Link-state routing protocol

First, each node needs to determine what other ports it is connected to, over fully working links; it does this using reachability protocol it runs periodically and separately with each of its directly connected neighbours.

Next, each node periodically (and in case of connectivity changes) sends a short message, the link-state advertisement, which:

• Identifies the node which is producing it.
• Identifies all the other nodes (either routers or networks) to which it is directly connected.
• Includes a 'sequence number', which increases every time the source node makes up a new version of the message.

This message is sent to all the nodes on a network. As a necessary precursor, each node in the network remembers, for every one of its neighbors, the sequence number of the last link-state message which it received from that node. When a link-state advertisement is received at a node, the node looks up the sequence number it has stored for the source of that link-state message: if this message is newer (i.e., has a higher sequence number), it is saved, the sequence number is updated, and a copy is sent in turn to each of that node's neighbors. This procedure rapidly gets a copy of the latest version of each node's link-state advertisement to every node in the network.

Networks running link state algorithms can also be segmented into hierarchies which limit the scope of route changes. These features mean that link state algorithms scale better to larger networks.

Finally, with the complete set of link-state advertisements (one from each node in the network) in hand, each node produces the graph for the map of the network. The algorithm iterates over the collection of link-state advertisements; for each one, it makes links on the map of the network, from the node which sent that message, to all the nodes which that message indicates are neighbors of the sending node.

No link is considered to have been correctly reported unless the two ends agree; i.e., if one node reports that it is connected to another, but the other node does not report that it is connected to the first, there is a problem, and the link is not included on the map.

The link-state message giving information about the neighbors is recomputed, and then flooded throughout the network, whenever there is a change in the connectivity between the node and its neighbors; e.g., when a link fails. Any such change will be detected by the reachability protocol which each node runs with its neighbors.

Each node independently runs an algorithm over the map to determine the shortest path from itself to every other node in the network; generally some variant of Dijkstra's algorithm is used. This is based around a link cost across each path which includes available bandwidth among other things.

A node maintains two data structures: a tree containing nodes which are "done", and a list of candidates. The algorithm starts with both structures empty; it then adds to the first one the node itself. The variant of a greedy algorithm then repetitively does the following:

• All neighbour nodes which are directly connected to the node are just added to the tree (excepting any nodes which are already in either the tree or the candidate list). The rest are added to the second (candidate) list.
• Each node in the candidate list is compared to each of the nodes already in the tree. The candidate node which is closest to any of the nodes already in the tree is itself moved into the tree and attached to the appropriate neighbor node. When a node is moved from the candidate list into the tree, it is removed from the candidate list and is not considered in subsequent iterations of the algorithm.

The above two steps are repeated as long as there are any nodes left in the candidate list. (When there are none, all the nodes in the network will have been added to the tree.) This procedure ends with the tree containing all the nodes in the network, with the node on which the algorithm is running as the root of the tree. The shortest path from that node to any other node is indicated by the list of nodes one traverses to get from the root of the tree, to the desired node in the tree.

With the shortest paths in hand, the next step is to fill in the routing table. For any given destination node, the best path for that destination is the node which is the first step from the root node, down the branch in the shortest-path tree which leads toward the desired destination node. To create the routing table, it is only necessary to walk the tree, remembering the identity of the node at the head of each branch, and filling in the routing table entry for each node one comes across with that identity.

Whenever a change in the connectivity map happens, it is necessary to recompute the shortest-path tree, and then recreate the routing table. Work by BBN Technologies discovered how to recompute only that part of the tree which could have been affected by a given change in the map. Also, the routing table would normally be filled in as the shortest-path tree is computed, instead of making it a separate operation.

In some cases it is reasonable to reduce the number of nodes that generate LSA messages. For instance, a node that has only one connection to the network graph does not need to send LSA messages, as the information on its existence could be already included in the LSA message of its only neighbor. For this reason a topology reduction strategy can be applied, in which only a subset of the network nodes generate LSA messages. Two widely studied approaches for topology reduction are:

1. Multipoint relays that are at the base of the Optimized Link State Routing Protocol (OLSR) but have also been proposed for OSPF[4]
2. Connected dominating sets that again have been proposed for OSPF[5]

With Fisheye State Routing (FSR) the LSA are sent with different time-to-live values to restrict their diffusion and limit the overhead due to control messages. The same concept is used also in the Hazy Sighted Link State Routing Protocol.