Forwarding OLSR traffic

Relaying of messages is what makes flooding in MANETS possible. OLSR specifies a default forwarding algorithm that uses the MPR information to flood packets. One is however free to make ones own rules for custom forwarding of custom messages. But all messages received that carries a type not known by the local node, must be forwarded according to the default forwarding algorithm. The algorithm can be outlined as:

1. If the link on which the message arrived is not considered symmetric, the message is silently discarded. To check the link status the link set is queried.

2. If the TTL carried in the message header is 0, the message is silently discarded.

3. If this message has already been forwarded the message is discarded. To check for already forwarded messages the duplicate set is queried.

4. If the last hop sender of the message, not necessarily the originator, has chosen this node as a MPR, then the message is forwarded. If not, the message is discarded. To check this the MPR selector set is queried.

5. If the message is to be forwarded, the TTL of the message is reduced by one and the hop-count of the message is increased by one before broadcasting the message on all interfaces.

The fact that all received unknown message types are forwarded using this approach makes flooding of special message-types possible even if these message-types are only known to a subset of the nodes.

Figures 3.4 and 3.5 shows the paths information is passed when being spread, first using regular flooding, then using MPR flooding. The number of retransmissions in a MPR scenario highly depends on the network topology and the MPR calculation algorithm. Using the same topology as in fig 3.2, a possible MPR calculation could lead to the black nodes in fig 3.3 being chosen as MPRs by the center node. As one can see, if the center node is to flood a message throughout the network, 4 retransmissions are done using MPR as opposed to 24 using traditional flooding.