Review: routing algorithms.

1

• 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.

2

• 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.

3
B
A B C D E
A (0, -) (8, C) (5, A) (4, E) (3, A) B
(4, B) (0, - ) (3, B) (3, C) (3, D) C
(2, C) (5, C) (0, - ) (2, C) (2, D) D
(5, E) (3, D) (3, B) (0, - ) (1, D) E
(2, E) (9, A) (8, A) (3, E) (0, - )
C
A
D
E
Routing table for node D.
Routing table for node A
Let dist(B, A) 4, dist(B, D) 1, dist (B, C)
10, what is Bs routing table in the next cycle?
4

• The count-to-infinity problem.
• A B C D E A B
  C D E
• 
  1 2 3 4
• 1
  3 2 3 4
• 1 2
  3 4 3 4
• 1 2 3
  5 4 5 4
• 1 2 3 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.

5

• 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.

6
Link state packets
B
1
4
2
Node A (A, seq, age, ((B, 4), (C, 4),(E,
2)) Node B (B, seq, age, ((A, 4), (D, 2), (C,
1)) . Node E (E, seq, age, ((A, 2), (D, 5))
C
A
4
6
2
D
E
5
7

• 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?