1. Communication Networks
Recitation 6
Routing
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2. Routing Problem: find an Optimal Path5 A R1 R2 R4 R3 R6 R7 R8 B 40 10 20 4 6 15 40
The example network shown on the picture indicates a set of internal connections and the metric value of each connection. Each router selects a first hop for a path to B, based on the total metric of each potential path to B.R1 for example, selects a first hop to R2, on the basis that a path of cost 39 passes through R2 and is the minimum cost from A to B.
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3. Distance Vector (RIP) Each node maintains a table:
(Destination, Cost, NextHop)
Each node sends updates to (and receives updates from) its directly connected neighbors
periodically (on the order of several seconds)
whenever its table changes (called triggered update)
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4. DV Updates Each update is a list of pairs:
(Destination, Cost)
Update local table if receive a “better” route
smaller cost
came from next-hop
Refresh existing routes; delete if they time out
Convergence rate
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5. RIP Table Processing
RIP routing tables managed by application-level process called routed (daemon)
Advertisements sent in UDP packets, periodically repeated
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6. Example - initial distances1
Distance to node B C
Info at
node A B C D E 7 A 7 ~ ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 ~ 2
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7. E receives D’s routes 1 Distance to node B C Info at node A B C D E 77 ~ ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 ~ 2
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8. E updates cost to C 1 Distance to node B C Info at node A B C D E 7 A7 ~ ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2
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9. A receives B’s routes 1 Distance to node B C Info at node A B C D E 77 ~ ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2
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10. A updates cost to C 1 Distance to node B C Info at node A B C D E 7 A7 8 ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2
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11. A receives E’s routes 1 Distance to node B C Info at node A B C D E 77 8 ~ 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2
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12. A updates cost to C and D 1 Distance to node B C Info at node A B C D7 A 7 5 3 1 A 8 2 B 7 1 ~ 8 C ~ 1 2 ~ 1 2 D ~ ~ 2 2 D E E 1 8 4 2
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13. Final distances 1 Distance to node B C Info at node A B C D E 7 A 6 56 5 3 1 A 8 2 B 6 1 3 5 C 5 1 2 4 1 2 D 3 3 2 2 D E E 1 5 4 2
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14. Final distances after link failure1
Distance to node B C
Info at
node A B C D E 7 A 7 8 10 1 A 8 2 B 7 1 3 8 C 8 1 2 9 1 2 D 10 3 2 11 D E E 1 8 9 11
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15. View from a node E’s routing table 1 Next hop B C dest A B D 7 A 1 145 B A 8 2 7 8 5 C 6 9 4 D 4 11 2 1 2 D E
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16. Some of the data in the table is based on loops!
E’s routing table 1
Next hop B C
dest A B D 7 A 1 14 5 B A 8 2 7 8 5 C 6 9 4 D 4 11 2 1 2 D E
E to A through D? Shortest path: E  D  E  A.
Cost: 5

17. And the loops might not be that simple…
E’s routing table 1
Next hop B C
dest A B D 7 A 1 14 5 B A 8 2 7 8 5 C 6 9 4 D 4 11 2 1 2 D E
E to A through B? Shortest path: E  B  C  D  E  A.
Cost: 14

18. All the loops in the table
E’s routing table 1
Next hop B C
dest A B D 7 A 1 14 5 B A 8 2 7 8 5 C 6 9 4 D 4 11 2 1 2 D E

19. Distance Vector: link cost changes
node detects local link cost change
updates distance table (line 15)
if cost change in least cost path, notify neighbors (lines 23,24)
we show distance to X only X Z 1 4 50 Y
The problematic entry
report 1
report 2
Chapter 5: Network Layer: Routing

20. Distance Vector: link cost changes
node detects local link cost change
updates distance table (line 15)
if cost change in least cost path, notify neighbors (lines 23,24)
we show distance to X only X Z 1 4 50 Y
algorithm
terminates
“good
news
travels
fast”
report 1
report 2
Chapter 5: Network Layer: Routing

21. Distance Vector: link cost changes
good news travels fast
bad news travels slow - “count to infinity” problem!
we show distance to X only X Z 1 4 50 Y 60 Y Y Y
algorithm
continues
on!
report 6
report 7
Chapter 5: Network Layer: Routing

22. Distance Vector: poisoned reverse
If Z routes through Y to get to X
(= Z learned its best dist. to X from Y):
Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)
will this completely solve count to infinity problem? X Z 1 4 50 Y 60
algorithm
terminates Y Y Y Y
Chapter 5: Network Layer: Routing

23. The bouncing effect dest cost dest cost 1 A 1 B A B 1 C 1 C 2 25 1 C
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24. C sends routes to B dest cost dest cost A ~ B A B 1 C 1 C 2 25 1 C
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25. B updates distance to A dest cost dest cost A 3 B A B 1 C 1 C 2 25 1 C
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26. B sends routes to C dest cost dest cost A 3 B A B 1 C 1 C 2 25 1 C4 B 1
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27. How are these loops caused?
Observation 1:
B’s metric increases
Observation 2:
C picks B as next hop to A
But, the implicit path from C to A includes itself!
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28. Solutions Split horizon/Poisoned reverse Works for two node loops
B does not advertise route to C or advertises it with infinite distance (16)
Works for two node loops
does not work for loops with more nodes
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29. Example where Split Horizon fails
When link breaks, C marks D as unreachable and reports that to A and B
Suppose A learns it first. A now thinks best path to D is through B. A reports a route of cost=3 to C.
C thinks D is reachable through A at cost 4 and reports that to B.
B reports a cost 5 to A who reports new cost to C.
etc... 1 A B C D
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30. Link State (OSPF) Link State Protocol
OSPF routing table has detailed information about each link : cost, reliability, etc.
This allows OSPF routers to optimize routing
Link
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31. Link State Updates
Routers send to all nodes (not just neighbors) information about directly connected links.
Each router calculates shortest cost path to all others (Dijkstra).
At each step of the algorithm, router adds the next shortest (i.e. lowest-cost) path to the tree.
Finds spanning tree routed on source router.
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32. Djikstra’s algorithm - step 110 3 2 9 6 4 5 7 2
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33. Djikstra’s algorithm - step 21 10 10 3 2 9 6 4 5 7 5 2
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34. Djikstra’s algorithm - step 31 14 8 10 3 2 9 6 4 5 7 5 2 7
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35. Djikstra’s algorithm - step 41 13 8 10 3 2 9 6 4 5 7 5 2 7
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36. Djikstra’s algorithm - step 51 9 8 10 3 2 9 6 4 5 7 5 2 7
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37. Djikstra’s algorithm - final1 9 8 10 3 2 9 6 4 5 7 5 2 7
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