• Review: routing algorithms.
– Choose the appropriate paths.
– Routing algorithms
• Flooding
• Shortest path routing (example).
– Dijkstra algorithm.
– Bellman-Ford algorithm.
• Shortest path routing algorithm is perfect if we know the
precise network topology.
– How to maintain the global network state information? This is
also an important part of practical routing algorithms.
• Distance Vector Routing:
– used at ARPANET till 1979, still used in WAN(RIP)
• Each router maintains a routing table, where each
node in the network has an entry in the table.
• Each entry contains two parts, the outgoing line for
the destination (node) and the distance (time, cost,
whatever) to the destination.
• Each router knows the distance to its neighbors
• In a certain period of time, each router sends the
table to each of its neighbors.
• When a router receives a table from each of its
neighbors, it updates its routing table, determines
the new minimum distance and the outgoing link for
the destination.
B
A
C
A
E
D
A
B
C
D
E
B
(0, -) (8, C)
(4, B) (0, - )
(2, C) (5, C)
(5, E) (3, D)
(2, E) (9, A)
Routing table for
node A
C
D
(5, A)
(3, B)
(0, - )
(3, B)
(8, A)
(4, E)
(3, C)
(2, C)
(0, - )
(3, E)
E
(3, A)
(3, D)
(2, D)
(1, D)
(0, - )
Routing table for
node D.
Let dist(B, A) = 4, dist(B, D) = 1, dist (B, C) = 10, what
is B’s routing table in the next cycle?
• The count-to-infinity problem.
A
B
#
1
1
1
1
C
#
#
2
2
2
D E
# #
# #
# #
3 #
3 4
A B C D E
1 2 3
4
3 2 3 4
3 4 3
4
5 4 5
4
5 6 5
6
7 6 7
6
7 8 7
8
…...
Good news propagates fast, bad news propagates slow.
Some heuristics (split horizon). But the heuristics do not
solve the problem.
• link state routing (currently used in the internet,
OSPF):
» The distance vector algorithm converges too slow.
• 1) discover its neighbors
• 2) measure the distance to each neighbor
• 3) construct a packet telling all that is learned
• 4) send the packet to all other nodes -- flooding
• 5) each node computes the shortest path to all other
nodes -- Dijsktra algorithm.
B
1
4
A
2
4
2
E
Link state packets
5
C
Node A: (A, seq, age, ((B, 4), (C, 4),(E, 2))
6
Node B: (B, seq, age, ((A, 4), (D, 2), (C, 1))
D
….
Node E: (E, seq, age, ((A, 2), (D, 5))
• If everything goes ok, every node will have the
whole information of the network in the next step,
algorithm converges faster than the distance vector
method.
• The count to infinite problem?
• Some of current research on the link state algorithm.
– Use broadcast tree instead of flooding
– Use local update to the fix the routing table instead of
running the full shortest path algorithm
– When the send the link state packet
» periodical or sudden change?