In most of the 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 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 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 node 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)

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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 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 reliable network
• Same for virtual circuit and datagram

Disadvantages –

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

• 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 above figure
  • A 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 is tried. A packet will always get through if path exists
• As all routes are tried, there will be atleast one route which is the shortest
• All nodes directly or indirectly connected are visited

Limitations –

• Flooding generates vast number of duplicate pakects
• 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 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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