Routing is the mechanism for finding the most cost-effective path from source to destination in a packet switching network. There are several desirable properties for routing algorithms including correctness, robustness, stability, fairness, and efficiency. Common routing strategies include fixed/static routing, flooding, random routing, flow-based routing, and adaptive/dynamic routing. Fixed routing selects predetermined routes that may only change if the network topology changes, while flooding explores all possible routes by sending every incoming packet out every outgoing line except the one it arrived on.

1. ROUTING AND ROUTING
ALGORITHMS
Aadil A. Bhat (1502CUKMr001)
Zarak Reeman (1502CUKMr001)

2. ROUTING
 Routing is a mechanism of finding out most cost effective
path from source to destination.
 Routing is one the most complex and crucial aspects of
Packet Switching Network Design
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3. DESIRABLE PROPERTIES
 Correctness and simplicity : Packets should be delivered to
correct destination. Routing algorithm should be simple.
 Robustness: Ability of the network to deliver packets via some
route even in face of failures.
 Stability: The algorithm should converge to equilibrium fast in
the face of changing environment.
 Fairness and optimality: Obvious requirement but conflicting.
 Efficiency: Minimum Overhead.
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4. DESIGN PARAMETERS
 Performance Criteria: Number of Hops, Cost Delay,
Throughput.
 Decision Time: Per Packet(Datagram), per secession (Virtual
Circuit).
 Decision place: Each node distributed, Central node
(centralized), originated note (source).
 Network Information Source: None, local, adjacent node,
nodes along route, all nodes. 4

5. ROUTING STRATEGIES
 Fixed (static) Routing.
 Flooding
 Random Routing
 Flow based Routing
 Adaptive/ Dynamic Routing
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6. FIXED ROUTING
 A route is selected for each source-destination pair
of nodes in the network.
 The routes are fixed, they may only change if there
is change in topology of the network.
 How fixed routing may be implemented???
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7. FIXED ROUTING- EXAMPLE
 A central routing matrix is created based on least-
cost path, which is stored at a Network Control
Centre.
 The matrix shows, for each source-destination pair
of nodes, the identity of the next node of the route.
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8. FIXED ROUTING- EXAMPLE(CONT.)
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9. LEAST COST PATH
 A cost is associated with each link.
 The simplest criterion is to choose the minimum-
hop route through the network.
 A generalization is least cost
 For any pair of attached stations, the least cost
route through the network is looked for.
 For either case, several well known algorithms exist
to obtain the optimum path.
i. Dijkstra’s algorithm
ii. Bellman- Ford algorithm
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10. DIJKSTRA ALGORITHM
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12. FIXED ROUTING
 Advantages
 Simple
 Works well in the reliable network
 Same for virtual circuit and datagram
 Disadvantages
 Lack of flexibility
 Doesn’t react to failures or network congestion.
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13. FLOODING
 Requires no network information whatsoever
 Every incoming packet to a node is sent out on
every outgoing line except the one it arrived on.
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14. FLOODING CHARACTERISTICS
 All possible routes between source and destination
are tried. A packet will go through if a path exists.
 As all routes are tried, at least one packet will pass
through the shortest route.
 All nodes, directly or indirectly are visited
 Limitations
 Flooding generates vast number of duplicate packets
 Suitable damping mechanism must be used
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15. SELECTIVE FLOODING
 A variation which is more practical is selective
flooding.
 The router do not send every incoming packet out on
every line, only those lines that go in approximately in
the direction of destination.
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16. UTILITIES OF FLOODING
 Flooding is highly robust and could be used to send
emergency messages.
 May be used to initially set up the route in a virtual
circuit.
 Flooding always chooses the shortest since it
explores every possible path in parallel.
 Can be useful for the dissemination of important
information to all nodes
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