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Fixed and Flooding Routing algorithms

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  • Difficulty Level : Medium
  • Last Updated : 13 Sep, 2021
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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)

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