Mechanism

A node running olsrd is constantly broadcasting 'Hello' messages at a given interval so neighbours can detect it's presence. Every node computes a statistic how many 'Hellos' have been lost or received from each neighbour thereby gaining information about the topology and link quality of nodes in the neighbourhood. The gained topology information is broadcasted as topology control messages (TC messages) and forwarded by neighbours that olsrd has chosen to be 'multipoint' relays.

The concept of multipoint relays is a new idea in proactive routing that came up with the OLSR draft. If every node rebroadcasts topology information that it has received, unnecessary overhead can be generated. Such transmissions are redundant if a node has many neighbours. Thus, an olsrd node decides which neighbours are favorable multipoint relays that should forward its topology control messages. Note that multipoint relays are only chosen for the purpose of forwarding TC messages. Payload is routed considering all available nodes.

Two other message types exist in OLSR that announce information: whether a node offers a gateway to other networks (HNA messages) or has multiple interfaces (MID messages). There is not much to say about what this messages do apart from the fact that they exist. HNA messages make olsrd very convenient when connecting to the Internet with a mobile device. When a mesh node roams around it will detect gateways into other networks and always choose the gateway that it has the best route to. However, olsrd is by no means bullet proof. If a node announces that it is an Internet gateway which it isn't because it never was or it is just offline at the moment -the other nodes will nevertheless trust this information. The pseudo-gateway is a black hole. To overcome this problem, a dynamic gateway plugin was written. The plugin will automatically detect at the gateway if it is actually connected and whether the link is still up. If not, olsrd ceases to send false HNA messages. It is highly recommended to build and use this plugin instead of statically enabling HNA messages.