Link sensing

To keep up-to-date information on what links exist between a node and its neighbors, the link set is maintained. In HELLO messages a node emits all information about the links to neighbors from the interface on which the HELLO is transmitted. When declaring links, the IP addresses of the actual interfaces making up the link is used. When declaring the neighbor state of neighbors not reachable on the interface on which the HELLO is transmitted, the main address of the neighbor node is used.

Figure: Nodes A and B runs OLSR on multiple interfaces. B uses the address of b1 as its main address. Nodes D and C runs on single interfaces(d1 and c1).

In a scenario like the one depicted in figure 3.12, A would send the following information in its HELLO message on interface a1:

• Its current link and neighbor state for d1.
• Its current link and neighbor state for c1.
• Its current neighbor state for the main address of node B which is b1.

When building a HELLO to be transmitted on a2, node A will include the following information:

• Its current neighbor state for d1.
• Its current neighbor state for c1.
• Its current link and neighbor state for b2.

Upon receiving a HELLO from a neighbor, a node checks to see if the HELLO message contains the IP address of the interface the message was received. The link set is then updated as follows:

• If no link entry exists for the tuple (originating IP, IP of received interface) then such an entry is created. The originating IP is fetched from the IP header of the received packet. Whenever a link entry is created a corresponding neighbor entry is created as well if no such entry exists.
• An asymmetric timer is then updated according to the validity time received. This timer decides for how long the link entry is to be considered asymmetric if the symmetric timer times out.
• If the address of the receiving interface is located in the received HELLO message, the symmetric timer is updated and the status of the link is updated if necessary. The status of the neighbor entry according to this link entry is also updated if necessary.
• Finally the actual holding time for this entry is set to be the maximum of the asymmetric timer and the symmetric timer.