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

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

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 Paper’s Contributions

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

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 {d ix j }, x is over all neighbors of i Neighbor k is next hop if d ik j = min j {d ix j } Overview of Routing Methods

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

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

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 DSDV Protocol

DSDV Operation Example Node 4 RT Structure

DSDV Operation Example Node 4 Advertised route table No broken links (even digits in units place)

DSDV Operation Example Updated Node 4 RT Structure

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, p i j ), p i j next hop to j G(j) forms a set of disjoint directed trees, rooted at j or NIL

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