- Recall that in the
link state
type route computation algorithm,
every node sends its
link state information
(information about the delays on its links to neighbor nodes)
to
all nodes
in the network.
- Examples of
link state routing methods used in the Internet are:
- Open Shortest Path First (OSPF)
- OSI's IS-IS
- You need to be aware of this
Chicken and Egg
dilemma in routing:
- The routing table
is used to forward packets to a destination
- We are trying to construct
the routing table for a node
In other words: the content of the routing table is not yet correct !!!
- Therefore:
The exchange of link state information CANNOT use information in the routing table in accomplishing its task (because the routing table has not yet been constructed) !!!
- The routing table
is used to forward packets to a destination
- It is so important that I want to repeat it:
A node using the link state route construction algorithm must send link cost information to every node in the network WITHOUT using routing tables !!! (After all, we are in the process of constructing the routing tables... If you can use the information in the routing table, why do you want to construct something that you already have ???)
- Sending information to
ALL node in the network
is caled
broadcasting
- A crucial step
in implementing
link state type route computation algorithm
is:
Develop a procedure that allow a node to broadcast their link state without the use of routing tables. (It's clear that this is a "chicken and egg" problem... You are developing a method to allow nodes to construct the routing tables. So... the routing tables have not been constructed, so you cannot use routing tables to send link state information to other nodes !)
- Flooding
is the basic algorithm to perform
broadcasting
without the use
of routing tables
- The Flooding algorithm:
- A node select a
Time To Live (TTL)
value that is
large enough
to allow the packet to reach
every node in the network
(set to the maximum distance between 2 nodes in network)
- A node send the message to
ALL its neighbors
- When a node send the
FLOOD message,
it decrements the value in the
Time To Live (TTL)
field.
-
If
TTL > 0 ,
the node forwards the
FLOOD message
to
all its neighbors
except the neighbor from it the message was received.
If TTL = 0 , the FLOOD message is the discarded
Example: Flooding
- Message contains a TTL field.
- Routers decrements TTL by 1 each times messaeg is forwarded.
- An incoming message is forwarded on all links, except on the link where the message was received.
- Message is not forwarded to next router if TTL becomes 0.
- A node select a
Time To Live (TTL)
value that is
large enough
to allow the packet to reach
every node in the network
(set to the maximum distance between 2 nodes in network)
- Reliable flooding:
- Each message contains a TTL and a version number
- Nodes maintains a last version number forwarded.
- Node will only forward a newer version
- The reason it is called reliable flooding is because the use of the sequence number allow nodes to identify the most recent copy of the information and prevent them from (re)transmitting old information.
- Recall that the information that are being broadcasted through flooding
are the link state information.
For example, node A will broadcast the following message to all nodes:
Imagine that all nodes doing this. Then, at the end, every node will know the topology of the entire network. In other word, every node in the network will have the following information:
Distance matrix To: From A B C D E ------+--------------------------------- A | * 5 10 * * B | 5 * 3 11 * C | 10 3 * 2 * D | * 11 2 * 3 E | * * * 3 *The matrix contains all the link state data from all nodes and the matrix is in fact another representation of the graph.
- Every node in the network will have the same information and each node will next run the well-known Dijkstra Algorithm on the graph to find the shortest paths.