1. Data Communications & Computer Networks
10. Introduction to Routing

2. Key points
Routing in circuit-switching networks has traditionally involved a static routing strategy with the use of alternate paths to respond to increased load.
Modern routing strategies provide more adaptive and flexible approaches.
The routing function of a packet-switching network attempts to find the least-cost route through the network, with cost based on number of hops, delay, bandwidth or other metrics.
Adaptive routing algorithms typically rely on the exchange of information about traffic conditions between nodes

3. Some Perspective on Routing …..
When we wish to take a long trip by car, we consult a road map.
The road map shows the possible routes to our destination.
It might show us the shortest distance, but, it can’t always tell us what we really want to know:
What is the fastest route!
Why is this not always obvious?
Question: What’s the difference between you and network packet?

4. Packets are Dumb, Students are Smart!
We adapt to traffic conditions as we go.
Packets depend on routers to choose how they get their destination.
Routers have maps just like we do. These are called routing tables.
What we want to know is:
How to these tables get constructed/updated?
How are routes chosen using these tables?

5. 1. Routing in Circuit-Switched Networks
Many connections will need paths through more than one switch
Need to find a route based on
Efficiency
Resilience
Public telephone switches are a tree structure
Static routing uses the same approach all the time
Dynamic routing allows for changes in routing depending on traffic conditions
Uses a peer structure for nodes

6. Alternate Routing
Alternative routing is a form of routing in circuit-switching networks
Possible routes between end offices are predefined
Originating switch selects appropriate route
Routes are listed in preference order
Different sets of routes may be used at different times

7. Alternate Routing Diagram
Switch X has 4 possible routes to destination switch Y.
Direct route is tried first.
If this trunk is unavailable (busy, out of service), the other routes will be tried in a particular order depending on the time period.
Eg, during weekday mornings, route b is tried next.

8. 2. Routing in Packet-Switched Networks
Routing is one of the most complex and crucial aspect of packet switched networks
Characteristics required
Correctness
Simplicity
Robustness
Stability
Fairness
Optimality
Efficiency

9. Elements of routing techniques for packet-switched networks
Performance criteria
Decision time
Decision place
Network information source
Network information update timing

10. Performance Criteria Used for selection of a route Number of hops Cost
Delay
Throughput (bandwidth)
In this example, the shortest path (fewer hops) from node 1 to node 6 is (cost=5+5=10) but the least-cost path is (cost=1+1+2=4)

11. Decision Time and Place
Decision time is determined by whether the routing decision is made on
Packet (datagram)
Routing decision made individually for each packet
Session (virtual circuit)
Routing decision is made at the time the VC is established
Decision place refers to which node (or nodes) in the network are responsible for the routing decision
Each node (distributed)
Central node (centralized)
Originating node (source)

12. Network Information Source and Update Timing
Routing decisions usually based on knowledge of network (not always)
Distributed routing (routing decision made by each node)
Nodes use local knowledge
May collect info from adjacent nodes
May collect info from all nodes on a potential route
Central routing
Collect info from all nodes
Update timing
When is network info held by nodes updated
Fixed - never updated
Adaptive - regular updates

13. Routing Strategies
Fixed
Flooding
Random
Adaptive

14. Fixed Routing
A single permanent route is configured for each source to destination pair nodes
Determine routes using a least cost algorithm
Routes are fixed, at least until there is a change in network topology

15. Fixed Routing Tables

16. Flooding No network info is required Operates as follows:
A packet is sent by a source node to every neighboring node
At each node, incoming packets retransmitted on every link except incoming link
Eventually a number of copies will arrive at destination
Each packet is uniquely numbered so duplicates can be discarded
Nodes can remember packets already forwarded to keep network load in bounds
Can include a hop count in packets

17. Flooding Example
A packet is to be sent from node 1 to node 6 and is assigned a hop count of 3.
On the 1st hop, 3 copies of packet are created.
For the 2nd hop of all these copies, a total of 9 copies are created.
One of these copies reaches node 6, which recognizes that it is the indented destination and does not retransmit.
However, the other nodes generate a total of 22 new copies for their 3rd and final hop.
All packets received from the 3rd hop are discarded.
In all, node 6 has received 4 additional copies of the packet.

18. Properties of Flooding
All possible routes are tried
Very robust
Could be used to send emergency messages
Because all routes are tried, at least one packet will have taken minimum hop count route
Can be used to set up virtual circuit
All nodes are visited
Useful to distribute information (e.g. routing)

19. Random Routing
Node selects one outgoing path for retransmission of incoming packet
Selection can be random or round robin
Can select outgoing path based on probability calculation
No network info needed
Route is typically not least cost nor minimum hop

20. Adaptive Routing Used by almost all packet switching networks
Routing decisions change as conditions on the network change
Failure (node or trunk)
Congestion (portion of network is heavily loaded with traffic)
Requires info about network
Decisions more complex
Tradeoff between quality of network info and overhead
Reacting too quickly can cause oscillation
Reacting too slowly can be irrelevant

21. Adaptive Routing - Advantages
Improved performance
Aid congestion control
Complex system
May not realize theoretical benefits

22. Classification Based on information sources Local (isolated)
Route to outgoing link with shortest queue
Can include bias for each destination
Rarely used - do not make use of easily available info
Adjacent nodes
All nodes

23. ARPANET Routing Strategies(1)
First Generation
1969
Distributed adaptive
Estimated delay as performance criterion
Bellman-Ford algorithm
Node exchanges delay vector with neighbors
Update routing table based on incoming info
Doesn't consider line speed, just queue length
Queue length not a good measurement of delay
Responds slowly to congestion

24. ARPANET Routing Strategies(2)
Second Generation
1979
Uses delay as performance criterion
Delay measured directly
Uses Dijkstra’s algorithm (appendix 10a)
Good under light and medium loads
Under heavy loads, little correlation between reported delays and those experienced

25. ARPANET Routing Strategies(3)
Third Generation
1987
Link cost calculations changed
Measure average delay over last 10 seconds
Normalize based on current value and previous results

26. Static vs. Dynamic Routing
Routes are static if they do not change.
Route table is loaded once at startup and all changes are manual
Eg. Computers at the network edge use static routing.
Routes are dynamic if the routing table information can change over time (without human intervention.
Eg. Internet routers use dynamic routing.

27. Dynamic Routing and Routers
To insure that routers know how to reach all possible destinations, routers exchange information using a routing protocol.
But, we cannot expect every router to know about every other router.
Too much Internet traffic would be generated.
Tables would be huge (> routes)
Algorithms to choose “best” path would never terminate.
How to handle this?

28. Autonomous Systems (AS)
Routers are divided into groups known as an autonomous systems (AS).
ASs communicate using an Exterior Routing Protocol (Intra-AS Routing)
Routers within an AS communicate using an Interior Routing Protocol (Inter-AS Routing)

29. Interior and Exterior Routing

30. Why different Intra and Inter-AS routing ?
Policy:
Inter-AS: administration wants control over how its traffic routed, who routes through its network.
Intra-AS: single administration, so no policy decisions needed
Scale:
hierarchical routing saves table size, reduced update traffic
Performance:
Intra-AS: can focus on performance
Inter-AS: policy may dominate over performance

31. Required Reading
Stalling Chapter 12

32. Review questions
What is the major tradeoff in the design of a routing strategy for a circuit-switching network?
Distinguish between static and alternate routing in a circuit-switching network.
What are the key requirements for a routing function for a packet-switching network?
What is fixed routing?
What is flooding?
What is the difference between static and dynamic routing? Give an example for each.
What are the advantages and disadvantages of adaptive routing?
How do Internet routers cope with the increasing number of routes?