Highly Dynamic Destination-Sequenced Distance-Vector Routing (DSDV) for Mobile Computers

1. Highly Dynamic Destination-Sequenced Distance-Vector Routing (DSDV) for Mobile Computers C. E. Perkins & P. Bhagwat Presented by Paul Ampadu

2. Introduction • Ad-hoc network is collection of mobile hosts that engage without centralized access point • Existing routing protocols • Lack dynamic self-starting capabilities • Too heavy computational burden on mobile hosts • Convergence characteristics unsuitable for ad-hoc • Wireless medium limited & variable range

3. Paper’s Contributions • Innovative distance-vector routing approach • Key idea: Operate each host as special router • Routing protocol modification to Bellman-Ford • Highly dynamic • Self-starting • Loop-free • Provides MAC-layer support

4. Overview of Routing Methods • Common objective: route packets along optimal path to destination • Each node i maintains for each destination j a preferred next-hop neighbor k • Using data packet’s destination identifier, forwarding using 2 broad routing methods: • Link-state and Distance-vector

5. Overview of Routing Methods • Link-state: Centralized shortest path problem • Each node maintains view of network topology • Periodically broadcasts link costs to all nodes • When nodes receive info, update view of network • Distance-vector: Distributed Bellman-Ford • Each node i maintains, for each destination j, a set of distances {dixj}, x is over all neighbors of i • Neighbor k is next hop if dikj = minj{dixj}

6. Distance-vector Routing • Computationally more efficient • Easier to implement • Requires less storage • Both temporary and lasting loops • Nodes choose routes in distributed fashion, based on possibly wrong stale information • Internodal coordination methods ineffective for rapid topological changes in ad-hoc networks

7. DSDV Protocol Approach: • Maintain distributed distance-vector attributes • Eliminate loops • Tag routing table entry with sequence number to distinguish stale routes from new ones • Compatible with base station operation • Layer 2 routing to easily detect broken link

8. DSDV Protocol • Routing Table (RT) contains not only destinations + metrics (e.g. num_hops), but also Sequence Number (SN) from destination • Each node transmits Network Protocol Data Unit (NPDU) containing SN, num_hops, dest. • Periodically (often enough) • Whenever new info available • SN freshness determines route reliability

9. 3 4 5 2 6 8 7 1 DSDV Operation Example Node 4 RT Structure

10. 3 4 5 2 6 8 7 1 DSDV Operation Example Node 4 Advertised route table No broken links (even digits in units place)

11. 3 4 5 2 6 8 1 7 1 DSDV Operation Example Updated Node 4 RT Structure

12. DSDV Properties Claim: DSDV guarantees loop-free paths to each destination, at all instants To see: • Routing tables of all N nodes form N trees rooted at destination • For destination j, directed graph G(j) defined by node i and arc (i, pij), pij next hop to j • G(j) forms a set of disjoint directed trees, rooted at j or NIL

13. Conclusion • Loop-free distance vector routing protocol suitable for ad-hoc networks • Preserves desirable distance-vector properties while solving looping problem with minimal complexity • Compatible with base-station operation • Can be modified to improve usage statistics