernels / release we need -- or Debian), we will almost certainly discontinue batman-adv and go one with sth. else. (Due to kernel deps it's easier to run batman-adv / olsr in parallel than two different version of batman-adv)
-> We'd by very happy if someone forks batman-adv to provide modules for recent kernels still using version 14
-> Maybe, some future version of batman-adv addresses backwards-compatibility. Maybe, in 2018 we will have a look at batman-adv again - noticing, that batman-adv remained stable (or migratable) over the last years and start using it again. Maybe, batman-adv will fit our needs, then.

Thanks for your time,
greetz, yanosz