---

Introduction to Routing Table construction

---

• Routing information
  • Recall that:

    Routers must know where to send/forward an IP packet The information needed to route a packet is stored in the routing table

    ---

    ---

  • The format of an entry in the routing table:

    (Destination IP network,     Next router IP address)

  ---

  ---

  ---

  ---

• Representing a computer network
  • A computer network can be represented by a graph.

    ---

  • Definitions: Graph:, nodes and edges

    A graph consists of: A set of nodes     and A set of edges connecting the nodes. Each edge on the graph has a cost associated with it. The cost represent the delay/time it takes for a packet to traverse the network link. Example: graph representing a communication network

  ---

  ---

  ---

  ---

  ---

  ---

• Cost of links
  • Link cost:

    Cost of a link = travel time (delay) of packets from the source node to the destination node

    ---

    ---

  • Recall that:

    Packet transmission time = queueing delay + Transmission delay

  ---

  ---

  ---

  ---

• Queueing delay
  Queueing delay:

  Queueing delay = time that a packet must waits before the packet is transmitted Graphically:

  ---

  ---

  Facts:

  Queueing delay is highly variable (i.e., constantly changing...) In congested links (i.e., long output queues), the queueing delay is very significant (much larger than than transmission time)

  ---

  ---

  • Conclusion:

    The best route from a source to a destination is: dynamic We must recompute the best routes from time to time...

  ---

  ---

  ---

  ---

• Shorted (best) routes and the Routing Table
  • Recall: shortest path (CS323)

    Shortest path = the path with the smallest total path cost from a source to a destination

    ---

  • Example:

    Network and the link costs: The shortest path from B ⇒ D (ties are broken arbitrarily): The shortest paths from node B to all other nodes:

    ---

    ---

  • From B's perspective, this is how the node B should forward packets:

    Destination | of packet | Next node --------------+------------ A | A B | - C | C D | A E | E F | F G | A

    Notice that:

    The next node is always a neighbor node of B !!!

    ---

    ---

    ---

  • Therefore, the routing table of node B is as follows:

  ---

  ---

  ---

  ---

• Measuring link costs

  • Facts:

    A node in the network is only aware of: The node's own immediate neighbor nodes (The node can send and receive frames from nodes that are directly connected to them) A node can compute the queueing delay: queue delay = queueing size (#bytes)/transmission speed A node can also measure (estiamte) the transmission delay to a neighbor nodes: Transmission delay = ~= 1/2 × round trip delay

  ---

  ---

  ---

  ---

• Overview routing algorithms
  • Types of routing algorithms:

    Centralized (Dijkstra) The link cost information from all nodes is sent to every node (So each node has the link cost of every link) Each node will compute the shortest paths on its own Distributed (distance vector algorithm) A gossip-based algorithm....