Network Wide Multi Link Optimization (technical documentation)

The original batman-adv multi link optimization worked on a local level - which is fairly easy to implement and works well as long as as all nodes are configured the same way and have similar link qualities. But local decision may be suboptimal network wide - for example, consider some dual radio mesh nodes where the first node starts on 5 GHz, but after some hops (alternating between 2.4 and 5 GHz) it stops at a 2.4 GHz-only device. If it had chosen the other (2.4 Ghz) link in the first hop, choosing the same frequency twice could have been avoided, and the total path quality would be better.

This limitation can be solved by relaying the information about multi-interfaces over the whole mesh. This requires some changes to routing and forwarding, which are to be described here.

Routing Table Per Interface

The main idea is to use n + 1 routing tables instead of just one, with n being the number of configured interfaces:

  • a routing table per incoming interface
  • an additional 'default' routing table for packets generated locally (e.g. from the soft-interface)

The routing table is chosen based on the interface the packet entered the system by.

OGM forwarding and penalties

To propagate different paths, the OGM forwarding is changed:

  • When there are multiple interfaces and an OGM is forwarded on the same interface, an additional penalty (e.g. "half duplex penalty) when re-forwarding the OGM on the same interface
    -> the metric with the applied penalty is stored locally
  • apply the strict "forward only from best neighbor" per interface

Path Metric computation
Each node, creates and sends its own OGMs to let all the other peers in the network build their routing table

  • on each interface, one OGM is sent as broadcast containing the initial metric value
  • A generic node receiving an OGM from a neighbor on interface I. When forwarding, it will consider the output interface O:
    • perform the usual B.A.T.M.A.N. metric computations, including accepting OGMs only from the best neighbor
    • compare incoming interface I and outgoing interface O, and apply special rules based on that (e.g. a penalty if I and O are the same, and the interface is a wifi interface)
    • mark the computed metric in a special originator table for the interface O
    • forward the OGM with this computed metric only on interface O
  • all metric computations above are also performed for a virtual 'default' interface O, which is used for local traffic.
Traffic forwarding
  • traffic forwarding works in the reverse direction:
    • When forwarding traffic, use the originator table of the interface where the packet was received
    • for locally generated traffic, use the default originator table

For example, consider the following illustration:

We consider all link qualities equal. What should happen according to the rules (after some OGMs):

  • OGMs from A are sent via A1 and A2
  • B applies the new penalty when forwarding packets from A1 again on B1, and also when forwarding from A2 on B2
  • therefore, As OGM received on B1 are only forwarded on B2 (because no penalty is applied here). They are not re-forwarded on B1 because for this interface, the packets received on B2 have a better quality.
  • finally (assuming equal link qualities everywhere), the TQ values in the OGMs from A forwarded by B are equal too - but the internal routing table include the interface switching
  • putting it all together and assuming the other nodes behave the same way: If a packet is sent by D on D1, it will follow the dashed path. If it is sent on D2, it will follow the solid path. The interface alternation was implemented by local routing table decisions but forwarded information (unlike the original local-only interface alternation).

Another example: path diversity

  • Node B is equipped with 2 interfaces (imagine two wifi cards with sectored antennas)
  • Node A is connected to B through B1 and uses B as next hop towards E
  • Node F is connected to B through B2 and uses B as next hop towards E
  • All the links are perfect
  • Packets sent by A intended to E are received by B on B1 and forwarded using B2
  • Packets sent by F intended to E are received by B on B2 and forwarded using B1
  • B's nexthop to E using B1 is different from B's nexthop to E using B2

The presented example is another consequence of the mechanism explained in this page.
Since the multi-interface optimisation is not per-link anymore but is now defined network wide, packets can possibly be routed through two completely different paths in order to reach the same destination.

This possibility is given by the fact that on the path from A to E B will choose G as best next-hop, while on the path from F to E B will choose C.

The result is that, with this new feature, multi-interface nodes can act as routing splitting point which leads to a first tempative of real multi-path routing.

Debug information

There is an implementation based on the description above currently pending for review. It offers some debugging facilities:

The default originator table can be found at its original place, using ''batctl o''. The following examples represents the topology shown in the top of the page, debug information obtained on node B, using OpenWRT and virtual machines:

root@OpenWrt:/# batctl o
[B.A.T.M.A.N. adv main-b82b9b2, MainIF/MAC: eth0/fe:f0:00:00:02:01 (bat0 BATMAN_IV)]
  Originator      last-seen (#/255)           Nexthop [outgoingIF]:   Potential nexthops ...
fe:f1:00:00:03:01    0.350s   (254) fe:f1:00:00:03:01 [      eth1]: fe:f1:00:00:03:01 (254)
fe:f1:00:00:01:01    0.800s   (255) fe:f1:00:00:01:01 [      eth1]: fe:f1:00:00:01:01 (255)
fe:f0:00:00:05:01    0.770s   (225) fe:f1:00:00:03:01 [      eth1]: fe:f0:00:00:03:01 (211) fe:f1:00:00:03:01 (225)
fe:f0:00:00:03:01    0.670s   (255) fe:f0:00:00:03:01 [      eth0]: fe:f1:00:00:03:01 (255) fe:f0:00:00:03:01 (255)
fe:f0:00:00:04:01    0.520s   (234) fe:f1:00:00:03:01 [      eth1]: fe:f1:00:00:03:01 (234) fe:f0:00:00:03:01 (222)
fe:f0:00:00:01:01    0.920s   (255) fe:f1:00:00:01:01 [      eth1]: fe:f1:00:00:01:01 (255) fe:f0:00:00:01:01 (254)

Comparing to the original topology, it shows that for destination E (fe:f0:00:00:05:01) when packets come in on 2.4 GHz (eth0, first table), they are preferably forwarded to C (fe:f1:00:00:03:01 ) on 5 GHz (eth1).

When packets are received on 5 GHz (eth1), there is not much difference because at node B or at node C, the same interface must be used. Therefore in the second table for node E (fe:f0:00:00:05:01) the two choices have pretty much the same TQ values (210 and 211).

Ideas for the future

  • Theoretically, a node generating traffic instead of using its own routing table could exploit the path diversity and route its traffic using all the routing tables of all the interfaces (the selection policy have to be defined..round robin would not work properly because when using two different paths TCP reordering may decrease the performance)