Fixed and Flooding Routing algorithms

In most situations, packets require multiple hops to make a journey towards the destination. Routing is one of the most complex and crucial aspects of packet-switched network design. 

Desirable Properties of Routing Algorithms:-  

• Correctness and Simplicity
• Robustness: Ability of the network to deliver packets via some route even in the face of failures.
• Stability: The algorithm should converge to equilibrium fast in the face of changing conditions in the network.
• Fairness and Optimality
• Efficiency: Minimum overhead.

Design Parameters of Routing Algorithms :  

• Performance Criteria: Number of hops, Cost(Send packet with high bandwidth path as the cost is less), Delay(Size of Queue), Throughput time(Number of packets delivered/time).
• Decision Time: When to decide to route a packet? Per-Packet(Datagram) or Per-session(Virtual-Circuit).
• Decision Place: Who will decide about routing? Each Node(distributed), Central Node (centralized),Originated Node (source) .
• Network Information Source: None, Local, Adjacent node, Nodes along the route, All nodes.
• Network Information Update Time: Continuous, Periodic, Major Load Change, Topology Change.

Routing Strategies :  

1. Fixed Routing
2. Flooding
3. Dynamic Routing
4. Random Routing
5. Flow-based Routing

Fixed Routing –  

• A route is selected for each source and destination pair of nodes in the network.
• The route is fixed; changes only if the topology of the network changes.

Fixed Routing: Example (1)  

Figure – A simple packet switching network with six nodes (routers)  

Figure – Central routing table based on least-cost path algorithm  

• A Central routing matrix is created based on the least-cost path which is stored in the network control center
• The matrix shows for each source-destination of the route, the identity of the next node on the route.
• Drawback: If the network control center fails, then everything will collapse. Hence it is not reliable.

Fixed Routing: Example (2)  

Figure – Routing table stored in different nodes of the network 

• Routing Table is created for each node. This is called a distributed routing algorithm
• Routing table can be created using the least-min path or min-hop reach method. Two famous path algorithms 
  1. Dijkstra Algorithm
  2. Bellman Ford Algorithm

Advantages –  

• Simple
• Works well in reliable network with stable load in a reliable network
• Same for virtual circuit and datagram

Disadvantages –  

• Lack of flexibility
• Doesn’t react to failure or network congestion

Flooding –  

• Requires no network information like topology, load condition, cost of diff. paths
• Every incoming packet to a node is sent out on every outgoing like except the one it arrived on.
• For Example in the above figure
  • An incoming packet to (1) is sent out to (2),(3)
  • from (2) is sent to (6),(4), and from (3) it is sent to (4),(5)
  • from (4) it is sent to (6),(5),(3), from (6) it is sent to (2),(4),(5), from (5) it is sent to (4),(3)

Characteristics –  

• All possible routes between Source and Destination are tried. A packet will always get through if the path exists
• As all routes are tried, there will be at least one route which is the shortest
• All nodes directly or indirectly connected are visited

Limitations –  

• Flooding generates a vast number of duplicate packets
• Suitable damping mechanism must be used

Hop-Count –  

• A hop counter may be contained in the packet header which is decremented at each hop. 
  with the packet being discarded when the counter becomes zero
• The sender initializes the hop counter. If no estimate is known, it is set to the full diameter of the subnet.
• Keep track of the packets which are responsible for flooding using a sequence number. Avoid sending them out a second time.

Selective Flooding: Routers do not send every incoming packet out on every line, only on those lines that go in approximately in the direction of the destination. 

Advantages of Flooding :  

• Highly Robust, emergency or immediate messages can be sent (eg military applications)
• Set up the route in virtual circuit
• Flooding always chooses the shortest path
• Broadcast messages to all the nodes

Reference – 
Data and Computer Communications 

Read next article – Routing Protocols Set 1 (Distance Vector Routing)