1. Overlay Networks + Internet routing has exhibited scalability - Internet routing is inefficient -Difficult to add intelligence to Internet Solution: Overlay networks (U. Washington, MIT)

2. Research Topic: Scalability Issues Complete virtual networks scale poorly MIT RON scales to about 50 or so nodes Primary challenge: overlay networks are modeled as a complete graph; Each node need to disperse O(n) information to O(n) neighbors resulting in O(n 3 ) traffic Solution: build sub-graphs Each node picks some random links (used in Yoid) + distributed algorithm - does not necessarily pick the good links Each node picks a few good links coming out of it (used in Narada) + distributed algorithm - does not guarantee connectivity! Graph theoretic algorithms by Roskind-Tarjan, Gabow + good approximate solutions to an NP-Complete problem - centralized algorithm Our approach: distributed, approximation to Roskind-Tarjan Real implementation on Planetlab nodes

3. Research Topic: Overlay Network Applications Coherent wide-area file systems (and object location mechanisms) on top of overlay networks Differs from existing object location schemes (which are based on distributed hash tables) in following ways: Provide mutable object semantics Provide different kinds of consistency Make the system “self-synchronizing” Meta-data (location information) needs to be persistent Need to exploit topology

4. Other research topics Evaluate mobile storage alternatives: What is the best file system for different storage technologies? File systems: unix file system, log-structure file system, and other variants Technologies: microdrives, compact flash, storage accessed through wireless Build a distributed file system for mobile devices Data exists at different locations with different kinds of connectivity At one extreme: can have disconnected operation Try to access the most accessible copy, try to minimize the amount of communication overhead to maintain consistency