This document gives a brief introduction into the major differences between B.A.T.M.A.N. IV and B.A.T.M.A.N. V. Links to the technical documentation for further reading are provided at the end of this document.
- Table of contents
- B.A.T.M.A.N. V
Separating neighbor discovery from mesh routing¶
The B.A.T.M.A.N. protocol originally used a single message type (called OGM) to determine the link qualities to the direct neighbors and spreading this link quality information through the whole mesh. This procedure is summarized on the B.A.T.M.A.N. concept page and explained in detail in the RFC draft published in 2008.
This approach was chosen for its simplicity during the protocol design phase and the first implementation. However, it also bears some drawbacks:
- Wireless interfaces usually come with packet loss varying over time, therefore a higher protocol transmission rate is desirable to allow a fast reaction on flaky connections. Other interfaces of the same host might be connected to Ethernet LANs / VPNs / etc which rarely exhibit packet loss or link state changes. Those would benefit from a lower protocol transmission rate to reduce overhead.
- It generally is more desirable to detect local link quality changes at a faster rate than propagating all these changes through the entire mesh (the far end of the mesh does not need to care about local link quality fluctuations). Other optimization strategies for reducing overhead, might be possible if OGMs weren't used for all tasks in the mesh at the same time.
B.A.T.M.A.N. V adopts the strategy of 'divide & conquer' to handle these different uses cases better: For neighbor discovery the Echo Location Protocol (ELP) is introduced. This packet type is never forwarded or rebroadcasted in the mesh. The Originator Messages version 2 (OGMv2) protocol remains responsible for flooding the mesh with link quality information and determining the overall path transmit qualities.
The task separation (neighbor discovery vs mesh routing) bears the following advantages:
- Reduced overhead, as OGMs can then be sent with a slower interval. The OGM propagation has a squared amount of overhead in worst case scenarios, therefore the the slower intervals are very desirable.
- Neighbor discovery and metric data collection can be performed individually, at different intervals or even different techniques.
- Effort for multiple interface handling can be reduced.
Throughput based metric¶
Since a packet loss based metric as used by B.A.T.M.A.N. IV isn't adequate to handle the increasing number devices & link types with little to no packet loss but very different throughput capabilities B.A.T.M.A.N. V uses packet throughput as mesh-wide metric. Depending on the link type batman-adv is able to determine the throughput automatically:
- wireless: Modern WiFi drivers export the estimated throughput per WiFi neighbor. This value is retrieved on a periodic basis and averaged before propagated in the mesh.
- wired: Most Ethernet capable devices export their theoretical throughput and duplex capabilities via the ethtool API.
- unknown/override: B.A.T.M.A.N. V allows to specify a throughput value per interface via generic netlink. Consequently. B.A.T.M.A.N. V will assume the specified throughput for any neighbor discovered over that interface.
- throughput meter (upcoming): If the throughput can not be queried via some API and is not manually configured, B.A.T.M.A.N. V will run a periodic throughput test with its built-in throughput test protocol.
Note: The WiFi neighbor throughput estimation relies on the WiFi driver being able to estimate the throughput. Commonly, the WiFi driver needs payload traffic to be sent to each neighbor for the estimation to be accurate. On idle links B.A.T.M.A.N. V will initiate payload traffic from time to time to feed the WiFi driver's estimation logic.
The path throughput between node A and node B is computed as the minimum between the throughput value of all given links on the path between node A and node B (other factors are also included in the computation - for further details please check our OGMv2 wikipage).
Though it appears natural to assume that B.A.T.M.A.N. IV and B.A.T.M.A.N. V are incompatible due to the different packet types and metrics it might be desirable to support both mesh protocols during the experimentation and/or migration phase. To support these use cases the batman-adv kernel module is able to run multiple independent mesh networks in parallel on the same host. From user space this ability can be managed by creating multiple batX mesh interfaces - each having assigned different slave interface(s) to run the mesh on. When such a mesh interface is created, batman-adv can be configured to use a specific mesh protocol (B.A.T.M.A.N. IV or B.A.T.M.A.N. V).
This allows different compatibility strategies:
- A 'compatibility node' could be placed in-between 2 incompatible mesh clouds (one running B.A.T.M.A.N. IV and the other running B.A.T.M.A.N. V). The compatibility node connects to both clouds via a distinct interface using the appropriate protocol. Consequently, the compatibility node will have 2 batX mesh interfaces - one configured with B.A.T.M.A.N. IV and the other with B.A.T.M.A.N. V. The incompatible mesh clouds can now be connected on the payload layer by either bridging or routing from one batX interface to the other.
- By logically separating the slave interfaces (for example via VLANs) each host could run both mesh protocols on its own 'logic' interface. In the case of VLANs, a given VLAN ID could be dedicated to B.A.T.M.A.N. IV while another is dedicated to B.A.T.M.A.N. V. This type of setup is ideal to perform direct comparison tests between the mesh protocols but bears the disadvantage of additional protocol overhead.