Fundamentals of Computer Networks ECE 478/578

1. Fundamentals of Computer NetworksECE 478/578 Lecture #17: Routing 2 Instructor: LoukasLazos Dept of Electrical and Computer Engineering University of Arizona

2. Broadcast Routing Broadcast Communication: All nodes of the network must receive the same message TV, radio are by default broadcast operations Unicasting to each node Source needs transmit message N times Addresses of all recipients must be known A total of N messages must traverse the network Flooding Inefficient use of bandwidth – Nodes receive the same message multiple times Does not scale with network size N Spanning Tree Routing Build a (minimum) spanning tree from source to all nodes Source need send the message only once Message is relayed a total of (N - 1) times

3. Building a Spanning Tree Kruskal’s Algorithm: Greedy approach Add the edge with the smallest weight that adds connects one or two new nodes together Repeat until all nodes are added Broadcasting All edges have the same weight Any spanning tree is a minimum spanning tree Spanning tree is the same regardless of the source of the message

4. Reverse Path Forwarding If packet arrives from a “preferred router” forward, otherwise discard Preferred router: router on the reverse shortest path to the source Source

5. Broadcasting in Wireless Multi-hop Nets Must take into account energy The Broadcast advantage SBn > SAn + ABn, broadcast to A and A will forward to B Else use one transmission to B BIP: Broadcast Incremental Power B S A S • T={S} • Add node j, which minimally increases the power required to reach new node • Repeat until all nodes are added to T • Greedy algorithm, that leads to a non-optimal solution A B C D E

6. Routing for Mobile Hosts Host moves from one network to another If IP is changed, certain services may not work

7. Optimizing Routing for Mobile Hosts

8. Routing in Ad Hoc Networks Ad Hoc Distance Vector Routing (AODV) Source node floods a RREQ message Every receiving hosts keeps a reverse path to where the packet came from Once the message reaches the destination, it replies with a RREP RREP is forwarded on a unicast path back to the source

9. Message Format Route Request Message Destination Sequence Number: Freshness of route to dest. Source Sequence Number: Freshness of RREQ Route Reply Message

10. Route Maintenance Active neighbors: Neighbors that maintain active paths

11. Dynamic Source Routing (DSR) The source determines the sequence of nodes that the packet must follow based on Cashed Information A route discovery phase Advantages of DSR No periodic route updates as in DV, saves energy resources No need for bidirectional links as in AODV Can quickly adapt to changes Disadvantages of DSR Route is specified on the packet header that can grow fairly long Uses flooding for routing discovery as well

12. Route Discovery in DSR Sender broadcasts a route request (RREQ) packet Similar format to AODV packet Each Intermediate node <Source address, request id> If same, discard The address of the intermediate node is already in the route record This is a loop - discard This node is the target Send a route reply (RREP) Else Append node’s address to the route record, and re-broadcast

13. Example: Routing with DSR A A A, B B C A A, B A, D D A, B, C E F A, D A, B, C, E A, B, D, G G A, D, F A, D, G H I A, D, F, H

14. Geographical Routing Route based on geographical location Send packet to the neighbor closest to the destination, based on geographic coordinates No need for route discovery, or routing tables A B C D E F G H I

15. Geographical Routing May Get Stuck A node is closest to the destination, yet there is no transition to the destination

16. Avoid local Minima Create a planar graph of the network Route in the perimeter until you hit the straight line